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Eclogite xenoliths recovered from kimberlites and alkali basalts are clinoproxene and garnet rich rocks formed at high pressures within the mantle. They are broadly 'basaltic' in composition and their genesis remains controversial. Early models suggested that they represent the metamorphic end product of (frozen) magma that never made it to shallow depths and instead crystallised at mantle pressures. However, most recent models favour an origin where basalts and gabbros generated at shallow pressures are subducted back into the mantle, converted into eclogite and that subduction is the mechanism by which eclogites become incorporated into the ancient subcontinental lithospheric mantle. The platinum-group elements (PGE) and Au are powerful tracers of mantle processes and their limited mobility during metamorphism and alteration allows them to be used as tools to see through later metamorphic events to the original formation mechanism. The present study presents new geochemical data, including the f...

Eclogite xenolith varieties at Kaalvallei include accessory-free bimineralic xenoliths, diamond-bearing eclogite, corundum-bearing eclogite, garnet-websterite, alkremite and spinel-bearing eclogite. The xenoliths can be accurately classified into previously defined Group I and Group II varieties on both petrographic and geochemical principles. Kaalvallei Group I eclogites (including diamond-bearing eclogite) are considered to derive from a heterogeneous protolith. Eclogite genesis might have been by residue formation associated with the dehydration and partial melting of a protolith consisting of variably mixed, subducted ocean floor basalt and sediment. Kaalvallei Group II eclogite xenoliths are likely to have formed through crystallization of small-volume melts within conduits in old, enriched subcontinental lithosphere. Kaalvallei websterite xenoliths might be petrogenetically related to Group II xenoliths. Isotopic data for Kaalvallei corundum-bearing eclogite and alkremite xenoliths do not provide constraints on petrogenesis. Spinel-bearing eclogite xenoliths are ultradepleted in virtually all trace elements, with very low light rare earth element contents, relatively high heavy rare earth element concentrations, extreme Sr/Sr ( 0 915) and extreme Nd/Nd (0 517) isotopic compositions. These xenoliths are considered to be the residues of a partial melting event.

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (

The Orapa and Jwaneng kimberlites are located along the western margin of the Kalahari Craton and the prevalence of eclogitic over peridotitic diamonds in both mines has recently been linked to lower P-wave velocities in the deep mantle lithosphere (relative to the bulk of the craton) to suggest a diamond formation event prompted by mid-Proterozoic growth and modification of preexisting Archean lithosphere (Shirey et al. 2002). Here we study peridotitic diamonds from both mines, with an emphasis on the style of metasomatic source enrichment, to evaluate their relationship with this major eclogitic diamond formation event. In their major element chemistry, the peridotitic inclusions compare well with a world-wide database but reveal differences to diamond sources located in the interior of the Western Terrane of the Kaapvaal block, where the classical mines in the Kimberley region are located. The most striking difference is the relative paucity of low-Ca (

Syngenetic garnet of eclogitic/pyroxenitic composition included in a polycrystalline diamond aggregate from the Venetia kimberlite, Limpopo Belt, South Africa shows multiple inclusions of spherules consisting of 61±5 vol% Fe3C (cohenite), 30±2 vol% Fe-Ni and 9±3 vol% FeS (troilite). Troilite forms shells around the native iron-cohenite assemblage, implying that both compositions were immiscible melts and were trapped rapidly by the silicate. It is proposed that this polycrystalline diamond-silicate-metallic spherule assemblage formed in very local pressure and fO2 conditions in cracks at the base of the subcratonic lithosphere from a C-H-O fluid that reacted with surrounding silicate at about 1,300–1,400 °C. In a mantle fluid consisting of CH4>H2O>H2 near fO2=IW, the H2 activity increases rapidly when carbon from the fluid is consumed by diamond precipitation, driving the oxygen fugacity of the system to lower values along the diamond saturation curve. Water from the fluid induces melting of surrounding silicate material, and hydrogen reduces metals in the silicate melt, reflected by an unusually low Ni content of the garnet. The carbon isotopic composition of δ13C=−13.69‰ (PDB) and the lack of nitrogen as an impurity is consistent with formation of the diamond from non-biogenic methane, whereas δ18O=7.4‰ (SMOW) of the garnet implies derivation of the silicate from subduction-related material. Hence, very localized and transient reducing conditions within the subcratonic lithosphere can be created by this process and do not necessarily call for involvement of fluids derived from subducted material of biogenic origin.

explain the isotopic variations of Group 1 kimberlites, but the We present major and trace element and Sr–Nd–Pb–Hf–Os isotopic available data do not allow us to rule out a major role for convecting data for the 76–58 Ma Western Cape melilitite province, an agemantle components in kimberlite genesis. Using the Western Cape progressive magmatic lineation in which primitive olivine melilitite melilitites as an analog, we propose a model in which the strong intrusives and alkali basalt lavas have been emplaced on the HIMU isotopic signature present in most Late Cretaceous to Recent southwestern margin of South Africa. The magmas range from alkaline magmatic provinces across Africa is supplied not by alkali basalts with strong HIMU isotopic and trace element affinities conventional mantle plumes, but rather by pods of recycled oceanic on the continental shelf to melilitites with kimberlite-like incompatible crust brought up from the deep mantle within a long-lived, and element compositions and EM 1 isotopic affinities on thick Protlaterally broad mantle upwelling feature located beneath the African erozoic lithosphere (i.e. Sr/Sri = 0·7029–0·7043, Nd(t) = plate. +5 to +2, Pb/Pbi = 20·5–18·1). The samples are characterized by radiogenic Os/Osi (0·15–0·21) and unradiogenic Hf isotope ratios, causing some of them to fall outside the compositional range of oceanic basalts. This suggests that they

The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m2 continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle.

Eclogite xenoliths from Orapa can be accurately classified as Group I or Group II on the basis of Na2O in garnet and K2O in clinopyroxene. Group I xenoliths are commonly diamondiferous while Group II xenoliths are diamond-free. Both xenolith varieties may contain graphite. Isotopic character is to some degree correlated with major- and trace-element chemistry. Group II samples with Ca-poor garnet have clinopyroxenes with radiogenic 87Sr86Sr (0.705–0.709) and the least radiogenic 143Nd144Nd (0.5122–0.5125). Group I eclogites with higher Ca and Fe in garnets have less radiogenic 87Sr86Sr (0.702–0.7066) and bulk-Earth or higher 143Nd144Nd ratios. Group I eclogites have more radiogenic 206Pb204Pb (18.6–19) than Group II xenoliths (16.5–18.6). In contrast, Group II xenoliths have more variable and, in some cases, more radiogenic 206Pb204Pb (36.6–39.3) than Group I xenoliths (38.3–38.4). The Sr, Sm, Nd and Pb concentrations of minerals in Orapa Group I eclogite xenoliths are much lower than in Group II samples. All the Group II xenoliths are inferred to be enriched in light rare-earth elements while Group I xenoliths are probably characterised in many cases by light rare-earth element depletion. Constituent garnet and clinopyroxene in both Group I and II eclogite xenoliths are essentially in isotopic equilibrium at the time of pipe emplacement. Mineral as well as calculated whole-rock 143Nd144Nd compositions of most of the Group I eclogites are too close to bulk-Earth and depleted-mantle estimates in order to obtain useful model age information. Depleted-mantle model ages derived from the much lower 143Nd144Nd compositions of the Group II eclogite xenoliths range from 661 to 1248 My, with an average clinopyroxene model age of 908 My and an average whole-rock model age of 1016 My. On the basis of an observed covariation of O and Sr isotopic compositions the entire Orapa Group I eclogite xenolith suite can be modelled as mixtures of oceanic basalt with or without a few percent of ocean floor sediment. The Group II xenoliths might have crystallised from a melt which derives from a protolith with time-averaged LREE depletion. Their radiogenic Sr isotope character could be due to interaction of the melt with metasomatised lithosphere, or might be a superimposed metasomatic signature.

One diamond-bearing and eight graphite-bearing eclogite xenoliths are described from the Bellsbank kimberlites, Cape Province, South Africa. Graphite mostly occurs as discrete grains which are commonly in the form of tabular prisms. Diamond is octahedral. Both Group I and Group II eclogite varieties are represented by the graphite-bearing specimens, while the single diamond-bearing eclogite is of the Group I variety. The carbon isotopic composition of the graphite varies from δ13C=−7‰ to δ13C=−2.8‰. This is within the range of carbon isotopic compositions for inclusion-free diamonds in kimberlite from this locality, suggesting that the carbon for the eclogites as well as some of the kimberlite diamonds are derived from the same source. The present day Nd isotopic compositions of clinopyroxene from three graphite-bearing xenoliths are slightly higher than the bulk earth estimate. Sr isotopic compositions of the clinopyroxene in these xenoliths vary from 87Sr/86Sr=0.703 to 87Sr/86Sr=0.706. This could be due to derivation of the xenoliths from a protolith with variable 87Sr/86Sr isotopic composition or could be the result of mixing between a low-Sr, high 87Sr/86Sr component and a high Sr, low 87Sr/86Sr component.

Concentrations of platinum-group elements (PGE) and Au have been determined in thirty-four samples of kimberlitic and alkaline igneous rocks (melilitite, carbonatite, and alkali basalt) from Brazil and southern Africa. The intrusions were grouped according to their lithophile element isotopic signatures (Group I, Group II, and transitional) and also their location (cratonic or off-craton) with respect to the Brazilian Sao Francisco and African Kaapvaal cratons. In contrast to the lithophile elements, the PGE signatures of Kaapvaal cratonic Group I and Group II kimberlites are not strongly fractionated relative to mantle and largely overlap, with Group II showing only a slight enrichment in Pd and Au relative to Group I. However, Kaapvaal cratonic and off-craton kimberlites show sharply contrasting PGE signatures; off-craton kimberlites have lower concentrations of Os, Ir, and Ru, lower Os/Ir ratios, and are more strongly fractionated relative to mantle. A similar distinction was observed between kimberlites and alkaline rocks flanking the margin of the Sao Francisco craton, where the high concentrations of Os, Ir, and Ru in the kimberlites make the PGE signature appear less fractionated than the alkaline rocks. Cratonic kimberlites have more primitive PGE signatures and higher concentrations of Os, Ir, and Ru than other mantle-derived melts (MORB, OIB, komatiites). The relatively high PGE concentrations and primitive signatures and the generally accepted view that kimberlites are low degree partial melts are difficult to reconcile with existing models of PGE fractionation during mantle melting. Given the spatial association between the primitive PGE signature and Archean cratonic lithosphere, we suggest that it might reflect a PGE component(s) which is/are inherited from the cratonic lithospheric mantle, either from the source region through melting or by post-melting assimilation. Suitable candidates might include: PGE-rich phases exposed in former oceanic podiform chromitites which have been tectonothermally converted to spinel-poor lithospheric peridotites; or fragments of deep mantle, PGE-rich, siderophile heterogeneities which were incorporated into the earliest cratonic lithosphere during extensive plume-driven melting or plate-tectonic-like stacking and underthrusting of oceanic lithosphere. Either process of cratonic development could allow the accumulation of an Os, Ir, and Ru-rich PGE component at depth in the lithospheric mantle. The absence of this component in off-craton kimberlites and shallow alkaline melts is suggested to reflect its concentration at depth in the earliest lithospheric keels.

Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has mineral compositions typical of low-T coarse textured garned peridotites, whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low-CaO content of garnet. Geothermobarometric calculations show both xenoliths equilibrated at temperatures above 1,100°C and pressures>55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different δ13C compositions between xenolith diamonds (-2.8 to-4.6‰) and diamonds in the kimberlite (generally

International audience; Mineralogical studies of deep-seated xenoliths and mineral inclusions in diamonds indicate that there is significant variability in oxygen fugacity within the Earth's upper mantle. This variability is consistent both with the occurrence of reduced (methane-bearing) or oxidized (CO2/carbonate-bearing) fluids. Invariably, direct sampling of reduced deep fluids is not possible as they are unquenchable and re-equilibrate with either the surrounding mantle or are affected by degassing. Key information about the nature of such fluids might be found in diamond if it were possible to study a population related to a single source. Usually, diamonds within a kimberlite pipe have different parageneses and can be shown to have formed at different times and depths. We studied 59 diamonds extracted from a single diamondiferous peridotite xenolith (with a volume of only 27 cm3), from the Cullinan mine (formerly called the Premier mine) in South Africa. Diamond sizes range from 0.0005 to 0.169 carats (0.1 to 33.8 mg). A correlation between the nitrogen contents of the diamonds (range 40 to 1430 ppm) and their nitrogen aggregation state (varying from 10 to 85% of IaB defects) is compatible with a single growth event. δ13C-values range from − 4.2‰ to − 0.1‰, with slight internal variability measured in the largest diamonds. Nitrogen isotope measurements show δ15N ranging from − 1.2‰ to + 7.2‰. On the centimeter scale of this upper mantle rock, the variations for nitrogen content, nitrogen aggregation state, carbon and nitrogen isotopic compositions, respectively, cover 64%, 75%, 15% and 23% of the ranges known for peridotitic diamonds. In spite of such large ranges, N-content, δ13C and δ15N within this diamond population are distinctly coupled. These relationships do not support a mixing of carbon sources, but are best explained by a Rayleigh distillation within the sub-continental mantle at depths > 150 km and T > 1200 °C, which precipitates diamonds from methane-bearing fluid(s). The involvement of this reduced metasomatic agent also suggests that the heterogeneous redox state of Archean cratons may mostly result from the heterogeneous nature of percolating fluids. The striking variability of the four determined parameters at cm scale may also account for the difficulty in interpreting these parameters in larger productions, such as those from a mine, because in these cases, the diamonds are mixed and sub-populations cannot be disentangled.

Polycrystalline diamonds (framesites) from the Venetia kimberlite in South Africa contain silicate minerals whose isotopic and trace element characteristics document remobilization of older carbon and silicate components to form the framesites shortly before kimberlite eruption. Chemical variations within the garnets correlate with carbon isotopes in the diamonds, indicating contemporaneous formation. Trace element, radiogenic, and stable isotope variations can be explained by the interaction of eclogites with a carbonatitic melt, derived by remobilization of material that had been stored for a considerable time in the lithosphere. These results indicate more recent formation of diamonds from older materials within the cratonic lithosphere.

The carbon isotope composition (δ13C) of primary graphite from 28 mantle derived eclogite xenoliths from kimberlites in southern Africa and USA range from ‐14.31 to ‐2.84%PDB, with a mean value of ‐6.2%. Both Group I and Group II eclogites are represented by anomalously low carbon isotope ratios. There is no simple correlation between isotopic composition and depth of origin, demonstrating carbon isotopic heterogeneity in the shallow upper mantle not previously recognized, but known from the deeper eclogite diamond population. Interpreted in the light of recent geochemical evidence supporting a subduction origin for some mantle eclogites and their diamonds, low δ13C graphite‐bearing eclogites could represent some of the earliest material emplaced beneath the cratons by subduction.