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.     

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.     

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.     

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.     

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.     

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.     

Mineral inclusions in fibrous diamonds: constraints on cratonic mantle refertilization and diamond formation

We analyzed mineral microinclusions in fibrous diamonds from the Wawa metaconglomerate (Superior craton) and Diavik kimberlites (Slave craton) and compared them with published compositions of large mineral inclusions in non-fibrous diamonds from these localities. The comparison, together with similar datasets available for Ekati and Koffiefontein kimberlites, suggest a general pattern of metasomatic alteration imposed on the ambient mantle by formation of fibrous diamond. Calcium and Fe enrichment of peridotitic garnet and pyroxenes and Fe enrichment of olivine associated with fibrous diamond-forming fluids contributes to refertilization of the cratonic mantle. Saline—carbonatitic—silicic fluid trapped by fibrous diamonds may represent one of the elusive agents of mantle refertilization. Calcium enrichment of peridotitic garnet and pyroxenes is expected in local mantle segments during fibrous diamond production, as Ca in the carbonatitic fluids is deposited into the surrounding mantle when oxidized carbon is reduced to diamond. Harzburgitic garnet evolves towards Ca-rich compositions even when it interacts with Ca-poor saline fluids. An unusual trend of Mg enrichment to Fo_95–98 is observed in some olivine inclusions in Wawa fibrous diamonds. The trend may result from the carbonatitic composition of the fluid that promotes crystallization of magnesian olivine and preferentially oxidizes the fayalite component. We propose a generic model of fibrous and non-fibrous diamond formation from carbonatitic fluids that explains enrichment of the mantle in mafic magmaphile and incompatible elements and accounts for locally metasomatized compositions of diamond inclusions.     

The stishovite paradox in the evolution of lower mantle magmas and diamond-forming melts (experiment at 24 and 26 GPa)

Experimental studies of phase relations in the oxide–silicate system MgO–FeO–SiO₂ at 24 GPa show that the peritectic reaction of bridgmanite controls the formation of stishovite as a primary in situ mineral of the lower mantle and as an effect of the stishovite paradox. The stishovite paradox is registered in the diamond-forming system MgO–FeO–SiO₂–(Mg–Fe–Ca–Na carbonate)–carbon in experiments at 26 GPa as well. The physicochemical mechanisms of the ultrabasic–basic evolution of deep magmas and diamondforming media, as well as their role in the origin of the lower mantle minerals and genesis of ultradeep diamonds, are studied.     

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.     

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.     

Diamond crystallization in the Fe-Co-S-C and Fe-Ni-S-C systems and the role of sulfide-metal melts in the genesis of diamond

Diamond crystals 0.1–0.8 carats were synthesized in experiments conducted in a BARS split-sphere multianvil high-pressure apparatus in the systems Fe-Co-S-C and Fe-Ni-S-C at a pressure of 5.5 GPa and temperature of 1300°C. The microtextures of the samples and the phases accompanying diamond (carbides, graphite, monoslufide solid solution, pentlandite, and taenite) are examined in much detail, the properties of metal-sulfide-carbon alloys are discussed, and issues related to the genesis of sulfide inclusions in diamonds and graphite crystallization in the diamond stability field are considered. The experiments demonstrate that diamonds can be synthesized and grow in pre-eutectic metal-sulfide melts with up to 14 wt % sulfur at relatively low P-T parameters, which correspond to the probable temperatures and pressures of natural diamond-forming processes at depths of approximately 150 km in the Earth’s upper mantle.     

The system ilmenite–carbonatite–carbon in the origin of diamond: Correlation between the titanium content and diamond potential of kimberlite

Experimental studies of melting relations in the system ilmenite–K–Na–Mg–Fe–Ca carbonatite–carbon at 8 GPa and 1600°C provide evidence for the effect of liquid immiscibility between ilmenite and carbonatite melts. It is shown that the solubility of ilmenite in carbonatitic melts is negligible and does not depend on its concentration in experimental samples within 25–75 wt %. However, carbonatite–carbon melts are characterized by a high diamond-forming efficiency. This means that the correlation between the concentration of TiO₂ and diamond content is problematic for mantle chambers and requires further, more complex, experimental studies.     

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.     

Inclusions in diamonds from the K14 and K10 kimberlites, Buffalo Hills, Alberta, Canada: diamond growth in a plume?

Analyses of mineral inclusions, carbon isotopes, nitrogen contents and nitrogen aggregation states in 29 diamonds from two Buffalo Hills kimberlites in northern Alberta, Canada were conducted. From 25 inclusion bearing diamonds, the following paragenetic abundances were found: peridotitic (48%), eclogitic (32%), eclogitic/websteritic (8%), websteritic (4%), ultradeep? (4%) and unknown (4%). Diamonds containing mineral inclusions of ferropericlase, and mixed eclogitic-asthenospheric-websteritic and eclogitic-websteritic mineral associations suggests the possibility of diamond growth over a range of depths and in a variety of mantle environments (lithosphere, asthenosphere and possibly lower mantle).

Eclogitic diamonds have a broad range of C-isotopic composition (δ¹³C=−21‰ to −5‰). Peridotitic, websteritic and ultradeep diamonds have typical mantle C-isotope values (δ¹³C=−4.9‰ av.), except for two ¹³C-depleted peridotitic (δ¹³C=−11.8‰, −14.6‰) and one ¹³C-depleted websteritic diamond (δ¹³C=−11.9‰). Infrared spectra from 29 diamonds identified two diamond groups: 75% are nitrogen-free (Type II) or have fully aggregated nitrogen defects (Type IaB) with platelet degradation and low to moderate nitrogen contents (av. 330 ppm-N); 25% have lower nitrogen aggregation states and higher nitrogen contents (30% IaB; <1600 ppm-N).

The combined evidence suggests two generations of diamond growth. Type II and Type IaB diamonds with ultradeep, peridotitic, eclogitic and websteritic inclusions crystallised from eclogitic and peridotitic rocks while moving in a dynamic environment from the asthenosphere and possibly the lower mantle to the base of the lithosphere. Mechanisms for diamond movement through the mantle could be by mantle convection, or an ascending plume. The interaction of partial melts with eclogitic and peridotitic lithologies may have produced the intermediate websteritic inclusion compositions, and can explain diamonds of mixed parageneses, and the overlap in C-isotope values between parageneses. Strong deformation and extremely high nitrogen aggregation states in some diamonds may indicate high mantle storage temperatures and strain in the diamond growth environment. A second diamond group, with Type IaA–IaB nitrogen aggregation and peridotitic inclusions, crystallised at the base of the cratonic lithosphere. All diamonds were subsequently sampled by kimberlites and transported to the Earth's surface.     

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).     

Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds

Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations. Received: 27 April 1999 / Accepted: 1 March 2000     

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.     

Review on Lithospheric Diamonds and Their Mineral Inclusions

Diamonds and their mineral inclusions are valuable for studying the genesis of diamonds, the characteristics and processes of ancient lithospheric mantle and deeper mantle. This has been paid lots of attentions by geologists both at home and abroad. Most diamonds come from lithospheric mantle. According to their formation preceded, accompanied or followed crystallization of their host diamonds, mineral inclusions in diamonds are divided into three groups: protogenetic, syngenetic and epigenetic. To determine which group the mineral inclusions belong to is very important because it is vital for understanding the data’s meaning. According to the type of mantle source rocks, mineral inclusions in diamonds are usually divided into peridotitic (or ultramafic) suite and eclogitic suite. The mineral species of each suite are described and mineralogical characteristics of most common inclusions in diamonds, such as olivine, clinopyroxene, orthopyroxene, garnet, chromite and sulfide are reviewed in detail. In this paper, the main research fields and findings of diamonds and their inclusions were described: ①getting knowledge of mineralogical and petrologic characteristics of diamond source areas, characteristics of mantle fluids and mantle dynamics processes by studying the major element and trace element compositions of mineral inclusions; ②discussing deep carbon cycle by studying carbon isotopic composition of diamonds; ③determining forming temperature and pressure of diamonds by using appropriate assemblages of mineral inclusions or single mineral inclusion as geothermobarometry, by using the abundance and aggregation of nitrogen impurities in diamonds and by measuring the residual stress that an inclusion remains under within a diamond ; ④estimating the crystallization ages of diamonds by using the aggregation of nitrogen impurities in diamonds and by determine the radiometric ages of syngenetic mineral inclusions in diamonds. Genetic model of craton lithospheric diamonds and their mineral inclusion were also introduced. In the end, the research progress on diamonds and their inclusions in China and the gap between domestic and international research are discussed.