Parental media of natural diamonds and primary mineral inclusions in them: Evidence from physicochemical experiment

A generalized diagram was constructed for the compositions of multicomponent heterogeneous parental media for diamonds of kimberlite deposits on the basis of the mantle carbonatite concept of diamond genesis. The boundary compositions on the diagram of the parental medium are defined by the components of minerals of the peridotite and eclogite parageneses, mantle carbonatites, carbon, and the components of volatile compounds of the C-O-H system and accessory phases, both soluble (chlorides, phosphates, and others) and insoluble (sulfides and others) in carbonate-silicate melts. This corresponds to the compositions of minerals, melts, and volatile components from primary inclusions in natural diamonds, as well as experimental estimations of their phase relations. Growth media for most natural diamonds are dominated by completely miscible carbonate-silicate melts with dissolved elemental carbon. The boundary compositions for diamond formation (concentration barriers of diamond nucleation) in the cases of peridotite-carbonate and eclogite-carbonate melts correspond to 30 wt % peridotite and 35 wt % eclogite; i.e., they lie in the carbonatite concentration range. Phase relations were experimentally investigated at 7 GPa for the melting of the multicomponent heterogeneous system eclogite-carbonatite-sulfide-diamond with a composition close to the parental medium under the conditions of the eclogite paragenesis. As a result, “the diagram of syngenesis” was constructed for diamond, as well as paragenetic and xenogenic mineral phases. Curves of diamond solubility in completely miscible carbonate-silicate and sulfide melts and their relationships with the boundaries of the fields of carbonate-silicate and sulfide phases were determined. This allowed us to establish the physicochemical mechanism of natural diamond formation and the P-T conditions of formation of paragenetic silicate and carbonate minerals and coexistence of xenogenic sulfide minerals and melts. Physicochemical conditions of the capture of paragenetic and xenogenic phases by growing diamonds were revealed. Based on the mantle carbonatite concept of diamond genesis and experimental data, a genetic classification of primary inclusions in natural diamond was proposed. The phase diagrams of syngenesis of diamond, paragenetic, and xenogenic phases provide a basis for the analysis of the physicochemical history of diamond formation in carbonatite magma chambers and allow us to approach the formation of such chambers in the mantle material of the Earth.     

Fundamentals of the mantle carbonatite concept of diamond genesis

In the mantle carbonatite concept of diamond genesis, the data of a physicochemical experiment and analytical mineralogy of inclusions in diamond conform well and solutions to the following genetic problems are generalized: (1) we substantiate that upper mantle diamond-forming melts have peridotite/eclogite–carbonatite–carbon compositions, melts of the transition zone have (wadsleyite ? ringwoodite)–majorite–stishovite–carbonatite–carbon compositions, and lower mantle melts have periclase/wüstite–bridgmanite–Ca-perovskite–stishovite–carbonatite–carbon compositions; (2) we plot generalized diagrams of diamondforming media illustrating the variable compositions of growth melts of diamonds and paragenetic phases, their genetic relationships with mantle matter, and classification relationships between primary inclusions; (3) we study experimentally equilibrium diagrams of syngenesis of diamonds and primary inclusions characterizing the diamond nucleation and growth conditions and capture of paragenetic and xenogenic minerals; (4) we determine the fractional phase diagrams of syngenesis of diamonds and inclusions illustrating regularities in the ultrabasic–basic evolution and paragenetic transitions in diamond-forming systems of the upper and lower mantle. We obtain evidence for physicochemically similar melt–solution ways of diamond genesis at mantle depths with different mineral compositions.     

Paragenetic relations of diamond with silicate and carbonate minerals in the carbonatite-diamond system: Experiments at 8.5 GPa

Phase relations of diamond and syngenetic minerals were experimentally investigated in the multicomponent system natural carbonatite-diamond at a pressure of 8.5 GPa and temperatures of 1300–1800°C (within the thermodynamic stability field of diamond). Under such conditions, the natural carbonatite of the Chagatai complex (Uzbekistan) acquires the mineralogy of Ca-rich eclogites (grospydites). The melting phase diagram of this system (syngenesis diagram) was constructed; an important element of this diagram is the diamond solubility curve in completely miscible carbonate-silicate melts (solubility values are 15–18 wt % C). The diamond solubility curve divides the phase diagram into two fields corresponding to (1) phase relations involving diamond-undersaturated melts-solutions of carbon with garnet as a liquidus phase (region of diamond dissolution) and (2) phase relations with diamond-saturated melts-solutions with diamond as a liquidus phase (region of diamond crystallization). During a temperature decrease in the region of diamond crystallization from carbonate-silicate melts, the crystallization of diamond is accompanied by the sequential formation of the following phase assemblages: diamond + garnet + melt, diamond + garnet + clinopyroxene + melt, and diamond + garnet + clinopyroxene + carbonate + melt, and the subsolidus assemblage diamond + garnet + clinopyroxene + carbonate is eventually formed. This is indicative of the paragenetic nature of silicate and carbonate minerals co-crystallizing with diamond and corresponding primary inclusions trapped by the growing diamond. A physicochemical mechanism was proposed for the formation of diamond in carbonate-silicate melts. The obtained results were used to analyze the physicochemical behavior of a natural diamond-forming magma chamber.     

Interaction of Titanium Minerals and Their Melts with Diamond-Forming Media (Experiments at 7–8 GPa)

Melting relations in the multicomponent diamond-forming systems of the upper mantle with a boundary of K–Na–Mg–Fe–Ca carbonate, phases of the model peridotite and eclogite, carbon, and titanium minerals from kimberlite (ilmenite FeTiO₃, perovskite CaTiO₃, and rutile TiO₂) were studied experimentally at 7–8 GPa and 1600–1650°C. Perovskite reacts with the formation of rutile in the diamond-forming silicate–carbonate melts. We discovered liquid immiscibility between melts of titanium minerals, on the one hand, and carbonate–carbon, peridotite–carbonate–carbon, and eclogite–carbonate–carbon diamond-forming melts, on the other. The solubility of titanium mineral in diamond-forming melts is negligible independent of their concentration in the experimental systems. Growth melts retain high diamond-forming efficiency. In general, the experimental results are evident for the xenogenic nature of titanium minerals in inclusions in diamond and, therefore, in diamond-forming melts. It is shown that the physicochemical factors that may correlate the diamond content with the concentration of Ti in kimberlite do not occur during the diamond genesis in silicate–carbonate–carbon parental melts containing titanium minerals and their melts.     

The physicochemical conditions of diamond formation in the mantle matter: experimental studies

Experimental studies of diamond formation in the alkaline silicate-carbon system Na₂O–K₂O–MgO–CaO–Al₂O₃–SiO₂–C were carried out at 8.5 GPa. In accordance with the diamond nucleation criterion, a high diamond generation efficiency (spontaneous mass diamond crystallization) has been confirmed for the melts of the system Na₂SiO₃–carbon and has been first established for the melts of the systems CaSiO₃–carbon and (NaAlSi₃O₈)₈₀(Na₂SiO₃)₂₀–carbon. It is shown that in completely miscible carbonate-silicate melts oversaturated with dissolved diamond-related carbon, a concentration barrier of diamond nucleation (CBDN) arises at a particular ratio of carbonate and silicate components. Study of different systems (eclogite–K-Na-Mg-Ca-Fe-carbonatite–carbon, albite–K₂CO₃–carbon, etc.) has revealed a dependence of the barrier position on the chemical composition of the system and the inhibiting effect of silicate components on the nucleation density and rate of diamond crystal growth. In multicomponent eclogite-carbonatite solvent, the CBDN is within the range of carbonatite compositions (<50 wt.% silicates). Based on the experimental criterion for the syngenesis of diamond and growth inclusions in them, we studied the syngenesis diagram for the system melanocratic carbonatite–diamond and determined a set of the composition fields and physical parameters of the system that are responsible for the cogeneration of diamond and various mineral and melt parageneses. The experimental results were applied to substantiate a new physicochemical concept of carbonate-silicate (carbonatite) growth media for most of natural diamonds and to elaborate a genetic classification of growth mineral, melt, and fluid inclusions in natural diamonds of mantle genesis.     

Experimental and thermodynamic study of the crystallization of diamond and silicates in a metal-silicate-carbon system

Silicate inclusions are widespread in natural diamonds, which also may contain rare inclusions of native iron. This suggests that some natural diamonds crystallized in metal-silicate-carbon systems. We experimentally studied the crystallization of diamond and silicate phases from the starting composition Fe_0.36Ni_0.64 + silicate glass + graphite and calculated the Fe mole fractions of the silicate phases crystallizing under these conditions. The silicates synthesized together with diamond had low Fe mole fractions [Fe/(Fe + Mg + Ca)] in spite of strong Fe predominance in the system. The Fe mole fractions of the silicates decreased in the sequence garnet-pyroxene-olivine, which is consistent with the results of our thermodynamic calculations. The Fe mole fraction of silicates under various redox conditions under which metal-carbon melts are stable drastically decreases with decreasing fo₂. The low Fe mole fractions of silicate inclusions in diamond from the Earth’s mantle can be explained by the highly reducing crystallization conditions, under which Fe was concentrated as a metallic phase of the magmatic melts and could be only insignificantly incorporated in the structures of silicates.     

Mineral equilibria of diamond-forming carbonate-silicate systems

Based on experimental and mineralogical data, the model of mantle carbonate-silicate (carbonatite) melts as dominating parental media for natural diamonds was substantiated. It was demonstrated that the compositions of silicate constituents of parental melts were variable and saturated with respect to mantle rocks, namely pyrope peridotite, garnet pyroxenite, and eclogite. Based on concentration contributions and role in diamond genesis, major (carbonate and silicate) and minor (admixture) components were distinguished. The latter components may be both soluble (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble (sulfides, metals, and carbides) in silicate-carbonate melts. This paper presents the results of a study of diamond crystallization in multicomponent melts of variable composition with carbonate components (K₂CO₃, CaCO₃ · MgCO₃, and K-Na-Ca-Mg-Fe carbonatite) and silicate components represented by model peridotite (60 wt % olivine, 16 wt % orthopyroxene, 12 wt % clinopyroxene, and 12 wt % garnet) and eclogite (50 wt % garnet and 50 wt % clinopyroxene). Carbonate-silicate melts behave like completely miscible liquid phases in experiments performed under the P-T conditions of diamond stability. The concentration barriers of diamond nucleation (CBDN) in melts with variable proportions of silicates and carbonates were determined at 8.5 GPa. In the peridotite system with K₂CO₃, CaCO₃ · MgCO₃, and carbonatite, they correspond to 30, 25, and 30 wt % silicates, respectively, and in the eclogite system, the CBDN is shifted to 45, 30, and 35 wt % silicates. In the silicate-carbonate melts with higher silicate contents, diamond grows on seeds, which is accompanied by the crystallization of thermodynamically unstable graphite. At P = 7.0 GPa and T = 1200−1800°C, we studied and constructed phase diagrams for the multicomponent peridotite-carbonate and eclogite-carbonate systems as a physicochemical basis for revealing the syngenetic relationships between diamond and its silicate (olivine, ortho- and clinopyroxene, and garnet) and carbonate (aragonite and magnesite) inclusions depending on the physicochemical conditions of growth media. The results obtained allowed us to reconstruct the evolution of diamond-forming systems. The experiments revealed similarity between the compositions of synthetic silicate minerals and inclusions in natural diamonds (high concentrations of Na in garnets and K in clinopyroxenes). It was experimentally demonstrated that the formation of Na-bearing majoritic garnets is controlled by the P-T parameters and melt alkalinity. Diamonds with inclusions of such garnets can be formed in alkalic carbonate-silicate (aluminosilicate) melts. A mechanism was suggested for sodic end-member dissolution in majoritic garnets, and garnet with the composition Na₂MgSi₅O₁₂ and tetragonal symmetry was synthesized for the first time.     

Physicochemical parameters of the material of mantle plumes: Evidence from the thermodynamic analysis of mineral inclusions in sublithospheric diamond

Thermodynamic analysis of equilibria involving minerals of the lower mantle of pyrolite composition and crystalline carbon-bearing compounds indicates that the range of oxygen fugacity values at which diamond can be formed is separated from the region in which Fe-rich metallic alloy is generated by a field in which Fe carbides are stable. This implies that diamond can be formed in the lower mantle under more oxidizing conditions than those thought to be dominant in this geosphere. The absence of a metallic phase from the lower-mantle diamond-bearing mineral assemblage is consistent with the high (approximately 1%) Ni concentration in the ferropericlase found as inclusions in diamonds (Fe-rich metallic alloy is able to intensely extract Ni). An elevated redox potential also follows from the occurrence of carbonate phases found among mineral inclusions in lower-mantle diamonds. The main reason for a local increase in oxygen fugacity in the lower mantle may be shifts of redox equilibria toward a decrease in the amount, and then the disappearance of the Fe-Ni alloy with increasing temperature. An important role in the formation of diamond may be played by the generation of carbonate-phosphate and silicate melts in high-temperature zones and the migration of these melts and their interaction with wall rocks.     

Peridotite–eclogite–carbonatite systems at 7.0–8.5 GPa: concentration barrier of diamond nucleation and syngenesis of its silicate and carbonate inclusions

Diamond crystallization in multicomponent melts of variable composition is studied. The melt carbonates are K₂CO₃, CaCO₃?MgCO₃, and K-Na-Ca-Mg-Fe-carbonatites, and the melt silicates are model peridotite (60 wt.% olivine, 16 wt.% orthopyroxene, 12 wt.% clinopyroxene, and 12 wt.% garnet) and eclogite (50 wt.% garnet and 50 wt.% clinopyroxene). In the experiments carried out under the PT-conditions of diamond stability, the carbonate-silicate melts behave like completely miscible liquid phases. The concentration barriers of diamond nucleation (CBDN) in the melts with variable proportions of silicates and carbonates have been determined at 8.5 GPa. In the system peridotite–K₂CO₃–CaCO₃?MgCO₃–carbonatite they correspond to 30, 25, and 30 wt.% silicates, respectively, and in the analogous eclogite–carbonate system, 45, 30, and 35 wt.%. In the silicate-carbonate melts with higher silicate contents seed diamond growth occurs, which is accompanied by the crystallization of thermodynamically unstable graphite phase. In the experiments with melts compositionally corresponding to the CBDN at 7.0 GPa and 1200–1700 °C, a full set of silicate minerals of peridotite (olivine, orthopyroxene, clinopyroxene, garnet) and eclogite (garnet, clinopyroxene) parageneses was obtained. The minerals occur as syngenetic inclusions in natural diamonds; moreover, the garnets contain an impurity of Na, and the pyroxenes, K. The experimental data indicate that peridotite-carbonate and eclogite-carbonate melts are highly effective for the formation of diamond (or unstable graphite) together with syngenetic minerals and melts, which agrees with the carbonate-silicate (carbonatite) model for the mantle diamond formation.     

Magmatic evolution of the material of the Earth’s lower mantle: Stishovite paradox and origin of superdeep diamonds (Experiments at 24–26 GPa)

The ultrabasic–basic magmatic evolution of the lower mantle material includes important physicochemical phenomena, such as the stishovite paradox and the genesis of superdeep diamonds. Stishovite SiO₂ and periclase–wüstite solid solutions, (MgO · FeO)_ss, associate paradoxically in primary inclusions of superdeep lower mantle diamonds. Under the conditions of the Earth’s crust and upper mantle, such oxide assemblages are chemically impossible (forbidden), because the oxides MgO and FeO and SiO₂ react to produce intermediate silicate compounds, enstatite and ferrosilite. Experimental and physicochemical investigations of melting phase relations in the MgO–FeO–SiO₂–CaSiO₃ system at 24 GPa revealed a peritectic mechanism of the stishovite paradox, (Mg, Fe)SiO₃ (bridgmanite) + L = SiO₂ + (Mg, Fe)O during the ultrabasic–basic magmatic evolution of the primitive oxide–silicate lower mantle material. Experiments at 26 GPa with oxide–silicate–carbonate–carbon melts, parental for diamonds and primary inclusions in them, demonstrated the equilibrium formation of superdeep diamonds in association with ultrabasic, (Mg, Fe)SiO₃ (bridgmanite) + (MgO · FeO)_ss (ferropericlase), and basic minerals, (FeO · MgO)_ss (magnesiowüstite) + SiO₂ (stishovite). This leads to the conclusion that a peritectic mechanism, similar to that responsible for the stishovite paradox in the pristine lower mantle material, operates also in the parental media of superdeep diamonds. Thus, this mechanism promotes both the ultrabasic–basic evolution of primitive oxide–silicate magmas in the lower mantle and oxide–silicate–carbonate melts parental for superdeep diamonds and their paradoxical primary inclusions.     

Inclusions in diamonds from Snap Lake kimberlites (Slave Craton,Canada): Geochemical features of crystallization

The results of integrated studies of inclusion-containing diamonds from kimberlites of the Snap Lake dike complex (Canada) are presented. Features of the morphology, defect–impurity composition, and internal structure of the diamonds were determined by optic and scanning microscopy. The chemical composition of crystalline inclusions (olivine, garnet, and pyroxene) in diamonds was studied using a microanalyzer with an electronic probe. The inclusions of ultramafic paragenesis in the diamond (87%) are predominant. Carbonates, sulfide and hydrated silicate phases were found only in multiphase microinclusions. The large phlogopite inclusion studied was similar in composition to earlier studied nanosize inclusions of high-silica mica in diamonds from Snap Lake kimberlites. Revealed features of studied diamonds and presence of high-silica mica suggest that diamonds from Snap Lake have formed as the result of interaction between enriched in volatile and titanium high-potassium carbonate–silicate melts and peridotitic substrate at the base of thick lithospheric mantle.     

Composite carbonate and silicate multiphase solid inclusions in metamorphic garnet from ultrahigh‐P eclogite in the Dabie orogen

Composite multiphase solid (MS) inclusions composed of carbonate and silicate minerals have been found for the first time in metamorphic garnet from ultrahigh‐P eclogite from the Dabie orogen. These inclusions are morphologically euhedral to subhedral, and some show relatively regular shapes approaching the negative crystal shape of the host garnet. Radial fractures often occur in garnet hosting the inclusions. The inclusions are primarily composed of variable proportions of carbonate and silicate minerals such as calcite, quartz, K‐feldspar and plagioclase, with occasional occurrences of magnetite, zircon and barite. They are categorized into two groups based on the proportions of carbonate and silicate phases. Group I is carbonate‐dominated with variable proportions of silicate minerals, whereas Group II is silicate‐dominated with small proportions of carbonates. Trace element analysis by LA‐ICPMS for the two groups of MS inclusions yields remarkable differences. Group I inclusions exhibit remarkably lower REE contents than Group II inclusions, with significant LREE enrichment and large fractionation between LREE and HREE in the chondrite‐normalized REE diagram. In contrast, Group II inclusions show rather flat REE patterns with insignificant fractionation between LREE and HREE. In the primitive mantle‐normalized spidergram, Group I inclusions exhibit positive anomalies of Zr and Hf, whereas Group II inclusions show negative anomalies of Zr and Hf. Nevertheless, both groups exhibit positive anomalies of Ba, U, Pb and Sr, but negative anomalies of Nb and Ta, resembling the composition of common continental crust. Group I inclusions have higher Ba and U contents than Group II inclusions. Combined with petrological observations, the two groups of MS inclusions are interpreted as having crystallized from composite silicate and carbonate melts during continental subduction‐zone metamorphism. The differences in trace element composition between the two groups are primarily attributed to the proportions of carbonate and silicate phases in the MS inclusions. The silicate melts were derived from the breakdown of hydrous minerals such as paragonite and phengite, whereas the occurrence of carbonate melts indicates involvement of carbonate minerals in the partial melting and thus has great bearing on recycling of supracrustal carbon into the mantle. The coexistence of silicate and carbonate melts in the eclogitic garnet provides insights into the nature of hydrous melts in the subduction factory.     

Nature of diamonds in Yakutian eclogites: views from eclogite tomography and mineral inclusions in diamonds

We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.     

Mineral inclusions in diamonds track the evolution of a Mesozoic subducted slab beneath West Gondwanaland

Three major suites of silicate inclusions in sublithospheric diamonds show evidence of formation at depths > 250 km, and for each suite there is evidence of their formation from subducted material. Two of these are the well known basic (majoritic garnet) and ultrabasic (MgSi-perovskite + ferropericlase) suites. The third, the recently recognised Ca-rich suite, is characterised by carbonate, Ca-Si-Ti minerals and some aluminous material. Carbon isotope ratios in the host diamonds and geochemical-petrological features of the inclusions themselves provide evidence for their derivation from subducted lithosphere materials. The diamonds hosting the basic and ultrabasic suites are suggested to form in fluids/melts resulting from the release of water caused by dehydration reactions affecting both the crustal and mantle portions of a subducting slab of ocean lithosphere. Conversely, the diamonds containing the Ca-rich suite are linked with the formation of carbonatitic melts. In the Juina kimberlite province of Brazil, all three suites have been found in close proximity. A model is presented whereby the formation of the suites occurs progressively during the subduction and stagnation of a single lithospheric slab, with all three suites being transported to the lithosphere by a plume with which the carbonatitic melts of the Ca-rich suite are associated. Nd-Sr isotopic data are presented for the Juina majoritic-garnet inclusions, which supports their formation from oceanic crust of Mesozoic age. In conjunction with published age data for a Ca-Si-Ti inclusion, the Juina (Brazil) sublithospheric inclusions document a series of events involving diamond formation during and following the emplacement of a subducted slab between ca 190 and 90 Ma beneath west Gondwanaland. This slab and related subducted slabs dating from the Palaeozoic at the Gondwanan margin may be the source of the widespread DUPAL geochemical anomaly in the South Atlantic and Indian Oceans. The kimberlites bringing the diamonds to the Earth's surface may have arisen from a superplume, developed from a graveyard of former Gondwanan stagnant slabs, at the Core-Mantle-Boundary.     

Olivine with diamond-imposed morphology included in diamonds. Syngenesis or protogenesis?

The identification of syngenetic inclusions in diamond (i.e. inclusions of minerals that crystallized at the same time and by the same genesis as their host) has long been of paramount importance in diamond studies. However, the widespread assumption that many or most inclusions in diamonds are syngenetic is based on qualitative morphological criteria and few direct measurements. In order to provide statistically significant information on inclusion–host genetic relations for at least one kimberlite, we have determined the crystallographic orientations of 43 olivine inclusions with diamond-imposed morphology, a feature generally interpreted to indicate syngenesis, in 20 diamonds from the Udachnaya kimberlite (Siberia). Our unprecedented large data set indicates no overall preferred orientation of these olivines in diamond. However, multiple inclusions within a single diamond frequently exhibit similar orientations, implying that they were derived from original single monocrystals. Therefore, regardless of the possible chemical re-equilibration during diamond-forming processes, at least some of the olivines may have existed prior to the diamond (i.e. they are protogenetic). Our results imply that a diamond-imposed morphology alone cannot be considered as unequivocal proof of syngenicity of mineral inclusions in diamonds.     

Homogenization of carbonate-bearing microinclusions in diamond at <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> parameters of the upper mantle

The staged high-pressure annealing of natural cubic diamonds with numerous melt microinclusions from the Internatsional’naya kimberlite pipe was studied experimentally. The results mainly show that the carbonate phases, the daughter phases in partially crystallized microinclusions in diamonds, may undergo phase transformations under the mantle P–T conditions. Most likely, partial melting and further dissolution of dolomite in the carbonate–silicate melt (homogenization of inclusions) occur in inclusions. The experimental data on the staged high-pressure annealing of diamonds with melt microinclusions allow us to estimate the temperature of their homogenization as 1400–1500°C. Thus, cubic diamonds from the Internatsional’naya pipe could have been formed under quite high temperatures corresponding to the lithosphere/asthenosphere boundary. However, it should be noted that the effect of selective capture of inclusions with partial loss of volatiles in relation to the composition of the crystallization medium is not excluded during the growth. This may increase the temperature of their homogenization significantly between 1400 and 1500°C.     

The mineralogy of Ca-rich inclusions in sublithospheric diamonds

This paper discusses mineralogy of Ca-rich inclusions in ultra-deep (sublithospheric) diamonds. It was shown that most of the Ca-rich majoritic garnets are of metabasic (eclogitic) affinity. The observed variation in major and trace element composition is consistent with variations in the composition of the protolith and the degree of enrichment or depletion during interaction with melts. Major and trace element compositions of the inclusions of Ca minerals in ultra-deep diamonds indicate that they crystallized from Ca-carbonatite melts that were derived from partial melting of eclogite bodies in deeply subducted oceanic crust in the transition zone or even the lower mantle. The occurrence of merwinite or CAS inclusions in ultra-deep diamonds can serve as mineralogical indicators of the interaction of metaperidotitic and metabasic mantle lithologies with alkaline carbonatite melts. The discovery of the inclusions of carbonates in association with ultra-deep Ca minerals can not only provide additional support for their role in the diamond formation process but also help to define additional mantle reservoirs involved in global carbon cycle.     

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ¹³C ~ ?25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (?5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth’s deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slab-derived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth’s deep carbon cycle.     

Trace element compositions of submicroscopic inclusions in coated diamond: A tool for understanding diamond petrogenesis

Trace element compositions of submicroscopic inclusions in both the core and the coat of five coated diamonds from the Democratic Republic of Congo (DRC, formerly Zaire) have been analyzed by Laser Ablation Inductively Coupled Mass Plasma Spectrometry (LA-ICP-MS). Both the diamond core and coat inclusions show a general 2-4-fold enrichment in incompatible elements relative to major elements. This level of enrichment is unlikely to be explained by the entrapment of silicate mantle minerals (olivine, garnet, clinopyroxene, phlogopite) alone and thus submicroscopic fluid or glass inclusions are inferred in both the diamond coat and in the gem quality diamond core. The diamond core fluids have elevated High Field Strength Element (Ti, Ta, Zr, Nb) concentrations and are enriched in U relative to inclusions in the diamond coats and relative to chondrite. The core fluids are also moderately enriched in LILE (Ba, Sr, K). Therefore, we suggest that the diamond cores contain inclusions of silicate melt. However, the Ni content and Ni/Fe ratio of the trapped fluid are very high for a silicate melt in equilibrium with mantle minerals; high Ni and Co concentrations in the diamond cores are attributed to the presence of a sulfide phase coexisting with silicate melt in the diamond core inclusions. Inclusions in the diamond coat are enriched in LILE (U, Ba, Sr, K) and La over the diamond core fluids and to chondrite. The coats have incompatible element ratios similar to natural carbonatite (coat fluid: Na/Ba ≈0.66, La/Ta≈130). The coat fluid is also moderately enriched in HFSE (Ta, Nb, Zr) when normalized to chondritic Al. LILE and La enrichment is related to the presence of a carbonatitic fluid in the diamond coat inclusions, which is mixed with a HFSE-rich hydrous silicate fluid similar to that in the core. The composition of the coat fluid is consistent with a genetic link to group 1 kimberlite.     

Experimental characterization of diamond crystallization in melts of mantle silicate-carbonate-carbon systems at 7.0–8.5 GPa

Diamond crystallization from carbon solutions in compositionally variable melts of model eclogite with dolomite [CaMg(CO₃)₂], potassium carbonate (K₂CO₃), and multicomponent K-Na-Ca-Mg-Fe carbonates was studied at 7.0–8.5 GPa. Concentration barriers for the nucleation of the diamond were determined at a standard pressure of 8.5 GPa for variable proportions of silicate and carbonate components in the growth solutions. They correspond to 35, 65, and 40 wt % of silicate components for systems with dolomite, K₂CO₃, and carbonatites, respectively. At higher contents of silicates in silicate-carbonate melts, the nucleation of diamond phase ceases, but diamond crystallization on seed crystals continues and is accompanied by the spontaneous crystallization of thermodynamically unstable graphite. In melts of the albite (NaAlSi₃O₈)-K₂CO₃-C compositions, the concentration barrier of diamond nucleation at 8.5 GPa is up to 90–92 wt % of the albite component, and diamond growth on seeds was observed in albite-carbon melts. Using mineralogical and experimental data, we developed a model of mantle carbonate-silicate (carbonatite) melts as the main parental media for natural diamonds; it was shown that the composition of the silicate constituent of such parental melts is variable and corresponds to the mantle ultrabasic-basic series. With respect to concentration contributions and dominant role in the genesis of diamond in the Earth’s mantle, major (carbonate and silicate) and minor or admixture components were distinguished. The latter include both soluble in carbonate-silicate melts (oxides, phosphates, chlorides, carbon dioxide, and water) and insoluble components (sulfides, metals, and carbides). Both major and minor components may affect the position of the concentration barriers of diamond nucleation in natural parent media.