Eclogitic Diamonds Research Articles

The Decarbonization of deeply subducted oceanic crusts and genesis of super-deep eclogitic diamonds: Progresses and challenges

The Decarbonization of deeply subducted oceanic crusts and genesis of super-deep eclogitic diamonds: Progresses and challenges

Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

Abstract Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between −6 (Kasai, Congo and Udachnaya, Siberia) and −0.1 (Udachnaya, Siberia) log units (relative to the fayalite–magnetite–quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/∑Fe values range between 0.03 and 0.19 for garnet and 0.18–0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/∑Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of ± 0.02 and ± 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0–5.2 (± 0.4) GPa and 720–1050 (± 60) °C for both high-MgO and low-MgO eclogites, slightly lower than previous P–T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, ∆logfo2 (FMQ) calculated using the available oxythermobarometric model varies from −1.7 to −0.6 log units for high-MgO eclogites and from −2.9 to 0.9 log units for low-MgO eclogites. Samples recording ∆logfo2 (FMQ) ≤ −1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of −1.2 (± 0.4) log units for high-MgO and −0.6 (± 1.1) log units for low-MgO eclogites suggest different redox conditions of basaltic protoliths during subduction worldwide. Previous geochemical studies on the same rock samples reported evidence of cryptic metasomatism in both garnet and clinopyroxene that we demonstrate being not recorded by their major elements, while modal metasomatism evidenced by the presence of phlogopite as a product of interaction with a kimberlitic melt only affects the MgO of the bulk rock. Therefore, we suggest that high Fe3+/∑Fe ratios for garnet (> 0.10) and for reconstructed bulk rocks in the case of both low-MgO and high-MgO samples cannot be due to metasomatic interaction with an oxidized fluid, but rather are the consequence of Fe3+ redistribution in an unusually oxidized mafic protolith upon metamorphism. Our results highlight the redox variability of eclogites of Archaean age at conditions more oxidized than present-day mid-ocean ridge basalts (MORBs) and imply an oxidizing nature of the convective mantle source where magma was formed with consequent speciation of C in the form of carbonate fluid explaining, therefore, the lack of eclogitic diamonds in V. Grib kimberlite pipe.

Reconstructions of mantle structure beneath the Anabar Shield kimberlites – Similarities and differences

Comparisons of mantle xenocrysts from Lower Triassic kimberlites in the Anabar Shield (Ary-Mastakh, Dyuken, Kuranakh and Orto-Yargyn fields) have shown essential differences from the xenocrysts in the Olenek River Basin (Chomurdakh field). Xenoliths in the Anabar Shield and its northern fields are very rare; they include mainly garnet dunites and harzburgites, and less commonly, pyroxenites and eclogites.PTXFO2 diagram reconstructions for the Boomerang pipe in the Ary-Mastakh field located in the suture zone of the Daldyn and Magan terranes have used monomineral thermobarometry to show that the Opx in rare lherzolitic pyropes formed between 6 and 7.5 GPa. Eclogites represent the mantle heated to the middle pyroxenite layer, and in these terranes the formation of Cr-less pyroxenites are linked to plume melt interactions with the eclogites. In the Dyuken, Kuranakh and Orto-Yargyn fields, the garnet advective trend starts from 7.5 GPa, while the asthenosphere – lithosphere boundary is found at 6 GPa, suggesting that the middle pyroxenite layer was heated and metasomatized. The lower and particularly mid-mantle parts of these fields also contain abundant eclogites. In the Chomur field, lherzolitic and pyroxenitic pyropes form from 7 GPa, while the captured materials mainly represent the upper mantle (4 GPa). All pipes show a similar mantle layering, consisting of seven parts and determined by the clustering of PT estimates for garnets, Cr-spinels, and pyroxenes. In the Boomerang pipe, the Cr-clinopyroxenes and pyropes show REE spectra with varying fan-shaped slopes, (La/Yb)n of 10–100 for pyropes, and HREE for garnets. Spider diagrams reveal peaks for Th, and troughs for U, Nb, Ta, and Pb. Eclogitic garnets and omphacites show minimum values of Eu and Nb, Ta, Zr, and Hf. REEs in ilmenites show a joint increase in LREE and HFSE for Mg to Fe-rich varieties with the degree of differentiation. Most depleted pyropes from Kuranakh have V-U shaped REE patterns, Ba and U peaks. The HFSE minima represents dunites from the arc and back-arc mantle, and the fertilization produces an increase in incompatible elements and sometimes large ion lithophile element (LILE) levels for lherzolitic pyropes. The pyroxenitic garnets display happed REE and the Cpx shows varying LILE and HFSE correlating with the (La/Yb)n.The Chomur pipe contains predominantly harzburgite-lherzolite garnets with minima Ba and Sr, in addition to various incompatible elements. Cpx shows similar variations with mainly depleted HFSE patterns. The marginal parts of the subcratonic lithospheric mantle (SCLM) of the Anabar Shield are extremely enriched in eclogitic deep-seated material. This is especially seen in the lower SCLM parts, demonstrating thermobarometric trends and features similar to the diamond inclusions from the Ebelyakh (Mayat) placers. The mantle column beneath several pipes (Los’, Universitetskaya, Kuranakh) contain Cr amphiboles distributed from the base of the lithosphere to the Moho. A comparison of the reconstructed SCLM beneath the Anabar and Chomurdakh fields reveal more fertile compositions of the mantle rocks and simpler structure for later field. The lower mantle lithospheric abundance and the PTX trends of the Kuranakh eclogites and the Orto-Yargyn kimberlites are similar to the eclogitic diamond inclusions from the Ebelyakh placers. This increases the possibility of finding diamonds deposits in the Anabar Shield kimberlites.

Carbonate-rich crust subduction drives the deep carbon and chlorine cycles.

The flux balances of carbon and chlorine between subduction into the deep mantle and volcanic emissions into the atmosphere are crucial for the habitability of our planet1,2. However, pervasive loss of fluids from subducting slabs has been thought to cut off the delivery of both carbon and chlorine to the deep mantle owing to their high mobility under hydrous conditions3,4. Our new high-pressure experiments show that most carbonates (>75 wt%) in carbonate-rich crustal rocks-one of the main subducting carbon reservoirs-survive devolatilization and hydrous melting in cold and warm subduction zones, indicating that their subduction has driven the deep carbon cycle since the Mesoproterozoic. We found that KCl and NaCl, respectively, become stable phases crystallizing from hydrous carbonatite melts with low chlorine solubility in warm and hot subduction zones, resulting in the sequestration of chlorine in the solid residue in downwelling slabs. Accordingly, the subduction of carbonate-rich rocks facilitated highly effective recycling of both chlorine and carbon into the deep mantle at intermediate stages of Earth's history and led to declining atmospheric pCO2 and the formation of carbon-rich and chlorine-rich mantle reservoirs since the Mesoproterozoic. This period of optimal carbon and chlorine subduction may explain the ages of eclogitic diamonds and the formation of the HIMU mantle source.

Experimental Modeling of Decarbonation Reactions, Resulting in the Formation of CO2 Fluid and Garnets of Model Carbonated Eclogites under Lithospheric Mantle P,T-Parameters

First experimental modeling of decarbonation reactions resulting in the formation of CO2-fluid and Mg, Fe, Ca, and Mn garnets, with composition corresponding to the garnets of carbonated eclogites of types I and II (ECI and ECII), was carried out at a wide range of lithospheric mantle pressures and temperatures. Experimental studies were performed on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS), in (Mg, Fe, Ca, Mn)CO3-Al2O3-SiO2 systems (with compositional variations according to those in ECI and ECII), in the pressure interval of 3.0–7.5 GPa and temperatures of 1050–1450 °C (t = 10–60 h). A specially designed high-pressure cell with a hematite buffering container—preventing the diffusion of hydrogen into the platinum capsule—was used, in order to control the fluid composition. Using the mass spectrometry method, it was proven that in all experiments, the fluid composition was pure CO2. The resulting ECI garnet compositions were Prp48Alm35Grs15Sps02–Prp44Alm40Grs14Sps02, and compositions of the ECII garnet were Prp57Alm34Grs08Sps01–Prp68Alm23Grs08Sps01. We established that the composition of the synthesized garnets corresponds strongly to natural garnets of carbonated eclogites of types I and II, as well as to garnets from xenoliths of diamondiferous eclogites from the Robert Victor kimberlite pipe; according to the Raman characteristics, the best match was found with garnets from inclusions in diamonds of eclogitic paragenesis. In this study, we demonstrated that the lower temperature boundary of the stability of natural garnets from carbonated eclogites in the presence of a CO2 fluid is 1000 (±20) °C at depths of ~90 km, 1150–1250 (±20) °C at 190 km, and 1400 (±20) °C at depths of about 225 km. The results make a significant contribution to the reconstruction of the fluid regime and processes of CO2/carbonate-related mantle metasomatism in the lithospheric mantle.

Caught in the act: Diamond growth and destruction in the continental lithosphere

Abstract Diamonds can form within eclogite (remnants of ancient subducted oceanic crust) in the subcontinental lithospheric mantle (SCLM), pieces of which may then be transported to the surface as xenoliths in kimberlites. These diamond-bearing xenoliths offer a rare opportunity to study diamonds in their host lithology. The morphology and surface features of diamonds record evidence of attack by diamond-aggressive melts/fluids, which results in resorption, rounding, and destruction of octahedral growth forms. The location and timing of such diamond-destruction processes remain controversial, and the kimberlite magma itself is often considered to be the primary culprit. We used X-ray computed tomography scanning to present a view of diamond morphology and distribution within 24 diamondiferous eclogites in unprecedented detail. These scans clearly capture diamond growth and destruction that occurred within the SCLM, prior to kimberlite entrainment. We show that euhedral diamonds in these eclogites are predominantly step-faced octahedra. This morphology is preserved even when the diamonds are exposed at the surface of the eclogites, indicating that kimberliteinduced resorption was not significant. Six eclogites contain only rounded diamonds with no distinct crystal faces, and their exposed surfaces on the exterior of the xenoliths are highly irregular, indicating diamond-destruction in the SCLM. In three cases, single xenoliths host both resorbed diamonds and step-faced octahedra, indicating multiple metasomatic events, some of which were diamond-aggressive and others diamond-friendly. These diamondiferous xenoliths provide snapshots of diamond growth and destruction in the SCLM, caught in the act.

Nature of slab-mantle interactions recorded by coupled δ13C–δ15N–δ18O signatures and elemental compositions of Koidu diamonds and their inclusions

Eclogitic diamond formation can be associated with interactions between subducted slabs and ambient mantle. To gain a better understanding of the extent of chemical exchange between slabs and ambient mantle and the mechanisms of coeval diamond formation, we examined 16 diamonds with eclogitic garnet inclusions from the Koidu kimberlite complex in Sierra Leone (West African Craton). We analyzed the 16 garnet inclusions for major element, trace element and oxygen isotope (δ18O) compositions and their host diamonds for carbon (δ13C) and nitrogen (δ15N) isotope compositions.The garnet inclusions have δ18O values ranging from +5.4 to +12.1‰ (median = +11.3‰) and all but two have δ18O values ≥+9.9‰. Such high δ18O values indicate a link to protoliths that had undergone extensive low-temperature alteration by seawater, which occurs in the uppermost basaltic layer of oceanic crust, prior to subduction. Diamonds hosting the high δ18O garnets have a crustal δ13C signature (−29.6 to −19.4‰) paired with δ15N values (−5.6 to +1.3‰, with one outlier at +9.9‰) implying typically only minor 15N enrichment. This apparent decoupling of δ18O and δ13C from δ15N indicates diamond formation in an eclogitic substrate where garnet oxygen isotope compositions are inherited from altered low-pressure protoliths, diamond carbon is principally derived from biogenic carbonate and organic matter, and diamond nitrogen is variably added by an external, mantle-derived fluid that prompted diamond formation. Of the two remaining garnet inclusions, one has mantle-like δ18O (+5.4‰) and a positive Eu anomaly, suggesting derivation from gabbroic protoliths originally located deep in the oceanic crust where significant alteration by seawater did not occur. Based on mantle-like δ13C (−4.7‰) and δ15N (−6.9‰), the associated diamond formed from mantle-derived fluids/melts. Relative to the other Koidu garnet inclusions, the final garnet has a small majorite component (formation pressure ∼8 GPa), high Mg# (79.0), elevated Cr# (0.90), and low δ18O (+6.3‰), suggesting encapsulation during infiltration of slab-derived melts into surrounding asthenospheric peridotite, associated with a high degree of chemical exchange between slab- and mantle-derived components.

Diamond formation beneath the Coromandel area, southwestern São Francisco Craton – The role of re-fertilization and subduction

The lithospheric mantle underpinning the Alto Paranaíba Igneous Province , southwestern São Francisco Craton , was investigated through a study of 82 diamonds and their mineral inclusions from the Coromandel (Douradinho River), Verde River (northern Coromandel), Abaeté, Frutal, and Romaria deposits. Mineral inclusion abundances show that lherzolite is the main diamond source-rock, followed by eclogite , harzburgite and minor websterite. The limited chemical depletion recorded in lherzolitic inclusions suggests a post-Archean origin or modification of the lithospheric mantle beneath this part of the São Francisco craton. Sinusoidal rare earth element patterns for lherzolitic garnet inclusions indicate variable but overall low degrees of metasomatism by fluids or silico‑carbonatitic (proto-kimberlitic) melts. These results contrast with previous studies on mineral inclusions in diamonds (Canastra range, southern Coromandel; Rio da Prata system, northeastern Coromandel; our new data on the Frutal area, southwestern Coromandel), where a dominance of harzburgitic inclusions documents a more depleted, likely Archean, cratonic root. Bulk rock compositions reconstructed from eclogitic and websteritic inclusions establish a clear link between subduction processes and diamond formation in the Coromandel region. Calculated bulk rare earth element patterns match typical sections of upper oceanic crust , where plagioclase accumulation is uncommon. Pyroxene inclusion-based geothermobarometry indicates that lherzolitic and websteritic diamonds formed and last resided along a ∼ 39 mW/m 2 model geotherm. Projection of eclogitic garnet-clinopyroxene pairs onto this 39 mW/m 2 geotherm places their origin near the base of the lithosphere (> 180 km depth). Low nitrogen contents and high aggregation states (up to 100 %B) indicate that eclogitic, websteritic, and part of the lherzolitic diamond populations experienced extended (2.0 Byr) mantle residence at high temperatures (> 1200 °C). The similarity in mantle residence temperatures and the shared presence of minor negative Eu anomalies for eclogitic and websteritic diamonds and their inclusions may be indicative of a common origin, following emplacement of subducted oceanic crust beneath the São Francisco Craton margin likely during a Paleoproterozoic orogeny (2.2–1.9 Ga). Based on our results, future exploration efforts in the Alto Paranaíba region should shift their focus from harzburgitic to lherzolitic and eclogitic indicator minerals to find new diamond deposits and kimberlites in this area. • Lherzolite and eclogite main diamond substrates in Alto Paranaíba Igneous Province. • Moderate olivine Mg# and rare G10 garnets indicate mild SCLM depletion. • Lherzolitic substrates affected by fluid or silico‑carbonatitic melt metasomatism. • Eclogitic and websteritic diamonds derived from base of lithosphere. • Eclogitic diamonds linked to Paleoproterozoic (2.2–1.9 Ga) subduction.

Eclogite Varieties and Their Positions in the Cratonic Mantle Lithosphere beneath Siberian Craton and Archean Cratons Worldwide

The pressure-temperature (PT) conditions and position of different groups of eclogites in the sub-cratonic lithospheric mantle (SCLM) worldwide were established using clinopyroxene Jd-Di and garnet thermobarometry. Beneath Siberia, Fe-eclogites found within the 3.0–4.0 GPa formed in Early Archean times. In the Middle and Late Archean, eclogites were melted during and after subduction. High-Mg eclogites (partial melts or arc cumulates) are related to low-T (LT) geotherms. Melt-metasomatized eclogites trace a high-temperature (HT) geotherm. Eclogitic diamond inclusions from Siberia mostly belong to the middle SCLM (MSCLM) part. Ca-rich eclogites from Precambrian Indian kimberlites are located in the MSCLM. In Phanerozoic time, they were located in the lithosphere base. In Proterozoic South Africa, Ca-rich eclogites and grospydites occur within 4.0–5.0 GPa and HT eclogite and diamond inclusions from the Premier pipe trace a HT geotherm at depths of 7.0–4.0 GPa, showing an increase in Fe upwards in the mantle section. Similar trends are common for eclogites worldwide. In the Wyoming craton, kimberlites captured eclogite xenoliths from the 4.0–2.5 GPa interval. Mantle eclogites have clinopyroxenes and garnet trace element patterns with high (La/Yb)n determined by KDs with melts and are magmatic. Flatter and bell-like REE patterns with Eu anomalies, HFSE troughs, and U and Pb peaks, are common for clinopyroxenes from MORB-type “basaltic” eclogites. High-Mg eclogites show less fractionated incompatible element branch in patterns. LILE-enrichments and HFSE troughs are typical for kyanite-bearing eclogites. Clinopyroxenes from diamond-bearing eclogites show lower REE, troughs in Nb and Zr, and peaks in Pb and U concentrations, compared to barren eclogites with round smooth trace element patterns and small depressions in Pb and Ba.

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism, thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (∼45%) and low-Cr megacrystic (∼39%) chemistries, with a significant proportion of eclogitic garnets (∼11%), and only subordinate contribution from harzburgitic garnets (∼5%) with variable Cr2O3 contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ∼1150–1250 °C) indicate their derivation from at least ∼220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ∼45 mW/m2 or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ∼50–65 kbar diamond window of the Olenek province by the same time.

Evidence for oxygen-conserving diamond formation in redox-buffered subducted oceanic crust sampled as eclogite

Cratonic eclogite is the product of oceanic crust subduction into the subcontinental lithospheric mantle, and it also is a fertile diamond source rock. In contrast to matrix minerals in magma-borne xenoliths, inclusions in diamond are shielded from external fluids, retaining more pristine information on the state of the eclogite source at the time of encapsulation. Vanadium is a multi-valent element and a widely used elemental redox proxy. Here, we show that that xenolithic garnet has lower average V abundances than garnet inclusions. This partly reflects crystal-chemical controls, whereby higher average temperatures recorded by inclusions, accompanied by enhanced Na2O and TiO2 partitioning into garnet, facilitate V incorporation at the expense of clinopyroxene. Unexpectedly, although diamond formation is strongly linked to metasomatism and xenoliths remained open systems, V concentrations are similar for bulk eclogites reconstructed from inclusions and from xenoliths. This suggests an oxygen-conserving mechanism for eclogitic diamond formation, and implies that eclogite is an efficient system to buffer fO2 over aeons of lithospheric mantle modification by subduction-derived and other fluids.

Formation of mixed paragenesis diamonds during multistage growth – Constraints from in situ δ13C–δ15N–[N] analyses of Koidu diamonds

Inclusion-bearing diamonds from the Koidu kimberlite complex, Sierra Leone (West African Craton) were analyzed in situ for carbon and nitrogen isotope compositions, nitrogen concentrations and nitrogen aggregation states. In a suite of 105 diamonds, 78% contain eclogitic mineral inclusions, 17% contain peridotitic mineral inclusions, and 5% − an unusually high proportion − contain co-occurring eclogitic and peridotitic mineral inclusions indicating a mixed paragenesis. Major and trace element compositions of mineral inclusions from two mixed paragenesis diamonds (one with omphacite + Mg-chromite, the other with eclogitic garnet + forsteritic olivine) were determined. The presence of positive Eu anomalies in centrally located omphacite and eclogitic garnet inclusions indicates derivation from subducted protoliths, formed as igneous cumulates in lower oceanic crust. Mg-chromite (Cr# 85.5; Mg# 65.2) and olivine (Mg# 94.5) inclusions, located in outer portions of the mixed paragenesis diamonds, have compositions indicative of derivation from strongly depleted cratonic peridotites. Given that the olivine Mg# of 94.5 is the highest reported to date for the West African Craton, the eclogitic and peridotitic inclusions in these mixed paragenesis diamonds cannot have precipitated during infiltration of peridotitic substrates by eclogite-derived fluids, as the consequent fluid-rock interaction should lead to Mg# lower than that for the original peridotitic diamond substrate. The different origins of eclogitic and peridotitic inclusions could be explained by physical transport of their host diamonds from eclogitic into peridotitic substrates, possibly along high-strain shear zones, before renewed diamond growth.Based on the δ13C–δ15N systematics of the entire inclusion-bearing diamond suite from Koidu, three major compositional clusters are identified. Cluster 1 (eclogitic diamond cores; δ13C = –33.2 to −14.4‰ and δ15N = −5.3 to +10.1‰) bears the isotopic signature of recycled crustal material (± a mantle component). Cluster 2 (peridotitic diamonds and including the core of a diamond containing omphacite + Mg-chromite; δ13C = −6.0 to −1.1‰ and δ15N = −4.2 to +9.7‰) likely involves mixing of carbon and nitrogen from subducted and mantle sources. Cluster 3 (rims of eclogitic diamonds and including the eclogitic garnet + olivine included diamond and the rim of the omphacite + Mg-chromite included diamond; δ13C = −7.8 to −3.6‰ and δ15N = −7.9 to −2.1‰) matches convecting mantle-derived fluids/melts. The distinct isotopic signatures of the three diamond clusters, together with differences in nitrogen aggregation and cathodoluminescence response between diamond cores and rims, suggest episodic diamond growth during multiple fluid/melt pulses.

Evidence of Subduction of the Paleoproterozoic Oceanic Crust in the Khapchan Belt of the Anabar Shield, Siberian Craton

Granulite complex on the left side of the Bol. Kuonamka River, below the mouth of the Khapchan River, is composed of melanocratic and mesocratic orthopyroxene–clinopyroxene crystalline schists and leucocratic orthopyroxene and orthopyroxene–clinopyroxene plagiogneisses. The granulites formed after mafic to felsic volcanoplutonic rocks with rare sedimentary interlayers. In terms of chemical composition, the mafic and ultramafic rocks correspond to pyroxenites, gabbronorites, and gabbro-diorites of the tholeiite series. The plagioschists and plagiogneisses were formed after rocks similar to diorites, tonalities, and trondhjemites or their volcanic analogs, and are distinguished by increased contents of Ba, Zr, Y, REE, Nb, and Ta. The concordant U-Pb age of zircon from the diorite protolith of orthopyroxene–clinopyroxene plagioschists is 2095 ± 10 Ma. The Lu-Hf isotopic composition of the zircon (eHf (T) = 6.5–12, THf(DM) = 1.98–2.22 Ga) indicates the correspondence of its source to the Paleoproterozoic depleted mantle. Two-pyroxene plagiogneisses with a concordant U-Pb zircon age of 2030 ± 17 Ma were formed after tonalites of the calc-alkaline series. They are characterized by well-pronounced negative Ti, Nb, Ta, P anomalies characteristic of subduction magmatism. All studied rocks of the Khapchan Belt have positive eNd(T) values from +2.3 to +4.2 and are interpreted as a juvenile suprasubduction complex. It is assumed that the subduction of oceanic crust of this age is associated with the formation of eclogitic diamond, which was noted earlier and widespread in placer deposits in the northeastern Siberian platform.

Redox state determination of eclogite xenoliths from Udachnaya kimberlite pipe (Siberian craton), with some implications for the graphite/diamond formation

The formation of diamonds within eclogitic rocks has been widely linked to the fate of carbon during subduction and, therefore, referred to conditions of pressure, temperature, and oxygen fugacity (fo2). Mantle-derived eclogite xenoliths from Udachnaya kimberlite pipes represent a unique window to investigate the formation of carbon-free, graphite–diamond-bearing and diamond-bearing rocks from the Siberian craton. With this aim, we exploited oxy-thermobarometers to retrieve information on the P–T–fo2 at which mantle eclogites from the Siberian craton equilibrated along with elemental carbon. The chemical analyses of coupled garnet and omphacitic clinopyroxene were integrated with data on their iron oxidation state, determined both by conventional and synchrotron 57Fe Mossbauer spectroscopy. The calculated fo2s largely vary for each suite of eclogite samples from 0.10 to − 2.43 log units (ΔFMQ) for C-free eclogites, from − 0.01 to − 2.91 (ΔFMQ) for graphite–diamond-bearing eclogites, and from − 2.08 to − 3.58 log units (ΔFMQ) for diamond-bearing eclogites. All eclogite samples mostly fall in the fo2 range typical of diamond coexisting with CO2-rich water-bearing melts and gaseous fluids, with diamondiferous eclogites being more reduced at fo2 conditions where circulating fluids can include some methane. When uncertainties on the calculated fo2 are taken into account, all samples essentially fall within the stability field of diamonds coexisting with CO2-bearing melts. Therefore, our results provide evidence of the potential role of CO2-bearing melts as growth medium on the formation of coexisting diamond and graphite in mantle eclogites during subduction of the oceanic crust.

Sm-Nd isochron ages coupled with C-N isotope data of eclogitic diamonds from Jwaneng, Botswana

Constraining the formation age of individual diamonds from incorporated mineral inclusions and assessing the host diamonds’ geochemical characteristics allows determination of the complex history of diamond growth in the sub-continental lithospheric mantle (SCLM). It also provides the rare opportunity to study the evolution of the deep cycling of volatiles over time. To achieve these aims, Sm-Nd isotope systematics are presented for 36 eclogitic garnet and clinopyroxene inclusions from 16 diamonds from the Jwaneng mine, Botswana. The inclusions and host diamonds comprise at least two compositional suites that record different ‘mechanisms’ of diamond formation and define two isochrons, one Paleoproterozoic (1.8 Ga) and one Neoproterozoic (0.85 Ga). There are indications of at least three additional diamond-forming events whose ages currently cannot be well constrained. The Paleoproterozoic diamond suite formed by large-scale (>100′s km), volatile-rich metasomatism related to formation and re-working of the Proto-Kalahari Craton. In contrast, the heterogeneous composition of the Neoproterozoic diamond suite indicates diamond formation on a small-scale, through local (<10 km) equilibration of compositionally variable diamond-forming fluids in different eclogitic substrates during the progressive breakup of the Rodinia supercontinent. The results demonstrate that regional events appear to reflect the input of volatiles (i.e., carbon-bearing) derived from the asthenospheric mantle, whereas local diamond-forming events mainly promote the redistribution of volatiles within the SCLM. The occurrence of isotopically light carbon analysed in distinct growth zones from samples of this study (δ13C < −21.1‰) provides further indication of a recycled origin for surface-derived carbon in some diamonds from Jwaneng. Determining Earth’s long-term deep carbon cycle using diamonds, however, requires an understanding of the nature and scale of specific diamond-forming events.

Diamondiferous and barren eclogites and pyroxenites from the western Kaapvaal craton record subduction processes and mantle metasomatism, respectively

Mineral major and trace elements combined with Sr isotopes of clinopyroxene are used to unravel the origins and evolution of mantle eclogite and pyroxenite xenoliths from the Doornkloof-Sover orangeite dike (western Kaapvaal craton), and to investigate the generation and destruction of diamond in these rocks. Two different eclogite types are present: (1) MgO-poor eclogites (MgO = 7.3 to 14.5 wt%; n = 43) with accessory diamond ± corundum and kyanite, high garnet grossular content (median Ca# = 0.25) and clinopyroxene jadeite content (0.39). Reconstructed bulk rocks are LREE-depleted (median La 0.29 ppm) and have low median Cr2O3 (0.06 wt%) and incompatible trace-element contents (e.g. Sr, Zr, Ba, Pb, Th), and high Li and transition metal abundances. Some are characterised by stepped REE patterns or steep slopes in the MREE, similar to eclogites affected by interaction with dehydration fluids generated in subduction zones. These fluids may also have deposited diamond in typically reducing eclogite assemblages at diamond-stable pressures. (2) MgO-rich eclogites and pyroxenites (MgO = 14.0 to 20.0 wt%; n = 29), which are barren and enriched in LREE (median La 1.39 ppm), Cr2O3 (0.25 wt%) and incompatible trace elements, with lower Li and transition metal abundances than the MgO-poor group. These are typical signatures of carbonated ultramafic melt metasomatism in the mantle lithosphere. Strontium isotopic compositions vary widely in both groups, but high Cr2O3 and Ba contents are dominantly associated with 87Sr/86Sr > 0.7055. This reflects interaction with metasomatic agents remobilised from ancient lithospheric metasomes, which eventually gave rise to regional orangeite magmatism. The presence of strong positive Eu anomalies in both groups, including two pyroxenites, requires low-pressure igneous protoliths, presumably derived from a ca. 3 Ga spreading ridge, as reported for other eclogite materials from the western Kaapvaal craton.Based on the proportions of MgO-poor and –rich eclogites and pyroxenites, approximately 40% of the diamond inventory were destroyed by mantle metasomatism centred at ~135 ± 15 km depth, overlapping a low-velocity anomaly (mid-lithospheric discontinuity). Two diamondiferous orangeites ≤20 km from Doornkloof-Sover contain significantly different eclogite xenolith populations: At Newlands, MgO-poor diamondiferous eclogites are present in addition to barren MgO-rich ones and pyroxenites, suggesting that the host orangeite sampled a source region equally affected by diamond-destructive mantle metasomatism, whereas at Bellsbank, all eclogites are MgO-poor and LREE-depleted. This may explain higher diamond grades reported for this locality compared to Newlands or Doornkloof-Sover.

Metasomatic Evolution of Coesite-Bearing Diamondiferous Eclogite from the Udachnaya Kimberlite

A coesite-bearing diamondiferous eclogite from the Udachnaya kimberlite (Daldyn field, Siberian craton) has been studied to trace its complex evolution recorded in rock-forming and minor mineral constituents. The eclogite sample is composed of rock-forming omphacite (60 vol%), garnet (35 vol%) and quartz/coesite (5 vol%) and contains intergranular euhedral zoned olivine crystals, up to 200 µm long, coexisting with phlogopite, orthopyroxene, clinopyroxene (secondary), K-feldspar, plagioclase, spinel, sodalite and djerfisherite. Garnet grains are zoned, with a relatively homogeneous core and a more magnesian overgrowth rim. The rim zones further differ from the core in having higher Zr/Y (6 times that in the cores), ascribed to interaction with, or precipitation from, a kimberlite-related melt. Judging by pressure-temperature estimates (~1200 °C; 6.2 GPa), the xenolith originated at depths of ~180–200 km at the base of the continental lithosphere. The spatial coexistence of olivine, orthopyroxene and coesite/quartz with K-Na-Cl minerals in the xenolith indicates that eclogite reacted with a deep-seated kimberlite melt. However, Fe-rich olivine, orthopyroxene and low-pressure minerals (sodalite and djerfisherite) likely result from metasomatic reaction at shallower depths during transport of the eclogite by the erupting kimberlite melt. Our results demonstrate that a mixed eclogitic-peridotitic paragenesis, reported previously from inclusions in diamond, can form by interaction of eclogite and a kimberlite-related melt.

Genesis of Diamondiferous Rocks from Upper-Mantle Xenoliths in Kimberlite

The conditions of genesis of diamondiferous ultrabasic and basic rocks from xenoliths in kimberlite were studied by combining the data from analytical investigations of their mineral phases and experimental results of the study of melting relations in the diamond-forming mineral systems of the upper mantle. The compositions of minerals in some samples of metasomatized diamondiferous eclogite associated with diamond-free eclogite from kimberlite of the Udachnaya pipe (Yakutia) were studied for the first time. The new results obtained in addition to the literature data were applied for generalization of estimates of genetically important characteristics of the chemical compositions of garnets, Ca-clinopyroxenes, and omphacites from diamond-bearing peridotite, pyroxenite, and eclogite. As a result, it was found that quite “fresh” minerals of diamondiferous rocks have typomorphic differences from the same minerals of diamond-free upper-mantle rocks. At the same time, it is significant that the compositions of minerals from diamondiferous rocks and paragenetic inclusions in diamonds are identical. These peculiarities of mineralogy of diamondiferous rocks are genetically significant; based on the mantle–carbonatite theory of the origin of diamond and associated mineral phases, this provides support for the same physicochemical origin of diamonds, minerals of diamondiferous rocks, and paragenetic inclusions in diamonds. Finally, the following genetic conclusions are made. (1) Completely miscible silicate (±oxide)–carbonate melts with dissolved carbon are the parental medium in petrogenesis of diamondiferous ultrabasic and basic rocks. (2) The physicochemically consistent formation of diamondiferous rocks and paragenetic inclusions of peridotitic and eclogitic minerals in diamonds occurred in the common diamond-forming chambers/reservoirs of parental melts; diamond-free peridotite, pyroxenite, and eclogite were the host mantle rocks for such chambers. (3) The origin of continuous series of diamondiferous peridotite–pyroxenite–eclogite rocks is controlled by the fractional ultrabasic–basic evolution of parental melts with exhaustion of olivine and orthorhombic pyroxene via the peritectic reactions. (4) Ascending flows of kimberlite magmas destroyed the parental chambers and captured diamonds with inclusions, individual minerals, their intergrowths, diamondiferous ultrabasic and basic rocks; at the entrance and exit from the chambers, they captured differentiated diamond-free host rocks of the mantle as well. (5) With further ascent from the mantle to the Earth’s crust, the material of diamond-forming chambers and diamond-free mantle was mixed in convecting kimberlite magma and was transported from the mantle to cumulative crustal chambers. (6) Kimberlite magmas were gradually solidified in stationary cumulative chambers with the release of highly compressed fluids; with an increase of pressure up to the critical values, they intruded into the rocks of the roof and ejected kimberlite with xenoliths of diamondiferous and mantle rocks to the surface with the formation of explosion pipes.

Deep carbon through time: Earth’s diamond record and its implications for carbon cycling and fluid speciation in the mantle

Diamonds are unrivalled in their ability to record the mantle carbon cycle and mantle fO2 over a vast portion of Earth’s history. Diamonds’ inertness and antiquity means their carbon isotopic characteristics directly reflect their growth environment within the mantle as far back as ∼3.5 Ga. This paper reports the results of a thorough secondary ion mass spectrometry (SIMS) carbon isotope and nitrogen concentration study, carried out on fragments of 144 diamond samples from various locations, from ∼3.5 to 1.4 Ga for P [peridotitic]-type diamonds and 3.0 to 1.0 Ga for E [eclogitic]-type diamonds. The majority of the studied samples were from diamonds used to establish formation ages and thus provide a direct connection between the carbon isotope values, nitrogen contents and the formation ages. In total, 908 carbon isotope and nitrogen concentration measurements were obtained. The total δ13C data range from −17.1 to −1.9 ‰ (P = −8.4 to −1.9 ‰; E = −17.1 to −2.1‰) and N contents range from 0 to 3073 at. ppm (P = 0 to 3073 at. ppm; E = 1 to 2661 at. ppm). In general, there is no systematic variation with time in the mantle carbon isotope record since > 3 Ga. The mode in δ13C of peridotitic diamonds has been at −5 (±2) ‰ since the earliest diamond growth ∼3.5 Ga, and this mode is also observed in the eclogitic diamond record since ∼3 Ga. The skewness of eclogitic diamonds’ δ13C distributions to more negative values, which the data establishes began around 3 Ga, is also consistent through time, with no global trends apparent.No isotopic and concentration trends were recorded within individual samples, indicating that, firstly, closed system fractionation trends are rare. This implies that diamonds typically grow in systems with high excess of carbon in the fluid (i.e. relative to the mass of the growing diamond). Any minerals included into diamond during the growth process are more likely to be isotopically reset at the time of diamond formation, meaning inclusion ages would be representative of the diamond growth event irrespective of whether they are syngenetic or protogenetic. Secondly, the lack of significant variation seen in the peridotitic diamonds studied is in keeping with modeling of Rayleigh isotopic fractionation in multicomponent systems (RIFMS) during isochemical diamond precipitation in harzburgitic mantle. The RIFMS model not only showed that in water-maximum fluids at constant depths along a geotherm, fractionation can only account for variations of <1‰, but also that the principal δ13C mode of −5 ± 1‰ in the global harzburgitic diamond record occurs if the variation in fO2 is only 0.4 log units. Due to the wide age distribution of P-type diamonds, this leads to the conclusion that the speciation and oxygen fugacity of diamond forming fluids has been relatively consistent. The deep mantle has therefore generated fluids with near constant carbon speciation for 3.5 Ga.

Composition of Reduced Mantle Fluids: Evidence from Modeling Experiments and Fluid Inclusions in Natural Diamond

Abstract —Experimental modeling in the C–O–H, C–O–H–N, and peridotite–C–O–H–N systems, combined with analyses of fluid inclusions in natural diamonds, is used to reconstruct the compositions of fluids that can be stable in the reduced mantle. Hydrocarbons (HCs) in the upper mantle can form either by reactions of carbonates with iron/wüstite and water or by direct hydrogenation of carbon phases (graphite, diamond, and amorphous carbon) interacting with reduced fluids. Carbon required for the formation of HCs can come from diamond, graphite, or carbonates. Mainly light alkanes are stable at the mantle pressures and temperatures in the C–O–H and C–O–H–N systems as well as in the peridotite–fluid system under ultrareduced to moderately reduced redox conditions at the oxygen fugacity from –2 to +2.5 lg units relative to the IW (Fe–FeO) buffer. Some oxygenated HCs can be stable in fluids equilibrated with carbonate-bearing peridotite. Ammonia and, to a lesser degree, methanimine (CH3N) are predominant nitrogen species in reduced fluids in the conditions of the subcratonic lithosphere or the Fe0-bearing mantle. The presence of HCs as common constituents of reduced mantle fluids is supported by data on inclusions from natural diamonds hosted by kimberlites of the Yakutian province and from placer diamonds of the northeastern Siberian craton and the Urals. Fluid inclusions have minor amounts of H2O, methane, and other light alkanes but relatively high concentrations of oxygenated hydrocarbons, while the H/(H + O) ratio varies from 0.74 to 0.93. Hydrocarbon-bearing fluids in some eclogitic diamonds have high CO2 concentrations. Also, the fluid inclusions have significant percentages of N2 and N-containing species, Cl-containing HCs, and S-containing compounds. Both the experimental results and the analyses of fluid inclusions in natural diamonds indicate that HCs are stable in the upper mantle conditions. The set of hydrocarbons, mainly light alkanes, might have formed in the mantle from inorganic substances. Further research should focus on the causes of the difference between experimental and natural fluids in the contents of methane, light alkanes, oxygenated hydrocarbons, and water and on the stability of N-, S-, and Cl-containing fluid components.