Physicochemical formation conditions of natural diamond deduced from experimental study of the eclogite-carbonatite-sulfide-diamond system

A diagram of the syngenesis of diamond, silicate, carbonate, and sulfide minerals and melts is compiled based on experimental data on phase relations in the heterogeneous eclogite-carbonate-sulfidediamond system at P = 7 GPa. Evidence is provided that silicate and carbonate minerals are paragenetic, whereas sulfides are xenogenic with respect to diamond. Diamond and paragenetic phases are formed in completely miscible carbonate-silicate growth melts with dissolved elemental carbon. Coherent data of physicochemical experiment and mineralogy of primary inclusions in natural diamonds allows us to prove the mantle-carbonatite theory of diamond origin. The genetic classification of primary inclusions in natural diamonds is based on this theory. The phase diagrams of syngenesis are applicable to interpretation of diamond and syngenetic minerals formation in natural magma sources. They ascertain physicochemical mechanism of natural diamond formation and conditions of entrapment of paragenetic and xenogenic mineral phases by growing diamonds.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Metasomatic Eclogitic Diamond Growth: Evidence from Multiple Diamond Inclusions

Diamond formation from metasomatic fluids, rather than from igneous melts, remains controversial but is paramount to our understanding of diamonds' mantle origin(s). Physical and chemical properties of diamonds, their inclusions, and host eclogites from the Mir kimberlite, Yakutia, Russia form the basis for our evaluation of diamond origin. Mir eclogitic diamonds and their multiple inclusions show a definite break in time and temperature between the formation of the core zones and the rims of the diamonds. Extreme changes in chemistry for multiple diamond inclusions (DIs) between the cores and the rims cannot be accounted for by magmatic fractional crystallization. Evidence also exists for large temperature decreases (40° to 140°C) from the cores to the rims of some diamonds. The distinct changes in nitrogen contents and aggregation states from cores to rims of diamonds would appear to reflect different residence times for these portions of the diamonds in the mantle- i.e., formation of cores and rims at vastly different times (e.g., 2 Gy). Many of the mineral-chemical characteristics, including C and N isotopes and N aggregation states of the diamond, can best be explained by crystallization of the diamonds after formation of the eclogite host. This suggests that the formation of the eclogite and the nucleation and growth of some diamonds are not coeval and possibly not cogenetic.

Most diamondiferous eclogite xenoliths probably have never experienced a major magmatic episode (i.e., complete melt stage) after subduction of their crustal protoliths into the mantle. Carbon isotopes in diamond, sulfur isotopes from sulfide DIs, and oxygen isotopes from eclogite minerals all point to crustal protoliths for many eclogites.

All of the factors above, taken as a whole, indicate that many eclogitic diamonds are the result of petrogenesis by metasomatism over a prolonged period of time. Introduction of metasomatic fluids facilitates the precipitation of the diamonds, either in tolo or as rims on previously formed diamonds. Inasmuch as some eclogites are considered to be igneous in origine.g., Group-A eclogites of Taylor and Neal (1989)-it is entirely possible that these eclogites may contain truly igneous diamonds. However, even some of these diamonds may have later metasomatic overgrowths.     

Liquid immiscibility in deep-seated magmas and the generation of carbonatite melts

This paper reviews the results of investigations of melt inclusions in minerals of carbonatites and spatially associated silicate rocks genetically related to various deep-seated undersaturated silicate magmas of alkaline ultrabasic, alkaline basic, lamproitic, and kimberlitic compositions. The analysis of this direct genetic information showed that all the deep magmas are inherently enriched in volatile components, the most abundant among which are carbon dioxide, alkalis, halides, sulfur, and phosphorus. The volatiles probably initially served as agents of mantle metasomatism and promoted melting in deep magma sources. The derived magmas became enriched in carbon dioxide, alkalis, and other volatile components owing to the crystallization and fractionation of early high-magnesium minerals and gradually acquired the characteristics of carbonated silicate liquids. When critical compositional parameters were reached, the accumulated volatiles catalyzed immiscibility, the magmas became heterogeneous, and two-phase carbonate-silicate liquid immiscibility occurred at temperatures of ≥1280–1250°C. The immiscibility was accompanied by the partitioning of elements: the major portion of fluid components partitioned together with Ca into the carbonate-salt fraction (parental carbonatite melt), and the silicate melt was correspondingly depleted in these components and became more silicic. After spatial separation, the silicate and carbonate-silicate melts evolved independently during slow cooling. Differentiation and fractionation were characteristic of silicate melts. The carbonatite melts became again heterogeneous within the temperature range from 1200 to 800–600°C and separated into immiscible carbonate-salt fractions of various compositions: alkali-sulfate, alkali-phosphate, alkali-fluoride, alkali-chloride, and Fe-Mg-Ca carbonate. In large scale systems, polyphase silicate-carbonate-salt liquid immiscibility is usually manifested during the slow cooling and prolonged evolution of deeply derived melts in the Earth’s crust. It may lead to the formation of various types of intrusive carbonatites: widespread calcite-dolomite and rare alkali-sulfate, alkali-phosphate, and alkali-halide rocks. The initial alkaline carbonatite melts can retain their compositions enriched in P, S, Cl, and F only at rapid eruption followed by instantaneous quenching.     

地幔岩中流体包裹体研究

地幔岩石中的流体包裹体代表地幔流体的样品。地幔流体包裹体可以存在从地幔来的金刚石,地幔捕虏体和岩浆碳酸岩中。研究这些岩石和矿物中的流体包裹体可以得出其所代表的地幔流体的温度、压力、成分和同位素。我们目前见到的这三类地幔岩石的包裹体主要可在橄榄石、辉石、金刚石、方解石和磷灰石中见到。这些包裹体可以粗略地分为CO2包襄体和硅酸盐熔融体包裹体。又可细分为四类包裹体：（1）富碳酸盐的硅酸盐熔融包裹体。这种包裹体在金刚石、地幔岩捕虏体和岩浆碳酸盐岩中见到,它又可分为结晶质熔融包裹体和玻璃包裹体。（2）CO2包裹体。这种包裹体大多见于地幔捕虏体中,在金刚石和岩浆碳酸岩中也可见到。（3）含硫化物的包裹体。这种包裹体见于地幔捕虏体中,与纯CO2包裹体和含CO2的熔融包裹体共存。（4）高密度的流体包裹体。这种包裹体见于金刚石中,是一种高盐度、高密度的含K、Cl和H2O的流体包裹体,又可分为高卤水包裹体和含卤水的富硅的碳酸盐岩浆包裹体。从对金刚石、地幔捕虏体和岩浆碳酸盐岩中流体包裹体的研究表明,地幔流体存在不均匀性和不混溶性。     

Diamond formation during metasomatism of mantle eclogite by chloride-carbonate melt

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).     

Ferropericlase—a lower mantle phase in the upper mantle

Experiments on compositions along the join MgO–NaA³⁺Si₂O₆ (A=Al, Cr, Fe³⁺) show that sodium can be incorporated into ferropericlase at upper mantle pressures in amounts commonly found in natural diamond inclusions. These results, combined with the observed mineral parageneses of several diamond inclusion suites, establish firmly that ferropericlase exists in the upper mantle in regions with low silica activity. Such regions may be carbonated dunite or stalled and degassed carbonatitic melts. Ferropericlase as an inclusion in diamond on its own is not indicative of a lower mantle origin or of a deep mantle plume. Coexisting phases have to be taken into consideration to decide on the depth of origin. The composition of olivine will indicate an origin from the upper mantle or border of the transition zone to the lower mantle and whether it coexisted with ferropericlase in the upper mantle or as ringwoodite. The narrow and flat three phase loop at the border transition zone—lower mantle together with hybrid peridotite plus eclogite/sediments provides an explanation for the varying and Fe-rich nature of the diamond inclusion suite from Sao Luiz, Brazil.     

Morphological Features of Diamond Crystals Dissolved in Fe<Subscript>0.7</Subscript>S<Subscript>0.3</Subscript> Melt at 4 GPa and 1400°C

An experimental study of the dissolution of natural and synthetic diamonds in a sulfur-bearing iron melt (Fe_0.7S_0.3) with high P–T parameters (4 GPa, 1400°С) was performed. The results demonstrated that under these conditions, octahedral crystals with flat faces and rounded tetrahexahedral diamond crystals are transformed into rounded octahedroids, which have morphological characteristics similar to those of natural diamonds from kimberlite. It was suggested that, taking into account the complex history of individual natural diamond crystals, including the dissolution stages, sulfur-bearing metal melts up to sulfide melts were not only diamond-forming media during the early evolution of the Earth, but also natural solvents of diamond in the mantle environment before the formation of kimberlitic melts.     

Diamond formation in sulfide pyrrhotite-carbon melts: Experiments at 6.0–7.1 GPa and application to natural conditions

Experiments at 6.0–7.1 GPa and 1500–1700°C were carried out to explore the boundary conditions of diamond nucleation and growth in pyrrhotite-carbon melt-solutions. Pyrrhotite is one of the main sulfide minerals of the pyrrhotite-pentlandite-chalcopyrite assemblage of mantle rocks and primary inclusions in diamond. Solutions of carbon in sulfide melts oversaturated with respect to diamond at the expense of the dissolution of starting graphite (thermodynamically unstable phase) are formed owing to the difference between the solubilities of graphite and diamond, which increases under the influence of temperature gradients in experimental samples. We determined the fields of carbon solutions in pyrrhotite melt showing labile and metastable oversaturation with respect to diamond, which correspond to the spontaneous nucleation of the diamond phase and diamond growth on seeds, respectively. The linear growth rate of diamond in sulfide-carbon melts is rather high (on average, 10 μ/min during the first 1–2 min from the onset of spontaneous crystallization). The nucleation density is estimated as 180 grains per cubic centimeter. Diamonds crystallized from sulfide melts show octahedral and spinel twin shapes. Diamond polycrystals were synthesized for the first time from a sulfide medium as intergrowths of skeletal (edge) or “cryptocrystalline” microdiamonds, from 1 to 100 μm in size, their spinel twins and, occasionally, polysynthetic (star-shaped) twins. During diamond growth from sulfidecarbon melts on smooth faces of cuboctahedral diamond seeds synthesized in metal systems, smooth-faced layer-by-layer step-like growth was observed on their octahedral (111) faces, whereas growth on the (100) cubic faces produced rough-surfaced layers of intergrown micropyramids, whose axes were oriented normal to the (100) face. The obtained experimental results were applied to the problem of diamond genesis under the conditions of the Earth’s mantle in the framework of the model of carbonate-silicate parental melts with blebs of immiscible sulfide melts.     

褶皱带橄榄岩-榴辉岩杂岩和稳定地块含金刚石岩筒的成因特征

金刚石母岩可以是榴辉岩、辉石岩、橄榄岩等多种岩石,它们与金刚石都是在地幔深处形成的,并上侵最终固结于地壳中。母岩中的金刚石等矿物在地壳中又发生准稳定生长。地壳中金伯利岩和钾镁煌斑岩岩浆作用对金刚石母岩侵入体进行改造,使原生金刚石发生破碎、溶解、再生长等一系列变化,并形成巨晶、劣质金刚石和黑金刚石等新类型,这些现象不是在金刚石母岩形成之初发生的。     

Diamonds and Their Mineral Inclusions,and What They Tell Us: A Detailed “Pull-Apart” of a Diamondiferous Eclogite

For the first time, three-dimensional, high-resolution X-ray computed tomography (HRXCT) of an eclogite xenolith from Yakutia has successfully imaged diamonds and their textural relationships with coexisting minerals. Thirty (30) macrodiamonds (≥1 mm), with a total weight of just over 3 carats, for an ore grade of some 27,000 ct/ton, were found in a small (4 × 5 × 6 cm) eclogite, U51/3, from Udachnaya. Based upon 3-D imaging, the diamonds appear to be associated with zones of secondary alteration of clinopyroxene (Cpx) in the xenolith. The presence of diamonds with secondary minerals strongly suggests that the diamonds formed after the eclogite, in conjunction with meta-somatic input(s) of carbon-rich fluids. Metasomatic processes are also indicated by the non-systematic variations in Cpx inclusion chemistry in the several diamonds. The inclusions in the diamonds vary considerably in major- and trace-element chemistry within and between diamonds, and do not correspond to the minerals of the host eclogite, whose compositions are extremely homogeneous. Some Cpx inclusions possess +Eu anomalies, probably inherited from their crustal source rocks. The only consistent feature for the Cpx crystals in the inclusions is that they have higher K₂O than the Cpx grains in the host.

The δ¹³C compositions are relatively constant at ?5% both within and between diamonds, whereas δ¹⁵N values vary from ?2.8% to ?15.8%. Within a diamond, the total N varies considerably from 15 to 285 ppm in one diamond to 103 to 1250 ppm in another. Cathodoluminescent imaging reveals extremely contorted zonations and complex growth histories in the diamonds, indicating large variations in growth environments for each diamond.

This study directly bears on the concept of diamond inclusions as time capsules for investigating the mantle of the Earth. If diamonds and their inclusions can vary so much within this one small xenolith, the significance of their compositions is a serious question that must be addressed in all diamond-inclusion endeavors.     

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.