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Reconstructed bulk rock compositions of eclogite from this study are shown in comparison with the Kimberley eclogite data from Jacob et al. (2009), Bellsbank eclogite data from Shu et al. (2016), with MORB and picrite compositions and a field for troctolites Perk et al. (2007): a Al2O3 vs. Na2O of calculated bulk rock compositions. Only group B1 samples overlap with MORB (data from Jenner and O’Neill 2012). Group B2 samples have the highest Al2O3 and Na2O contents while group A samples the lowest Al2O3 and Na2O contents. The latter are interpreted as pyroxene dominated ol (opx) + cpx cumulates as indicated by the black arrow. b Concentrations of Ni vs. Mg# compared with MORB and intraplate and Archean cratonic picrites from GEOROC (http://georoc.mpch-mainz.gwdg.de). c Mg# vs. SiO2 for calculated bulk rock compositions. The vast majority of eclogites have lower SiO2 and higher Mg# compared to modern MORB samples. Group B1 samples with higher Mg# and lower SiO2 than MORB can be interpreted as residues of partial melting in the eclogite stability field. Protoliths for Group A samples with higher SiO2 contents and Mg# were probably cpx-rich cumulates
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We describe the petrography and mineral chemistry of sixteen eclogite and garnet pyroxenite xenoliths from the reworked Boshof road dump (Kimberley) and define three groups that stem from different depths. Group A, the shallowest derived, has low HREE (heavy rare earth element) abundances, flat middle to heavy REE patterns and high Mg# [= 100·Mg/(M...
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... More significantly, these hydrous pyroxenites begin to melt at temperatures 300-350 °C lower than anhydrous peridotite, the hitherto expected mantle source for many Ni-Cu-(PGE) sulfide deposits. These experimental studies (Funk and Luth 2013;Foley et al. 2022;Foley and Ezad 2024;Shu 2023) also show that the hydrous minerals (Ca-amphibole, phlogopite, K-richterite, minor apatite) dominate contributions to the initial melts, whereas clinopyroxene remains in the residue and is commonly a peritectic phase. Amphibole, where present, melts rapidly and completely within about 50 °C of the solidus, resulting in melts with compositions broadly similar to the amphibole, namely basanite/nephelinite melts in the case of calcic amphibole, and lamproitic melts in the case of the alkali amphibole K-richterite . ...
... The concentrations of Ni in the hydrous minerals phlogopite, amphibole and apatite are surprisingly high across many mantle lithologies and tectonic settings. It is common for phlogopite to contain between 1000 and 2000 ppm Ni, and this remains the case for phlogopite that coexists with olivine from both on-craton (up to 2250 ppm; Le Roux and Class 2016; Grégoire et al. 2002;Shu et al. 2018) and (2013) off-craton settings (max. 2090 ppm; Glaser et al. 1999;Ionov et al. 1997). ...
Magmatic Ni–sulfide ore deposits are generally associated with basaltic to komatiitic igneous rocks that originate by partial melting of the mantle, which is usually modelled as a uniform four-phase peridotite. Existing models accept that the key metal contributors to mantle melts are olivine (Ni) and sulfide (Cu, platinum group elements (PGEs) and minor Ni). However, melting in the mantle commonly begins in volumetrically minor mantle assemblages such as hydrous pyroxenites that occur as veins in the peridotite mantle, which are rich in the hydrous minerals phlogopite, amphibole and apatite. The contribution of hydrous pyroxenites to the metal endowment of mantle melts may have been underestimated or overlooked in the past, partly because evidence of their input is partially erased as melting intensifies to involve peridotite.
Here, we compile new results from experiments and natural rocks which demonstrate that the hydrous minerals such as phlogopite, amphiboles and apatite all have high partition coefficients for Ni (3–20) and may be important repositories for Ni in mantle sources of igneous rocks. This implies that hydrous minerals hosted in metasomatic mantle lithologies such as hydrous pyroxenites may be important contributors to some magmatic Ni–sulfide ore systems. Hydrous pyroxenites contain hydrous minerals in large modal abundances up to 30–40 vol% in addition to clinopyroxene and a few vol% of oxide phases, such as rutile and ilmenite. These mantle lithologies are commonly associated with cratonic and continental regions, where low-temperature, low-degree volatile-rich melts commonly modify lithospheric peridotite mantle, depositing variable hydrous pyroxenites.
The lower melting temperatures of hydrous minerals in hydrous pyroxenite lithologies also means that the generation of magmatic ore deposits may not require a major thermal perturbation such as a plume, as the melting temperatures of hydrous pyroxenites lie around 300–350 °C lower than dry peridotites. Partial melts of hydrous pyroxenite are more voluminous at low temperatures than melts of peridotite would be. Furthermore, it is argued in the following that they would contain similar or even higher concentrations of Ni. Thus, predictive exploration models should consider domains of the lithospheric mantle where hydrous pyroxenites may be localised and concentrated, as they may have been episodically melted throughout the long-lived geological evolution of cratonic blocks, yielding Ni-rich melts that may be hosted in conduits of varying size and geometry at various crustal levels.
... The Alfeu-I pyroxenes suggest two distinct genetic episodes, as the clinopyroxenes have an eclogite signature and the orthopyroxenes contain a peridotite signature (Fig. 5A), which is associated with the high Cr content. Figure 5A (Cr 2 O 3 versus Al 2 O 3 and Cr versus Fe versus Na molar) shows that Alfeu-I clinopyroxenes plot in the eclogites and Cr-poor megacrysts fields, which indicates that their mantle source may have been metasomatized by fluids derived from a subducted slab (Shu et al. 2018, Skuzovatov et al. 2022. The chondrite-normalized garnet and clinopyroxene trace element patterns are similar (Figs. ...
... It has been suggested that pyroxenite xenoliths entrained by basalts could be of cumulative origin, the products of melt-peridotite interaction, or represent the remnant of recycled oceanic crust (Yu et al., 2010;Zhang et al., 2010;Xiong et al., 2014;Shu et al., 2018;Guo et al., 2020;Zhao et al., 2021). Based on the various strands of evidence (detailed discussion is summarized in Text S3 of the Supplementary Material), we speculate that olivine websterite KSM5 can be explained by silica-rich melt-peridotite interaction that converts olivine to orthopyroxene (Kelemen et al., 1990), leading to silica enrichment via intensive refertilization. ...
... Mantle eclogites are usually dated using U-Pb, Pb-Pb, Lu-Hf, and Re-Os isotope systems (Pearson et al., 1995a(Pearson et al., , 1995bAulbach et al., 2009;Smart et al., 2017). The Sm-Nd and Rb-Sr isotope dating yielded an extremely wide scatter of obtained isochron and model ages, which caused difficulties with their interpretation (e.g., Jacob, 2004;Shu et al., 2018;Shchukina et al., 2019). ...
... The studied samples were metasomatized by kimberlite melt and all obtained isotope ages likely correspond to the age of the V. Grib kimberlite pipe. However, it was shown that Rb-Sr and Sm-Nd isotope systems in eclogitic minerals could retain evidence for earlier events (Shu et al., 2018;Shchukina et al., 2019). A slope of Sm-Nd regression line constructed using all minerals from garnet pyroxenites gives an age of 370 ± 50 Ma, MSWD = 70 (Supplementary 4, ESM_4.pdf), ...
We investigated mantle eclogite and garnet pyroxenite xenoliths from the V. Grib kimberlite located in the Arkhangelsk diamond province. The eclogites in the lithospheric mantle beneath the Arkhangelsk province were strongly modified by metasomatic processes, which totally obliterated the primary features of protolith. Detailed studies of the xenoliths allowed us to distinguish the following metasomatic events: (1) early mantle metasomatism and (2) interaction with kimberlite melt. During the multiple early mantle metasomatism, primary clinopyroxene and garnet were replaced by metasomatic clinopyroxene, garnet, amphibole, calcite, and phlogopite under the influence of carbonated ultramafic melts. The impact of kimberlite melt caused the dissolution and recrystallisation of solid-phase inclusions and formation of melt pockets consisting of serpentine, chlorite, carbonate, spinel, perovskite, amphibole, recrystallized garnet, and clinopyroxene. En route to the surface in kimberlite melt, the xenoliths were disintegrated and primary garnet and clinopyroxene were metasomatized with increasing Ti and Cr contents, up to formation of high-Cr megacrysts. The garnet pyroxenites are represented by high-Ca, low-Mg and low-Ca, high-Mg types. It is shown that the high-Ca, low-Mg garnet pyroxenites can be the final products of the eclogite xenolith metasomatism by carbonated ultramafic melts. The low-Ca, high-Mg pyroxenites were derived through the interaction of a partial eclogite melt with depleted peridotites.
... The low incompatible element contents of some cratonic eclogites may derive from partial melt extraction from their crustal protoliths or following subduction and eclogite metamorphism (Viljoen et al. 2005;Shu et al. 2018), which could account for the low incompatible element contents of the Type II eclogites. During subduction of oceanic crust, basalt and gabbro may experience partial melting and become incompatible element depleted. ...
... Partial melting of anhydrous eclogite (using an inferred eclogite mineralogy of 50% garnet and 50% clinopyroxene) leads to incompatible element depletion in eclogite residues ( Fig. 12a; (Perk et al., 2007). Smart et al. 2017b;Shu et al., 2018), but increasing Zr/Hf ratios that evolve away from the Type II L eclogite compositions (Fig. 12b). Therefore, the combination of LREE N -depleted compositions and the extremely varied, low Zr/Hf ratios of the Type II L eclogites in particular cannot be explained solely by partial melting. ...
... Mineral-melt partition coefficients are from Green et al. (2000). Modelling is after Shu et al. (2018). (a) Bulk REE N patterns for eclogite melt residues (dashed lines) are compared with the median Roberts Victor Type II L and II H compositions. ...
The origin of the eclogites that reside in cratonic mantle roots has long been debated. In the classic Roberts Victor kimberlite locality in South Africa, the strongly contrasting textural and geochemical features of two types of eclogites have led to different genetic models. We studied a new suite of 63 eclogite xenoliths from the former Roberts Victor Mine. In addition to major- and trace-element compositions for all new samples, we determined 18O/16O for garnet from 34 eclogites. Based on geochemical and textural characteristics we identify a large suite of Type I eclogites (n = 53) consistent with previous interpretations that these rocks originate from metamorphosed basaltic-picritic lavas or gabbroic cumulates from oceanic crust, crystallised from melts of depleted MORB mantle. We identify a smaller set of Type II eclogites (n = 10) based on geochemical and textural similarity to eclogites in published literature. We infer their range to very low δ18O values combined with their varied, often very low Zr/Hf ratios and LREE-depleted nature to indicate a protolith origin via low-pressure clinopyroxene-bearing oceanic cumulates formed from melts that were more depleted in incompatible elements than N-MORB. These compositions are indicative of derivation from a residual mantle source that experienced preferential extraction of incompatible elements and fractionation of Zr-Hf during previous melting.
... Generally, there are several indices of partial melting recorded in eclogite WR composition: strong depletion of LREE towards heavy rare earth elements (HREE) (e.g. [53,54]), fractionation of the middle rare earth elements (MREE) to HREE (e.g., [54]), a simultaneous increase in the MgO content and decrease in the SiO 2 content relative to the protolith (e.g., [55,56]), and an increase in the Sm/Nd ratio to the Nd content [5]. The LREE depletion in WR REE spectra was observed only in the three eclogites with the oceanic gabbro protoliths (Figure 5(a)). ...
... Generally, there are several indices of partial melting recorded in eclogite WR composition: strong depletion of LREE towards heavy rare earth elements (HREE) (e.g. [53,54]), fractionation of the middle rare earth elements (MREE) to HREE (e.g., [54]), a simultaneous increase in the MgO content and decrease in the SiO 2 content relative to the protolith (e.g., [55,56]), and an increase in the Sm/Nd ratio to the Nd content [5]. The LREE depletion in WR REE spectra was observed only in the three eclogites with the oceanic gabbro protoliths (Figure 5(a)). ...
... It is well documented that mantle metasomatism enriches mantle rocks with incompatible elements and, therefore, should also increase their water content (e. g., [1, 3, 15]). The primary evidence that demonstrates eclogite metasomatic enrichment is elevated LREE relative to HREE in the WR profile and the disturbance of the initial isotopic compositions (i.e., cryptic metasomatism; e.g., [7,11,54]), as well as the growth of secondary minerals (e.g., phlogopite) and its effect on the composition of the primary mineral assemblage (i.e., modal metasomatism; [57,58]). ...
The water content in the garnet and clinopyroxene in the mantle eclogites from the V. Grib kimberlite pipe (Arkhangelsk Diamondiferous Province, NW Russia) was analysed using Fourier transform infrared spectrometry. The results show that all clinopyroxene grains contained structural water at concentrations of 39 to 247 ppm, whereas two garnet samples contained detectable water at concentrations of 211 and 337 ppm. The low-MgO eclogites with oceanic gabbro precursors contained significantly higher water concentrations in the omphacites (70–247 ppm) and whole rock (35–224 ppm) compared to those with oceanic basalt protoliths (49–73 ppm and 20–36 ppm, respectively). The incorporation of water into the clinopyroxene may be associated with vacancies at the M2 site, Al in the tetrahedral position, and the elements that filled the M2 site (mostly Na and Ca). The highest water content in the omphacite was detected in a nonmetasomatised sample and was assumed to represent residual water that survived during subduction. Other eclogite samples showed signs of modal and/or cryptic metasomatism and contained less water in the omphacites compared to the nonmetasomatised sample. The water content was heterogeneous within the eclogite section of the sampled lithospheric mantle. The lack of distinct and uniform correlations between the indices of eclogite modification and their water content indicated that the saturation with water was disturbed during their residence within the lithospheric mantle.
... Kyanite-corundum-bearing eclogites may have plagioclase-rich protoliths, as indicated by non-mantle d 18 O ratios, Eu anomalies and characteristic reconstructed rock compositions (e.g. Jacob, 2004;Dongre et al., 2015;Aulbach & Jacob, 2016;Shu et al., 2018). At Udachnaya, an Archean age of eclogites has been suggested based on mineral and whole-rock isochrons and model ages (Pearson et al., 1995a;Jacob et al., 2005). ...
... Bultfontein (Shu et al., 2018) Udachnaya (Alifirova et al., 2015) Udachnaya (our unpublished data) ...
Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. In order to shed new light on this issue, seven graphite-diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite 87Sr/86Sr, acquired in situ by laser-ablation multicollector ICPMS. The calculated P-T conditions for basaltic-group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4.8 – 6.5 GPa and temperatures of 1060 – 1130 °C, while gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4.8 – 5.8 GPa), but a larger range in temperature (990 – 1260 °C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine ± clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their LREE-depletion was the result of < 10 % partial melting during subsequent subduction and emplacement into the cratonic lithosphere. Extremely unradiogenic 87Sr/86Sr (0.70091 – 0.70186 for six of seven samples) not only provide new evidence for the Archean age (2.5 – 2.9 Ga) of Yakutian graphite-diamond-bearing eclogites and for formation of their protoliths in a depleted mantle source, but also suggest that they were not significantly metasomatically overprinted after their formation, despite their extended residence in the cratonic mantle lithosphere.
The mineralogical and petrographic features indicate that the primary mineral association includes garnet, omphacite, ± coesite, ± kyanite, ± rutile, graphite, and diamond. Graphite occurs in the samples in the form of idiomorphic crystals (the longest dimensions being up 0.4 to 1 mm) in garnet and kyanite and extends beyond their grain boundaries. Diamonds occur as octahedral cubic transparent, slightly colored or bright-yellow crystals as big as 0.1 to 2 mm. Furthermore, idiomorphic and highly ordered graphite occurs as inclusions in diamond in four samples. The carbon isotope composition for diamond and graphite has a narrow range (-4 to -6.6 ‰) for both groups (gabbroic and basaltic), indicating a mantle source and limiting the role of subducted isotopically light biogenic carbon or reduction of isotopically heavy carbonate in diamond crystallization. Importantly, the presence of graphite and diamond inclusions in garnet, omphacite, and kyanite in three samples indicates a co-formation close in time to eclogitization. Combined, the petrographic and geochemical evidence suggests that both polymorphic carbon modifications can form in the diamond stability field, as also suggested by experiments and some natural examples, although the exact mechanism remains unresolved. Furthermore, this study provides natural evidence that graphite can be preserved (metastably) deep within the diamond stability field, without recrystallizing into diamond, for a long time – ≥ 2.5 Ga.
... Indeed, various effects of metasomatism in eclogites have been described, such as addition of hydrous minerals or carbonates, enrichments in MgO and incompatible elements as well as enriched isotopic compositions (e.g. Czas et al., 2018;Heaman et al., 2002Heaman et al., , 2006Hills & Haggerty, 1989;Huang et al., 2012Huang et al., , 2014Ireland et al., 1994;De Stefano et al., 2009;Jacob et al., 2009;Misra et al., 2004;Pyle & Haggerty, 1998;Shu et al., 2018;Smart et al., 2009Smart et al., , 2014Spetsius & Taylor, 2002;Taylor et al., 1996;Viljoen et al., 1996;Zedgenizov et al., 2018). External introduction or internal production of a carbonatite-or kimberlite-like ultramafic melt has been implicated in many of these instances, and such melts have been shown to be temporally and genetically related to kimberlite magmatism (e.g. ...
The mineralogy, chemical composition and physical properties of cratonic mantle eclogites with oceanic crustal protoliths can be modified by secondary processes involving interaction with fluids and melts, generated in various slab lithologies upon subduction (auto‐metasomatism) or mantle metasomatism after emplacement into the cratonic lithosphere. Here, we combine new and published data to isolate these signatures and evaluate their effects on the chemical and physical properties of eclogite. Mantle metasomatism involving kimberlite‐like, ultramafic carbonated melts (UM carbonated melts) is ubiquitous though not pervasive, and affected between ~20–40% of the eclogite population at the various localities investigated here, predominantly at ~60–150 km depth, overlapping cratonic mid‐lithospheric seismic discontinuities. Its hallmarks include lower jadeite component in clinopyroxene and grossular component in garnet, an increase in bulk‐rock MgO ± SiO2 and decrease in FeO and Al2O3 contents, and LREE‐enrichment accompanied by higher Sr, Pb, Th, U and in part Zr and Nb, as well as lower Li, Cu ± Zn. This is mediated by addition of a high‐temperature pyroxene from a UM carbonated melt, followed by redistribution of this component into garnet and clinopyroxene. As clinopyroxene‐garnet trace‐element distribution coefficients increase with decreasing garnet grossular component, clinopyroxene is the main carrier of the metasomatic signatures. UM ultramafic melt‐metasomatism at >150 km has destroyed the diamond inventory at some localities. These mineralogical and chemical changes contribute to low densities, with implications for eclogite gravitational stability, but negligible changes in shear‐wave velocities, and, if accompanied by H2O‐enrichment, will enhance electrical conductivities compared to unenriched eclogites.
... Before the chemical composition of eclogites can be used to infer their origin, one must first evaluate whether they have experienced any metasomatism, which may affect xenolithic eclogites both during their residence in the cratonic lithosphere and/or during entrainment in the host kimberlite (Barth et al. 2001;Huang et al. 2012Huang et al. , 2014Shu et al. 2018). Metasomatism can alter the chemical and isotopic compositions of a rock at different scales, cryptically to modally, obscuring the primary composition to varying degrees (Harte 1987). ...
... Therefore, it is important to evaluate the effects of metasomatic processes on the primary compositions of these xenoliths. Several lines of evidence suggest that most eclogites in this study have not been strongly metasomatized: (1) phlogopite and amphibole, which are typically added to the cratonic mantle during modal metasomatism (e.g., eclogites from Kimberley, Kaapvaal craton; Jacob et al. 2009;Shu et al. 2018), are not observed in eclogites in this study. (2) Interaction with low-volume alkaline melts can lead to TiO 2 and high field-strength element (HFSE) enrichment in garnet (e.g., Pearce 2008). ...
The petrology, mineral major and trace-element concentrations, and garnet oxygen isotopic composition of low-MgO (11–16 wt%) eclogites from the Obnazhennaya kimberlite, Siberian craton, are used to infer their petrogenesis. These eclogites contain two types of compositionally distinct garnet: granular coarse garnet, and garnet exsolution (lamellae and fine-grained garnet) in clinopyroxene. The former record higher temperatures at lower pressures than the latter, which record the last stage of equilibrium at moderate pressure–temperature conditions 2.3–3.7 GPa and 855–1095 °C in the upper mantle at the time of entrainment. Although derived from the garnet stability field, these rocks have low-pressure cumulate protoliths containing plagioclase, olivine, and clinopyroxene as reflected by pronounced positive Eu and Sr anomalies in all eclogites, and low heavy rare earth element (HREE) contents in both minerals and reconstructed bulk rocks for a number of samples. Major elements, transition metals, and the HREE compositions of the reconstructed whole rocks are analogous to modern oceanic gabbro cumulates. Despite geochemical signatures supporting an oceanic crust origin, mantle-like δ18O of the garnets (5.07–5.62‰) for most samples indicates that the protoliths either did not interact with seawater or have coincidently approximately normal igneous values. Some of the eclogite xenoliths have lower SiO2 contents and depleted light REE ((Nd/Yb)N < 1) compared to modern oceanic gabbros, suggesting that they experienced partial melting. Positively inclined middle to heavy REE patterns ((Dy/Yb)N < 1) of the reconstructed bulk rocks mostly result from repeated partial melting in the eclogite stability field, based on melting model calculations. We, therefore, suggest that the Obnazhennaya low-MgO eclogites may represent the gabbroic section of subducted or foundered basaltic crust that underwent continued partial melting processes at high pressures, where garnet was the main residual phase.
... Before the chemical composition of eclogites can be used to infer their origin, one must first evaluate whether they have experienced any metasomatism, which may affect xenolithic eclogites both during their residence in the cratonic lithosphere and/or during entrainment in the host kimberlite (Barth et al. 2001;Huang et al. 2012Huang et al. , 2014Shu et al. 2018). Metasomatism can alter the chemical and isotopic compositions of a rock at different scales, cryptically to modally, obscuring the primary composition to varying degrees (Harte 1987). ...
... Therefore, it is important to evaluate the effects of metasomatic processes on the primary compositions of these xenoliths. Several lines of evidence suggest that most eclogites in this study have not been strongly metasomatized: (1) phlogopite and amphibole, which are typically added to the cratonic mantle during modal metasomatism (e.g., eclogites from Kimberley, Kaapvaal craton; Jacob et al. 2009;Shu et al. 2018), are not observed in eclogites in this study. (2) Interaction with low-volume alkaline melts can lead to TiO 2 and high field-strength element (HFSE) enrichment in garnet (e.g., Pearce 2008). ...
