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Asthenospheric source of Neoproterozoic and Mesozoic kimberlites from the North Atlantic craton, West Greenland: New high-precision U–Pb and Sr–Nd isotope data on perovskite
... Ultramafic carbonate-rich dykes were emplaced in the center of the NAC at Tikiusaaq at around 160 Ma and, similar to the Maniitsoq kimberlite dykes, are interpreted to derive from the convective upper mantle based on their Sr-Nd-Hf-Pb-Li isotopic compositions (Tappe et al., 2012. They have relatively enriched K 2 O compositions (0.92 -1.40 wt.% ...
... U-Pb dating of groundmass perovskite provides emplacement ages of between 157.4 -165.9 Ma for the Tikiusaaq kimberlite dykes (Tappe et al., 2012). For detailed field relations plus petrographic and mineralogical descriptions of the samples used in this study see Steenfelt et al. (2006) and Tappe et al. (2009). ...
... The temporal and spatial relationship of kimberlites and related rocks including carbonatites is widely acknowledged (Dalton and Presnall, 1998;Woolley and Kjarsgaard, 2008;Tappe et al., 2020b), with the coexistence of kimberlite and carbonatite intrusives commonly (but not exclusively) explained by exsolution and removal of a substantial fluid/carbonate phase during kimberlite magma ascent through the shallow cratonic mantle and crust . The enriched halogen compositions of carbonatite magmas (Mangler et al., 2014) suggest that the exsolution of a carbonate melt phase via liquid immiscibility from a carbonated silicate magma could offer a solution to the extreme Cl and Br depletion observed in kimberlites from the NAC, especially because kimberlites and carbonatites are frequently observed to coexist in West Greenland (Larsen and Rex, 1992;Tappe et al., , 2011Tappe et al., , 2012. However, for Nigerdlikasik and Pyramidefjeld in South-West ...
Kimberlite magmatism occurs as a result of volatile fluxed melting of the convecting upper mantle underlying cratonic lithospheric mantle regions. During passage to the Earth's surface, proto-kimberlite magma can interact with, and assimilate, variably enriched cratonic mantle producing hybrid melts consisting of asthenospheric and cratonic mantle components including contributions from metasomatic domains. The halogen elements (F, Cl, Br, I) and chlorine isotope ratios (³⁷Cl/³⁵Cl) are increasingly used as tracers of recycled crustal materials within the Earth's mantle yet are only rarely reported in analyses of kimberlites. As a result, the origin and distribution of halogens in kimberlite magmas is poorly constrained. Here, we present novel, combined elemental (F, Cl, Br) and isotopic (δ³⁷Cl) halogen data for 14 fresh kimberlite samples from the North Atlantic Craton (NAC) of West and South-West Greenland.
The F composition of kimberlites from the NAC appears to be controlled by melting in the convecting upper mantle with minimal effect from interaction with metasomatized lithospheric mantle or volatile loss during or after emplacement. By contrast, Cl and Br in the studied samples have undergone significant devolatilization during kimberlite dyke emplacement and post-emplacement processes, whereby up to 99% of the original halogen budget was removed.
Whilst all the studied kimberlites broadly follow the same geochemical pattern, there exists some regional variability in their halogen systematics. The northern NAC kimberlite sampled at Majuagaa has mantle-like δ³⁷Cl values of −0.2 to −0.5‰ [versus SMOC (standard mean ocean chloride)]. In contrast, kimberlite dykes from Nigerdlikasik and Pyramidefjeld near the southern craton margin display positive δ³⁷Cl values of +0.4‰ to +1.3‰, in addition to a relative Cl and Br enrichment, which is consistent with the assimilation of recycled crust-derived halogens by the kimberlite magmas. The data support a scenario in which recycled halogens were sampled either from within an OIB-type reservoir in the convecting mantle or through interaction with subduction-modified lithospheric mantle reservoir during eruption. We prefer a scenario in which the ascending kimberlite magmas assimilated Cl-rich, metasomatized regions within cratonic mantle lithosphere.
... Ga, 0.14-0.10 Ga), followed by opening of the North Atlantic and craton breakup along the Labrador Sea margins, caused successive thinning of the cratonic lithospheric mantle (Tappe et al. 2007(Tappe et al. , 2011(Tappe et al. , 2012Larsen et al. 2009). The attendant intrusion of kimberlites, ultramafic lamprophyres, and alkali basalts into the GNAC and bordering mobile belts exhumed mantle xenoliths and xenocrysts, including diamond (Bernstein et al. 1998(Bernstein et al. , 2006(Bernstein et al. , 2013Wittig et al. 2008Wittig et al. , 2010Sand et al. 2009;Bizzarro and Stevenson 2003;Tappe et al. 2011a, b;Aulbach et al. 2017). ...
... A relatively high geothermal gradient determined for garnet peridotite xenoliths from the ca. 150 Ma Pyramidefjeld and Midternaes kimberlites in the SW-GNAC (corresponding to a surface heat flow of 42-44 mW/m 2 ; Nielsen et al. 2008), compared to that for mantle xenoliths from 590 to 550 Ma kimberlites and ultramafic lamprophyres in the NW-GNAC (~ 38 mW/m 2 ; Sand et al. 2009), was ascribed to intervening heating and lithosphere thinning during incipient rifting of the NAC (Tappe et al. 2007(Tappe et al. , 2012Larsen et al. 2009). ...
... As in previous Re-Os isotope studies on Greenland peridotite xenoliths, there is no correspondence between Re/Os and 187 Os/ 188 Os that would permit an isochron approach to dating events in the lithospheric mantle (Wittig et al. 2010) (Fig. 5d). The unusually strong disturbance of this otherwise robust isotope system is likely due to multiple rifting episodes affecting the GNAC, that were all accompanied by migration of small-volume carbonate-rich melts derived from the underlying convecting mantle (Tappe et al. 2007(Tappe et al. , 2012, combined with its setting at the southern craton margin where it was affected by the Palaeoproterozoic Ketilidian orogeny (Fig. 1). ...
We report highly siderophile element (HSE) abundances and Re-Os isotope compositions, obtained by isotope dilution induc-tively coupled plasma mass spectrometry, of olivine separates from a suite of multiply metasomatised peridotite xenoliths entrained in kimberlites from SW Greenland. Combined with petrographic and compositional observations on accessory base metal sulphides (BMS), the results reveal new insights into the chemical, physical and mineralogical effects of multi-stage rifting and associated melt percolation on the Archaean lithospheric mantle. Refertilised lherzolites are dominated by rare to frequent small (tens of µm) BMS inclusions in olivine, whereas modally metasomatised phlogopite-bearing lherzolite and wehrlites have higher proportions of more Ni-rich BMS, including abundant large interstitial grains (hundreds of µm). The olivine separates display depleted HSE systematics with Primitive Upper Mantle (PUM)-normalised Pd/Ir of 0.014-0.62, and have both depleted and enriched 187 Os/ 188 Os (0.1139-0.2724) relative to chondrite that are not correlated with 187 Re/ 188 Os. Four out of ten olivine separates retain similarly depleted Os corresponding to Re-depletion model ages of 2.1-1.8 Ga. They may reflect Palaeoproterozoic refertilisation (lherzolitisation) during Laurentia plate assembly, with re-introduction of clinopyroxene and Os-rich BMS into the originally refractory mantle lithosphere by asthenosphere-derived basaltic melts, followed by recrystallisation and occlusion in olivine. Unradiogenic Os is observed regardless of lithology, including from peridotites that contain abundant interstitial BMS. This reflects addition of Os-poor BMS (<< 1 ppm) during more recent wehrlitisation and phlogopite-introduction, and control of the Os isotopic signature by older Os-rich BMS that precipitated from the basaltic melt. Depletions in compatible HSE (< 0.5 × PUM for Ru, Ir, Os) in all, but one olivine separate reflect nugget effects (amount of depleted vs. metasomatic BMS inclusions) and/or loss due to sulphide dissolution into oxidising small-volume melts that invaded the lithosphere during recurrent rifting, the latter supported by similar depletions in published bulk peridotite data. Combined, these multiple metasomatic events destroyed all vestiges of Mesoarchaean or older inheritance in the olivine separates investigated here, and highlight that caution is needed when interpreting Proterozoic Os model ages in terms of Proterozoic lithosphere stabilisation.
... The resulting alkaline rocks can offer valuable insights into mantle dynamics (Molnar et al., 1993;Tapponnier et al., 2001) and allow a better understanding of the tectonic and topographic evolution of the India-Asia collision zone (see the review by Wang et al. [2014]). However, inevitable contamination and hydrothermal alteration at crustal levels make it difficult to constrain the nature of sources through bulk-rock data from mantle-derived rocks (Tappe et al., 2012;Sun et al., 2014). The lack of accurate and precise age data on alkaline rocks with mantle affinity also leads to ambiguities in their correlations with regional tectonic processes on various scales and hampers understanding how the Tibetan Plateau may have grown laterally in space and time. ...
... Perovskite (CaTiO 3 ) is a common accessory mineral in SiO 2 -undersaturated rocks and serves as a major carrier of Th, U, Sr, and rare earth elements (Chakhmouradian et al., 2013). It is thus suitable for U-Th-Pb dating, and its radiogenic isotopes can also provide reliable information about magmatic sources and processes (Yang et al., 2009;Tappe et al., 2012). Here we present a combined study of in situ U-Pb dating, trace element analysis, and Sr-Nd isotope compositions on perovskites extracted from melilitite samples in the West Qinling orogenic belt. ...
... The early-crystallizing perovskites can better preserve isotopic signatures of the primary magma than the bulk-rock samples (Tappe et al., 2012;Sun et al., 2014). In situ Sr-Nd isotopic analyses indicate that most perovskites display lower 87 Sr/ 86 Sr i and slightly higher ε Nd (t) than the melilitite and entrained mantle xenoliths (Fig. 3). ...
Mantle-derived alkaline magmatism along major strike-slip faults provides unique insights into the lateral growth of the Tibetan Plateau. Here we use the geochemistry of perovskites from the West Qinling melilitite to probe into the nature and dynamics of sub-lithospheric mantle beneath the northeastern Tibetan Plateau. The texture and chemical composition of perovskites indicate their early crystallization from a CO2-rich melilitite magma. Most perovskite crystals have moderately depleted Sr-Nd isotopic compositions, whereas a few grains
exhibit high 87Sr/86Sri and low εNd(t). Together with the bulk-rock geochemistry of the melilitite, the perovskite Sr-Nd isotope data imply that the primary magma parental to the melilitite was most likely derived from seafloor subduction–modified asthenosphere and underwent interaction with lithospheric mantle during ascent. In situ U-Pb dating of the perovskites demonstrates the temporal correlation between the melilitite magmatism and Kunlun strike-slip faulting in the early Miocene. These findings indicate the fundamental role of India-Asia convergence in driving outward plateau growth through strike-slip extrusion and in reactivating long-lived lithospheric zones of weakness for evacuating low-volume asthenospheric melts.
... One mineral that meets all of the above criteria is perovskite (ideally CaTiO 3 ), occurring as a widespread and in some cases, volumetrically significant groundmass phase in kimberlites (Chakhmouradian and Mitchell, 2000). Uranium\Th\Pb geochronology of this mineral using a variety of mass-spectrometric techniques has become an essential aspect of kimberlite research (Heaman and Kjarsgaard, 2000;Batumike et al., 2008;Li et al., 2010;Donatti-Filho et al., 2013-this volume;Tappe et al., 2012;Wu et al., 2013-this issue). In addition, the importance of perovskite as a source of isotopic (Sr, Nd and Pb) evidence for unraveling the mantle sources of kimberlites has been increasingly recognized in recent years (Paton et al., 2007;Woodhead et al., 2009;Yang et al., 2009;Wu et al., 2010Wu et al., , 2013Tappe et al., 2012). ...
... Uranium\Th\Pb geochronology of this mineral using a variety of mass-spectrometric techniques has become an essential aspect of kimberlite research (Heaman and Kjarsgaard, 2000;Batumike et al., 2008;Li et al., 2010;Donatti-Filho et al., 2013-this volume;Tappe et al., 2012;Wu et al., 2013-this issue). In addition, the importance of perovskite as a source of isotopic (Sr, Nd and Pb) evidence for unraveling the mantle sources of kimberlites has been increasingly recognized in recent years (Paton et al., 2007;Woodhead et al., 2009;Yang et al., 2009;Wu et al., 2010Wu et al., , 2013Tappe et al., 2012). ...
... Whereas fractionation of olivine, spinel and possibly other ferromagnesian phases (e.g., Pasteris, 1983;Eccles et al., 2004;Tappe et al., 2012) is expected to cause depletion of evolved kimberlites in certain compatible trace elements, accompanied by their enrichment in Ba, Sr, Nb and other elements not readily incorporated in the early-crystallizing mineral assemblage (e.g., Fig. 6a and b), it does not explain significant spread in La/Yb and Th/U ratios (59-625 and 0.8-18.5, respectively), or "anomalous" Nb/Ta and Zr/Hf values in some kimberlites ( Fig. 6c and d). ...
... Several failed rifting episodes at 2Á0 Ga, 1Á4-1Á2 Ga and 0Á61-0Á55 Ga led to the emplacement of mafic dyke swarms and of volumetrically minor ultramafic alkaline magmas. This lithospheric stretching affected the underlying continental lithospheric mantle, leading to its partial destruction and replacement with asthenospheric mantle (Tappe et al., 2007(Tappe et al., , 2012. At 140-100 Ma, the development of rift basins and emplacement of alkaline basalts signalled the beginning of the opening of the North Atlantic Ocean, with breakup of the NAC along the Labrador Sea margins at 60-30 Ma (Tappe et al., 2007Larsen et al., 2009). ...
... A lower geothermal gradient ($38 mW m -2 ) was obtained for mantle xenoliths from the NW-GNAC, which were entrained during 590-550 Ma emplacement of kimberlites and ultramafic lamprophyres (Sand et al., 2009). This suggests that substantial heating and related lithosphere thinning, related to incipient breakup of the NAC, occurred in the time interval up to the Jurassic kimberlite emplacement at c. 150 Ma in the southwestern part of the craton (Tappe et al., 2007(Tappe et al., , 2012Larsen et al., 2009). These processes were probably accompanied by decompression owing to rifting, during which garnet was destabilized, as observed in Antarctica and the East African Rift (see Foley et al., 2006;Kaeser et al., 2006). ...
... Whereas the average initial (at $150 Ma) 143 Nd/ 144 Nd for cpx of 0Á51270 (e Nd,i þ5Á1) is identical to that obtained for the nearby c. 160 Ma Tikiusaaq kimberlite dykes (0Á51268), the inital 87 Sr/ 86 Sr of 0Á70375 is distinctly higher than in the kimberlite (0Á70334; Tappe et al., 2012). This suggests that the mantle lithosphere beneath the SW-GNAC interacted with a metasomatic agent that was isotopically similar but not identical to the host kimberlite. ...
The cratonic part of Greenland has been a hotspot of scientific investigation since the discovery of some of the oldest crust on Earth and of significant diamond potential in the underlying lithospheric mantle, the characterisation of which remains, however, incomplete. We applied a detailed petrographic and in situ analytical approach to a new suite of fresh kimberlite-borne peridotite xenoliths, recovered from the North Atlantic craton in SW Greenland, in order to unravel the timing and nature of mantle metasomatism, its link to the formation of low-volume melts (e.g. kimberlites), and to geophysically detectible discontinuities. Two types of mineralogies and metasomatic styles, occurring at two depth intervals, are recognised: (1) Lherzolites, harzburgites and dunites, some phlogopite-bearing, from ∼100-170 km depth form continuous trends towards lower mineral Mg# at increasing TiO2, MnO and Na2O and decreasing NiO contents. These systematics are ascribed to metasomatism by a hydrous silicate melt precursor to ca. 150 Ma kimberlites, in the course of rifting, decompression and lithosphere thinning. This metasomatism was accompanied by progressive garnet-breakdown, texturally evident by pyroxene-spinel assemblages occupying former coarse grains and compositionally by increasing concentrations of elements that are compatible in garnet (Y, Sc, In, HREE) in newly-formed clinopyroxene. Concomitant sulphide saturation is indicated by depletion in Cu, Ni and Co. The residual, more silica-undersaturated and potentially more oxidising melts percolated upwards and metasomatised the shallower lithospheric mantle, which is composed of (2) phlogopite-bearing, texturally equilibrated peridotites, including wehrlites, showing evidence for recent pyroxene breakdown. These lithologies occur at ∼90-110 km depth and are inferred to have highly depleted protoliths. They are compositionally distinct from lherzolites, with olivine having higher Ca/Al, but lower Al and V contents. While low Al may in part reflect lower equilibration temperatures, low V is ascribed to a combination of intrinsically more oxidising mantle at lower pressure and oxidative metasomatism. The intense metasomatism in the shallow cratonic mantle lithosphere contrasts with the strong depletion recorded in the northwestern part of the craton, which at 590-550 Ma extended to > 210 km depth, and suggests loss of ∼40 km of lithospheric mantle, also recorded in the progressive shallowing of magma sources during the breakup of the North Atlantic craton. The concentration of phlogopite-rich lithologies in a narrow depth interval (∼90-110 km) overlaps with a negative seismic velocity gradient that is interpreted as a mid-lithospheric discontinuity beneath western Greenland. This is suggested to be a manifestation of small-volume volatile-rich magmatism, which paved the way for Mesozoic kimberlite, ultramafic lamprophyre, and carbonatite emplacement across the North Atlantic craton.
... The rifted North Atlantic craton, with fragments preserved in eastern Canada and Greenland (Fig. 1A, B), represents one of the best natural laboratories to study the relationships, if any, between cratonic carbonatites and kimberlites as well as ultramafic lamprophyres (Tappe et al., 2007). Several kimberlite dyke swarms of Neoproterozoic and Mesozoic age occur along the passive margin of West Greenland, and some of these dyke swarms are associated with contemporaneous intrusive carbonatite complexes (Larsen et al., 2009;Tappe et al., 2012). We have undertaken the first radiogenic Sr-Nd-Hf-Pb and stable Li-C-O isotope study of the intrusive carbonatite complex and associated kimberlite dykes at Tikiusaaq in West Greenland (Fig. 1C). ...
... Between 1400 and 1200 Ma, H 2 O-rich ultrapotassic melts (i.e., olivine lamproites) erupted along the craton margins, likely as a consequence of hydrous redox melting (Tappe et al., 2007). Carbonate-rich magmatism (i.e., kimberlites, ultramafic lamprophyres, carbonatites) appeared across the entire craton between 600 and 540 Ma (Tappe et al., 2006(Tappe et al., , 2011, and again between 220 and 150 Ma (Larsen et al., 2009;Tappe et al., 2012). The widespread carbonate-rich magmatism was likely a consequence of gradually more oxidizing conditions beneath the thinning craton root, which resulted in H 2 O-and CO 2 -rich mantle melting conditions at relatively low temperatures (Foley, 2011). ...
... Seafloor-spreading was preceded by extensive continental rifting of the North Atlantic craton, which started at ca. 140 Ma. The pronounced carbonatite and kimberlite magmatic activity in West Greenland between 220 and 150 Ma was due to increasing volatile-fluxing and incipient thinning of the destabilizing cratonic mantle lithosphere (Tappe et al., 2007(Tappe et al., , 2012Larsen et al., 2009). The box indicates the margin of West Greenland that is shown in more detail in Panel C. (C) Geology of the on-and off-shore sectors of West Greenland adapted from Chalmers and Pulvertaft (2001) and Larsen et al. (2009). ...
Kimberlite and carbonatite magmas that intrude cratonic lithosphere are among the deepest probes of the terrestrial carbon cycle. Their co-existence on thick continental shields is commonly attributed to continuous partial melting sequences of carbonated peridotite at >150 km depths, possibly as deep as the mantle transition zone. At Tikiusaaq on the North Atlantic craton in West Greenland, approximately 160 Ma old ultrafresh kimberlite dykes and carbonatite sheets provide a rare opportunity to study the origin and evolution of carbonate-rich melts beneath cratons. Although their Sr–Nd–Hf–Pb–Li isotopic compositions suggest a common convecting upper mantle source that includes depleted and recycled oceanic crust components (e.g., negative coupled with ⁷Li), incompatible trace element modelling identifies only the kimberlites as near-primary low-degree partial melts (0.05–3%) of carbonated peridotite. In contrast, the trace element systematics of the carbonatites are difficult to reproduce by partial melting of carbonated peridotite, and the heavy carbon isotopic signatures (−3.6 to ¹³C for carbonatites versus −5.7 to ¹³C for kimberlites) require open-system fractionation at magmatic temperatures.
... The Sarfartôq and Sisimiut alkaline provinces are part of an extensive belt of epizonal ultramafic dykes and sheets, which resulted from the interaction between asthenosphere-derived carbonate-rich melts and variably metasomatized SCLM (Nelson, 1989;Larsen and Rex, 1992;Nielsen et al., 2009;Tappe et al., 2011;Pilbeam et al., 2013). The magmatic bodies originated during a prolonged period of regional magmatism during the Late Neoproterozoic (590-550 Ma; Tappe et al., 2012); they intruded into the heterogeneous gneiss-dominated basement that extends from the Archean NAC in the south, across the Ikertooq shear zone, to the c. 1.8-Ga Nagssugtôqidian Orogen in the north (Fig. 1). Nagssugtôqidian reworking involved tectono-magmatism and -following Van Gool et al. (2002) -southward subduction of terranes north of the Ikertooq boundary. ...
... The Sarfartôq location is on the northernmost margin of the NAC. Magmatism at this location ended with the simultaneous emplacement of dykes and sheets comprising aillikite -technically aillikite straddling the kimberlite field based on groundmass paragenesis ) -calcite-kimberlite and REE-rich carbonatite (Larsen and Rex, 1992;Mitchell et al., 1999;Hutchison and Heaman, 2008;Hutchison and Frei, 2009;Tappe et al., 2012). Sarfartôq mantle xenocrysts and xenoliths locally include high-Cr/Ca 'G9' and 'G10' pyrope, chromite, and sporadic Mengel et al. (1998), showing the location of the Sisimiut (SS) and Sarfartôq (SR) alkaline provinces and the Archean-Proterozoic terrane boundary from Nielsen et al. (2009). ...
... Neoproterozoic magmatism in the region (590-550 Ma; Tappe et al., 2012), yet they also outlast the duration of xenolith transport (4 ± 1 h) by c. 5 orders of magnitude. The grain-scale metasomatism and mass transfer recorded in garnet thus represents a previously unrecognized stage of melt metasomatism associated with individual dykes within the investigated alkaline provinces. ...
... That is why the issues of the occurrence of ilmenite, its mantle sources, and its genetic connection with kimberlite melt continue to be discussed. Potential origins of Ilm macrocrysts and megacrysts include: (I) the disaggregation of Ilm-bearing lithospheric mantle lithologies [1,7,8]; (II) crystallization within the asthenosphere [9][10][11][12]; (III) crystallization from an asthenospheric melt within the lithosphere associated with kimberlite magmatism [4,[13][14][15][16][17]; a modern take on this previous model is (IV) formation in a "metasomatic aureole" surrounding the (proto-) kimberlite melt and or previous pulses of failed (proto-)kimberlite melt, alongside other megacryst suite minerals and sheared xenoliths [18][19][20][21][22]. ...
... The similarity of Rb-Sr, Sm-Nd, and Lu-Hf isotope systematics, the same age of formation [11,15,16,18,28,34] for kimberlites and low-Cr megacryst association of minerals (to which Ilm belongs) testify to a single primary asthenospheric source for them. The similar or almost identical compositions of Ilm in different pipes of one cluster can be accounted for by the existence of a common magmatic supply channel. ...
The main regularities of the saturation of kimberlite rocks with the accessory mineral Mg-ilmenite (Ilm), the peculiarities of the distribution of Ilm compositions in individual pipes, in different clusters of pipes, in diamondiferous kimberlite fields, are considered as the example of studies carried out within the Yakutian kimberlite province (Siberian Craton). Interpretation of different crystallization trends in MgO-Cr2O3 coordinates (conventionally named “Haggerty’s parabola”, “Steplike”, “Hockey stick”, as well as the peculiarities of heterogeneity of individual zonal and polygranular Ilm macrocrysts made it possible to propose a three-stage model of crystallization Ilm: (1) Mg-Cr poor ilmenite crystallizing from a primitive asthenospheric melt; (2) Continuing crystallization in the lithospheric contaminated melt by MgO and Cr2O3; (3) Ilmenite subsequently underwent sub-solidus recrystallization in the presence of an evolved kimberlite melt under increasing oxygen fugacity (ƒO2) conditions.
... This is best illustrated when comparing groundmass perovskite (with less radiogenic 87 Sr/ 86 Sr, Fig. 6a) to their host kimberlite (whose 87 Sr/ 86 Sr ratios extend to more radiogenic values at more or less unchanged epsilon Nd values, Fig. 6b). A similar trend is observed in kimberlites from Western Greenland kimberlites (Fig. 4 in Tappe et al., 2012). It is worth noticing that the Sr isotope range in salty kimberlites is bracketed on one side by the perovskite composition (Kamenetsky et al., 2009), and on the other side by the chloride-carbonate nodule composition (Fig. 6). ...
... Furthermore, in kimberlites worldwide, Sr isotopes display a range that extends to radiogenic Sr compositions at nearly constant Nd isotope values. The origin of such a range is most often interpreted as the result of overprint during magma cooling in the presence of fluids (Tappe et al, 2012). However, in the absence of any sample of these hypothetical fluids, this interpretation has remained highly speculative. ...
Deep, carbonate-rich melts are key constituents of kimberlites and are crucial for understanding the cycle of volatile elements in the mantle. On the Siberian craton, the Udachnaya-East kimberlite hosts extremely well-preserved nodules composed of chlorides + carbonates + sulfates, that do not present any relict sedimentary textures. These salty nodules display textures that are commonly observed in quenched liquids and may thus represent the very last stage liquid of the kimberlite. Alternatively, they could represent assimilated sedimentary material, or even post-magmatic hydrothermal alteration, because kimberlites are known to ascend through the lithosphere while assimilating material from their wall rocks.
Here we focus specifically on those chloride-carbonate nodules, which are composed of 70% chloride + 30% alkali-carbonate and sulfate, and used two radiogenic systems (Rb-Sr, Sm-Nd) and the isotopic composition of sulfur, in addition to their major and trace element compositions (n=3). We then compared the results with the same geochemical data on host kimberlites (n=4), sedimentary cover (n=3) and hydrothermal veins (n=3).
Taken together, our results show that the nodules are not the product of a contamination by the Cambrian sedimentary cover. Trace element patterns of the nodules display extreme enrichments in the same elements that are relatively depleted in the host kimberlite but also in kimberlites worldwide (K, Rb, Sr, Pb), suggesting that chloride-carbonate nodules are snapshots of the latest stage liquid present in the kimberlite system. Their isotopic compositions (Rb-Sr, Sm-Nd and δ³⁴S) are consistent with a common magmatic source with their host kimberlite. We propose that chloride-carbonate nodules record a missing compositional endmember, which could explain the trend towards more radiogenic Sr isotope ratios at nearly constant Nd signatures observed in their host kimberlite, as well as in other kimberlites worldwide. This observed trend suggests the presence of a recycled component with high Rb/Sr (such as salts or terrigenous sediments) in the mantle sampled by some kimberlites, either in the lithosphere or the asthenosphere. This study highlights that the role of alkalies and halogens may have been underestimated in the genesis of kimberlites at depths where diamonds are stable, as well as in more evolved magmatic stages. Segregations of chlorides and carbonates occur specifically in sulfate-bearing kimberlites, which may thus sample a mantle domain in which sulfates with δ³⁴S > 0‰ are dominant. The existence of such a reservoir could explain the apparent imbalance observed between the chondritic value (δ³⁴S of 0‰) and the negative S isotopic compositions of mantle sulfides (MORB and peridotites).
... Whole-rock 87 Rb/ 86 Sr and 87 Sr/ 86 Sr i ratios vary between 0.704 and 0. 708, while εNd i values range from 1.9 to 4.3. The respective 87 Sr/ 86 Sr i and εNd i values of 0.704 and 4.3 for Novinka kimberlite (Table 3) S16) are due to the 'secondary' post-emplacement processes (e.g., low-temperature hydrothermal overprint; Tappe et al. 2012) or may relate to the interaction with crustal fluids, showing high Sr values similar to the country rocks. ...
... Existing models propose a LAB transition zone, i.e. asthenospheric mantle source for parental kimberlite melts (e.g. Ringwood et al. 1992;Nowell et al. 2004;Field et al. 2008;Arndt et al. 2010;Tappe et al. 2012;Stamm and Schmidt 2017;Foley et al. 2019;Giuliani et al. 2020). Some authors agree that kimberlite melts argue strongly for a source located in the convecting mantle or sublithospheric mantle (Griffin et al. 2000;Davies et al. 2001Choukroun et al. 2005EDQ9 Pearson et al. 2019). ...
Petrographic, geochemical and mineralogical characteristics of diamond deposits from the Upper Muna field have been investigated. Geochemically, diamondiferous kimberlites from Upper Muna belong to the most widespread Fe-Mg-rich rocks in the Yakutian kimberlite province (average FeO total = 8.4 wt%, MgO = 32.36 wt%, TiO 2 = 1.6 wt.%). Striking mineralogical features of Upper Muna kimberlites are: 1) abundance of monticellite and perovskite in the groundmass; 2) rare occurrence of Mg-ilmenite; 3) abundance of phlogopite megacrysts (up to 8 cm across); 4) coexistence of low-Cr (0.1–4wt. % Cr 2 O 3 , with 0.8–1.2 wt.% TiO 2 ), and high-Cr (3–8 wt.% Cr 2 O 3 , with 0.1-0.6 wt.% TiO 2 ) garnet megacrysts with contrasting REE patterns. The compositional features of groundmass minerals, the relatively low CaO and CO 2 contents in kimberlites, and few deuteric alteration in Upper Muna kimberlites suggest high-temperature melt crystallization during pipe emplacement. Based on the compositional data of garnet and Cr-diopside from megacrysts and peridotites, we suggest a poor Cr dunite-harzburgitic and lherzolitic mantle source beneath the Upper Muna field where Cr-diopside crystallized within a wide P-T range (40–65 kbar and 900–1350 °C). Mineral geochemistry, trace element distribution and Sr-Nd isotope variations of Upper Muna kimberlites are typical for group I kimberlites and reflect a deep-seated asthenospheric (convective mantle) source for the kimberlites.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.5442956
... Kimberlites (formerly known as Group 1 kimberlite), primarily represent mantle sources having inputs from the convecting mantle (e.g. Foley, 1992;Tappe et al., 2008;Tappe et al., 2012;Pearson et al., 2019;Woodhead et al., 2019). Kaapvaal lamproites (formerly known as Group 2 kimberlite or orangeite) on the other hand represent lithosphere sourced/ influenced magmas (e.g. ...
... Deep sourced metasomatizing fluids/ melts emanating from the upwelling mantle (e.g. Foley, 1992;Tappe et al., 2012), are therefore responsible for the enrichment of source regions of the Wajrakarur kimberlites including that of the P-12. Dongre et al. (2020), based on the available and newly dated precise ages, observed a significant and a very specific time period of kimberlite magmatism which occurred during 1050-1153 Ma in the Indian shied. ...
Petrology, bulk-rock geochemistry, and perovskite U-Pb age for the P-12 kimberlite pipe from the Wajrakarur kimberlite field, Eastern Dharwar craton (EDC) of southern India is reported. Perovskites yielded a high-precision U-Pb age of 1122± 7.7 Ma, taken to be an emplacement age of the host P-12 kimberlite pipe. The groundmass of coherent facies P-12 kimberlite contains monticellite, clinopyroxene, andradite, atoll spinel with titanomagnetite trend, and perovskite with an elevated REE contents. Phlogopite shows restricted Al2O3 and TiO2 contents. Furthermore, olivines with a wider and higher range of core compositions (i.e. Mg# = 84-94) and multi-granular nodules are the hallmark features of the P-12 pipe. This assorted primary mineral content and its composition indicates the transitional nature of the P-12 towards the Kaapvaal lamproites. However, concentrations of bulk-rock major and trace elements in the P-12 and other Wajrakarur kimberlites are similar to the global hypabyssal magmatic kimberlites. Large ion lithophile and high field strength elements (e.g. Ba and Nb) and their ratios (e.g. La/Nb and Th/Nb) suggest the presence of a heterogeneous and lithosphere influenced mantle source region which have been severely overprinted by metasomatizing fluids/melts emanating from the deep sourced upwelling mantle. The presence of such mixed and metasomatized mantle source regions likely to be an important factor for the transitional nature of the P-12 and other Mesoproterozoic kimberlites.
Based on the availability of the newest emplacement ages, we propose a geodynamic model for the origin of kimberlites in the Indian subcontinent. The U-Pb age of 1122± 7.7 Ma for the P-12 pipe shows its close temporal association to the emplacement of the recently proposed 1110 Ma Large Igneous Province (LIP), with plume center beneath the NW part of the Kalahari craton. Emplacement of the P-12 and other contemporaneous Indian kimberlites, therefore, marks the impingement of mantle plume which contributed heat and triggered partial melting of metasomatized lithospheric mantle without melt input. The eruption phase of ∼100 million years (i.e. 1050- 1153 Ma), for the kimberlites and related rocks in the Indian shield, does not appear to be continuous and can be separated into several short-durational magmatic events. For this reason, small-volume, volatile-rich magmatism during the Mesoproterozoic time in India is linked to the presence of a number of LIPs and associated mantle plumes during Columbia to Rodinia supercontinent transition and assembly of cratonic blocks of the latter.
... The well-documented rifted North Atlantic Craton, with fragments preserved in eastern Canada and Greenland, contains alkaline intrusives, such as carbonatites and kimberlites, as well as ultramafic lamprophyres (Tappe et al., 2006(Tappe et al., , 2007(Tappe et al., , 2012(Tappe et al., , 2017. The rifted North Atlantic craton records a lengthy history of mantle derived volatile-rich magmatism that formed during three failed extensional events prior to the opening of the Labrador Sea (Tappe et al., 2006(Tappe et al., , 2009. ...
... The rifted North Atlantic craton records a lengthy history of mantle derived volatile-rich magmatism that formed during three failed extensional events prior to the opening of the Labrador Sea (Tappe et al., 2006(Tappe et al., , 2009. The main carbonate-rich (i.e., kimberlites, ultramafic lamprophyres, carbonatites) magmatic events occurred during 600e540 Ma (Tappe et al., 2006(Tappe et al., , 2011 and between 220 Ma and 150 Ma (Larsen et al., 2009;Tappe et al., 2012). The prevalent carbonate-rich magmatism through the area has been explained by H 2 O-and CO 2 -rich mantle melting conditions at relatively low temperatures during thinning of the craton root as a consequence of gradually more oxidising conditions during the Mesozoic. ...
Reactivation of metasomatized mantle lithosphere may occur during continental extension, which is an important component of plate tectonics. The lower most part of the metasomatized domains in the subcontinental mantle lithosphere can be locally enriched in CO2. Therefore, partial melting of these metasomatized domains may play a crucial role in the global carbon cycle. However, little is known about this process and up until now few numerical constraints are available. Here we address this knowledge gap and use a 2-D high resolution petrological-thermomechanical model to assess lithospheric rifting, CO2 degassing and melting. We test 4 lithospheric thicknesses: 90, 110, 130 and 200 km with a 10 km thick metasomatized layer at the base using CO2 of 2 wt.% in the bulk composition. The carbonate enriched layer is stable below ∼3 GPa (>110 km) for a temperature of 1300 °C; therefore, we only observe degassing patterns for lithospheric models that are 130 km and 200 km thick. The metasomatized layer for the 130 km thick lithosphere mostly comprises carbonatite melting, whereas in the 200 km thick scenario propagation of melt development from kimberlites to carbonatites occurs as the metasomatic mantle is exhumed during extension. The numerical models fit well into natural rifting zones of the European Cenozoic Rift System for young (shallow) and of the North Atlantic Rift for old (thick) lithosphere. © 2018 China University of Geosciences (Beijing) and Peking University
... divergent zones, convergent zones, and hot spots). In general, three petrogenetic models have been proposed for carbonatites, including: 1) derivation of parental magmas by direct melting of a carbonate-bearing mantle (Chakhmouradian, 2006;Harmer and Gittins, 1998;Sweeney, 1994); 2) immiscible separation of carbonate and alkaline silicate melts from an initially homogeneous magma (Halama et al., 2005;Ivanov et al., 2010); 3) derivation of CO 2 -rich silicate magmas by crystal fractionation (Tappe et al., 2012;Veksler et al., 1998). These models could be used to explain the genesis of most of carbonatite, but have difficulties to interpret carbonatites from Bayan Obo, Hannuoba and Dalihu in North China Craton (NCC) (Chen et al., 2016;Fan et al., 2016;Ling et al., 2013;Liu et al., 2008;Liu et al., 2015;Mitchell, 2005;Yang et al., 2017;Yang et al., 2009). ...
... Several genesis models of carbonatite have been proposed (Chakhmouradian, 2006;Halama et al., 2005;Harmer and Gittins, 1998;Ivanov et al., 2010;Sweeney, 1994;Tappe et al., 2012;Veksler et al., 1998). Both liquid immiscibility and crystal fractionation have been invoked to explain derivation of small-volume carbonate melts from a hybrid alkali-rich carbonate-silicate magma (Halama et al., 2005;Van Groos and Wyllie, 1963;Verhulst et al., 2000). ...
... Iceland mantle plume track is from Lawver and Müller (1994). Geochronological data of subcontinental lithospheric mantle (SCLM) xenolith are from Wittig et al. (2010) recorded the gradual thinning processes of the southwestern margin (Tappe et al. 2007(Tappe et al. , 2012. ...
... The strong heating of the NAC lithosphere started during the Jurassic (Hutchison et al. 2007), when the SCLM was still thick enough to produce kimberlites on the NAC's western margin (Tappe et al. 2012), and was enhanced during the Cenozoic, when the Iceland mantle plume impinged beneath Greenland and successively generated rifts along its western and eastern margins. The early rifts linked the preweakening zones along the margins, which had been gradually thinned by repeated convergence and extension, and formed some relatively connected thinned margins. ...
Destruction of cratonic lithosphere has been demonstrated by numerous studies. However, the driving forces and mechanisms of craton destruction are still unclear. Subduction, collision, and mantle plumes, some of the most important driving forces for geological events, may also be responsible for lithospheric destruction. However, their relationships in the destruction process, including which is the major driving force and how they interact with each other, are not understood sufficiently. In this contribution, the North China Craton is used as an example of craton destruction by subduction and collision and is compared with the North Atlantic and Tanzania Cratons, which have been strongly affected by mantle plumes. The comparison is mainly based on their tectonic history and geophysical and geochemical data. It is suggested that craton destruction can be caused by the interaction of subduction, collision, and mantle plumes rather than any single mechanism. The lithospheric thinning of cratons is started from the cratonic margin, is enhanced by repeated faulting and convergence, and is finally extended to the cratonic interior via interaction among tectonic extension, convective removal, rheological weakening, and/or delamination.
... The carbonatite-ultramafic lamprophyre-kimberlite province in southern West Greenland includes a carbonatite complex and dike swarms described as ultramafic lamprophyre and kimberlite ( Fig. 27a; Secher and Larsen, 1980;Scott, 1981;Larsen, 1991;Nielsen et al., 2009;Sand et al., 2009;Steenfelt et al., 2009b;Tappe et al., 2009Tappe et al., , 2012. The~90 km 2 Sarfartoq Carbonatite Complex is located~50 km southwest of the Kangerlussuaq international airport (Figs. 2, 27). ...
... Kimberlite dikes in the vicinity are generally less than 1 m wide with a few up to 4 m in width. Mantle nodules are scarce, but xenocrysts of olivine and peridotitic garnet are common (Steenfelt et al., 2006;Steenfelt and Sand, 2010;Tappe et al., 2012). ...
Greenland is the largest island on Earth, with 80% of its area covered by a thick ice sheet. The coastal areas are underlain by variable rocks ranging from Eoarchean to the most recent ages. Greenland has a mineral exploration tradition since its colonization in the 18th century, and mining of cryolite started in 1854. Since the 1960s, the country is explored systematically for various commodities, which however resulted only in limited mining activity in only a few successful mines. Most exploration has been based on prospecting followed by exploration around the exposed mineralization.
... Debate, however, continues regarding the genetic relations (or lack thereof) between carbonatites and their associated silicate rocks. Three major petrogenetic models proposed previously for carbonatites include: (1) derivation of their parental magmas by direct melting of a carbonate-bearing mantle source (e.g., Chakhmouradian, 2006;Harmer and Gittins, 1998;Sweeney, 1994), (2) immiscible separation of carbonate and alkaline silicate melts from an initially homogeneous magma (e.g., Halama et al., 2005;Kjarsgaard and Hamilton, 1989), and (3) derivation of carbonatites from CO 2 -rich silicate magmas by crystal fractionation (e.g., Tappe et al., 2012;Veksler et al., 1998). Identification of the exact process (or processes) responsible for the formation of carbonatitic magmas is complicated by widespread Na-K metasomatism (fenitization) associated with carbonatites, and manifesting the loss of alkalis and volatiles from their parental magma (Le Bas, 1989). ...
... Crystal fractionation, which has been invoked to explain intimate association of carbonatites and alkaline silicate rocks in some localities (e.g., Tappe et al., 2012;Veksler et al., 1998), will also lead to a predictable pattern of trace-element distribution. Partition coefficients (D) for trace elements between apatite and silicate and carbonate melts (SM and CM, respectively), determined by Prowatke and Klemme (2006) a b Fig. 9. Trace-element abundances of carbonatites normalized to the primitive mantle (a) and REE abundances normalized to chondrite (b). ...
The South Qinling orogen in central China hosts carbonatites occurring as stocks associated with syenites and collectively regarded as the Miaoya intrusive complex. The complex hosts economic resources of rare-earth elements (REE) and Nb. The Miaoya syenites are strongly metasomatized at the contact with the carbonatites and cross-cut by carbonate and felsic veinlets. Small oscillatory-zoned crystals of zircons from the syenites give a concordant U-Pb age of 147 ± 0.5 Ma, which differs significantly from the ages of both large magmatic zircon grains from the syenites and primary monazite from the carbonatites (766 Ma and 234 Ma, respectively). To account for the possibility that the Miaoya syenites are coeval and cogenetic with the carbonatites, the trace-element budget of both rock types was examined in detail. The Miaoya carbonatite contains primary REE-rich fluorapatite and monazite, which precipitated earlier than the rock-forming REE-poor calcite, indicating that the primary carbonatitic magma was rich in REE. The compositions of the parental syenitic and carbonatitic magmas, calculated on the basis of the trace-element composition of primary fluorapatite in the two rock suites, show that the carbonatitic magma contained higher Sr and REE (La-Tb), but lower Ba, Pb, Th, U, Nb and Ta levels in comparison with the syenitic melt. These differences are inconsistent with derivation of the Miaoya rocks from a homogeneous carbonate-silicate melt by immiscibility or crystal fractionation. It is therefore concluded that the carbonatitic magma at Miaoya was generated directly in the mantle. Emplacement of the carbonatites in the South Qinling orogen marked transition to a postorogenic regime, and was preceded by oceanic crust subduction and closure of the Mianlue Ocean in the Triassic. Our models show that melting of the Mianlue crust and up to 10 wt.% of sediments cannot produce the levels of REE enrichment observed in the Miaoya carbonatites. More complex models, involving recycling of the Mianlue oceanic crust and a REE-rich carbonate liquid from an old deep-seated mantle source are required to explain the observed trace-element characteristics of the Miaoya carbonatites.
... Alkaline rocks are known to be limited on the Earth. They are found mainly in the areas of continental intraplate magmatism, rifting zones, and mantle hot spots (Tappe et al., 2012;Nosova et al., 2020) and, much less frequently, as part of volcanic-plutonic complexes of active continental margins and collisional zones. Most researchers consider alkaline rocks to be a product of magmatism associated with deep mantle sources. ...
A geochronological and isotope-geochemical study of alkaline basalts from three areas of young magmatism within the northeastern part of the Arabian Plate (Southeastern Turkey), Batman, Kurtalan and Alemdağ, was carried out. The obtained isotope data have indicated that the volcanism in the studied region developed over a 5-Ma period from the end of Miocene to the middle Pleistocene during four pulses separated by breaks in magmatic activity: 6.1-4.9 Ma (Batman area, hawaiites), ~3.0 Ma (Alemdağ plateau, phase I, basalts), 2.0-1.9 Ma (Alemdağ plateau, phase II, tephrites), and 1.5-1.3 Ma (Alemdağ plateau, phase III, basalts; Kurtalan area, basalts). A comparison of spatial-temporal changes of magmatic activity evolution in the studied part of the Arabian Plate and within the largest basalt plateau of Arabian foreland, Karacadağ Plateau , located to the west, was carried out. The results of Sr-Nd-Pb isotope-geochemical studies show that the development of young basalt volcanism in the Arabian Plate was characterized at different time by the contribution of various mantle sources in magma generation under this region. Initial pulses of magmatic activity are associated with melting of Arabian subcontinental lithospheric mantle (SCLM). The processes of fractional crystallization combined with crustal assimilation (AFC) have played an important role in the petrogenesis of lavas as well. Later, a deep mantle source (PREMA) with a depleted isotopic composition played a leading role in the formation of basaltic magmas of increased alkalinity. The melts generated by this source were mixed with the SCLM material in various proportions at different stages of magmatism with a limited participation of AFC processes in the petrogenesis of the rocks. It was concluded that young basalt volcanism of increased alkalinity in the northeast of the Arabian Plate is not related to the collision of the Eurasian and Arabian plates genetically, but presumably manifested here as a result of the migration of the initial rift geodynamic setting from the Red Sea basin to the north along Levantine and East Anatolian transform faults due to directed convection flows in the lower part of mantle under this part of the Earth.
... Growing evidence from phase equilibrium experiments indicates that carbonatite can be generated by low-degree partial melting of primary mantle and by the differentiation of carbonated silicate melts, i.e. liquid immiscibility and/or crystal fractionation (Freestone & Hamilton, 1980;Kjarsgaard & Hamilton, 1988Wallace & Green, 1988;Gittins, 1989;Sweeney, 1994;Lee & Wyllie, 1994, 1998Veksler et al. 1998b;Dalton & Presnall, 1998;Chakhmouradian, 2006;Brooker & Kjarsgaard, 2011;Tappe et al. 2012;Cheng et al. 2017Cheng et al. , 2018. The term 'primary carbonatite' implies a near-solidus partial melt in equilibrium with CO 2 -rich peridotitic mantle (Eggler, 1989). ...
The Qieganbulake deposit associated with a mafic–ultramafic–carbonatite complex in the Kuluketage block is not only the world’s second-largest vermiculite deposit, but also a medium-size carbonatite-related phosphate deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and mafic–ultramafic rocks are cogenetic and formed synchronously at c . 810 Ma. Geochemical characteristics and Sr–Nd–Hf–S isotopes (( ⁸⁷ Sr/ ⁸⁶ Sr) i = 0.70581–0.70710; ϵ Nd (t) = −0.20 to −11.80; ϵ Hf (t) = −7.5 to −10.3; δ ³⁴ S = +0.7 ‰ to +3.0 ‰ (some sulfides with high δ ³⁴ S values (+3.2 to +6.6) were formed by late hydrothermal sulfur)), in combination with mineral compositions and previous research, strongly indicate that the Qieganbulake mafic–ultramafic–carbonatite complex formed via extensive crystal fractionation/cumulation and liquid immiscibility of a carbonated tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids and sediment input in a continental rift setting. The coupled enriched Sr–Nd isotopic signatures, in combination with previous research, suggest that the enriched subcontinental lithospheric mantle could have been metasomatized by asthenospheric mantle melts to different degrees. The Qieganbulake carbonatite-related phosphate ores were the products of normal fractional crystallization/cumulation of P–Fe ³⁺ complex enriched carbonatite magma in high oxygen fugacity conditions, which was generated by liquid immiscibility of CO 2 –Fe–Ti–P-rich residual magma undergoing high differentiation.
... Kimberlites, in contrast, are sourced from the asthenospheric convecting mantle close to the Bulk Silicate Earth (BSE) in Sr-Nd isotope space (Mitchell, 1995;Tappe et al., 2012;Pearson et al., 2019). Lamprophyres and related carbonatites rarely occur spatially and temporally close to kimberlites. ...
We investigate relationships between melt sources for lamprophyres, kimberlites and other alkaline carbonate-
rich melts by studying rocks of the Terskiy Coast (Northwest Russia) situated between the coeval Devonian Kola
Alkaline Carbonatite (KACP) and Archangelsk Kimberlite Provinces (AKP). This study reports Sr-Nd-Pb isotope
systematics of lamprophyres, carbonatites, turjaite and foidite dykes, Turij Mys massif turjaites, perovskite and
garnet from the dykes, and the Ermakovskaya-7 kimberlite, the only kimberlite of the Terskiy Coast. Principal
Components Analysis was used to classify the data by five isotopic variables into rock groups T1, T2 and T3. A
factor analysis technique was then applied to new Sr-Nd-Pb data and the KACP literature data to portray the
analyses as three clusters on a planar 2-dimentional array. The first of these clusters, T1 is characterized by high
combined parameter f(Nd, Pb) > 0.5137 and moderately radiogenic (87Sr/86Sr)375. T1 rocks are mostly ultra-
mafic and plot along the Kola Carbonatite Line in Sr-Nd space. Group T2 is recognized by the presence of a linear
correlation between Sr isotope compositions and f(Nd, Pb); the extended span of εNd is combined with a
moderately radiogenic (87Sr/86Sr)375, and this group includes the least radiogenic Pb compositions. T2 rocks are
petrographically diverse, ranging from kimberlites to phonolites. T3 rocks form a cloud of points with elevated
(87Sr/86Sr)375 > 0.7041 and widely varying f(Nd, Pb); they are mostly alkaline lamprophyres. All three groups of
studied rocks record a contribution from the depleted mantle, while the mantle beneath the AKP kimberlites that
was metasomatized just prior to eruption contributed to T1 rocks, and the Paleoproterozoic metasomatized
lithospheric mantle contributed to T2 rocks. T3 rocks record 10% contamination by the lower crust, as recorded
by the Sr-Nd isotopic shift between the whole rock and garnet compositions. A geographic position of
Ermakovskaya-7, Zolotitsa and Mela kimberlites within the adjoining areas of the KACP and AKP stretched along
the strike of the Lapland-Kola belt results in their distinct low εNd(375) signatures highlighting a local control on
melt generation by the ancient metasomatized mantle. The repeated generation, over billions of years, of melts in
spatially restricted areas explains the observed contribution of ancient fеnitized crust and metasomatized mantle
to magmas from large alkaline or kimberlite provinces
... These magmas show enrichment in large ion lithophile elements (Cs, Ba, K) indicative of a metasomatized SCLM component in their source. Following, prevalent Neoproterozoic CO 2rich silica undersaturated ultramafic magmatism is evident by the eruption of diamondiferous lamprophyres (600-550 Ma), aillikites/carbonatites (610-550 Ma), and Group I kimberlites in the area (Davies et al., 2004;Tappe et al., 2005Tappe et al., , 2007Tappe et al., , 2012Tappe et al., , 2017. Similar magmatism with elevated La/Sm and K 2 O/SiO 2 continued during late-Triassic to early Cretaceous with the emplacement of aillikites and monchiquites (Larsen et al., 2009). ...
Here we report the nitrogen characteristics and composition of high-density fluid (HDF) trapped in microinclusions in a suite of fibrous diamonds from the ~142 Ma Chidliak CH-7 kimberlite pipe, the Hall Peninsula, southern Baffin Island, Nunavut. Within these diamonds, we observe three populations based on the chemistry of the encapsulated HDFs, the diamond's nitrogen aggregation states, and the diamond color. ‘Chidliak C' diamonds contain highly silicic HDFs, have nitrogen in A- and C-centers (with 5–20% in C-centers), and a characteristic intense yellow color. ‘Chidliak A' diamonds contain silicic to low-Mg carbonatitic HDFs, carry nitrogen solely in A-centers, and are mostly colorless. A third population, ‘Chidliak B', has grey color and distinctive low-K2O silicic to low-Mg carbonatitic HDF compositions and overall smoother and less fractionated trace element pattern relative to ‘Chidliak C' and ‘Chidliak A' diamonds; they carry nitrogen in A- and B-centers (with ~15% in B-centers) and are characterized by a grey hue. An eclogitic paragenesis of all diamonds is evident by the HDF compositional variation as well as the presence of omphacitic clinopyroxene inclusions. The appearance of a diamond with A- and B-centers in its octahedral core and A- and C-centers in its coat suggests formation at two distinct events at a similar depth. Combined with pressure and mantle residence estimates based on nitrogen aggregation considerations, we argue that the three diamond populations formed at the relatively shallow region of the lithosphere (likely <180 km) during distinct metasomatic events in the North Atlantic Craton (NAC) since the Proterozoic. The youngest event by silicic HDFs took place close in time to kimberlite activity at 142–157 Ma, as evident by the preservation of nitrogen C-centers in ‘Chidliak C' diamonds. A link between this event and the mid-lithosphere discontinuity (MLD) in eclogitic portions of the cratonic lithosphere in Chidliak is plausible. The timing of ‘Chidliak A' diamonds formation by more carbonatitic HDFs is less well constrained, but can be related to Ca-rich metasomatism observed in local peridotite xenoliths and/or alkaline magmatism between 610 and 550 Ma. A possible link between the formation of ‘Chidliak B' diamonds and the timing of Mesoproterozoic olivine lamproite magmatism ca. 1400 Ma is suggested based on the HDF trace element composition and the aggregated nature of nitrogen in these diamonds. The nitrogen systematics and eclogitic source of the fibrous diamonds are comparable with those observed for previously studied gem-quality diamonds from Chidliak. We suggest that these similarities show a temporal connection and mutual crystallization of the two diamond types. This strengthens the involvement of HDFs in the formation of gem-quality diamonds.
... Two alternative explanations for these young T RD erupt ages are proposed: recent melt depletion related to carbonate-rich magmatism (i.e. kimberlites, ultramafic lamprophyres, carbonatites) that appeared across the NAC between 610 and 540 Ma (e.g., the Torngat ultramafic lamprophyres from northern Labrador; Tappe et al., , 2008Tappe et al., , 2011a, and between 220 and 150 Ma (Tappe et al., 2012); or the ages do not have geological meaning and are merely a product of the integrated signatures of multiple metasomatic events affecting these peridotites that introduced radiogenic 187 Os. The second view is more likely because these 'young' samples generally have high and uniform suprachondritic (Pd/Ir) N and (Pt/Ir) N ratios (Fig. 7E & F), mirroring the effect of BMS addition during metasomatic overprinting, as also found in the GNAC peridotites (Wittig et al., 2010). ...
A suite of peridotite xenoliths from the Chidliak kimberlite province provides an ideal opportunity to assess the age of the mantle lithosphere beneath the eastern Hall Peninsula Block (EHPB) in southern Baffin Island, Nunavut and to provide constraints on the lithospheric architecture of this region. The new dataset comprises highly siderophile element (HSE) abundances and Re-Os isotopic compositions for 32 peridotite xenoliths sampled from four Late Jurassic-Early Cretaceous kimberlite pipes (CH-1, -6, -7, and -44). These peridotites represent strongly depleted mantle residues, with bulk-rock and olivine chemistry denoting melt extraction extents of up to 40%. The vast majority of samples show PPGE (Pt and Pd) depletion relative to IPGE (Os, Ir, and Ru) ((Pt/Ir)N: 0.10–0.96, median = 0.57; (Pd/Ir)N: 0.03–0.79, median = 0.24), coupled with mostly unradiogenic Os isotopic compositions (187Os/188Os = 0.1084–0.1170). These peridotites display strong correlations between 187Os/188Os and melt depletion indicators (such as olivine Mg number and bulk-rock Al2O3, (Pd/Ir)N), suggesting that an ancient (~2.8 Ga) melt depletion event governed the formation of the Chidliak lithosphere. The prominent mode of TRDerupt model ages at ca. 2.8 Ga matches the main crust-building ages of the EHPB, demonstrating temporal crust-mantle coupled in the Meso-Neoarchean. These ancient melt-depletion ages are present throughout the depth of the ~200 km thick lithospheric mantle column beneath Chidliak. The Meso-Neoarchean formation age of the EHPB mantle broadly coincides with the timing of stabilization of the lithospheric mantle beneath the Greenlandic portion of the North Atlantic Craton (NAC). This, along with the similarity in modal mineralogy, chemical composition and evolutionary history, indicates that the EHPB, southern Baffin Island was once -contiguous with the Greenlandic NAC. The mantle lithosphere beneath both the EHPB and the NAC show a similar metasomatic history, modified by multiple pulses of metasomatism. These multiple metasomatic events combined to weaken and thin the lithospheric mantle, culminating in the formation of the Labrador Sea and Davis Strait separating the EHPB from the Greenlandic NAC in the Paleocene.
... Terrane is largely comprised of Mesoarchean TTG-suite tonalitic and dioritic orthogneisses (Gardiner et al., 2019), with subordinate supracrustal belts (Hollis et al., 2005), ultramafic layered sequences (Szilas et al., 2015(Szilas et al., , 2017Goutana et al., 2018;Aarestrup, et al., 2020), metamafics (norites, gabbros) (Gardiner et al., 2019;Waterton et al., 2020a), carbonatites (Steenfelt, 2004), kimberlites (Tappe et al., 2012;Gardiner et al., 2020) and minor late tectonic tonalitic, granitic and dioritc rocks (Steenfelt et al., 2005: Gardiner et al., 2019. ...
Investigated here is the petrogenesis of the ~3 Ga Maniitsoq Norite Belt, Akia terrane, South West
Greenland. The belt is composed of two main lithologies: Plagioclase-rich norites and pyroxene-rich
melanorites. Both lithologies represent magmatic accumulations of respective dominant minerals. The
Fossilik intrusion is the largest intrusion and the type-locality of the noritic portion of the belt. The belt
is hosted by ~3018 ± 13 Ma regional Akia felsic crust, comprised mainly of tonalite-trondhjemite-
granodiorite (TTG) and dioritic orthogneiss. Fossilik norites are SiO2-rich (53wt% SiO2), TiO2-poor
(<1 wt% TiO2) and have high bulk-rock Mg# (~ 0.75). Trace element patterns show highly enriched
light Rare-Earth elements, depleted heavy Rare-Earth elements, negative anomalies in Nb, Ta and Ti,
consistently subchrondritic Eu anomalies and variable Zr and Hf anomalies. The Maniitsoq Norite Belt
has a emplacement ages of 3013 ± 1 Ma, with magmatism continuing for ~12 Myr, making its
emplacement age coeval with TTG production. The Akia terrane was subject to a widespread high-T,
low-P metamorphic event (~800o
C and <9 Kbar) soon after the belts emplacement, resulting in the
complete recrystallisation of the Maniitsoq Norite Belt. Peak metamorphic conditions are further
constrained using two-pyroxene and biotite thermobarometry. The high thermal gradients and higher
heat production in the Archaean indicate the belt was intruded into thin crust. The Fossilik intrusion
displays almost ‘dry’ granulite-facies assemblages dominated by plagioclase (~65 vol%). Mafic
assemblages consist of pyroxene (orthopyroxene > clinopyroxene), minor biotite and amphibole. All
samples show retrogression to amphibolite-facies assemblages. However, pegmatitic magmas(~2.7 and
~2.2 Ga) cutting the Fossilik intrusion result in the intense localised retrogression of the norites. Mafic
assemblages display complete epitaxial replacement to amphibole and biotite (hornblende and
actinolite). Amphibolite-facies samples show enrichment in fluid-mobile elements supplied by granitic
magma, alongside metasomatic alteration of mineral chemistry. Field relationships, major element
compositions and trace element ratios combined with isotope characteristics shows the Maniitsoq
Norite Belt was subjected to high degrees of crustal contamination by the crustal TTG, and that the
mantle source was heavily depleted. Retrogression is seen to dramatically increase the degree of
apparent crustal contamination, resulting in overestimates. The Fossilik intrusion and the Maniitsoq
Norite Belt shows superficial similarities to noritic material associated with ultramafic complexes in
the Akia terrane and are therefore found not be sections of a disassembled mafic complex. The
Maniitsoq Norite Belt shows little affinity to the ‘boninite-like’ norite series from 2.7 – 2.0 Ga, and
instead shows greater similarities to ‘SHMB-like’ petrogenesis. The belt is a series of crustally
contaminated mantle-melt intrusions. No evidence found here supports the theory of a giant bolide
impact in the Maniitsoq region.
... By the mid-Jurassic, sufficient subsidence of the Nova Scotian-Moroccan rift shoulder had taken place for sediment to accumulate on the Scotian Shelf (Fig. 11A), and at least the lower drainage basin was arid (Jansa and Wade, 1975). In the region of what would become the Labrador rift, various intrusions in the range of 166-160 Ma (Larsen et al., 2009;Tappe et al., 2012) suggest the onset of rifting by the mid-Jurassic. In the western part of the basin, our new detrital K-feldspar data from Middle to Late Jurassic sandstones indicate predominant supply from the Meguma terrane and offshore German Bank pluton (Fig. 9), in line with varietal heavy minerals and detrital geochronology of muscovite and monazite . ...
Provenance analysis provides a powerful means to understand, connect, and reconstruct source-to-sink systems and Earth surface processes, if reliable toolkits can be developed, refined, and applied. Deciphering sediment routing to the Scotian Basin, offshore eastern Canada, is marred by sedimentary recycling but is critical to understanding the evolution of the Canadian margin in response to the evolving Labrador rift. In this study, Pb isotopes in detrital K-feldspars were fingerprinted in 13 wells across the Scotian Basin to track first-cycle sand supply. Unlike previous approaches, which utilized less labile proxies such as zircon, detrital K-feldspars are unlikely to survive multiple sedimentary cycles.
The Pb-isotopic data reveal a dynamic seesaw effect between hinterland sources across the Jurassic-Cretaceous boundary, reflecting the complex interplay between the northward propagation of uplift along the rising Labrador rift flank and the reactivation of fault systems in the lower drainage basin. Pb isotopes in K-feldspar record progressively increasing long-distance supply from eastern Labrador, as early as the Callovian in the central basin, alongside diminishing but persistent local sourcing from adjacent Appalachian terranes. Comparison with more resilient mineral proxies, notably zircon, appears to confirm recycling in the lower drainage basin and highlights the limitations of using a single mineral proxy in isolation.
This case study serves as an example of the growing potential of multiproxy provenance toolkits not only to decipher sediment-routing corridors in paleodrainage systems, but to better define and connect the drivers, mechanisms, and spatial and temporal ranges of Earth surface processes and tectonic events.
... Forty-two metasomatised peridotite xenoliths from the Mesozoic Pyramidefjeld and Midternaes kimberlite comprise garnet-free lherzolites, harzburgites and reaction dunites from 100 to 170 km depth, and texturally equilibrated reaction dunites and olivine-rich phlogopite-bearing wehrlites from 90 to 110 km depth (Aulbach et al., 2017) (Table S-1). Based on microstructural evidence for garnet break-down, the metasomatic overprint may be linked to the latest, Mesozoic, rifting event, which was preceded by multiple failed rifting episodes and accompanied by partial destruction of the lithospheric mantle (Tappe et al., 2012). The intense metasomatism is proposed to have been oxidising based on the less compatible behaviour of V in wehrlites compared to other peridotites (Aulbach et al., 2017). ...
doi: 10.7185/geochemlet.2031 Continental rifting has been linked to the thinning and destruction of cratonic lithosphere and to the release of enough CO 2 to impact the global climate. This fundamental plate tectonic process facilitates the infiltration and mobilisation of small-volume carbonated melts, which may interact with mantle peridotite to form wehrlite through the reaction: enstatite þ dolomite (melt) = forsterite þ diopside þ CO 2 (vapour). Application to mantle xenolith suites from various rifts and basins shows that 2.9 to 10.2 kg CO 2 are released per 100 kg of wehrlite formed. For the Eastern Rift (Africa), this results in estimated CO 2 fluxes of 6.5 ± 4.1 Mt yr −1 , similar to estimates of mantle contributions based on surficial CO 2 surveys. Thus, wehrlite-bearing xenolith suites can be used to monitor present and past CO 2 mobility through the continental lithosphere, ultimately with diffuse degassing to the atmosphere. They may also reveal the CO 2 flux in lithospheric provinces where carbonated melts or continent-scale rifts are not observed at the surface.
... The initial Sr isotopic ratio is lower (0.7029) than 87 Sr/ 86 Sr values of whole-rocks, whereas the εNd (T) value (4.5) lies within the whole-rock interval. Similar results were obtained for perovskite and the host kimberlites by Tappe et al. (2012). Compared to Chadobets, perovskite from the Ilbokich aillikite has lower Sr (1800 ppm) and Nd (579 ppm) contents, higher 87 Sr/ 86 Sr ratio (0.7034) and lower εNd(T) value (3.4). ...
To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr–Nd–Pb–C–O isotope data was obtained. Our new ²⁰⁶Pb/²³⁸U perovskite age (399 ± 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and ⁴⁰Ar/³⁹Ar dating yielded a Middle Triassic age (243 ± 3 Ma and 241 ± 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks.
Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to ×200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33–84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow ⁸⁷Sr/⁸⁶Sr0 range (0.7032–0.7042) and εNd(T) (4.03–3.97). Chadobets UMLs have a similar Sr isotopic signature (⁸⁷Sr/⁸⁶Sr0: 0.7031–0.7043) and a more depleted Nd isotopic signature (εNd(T) 4.09–5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between ²⁰⁶Pb/²⁰⁴Pb = 18.4–19.0, ²⁰⁸Pb/²⁰⁴Pb = 38.3–38.8, and are characterized by a narrow ²⁰⁷Pb/²⁰⁴Pb ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between ²⁰⁶Pb/²⁰⁴Pb = 18.0–18.4, ²⁰⁸Pb/²⁰⁴Pb = 37.8–38.4 and ²⁰⁷Pb/²⁰⁴Pb ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable δ¹⁸O values from +12.1 and up to +20.5‰ (SMOW). The isotopic composition of δ¹³C values range from −1.3‰ to −7.1.
Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps.
... Radiogenic isotope systematics (Nowell et al., 2004;Woodhead et al., 2009;Tappe et al., 2012), the Mg#, Cr, and Ni contents of kimberlite melts (e.g., Price et al., 2000;Soltys et al., 2018), sub-lithospheric xenoliths (Sautter et al., 1991;Haggerty, 2017), as well as deeply derived diamonds (Stachel, 2001;Harte, 2010;Kaminsky, 2012;Pearson et al., 2014) in kimberlites globally suggest that the source of kimberlites occurs in the asthenosphere (Boyd and Nixon, 1975;Pasteris, 1979;Pearson et al., 1995). ...
We present new major element geochemical data, and review the existing data for ilmenite macrocrysts, megacrysts, as well as ilmenite in mantle xenoliths from four diamondiferous kimberlite fields in the Yakutian province. This combined data set includes 10,874 analyses of ilmenite from 94 kimberlite pipes. In the studied samples we identify various different ilmenite compositional distributions (e.g., “Haggerty's parabola”, or “Step-like” trends in MgO-Cr2O3 bivariate space), which are common to all kimberlites from a given cluster, but the compositional distributions differ between clusters. We propose three stages of ilmenite crystallization:
(1) Mg-Cr poor ilmenite crystallising from a primitive asthenospheric melt (the base of Haggerty's parabola on MgO-Cr2O3 plots).
(2) This primitive asthenospheric melt was then modified by the partial assimilation of lithospheric material, which enriched the melt in MgO and Cr2O3 (left branch of Haggerty’s parabola).
(3) Ilmenite subsequently underwent sub-solidus recrystallization in the presence of an evolved kimberlite melt under increasing oxygen fugacity (ƒO2) conditions (right branch of Haggerty’s parabola in MgO-Cr2O3 plots).
Significant differences in the ilmenite compositional distribution between different kimberlite fields are the result of diverse conditions during subsequent ilmenite crystallization in a kimberlite melt ascending through the lithospheric mantle, which have different textures and compositions beneath the studied kimberlite fields. We propose that a TiO2 fluid formed due to immiscibility of an asthenospheric melt with low Cr and high Ti contents. This fluid infiltrated lithospheric mantle rocks forming Mg-ilmenite. These features indicate a genetic link between ilmenite and the host kimberlite melt.
... Ga Nd model ages of other Mesoproterozoic Wajrakarur kimberlites (Chalapathi Dongre et al., 2017), and this clearly points at a similarity in the timing of mantle source enrichment for all of the varied Mesoproterozoic intrusions of the WKF. The enrichment of the mantle source region(s) may be related to metasomatizing melts/fluids emanating from upwelling asthenospheric mantle (McKenzie, 1989;Foley, 1992;Tappe et al., 2012). Hence the assemblage of carbonatite-kimberlite-ultramafic lamprophyre-orangeite-olivine lamproite intrusions of the WKF may have formed by incipient partial melting of contrasting metasomatic assemblages that were produced in EDC lithospheric mantle during the Fig. 10. ...
Detailed mineralogy, bulk rock major, trace and Sr–Nd isotope compositions, and ⁴⁰Ar/³⁹Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India, is reported. Based on the presence of Ti-rich phlogopite, high Na/K content in amphibole, Al- and Ti-rich diopside, a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass, the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre (i.e., aillikite). An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr–Nd isotope geochemistry. Sr–Nd isotope data are consistent with a common, moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India. A phlogopite-rich groundmass ⁴⁰Ar/³⁹Ar plateau age of 1115.8 ± 7.9 Ma (2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053–1155 Ma emplacement ages of kimberlites and related rocks in India. The presence of ultramafic lamprophyres, carbonatites, kimberlites, and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle. The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic (ca. 1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari, Australian, West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly. In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia. This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia. It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent, with the Indian block located on its periphery, could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic, volatile rich rocks in India due to comparatively low thermal effects from the distant plume.
... Source region of kimberlites remains controversial and various competing models are in vogue involving (i) sub-continental lithospheric mantle (SCLM; Skinner, 1989;Heaman, 1989;Tainton McKenzie, 1994;Le. Roex et al., 2003;Chalapathi Rao et al., 2004;Donnelly et al., 2011), (ii) asthenospheric convecting mantle (Mitchell, 1995(Mitchell, , 2006Paton et al., 2007;Wu et al., 2010;Tappe et al., 2012), (iii) transition zone (Ringwood et al., 1992;Nowell et al., 2004;Paton et al., 2009;Tappe et al., 2013a,b), (iv) core-mantle boundary (Haggerty, 1999;Collerson et al., 2010;Torsvik et al., 2010) and (v) even from the sources involving cratonic lithosphere and asthenosphere (Griffin et al., 2000;Tappe et al., 2011). ...
Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite (Pipe-16) from the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Ti-rich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of (i) liquidus mineral composition, (ii) bulk-rock chemistry, and (iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The TDM Nd model age (∼2.0 Ga) of the Ahobil kimberlite is (i) significantly older than those (1.5–1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC, (ii) indistinguishable from those of the Mesoproterozoic EDC lamproites, and (iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond. © 2018 China University of Geosciences (Beijing) and Peking University
... Extensive geochronology programs on large kimberlite fields have demonstrated that the duration of magmatic activity can be on the order of several tens of millions of years (Tappe Tappe_G45097 1st pages et al., 2012;Heaman et al., 2015). However, the formation of individual kimberlite pipes, plus cooling of volcaniclastic infill, is commonly thought to be complete within centuries to a few millennia at most (Afanasyev et al., 2014). ...
Volcanic pipes, or maar-diatreme volcanoes, form during explosive eruptions of mantle-derived magmas near Earth's surface. Impressive examples are the carrot-shaped, downward tapering structures formed by kimberlite magmas. Kimberlites originate from >150 km depth within Earth's mantle beneath thick continental roots, away from tectonic plate margins. Kimberlite pipes can be significant diamond deposits, and the complex architecture revealed during exploration and mining is ascribed to repeated magma injections leading to multiple eruptions. Repeated magmatic pulses cause diatremes to widen and grow downward, forming kilometer-sized subterranean structures. However, the time-resolved evolution of kimberlite pipe systems is largely unknown. We present the first U/Pb perovskite ages for newly discovered kimberlite dikes (1139.8 ± 4.8 Ma) that cut through the volcaniclastic infill of the Premier kimberlite pipe (1153.3 ± 5.3 Ma) at Cullinan Diamond Mine, South Africa. The ages reveal that renewed kimberlite volcanic activity occurred, at a minimum, 3 m.y. after the main pipe formation. This finding suggests that the largest kimberlite pipes, and maar-diatreme volcanoes in general, may be magmatically active for several millions of years, which conflicts with this volcanism being described as 'monogenetic' at millennia time scales. Exemplified by Tier-1 diamond deposits on the Kaapvaal craton, long-lasting kimberlite volcanic activity may be an important factor in growing large diatremes, plus enabling effective transport of mantle cargo from the diamond stability field to Earth's surface.
... Moreover, they are inferred in the sources of smallvolume volatile-rich melts intruded in cratons around the globe (e.g. Foley et al., 2002b;Foley, 2008;Tappe et al., 2007Tappe et al., , 2012Rosenthal et al., 2009), and provide the mantle ''conditioning" necessary for successful kimberlite eruption (Giuliani et al., 2014). It is worth noting that MARID and glimmerite xenoliths also occur within kimberlites of the Orapa cluster (Letlhakane; Stiefenhofer et al., 1997). ...
Major- and trace-element compositions of garnet and clinopyroxene, as well as 87Sr/86Sr in clinopyroxene and δ18O in garnet in eclogite and pyroxenite xenoliths from Orapa, at the western margin of the Zimbabwe craton (central Botswana), were investigated in order to trace their origin and evolution in the mantle lithosphere. Two groups of eclogites are distinguished with respect to 87Sr/86Sr: One with moderate ratios (0.7026-0.7046) and another with 87Sr/86Sr >0.7048 to 0.7091. In the former group, heavy δ18O attests to low-temperature alteration on the ocean floor, while 87Sr/86Sr correlates with indices of low-pressure igneous processes (Eu/Eu*, Mg#, Sr/Y). This suggests relatively undisturbed long-term ingrowth of 87Sr at near-igneous Rb/Sr after metamorphism, despite the exposed craton margin setting. The high-87Sr/86Sr group has mainly mantle-like δ18O and is suggested to have interacted with a small-volume melt derived from an aged phlogopite-rich metasome.
The overlap of diamondiferous and graphite-bearing eclogites and pyroxenites over a pressure interval of ~3.2 to 4.9 GPa is interpreted as reflecting a mantle parcel beneath Orapa that has moved out of the diamond stability field, due to a change in geotherm and/or decompression. Diamondiferous eclogites record lower median 87Sr/86Sr (0.7039) than graphite-bearing samples (0.7064) and carbon-free samples (0.7051), suggesting that interaction with the – possibly oxidising – metasome-derived melt caused carbon removal in some eclogites, while catalysing the conversion of diamond to graphite in others. This highlights the role of small-volume melts in modulating the lithospheric carbon cycle. Compared to diamondiferous eclogites, eclogitic inclusions in diamonds are restricted to high FeO and low SiO2, CaO and Na2O contents, they record higher equilibrium temperatures and garnets have mostly mantle-like O isotopic composition. We suggest that this signature was imparted by a sublithospheric melt with contributions from a clinopyroxene-rich source, possibly related to the ca. 2.0 Ga Bushveld event.
... Whilst there is an overall consensus for the role of sub-continental lithospheric mantle (SCLM) as the source region of orangeites (e.g., Fraser and Hawkesworth, 1992;Tainton and McKenzie, 1994;Mitchell, 2006;Coe et al., 2008;Chalapathi Rao et al., 2011a;Giuliani et al., 2015), the depth of melting and the source region of kimberlites are unconstrained and remain controversial. Based on bulk-rock geochemistry, high-pressure experimental studies, and entrained diamond inclusions, a number of potential reservoirs have been invoked for kimberlites that range from a metasomatised (enriched) cratonic SCLM (Skinner, 1989;Tainton and McKenzie, 1994;Le Roex et al., 2003;Chalapathi Rao et al., 2004;Harris et al., 2004;Becker and Le Roex, 2006;Agashev et al., 2008;Donnelly et al., 2011;Griffin et al., 2014), solely asthenospheric (convecting) mantle (Mitchell, 1995(Mitchell, , 2006Wu et al., 2010;Tappe et al., 2012Tappe et al., , 2014, transition zone or lower mantle (Ringwood et al., 1992;Bizzarro et al., 2002;Nowell et al., 2004;Paton et al., 2009), the core-mantle boundary (Haggerty, 1999;Torsvik et al., 2010;Collerson et al., 2010) and also multiple reservoirs involving the asthenosphere and cratonic lithosphere (Tappe et al., 2011). Before constraining the genesis of the FALC kimberlites under study, it is imperative to assess the role of late-stage deuteric alteration and crustal contamination, in comparison with primitive kimberlite magma compositions that are reported. ...
The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to − 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6–0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.
... Whilst there is an overall consensus for the role of sub-continental lithospheric mantle (SCLM) as the source region of orangeites (e.g., Fraser and Hawkesworth, 1992;Tainton and McKenzie, 1994;Mitchell, 2006;Coe et al., 2008;Chalapathi Rao et al., 2011a;Giuliani et al., 2015), the depth of melting and the source region of kimberlites are unconstrained and remain controversial. Based on bulk-rock geochemistry, high-pressure experimental studies, and entrained diamond inclusions, a number of potential reservoirs have been invoked for kimberlites that range from a metasomatised (enriched) cratonic SCLM (Skinner, 1989;Tainton and McKenzie, 1994;Le Roex et al., 2003;Chalapathi Rao et al., 2004;Harris et al., 2004;Becker and Le Roex, 2006;Agashev et al., 2008;Donnelly et al., 2011;Griffin et al., 2014), solely asthenospheric (convecting) mantle (Mitchell, 1995(Mitchell, , 2006Wu et al., 2010;Tappe et al., 2012Tappe et al., , 2014, transition zone or lower mantle (Ringwood et al., 1992;Bizzarro et al., 2002;Nowell et al., 2004;Paton et al., 2009), the core-mantle boundary (Haggerty, 1999;Torsvik et al., 2010;Collerson et al., 2010) and also multiple reservoirs involving the asthenosphere and cratonic lithosphere (Tappe et al., 2011). Before constraining the genesis of the FALC kimberlites under study, it is imperative to assess the role of late-stage deuteric alteration and crustal contamination, in comparison with primitive kimberlite magma compositions that are reported. ...
... The lack of characteristic negative Nbe and Taeanomalies in the Indian orangeites excludes a subductionrelated source enrichment process (Coe et al., 2008). Enrichment of mantle source regions has also been attributed to metasomatizing alkaline fluids or melts derived from upwelling asthenospheric mantle, which modify the overlying sub-continental lithospheric mantle (McKenzie, 1989;Foley, 1992;Le Roex et al., 2003;Tappe et al., 2012). Therefore, it is speculated that tectonic processes related to a 1100 Ma, short-lived plume activity, may have provided the heat resulting in (1) the initiation of partial melting, (2) the generation of the kimberlites on the EDC and the Bundelkhand Craton, (3) the depletion of the mantle source regions and (4) subsequently, or even contemporaneously, the enrichment of the already depleted source regions of the Indian orangeites. ...
New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic (ca. 1100 Ma) and late Cretaceous (ca. 90 Ma) kimberlites in the Timmasamudram cluster (TKC) of the Wajrakarur kimberlite field (WKF), Eastern Dharwar Craton, southern India, are presented. On the basis of groundmass mineral chemistry (phlogopite, spinel, perovskite and clinopyroxene), bulk-rock chemistry (SiO2, K2O, low TiO2, Ba/Nb and La/Sm), and perovskite Nd isotopic compositions, the TK-1 (macrocrystic variety) and TK-4 (Macrocrystic variety) kimberlites in this cluster are here classified as orangeites (i.e. Group II kimberlites), with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India, as well as the Kaapvaal Craton in South Africa. The remaining kimberlites (e.g., TK-2, TK-3 and the TK-1 microcrystic variant), are more similar to other 1100 Ma, Group I-type kimberlites of the Eastern Dharwar Craton, as well as the typical Group I kimberlites of the Kaapvaal Craton. Through the application of geochemical modelling, based on published carbonated peridotite/melt trace element partition coefficients, we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised, carbonated peridotite.
Depleted mantle (TDM) Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasomatic enrichment of their source regions are broadly similar to that of the Mesoproterozoic kimberlites of the EDC. The younger, late Cretaceous (ca. 90 Ma) TK-1 (macrocrystic variant) and TK-4 kimberlites, as well as the orangeites from the Bastar Craton, share similar Nd model ages of 1100 Ma, consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent. The presence of late Cretaceous diamondiferous orangeite activity, presumably related to the location of the Marion hotspot in southern India at the time, suggests that thick lithosphere was preserved, at least locally, up to the late Cretaceous, and was not entirely destroyed during the breakup of Gondwana, as inferred by some recent geophysical models.
Textural, mineralogical and mineral compositional observations in a suite of Neoproterozoic aillikite and calcite kimberlite dykes from southern West Greenland point to consistent variations in melt major element compositions amongst these silica-undersaturated magma types. The aillikites have notably higher bulk SiO2/CO2, H2O/CO2 and K2O compared to calcite kimberlite. Bulk rock arrays, together with field and petrographic observations, emphasize that flow sorting of olivine and other crystalline phases during magma emplacement is important in controlling the compositions of individual samples from these ultramafic dykes. Flow sorting together with variable overall proportions of entrained lithospheric mantle material result in scatter on element–element plots, which makes the interpretation of regional scale major and trace element geochemical datasets difficult. We argue that a significant proportion of the regional Ni—MgO variation in the ultramafic dyke suite of SW Greenland is due to variation in the proportion of an entrained refractory lithospheric mantle component. Therefore, ratios of elements to MgO can be used as proxies for melt compositions. Ratios of SiO2, TiO2, Al2O3, FeO and K2O over MgO are systematically higher, and CO2/MgO lower, in aillikites compared to calcite kimberlites. The trace element patterns of the calcite kimberlite and aillikite dykes show strong similarities in incompatible element concentrations, resulting in overlapping ratios for the highly to moderately incompatible elements. However, differences in Zr-Hf concentrations between rock types imply differences in mantle source mineralogy. Guided by our observations, we present mixing models that demonstrate that partial flux-melting of phlogopite–ilmenite metasomes within the cratonic mantle lithosphere is capable to produce the geochemical characteristics of aillikites and mela-aillikites in West Greenland. Fusion of cratonic metasomes was initiated by infiltrating asthenosphere-derived carbonatitic melts previously identified as the parental liquids to calcite kimberlite.
A model is developed to test the hypothesis that kimberlites can form by low-degree melting of asthenospheric mantle followed by entrainment and assimilation of lithospheric mantle. The developed model uses inversion calculations based upon rare earth and compatible trace elements. For kimberlites (s.s.), an equation describing mass balance between a melt of unknown composition and a contaminant end-member of xenocrystic/assimilated material from the lithospheric mantle is inverted. This allows calculation of the mass fraction of xenocrystic minerals from the lithospheric mantle (olivine, orthopyroxene, clinopyroxene, garnet, ilmenite) entrained in the kimberlitic magma, as well as the source mineralogy and melt degree in the source region.
The composition of the parental melt prior to interaction with the lithosphere is not assumed a priori but is calculated by the model. The CO2, H2O, K2O and P2O5 contents of the source are estimated assuming batch melting and the inversion models. The range and coupling of the model parameters are found using a non-linear most-squares inversion procedure, and the model space is visualised using a Self-Organising Map approach. Our earlier work supporting assimilation of xenocrystic opx is, however, not a precondition but provides a post-processing constraint, as well as the selection of a more likely set of solutions from the Self-Organising Map.
The calculation is applied to a data set from the Majuagaa kimberlite dyke (southern West Greenland) including added whole rock analyses for CO2 and H2O. Major variations in whole rock compositions are related to flow differentiation of olivine macrocrysts. The textures of opx, cpx, gt and ilm megacrysts show evidence for reaction with the transporting melt and physical erosion in the kimberlitic mush. Using the bulk rocks in our inversion scheme results in a silico-carbonatite parental melt with major element concentrations consistent with experimental melts. The ol, opx, and cpx mass fractions in the source are not well-resolved by this calculation, but the proportion of gt in the source is comparatively well defined at 15-22 wt.% and with cpx to less than 14 wt.%. The source assemblage required is 36-80 wt% ol, 2-49 wt.% opx, 0-6wt% cpx, and 15-19 wt.% gt. This suggests a peridotitic rather than an eclogitic source.
The inversion model gives an overall mass fraction of xenocrystic material in the Majuagaa kimberlite magma of 41-51 wt.% The mass fractions of the xenocryst phases are: 71-85 wt.% ol, 0-13 wt.% opx, 5±1 wt. % gt, and 10-14 wt.% ilm. There is less than 3 wt.% cpx in the xenocrystic and assimilated assemblage. These results agree with petrographic observations. Processing the model results using the Self-Organising Map clearly displays the extent and coupling within the statistically acceptable region of the model space and leads us to a preferred model of 49 wt% xenocrysts with a xenocryst assemblage of 71-76 wt.% ol, 8-13 wt.% opx, 4 wt.% gt and 12 wt.% ilm.
A source with a REE pattern similar to that of primitive mantle is sufficient to form the parental melt and consistent with generation of the initial kimberlite melt in the convecting mantle. Calculated CO2 and H2O concentrations in the source of the Majuagaa kimberlite of 230-860 μg/g and 223-741 μg/g, respectively, are within the range of independent convecting mantle estimates. This is equivalent to <0.17 wt.% magnesite and the H2O budget of the mantle source can be accommodated via storage in nominally anhydrous silicate phases.
When applied to Majuagaa kimberlite, the inversions are consistent with a conceptually simple model of kimberlite formation. (1) low degree melting in carbonated astenospheric peridotite, (2) melt extraction and concentration, and (3) entrainment and reaction with lithospheric mantle material.
Despite being volumetrically minor components, carbonate-rich ultramafic magmas like aillikites represent good candidates to investigate the compositional variations in plume and/or lithospheric mantle sources because they represent low-degree melts which preferentially sample highly fusible components including recycled crustal material. To gain new insights into the composition of the plume-related magmas and, more broadly, the petrogenesis of ultramafic lamprophyres, we have undertaken the first comprehensive study of bulk rock and mineral (olivine and Ti-magnetite) highly siderophile element (HSE) abundances and Re-Os isotopes combined with in situ major-, trace-element and Sr-Nd isotope analyses of apatite and perovskite from the Permian Wajilitag aillikites of the Tarim large igneous province, China. The Wajilitag aillikites have high PPGE (Pt and Pd) contents relative to IPGE (Os, Ir and Ru), which can be ascribed to low-degree partial melting and/or fractionation of olivine and laurite. Measured ¹⁸⁷Os/¹⁸⁸Os ratios are moderately to highly radiogenic (0.186-0.313) with age-corrected γOs values up to +113. In situ Sr and Nd isotope analyses of apatite phenocrysts (⁸⁷Sr/⁸⁶Sr(i) = 0.70349-0.70384; εNd(i) = +1.3 to +4.9) and fresh perovskite grains (⁸⁷Sr/⁸⁶Sr(i) = 0.70340-0.70390; εNd(i) = +1.3 to +3.8) exhibit limited variability both within and across samples from different aillikite dykes and the only volcanic pipe in the area. These Nd isotopic values resemble those from bulk-rock samples (εNd(i) = +1.9 to +5.2), whereas Sr in apatite and perovskite extends to marginally less radiogenic values than the bulk-rock compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70362-0.70432). The moderately depleted Sr-Nd isotope compositions of magmatic apatite and perovskite, and the previously reported mantle-like C isotope values of these samples suggest that the aillikites and their carbon probably derived from a sub-lithospheric (plume) source with minimal contribution of deeply subducted material. Conversely, the radiogenic Os isotope compositions of the Tarim aillikites and separated minerals require some contribution from recycled crustal material in the plume source. Mass balance calculations suggest that the radiogenic Os isotopes and moderately depleted Sr-Nd isotopes can be reproduced by less than one third of eclogite component addition to a moderately depleted mantle source. We conclude that the combination of complementary isotopic systems can enlighten contributions from different components to mantle-derived magmas and, in this case, clarifies the occurrence of carbon-free subducted oceanic crust in the Tarim plume.
Monomineralic millimeter-sized olivine nodules are common in kimberlites worldwide. It is generally thought that such ‘dunitic nodules’ originate from the base of the cratonic lithosphere and that their formation marks the onset of deep-rooted kimberlite magmatic plumbing systems. However, thermobarometric constraints to support such a model have been lacking thus far.
This study focuses on the petrography and textures, as well as on pressure–temperature estimations, of well-preserved dunitic nodules from the Quaternary Igwisi Hills kimberlite lavas on the Tanzania craton, with the ultimate goal to constrain their origins. We utilize EBSD-determined textural information in combination with olivine geochemistry data determined by EPMA and LA-ICP-MS methods. We find that host olivine grains in these nodules are compositionally similar to olivine in garnet-facies cratonic mantle peridotites, and such an association is supported by garnet inclusions within olivine. Projection of Al-in-olivine temperatures onto a regional geotherm suggests that the host olivine grains equilibrated at ~100-145 km depth, which points to origins from mid-lithospheric levels down to the lower cratonic mantle if a depth of 160-180 km is considered for the lithosphere–asthenosphere transition beneath the Tanzania craton. These first pressure–temperature estimates for dunitic nodules in kimberlites suggest that their formation also occurs at much shallower depths than previously assumed.
Recrystallized olivine grains (i.e., neoblasts) show random crystallographic orientations and are enriched in minor and trace elements (e.g., Ca, Al, Zn, Sc, V) compared to the host olivine grains. These features link neoblast formation to melt-assisted recrystallization of cratonic mantle peridotite, a process that persisted during kimberlite magma ascent through the lower half of thick continental lithosphere. Partial recrystallization of olivine-rich mantle xenoliths makes these materials texturally weaker and subsequent liberation of mineral grains promotes the assimilation of compositionally ‘unstable’ orthopyroxene in rising carbonate-rich melts, which is considered to be an important process in the evolution of kimberlite magmas.
Dunitic nodules in kimberlites and related rocks may form as melt–rock equilibration zones along magmatic conduits within the lower half of the cratonic mantle column all the way up to mid-lithospheric depth. Such an origin potentially links dunitic nodules to olivine megacrysts, which are equally considered as melt/fluid-assisted recrystallization products of peridotitic mantle lithosphere along the ascent pathways of deep-sourced CO2-H2O-rich ultramafic melts.
We studied the petrography, mineralogy, thermobarometry and whole-rock chemistry of 120 peri-dotite and pyroxenite xenoliths collected from the 156-138 Ma Chidliak kimberlite province (Southern Baffin Island). Xenoliths from pipes CH-1,-6,-7 and-44 are divided into two garnet-bearing series, dunites-harzburgites-lherzolites and wehrlites-olivine pyroxenites. Both series show widely varying textures, from coarse to sheared, and textures of late formation of garnet and clinopyroxene. Some samples from the lherzolite series may contain spinel, whereas wehrlites may contain ilmenite. In CH-6, rare coarse samples of the lherzolite and wehrlite series were derived from P ¼ 2Á8 to 5Á6 GPa, whereas predominant sheared and coarse samples of the lherzolite series coexist at P ¼ 5Á6-7Á5 GPa. Kimberlites CH-1,-7,-44 sample mainly the deeper mantle, at P ¼ 5Á0-7Á5 GPa, represented by coarse and sheared lherzolite and wehrlite series. The bulk of the pressure-temperature arrays defines a thermal state compatible with 35-39 mW m-2 surface heat flow, but a significant thermal disequilibrium was evident in the large isobaric thermal scatter, especially at depth, and in the low thermal gradients uncharacteristic of conduction. The whole-rock Si and Mg contents of the Chidliak xenoliths and their mineral chemistry reflect initial high levels of melt depletion typical of cratonic mantle and subsequent refertilization in Ca and Al. Unlike the more orthopyroxene-rich mantle of many other cratons, the Chidliak mantle is rich ($83 vol%) in forsteritic olivine. We assign this to silicate-carbonate metasomatism, which triggered wehrlitiza-tion of the mantle. The Chidliak mantle resembles the Greenlandic part of the North Atlantic Craton, suggesting the former contiguous nature of their lithosphere before subsequent rifting into separate continental fragments. Another, more recent type of mantle metasomatism, which affected the Chidliak mantle, is characterized by elevated Ti in pyroxenes and garnet typical of all rock types from CH-1,-7 and-44. These metasomatic samples are largely absent from the CH-6 xenolith suite. The Ti imprint is most intense in xenoliths derived from depths equivalent to 5Á5-6Á5 GPa where it is associated with higher strain, the presence of sheared samples of the lherzolite series and higher temperatures varying isobarically by up to 200 C. The horizontal scale of the thermal-metasomatic imprint is more ambiguous and could be as regional as tens of kilometers or as local as <1 km. The timescale of this metasomatism relates to a conductive length-scale and could be as short as <1 Myr, shortly predating kimberlite formation. A complex protracted meta-somatic history of the North Atlantic Craton reconstructed from Chidliak xenoliths matches em-placement patterns of deep CO 2-rich and Ti-rich magmatism around the Labrador Sea prior to the craton rifting. The metasomatism may have played a pivotal role in thinning the North Atlantic Craton.
Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit.
The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO+MgO+FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive δ¹⁸OSMOW (7.29 to 15.53‰) and negative δ¹³CPDB (-5.26 to -10.08‰); shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.
The parent magma and mantle source of the Mesoproterozoic Kiglapait intrusion, the largest and youngest troctolitic intrusion in the Nain Plutonic Suite of coastal Labrador, Canada, are evaluated using an integrated Pb-Sr-Nd-Hf isotopic and trace element framework. The bowl-shaped Kiglapait intrusion crystallized mostly as a closed system, forming an 8 km-thick stack of differentiated cumulates from the troctolitic Lower Zone (0-84% solidified or PCS) through the olivine gabbroic to ferrosyenitic Upper Zone (84-100 PCS). Whole-rocks and mineral separates (plagioclase, clinopyroxene, bulk mafic minerals) were analysed from eight different stratigraphic locations from 5 to 89·3 PCS. This new dataset is complemented by Pb-Sr-Nd TIMS isotopic analyses of samples spanning the entire magmatic stratigraphy of the Kiglapait intrusion (29 samples, 0·12 to 99·99 PCS) and from the Nain Plutonic Suite (26 samples) for regional comparisons. There is no significant change in initial Nd and Hf isotopic ratios from the base to the top of the intrusion. In contrast, initial 87 Sr/ 86 Sr steadily increases in the Upper Zone due to progressively increasing small amounts of assimilation of country rock with a composition similar to local Proterozoic supracrustal rocks and Archean gneiss. The Pb isotopic analyses, both MC-ICP-MS and TIMS, reveal differences between less radiogenic plagioclase and more radiogenic mafic separates in nearly all samples. This feature is attributed to cryptic alteration from interaction with a higherature, externally-derived, hydrothermal fluid in oxygen isotope equilibrium with the host anorthositic rocks. The Pb isotope composition of the Kiglapait parent magma was recovered through systematic analysis of leached plagioclase separates (and corresponding leachate solutions). Trace element modeling combined with Pb-Sr-Nd-Hf isotopic constraints indicate that the Kiglapait parent magma was derived from depleted mantle (∼95%) with a small contribution (∼5%) from the lower crust assimilated during ponding at the Moho or during transit through the crust. This geochemical model is extended to the origin of other troctolitic and anorthositic magmas in the Nain Plutonic Suite at a regional scale. Most of the Pb-Sr-Nd isotopic compositions of the Nain anorthosites are compatible with crystallization from melts that originated from the mantle and that assimilated variable extents of crustally-derived melts (3-30%). The multi-isotopic and trace element geochemical framework developed for the Kiglapait intrusion in this study can be applied to the investigation of the source, parent magma, differentiation processes and post-crystallization changes in layered intrusions and anorthosites throughout the geological record.
The AT2-1 well in the Davis Strait between Canada and Greenland penetrated an approximately 1.2 km thick sequence of alkaline volcanic rocks with some intercalated sediments at depths between 3690 to 4850 m. These volcanic rocks can be mapped on 2D seismic data and constitute a cone-shaped 5 km × 10 km wide and >1.2 km high structural high named the Atammik Volcano. This sequence comprises two distinct parts, an upper part of phono-tephrite to basaltic trachy-andesite and a lower part of tephriphonolite and phonolite. Rock textures and structures testify to a volcanic origin, with the uppermost units showing textural evidence of being subaerially extruded. Zircon crystals found in a sample of phonolite from 4453 m were dated by in situ laser ablation ICP–MS technique to yield ages between 98 and 93 Ma, indicating a maximum age of the formation of the phonolitic volcano of 93 Ma (Turonian). Further, detrital zircons from the clastic material have been dated yielding Archean ages. The gamma ray log indicates three internal cycles within the phonolites, each cycle displaying a stratigraphically upwards decrease in potassium content, suggesting the existence of a longer-lived system undergoing repeated magmatic differentiation and eruption events. The upper volcanic sequence is less evolved and less alkaline than the lower, suggesting a change in primary magma compositions towards progressively higher degrees of melting of the underlying mantle. This fits into a scheme of progressively higher degrees of melting with time, which in a regional context probably corresponds to a rifting event.
Important implications for the interior workings, constituent, circulation between crust and mantle, convection between core and mantle of the Earth can be drawn by studying diamonds and their hosted rocks. Based on the geological comparison of metallogenic kimberlites from super-giant deposits in Botswana and Mengyin and Wafangdian deposits in China, some exploration suggestions and prospecting clues are present as follows: (1) Kimberlite is an unique diamondiferous rock in Botswana. Whereas, lamproite is main hosted-rock in South China craton including two important lamproite zones along the Jiangnan orogenic belt and northern margin of South China craton. Kimberlite is dominantly distributed in the North China craton, which is composed of three kimberlitic zones along Tanlu fault, Trans-North China orogen and Northern margin of North China. Two industrial value diamondiferous kimberlite deposits are distributed in the Tanlu zone. (2) In-situ U-Pb age and Sr, Nd isotopic data of perovskites show that 86-97 Ma Orapa kimberlites and 456-470Ma Mengyin and Wafangdian kimberlites have low 87Sr/86Sr ratios of 0.703-0.705, medium εNd(t) values ranging from -0.09 to 5, indicating that primary kimberlitic magmas were likely derived from primitive mantle or convective lower mantle. (3) Primary kimberlite morphology in Botswana dominantly occur as pipes, while in China mainly occur as irregular fissures, expressed as dykes and lesser extent sills. Crater facies are pervasively observed in Orapa and Jwaneng kimberlite pipes in comparison with hypabyssal (or root zone) facies in Mengyin and Wafangdian pipe clusters. (4) Orapa A/K1 and Jwaneng mines are few dimonderious kimberlitic pipes yielding predominantly eclogitic xenoliths and E type diamond. In constract, Letlhakane, Damtshaa and Karowe mines also in Orapa cluster, Mengyin and Wafangdian mines from the Tanlu kimberlite belt have mainly peridotite xenoliths, P type and E type diamonds. (5) Some exploration suggestions and prospecting clues of diamondiferous kimberlites are represented as follows: (A) Deep faults cutting through on-craton and off-craton subcontinental lithospheric mantle play role in the emplacement of kimberlites; (B) Soil sampling for kimberlite indicator minerals such as picroilmenite and garnet, Cr-rich rutile, Cr-spinel and Cr-diopside is primary exploration tool; (C) Geophysical surveys such as aeromagnetic mothed should be combined with soil sampling for better prospecting results. (6) Diamondiferous prospecting target areas in the Tanlu kimberlite zone, Jiangnan lamproite zone and Tarim block should be further intensified. Illuvial type diamond deposits in China have great potential for mineralization.
Voluminous, subaerial, ultra-depleted, 62 Ma, primary picritic lavas lie on conjugate volcanic margins on both sides of Davis Strait separating Baffin Island and West Greenland. Temporally, these picrites erupted just prior to, and coeval with, the initiation of sea-floor spreading in Labrador Sea and Baffin Bay. Petrogenetically, the chemical characteristics of these picrites (MgO = 18–21 wt%; K 2 O = 0.01–0.20 wt%; ⁸⁷ Sr/ ⁸⁶ Sr i ≈ 0.7030; εNd i ≈ +5.2–8.6; ³ He/ ⁴ He ≤ 49.5R A ) are those of D-MORBs that demand derivation only by partial melting of highly incompatible-element depleted subcontinental lithospheric mantle (SCLM) at a pressure of ~ 4 GPa, followed by rapid ascent to the surface, but do not necessarily require high temperatures or high degrees of partial melting. Tectonically, these picrites formed near Paleoproterozoic suture zones in the SCLM of thick Paleoproterozoic cratonic terranes during Paleogene rifting between Greenland and North America. Structurally, the picrites are related to the major intersection of a NNW-trending lithospheric thinning under Baffin Bay and the ~E-W-trending thickened lithosphere of the Paleoproterozoic Nagssugtoqidian Fold Belt. During the late Mesozoic, ENE extension that thinned the mantle lithosphere and created normal-faulted basins. Elastic finite-element models and present-day studies of crustal extension show that the thicker Nagssugtoqidian Fold Belt underwent less thinning and extension than the Baffin Bay lithosphere. These extensional disparities occurred at the orthogonal intersection of pre-existing ~E-W-trending strike-slip faults in the thicker Nagssugtoqidian Fold Belt with the incipient spreading under Baffin Bay, and likely resulted in the formation of one or more pull-apart basins. Because the strike-slip faults are ancient suture zones, trans-tension within these suture zones easily reached depths of ~120 km, not only creating adiabatic decompression melting in the SCLM, but also forming an open pathway for the picritic melts to rapidly reach the surface. This purely tectonic model requires no spatially or temporally improbable deep mantle plume for generation of the Paleocene picrites of Davis Strait.
A comprehensive, internally consistent U-Pb and Lu-Hf isotope data set for 93 mantle-derived zircons from the Yakutian kimberlite province confirms and further refines the four major episodes of kimberlite magmatism on the Siberian craton: 421-409 Ma (Late Silurian-Early Devonian), 358-353 Ma (Late Devonian-Early Carboniferous), 226-218 Ma (Late Triassic), and 161-144 Ma (Middle-Late Jurassic). The relatively narrow, constant range of εHf values between +2 and +10 for both the Paleozoic and Mesozoic mantle-derived zircons (and by inference kimberlites) suggests that the volatile-rich magmas were repeatedly sourced from the convecting upper mantle beneath the Siberian craton. This finding is in keeping with the narrow and constant range of εNd values for groundmass perovskites from the Yakutian kimberlite province between +1.8 and +5.5 between 420 and 150 Ma. Our preferred model implies that the convecting upper mantle beneath the Yakutian kimberlite province 'recovered' rapidly back to ambient conditions shortly after the giant plume-related flood volcanic event that produced the Siberian Traps at 250 Ma. Although close spatial relationships exist between kimberlites and flood basalts on the Siberian craton during both the Paleozoic and Mesozoic, exact timing of the igneous events and the isotopic compositions of the diverse deep-sourced melting products rule out any direct genetic links.Besides the highly economic kimberlite-hosted diamond deposits of Late Devonian age (e.g., Mir and Udachnaya), the Siberian craton also contains significant Mesozoic placer diamond deposits (e.g., along the Anabar river), for which lamproite sources have been suggested recently. Our study shows that mantle-derived zircon megacryst fragments from the Ebelyakh placer deposit have Late Triassic ages of ca. 224 Ma. Their long-term depleted Hf isotopic compositions (+8.5 εHf) suggest that the alluvial diamonds were sourced from asthenosphere-derived Triassic kimberlites rather than from lithospheric mantle derived isotopically enriched lamproites.
We present high-precision isotope dilution data for Os, Ir, Ru, Pt, Pd and Re in Group I and Group II kimberlites from the Karelian craton, as well as 2 samples of the Premier Group I kimberlite pipe from the Kaapvaal craton. The samples have, on average, 1.38. ppb Pt and 1.33. ppb Pd, with Pt/Pd around unity. These PGE levels are markedly lower, by as much as 80%, than those reported previously for kimberlites from South Africa, Brazil and India, but overlap with PGE results reported recently from Canadian kimberlites. Primitive-mantle-normalised chalcophile element patterns are relatively flat from Os to Pt, but Cu, Ni and, somewhat less so, Au are enriched relative to the PGE (e.g., Cu/Pd. >. 25.000). Pd/Ir ratios are 3,6 on average, lower than in most other mantle melts. The PGE systematics can be largely explained by two components, (i) harzburgite/lherzolite detritus of the SCLM with relatively high IPGE (Os-Ir-Ru)/PPGE (Rh-Pt-Pd) ratios, and (ii) a melt component that has high PPGE/IPGE ratios. By using the concentrations of iridium in the kimberlites as a proxy for the proportion of mantle detritus in the magma, we estimate that the analysed kimberlites contain 3-27% entrained and partially dissolved detritus from the sub-continental lithospheric mantle, consistent with previous estimates of kimberlites elsewhere (Tappe S. et al., 2016, Chem. Geol. http://dx.doi.org/10.1016/j.chemgeo.2016.08.019 ). The other major component in the samples is melt, modelled to contain an average of 0.85. ppb Pt and 1.09. ppb Pd. Assuming that Group II kimberlites are derived from relatively metasomatised SCLM, our data suggest that the metasomatised Karelian SCLM is relatively poor in Pt and Pd. If our data are representative of other Group II kimberlites elsewhere, this result could imply that the PGE enrichment in certain continental large igneous provinces, including Bushveld, is not derived from melting of metasomatised SCLM.
Metamorphic perovskite associated with diopside + chlorite occurs in blackwalls surrounding rodingitized mafic dikes that are included in ultrahigh-pressure (UHP) serpentinized ultramafic rocks from Changawuzi, Chinese southwestern Tianshan. Perovskite forms discrete crystals in the chlorite-rich matrix or appears at the rim of ilmenite. The metamorphic perovskites from the blackwalls are characterized by high concentrations of REE, Nb, Ta, Th and U. The primitive mantle normalized patterns of perovskite are two to three orders of magnitude higher but parallel to the bulk rock patterns, demonstrating that perovskite is a major host of these incompatible trace elements in the ultramafic rocks. The bulk-rock major and trace element compositions of rodingite dikes and associated blackwalls display variable patterns, consistent with different mafic protoliths of the rodingites.Perovskites from three samples were analysed for U-Pb by in-situ secondary ion mass spectrometry. Two samples contained essentially common Pb whereas one sample had enough radiogenic Pb to define a discordia in the Tera-Wasserburg diagram with an age intercept of 317 ± 11 Ma. If all analyses are pooled together, a 238U-206Pb age of 308 ± 5 Ma is obtained. These ages are in agreement with the age of peak UHP metamorphism in Chinese southwestern Tianshan. In-situ Nd isotopes of perovskite from the three samples were obtained by laser ablation multi-collector inductively coupled plasma mass spectrometry. The εNd(t)310 of two samples are 8.37 ± 0.14 and 7.88 ± 0.25, respectively, which suggests that they have similar protoliths. These samples plot close to a 310 Ma Sm-Nd isochron. In contrast, the third sample has a different εNd(t)310 of 0.90 ± 0.51, indicating a completely different protolith. Thus, in-situ analyses of perovskite can provide important information on the protolith as well as the metamorphic evolution of reaction zones between mafic and ultramafic rocks.
Perovskite (CaTiO3) has become a very useful mineral for dating kimberlite eruptions, as well as for constraining the compositional evolution of a kimberlitic magma and its source. Despite the undeniable potential of such an approach, no similar study had been done in Angola, the fourth largest diamond producer in Africa. Here we present the first work of in situ U–Pb geochronology and Sr–Nd isotope analyses of perovskite in six Angolan kimberlites, supported by a detailed petrographic and geochemical study of their perovskite populations. Four types of perovskite were identified, differing in texture, major- and trace-element composition, zoning patterns, type of alteration and the presence or absence of inclusions. Primary groundmass perovskite is classified either as anhedral, Na-, Nb- and LREE-poor perovskite (Ia); or euhedral, strongly zoned, Na-, Nb- and LREE-rich perovskite (Ib). Secondary perovskite occurs as reaction rims on ilmenite (IIa) or as high Nb (up to 10.6 wt% Nb2O5) perovskite rims on primary perovskite (IIb). The occurrence of these four types within the Mulepe kimberlites is interpreted as an evidence of a complex, multi-stage process that involved mingling of compositionally different melts.
U–Pb dating of these perovskites yielded Lower Cretaceous ages for four of the studied kimberlites: Mulepe 1 (116.2 ± 6.5 Ma), Mulepe 2 (123.0 ± 3.6 Ma), Calonda (119.5 ± 4.3 Ma) and Cat115 (133 ± 10 Ma). Kimberlite magmatism occurred in NE Angola likely due to reactivation of deep-seated translithospheric faults (> 300 km) during the break-up of Gondwana. Sr–Nd isotope analyses of four of these kimberlites indicate that they are Group I kimberlites, which is consistent with the petrological observations.
Geological and structural mapping of Paleoproterozoic Kimozero kimberlite with account for lithological facies and geochemical specialization provides evidence for the multiphase structure of the kimberlite pipe, which underwent fragmentation as a result of shear–faulting deformations. Two geochemical types of kimberlite (magnesium and carbonate) are distinguished.
The lithospheric mantle beneath continents is often the same age as the superjacent crust, but remains less well understood. Analysis based on a large database of xenoliths and xenocrysts shows that mantle domains that stabilized during different geologic eons have distinctly different mean compositions. There is a secular evolution from depleted Mg-rich low-density Archean mantle to more fertile, denser Phanerozoic mantle; the most significant differences are between the Archean and Proterozoic mantle. The compositional variations produce differences in the density and elastic properties of lithospheric mantle of different age. Archean and Proterozoic mantle roots are highly buoyant; they cannot be delaminated but require mechanical disaggregation (lithospheric thinning and/or rifting) and infiltration of upwelling fertile material to be destroyed or transformed. In contrast, Phanerozoic subcontinental lithospheric mantle is denser than the asthenosphere for observed thicknesses (∼100 km) and can "delaminate" under stress. The contrasting properties of different mantle domains require lateral contrasts in composition, density, thickness, and seismic response in the present-day subcontinental lithospheric mantle. They also suggest a secular evolution in Earth's geodynamics from Archean to Proterozoic time, and an increased importance for lithosphere-delamination processes in Phanerozoic orogens.
The system forsterite-nepheline-quartz is a useful simple system analogue of melting relations in upper-mantle peridotite. The liquidus phase fields at 28 kbar differ from those at low P by expansion of the enstatite field at the expense of forsterite. The system illustrates a large field of liquid compositions, from model basanites to model quartz tholeiites, which can be derived from one peridotite source. The effect of C-H-O volatiles on melting relationships has been explored with H2O, CH4 and CO2-vapour saturated experiments. The effect of water is to expand the olivine field and depress liquidus T by 350-400oC, but liquids at low degrees of melting of a model peridotite remain Ne- normative. The effect of CO2 is most marked with liquids moving to increasingly undersaturated compositions. Methane saturation produces a similar liquidus depression but results in OH- solution, low carbon solubility and a reduced melt structure, i.e. Si:O <1:2. The studies in the simple Fo-Ne-Q system are matched by melting studies of several peridotite compositions and by liquidus studies on a variety of magnesian primary magmas from different tectonic settings.-J.M.H.
Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thick-ness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consist-ent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation re-quires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar ' garnet signature' to all OIB melts (e.g. [Sm/Yb] N 41); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution.The dilution effect is also re-corded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobaro-metry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such informa-tion. For plate ages up to $80 Ma, we demonstrate that the geophy-sically defined base of the growing oceanic lithosphere corresponds to both an isotherm ($11008C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H 2 O^CO 2 -bearing mantle peridotite is stable under conditions of T 11008C and P 3 GPa ($90 km), this solidus is essentially isothermal (i.e. dT/dP $ 0 in P^T space) with T $11008C) at depths 90 km, but becomes isobaric (i.e. dP/dT $ 0 in P^T space) at the $90 km depth. The latter explains why older (470 Ma) oceanic lithosphere cannot be thicker than $90 km with-out the need to invoke physically complex processes such as convective removal.
A new interpretation of seismic reflection data on the continental margin of northern Labrador shows basement structures similar to those observed on other volcanic rifted margins. Seaward dipping reflections, inner flows, volcanic plateaus, and lava deltas are observed on these data. Magnetic chron 27n (∼61 Ma) is coincident with the volcanic plateaus, connecting these features to the Paleocene volcanism farther north in Davis Strait. Therefore, we are able to extend the region of volcanism about 500 km south along the margin. Similar structures are also observed on the conjugate west Greenland margin. Below the volcanic plateaus, gravity and wide-angle seismic data show thick igneous crust, which is fairly symmetrically distributed across the conjugate margins. However, the geometry of the thinned continental crust is not symmetrical: a narrower zone of thinning is observed off northern Labrador. The thick igneous crust lies seaward of thinned continental crust, which exhibits little or no underplating by the magmatic event. This observation is compatible with recent models for the formation of nonvolcanic margins, followed by a magmatic event late in the rift evolution. In Mesozoic time rifting of the cold, thick cratonic lithosphere occurred between Greenland and North America with minimal volcanic activity. Then in Late Cretaceous time, the zone of rifting narrowed and localized seaward of the shelf, creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the Paleocene magmas were eventually channeled. This scenario is supported by studies of Mesozoic igneous rocks on land in coastal west Greenland and Labrador.
Mantle plumes are columns of hot, solid material that originate deep in the mantle, probably at the core-mantle boundary. Laboratory and numerical models replicating conditions appropriate to the mantle show that mantle plumes have a regular and predictable shape that allows a number of testable predictions to be made. New mantle plumes are predicted to consist of a large head, 1000 km in diameter, followed by a narrower tail. Initial eruption of basalt from a plume head should be preceded by similar to 1000 m of domal uplift. High-temperature magmas are expected to dominate the first eruptive products of a new plume and should be concentrated near the centre of the volcanic province. All of these predictions are confirmed by observations.
The depth of origin of kimberlite magmas remains controversial. Hypothesized sources include the asthenosphere, the lower part of the upper mantle, and the lower mantle. Based on Sr-Nd isotope systematics, kimberlites are divided into two groups. Group 1 kimberlites have Sr-Nd isotope compositions similar to bulk earth. These compositions are consistent with a kimberlite source that is either a relatively unfractionated ocean-island basalt (OIB) like source, or a mixture of depleted mid-oceanic-ridge basalt and enriched mantle components. In order to distinguish between these two possibilities, we performed noble gas analyses on olivine crystals separated from extremely fresh kimberlites from South and West Greenland. We detected OIB-like (Loihi-type) high 3He/4He ratios of up to 26.6 RA in kimberlites. This clearly indicates that group 1 kimberlite magmas have a source similar to that of mantle plumes, implying that the source of kimberlite magma is in the lower mantle.
In situ Sr isotopic analyses of kimberlitic perovskite are more
representative of primary magmatic compositions than conventional
bulk-rock analyses of the same samples, because the latter are variably
compromised by contamination and alteration processes. Bulk-rock Sr
isotopic data obtained for 18 intrusions from the adjacent Narayanpet
and Wajrakarur kimberlite fields of the Dharwar Craton, India, exhibit a
high degree of scatter (˜0.701-0.709) and have
indistinguishable initial isotope ratios. In contrast, laser ablation
perovskite results display strikingly uniform and distinct initial
87Sr/86Sr compositions for each field of
0.70312-0.70333 and 0.70234-0.70251, respectively. The
increased resolution provided by these new data permits the evaluation
of key aspects of kimberlite genesis. It is argued that lithospheric and
crustal contamination had a negligible impact on the perovskite Sr
isotope compositions, and that these values are representative of the
primary melt component in each field. The results cast doubt on models
of kimberlite formation that invoke either the small degree melting of
metasomatized subcontinental lithospheric mantle, or derivation from
unusually enriched asthenospheric mantle. The data are more compatible
with a source region for the Dharwar kimberlites that was similar to a
common mantle component such as prevalent mantle (PREMA) or focal zone
(FOZO).
The composition of the subcontinental lithospheric mantle (SCLM) is broadly related to the tectonothermal age of the overlying
crust, suggesting a secular change in SCLM-forming processes. Most estimated compositions of Archean SCLM, based on well-studied
suites of xenoliths and xenocrysts, are depleted garnet lherzolites with high orthopyroxene/olivine. However, these compositions
make it difficult to account for the high shear-wave velocities measured in the cores of large cratons, and predict deeper
geoid anomalies and higher elevations than are observed in most cratons. Global and regional seismic tomography indicates
that most cratonic xenolith suites represent material from the lower-velocity margins of lithospheric blocks. This implies
that previous compositional estimates are strongly biased toward metasomatized material. We suggest that most Archean SCLM
originally consisted of highly depleted dunites/harzburgites, similar to the Archean orogenic massifs of western Norway. Incorporation
of such rocks in the cold upper parts of the cratonic SCLM satisfies the seismic and gravity data, suggesting that large volumes
of these rocks are preserved in the cores of cratons, but are poorly sampled by volcanic rocks. The roots of most Proterozoic
shields probably consist of refertilized Archean SCLM; the juvenile SCLM beneath Proterozoic and Phanerozoic mobile belts
reflects only moderate depletion of Primitive Mantle compositions. Rather than a gradual evolution in SCLM-forming processes,
we suggest a sharp dichotomy between Archean and younger tectonic regimes. The differences in buoyancy and viscosity between
these two types of SCLM have played a major role in the construction, preservation and recycling of continental crust. If
originally Archean SCLM is more widespread than currently recognized, models of crustal growth rates and recycling may need
to be revised.
Partial melting of magnesite-bearing peridotites was studied at 610 GPa and 1300-1700 degrees C. Experiments were performed in a multianvil apparatus using natural mineral mixes as starting material placed into olivine containers and sealed in Pt capsules. Partial melts originated within the peridotite layer, migrated outside the olivine container and formed pools of quenched melts along the wall of the Pt capsule. This allowed the analysis of even small melt fractions. Iron loss was not a problem, because the platinum near the olivine container became saturated in Fe as a result of the reaction Fe2SiO4Ol = Fe-FePt alloy + FeSiO3Opx + O-2. This reaction led to a gradual increase in oxygen fugacity within the capsules as expressed, for example, in high Fe3+ in garnet. Carbonatitic to kimberlite-like melts were obtained that coexist with olivine + orthopyroxene + garnet +/- clinopyroxene +/- magnesite depending on P-T conditions. Kinetic experiments and a comparison of the chemistry of phases occasionally grown within the melt pools with those in the residual peridotite allowed us to conclude that the melts had approached equilibrium with peridotite. Melts in equilibrium with a magnesite-bearing garnet lherzolite are rich in CaO (2025 wt%) at all pressures and show rather low MgO and SiO2 contents (20 and 10 wt%, respectively). Melts in equilibrium with a magnesite-bearing garnet harzburgite are richer in SiO2 and MgO. The contents of these oxides increase with temperature, whereas the CaO content becomes lower. Melts from magnesite-free experiments are richer in SiO2, but remain silicocarbonatitic. Partitioning of trace elements between melt and garnet was studied in several experiments at 6 and 10 GPa. The melts are very rich in incompatible elements, including large ion lithophile elements (LILE), Nb, Ta and light rare earth elements. Relative to the residual peridotite, the melts show no significant depletion in high field strength elements over LILE. We conclude from the major and trace element characteristics of our experimental melts that primitive kimberlites cannot be a direct product of single-stage melting of an asthenospheric mantle. They rather must be derived from a previously depleted and re-enriched mantle peridotite.
The current (and past) mantle adiabat beneath continents is hot enough
that ascending carbonate-rich domains partially melt liberating
kimberlites. Over the last 2 Ga, most of these kimberlites have been
trapped and frozen in the deep continental lithosphere, forming a
massive CO2 reservoir. Geochemical studies of kimberlites
that reached the surface indicate the pervasiveness of their effects
within the deep lithosphere. These magmas extensively reacted with the
deep lithosphere on the way up. Conversely, studies of deep mantle
xenoliths brought up in kimberlites detect repeated episodes of
metasomatism by these fluids. The heat balance between convection and
conduction through the lithosphere provides gross constraints on the
cumulative global effects of this process. Stagnant lid (including
chemical lid) convection supplies heat to the base of continental
lithosphere in equilibrium with the conductive heat flow. The thermal
gradient beneath continents is constrained by xenolith geotherms and
mantle heat flow q is ˜20 mW m-2. The upwelling
velocity V at the base of the thermal boundary layer is
˜q/TηρC, where ρC is volume specific heat,
4 MJ m-3 and Tη is the temperature to change
viscosity by a factor of e, ˜60 K. The upwelling velocity is thus
˜2 km per million years; the mass of the mantle circulates beneath
continents over ˜2 Ga. The rate that plate tectonics recirculates
the mantle is comparable and limits the supply carbonate-rich undepleted
material to the melting zone of upwelling stagnant lid convection and
thus the production of kimberlites. Overall the amount of CO2
emplaced by kimberlites into continental lithosphere is a significant
fraction (˜1/2) of that in the convecting mantle, resulting in a
major deep lithospheric reservoir of CO2 of ˜5000
× 1018 moles, an equivalent thickness of ˜2 km
carbonatized rock. This process, however, is insufficient to affect the
overall density of deep lithosphere. The observed linearity of the
xenolith geotherm precludes equivalent thicknesses of several kilometers
depending on the radioactive element concentration of actual
kimberlites. Kimberlites are partial melts often of subducted
carbonatized oceanic crust and hence have CO2 to radioactive
element ratios higher than the bulk mantle. Most kimberlites remain
within the deep lithosphere because ascending dikes do not usually
penetrate the region of horizontal compression in the strong cool part
of the lithosphere. Conversely stresses relax quickly within the warm
deep lithosphere allowing dikes to repeatedly intrude.
Carbon is the fourth most abundant element in the solar system. In the Earth carbon is in atmospheric CO2, limestone, other organic products, graphite and trace diamond; interstellar diamond, however, is ubiquitous. Diamond is well known for some unique physical and chemical properties, but it is perhaps less well known that the mineral is geologically ancient (3.3 Ga), that its origins are deep in the mantle (> 180 km), and that diamonds are among the deepest solid objects to reach the surface of the Earth; rare diamonds are from the transition zone (400–670 km), and other diamonds possibly nucleated in the lower mantle (> 670 km). Transport to the surface is in volatile (COHNS)-charged highly explosive kimberlite and lamproite volcanoes. These volcanoes are sited exclusively in the oldest (> 1.7 Ga), tectonically most stable, and thickest (∼ 200 km) regions of crust and upper mantle lithosphere. The energetics required for volcanism are so exceptional and the sources so deep that possible connections between and among the core, geomagnetism, plumes and diamonds are explored. Some correlations are established and others are implied. The results are sufficiently enticing to propose that kimberlites and geographically and temporally associated carbonatites are continental recorders of plumes dating back to at least 2.8 Ga, and that some diamonds may have recorded core events dating back to 3.3 Ga, or possibly earlier.
The intersection between geological sciences and human health, termed medical geology, is gaining significant interest as we understand more completely coupled biogeochemical systems. An example of a medical geology problem largely considered solved is that of lead (Pb) poisoning. With aggressive removal of the major sources of Pb to the environment, including Pb-based paint, leaded gasoline, and lead pipes and solder, the number of children in the United States affected by Pb poisoning has been reduced by 80%, down to a current level of 2.2%. In contrast to this national average, however, about 15% of urban children exhibit blood Pb levels above what has been deemed "safe" (10 pg per deciliter); most of these are children of low socio-economic-status minority groups. We have analyzed the spatial relationship between Pb toxicity and metropolitan roadways in Indianapolis and conclude that Pb contamination in soils adjacent to roadways, the cumulative residue from the combustion of leaded gasoline, is being remobilized. Developing strategies to remove roadway Pb at the source is a matter of public health and social justice, and constitutes perhaps the final chapter in this particular story of medical geology.
Data on the diversity and relative abundance of igneous rock types associated with carbonatite have been compiled for 477 occurrences, which represent 90% of the 527 occurrences of carbonatite known to the authors. The carbonatites have been subdivided into magmatic carbonatites (84%) and carbohydrothermal carbonatites (16%) on the basis of their mineralogy and their intrusive or extrusive characteristics. The carbohydrothermal carbonatites are associated with a variety of silicate rock-types, but nepheline syenite and syenite are the characteristic ones. For the magmatic carbonatites, 24% of these localities have no associated igneous silicate rocks, whereas a diverse range of silicate rock-types are recognized to be associated with the other 76%. The silicate rocks associated with these carbonatites are listed, quantified and described. We distinguish seven main associations with silicate rocks. These are (in decreasing order of abundance): 1) nephelinite-ijolite, 2) phonolite - feldspathoidal syenite, 3) trachyte-syenite, 4) melilitite - melilitolite, 5) lamprophyre, 6) kimberlite, 7) basanite - alkali gabbro. A significant proportion of the nephelinite-ijolite, melilitite-melilitolite and carbonatite only associations also contain ultramafic rocks, which are interpreted as cumulates. Extrusive carbonatite is found at forty-six localities; these are characterized by the high proportion that contain mantle debris (xenoliths and xenocrysts) and the occurrence of melilite-bearing silicate rocks in more than one third of them. The presence of mantle debris in some carbonatite occurrences and the absence of associated silicate rocks from others are taken as evidence of derivation of these carbonatites directly from the mantle. Most other carbonatites are considered to have been generated by differentiation from magmas represented by the associated silicate rocks that are the result of partial melting in a metasomatized lithosphere. Carbonatites can be generated in a number of ways, but the close spatial and temporal association with a broad spectrum of the silicate melts implies a close relationship with them. However, certain associated silicate rocks are not strictly consanguineous with the carbonatite, but are interpreted as products of independent melting events at shallower levels in the lithosphere.
Cratonic lithospheric mantle plays an integral role in defining the physical behaviour of ancient continents and their mineral potential. Bulk compositional data show that modern-day melting residues from a variety of tectonic settings can be as depleted in Al and Ca as cratonic peridotites. Cratonic peridotites are strongly affected by secondary introduction of pyroxenes and garnet such that the extent and depth of melting cannot be reliably determined. Olivine compositions are probably the most reliable tracer of the original melting process and indicate that typical cratonic peridotites have: experienced 40% or more melt extraction. Homogeneous levels of depletion indicated by olivine compositions, combined with mildly incompatible trace element evidence, indicate that melting took place at shallow depths, dominantly in the spinel stability field. Consideration of melt production models shows that shallow (<3 GPa) anhydrous melting is not capable of producing residues dominated by large degrees of melt extraction. Instead, a critical role for water is indicated, implicating the formation of cratonic peridotites within Archaean subduction zones. This melting occurred in the Ncoarchaean in some cratonic blocks, initially forming dunitic residues that are still evident ill the xenolith inventory of some cratons. Release and migration up-section of siliceous melt produced during orthopyroxene breakdown metasomatizes the proto-lithospheric via re-enrichment in orthopyroxene crystallizing from this hydrous Si-rich melt, forming the variably orthopyroxene-rich refractory harzburgites typical of most cratonic roots. Melting in Archaean subduction zones is followed by subduction stacking to form the cratonic root. Gravitational forces may then be responsible for the loss of imbricated matic crust during periods of transient thermal and physical disturbances prior to final long-term tectonic stability. Most diamonds form in the base of these cratonic roots during pulses of thermal or tectonic activity, initially during root construction and subsequently associated with large-scale regional lithospheric events that may be correlated to pulses ill global mantle dynamic evolution.
Shallow (garnet-free), depleted cratonic mantle, occurring as xenoliths in kimberlites and alkaline basaltic lavas, has a high Mg# (100 × Mg/(Mg + Fe) > 92) and is poor in Al and Ca compared to off-cratonic mantle. Here we compile data for many suites of shallow cratonic mantle xenoliths worldwide, and demonstrate a remarkably small range in their olivine Mg#, with an average of ≈92.8. Via comparison with data for experimental melting of mantle peridotite compositions, we explain consistent olivine Mg# as the result of mantle melting and melt extraction to the point of orthopyroxene exhaustion, leaving a nearly monomineralic olivine residue.
Hypabyssal kimberlites are similar in their mineralogy and geochemistry across the world, and thus form a distinct magma type that is produced repeatedly in time and space. This small volume magma is derived from the asthenosphere (200–?650 km), and as it nears the surface consists of olivine macrocrysts, i.e. xenocrysts (~ 25 vol.%), olivine phenocrysts (~ 25 vol.%), entrained in a Ba–Ti–Mg-rich, Al-poor carbonate-silicate liquid containing high, but unknown amounts, of dissolved CO2, CH4 and H2O. Hypabyssal kimberlites are shown to be hybrid rocks formed by mixing with, and assimilation of, mantle-derived lherzolite and harzburgite. Orthopyroxene is not stable in these magmas and its resorption leads the precipitation of primary liquidus olivine. The magma crystallizes rapidly at crustal levels to uniform holocrystalline rocks that do not contain glass. In addition to spinel, perovskite, monticellite, apatite, and phlogopite–kinoshitalite, primary carbonates are typically present as low pressure liquidus phases. Serpentines (primary polygonal serpentine and chrysotile) and carbonate commonly crystallize together from volatile-rich (CO2 + H2O) deuteric residua as a polycrystalline mesostasis or discrete segregations; the latter are not in-filled vesicles. Kimberlite magmas have a very long crystallization interval ranging from > 1200 to ~ 300 °C, and there is a continuum from the magmatic crystallization of silicate and oxide minerals to carbo-hydrothermal residua forming the calcite-serpentine mesostasis. It is concluded that there are no hypabyssal kimberlites which are representatives of the initial primary magma; all kimberlites are hybrid and contaminated magmas which have undergone crystallization in the mantle prior to crystallization of the groundmass-mesostasis assemblages.
Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.
Exploration for diamonds in West Greenland has experienced a major boost within the last decade following the establishment of world-class diamond mines within the nearby Slave Province of the Canadian Arctic. Numerous companies have active programmes of diamond exploration and increasingly larger diamonds have been discovered, notably a 2.392 carat dodecahedral stone recovered by the Canadian exploration company Hudson Resources Inc. in January 2007. The Geological Survey of Denmark and Greenland (GEUS) is currently carrying out several studies aimed at understanding the petrogenesis of diamondiferous kimberlites in Greenland and the physical and chemical properties of their associated mantle source regions (e.g. Hutchison 2005; Nielsen & Jensen 2005).
Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH4- and H-2-bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks. This oxidation mechanism causes an increase in the activity of H2O by the reaction of CH4 with O-2, and the increased aH(2)O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO2 and H2O is lower than in conditions with H2O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO2-undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO2-rich picrites, whereas melts produced by CRM are always SiO2-undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting. The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time. The general decrease of oxygen fugacity (fO(2)) of c. 0 center dot 7 log units per 1 GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200-300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fO(2) than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3 center dot 5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite-magnetite-quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2 center dot 7 Ga. However, the lower fO(2) of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volaile species in melts, which are H2O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO2-poor early atmosphere.
It is now well established that the early continental crust was
formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geo- logic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shal- low subduction. Distinguishing between these scenarios has impor- tant implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to con- strain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trond- hjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG gran- itoids from the NAC, and their elevated garnet δ18O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stack- ing of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.
Growing interest in the mineral perovskite (CaTiO3) as a U–Pb chronometer and archive of near-primary Sr and Nd isotope compositions in magmatic systems highlights the need for well-characterized mineral standards. Based on a conventionally determined, high-precision Sr and Nd isotope data set, we propose perovskite from the alkaline Ice River intrusion as a natural reference material for Sr and Nd isotope ratio determinations. Ice River perovskite has mean present-day 87Sr/86Sr (TIMS) and 143Nd/144Nd (ID-MC-ICP-MS) ratios of 0.702838 ± 51 and 0.512581 ± 32, respectively (2-sigma uncertainties). The TIMS 206Pb/238U age of 361.7 ± 1.0 Ma (2-sigma), determined on the same crystal fragments as the Sr and Nd isotope compositions, falls within the accepted range of 355–372 Ma for the Ice River intrusion. Although there is a ~ 2.2% difference between the ages of Ice River perovskite (~ 355–363 Ma) reported in various U–Pb studies (determined by TIMS, SIMS, and LA-MC-ICP-MS methods), the material investigated here has the currently best-calibrated U–Pb systematics for perovskite. We therefore recommend the 206Pb/238U age of 361.7 ± 1.0 Ma as a reference value for use in geochronology studies.Perovskite is a prime target for petrogenetic studies of kimberlites and related alkaline rocks, because it preserves primary magmatic signatures. The ability to combine important petrogenetic information with high-resolution U–Pb emplacement ages at mineral scale will be instrumental for an improved understanding of deep magmatism beneath cratons and areas of rifted continental lithosphere.
The timing and duration of kimberlite and related magmatism in southern West Greenland have been investigated by determining precise U–Pb perovskite/pyrochlore and Rb–Sr phlogopite ages for 37 samples collected in the Sisimiut, Sarfartoq and Maniitsoq regions. A summary of these results indicate that this magmatism occurred during three main periods within a time span of more than 1 b.y.: 1) Mesoproterozoic (1284–1209 Ma), 2) Neoproterozoic (604–555 Ma), and 3) Jurassic (166–158 Ma).Rb–Sr phlogopite ages were determined for four Mesoproterozoic ultramafic lamprophyre samples collected ~ 50 km ESE to SSE of Sisimiut and they have broadly similar ages between 1284 and 1209 Ma. The most precise ages are between 1246 and 1209 Ma and are interpreted to be a distal manifestation of alkaline and ultrapotassic magmatism related to the ~ 1300–1130 Ma Gardar rift ~ 800 km further south.The majority of the new dates (28) are from dykes and sheets emplaced over an ~ 50 m.y. time span during the Neoproterozoic, a period dominated by ultramafic lamprophyres, some of which are transitional into kimberlites. Initiation of this alkaline magmatism occurred at 604 Ma, however the majority of dykes and sheets were emplaced in the period between 585–555 Ma with possible pulses at 578.7 ± 1.4 and 556.7 ± 1.2 Ma. Calcite kimberlites in the Maniitsoq region and the Sarfartoq carbonatite are constrained to the youngest period of magmatism at ~ 560 Ma. Many of the kimberlites and ultramafic lamprophyres in the Neoproterozoic West Greenland alkaline province were emplaced following the opening of the Iapetus Ocean at about 615 Ma and are interpreted to be linked to continental rifting.The youngest alkaline and ultrapotassic magmatism in West Greenland (Mesozoic to Paleogene) extends for more than 300 km along the coast and the ages obtained in this study for two Jurassic carbonatite complexes, the 165 Ma Qaqarssuk complex near Maniitsoq and the 158 Ma Tikiusaaq complex east of Nuuk, indicate that carbonatites are also an integral part of this Jurassic alkaline province. There is a peak in alkaline and ultrapotassic magmatism in West Greenland during the period 165–145 Ma, possibly indicating a period of increased extension during the opening of the North Atlantic Ocean.
Perovskite is a common matrix mineral and a major carrier of U, Th, Sr and Nd in kimberlite/lamproite magmas. A combined U-Pb age and Sr, Nd and Pb isotopic study of terrestrial perovskite demonstrates that Cretaceous-Jurassic kimberlites in eastern North America are isotopically similar to ocean island basalts. The initial Sr and Nd isotopic compositions for two perovskite populations from the Elliott County kimberlite, Kentucky (0.70403, 0.51266 and 0.70485, 0.51248, respectively) provide evidence that some kimberlite magmas experience substantial isotopic modification in the subcontinental mantle during their evolution. The approximately colinear Sr and Nd isotopic results for perovskite can be explained by the existence of at least three isotopically distinct mantle reservoirs beneath eastern North America. The kimberlitic perovskite data alone extend from PREMA-like compositions to values estimated for primitive upper mantle while the perovskite data from the Prairie Creek lamproite provide evidence for the existence of a third mantle component that experienced an ancient (Late Proterozoic?), LREE and Rb enrichment event.
High relative abundances of primordial 3He are commonly found in ocean island basalts (OIB) thought to be derived from mantle plumes, and high 3He/4He ratios have been used to distinguish plume-type from non-plume OIBs. In simple plume models, one expects to find the highest 3He/4He ratios in the axial part of the plume conduit, which is sampled during the shield building stage of the volcanoes. However, the actual locus of the highest 3He/4He ratios is sometimes significantly displaced. This is best documented for the Hawaiian plume, where the highest-3He/4He basalts are found on Loihi, a volcano tens of kilometers ahead of the inferred plume center, and 3He/4He ratios decrease systematically toward MORB-type values during the main and late phases of eruption. We propose that this effect is caused by small amounts of carbonatite melt formed in plumes as they rise through the transition zone. If the plume conduit is tilted by plate-driven upper mantle flow, the carbonatite melt infiltrates more vertically due to its low density and viscosity and is thus displaced from the plume center. Helium, if partitioned into the carbonatite melt, will also be displaced from the plume center. To test this model we use a numerical simulation of the Hawaiian plume interacting with the fast-moving Pacific lithosphere. We obtain vertical separation velocities of the carbonatite melt on the order of a meter/year. Consequently, helium and carbon, initially located in the plume center at >450km depth, are laterally displaced by 50 to 80km in the shallow mantle, depending on grain size, porosity and melt production rate. This can explain why the highest 3He/4HE ratios (R/Ra up to 39; R/Ra≡(3He/4He)sample/(3He/4He)atmosphere) occur on pre-shield Loihi, why they decline during the shield phases of Mauna Loa, Mauna Kea and Haleakala, and why post-shield and rejuvenated Hawaiian volcanism delivers only low 3He/4He ratios (R/Ra=8–10). Our results quantify the potential role of carbonatite liquids in transporting helium in the Hawaiian conduit, and they appear to apply also to other plumes tilted by upper-mantle ‘wind’.
The Majuagaa kimberlite dike in the Maniitsoq region, southern West Greenland, is 564 Ma old, 2.5 km long, and up to 2 m wide. It is well exposed and very fresh, allowing detailed petrographic and chemical investigations. Little or no serpentinization is observed, and primary petrographic characteristics and grain morphologies are preserved. The dike is diamondiferous and carries xenoliths (up to 30 cm), megacrysts and macrocrysts characteristic of kimberlite. On the bases of the paragenesis and compositions of groundmass phlogopite, geikielite and Mg-Ti-enriched spinel, the dike is classified as kimberlite, despite being carbonate-rich and the apparent absence of monticellite characteristic of many occurrences of bonafide hypabyssal kimberlite. Olivine megacrysts, macrocrysts and microcrysts are all xenocrystic, whereas microphenocrysts are xenocrystic microcrysts overgrown by equilibrium olivine. Most olivine (max. 37 wt.%) is found to be xenocrystic on the basis of Ni mass balance. The average bulk composition of the melt involved in the dike compares well with Group-1 kimberlite. Corrected for xenocrystic olivine and ilmenite, the melt has the composition of a silicocarbonatite and compares with experimental melts formed at very low degrees of partial melting of CO2-bearing lherzolite.
The Labrador Sea is a small oceanic basin that developed when the North American and Greenland plates separated. An initial period of stretching in Early Cretaceous time formed sedimentary basins now preserved under the continental shelves and around the margins of the oceanic crust. The basins subsided thermally during Late Cretaceous time and a second episode of tectonism took place during latest Cretaceous and early Paleocene time, before the onset of sea-floor spreading in mid-Paleocene time. Around the northern Labrador Sea, Davis Strait and in southern Baffin Bay, voluminous picrites and basalts were erupted at and shortly after the commencement of sea-floor spreading. Volcanism occured again in early Eocene time at the same time as sea-floor spreading commenced in the northern North Atlantic. Farther southeast, along the Labrador and southern West Greenland margins, oceanic crust is separated from continental crust by highly stretched but non-magmatic transition zones which developed before sea-floor spreading. A complex transform zone, which developed during sea-floor spreading in late Paleocene and early Eocene time, separates continental and oceanic crust along the Baffin Island margin. The Greenland and Labrador ocean- continent transitions are asymmetric across the only available conjugate cross-sections. However, a cross-section through the Labrador margin farther north resembles the Greenland cross-section in the conjugate pair more than it does the Labrador cross-section of this pair. Consideration of the geological history of the area suggests that the non-magmatic transition zones may have formed by slow extension of a few millimetres per year through a period of 53 Ma during Cretaceous and early Paleocene time.
Mesozoic to Palaeogene intrusive igneous rocks in West Greenland range from a large, coast-parallel dyke swarm to small, poorly defined dyke swarms or single intrusions. New age and geochemical data indicate that intrusion forms and melt compositions changed with time, dependent on changing stress fields and increasing lithospheric attenuation. During the period c . 220–150 Ma (Late Triassic to Late Jurassic) incipient stretching is reflected in the production of highly alkaline, volatile-rich melts formed in small volumes in the deep lithosphere. Around 150 Ma (Kimmeridgian), increased extension took place and melts were intruded in a 60 km long swarm of scattered alkaline dykes. In the Early Cretaceous, 140–133 Ma, the regional stress field was intense, upwelling asthenospheric mantle started to melt, and alkali basaltic magmas were emplaced in a 400 km long coastal dyke swarm parallel to large linear faults offshore. In the Palaeocene, continental break-up took place and flood basalts (62–60 Ma) were extruded in the Nuussuaq Basin. Large basalt sills and dykes extend the region with Palaeocene activity 150 km southwards and form a link between the Nuussuaq Basin and the Sisimiut Basin offshore. Dykes with ages of 57–51 Ma indicate widespread younger volcanic activity.
Supplementary material
Sample details, Ar/Ar data and plots, and Rb–Sr isochrons are available at http://www.geolsoc.org.uk/SUP18374 .
nle compositions of liquids in equilibrium with lherzolite oner a range of temperatures (1380-1505 degrees C) above the carbonated lherzolite solidus have been determined in the system CaO-MgO-Al2O3-SiO2-CO2 at 6 GPa. Melt compositions show systematic variation with temperature from carbonatitic (Mg/Ca ratio 1; 5 wt % SiO2) at the solidus (1380 degrees C) through intermediary compositions to kimberlitic (Mg/Ca ratios >2; >25 wt % SiO2) 70-100 degrees C above the solidus. For melting of model lherzolite with a CO2 content of 0.15 wt %; this continuous change in melt composition from carbonatitic to kimberlitic fakes place in the melting range 0-1%. Our data are thus consistent with an origin for group 1B kimberlites by low-degree partial melting of carbonated; garnet lherzolite at pressures of at least <10 GPa. Furthermore, the observed carbonatite-kimberlite continuum in melt compositions supports petrogenetic links between carbonatites and kimberlites in the mantle source region by small variations in the melt fraction. Carbonatites are associated with kimberlites in mobile belts adjacent to cratons, such as in the Sarfartoq region in west Greenland. Here, a continuum of rock compositions that range from kimberlite through ultramafic lamprophyres to dolomitic carbonatites is in very good agreement with the experimental data at 6 GPa, consistent with the variations in magma compositions in the Sarfartoq region being Produced mainly by variations in the amount of melting at the source. Our data suggest that a similar origin may apply to other carbonatite-kimberlite-ultramafic alkaline rock associations.
comparable with those of alkali basalts. Rb–Sr mica ages, >72 Ma, demonstrate that kimberlite volcanism occurred between 5 and 10 my after the inferred passage of the Discovery plume beneath the Gibeon region. Sr–Nd–Pb isotope relationships of the kimberlite and megacrysts are distinct from that of the inferred INTRODUCTION plume and hence it is argued that the plume contributed little mass Kimberlites have been subdivided into Group I (basaltic) to the volcanism. The megacryst suite has a strong DUPAL Pb and Group II (micacous) on the basis of their petisotope signature. Two hypotheses can explain the genesis of the rography and geochemistry (Smith, 1983). Low Crkimberlite and megacryst suites. The first is that the DUPAL Pb megacrysts are a characteristic of the majority of Group isotope signature is derived from the lower mantle. The megacryst I kimberlites. These megacrysts, also termed discrete suite therefore represents the high-pressure crystallization product of nodules, comprise large (>1 cm), rounded, fractured deep plume-related magmatism. This magmatism interacts with the single crystals. Mutual inclusions and lamella intersub-continental lithospheric mantle (SCLM) to produce the kim- growths define a suite that consists predominantly of berlite magmatism. Alternatively, fluid-rich melts derived from pyrope garnet, magnesian ilmenite (picro-ilmenite), the Discovery plume migrated under the lithosphere and become clinopyroxene (sub-calcic to varying degrees), orthoconcentrated in areas that were recently thermally perturbed as- pyroxene, zircon and possibly olivine. These minerals thenosphere, causing small degrees of melting and kimberlite mag- are compositionally distinct from those found in asmatism. In this scenario the megacrysts represent polybaric sociated peridotite xenoliths, which coupled with differfractionation products from ‘basaltic’ asthenospheric-derived melts ent deformation rules out derivation from peridotites that ponded at the base of, but underwent interaction with, the sub- or eclogites (e.g. Mitchell, 1987). The association with continental lithosphere. Storage of the megacrysts for an extended kimberlite, plus the homogeneity of single crystals, which can be up to 40 cm across, led workers to period (>10 and <100 my) is required to explain the homogeneous
We present new volatile, trace element, and radiogenic isotopic compositions for rejuvenated-stage lavas erupted on Niihau and its submarine northwest flank. Niihau rejuvenated-stage Kiekie Basalt lavas are mildly alkalic and are isotopically similar to, though shifted to higher 87Sr/86Sr and lower 206Pb/204Pb than, rejuvenated-stage lavas erupted on other islands and marginal seafloor settings. Kiekie lavas display trace element heterogeneity greater than that of other rejuvenated-stage lavas, with enrichments in Ba, Sr, and light-rare earth elements resulting in high and highly variable Ba/Th and Sr/Ce. The high Ba/Th lavas are among the least silica-undersaturated of the rejuvenated-stage suite, implying that the greatest enrichments are associated with the largest extents of melting. Kiekie lavas also have high and variable H2O/Ce and Cl/La, up to 620 and 39, respectively. We model the trace element concentrations of most rejuvenated-stage lavas by small degrees (~1% to 9%) of melting of depleted peridotite recently metasomatized by a few percent of an enriched incipient melt (0.5% melting) of the Hawaiian plume. Kiekie lavas are best explained by 4% to 13% partial melting of a peridotite source metasomatized by up to 0.2% carbonatite, similar in composition to oceanic carbonatites from the Canary and Cape Verde Islands, with lower proportion of incipient melt than that for other rejuvenated-stage lavas. Primary H2O and Cl of the carbonatite component must be high, but variability in the volatile data may be caused by heterogeneity in the carbonatite composition and/or interaction with seawater. Our model is consistent with predictions based on carbonated eclogite and peridotite melting experiments in which (1) carbonated eclogite and peridotite within the Hawaiian plume are the first to melt during plume ascent; (2) carbonatite melt metasomatizes plume and surrounding depleted peridotite; (3) as the plume rises, silica-undersaturated silicate melts are also produced and contribute to the metasomatic signature. The metasomatic component is best preserved at the margins of the plume, where low extents of melting of the metasomatized depleted mantle surrounding the plume are sampled during flexural uplift. Formation of carbonatite melts may provide a mechanism to transfer plume He to the margins of the plume.
A new diapiric model for kimberlite genesis takes into account recent interpretations of peridotiteCO2-H20 melting relationships. A minor thermal perturbation at depth might trigger release of reduced vapors with major components C-H-O. Where these volatile components cross the estimated solidus boundary near 260 km, partial melting occurs, the density inversion causes diapiric uprise along adiabats, and the partially melted diapirs begin to crystallize at 100 to 80-km depth, where they reach a temperature maximum (thermal barrier) on the solidus. The released vapor enhances the prospects for crack propagation through overlying lithosphere in tension, and this could produce an initial channel to the surface. Magma separation could then occur from progressively greater depths, releasing COA-undersaturated kimberlitic magma for independent uprise through the established conduit, quite unaffected by the thermal barrier on the solidus of peridotite-CO:-H:O. Cooler diapirs would cross the solidus at somewhat greater depth, solidifying to phlogopite-dolomite-peridotite with the release of aqueous solutions. These solutions are likely candidates for the mantle metasomatism commonly considered to be a precursor for the generation of kimberlites and other alkalic magmas. According to this model the metasomatism is a consequence of kimberlite magmatism rather than its precursory cause.
Kimberlites can be viewed as time capsules in a global plate tectonic framework. Their distribution illustrates clustering in time and space. Kimberlite ages span the assembly and break-up of a number of supercontinents, such as Rodinia and Gondwana. These supercontinents show time lines with (i) broad periods devoid of kimberlite magmatism corresponding to times of continent stability, and (ii) narrow kimberlite emplacement windows corresponding to times of fundamental plate reorganizations. This episodicity implies that kimberlite emplacement events are intrinsically related to particular stages in the life cycle of supercontinents. The onset of kimberlite magmatism is closely associated with thermal perturbations (thermal insulation, mantle upwelling?) beneath a stagnant or sluggish supercontinent. These perturbations may have caused uplift and the onset of continental break-up through fracture zones propagating into the supercontinent. Subsequent spreading and ocean floor development is marked by apparent cusps and jogs in plate motion paths. Resultant strain is accommodated along trans-lithospheric corridors with episodic uplift and erosion and focused kimberlite melt migration. The corridors are manifest as discontinuities in the lithosphere mantle, measured as geophysical gradients and as changes in mantle lithosphere composition. Within the crust, these corridors are expressed as (a) terrane boundaries, (b) incipient continental rifts, (c) fracture zones, or (d) major dyke swarms. Some kimberlite populations are clustered along parallel sets of corridors widely distributed across a large part of a subcontinent and repeated magmatism is seen within many of the clusters. The association of kimberlite occurrences with discontinuities may be ascribed to favorable conditions for melt production and to resultant melt focusing along high strain zones that contain fractures and faults. Such conditions may be attained during different stages in the evolution of continents: (a) supercontinent formation; (b) incipient rifting (driven by far-field stresses?) and onset of continental break-up; and (c) strain accommodation along the continental continuation of oceanic fracture zones during spreading. Type (c) may show concomitant kimberlite magmatism in separate continents after break-up.
Lamprophyre dykes intruded into the Archaean of South West Greenland are believed to coincide with the early stages of rifting in the Labrador Sea area. They yield K/Ar and fission-track ages of 150–115 m.y. which are similar to the apatite fission-track ages from the host rocks but which predate the formation of ocean floor. The strongly undersaturated, often K-rich magmas exhibit the chemical characters of modern rift volcanism with many types present, and the paragenesis and their mineral chemistry reflect the composition and the different physical conditions in the evolution of the lamprophyres. Crystal fractionation and carbonate unmixing are believed to be the main factors in the evolution of the lamprophyre dykes from a primary mantle derived magma. However, the most undersaturated and evolved lamprophyre types cannot be explained in this way, and more complex models must be considered.
A critical examination of the extent to which geodynamic information on the initial mantle depletion and accretion event(s) is preserved in kimberlite-borne cratonic SCLM peridotite xenoliths is attempted by using new major and trace element data of whole-rock peridotites (n=55) sampled across the North Atlantic Craton (NAC; West Greenland). We also present additional whole-rock trace element data of mantle xenoliths from Somerset Island, the Slave and Kaapvaal cratons for comparison.Peridotites comprising the West Greenland SCLM are distinctly more olivine-rich and orthopyroxene-poor than most other cratonic peridotites, in particular those from the Kaapvaal craton. The West Greenland peridotites have higher Mg/Si but lower Al/Si, Al2O3 and CaO than cratonic mantle from the Kaapvaal Craton. We suggest that the more orthopyroxene depleted, harzburgite to dunite character of the NAC peridotites reflects more of the original melting history than peridotites from other cratons and in that sense may be more typical of cratonic lithosphere compositions prior to extensive modification. Despite this, some modal and cryptic metasomatism has clearly taken place in the West Greenland lithosphere. The insensitivity of major elements to pressure of melting at high degrees of melt extraction combined with the ease with which these elements may be changed by modal metasomatism mean that we cannot confidently constrain the depth of melting of peridotites using this approach.Mildly incompatible trace elements offer much more promise in terms of providing geodynamic information about the original Archean melting regime. The very low, systematically varying heavy REE abundances in NAC whole-rock peridotites and in peridotites from all other cratons where high-quality data are available provide ubiquitous evidence for a shallow melting regime in the absence of, or to the exhaustion of garnet. This finding explicitly excludes large extents of deep (iso- and polybaric) melting, which results in high initial garnet abundances and increasing heavy REE abundances. This evidence renders models that invoke large plume-like melting environments redundant in explaining SCLM formation and suggests broadly modern plate tectonic environments are responsible for the depletion of cratonic SCLM. A combination of the shallow melting environment and uniformly high levels of depletion indicate that melting to form the NAC lithosphere and that of other cratons probably took place in a subduction-zone environment.
The once-contiguous North Atlantic craton (NAC) is crosscut by the Labrador Sea that opened during the Early Cenozoic after extensive Mesozoic continental rifting and removal of cratonic mantle. This large-scale structural change within the cratonic lithosphere was followed at about 150 Ma by the cessation of ultrapotassic and potassic-to-carbonatitic magma production, which had prevailed throughout much of the NAC history. At Aillik Bay, a sequence of olivine lamproites (1374.2±4.2 Ma, 2σ), aillikites/carbonatites (590–555 Ma), and nephelinites (141.6±1.0 Ma, 2σ) erupted through the southern NAC edge on the present-day Labrador Sea margin. Links between these alkaline magma types with diverse petrogeneses as a consequence of large-scale processes in the lithospheric mantle over a period of 1200 Myr are demonstrated utilizing their Sr–Nd–Hf–Pb isotope compositions.The Mesoproterozoic olivine lamproites are characterized by unradiogenic Nd (εNd(i)=−8.4 to −5.4), Hf (εHf(i)=−11 to −7.8), and Pb (206Pb/204Pb(i)=14.2–14.8) but moderately radiogenic Sr isotope compositions (87Sr/86Sr(i)=0.7047–0.7062) fingerprinting long-term enriched cratonic mantle, which must have reached to depths of more than 150 km at this time. In contrast, Neoproterozoic carbonate-rich aillikites and carbonatites have fairly radiogenic Nd (εNd(i)=0.1–1.8), Hf (εHf(i)=−0.9 to +2.6), and Pb (206Pb/204Pb(i)=17.5–18.8) but unradiogenic Sr isotope compositions (87Sr/86Sr(i)=0.7033–0.7046) that point to the involvement of convective upper mantle material during melting. Simple binary mixing calculations coupled with the observation that carbonate-rich magmatism prevailed for over 30 Myr in the area imply a complex pattern of lithosphere–asthenosphere interaction at depths between ∼180 and 140 km. The Cretaceous nephelinites have slightly unradiogenic Nd (εNd(i)=−4 to −1.4), moderately radiogenic initial 87Sr/86Sr (0.7044–0.7062), but initial εHf (−3.3 to +1.4) similar to the aillikites and highly radiogenic Pb (206Pb/204Pb(i)=19.1–20.2) isotope compositions. Their sodic mafic alkaline nature reflects partial melting at a higher level of the cratonic mantle tapping metasomatic components that had been introduced during the >30 Myr of Neoproterozoic aillikite/carbonatite magmatism.The new 40Ar/39Ar age and Sr–Nd–Hf–Pb isotope data, along with petrological arguments, suggest that at least 30 km of the cratonic mantle beneath the southern NAC edge had been replaced by the hotter upwelling asthenosphere between ca. 550 Ma, when a thick diamond-bearing lithosphere was present, and 150 Ma. This lithospheric thinning presumably occurred shortly prior to Cretaceous continental rifting in response to enhanced plate-tectonic stresses focused at this zone of persistent lithospheric weakness. It appears, however, that the recurrent volatile-rich alkaline magmatism and associated mantle metasomatism played an important role in destroying the structural integrity of the cratonic mantle thereby aiding the subsequent lithosphere thinning.
Rare earth fractionation in kimberlites implies that they were produced by partial melting in the presence of residual garnet, in accordance with the widely held belief that kimberlites were formed by small degrees of partial melting of a garnet lherzolite lithology in the upper mantle. However, recent discoveries in some kimberlites of diamond xenocrysts containing syngenetic inclusions of majorite, and of xenoliths which originally contained majoritic garnet are suggestive of a deeper, transition zone origin for kimberlites. Experiments on a synthetic Group I kimberlite were carried out using an MA-8 apparatus to evaluate this possibility. At 16 GPa and 1650°C, majorite garnet (13% Al2O3) and beta-M2SiO4 crystallize together on the liquidus, showing that this kimberlite magma could have been produced by a small degree of partial melting of a majorite + beta-M2SiO4 (or gamma-M2SiO4) assemblage in the transition zone (400-650 km). However, the first appearance of garnet well below the liquidus at 10 GPa implies that this typical kimberlite composition could not have been produced by a small degree of partial melting of garnet peridotite at depths shallower than 300 km, and casts doubt on conventional models of kimberlite petrogenesis. Isotopic, trace element and geochemical similarities imply a genetic relationship between kimberlites and ocean island basalts (OIBs). However, kimberlites were derived from a source possessing a higher Mg-number, and lower Na2O, Al2O3 and CaO contents than the OIB source. It is proposed that the ultimate source regions both of kimberlites and OIBs lie in the transition zone, in a boundary layer comprised of mixed domains of subducted former harzburgite and aesthenospheric pyrolite. The boundary layer was refertilized by partial melts derived from garnetite (former subducted oceanic crust) trapped on the 650 km discontinuity.
We present 104 whole-rock geochemical analyses of hypabyssal kimberlite from the Lac de Gras field. Screens using Yb versus Al2O3 and ln Si/Al versus ln Mg/Yb effectively discriminate crustally contaminated samples. The remaining “non-contaminated” kimberlites samples have variable (5 to 50%) entrainment of cratonic peridotite. It is problematic to effectively screen for small amounts (20%) contents of peridotite contamination. We utilize the Lac de Gras data suite to calculate, by two different methods, parent magma compositions and identify two (and potentially three) geochemically distinct parent magma types. The Lac de Gras parent magma compositions are compared to those calculated from other localities in Canada, Greenland, South Africa and Russia. Together, these calculated parent magmas define a range, albeit limited, of viable, yet distinct, kimberlite parent magma compositions. Geochemically, kimberlite parent magmas have high volatile contents (H2O and CO2), high MgO, and low SiO2, Al2O3 and alkalis, with K>Na and Na+K/Al
Kimberlites which intruded the Sisimiut (formerly Holsteinsborg) region of central west Greenland during the Early Palaeozoic have initial between 0.7028 and 0.7033 and ϵNd between + 1.3 and + 3.9. Mid-Proterozoic potassic lamproites from the same region have initial between 0.7045 and 0.7060, ϵNd between −13 and −10 and unradiogenic initial Pb isotopic compositions. The isotopic data favour an asthenospheric mantle source for the kimberlite magmas, in common with “basaltic” kimberlites from other localities, whereas the lamproite magma sources evolved in isolation from the convecting mantle for > 1000 Ma, probably within the subcontinental lithospheric mantle of the Greenland craton, prior to emplacement of the lamproites.
A clear-cut temporal and spatial relationship between small-volume, volatile-rich and highly potassic continental melt fractions, such as kimberlites and related rocks, and large-volume continental flood basalts exists in several Large Igneous Provinces (LIPs). Many of these LIPs are also widely regarded as products of mantle plume–lithosphere interactions. The small-volume melts either immediately pre-date or post-date or even are co-eval with the main flood basalt event. The overlap of ages between the flood basalts and the kimberlites very likely reflects a cause and effect relationship via mantle plumes. Recently discovered end-Cretaceous diamondiferous kimberlites (orangeites) in the Bastar craton of central India which are synchronous with the flood basalts, carbonatites, lamprophyres and alkaline rocks of the Deccan LIP provide an opportunity to re-evaluate the role of mantle plume–lithosphere interactions in the generation of these disparate magmas. The geographical zonation of the kimberlite–lamprophyre–carbonatite–alkaline rock spectrum in the Deccan LIP is inferred to reflect variable thickness of the pre-Deccan Indian lithosphere with a thinner lithosphere along the known rift zones of northwestern and western India and a thickened lithosphere underlying the Bastar craton of central India. This heterogeneity is thought to have controlled the volume of melt generation and melt ascent, as well as the ultimate alkaline magma type. These findings are supported by the regional lithospheric thickness map, generated from converting seismic shear wave velocities into temperature profiles, which clearly depicts that the present-day lithosphere beneath the Bastar craton is thicker than that in western and NW India where the centre of the Deccan plume-head was located. Thermal weakening of the sub-Bastar craton due to mantle plume–lithosphere interaction at the end-Cretaceous resulting in a thin-spot is suggested to have controlled the Deccan-related mafic dyke emplacement in the Bastar craton.