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![Normalized multi-element plots [see Thompson et al. (1984) for factors] for PVF lavas. Rounded plagioclase macrocrysts cause the small positive Sr anomalies in two Low-K Series samples. Analytical reproducibility is illustrated by duplicate analyses of a Namibian picrite (RNT & CJO, unpublished).](profile/Sally-Gibson/publication/30051036/figure/fig6/AS:377559939338242@1467028712157/Normalized-multi-element-plots-see-Thompson-et-al-1984-for-factors-for-PVF-lavas.png)
Normalized multi-element plots [see Thompson et al. (1984) for factors] for PVF lavas. Rounded plagioclase macrocrysts cause the small positive Sr anomalies in two Low-K Series samples. Analytical reproducibility is illustrated by duplicate analyses of a Namibian picrite (RNT & CJO, unpublished).
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The <80 ka basalts–basanites of the Potrillo Volcanic Field (PVF) form scattered scoria cones, lava flows and maars adjacent to the New Mexico–Mexico border. MgO ranges up to 12·5%; lavas with MgO < 10·7% have fractionated both olivine and clinopyroxene. Cumulate fragments are common in the lavas, as are subhedral megacrysts of aluminous clinopyrox...
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... Low-K Series Figure 9 shows the normalized abundances in PVF lavas of a range of elements that are incompatible during anhydrous mantle fusion. The predominant Main Series lavas have coherent patterns on this plot, all closely resembling those of mildly alkalic OIB. ...
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... mantle fusion. The predominant Main Series lavas have coherent patterns on this plot, all closely resembling those of mildly alkalic OIB. The normalized patterns of all element abundances except K, Rb, U and Th in the Low-K Series lavas are within the same range as the Main Series. In two Low-K patterns, Sr is slightly relatively enriched (Fig. 9), as would be anticipated if these two lavas contained dissolved sodic plagioclase megacrysts (see above). Normalized U and Th abund- ances in the Low-K lavas differ little from those in the Main Series. K is, of course, deficient in all the Low-K lavas, with variable K/Nb. Rb relative abundances are extremely ...
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... small volcanic centres, each evolving independently and spread over a large area (Fig. 1). The possibility that all these centres are fed from a single large underlying magma chamber can be discounted because convection within such a reservoir would eliminate the sorts of variations in incompatible elements that characterize the Low-K PVF lavas (Fig. 9). In such circumstances, it is quite reasonable that minor local events, affecting individual magma batches, caused the sorts of random minor variations in the normalized trace-element patterns of the Low-K Series lavas that occur in Fig. ...
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... would eliminate the sorts of variations in incompatible elements that characterize the Low-K PVF lavas (Fig. 9). In such circumstances, it is quite reasonable that minor local events, affecting individual magma batches, caused the sorts of random minor variations in the normalized trace-element patterns of the Low-K Series lavas that occur in Fig. ...
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Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and
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equilibrated in the spinel peridotite stability field (T = 850-1150°C). They attest to the existence of a
metasomatized refractory upper mantle...
Pyroxenites provide important information on mantle heterogeneity and can be used to trace mantle evolution. New major and trace element and Sr-, Nd-, and Hf-isotope analyses of minerals and wholerock samples of garnet websterites entrained in basanite tuffs in Bullenmerri and Gnotuk maars, southeastern Australia, are here combined with detailed pe...
Kilbourne Hole maar, Potrillo volcanic field (PVF), erupted basanite approximately 10 ka ago and thus the melt composition represents essentially the present-day thermal and compositional character of this segment of the continental mantle. Mantle xenoliths are lherzolite (cpx mode 10-25 percent; Cr number of spinel approx. 0.1) and more depleted h...
We present major- and trace-element analyses of mineral phases present in a suite of 16 garnet-peridotite xenoliths from the western terrane of the Kaapvaal craton. The xenoliths were entrained by a Group II Finsch kimberlite at 118 Ma, shortly prior to a major metasomatic event that caused widespread enrichment of the Kaapvaal lithospheric mantle....
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... Furthermore, high-Nb basalts with PREMA-like isotopic compositions occur in both oceanic and continental settings (Fig. 4), which negates an exclusive link to continental lithosphere. We recognize that the elemental compositions of some continental sodic basalts may require derivation-in part or in whole-from a lithospheric source region 29 . In such cases, many basalts retain their PREMA-like isotopic compositions because they are dominated by the signature of asthenosphere-derived metasomatic melts that transferred their compositions to the basal lithosphere shortly before magma genesis 29 . ...
... We recognize that the elemental compositions of some continental sodic basalts may require derivation-in part or in whole-from a lithospheric source region 29 . In such cases, many basalts retain their PREMA-like isotopic compositions because they are dominated by the signature of asthenosphere-derived metasomatic melts that transferred their compositions to the basal lithosphere shortly before magma genesis 29 . Here, we propose that the similarity of trace elements and radiogenic isotopes between kimberlites and silicaundersaturated high-Nb basalts reflects derivation from common carbon-bearing mantle sources containing fusible PREMA-like components at differing depths. ...
Studies of ocean island basalts have identified a Prevalent Mantle (PREMA) component as a fundamental feature of mantle geochemical arrays; however, its origin and distribution are highly controversial, including its potential link to plumes sourced in low-shear-wave velocity provinces (LLSVPs) above the core-mantle boundary. In this study, we interrogate the compositional systematics of ~ 3500 Cenozoic oceanic and continental sodic basalts to provide insights into the origin and distribution of PREMA. We find that low-degree basaltic melts with high Nb concentrations located away from deep-mantle plumes have PREMA-like Sr-Nd-Hf isotopic signatures, implying that PREMA is highly fusible and not exclusively associated with LLSVPs. Geochemical modelling and mantle convection simulations indicate that PREMA could have been generated soon after Earth accretion, experiencing only minimal melting or enrichment, and then scattered throughout the upper mantle, rather than being the result of mixing between depleted and enriched mantle components.
... Using the maximum and minimum age uncertainties, the distribution in these ages suggests a possible eruption duration of 2.9 to 2.2 Myr, respectively. Notably a basanite sample (C311515, 55.7 ± 0.4 Ma) has an age intermediate to nephelinite and phonolite, permissive with more Si-saturated samples (Janney et al. 2002;Janney and Roex 2003), East African Rift (EAR) volcanic rocks (Rosenthal et al. 2009), Brazilian kamafugites (Carlson et al. 2007), Rio Grande rift basanites (Thompson et al. 2005), Central European Volcanic Province basanites (CEVP; Pfänder et al. 2018), and High Arctic LIP (Dockman et al. 2018). FCC volcanic rocks are broken into Groups 1-3 samples and phonolite differentiates. ...
... (Fig. 6) and have generally less radiogenic Os for a given Os content than potassic equivalents (Fig. 7). The majority of silica-undersaturated continental intraplate alkaline volcanic rocks are thought to originate from the thermal boundary layer (TBL)-the region of the lithosphere between the convecting asthenosphere (> 1250° C) and the mechanical boundary layer (MBL) of the lithospherewhere mantle rocks behave rigidly (< 750° C; e.g., Wilson et al. 1995;Thompson et al. 2005). As such, the TBL is not at a fixed depth/pressure but is of variable depth due to the degree of lithospheric thinning. ...
... As such, the TBL is not at a fixed depth/pressure but is of variable depth due to the degree of lithospheric thinning. The FCC sampled a broadly enriched mantle source based on Sr-Nd-Hf-Os isotopes, distinct from earlier (130-80 Ma) mantle sources of HALIP ( Fig. 6; e.g., Dockman et al. 2018), and more enriched than partial melts from the Rio Grande (e.g., Thompson et al. 2005). The FCC mantle source region is less enriched than low-degree partial melts from the EAR, Brazil, the Rhine Graben, or the Colorado Plateau (e.g., Carlson and Nowell 2001;Carlson et al. 2007;Rosenthal et al. 2009;Pfänder et al. 2018). ...
Melilitite, nephelinite, basanite, and alkali basalt, along with phonolite differentiates, form the Freemans Cove Complex (FCC) in the south-eastern extremity of Bathurst Island (Nunavut, Canada). New ⁴⁰Ar/³⁹Ar chronology indicates their emplacement between ~ 56 and ~ 54 million years ago within a localized extensional structure. Melilitites and nephelinites, along with phonolite differentiates, likely relate to the beginning and end phases of extension, whereas alkali basalts were emplaced during a main extensional episode at ~ 55 Ma. The melilitites, nephelinites, and alkali basalts show no strong evidence for significant assimilation of crust, in contrast to some phonolites. Partial melting occurred within both the garnet- and spinel-facies mantle and sampled sources with He, O, Nd, Hf, and Os isotope characteristics indicative of peridotite with two distinct components. The first, expressed in higher degree partial melts, represents a relatively depleted component (“A”; ³He/⁴He ~ 8 RA, εNdi ~ + 3 εHfi ~ + 7, γOsi ~ 0). The second was an enriched component (“B” ³He/⁴He < 3 RA, εNdi < – 1 εHfi < + 3, γOsi > + 70) sampled by the lowest degree partial melts and represents carbonate-metasomatized peridotite. Magmatism in the FCC shows that rifting extended from the Labrador Sea to Bathurst Island and reached a zenith at ~ 55 Ma, during the Eurekan orogeny. The incompatible trace-element abundances and isotopic signatures of FCC rocks indicate melt generation occurred at the base of relatively thin lithosphere at the margin of a thick craton, with no mantle plume influence. FCC melt compositions are distinct from other continental rift magmatic provinces worldwide, and their metasomatized mantle source was plausibly formed synchronously with emplacement of Cretaceous kimberlites. The FCC illustrates that the range of isotopic compositions preserved in continental rift magmas are likely to be dominated by temporal changes in the extent of partial melting, as well as by the timing and degree of metasomatism recorded in the underlying continental lithosphere.
... Numerous previous studies have investigated lithospheric thickness, mantle potential temperature and magmatic source compositions of the Rio Grande Rift (e.g. Thompson et al., 2005;Afonso et al., 2016;Hopper and Fischer, 2018), which makes it an ideal region to test our modelling framework. In particular, there is evidence of a mantle upwelling with elevated asthenospheric potential temperatures beneath the region, that triggered magmatic activity and largescale regional uplift (e.g. ...
... In particular, there is evidence of a mantle upwelling with elevated asthenospheric potential temperatures beneath the region, that triggered magmatic activity and largescale regional uplift (e.g. Thompson et al., 2005;Klö cking et al., 2018). ...
... In addition, the data set has been screened by La/Ba and La/Nd ratios to remove samples with a subduction signature attributed to a lithospheric mantle source (Fitton et al., 1991). Since there is evidence of clinopyroxene fractionation in samples with <10.7 wt% MgO (Thompson et al., 2005), we correct major and trace element compositions through reverse crystallisation of clinopyroxene in Petrolog3 (Danyushevsky and Plechov, 2011). Beyond the threshold of 10.7 wt% MgO, the major element compositions of all samples are further corrected for olivine fractionation by incremental addition until the Ni content of olivine in equilibrium with the corrected melt reaches 3500 ppm (Korenaga and Kelemen, 2000). ...
The chemical composition of erupted basalts provides a record of the thermo-chemical state of their source region and the melting conditions that lead to their formation. Here we present the first probabilistic inversion framework capable of inverting both trace and major element data of mafic volcanic rocks to constrain mantle potential temperature, depth of melting, and major and trace element source composition. The inversion strategy is based on the combination of i) a two-phase multi-component reactive transport model, ii) a thermodynamic solver for the evolution of major elements and mineral/liquid phases, (iii) a disequilibrium model of trace element partitioning and iv) an adaptive Markov chain Monte Carlo algorithm. The mechanical and chemical evolution of melt and solid residue are therefore modelled in an internally- and thermodynamically-consistent manner.
We illustrate the inversion approach and its sensitivity to relevant model parameters with a series of numerical experiments with increasing level of complexity. We show the benefits and limitations of using major and trace element compositions separately before demonstrating the advantages of a joint inversion. We show that such joint inversion has great sensitivity to mantle temperature, pressure range of melting and composition of the source, even when realistic uncertainties are assigned to both data and predictions. We further test the reliability of the approach on a real dataset from a well-characterised region: the Rio Grande Rift in western North America. We obtain estimates of mantle potential temperature (∼ 1340 °C), lithospheric thickness (∼ 60 km) and source composition that are in excellent agreement with numerous independent geochemical and geophysical estimates. In particular, this study suggests that the basalts in this region originated from a moderately hot upwelling and include the contribution from a slightly depleted source that experienced a small degree of melt or fluid metasomatism. This component is likely associated with partial melting of the lower portions of the lithosphere. The flexibility of both the melting model and inversion scheme developed here makes the approach widely applicable to assessing the thermo-chemical structure and evolution of the lithosphere-asthenosphere system and paves the way for truly joint geochemical-geophysical inversions.
... Field for hornblendite melt is from Pilet et al. (2008), hydrous peridotite from Hirose and Kawamoto (1995), silica-excess pyroxenite from Tsuruta and Takahashi (1998) and Pertermann and Hirschmann (2003), and silica-deficient pyroxenite melts from Lambart et al. (2009Lambart et al. ( , 2013. Arrowed lines show melting trends of increasing degrees of melting of anhydrous lherzolite (Thompson et al., 2005). et al., 2013) but cannot produce melts with FeO contents as high as observed in the Youkou basanites. ...
The Adamawa Volcanic Massif of the Cameroon Volcanic Line (CVL) is dominated by silica-undersaturated basaltic lavas, which have been attributed to plume volcanism. However, the relative contributions of peridotite and pyroxenite sources have not been assessed. Here, we present mineral and whole-rock data for the lavas to consider this issue and to better understand their petrogenesis and source mineralogy. The lavas are porphyritic, with phenocrysts and micro-phenocrysts of olivine, and to a lesser extent, clinopyroxene, in a groundmass of plagioclase, oxides, and apatite. The lava compositions are all basanites. They are silica-undersaturated and are primitive, mantle-derived melts. The basanites display lower CaO and CaO/Al2O3 ratio but higher Fe/Mn values than expected for melts of peridotite, consistent with an appreciable contribution from melting of pyroxenite in the source.
Olivine phenocrysts are characterized by low CaO and NiO contents but higher Fe/Mn ratios than those of olivine in melts of peridotitic sources, supporting the contribution of pyroxenite to the primary melts, and that the main mantle source lithology for the basanites was a mixture of spinel-facies peridotite and silica-deficient pyroxenite. The presence of both pyroxenite and peridotite xenoliths in this area attests to the contribution of these two mantle lithologies to the primary melts, and their relative contribution varies on a very local scale. Significant contamination by crustal materials during magma ascent is ruled out. The Youkou basanites provide a key for improving the general understanding of melt production in lithologically heterogeneous mantle beneath the CVL. Beneath the Youkou volcano, partial melting of pyroxenite by decompression formed silica-deficient melts at 60-80 km depth, which modified the peridotite mantle source by a low degree melt enrichment process indicated by the enrichment in incompatible elements. This modified mantle source then underwent low-degree melting to produce the basanitic melts, which are concentrated in a linear zone focused by the combination of the northern edge of the Congo craton and the Central African Shear Zone.
... Different geochemical reservoirs and processes can contribute to their petrogenesis. These include, (i) delaminated subcontinental lithospheric mantle (SCLM) recycled into the asthenosphere (e.g., Zindler and Hart, 1986), (ii) metasomatically enriched asthenospheric mantle (e.g., Aldanmaz et al., 2006), (iii) thermal boundary layer enriched by mantle plume/asthenospheric melts (e.g., Hasse et al., 2004;Thompson et al., 2005), (iv) SCLM metasomatized by mantle plumes (e.g., Chandra et al., 2019), and (v) SCLM enriched by subduction of crustal material (e.g., Goodenough et al., 2002;Paul et al., 2020;Zhao and McCulloch, 1993). ...
The Singhbhum Craton in eastern India is host to at least seven sets of mafic dyke swarms. Four previously dated swarms (studied here) include the NNE-trending Keshargaria (ca. 2.80 Ga) and Ghatgaon (ca. 2.76-2.75 Ga) swarms, the ENE-trending Kaptipada swarm (ca. 2.26 Ga), and the ESE-trending Pipilia swarm (ca.1.76 Ga). The dykes range in composition from basalt to andesite and have transitional tholeiitic to calc-alkaline affinities. They show intra-and inter-swarm geochemical and Sr-Nd isotopic heterogeneities and have SiO 2 content ranging from 47 to 60 wt.%. The chondrite normalized REE patterns show enrichment in LREE and the primitive-mantle normalized multi-element patterns show elevated U, Th, Cs, Rb, K, and Pb; and depletion in Nb, Ta, and Ti. These characteristics indicate involvement of crustal component in the petrogenesis of these dykes. The dykes of different swarms have variable 87 Sr/ 86 Sr i and ε Nd(i) values, which define a crust-like isotopic growth trajectory with time from a common chondritic to depleted source that was enriched contemporaneously with the formation of the crustal rocks of the Singhbhum Craton. The isotope data indicate involvement of older enriched crustal material in the petrogenesis of these dykes. Variable but mostly high (compared to similarly evolved magmas) Ni (40-590 ppm), Cr (40-1110 ppm), and V (120-434 ppm) contents particularly of the most primitive dykes indicate that parental melts were in equilibrium with mantle peridotite and experienced only minor fractional crystallization of olivine, pyroxene, and magnetite. The Sr-Nd isotope ratios do not show any correlation with differentiation indices which indicates that the melts were not modified significantly by crustal assimilation during ascent and emplacement. The crust-like secular trend of the Sr and Nd isotopic compositions suggests that the enriched crustal material was incubated in the mantle (i.e., metasomatized lithospheric mantle) for a long time and this source was periodically tapped leading to multiple dyke emplacement events over at least 1 Gyr. The recycled crustal material played a role in metasomatizing the subcontinental lithospheric mantle prior to ca. 2.80 Ga. Mantle plume activity triggered melting of the metasomatized lithospheric mantle many times, leading to the emplacement of mafic dykes of different generations across the craton.
... low degrees of mantle melting and/or derivation from mantle source(s) rich in incompatible elements, such that alkali basalts might be taken as deep probes of enriched domains in the upper mantle (Farmer 2014). Alkali basaltic rocks, like other types of basaltic rocks, are derived from magmas generated in the lithospheric and sub-lithospheric mantle, although their relative contribution is a subject of continuous debate (Thompson et al. 2005;Xu et al. 2005;Jung et al. 2006;Kuritani et al. 2009;Zeng et al. 2010;Pilet et al. 2008). An improved understanding of this aspect has direct bearing on the architecture of the lithosphere and its tectonic modification. ...
... These authors showed that (Pilet et al. 2008), hydrous peridotite (Hirose and Kawamoto 1995), and silica-excess pyroxenite (Tsuruta and Taka-hashi 1998;Pertermann and Hirschmann 2003). Also shown are melting trends with increasing degrees of melting of anhydrous lherzolite (Thompson et al. 2005). The field for silica-deficient pyroxenite melts is projected from the same compiled data as in Fig. 9 (a) Roux et al. 2011) relative to peridotite-derived melts, pyroxenite-derived melts are expected to have low Mn/Fe, Co/Fe, Ni/Co, Mn/ Zn and high Zn/Fe because of differences in the partitioning behavior of FRTE between assemblages rich in olivine and orthopyroxene, and those rich in garnet and clinopyroxene. ...
A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57–66) and Na2O/K2O (~ 2.0–6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb–Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr–Nd isotopic compositions (87Sr/86Sr = 0.7037–0.7048 and 143Nd/144Nd = 0.5128–0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic–ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes.
... A systematic study on the primary magmas has revealed that pyroxenite is the principal lithology in the mantle source of Late Mesozoic basalts in SE China (Zeng et al., 2016). Following the method in Thompson et al. (2005), the calculated primary melts were plotted on the CIPW normative Di-Ol-Hy-(Ne+Lc)-Qz projection, along with experimentally determined mantle-derived melts from peridotite, carbonated peridotite, carbonated eclogite, hornblendite, clinopyroxene-hornblendite, silica-deficient pyroxenite and silica-excess pyroxenite (Fig. 11). The Antang basalts plot within the compositional range of silica-deficient garnet-pyroxenite melts. ...
... Therefore, the best interpretation is that the source of the Antang basalts was a Fig. 11. CIPW normalization plotted in Ne + Lc (nepheline + leucitite), Ol (olivine), Di (diopside), Hy (hypersphene), Q (quartz) space (Thompson et al., 2005). Melt compositions in a range of partial melting experiments on various sources, including: hornblendite, cpx-hornblelndite, carbonated peridotite, carbonated eclogite, silica-deficient pyroxenite, silica-rich pyroxenite, and peridotite. ...
The petrogenesis of high-μ (HIMU, μ = 238U/204Pb) basalt in a continental setting remains highly controversial. Here we present a comprehensive geochemical (including major and trace element and isotopic compositions of whole-rock, olivine and melt inclusion) data on Jurassic HIMU-like basalts from the interior (Antang) of South China. This study identified high-H2O (up to 5.0 wt%) olivine-hosted melt inclusions in these basalts. After stripping off the effects of degassing, post-entrapment crystallization and kinetic diffusion, the primary magma of the Antang basalts was estimated to contain ≥2.65 wt% H2O and the corresponding mantle source contains >1000 ppm H2O, which is much higher than the water content of convecting asthenosphere and lower mantle. In addition, the relative depletion of fluid-mobile elements and positive Nb-Ta anomalies in bulk rocks and melt inclusions, and the distance far away from the contemporaneous paleo-Pacific subduction zone, preclude an origin from a slab fluid-metasomatized mantle. The strong depletion of CaO in both olivine phenocrysts and whole-rock compositions further indicates that the basaltic magmas originated from an eclogitic or pyroxenitic mantle source. The combined geochemical data suggest that the Antang basalts were most likely derived from a hydrous source, which was composed mainly of ancient recycled oceanic crust that had stagnated in the mantle transition zone (MTZ) for more than one billion years. Our results therefore provide a new perspective on the MTZ origin of intracontinental basalts and imply that the hydrous MTZ can preserve recycled crustal components for long periods of time and form distinct mantle reservoirs for intraplate basalts.
... One international standard (NBS 688) and three blanks were prepared parallel with the rock samples. More details on dissolution methods are presented in Ottley et al. (2003) and Thompson et al. (2005). Calibration of the ICP-MS was achieved by means of in-house standards and international reference materials (W-2, BHVO-1, AGV1, BE-N and NBS 688), together with procedural blanks. ...
Petrography of seven saucer-shaped sills of the Faroe Islands. S1.1. The Streymoy and Kvívík sills (Fig. S1.1) Intergranular plagioclase constitutes ~35 % in most samples, occurring as 0.07-0.75 mm randomly oriented subhedral laths or as 0.75-1 mm equant anhedral grains. Additional 1.5 to 3 mm subhedral chemically zoned plagioclase phenocrysts count for 15-20 %. Intergranular anhedral equant 0.07-0.75 mm grains of clinopyroxene count for ~40 % of these rocks. Randomly strewn <0.25 mm anhedral equant olivine grains, partly altered to phyllosilicates, make up 2-6 % of rock volume joined by <3 % oxide grains of roughly Fig. S1.1. Photomicrographs of the Streymoy and Kvívík sills. a) General distribution of most common minerals under planed polars. b) Chemically zoned plagioclase phenocryst under crossed polars. c) Partially resorbed olivine phenocryst under crossed polars (yellow dotted line). d) Closer view of a) showing distribution of tiny partially resorbed olivines under plane polars. plag = plagioclase; cpx = clinopyroxene; ol = olivine; ox = oxide. similar sizes. A few larger 0.5-1 mm partially resorbed olivine grains constitute <1 % of these intrusions.
... This anomaly could be explained either by (i) higher mantle water contents in the anomalous zone compared to the adjacent mantle, (ii) the presence of partial melts, or (iii) removal and replacement of the lithosphere by hotter asthenosphere West et al., 2004). It has been suggested that KH peridotite xenoliths are sampling asthenosphere or new lithosphere compositionally similar to the asthenosphere below (Byerly and Lassiter, 2012;Gao et al., 2004;Kil and Wendlandt, 2004;Perry et al., 1988;Rowe et al., 2015;Thompson et al., 2005). However, it is Rio Puerco that is located in the northern section of the rift where the slow seismic velocity anomaly has been detected West et al., 2004). ...
... Moreover, RP xenoliths have trace element and isotopic compositions that indicate extensive metasomatism typical of the subcontinental lithosphere, and compositional characteristics of the lavas from this area also rule out an asthenospheric origin (Byerly and Lassiter, 2012;Perry et al., 1988). On the other hand, lithosphere thicknesses are similar at RP and KH: $50-75 km and $40-80 km thick respectively (Achauer and Masson, 2002;Cordell et al., 1991;Thompson et al., 2005). Equilibration pressure estimates are also similar: 1-1.8 GPa (based on phase stability diagrams) at KH and 1.1-1.8 ...
... First, realistic pressure-temperature conditions the peridotites could come from near the LAB must be estimated. The LAB is estimated to be at 40-80 km depth beneath KH (Achauer and Masson, 2002;Cordell et al., 1991;Thompson et al., 2005). Using the geotherms for the KH area of Kil and Wendlandt (2004), this corresponds to temperatures of 850-1450°C. ...
The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5–184 · 10²¹ Pa·s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 ± 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from <0.5 to 120 ppm H2O in peridotites and from 227 to 400 ppm H2O in pyroxenites. A composite KH xenolith of a harzburgite cross-cut by a clinopyroxenite vein shows this heterogeneity at the cm scale. The second contribution of this study stems from the majority of the KH peridotites and two of the RP peridotites having major and trace elements that can be explained by partial melting without any need to invoke metasomatic processes. This allows to show that, prior to modelling the water content variation of each peridotite mineral during melting, a correction for sub-solidus equilibration has to be applied to the water contents of the minerals. Sub-solidus equilibration also provides an explanation for the discrepancy between the clinopyroxene/orthopyroxene ratio of water contents in natural peridotites worldwide and in laboratory experiments on water partitioning in peridotite minerals. Finally, the cryptically metasomatized peridotites, rare at KH and abundant at RP, as well as the pyroxenites, permit to decipher the origin and water contents of the metasomatic melts that affected the continental lithosphere beneath the Rio Grande Rift. Trace element modelling of the metasomatized KH and RP peridotites are consistent with metasomatism via melts that are of subduction origin. Melts in equilibrium with peridotites contain more water at RP (∼1 wt.% H2O) than at KH (∼0.5 wt.% H2O), although this did not result in a more water-rich mantle lithosphere at RP. Rio Puerco lies within the northern Rio Grande rift, proposed to have been affected by a flat slab subduction, which may explain the more hydrous and extensive metasomatism compared to the south, where KH is located.
... Continental intraplate alkaline volcanic rocks (CIAV) comprise a wide spectrum of sodic and potassic compositions ranging from alkali basalts, picrites and basanites through to more evolved eruptive products that include nephelinites, carbonatites, melilitites, and kimberlites. The origin of some of these rock types are not unequivocal, with petrogenetic models ranging from pure incipient rift-related sources (e.g., Thompson et al. 2005), to 'hotspot' or 'plume' related origins (e.g., Haggerty 1999. Finding a likely source for these volcanic rocks is not made any less ambiguous when experimental and geochemical data are considered as many of these lavas are thought to derive from close to the boundary layer that separates the convecting and conducting mantle (e.g., Foley 1992;Day et al. 2005), i.e., both the asthenosphere and SCLM can be implicated in the genesis of these magmas. ...
... CFB above). Alternatively, melting of metasomatized lithosphere during rifting events (e.g., Carlson and Nowell 2001;Thompson et al. 2005) may also be responsible for the HSE abundances and Re-Os systematics of some CIAV, such as the Newer volcanic rocks, Australia (Vogel and Keays 1997). Similarly, carbonatites may also ultimately originate from mafic as opposed to ultramafic sources due to their close association with other ultrapotassic rocks (e.g., Gudfinnsson and Presnall 2005). ...
