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Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples. Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds. Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes. Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget. [ABSTRACT FROM AUTHOR]

Water in Earth's upper mantle is a minor and yet critically important component that dictates mantle properties such as strength and melting behavior. Minerals with stoichiometric water, such as those of the humite group, are important yet poorly characterized potential reservoirs for volatiles in the upper mantle. Here, we report observation of hydroxyl members of the humite group as inclusions in mantle-derived diamond. Hydroxylchondrodite and hydroxylclinohumite were found coexisting with olivine, magnesiochromite, Mg-bearing calcite, dolomite, quartz, mica, and a djerfisherite-group mineral in a diamond from Brazil. The olivine is highly forsteritic (Mg# 97), with non-mantle-like oxygen isotope composition (18O +6.2%), and is associated with fluid inclusions and hydrous minerals--features that could be inherited from a serpentinite protolith. Our results constitute direct evidence for the presence of deserpentinized peridotitic protoliths in subcratonic mantle keels, placing important constraints on the stability of hydrous phases in the mantle and the origin of diamond-forming fluids. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds. [ABSTRACT FROM AUTHOR]

Heamanite-(Ce) (IMA 2020-001), ideally (K0.5Ce0.5)TiO3, is a new perovskite-group mineral found as an inclusion in a diamond from the Gahcho Kué mine in the Northwest Territories, Canada. It occurs as brown, translucent single crystals with an average maximum dimension of ~80 μm, associated with rutile and calcite. The luster is adamantine, and the fracture conchoidal. Heamanite-(Ce) is the K-analog of loparite-(Ce), ideally (NaCe)Ti2O6. The Mohs hardness is estimated to be 5½ by comparison to loparite-(Ce), and the calculated density is 4.73(1) g/cm3. Electron microprobe wavelength-dispersive spectrometric analysis (average of 34 points) yielded: CaO 10.70, K2O 7.38, Na2O 0.16, Ce2O3 13.77, La2O3 8.22, Pr2O3 0.84, Nd2O3 1.59, SrO 6.69, BaO 2.96, ThO2 0.36, PbO 0.15, TiO2 45.77, Cr2O3 0.32, Al2O3 0.10, Fe2O3 0.09, Nb2O5 0.87, UO3 0.01, total 99.98 wt%. The empirical formula, based on 3 O atoms, is: [(K0.268Na0.009)Σ0.277(Ce0.143La0.086Pr0.009Nd0.016)Σ0.254(Ca0.326Sr0.110Ba0.033Pb0.001)Σ0.470Th0.002]Σ1.003 (Ti0.979Nb0.011Cr0.007Al0.003Fe0.002)Σ1.002O3. The Goldschmidt tolerance factor for this formula is 1.003. Heamanite-(Ce) is cubic, space group Pm3m, with unit-cell parameter a = 3.9129(9) Å, and volume V = 59.91(4) Å3 (Z = 1). The crystal structure was solved using single-crystal X‑ray diffraction data and refined to R1(F) = 2.61%. Heamanite-(Ce) has the aristotypic perovskite structure and adopts the same structure as isolueshite and tausonite. The six strongest diffraction lines are [dobs in angstroms (I in percentages) (hkl)]: 2.764 (100) (110), 1.954 (41) (200), 1.596 (36) (211), 1.045 (16) (321), 1.236 (13) (310), and 1.382 (10) (220). The Raman spectrum of heamanite-(Ce) shows two broad bands at 560 and 787 cm−1, with no bands observed above 1000 cm−1. Heamanite-(Ce) is named after Larry Heaman, a renowned scientist in the field of radiometric dating applied to diamond-bearing kimberlites, mantle-derived eclogites, and lamprophyre dikes. The dominant REE should appear as a Levinson suffix, hence heamanite-(Ce). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Billions of years ago, pieces of subducted oceanic crust were trapped as eclogites in the mantle keels beneath continents. Comparing eclogitic minerals protected inside and evolving outside of ancient diamonds reveals no change in redox state. [ABSTRACT FROM AUTHOR]

Yellow diamonds from the CH-7 (Chidliak) and the Misery (Ekati Mine) kimberlites in northern Canada are characterised for their nitrogen characteristics, visible light absorption, internal growth textures, and carbon isotope compositions. The diamonds are generally nitrogen-rich, with median N contents of 1230 (CH-7) and 1030 at.ppm (Misery). Normally a rare feature in natural diamonds, single substitutional nitrogen (C centres) and related features are detected in infrared absorption spectra of 64% of the studied diamonds from CH-7 and 87% from Misery and are considered as the major factor responsible for their yellow colouration. Episodically grown diamonds, characterised by colourless cores containing some nitrogen in the fully aggregated form (B centres) and yellow outer layers containing C centres, occur at both localities. Carbon isotope compositions and N contents also are significantly different in such core and rim zones, documenting growth during at least two temporally distinct events and involving different diamond forming fluids. Based on their nitrogen characteristics, both the yellow diamonds and yellow rims must have crystallized in close temporal proximity (<<1 Ma) to kimberlite activity at CH-7 and Misery. [ABSTRACT FROM AUTHOR]

The presence of diamonds of lherzolitic, eclogitic and websteritic paragenesis in proximal alluvial deposits on the southwestern edge of the São Francisco Craton documents the incorporation of subducted oceanic crust and associated metasomatism through slab dehydration fluids affecting the local SCLM. To better constrain the subduction-association of diamond substrates and metasomatic events, we conducted a combined study of the δ13C-δ15N-[N] characteristics of 81 diamonds and the δ18O values of four of their eclogitic garnet inclusions. Diamond carbon isotope compositions range from −25.5 to +0.5‰, with 13C-depleted diamonds (≤ −7‰) being exclusively of eclogitic/websteritic paragenesis while the 13C-enriched (≥ −2‰) tail of the distribution is related to diamonds with lherzolitic inclusions. Nitrogen isotope values range from −14.2 to +25.5‰, with about half of the values being positive. A general absence of coherent trends in δ13C-δ15N-[N] across growth zones implies that diamond formation did not occur under fluid-limited conditions. Instead, the observed heterogeneity in carbon and nitrogen isotope compositions reflects contributions of distinct source reservoirs hosted in both altered oceanic crust and Earth's mantle. Nitrogen contents peak around a δ15N value of −3.5‰, indicating that more N-rich fluids, presumably representing a primitive endmember composition, have a mantle-like δ15N signature. While positive and negative δ15N values occur equally near the δ13C mantle value (−5 ± 2‰), 13C-depleted diamonds have nitrogen isotope compositions skewed towards positive values. 13C depletion and 15N enrichment is a signature of biogenic carbonates/organic matter and low-T clays in uppermost, basaltic sections of oceanic crust that experienced low-temperature seawater alteration prior to subduction. Correspondingly, the oxygen isotope compositions of eclogitic garnet inclusions fall in a restricted range between +5.5‰ to +7.0‰. For three of the four samples, the stable isotope signatures of inclusions and host diamonds display perfect agreement, with the intensity of seawater alteration signatures, in the form of garnet inclusion 18O enrichment and host diamond 13C depletion and 15N enrichment, increasing together. For the fourth sample, the δ18O signature of the garnet inclusion (+5.5‰) and δ13C-δ15N signatures of the diamond host (−25 and + 19‰, respectively) are decoupled. While the mantle-like δ18O signature indicates a diamond substrate derived from deeper levels in oceanic crust (e.g., deep sheeted dikes), the diamond-forming fluids must have originated from sources that originally resided near the sea water interface. A viable mode of mixing such disparate isotopic signatures is the interaction of diamond-forming fluids derived from shallow oceanic crust altered at low-temperatures with eclogitized substrates originally formed in deeper levels of oceanic crust. This process likely occurred during diamond formation in a tectonic subduction mélange, which juxtaposes deeper and shallower levels of oceanic crust. In the lithospheric mantle above the subcreted oceanic slab, the elevated carbon isotope and highly variable nitrogen isotope compositions of diamonds formed in lherzolitic substrates likely relate to devolatilization and/or melting of principally oceanic sediments containing marine carbonates and clays and subsequent mixing with mantle-derived volatile components. In combination, our diamond and inclusion stable isotope data provide insight into multiple processes that promote diamond formation both inside subducted slabs accreted to the São Francisco cratonic keel and in adjacent subcontinental lithospheric mantle. • Diamonds record both mantle and crustal isotopic signatures. • Eclogitic and websteritic diamonds are 13C-depleted, lherzolitic diamonds are 13C-enriched. • Mantle fluids are more nitrogen-rich than subduction fluids. • Garnet inclusions δ18O reflects original stratigraphic position within oceanic crust. • Diamond formation within subduction mélange. [ABSTRACT FROM AUTHOR]

Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ13C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ13C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ13C values) and carbonates (high δ13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H 2 O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ13C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH 4 -bearing fluids. Unlike mixed CH 4 -CO 2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking. [ABSTRACT FROM AUTHOR]

Three diamonds from Sao Luiz, Brazil carrying nano- and micro-inclusions of molecular δ-N2 that exsolved at the base of the transition zone were studied for their C and N isotopic composition and the concentration of N utilizing SIMS. The diamonds are individually uniform in their C isotopic composition and most spot analyses yield δ13C values of −3.2 ± 0.1‰ (ON-SLZ-390) and − 4.7 ± 0.1‰ (ON-SLZ-391 and 392). Only a few analyses deviate from these tight ranges and all fall within the main mantle range of −5 ± 3‰. Most of the N isotope analyses also have typical mantle δ15N values (−6.6 ± 0.4‰, −3.6 ± 0.5‰ and − 4.1 ± 0.6‰ for ON-SLZ-390, 391 and 392, respectively) and are associated with high N concentrations of 800-1250 atomic ppm. However, some N isotopic ratios, associated with low N concentrations (<400 ppm) and narrow zones with bright luminescence are distinctly above the average, reaching positive δ15N values. These sharp fluctuations cannot be attributed to fractionation. They may reflect arrival of new small pulses of melt or fluid that evolved under different conditions. Alternatively, they may result from fractionation between different growth directions, so that distinct δ15N values and N concentrations may form during diamond growth from a single melt/fluid. Other more continuous variations, in the core of ON-SLZ-390 or the rim of ON-SLZ-392 may be the result of Rayleigh fractionation or mixing. [ABSTRACT FROM AUTHOR]

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/m2 model geotherm. Projection of eclogitic garnet-clinopyroxene pairs onto this 39 mW/m2 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. [ABSTRACT FROM AUTHOR]

Mixed-habit diamonds have experienced periods of growth where they were bounded by two surface forms at the same time. Such diamonds are relatively rare and therefore under-investigated. Under certain physical and chemical conditions, smooth octahedral faces grow concurrently with rough, hummocky cuboid faces. However, the specific conditions that cause this type of growth are unknown. Here we present a large array of spectroscopic data in an attempt to investigate the impurity and carbon-isotope characteristics, as well as growth conditions, of 13 large (>6 mm diameter) plates cut from mixed-habit diamonds. The diamonds all generally have high nitrogen concentrations (>1400 ppm), with the octahedral sectors enriched by 127--143% compared to their contemporary cuboid sectors. Levels of nitrogen aggregation are generally low (2--23% IaB) with no significant difference between sectors. IR-active hydrogen features are predominantly found in the cuboid sectors with only very small bands in the octahedral sectors. Platelet characteristics are variable; only one sample shows a large B' band intensity in the octahedral sector, with no platelets occurring in the cuboid sector. Other samples either show a small B' band in both sectors, or just in the cuboid sector, or none at all. These data support a model that shows the concentration-adjusted aggregation rate of nitrogen to be the same in both sectors, whereas the subsequent platelet development is reduced in the cuboid sectors. This is because the interstitial carbon atoms have interacted with disk-crack-like defects only found in cuboid sectors, which in turn reduces their chances of aggregating to form platelets. These disk-crack-like defects are also thought to be the most likely site for the IR-active hydrogen features and they maybe intrinsic to cuboid growth in mixed-habit diamonds. When they are graphitized, as they are in all of the diamonds in this study, this may reflect a heating event prior to volcanic exhumation. Spectroscopic analysis of the green cathodoluminescence exhibited by all of the diamonds shows nickel centers to be present in only the cuboid sectors. Carbon isotope data, obtained by secondary ion mass spectrometry, show very little variation in seven of the diamonds. The total range of 217 analyses is -7.94 to -9.61 (±0.15)‰, and the largest variation in a single stone is 0.98‰. No fractionation in carbon isotopes is seen between octahedral and cuboid sectors at the same growth horizon. These data suggest that the source fluid chemistry, as well as pressure, temperature, and oxygen fugacity were very stable over time, allowing such large volumes of mixed-habit growth to occur. The high concentration of impurities, namely nitrogen and hydrogen, is probably the critical factor required to cause mixed-habit growth. The impurity and isotopic data fall in line with previous modeling based on diamond growth from reduced carbonates with the loss of a 13C-enriched CO2 component. [ABSTRACT FROM AUTHOR]

Abstract: In the central Slave craton, eclogitic diamonds are suggested to have formed during Paleoproterozoic subduction in a meta-gabbroic substrate representing former lower oceanic crust that interacted with serpentinite-derived fluids at high fluid–rock ratios. In order to assess the behaviour of chalcophile and siderophile elements (CSE) during this process, we measured trace-element concentrations of sulphide inclusions in diamonds from the Diavik mine by laser ablation ICPMS. The nitrogen systematics of the diamonds (average N concentration of ∼600ppm and aggregation state 4% N as B-aggregates) indicate average mantle residence temperatures of ∼1050°C for a 1.85Ga formation age, corresponding ∼170km depth. Based on the available evidence from natural samples and experiments, we suggest that the highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pd, Pt, Re) except Au behaved largely conservatively during fluid-induced metamorphism, which may point to a reducing and Cl-poor nature of the fluid. The abundances of the moderately siderophile and chalcophile elements Cr, Co, Ni, Cu, Ag, Sn, Mo and W may also have changed little, whereas As, Sb, Tl, Pb and Bi may have been mobilised from the subducting lower oceanic crust. The partitioning of CSE in eclogite and geochemical behaviour during oceanic crust formation was assessed for inferred conservative elements. Assuming an average sulphide mode of 0.3wt.% for the oceanic crust, its abundances of HSE, Cu, Mo, Se and Te can mostly be accounted for by sulphide minerals alone. Lithophile behaviour of W, Cd, In and Sn and enrichment in residual melts may explain their lower abundances in the gabbroic eclogitic sulphide inclusions compared to MORB sulphide. These elements, as well as Cr, Co, Ni, Zn and Ga require additional host phases both in eclogite, where rutile partitions significant amounts of Cr, Zn, W, Ga and Sn, and in the oceanic crust. [Copyright &y& Elsevier]

Abstract: Mineral inclusions in diamonds from the Renard kimberlites, on the eastern Superior Craton, document an almost exclusively peridotitic inclusion suite with an unusually high abundance of SiO2 phases (coesite and presumably retrograde quartz). Whilst coesite is often associated with eclogitic diamond sources, there is an almost complete absence of unequivocally eclogitic mineral inclusions at Renard. Consequently, the presence of abundant SiO2 inclusion phases is likely caused by influx of CO2 within localised regions of the lithospheric mantle with very high fluid/rock ratios. This caused a progression of carbonation reactions within the host peridotite, likely along veins, locally consuming all olivine, orthopyroxene, and clinopyroxene present and producing free SiO2. Nitrogen contents and aggregation states of the diamonds indicate normal mantle residence temperatures, predominantly between 1100 and 1200°C. Co-variations of carbon isotopic composition and nitrogen content suggest formation from both reducing (CH4) and oxidising (CO3 2− or CO2 bearing) melts/fluids. The observation of single diamonds with non-isothermal co-variations of nitrogen content and aggregation state, or containing disequilibrium inclusion parageneses (e.g., variations in olivine Mg# up to 0.5 within single diamonds), indicates that Renard diamonds grew during a number of precipitation events within a physically and chemically changing environment. [Copyright &y& Elsevier]

Abstract: A suite of 23 diamondiferous microxenoliths from the Ekati kimberlites in the central Slave craton was studied to elucidate the tectonothermal evolution of the lithosphere during craton formation and modification along craton margins, and associated diamond formation. Major- and trace-element abundances of eclogites (n=16) reveal the presence of two compositionally distinct suites: a group of three eclogites with typical gabbroic trace-element patterns (flat REEN and low ΣREE, positive SrN and PbN, negative ZrN anomalies) and a group of 13 eclogites that are characterised by lower Mg#, Cr, Sc and Sr combined with higher Al2O3, TiO2 and Na2O, and a conspicuously stepped REE pattern with lower LREE/HREE. Inclusions in diamonds from the central Slave craton consistently plot with the latter group. The unusual trace-element characteristics of the second group are strikingly similar to those produced along fluid conduits in younger subduction zones, where infiltration of a metasomatic fluid induced eclogitisation. Eclogitic diamonds contain an average of 540at.ppm nitrogen that is poorly aggregated (12% in the B centre), similar to eclogitic diamonds recovered from the A154 kimberlite (Diavik Mine, ~30km southeast) that were dated to 1.85Ga and linked to subduction at the western craton margin. This suggests that (1) by 1.85Ga subduction-related reactive fluid transport and element mobilisation similar to younger examples were established, and (2) this fluid flow not only catalysed eclogitisation of subducting oceanic crust, but also was a prerequisite for the majority of eclogitic diamond formation beneath the central Slave craton. Harzburgitic and lherzolitic diamondiferous microxenoliths (n=4) may document diamond formation in the Slave craton prior to emplacement of eclogites. Peridotitic garnets have sinusoidal REE-patterns with strong depletions in Ti, consistent with carbonate-melt or fluid metasomatism that may have also prompted diamond growth. Their temperatures of last equilibration (~1020 to 1300°C), combined with diamond N characteristics are consistent with precipitation precontemporaneously to ~3.3Ga formation of the deep Slave lithosphere from carbonatitic melts or fluids derived from below the lithosphere–asthenosphere boundary. [Copyright &y& Elsevier]

Peridotitic sulphide inclusions in diamonds from the central Slave craton constrain the age and origin of their subcontinental lithospheric mantle (SCLM) sources. These sulphides align with either a ca. 3.5 Ga (shallow SCLM) or a ca. 3.3 Ga isochron (deep SCLM) on a Re-Os ischron diagram, with variably enriched initial Os/Os. Since some Archaean to recent plume-derived melts carry a subducted crust (eclogite) signature and some cratonic SCLM may have been generated in plumes by extraction of komatiitic liquids, we explain these data by subduction of evolved lithospheric material (shallow SCLM) and melting in a hybrid mantle plume that contains domains of recycled eclogite (deep SCLM), respectively. In upwelling hybrid mantle, eclogite-derived melts react with olivine in surrounding peridotites to form aluminous orthopyroxene, convert peridotite to pyroxenite and confer their crustal isotope signatures. We suggest that it is subsequent to orthopyroxene enrichment of peridotite in an upwelling plume that partial melting of this Al- and Si- enriched source generated komatiites and complementary ultradepleted cratonic mantle residues. Although subduction is needed to explain some cratonic features, melting of a hybrid plume source satisfies several key observations: (1) suprachondritic initial Os/Os in subsets of lithospheric mantle samples and in some coeval Archaean komatiites; (2) variable enrichment of cratonic mantle by high-temperature aluminous orthopyroxene; (3) high Mg# combined with high orthopyroxene content in cratonic mantle due to higher melt productivity of an Al- and Si-richer source; (4) variable orthopyroxene enrichment possibly linked to varying mantle potential temperatures (Tp), plume buoyancy and resultant eclogite load and/or variable availability of subducted material in the source; and (5) absence of younger analogues due to a secular decrease in Tp. Most importantly, this model also alleviates a mass balance problem, because it predicts a hybrid mantle source with variably higher SiO and AlO than primitive mantle, and, contrary to a primitive mantle source, is able to reconcile compositions of komatiites and complementary cratonic mantle residues. [ABSTRACT FROM AUTHOR]

Abstract: The diamondiferous Carolina kimberlite (Rondônia State, Brazil) is located within Proterozoic basement rocks (1.8 to 1.2 Ga) of the Amazon Craton. This “unconventional” post-Archean setting is consistent with a lack of harzburgitic (G10) garnets in heavy media concentrate from the kimberlite. Diamonds from Carolina have high nitrogen contents and in part highly negative carbon isotopic values suggesting derivation predominantly from eclogitic portions of the underlying lithospheric mantle. This is consistent with the abundance and chemistry of eclogitic garnet xenocrysts, which make up 13% of the garnets analysed: just over half of the eclogitic garnets classify as Group I (>0.07 wt.% Na2O), which is considered to be an indication of good diamond potential. Based on nitrogen contents and aggregation states, the majority of the Carolina diamonds indicate time averaged residence temperatures between 1100 and 1150 °C (at 1.5 Ga mantle residence). Platelet degradation was noted in the majority of diamonds, suggesting that their mantle source was affected by a transient heating event. Geothermobarometry on clinopyroxene grains derived from both surficial samples and kimberlite core indicates two distinct model geotherms: a hot “Somerset Island type” geotherm (44 mW/m2), and a colder “Slave type” geotherm (38 mW/m2). Grains from the kimberlite drill core exclusively reflect the lower model geotherm, whereas clinopyroxenes from surficial samples depict both gradients. Given the Triassic age (230 Ma, Rb–Sr model age on phlogopite) of the Carolina kimberlite, it is speculated that a younger generation of Cretaceous–Tertiary kimberlites in the Pimenta Bueno area may represent the source of “hotter” mantle xenocrysts seen in surficial samples. The implied change in geotherm reflects a large scale, possibly plume related, heating episode occurring between the two kimberlite events (i.e. between the Jurassic and Cretaceous) that may relate to the opening of the South Atlantic. [Copyright &y& Elsevier]

Abstract: Sulphide inclusions from 35 eclogitic and 7 peridotitic diamonds from the Diavik kimberlites in the central Slave craton have been characterized to address questions of diamond age and craton formation. Eclogitic sulphide inclusions occur in diamonds with mantle-like δ 13C (−4.94±0.72 1σ) and low N aggregation states (%N as B=8.2±10.0, average N contents of 720 ppm) indicative of relatively low mantle residence temperatures. A 1.86±0.19 Ga Re–Os age array for eclogitic sulphides with suprachondritic initial 187Os/188Os of 0.13 (±0.10) indicates a close temporal link between eclogitic diamond formation, eclogite emplacement and collisional events affecting the Slave craton. Sulphides in peridotitic diamonds plot on older, previously established 3.3 and 3.5 Ga isochrons, consistent with higher average N aggregation states (~20%) despite lower N contents (~230 ppm) for their host diamonds compared to eclogitic diamonds. Two intriguing observations emerge from a comparison of diamond populations and formation ages between the Slave and Kaapvaal that indicate fundamentally different and common diamond formation mechanisms, respectively: (1) Despite the general abundance of peridotitic silicate inclusions, peridotitic sulphide inclusions are rare in the Kaapvaal and occur in relatively young diamonds whereas in the central Slave there is a sizable Archaean population. (2) Compared to the distribution of silicate inclusions in diamonds, both cratons have an overabundance of eclogitic relative to peridotitic sulphide inclusions. During Kaapvaal lithospheric mantle formation, large melting intervals, as gauged by extremely depleted silicate inclusions in diamonds, led to exhaustion of sulphide in the residue. Formation of peridotitic sulphide inclusion-bearing diamonds occurred only significantly later, after re-sulphidation accompanying metasomatism. By contrast, the less depleted deep lithospheric mantle beneath the central Slave craton may have formed during plume subcretion, leading to smaller melting intervals due to the presence of a pre-existing lithospheric mantle lid, thereby allowing for coeval precipitation of sulphide inclusion-bearing diamonds. Abundant eclogitic sulphide inclusion-bearing diamonds that can be related to accretionary processes along the edges of the Slave and Kaapvaal craton indicate that sulphide-saturated eclogite is a fertile source for diamond formation. Reduced fluids from dehydration of underlying seawater-altered peridotite may react with the overlying oceanic crust to precipitate eclogitic sulphide-bearing diamonds penecontemporaneously with metamorphism and tectonic emplacement of eclogite into the subcratonic lithosphere. [Copyright &y& Elsevier]

Abstract: The carbon isotopic composition, nitrogen content and aggregation state of 95 inclusion bearing diamonds, collected from the production of Damtshaa kimberlites, were investigated. The Damtshaa kimberlites belong to the group of southern African kimberlites showing 12C enrichment in eclogitic compared to peridotitic diamonds. The results were compared with similar studies of the nearby Orapa and Letlhakane kimberlites, providing the opportunity to examine variations in mantle composition on the kilometer scale. While Orapa, Letlhakane and Damtshaa diamonds may share some overall similarities in their δ13C distribution, there are significant differences among them. These differences support the concept that diamonds from different kimberlite pipes have distinctive features in their δ13C distribution and that this is true for kimberlite occurrences which are spatially separated by only a few tens of km. Compared to Orapa, peridotitic garnet inclusions from Damtshaa and Letlhakane are derived, in part, from highly depleted dunitic–harzburgitic reservoirs extending to greater depth. In all three kimberlites diamonds with eclogitic garnet inclusions displaying comparatively low Al2O3/Cr2O3 ratios, characteristic for mantle compositions, show a wide range in carbon isotopic composition from about −4 to −18 ‰ vs. PDB. The observed relationship between δ13C and Al2O3/Cr2O3 ratios indicates the presence of 13C depleted mantle sources that cannot be explained by subduction. Estimates of possible pressure/temperature conditions, based on the Fe/Mg distribution coefficients of coexisting eclogitic garnets and clinopyroxenes and the Ca mole fraction in garnets, lead to the conclusion that beneath the Orapa, Letlhakane, and Damtshaa kimberlites there is a mantle zone depleted in 13C, approximately 20–30 km thick and extending over an area of at least 200 km2. [Copyright &y& Elsevier]

Abstract: The carbon isotopic composition (δ 13C) of diamonds containing peridotitic, eclogitic, websteritic and ultra-deep inclusions is re-evaluated on a detailed level. Applying a binning interval of 0.25‰, the previously recognized mode of peridotitic and eclogitic diamonds at about −5‰ is shown to reflect at least two subpopulations with abundance peaks at ∼−5.75 to −4.75‰ and ∼−4.50 to −3.50‰. Within the peridotitic suite, diamonds with lherzolitic inclusions overall show higher δ 13C values. Evolution away from a δ 13C value of ∼−5‰, towards both 13C depleted and enriched compositions, is accompanied by decreasing maximum nitrogen contents of peridotitic diamonds. In combination with data on diamonds synthesized under reducing (metal melts) and more oxidizing conditions (carbonate–silicate interactions), this is taken to indicate that nitrogen is a compatible element in diamond that becomes depleted in the growth medium during progressive diamond precipitation. The observed co-variations of nitrogen content and δ 13C around −5‰ can then be modelled as reflecting closed system Rayleigh fractionation during crystallization of diamond from fluids/melts that are both reducing (i.e. methane bearing; evolution from ∼−5 to −10‰) and oxidizing (i.e. CO3 2− bearing; evolution from starting points varying between ∼−9 to −5‰ and extending to about 0‰). Lherzolitic diamonds are believed to be mainly derived from diamond forming events subsequent to precipitation of predominantly Mesoarchean harzburgitic diamonds. The shift of lherzolitic diamonds towards higher δ 13C values thus may relate to a temporal evolution, with carbonate bearing fluids with an initial isotopic composition ranging between about −5.5 and −1.5‰, derived from subducting oceanic crust, becoming increasingly important subsequent to the Mesoarchean. Devolatilization of marine carbonates (δ 13C ∼0‰) drives their isotopic composition towards mantle like values and may explain the fairly large range in δ 13C for the assumed initial fluids/melts derived from them. The carbon isotopic composition of eclogitic diamonds appears to be dominated by open system fraction processes involving CO2 escape. Closed system Rayleigh fractionation processes are likely to operate as well, as indicated by the similarity of the bimodal δ 13C frequency distribution near −5‰ for eclogitic and peridotitic diamonds. The absence of a prominent mode at ∼−5‰ among eclogitic diamonds from the Amazon and Kimberley (Australia) cratons, as well as the strongly negative carbon isotopic composition of eclogitic diamonds containing majorite garnet inclusions at Jagersfontein (for which the pressure of formation is far too high to consider CO2 escape) may suggest that at least in some instances subducted organic matter is re-precipitated in the form of eclogitic diamonds. [Copyright &y& Elsevier]

Sulfide inclusions in diamonds from the 90-Ma Jagersfontein kimberlite, intruded into the southern margin of the Kaapvaal Craton, were analyzed for their Re–Os isotope systematics to constrain the ages and petrogenesis of their host diamonds. The latter have δ13C ranging between −3.5 and −9.8‰ and nitrogen aggregation states (from pure Type IaA up to 51% total N as B centers) corresponding to time/temperature history deep within the subcontinental lithospheric mantle. Most sulfides are Ni-poor ([Ni + Co]/Fe = 0.05–0.25 for 15 of 17 inclusions), have elevated Cu/[Fe + Ni + Co] ratios (0.02–0.36) and elemental Re–Os ratios between 0.5 and 46 (12 of 14 inclusions) typical of eclogitic to more pyroxenitic mantle sources. Re–Os isotope systematics indicate two generations of diamonds: (1) those on a 1.7 Ga age array with initial 187Os/188Os (187Os/188Osi) of 0.46 ± 0.07 and (2) those on a 1.1 Ga array with 187Os/188Osi of 0.30 ± 0.11. The radiogenic initial Os isotopic composition for both generations of diamond suggests that components with high time-integrated Re–Os are involved, potentially by remobilization of ancient subducted oceanic crust and hybridization of peridotite. A single sulfide with higher Os and Ni content but significantly lower 187Os/188Os hosted in a diamond with less aggregated N may represent part of a late generation of peridotitic diamonds. The paucity of peridotitic sulfide inclusions in diamonds from Jagersfontein and other kimberlites from the Kaapvaal craton contrasts with an overall high relative abundance of diamonds with peridotitic silicate inclusions. This may relate to extreme depletion and sulfur exhaustion during formation of the Kaapvaal cratonic root, with the consequence that in peridotites, sulfide-included diamonds could only form during later re-introduction of sulfur. [ABSTRACT FROM AUTHOR]

Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the diamondiferous microxenoliths is lherzolitic (G9) in paragenesis, in stark contrast to a predominantly harzburgitic (G10) inclusion paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic paragenesis, compared to the harzburgitic paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however, Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo95) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to observations for inclusion garnets. Sinusoidal REEN patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE) fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite appears to have had no destructive effect on diamond content. [ABSTRACT FROM AUTHOR]

Abstract: Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ 13C distribution with peaks at −5‰ and −17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ 13C<−10‰) indicates a predominantly eclogitic paragenesis. The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions. [Copyright &y& Elsevier]

With an age of ca. 2.7 Ga, greenschist facies volcaniclastic rocks and lamprophyre dikes in the Wawa area (Superior Craton) host the only diamonds emplaced in the Archean available for study today. Nitrogen aggregation in Wawa diamonds ranges from Type IaA to IaB, suggesting mantle residence times of tens to hundreds of millions of years. The carbon isotopic composition (δ13C) of cube diamonds is similar to the accepted mantle value (− 5.0‰). Octahedral diamonds show a slight shift (by + 1.5‰) to isotopically less negative values suggesting a subduction-derived, isotopically heavy component in the diamond-forming fluids. Syngenetic inclusions in Wawa diamonds are exclusively peridotitic and, similar to many diamond occurrences worldwide, are dominated by the harzburgitic paragenesis. Compositionally they provide a perfect match to inclusions from diamonds with isotopically dated Paleo- to Mesoarchean crystallization ages. Several high-Cr harzburgitic garnet inclusions contain a small majorite component suggesting crystallization at depth of up to 300 km. Combining diamond and inclusion data indicates that Wawa diamonds formed and resided in a very thick package of chemically depleted lithospheric mantle that predates stabilization of the Superior Craton. If late granite blooms are interpreted as final stages of cratonization then a similar disconnect between Paleo- to Mesoarchean diamondiferous mantle lithosphere and Neoarchean cratonization is also apparent in other areas (e.g., the Lac de Gras area of the Slave Craton) and may suggest that early continental nuclei formed and retained their own diamondiferous roots. [ABSTRACT FROM AUTHOR]

Sixty-eight alluvial diamonds from three placer deposits in Brazil (Arenapolis in the State of Mato Grosso, Boa Vista in the State of Roraima, and Canastra in the State of Minas Gerais) are characterized by similar crystal shapes, body colors, and surface textures, which are related to growth or resorption processes. The concentrations and the aggregation states of nitrogen impurities in the diamonds, as well as in their carbon isotope compositions, are also similar. A higher proportion of diamonds with radiation spots distinguishes the diamonds from Boa Vista from the other deposits. The majority of the diamonds from Arenapolis (∼70%) exhibit transport-related abrasion textures. In contrast, diamonds from Boa Vista and Canastra are characterized by the absence or a low abundance (<15%) of such abrasion textures, which indicates that the diamonds are derived from nearby kimberlitic sources. At this time, kimberlitic sources have been located only in the proximity of the Canastra placer deposits. The composition of the mineral inclusions is similar for diamonds from all three deposits. The diamonds formed in a strong to moderately depleted peridotitic mantle, with only minor involvement of eclogitic sources. Pressure mad temperature estimates for the diamonds from Boa Vista are similar to the estimates for diamonds from other deposits worldwide and are consistent with a geothermal gradient of 40 to 42 m W/m² surface heat flow. [ABSTRACT FROM AUTHOR]

The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions. [ABSTRACT FROM AUTHOR]

Inclusions of majoritic garnet in diamonds from the Jagersfontein kimberlite formed at unusually great depths of ∼250 to >500 km in the asthenosphere and transition zone. The original host rocks were derived from a much shallower, basaltic (eclogitic) source. The presence of negative Eu anomalies in all majoritic garnets requires a crustal origin, thereby linking these very deep diamond sources to subducting oceanic crust. The carbon isotope values (δ13C) of the host diamonds fall within a narrow range at ∼-20‰, which is fundamentally different from the broad range (-24‰ to -2‰) and bimodal distribution of carbon isotopes of Jagersfontein diamonds that formed in the shallower lithosphere. This indicates that majoritic garnet-bearing diamonds at Jagersfontein inherited their light carbon isotopic composition directly from organic matter contained in a subducting slab. These diamonds were likely formed by direct conversion from graphite, well within the diamond stability field. [ABSTRACT FROM AUTHOR]

The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from -20.3‰ to -0.5‰ (n=48) for peridotitic diamonds and from -38.5‰ to -1.6‰ (n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from -8.5‰ to -2.7‰ (n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from -10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. [Copyright &y& Elsevier]

About half the diamonds studied from the Cenozoic placer deposits along the Namibian coast belong to the peridotitic suite. The peridotitic mantle source is heterogeneous ranging from lherzolitic to strongly Ca depleted (down to 0.24 wt.% CaO in garnet) and shows large variations in Cr/Al ratio, illustrated by very low to very high Cr2O3 contents in garnet (2.6–17.3 wt.%). The Cr-rich end of this range includes exceptionally high Cr2O3 contents in Mg-chromite (70.7 wt.%) and clinopyroxene (3.6 wt.%). Garnet-olivine thermometry appears to indicate two groups, one that equilibrated at temperatures between 1200 and 1220°C and a second between 960 and 1100°C. Combined estimates of pressure and temperature based on garnet-orthopyroxene pairs indicate a large variance in geothermal gradients, corresponding to 38–42 mW/m2 surface heat flow.The trace-element composition of peridotitic garnet inclusions (determined by SIMS) also indicates large diversity. Two principal groups, corresponding to different styles of metasomatic source enrichment, are recognized. The first group ranges from extremely LREEN-depleted patterns, through trough-shaped REEN to sinusoidal patterns with the position of the first peak gradually moving from the LREEN to the MREEN. This series of REE patterns is interpreted to reflect a range of metasomatic agents with decreasing LREE/HREE. Only in the case of the two garnets with REEN peaking at Sm–Eu is this process connected with enrichment in Zr, without significant introduction of Y and Ti. The metasomatism responsible is interpreted as reflecting percolation of CHO-fluids through harzburgite under sub-solidus conditions. A second group of garnets shows an increase from LREEN–MREEN and almost flat (lherzolitic garnet) to moderately declining MREEN–HREEN at super-chondritic levels. This second style of metasomatism is caused by an agent carrying HFSE and showing only moderate enrichment in LREE over HREE, which points towards silicate melts. [Copyright &y& Elsevier]

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REEN, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREEN to MREEN, fairly flat MREEN–HREEN), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.Eclogitic garnet inclusions show “normal” REEN patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREEN–HREEN slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle. [Copyright &y& Elsevier]

A comparison of the diamond productions from Panda (Ekati Mine) and Snap Lake with those from southern Africa shows significant differences: diamonds from the Slave typically are un-resorbed octahedrals or macles, often with opaque coats, and yellow colours are very rare. Diamonds from the Kaapvaal are dominated by resorbed, dodecahedral shapes, coats are absent and yellow colours are common. The first two features suggest exposure to oxidizing fluids/melts during mantle storage and/or transport to the Earth''s surface, for the Kaapvaal diamond population.Comparing peridotitic inclusions in diamonds from the central and southern Slave (Panda, DO27 and Snap Lake kimberlites) and the Kaapvaal indicates that the diamondiferous mantle lithosphere beneath the Slave is chemically less depleted. Most notable are the almost complete absence of garnet inclusions derived from low-Ca harzburgites and a generally lower Mg-number of Slave inclusions.Geothermobarometric calculations suggest that Slave diamonds originally formed at very similar thermal conditions as observed beneath the Kaapvaal (geothermal gradients corresponding to 40–42 mW/m2 surface heat flow), but the diamond source regions subsequently cooled by about 100–150 °C to fall on a 37–38 mW/m2 (surface heat flow) conductive geotherm, as is evidenced from touching (re-equilibrated) inclusions in diamonds, and from xenocrysts and xenoliths. In the Kaapvaal, a similar thermal evolution has previously been recognized for diamonds from the De Beers Pool kimberlites. In part very low aggregation levels of nitrogen impurities in Slave diamonds imply that cooling occurred soon after diamond formation. This may relate elevated temperatures during diamond formation to short-lived magmatic perturbations.Generally high Cr-contents of pyrope garnets (inside and outside of diamonds) indicate that the mantle lithosphere beneath the Slave originally formed as a residue of melt extraction at relatively low pressures (within the stability field of spinelperidotites), possibly during the extraction of oceanic crust. After emplacement of this depleted, oceanic mantle lithosphere into the Slave lithosphere during a subduction event, secondary metasomatic enrichment occurred leading to strong re-enrichment of the deeper (>140 km) lithosphere. Because of the extent of this event and the occurrence of lower mantle diamonds, this may be related to an upwelling plume, but it may equally just reflect a long term evolution with lower mantle diamonds being transported upwards in the course of “normal” mantle convection. [Copyright &y& Elsevier]

A multiphase inclusion (KK-83) in a diamond from the Kankan deposits in Guinea, resulting from complex reactions between primary lower mantle phases, was examined in detail using analytical transmission electron microscopy. The inclusion consists of a large (300 μm) diopside crystal with a small symplectitic intergrowth in one corner, comprising olivine and tetragonal almandine pyrope phase (TAPP). The olivine part of the symplectite contains small exsolutions of Mg-Al-chromite and rare Ca-carbonate. A second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle. The clinopyroxene formed through a series of reactions at the expense of touching inclusions of CaSi- and MgSi-perovskite during convective ascent of the diamond through the transition zone. The proposed reaction sequence is: MgSiPvk+CaSiPvk→MgSiIlm+CaSiPvk→ringwoodite+stishovite+CaSiPvk→clinopyroxene+relict ringwoodite. The high Al-content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene. A slight deficiency of SiO2 during the diopside forming reaction leads to a small amount of relict ringwoodite (<0.1 vol% of the total inclusion is occupied by ringwoodite) coexisting with diopside. The ringwoodite captures the observed high amount of Al. Subsequent breakdown of ringwoodite to wadsleyite+TAPP produces the observed symplectitic intergrowth. During the phase transformation of wadsleyite to olivine, exsolution of Mg-Al-chromite occurs further decreasing the original solubility of Al, Cr and Ti, in accordance with experimental data on the element partitioning between wadsleyite and olivine. Based on these observations, TAPP can form as a retrograde phase within the transition zone of the Earth’s mantle and is not restricted to the upper part of the lower mantle. High Fe3+-contents may favour its formation. [Copyright &y& Elsevier]

Analyzes the trace-element concentrations in eclogitic and websteritic inclusions in diamonds from Venetia in South Africa. Use of ion microprobe; Similarities in the slopes of garnets from different parageneses; Occurrence of negative and positive europium anomalies.

Part I. Presents information on a study which examined the chemistry of normal lithospheric inclusion paragenesis and the diamond populations derived from the asthenosphere and the transition zone from the Kankan district in Guinea. Methodology; Discussion on the major elements of inclusions; Discussion on the trace elements of inclusions.

Presents information on rare and unusual mineral inclusions found in diamonds from Mwadui, Tanzania. Findings of studies conducted on syngenetic diamonds from Mwadui; Details on the composition of these diamonds; Reference to the presence of groundmass nepheline and analcime of kimberlites.

Presents a study on the trace element chemistry of silicate inclusions in diamonds from Akwatia, Ghana. Analytical methodology used to carry out the study; Information on mineral inclusion chemistry; Discussion on the study.

Presents information on diamonds and their syngenetic mineral inclusions from placer deposits along the Birim River, Ghana. What was indicated by inclusion contents; Difference between Akwatian diamond inclusions and others; Properties of these diamonds; Details on how these diamonds were studied.

• Superdeep DO-27 diamonds include a complete predicted lower mantle mineralogy. • Diamond formation occurred during metasomatism of subducted slab harzburgite. • We determined two new superdeep diamond formation ages, at 1679 ± 13 and 998 ± 18 Ma. • These ages constrain onset of cold deep subduction into the lower mantle to ≥∼1.7 Ga. Cold deep subduction – a hallmark of modern-style plate tectonics – is the dominant mechanism for recycling crustal materials into Earth's deep mantle, potentially changing its composition, heat budget and dynamics. The onset of cold deep subduction remains contentious and is largely constrained by crustal rocks, despite subduction being driven by mantle processes. Here we use the chemical and chronological information from sublithospheric mantle-derived minerals entrapped in diamonds from the DO-27 kimberlite, Slave Craton, Canada, to better constrain the timing and products of cold deep subduction reaching Earth's lower mantle. DO-27 sublithospheric diamonds preserve a complete lower mantle mineral assemblage: Ca-silicates (retrogressed CaSi-perovskite) ± enstatite (retrogressed bridgmanite) ± ferropericlase. The high Mg#s (molar Mg/(Mg+Fe)) of retrogressed bridgmanite (median Mg#=95) and ferropericlase (median Mg#=86) implicate a harzburgitic host rock for the diamonds. This strongly melt-depleted signature contrasts with the high abundance of Ca-silicate inclusions, some of which are variably enriched in incompatible elements and have enriched Sr-Nd-Pb isotopic signatures that collectively indicate the host rock was metasomatized by carbonatitic melts released from a relatively cold subducting slab in the lower mantle. The U-Pb systematics of the Ca-silicates define two ages of diamond crystallisation: 998 ± 18 Ma and 1679 ± 13 Ma. This is the first direct evidence of cold deep subduction in Earth's lower mantle at ∼1.7 Ga (Paleoproterozoic), in agreement with the oldest crustal low-temperature, high-pressure eclogites and is much earlier than previous Neoproterozoic estimates. [ABSTRACT FROM AUTHOR]

The article discusses the precipitation of diamonds from high-density carbon-hydrogen-oxygen (CHO) fluids. It mentions the emergence of a precise scenario on the chemical and mineralogical composition of diamond substrates. Also outlined are the appoaches in constraining the diamond-forming processes including the use of model calculations to yield the carbon content and speciation of the CHO fluids and the study on the presence of some fluid inclusions in diamonds like the hydrous saline.

Abstract: We report the geochemical and oxygen isotope compositions for eclogitic mineral inclusions in diamonds hosted by high-MgO eclogite xenoliths from the Jericho kimberlite, Canada. These data are used to constrain the nature and evolution of the eclogite protolith. The garnet and clinopyroxene diamond inclusions (DIs) are compositionally different than their host eclogite counterparts. In particular, garnet DIs have much lower Mg-numbers (54 vs. 82) and Cr2O3 contents (0.1 vs. 0.6wt.%) and higher CaO contents (7.6 vs. 4.3wt.%) than host eclogite garnet. DI and host eclogite clinopyroxenes are more similar but differences include lower Mg-numbers (78–81 vs. 93) and higher Na2O contents (2.3 vs. 1.8wt.%) in the DIs. The DIs lack typical shallow oceanic crust signatures such as strong positive Eu and Sr anomalies, and oxygen isotope compositions that deviate significantly from the pristine mantle average. On the contrary, both the Jericho DIs and host eclogite garnets have small negative Eu and Sr anomalies, fractionated HREE patterns ((LuN/GdN)~3–5) and pristine mantle-like δ18O values of 5.2–6.0‰, indicating that shallow, plagioclase-rich oceanic crust protoliths are unlikely. The eclogitic DI trace-element characteristics require that both garnet and plagioclase were present in the protolith, which likely crystallized in the shallow upper mantle. DI-based reconstructed whole-rock eclogite compositions have higher Mg-numbers and lower Al2O3 contents than found in typical basaltic or gabbroic oceanic crust, and are similar to pyroxenitic veins found in orogenic peridotite massifs. Due to the lack of clear oceanic crust signatures and the mantle-like δ18O values of the studied DIs, we propose that the Jericho diamond eclogites originally crystallized as pyroxenite cumulates that formed veins within the oceanic mantle lithosphere. Following partial melt extraction, the eclogite protoliths were subducted into the diamond stability field beneath the evolving Slave craton. Hence, the Jericho DIs and host high-MgO eclogites may represent an example of eclogite formation in an oceanic setting without the diagnostic ‘crustal signatures’ that are typically observed in cratonic eclogite xenolith suites worldwide. [Copyright &y& Elsevier]

Abstract: Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5–82ppm) and extremely low δ13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200ppm) and δ13C values from −3.5‰ to −5.3‰. Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction. SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ13C–N yielded a partition coefficient (K N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ13C values of diamond cores (−4‰) dictate the growth medium had higher δ13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates. [Copyright &y& Elsevier]

Abstract: Diamonds in South Australia occur in kimberlites at Eurelia (Orroroo), and in placer deposits, which include the Springfield Basin and the historic Echunga goldfield. To identify the kimberlitic and mantle sources of the placer diamonds, and to determine any possible connections between the placer diamonds and the diamonds from the Eurelia kimberlites, we examined the physical and compositional characteristics, and the mineral inclusion content of 122 diamonds from the Springfield Basin and 43 diamonds from kimberlites at Eurelia. Additional morphological data for three Echunga diamonds are also given. Most of the diamonds from the Springfield Basin are similar to the diamonds from Eurelia with respect to their crystal shapes, surface textures, and colors. The diamond populations from both areas are characterized by a high abundance of low-nitrogen (<100 ppm) diamonds with variable nitrogen aggregation states. The stable carbon isotope compositions of the Springfield Basin diamonds are similar to the Eurelia diamonds with δ 13C values in the range −20.0 to −2.5‰, and a mode at −6.5‰. Ferropericlase inclusions in two diamonds from the Springfield Basin are consistent with ferropericlase-bearing mineral inclusion assemblages found in the Eurelia diamonds and indicate that part of the diamond population from both areas is of sublithospheric origin. One diamond from the Springfield Basin contained an inclusion of lherzolitic garnet. The overall similarities between the Springfield Basin and Eurelia diamonds indicates that the bulk of the Springfield Basin diamonds are derived from kimberlitic sources that are similar (or identical) to those at Eurelia. However, three diamonds from the Springfield Basin are markedly distinct. These have well-developed crystal shapes, large sizes, yellow body colorations, and brown irradiation spots. The brown irradiation spots and abrasion textures provide evidence that these diamonds are much older than the other diamonds in the Springfield Basin, and that they are derived from distal kimberlitic sources. The diamonds are most likely derived from Permian glacigene sediments and may ultimately be sourced from kimberlites on the East Antarctic craton. Abrasion textures and brown irradiation spots are also present on diamonds from Echunga. This provides a link to the three “old” Springfield Basin diamonds and other alluvial diamonds in Eastern Australia, and suggests that Permian glaciations caused a widespread distribution of diamonds over large parts of southern Australia, which at that time was part of the supercontinent Gondwana. [Copyright &y& Elsevier]

Diamonds from Jurassic kimberlites at Eurelia, South Australia, contain coexisting inclusions of ferropericlase and MgSi-perovskite that provide evidence for their deep (>670 km) lower mantle origin. Eurelia diamonds formed from mixed carbon sources, likely including subducted carbonate, as indicated by a trend toward isotopically heavy carbon compositions (δ13C = 0‰) and low nitrogen concentrations (<100 ppm) in highly aggregated states. The discovery of lower mantle diamonds at Eurelia extends the area of known Mesozoic kimberlites carrying sublithospheric diamonds within continental fragments of Gondwana. The alignment of the kimberlite localities with the former Gondwana subduction margin and the presence of crustal signatures in the composition of the sublithospheric diamonds provide evidence that deeply subducted remnants of the proto-Pacific plate are the ultimate source of the diamonds. The kimberlite magmatism and the widespread emplacement of Jurassic to Early Cretaceous large igneous provinces in southern Gondwana are also attributed to this subduction process. [ABSTRACT FROM AUTHOR]

Lithium abundances in inclusions in diamonds of peridotitic, websteritic, eclogitic (including garnets with majoritic component) and lower mantle paragenesis were determined with the ion microprobe. The partitioning of Li both between olivine and clinopyroxene and clinopyroxene and garnet appears to be pressure-dependent in peridotites. More importantly, garnets containing a majorite component show a dramatic increase in Li solubility so that even for mildly majoritic garnets Dcpx/majorite is smaller than one. Lower mantle ferropericlase is a sink for lithium, containing several hundred times more Li than coexisting Mg-perovskite. Lithium decreases in olivine inclusions with increasing forsterite content, which we interpret as reflecting a depletion event in the protoliths of the inclusions. We also infer that lithium was not re-enriched in the silicate phases by a metasomatic agent during diamond formation unlike other trace elements like REE and HFSE. Depletion on partial melting and no re-enrichment also seems to hold for the eclogitic suite of inclusions where cpx''s (and deduced bulk composition) have similar Li abundances as in the peridotitic suite and are significantly lower than in crustal eclogites. For the lower mantle bulk, lithium estimates give values four to five times higher than primitive mantle. This may relate either to local enrichment through fluid/melt metasomatism similarly as for spinel lherzolites or high Li abundances in lower mantle diamonds sources are inherited from crustal protoliths. [Copyright &y& Elsevier]

High pressure high temperature (HPHT) treatment has long been applied in the gem trade for changing the body colour of diamonds. The identification of HPHT-treated diamonds is a field of on-going research in gemological laboratories, as different parameters of treatment will result in either the creation or the destruction of a variety of lattice defects in diamonds. Some features that exist in treated diamonds can also be found in natural diamonds, and consequently must not be employed for the separation of treated and natural diamonds. In this research, we investigated the properties of 11 natural yellow diamonds (directly obtained from the Ekati Diamond Mine to ensure that they are untreated) before and after HPHT treatment, conducted at a temperature of 2100 °C and a pressure of 6 GPa for 10 min. We report spectroscopic data and fluorescence characteristics, collected using PL mapping, FTIR mapping and fluorescence imaging showing the distribution of lattice defects and internal growth structures. PL mapping indicates Si V defects exist in one of the nitrogen-rich natural diamonds prior to treatment. Silicon-related defects can also be created by HPHT treatment, and they seem to show a relationship with pre-existing NV− centres. SIMS analysis was conducted to confirm the presence of silicon in these diamonds. The increase in the hydrogen-related infrared absorption peak at 3107 cm−1 (VN 3 H) is very strong in some diamonds that do not form B-centres during treatment. NVH was observed in our HPHT-treated natural diamonds, so it is possible that this strong increase in VN 3 H suppresses the aggregation of A- to B-centres as the newly formed A-centres were captured by NVH lattice defects to form VN 3 H. HPHT-altered and HPHT-induced platelet peaks are different from their natural counterparts in peak width and shape. Strong green fluorescence over a large area of a diamond, which is linked to relatively high concentration of H3 centres, was produced after HPHT treatment. We are confident that the unusual platelet peaks and strong emission of H3 centres are reliable indicators for HPHT-treated diamonds as they are not observed in untreated natural diamonds. Unlabelled Image • Si V defect is detected in a nitrogen-rich natural diamond. • Silicon-related defect can be created by HPHT treatment and it shows a relationship with pre-existing NV- centre. • Platelet peaks become broader and more symmetrical in HPHT-treated diamonds. • Episodically grown diamonds result in a mixed infrared absorption spectrum which shows A-, B- (and platelets) and C-centres. • The strong increase in infrared absorption peak at 3107 cm-1 (VN 3 H) possibly suppresses the nitrogen aggregation. [ABSTRACT FROM AUTHOR]

A long-standing unresolved problem in understanding Earth's deep carbon cycle is whether crustal carbon is recycled beyond arc depths. While isotopic signatures of eclogitic diamonds and their inclusions suggest deep recycling of crustal material, the crustal carbon source remains controversial; seafloor sediment – the widely favored crustal carbon source – cannot explain the combined carbon and nitrogen isotopic characteristics of eclogitic diamonds. Here we examined the carbon and oxygen isotopic signatures of bulk-rock carbonate for 80 geographically diverse samples from altered mafic-ultramafic oceanic crust (AOC), which comprises 95 vol% of the crustal material in subducting slabs. The results show: (i) AOC contains carbonate with δ 13 C values as low as −24‰, indicating the presence of biogenic carbonate; (ii) carbonate in AOC was mainly formed during low-temperature (<100 °C) alteration processes. Modeling accounting for this newly recognized carbon source in the oceanic crust with formation temperatures <100 °C yields a global carbon influx of 1.5±0.3 × 1012 mol C/yr carried by subducting AOC into the trench, which is 50–90% of previous estimates, but still of the same order of the carbon influx carried by subducting sediments into the trench. The AOC can retain carbon better than sediment during subduction into the asthenosphere, transition zone and lower mantle. Mixing of asthenospheric and AOC fluids provides the first consistent explanation of the diverse record of carbon and nitrogen isotopes in diamonds, suggesting that AOC, instead of sediment, is the key carrier of crustal carbon into the deep mantle. • AOC contains carbonate with extremely variable δ 13 C values from −24‰ to +11‰. • An input flux of 1.5 × 1012 mol C/yr is contributed by AOC into global trench. • This amount is comparable to C input flux by sediment into global trench. • Recycling of AOC solely well explains C-N isotopic feature of eclogitic diamond. • AOC, rather than sediment, is the key carrier for C recycling into the deep mantle. [ABSTRACT FROM AUTHOR]

Ferropericlase [(Mg,Fe)O] is one of the major constituents of Earth's lower mantle and the most abundant mineral inclusion in sub-lithospheric diamonds. Although a lower mantle origin for ferropericlase inclusions has often been suggested, some studies have proposed that many of these inclusions may instead form at much shallower depths, in the deep upper mantle or transition zone. No straightforward method exists to discriminate ferropericlase of lower-mantle origin without characteristic mineral associations, such as co-existing former bridgmanite. To explore ferropericlase-diamond growth relationships, we have investigated the crystallographic orientation relationships (CORs), determined by single-crystal X-ray diffraction, between 57 ferropericlase inclusions and 37 diamonds from Juina (Brazil) and Kankan (Guinea). We show that ferropericlase inclusions can develop specific (16 inclusions in 12 diamonds), rotational statistical (9 inclusions in 7 diamonds) and random (32 inclusions in 25 diamond) CORs with respect to their diamond hosts. All measured inclusions showing a specific COR were found to be Fe-rich (X FeO > 0.30). Coexistence of non-randomly and randomly oriented ferropericlase inclusions within the same diamond indicates that their CORs may be variably affected by local growth conditions. However, the occurrence of specific CORs only for Fe-rich inclusions indicates that Fe-rich ferropericlases have a distinct genesis and are syngenetic with their host diamonds. This result provides strong support for a dual origin for ferropericlase in Earth's mantle, with Fe-rich compositions likely indicating redox growth in the upper mantle, while more Mg-rich compositions with random COR mostly representing ambient lower mantle trapped as protogenetic inclusions. • Ferropericlase is the most abundant mineral inclusion within super-deep diamonds. • Mg-rich ferropericlase are remnants of pre-existing minerals within diamonds. • Diamonds containing Mg-rich ferropericlases formed in lower mantle. • Fe-rich ferropericlase simultaneously crystallized with their diamond hosts. • Fe-rich ferropericlase-bearing diamonds formed in the deep upper mantle/transition zone. [ABSTRACT FROM AUTHOR]