V. S. Shatsky

Evidence of fluid inclusions in metamorphic microdiamonds from the Kokchetav massif, northern Kazakhstan

Abstract Microdiamonds from garnet clinopyroxenites of the Kokchetav massif (northern Kazakhstan) and associated alluvial diamonds have been investigated using infrared spectroscopy. All diamonds have high nitrogen contents (from 747 up to 4488 ± 20% at. ppm) and are of type Ib-IaA. Nitrogen data are consistent with a diamond formation over a narrow temperature range. Microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions, similar to those ocurring in fibrous coated diamonds from kimberlite, which suggest that diamonds grew from a C-H-O fluid. In contrast, alluvial microdiamonds do not contain any carbonate inclusions and H2O is absent or present in only minor amounts. Alluvial diamonds are considerably more N-rich but have a similar range of nitrogen aggregation states. It is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites. The Kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant Ib centres, have high nitrogen contents and show different populations depending on host rock type. These characteristics support a metamorphic origin for the microdiamonds from the UHPM rocks of the Kokchetav massif.

Carbonatitic melts in cuboid diamonds from Udachnaya kimberlite pipe (Yakutia): evidence from vibrational spectroscopy

Abstract Micro-inclusions (1 −10 μm) in 55 diamonds of cubic habit from the Udachnaya kimberlite pipe have been studied using vibrational spectroscopy. This has revealed a multiphase assemblage in cuboid diamonds from the Udachnaya kimberlite pipe. This assemblage includes carbonates, olivine, apatite, graphite, water and silicate glasses. The micro-inclusions preserve the high internal pressure and give confidence that the original materials were trapped during growth of the host diamond. The internal pressures, extrapolated to mantle temperatures, lie within the stability field of diamond and the relatively low temperatures are typical for the formation of cuboid diamonds. In contrast to previously reported data for African diamonds, the micro-inclusions in the cuboids from Udachnaya are extremely carbonatitic in composition (H2O/(H2O+CO2) ≈5−20%) with the observed assemblage of microinclusions similar to some types of carbonatites. The low water and silica content testify that the material in the micro-inclusions of the Udachnaya diamonds was near-solidus carbonatitic melt. Vibrational spectroscopy has provided the evidence of carbonatitic melts in cuboid diamonds.

The role of mantle ultrapotassic fluids in diamond formation.

Analysis of data on micro- and nano-inclusions in mantle-derived and metamorphic diamonds shows that, to a first approximation, diamond-forming medium can be considered as a specific ultrapotassic, carbonate/chloride/silicate/water fluid. In the present work, the processes and mechanisms of diamond crystallization were experimentally studied at 7.5 GPa, within the temperature range of 1,400–1,800°C, with different compositions of melts and fluids in the KCl/K2CO3/H2O/C system. It has been established that, at constant pressure, temperature, and run duration, the mechanisms of diamond nucleation, degree of graphite-to-diamond transformation, and formation of metastable graphite are governed chiefly by the composition of the fluids and melts. The experimental data suggest that the evolution of the composition of deep-seated ultrapotassic fluids/melts is a crucial factor of diamond formation in mantle and ultrahigh-pressure metamorphic processes.

The origin and formation of metamorphic microdiamonds from the Kokchetav massif, Kazakhstan: a nitrogen and carbon isotopic study

Abstract This study reports δ13C, δ15N and N-content values for microdiamonds from ultrahigh-pressure metamorphic rocks of the Kokchetav massif in Kazakhstan. Both alluvial diamonds and in-situ diamonds from a garnet–clinopyroxene rock and a marble (i.e. a garnet–pyroxene dolomitic rock) were investigated. In-situ diamonds were analysed in batches, because of their small size (average 40 μm), whereas the larger alluvial diamonds were analysed individually. The latter group has δ13C-values ranging from −15.92‰ to −10.57‰, δ15N from −1.8‰ to +1.1‰ and N-contents from 2300 to 3650 ppm. Diamonds from the garnet–clinopyroxene rock yield mean values of −10.50‰ for δ13C, +5.9‰ for δ15N and a high average nitrogen content of 11,150 ppm. Values for diamonds in marble are −10.19‰, +8.5‰ and 2650 ppm, respectively. For diamonds from garnet–clinopyroxene rock and marble, there is more nitrogen released by bulk combustion than estimated by infrared (IR) spectroscopy, the differences being of about 7000 and 1500 ppm, respectively. These differences suggest that a significant quantity of nitrogen is IR-inactive and may be present as fluid inclusions. Their carbon and nitrogen isotopic compositions are compatible with an in-situ crystallisation of diamond from dominantly metasedimentary sources, suggesting that sedimentary nitrogen can be subducted to very high pressures. Carbon isotopic fractionation between coexisting carbonate and diamond suggests crystallisation temperatures before the peak of metamorphism at temperatures probably below 700°C and deduced pressures of 3 GPa. Relative to the isotopic data reported for sediments, metasediments and in-situ diamonds, the slightly 15N-depleted compositions of alluvial diamonds is striking. These values suggest that the contribution of any metasedimentary source is unlikely and may point toward a mafic/ultramafic protolith.

Tectonic Setting and Petrology of Ultrahigh-Pressure Metamorphic Rocks in the Maksyutov Complex, Ural Mountains, Russia

The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone. The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic composition...

Garnet-biotite-clinozoisite gneiss: a new type of diamondiferous metamorphic rock from the Kokchetav Massif

Clinozoisite gneisses were studied from the Barchi-Kol area, located 17 km to the west of the Kumdy-Kol microdiamond deposit at Kokchetav massif (Northern Kazakhstan). As distinct from the deposit, the studied rocks are characterized by predominance of diamonds of octahedral habit. Ultrahigh-pressure mineral assemblages were investigated mainly as inclusions in zircons from these rocks. It is established that nucleation and growth of some zircon grains began at the peak of metamorphism (T = 950–1000°C and P > 40 kbar) and continued while temperature and pressure decreased to T = 650–750°C and P = 10–12 kbar, Ultrahigh-pressure metamorphic conditions for clinozoisite gneisses from the Barchi-Kol area are comparable with those for the Kumdy-Kol microdiamond deposit and correspond to T = 900–1000°C and P > 40 kbar. High P-T of metamorphism and bulkrock composition did not affect the morphology of the diamond crystals. The abundance of fluid or melt is proposed to be responsible for the variable extent of the completeness of cuboid reshape reaction, resulting in the formation of octahedral diamond crystals. The preservation of coesite as inclusions within zircon grains, as well as within garnets, suggests the rapid cooling and fast exhumation of the studied rocks. Based on concentration profiles in garnets from diamondiferous clinozoisite rocks, the duration of retrograde metamorphism is estimated to be less than 0.1 Ma.

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan New insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Merwinite in diamond from São Luiz, Brazil: A new mineral of the Ca-rich mantle environment

Abstract Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca2.85Mg0.96Fe0.11Si2.04O8) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the São Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO3 in the core and (Mg0.86Fe0.14)2SiO4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO3 and merwinite) and diamond at an early stage, and (Mg0.86Fe0.14)2SiO4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

Diamond Formation in UHP Dolomite Marbles and Garnet-Pyroxene Rocks of the Kokchetav Massif, Northern Kazakhstan: Natural and Experimental Evidence

Based upon detailed studies of diamondiferous metamorphic rocks, many authors share the opinion that diamonds crystallize in the field of their thermodynamic stability. Nevertheless, some problems remain, and the most important questions are as follows: (1) What is the pressure under which diamond crystallized? (2) Does the composition of diamondiferous rocks correspond to the medium of diamond crystallization? (3) Why are microdiamonds irregularly distributed in dolomite marbles and garnet-pyroxene rocks? (4) What is the role of carbonates in diamond genesis? To answer these questions, we carried out petrographic and mineralogical studies and experimentally modeled the process of microdiamond crystallization in diamondiferous garnet-pyroxene rocks and dolomite marbles. Diamondiferous marbles and garnet-pyroxene rocks occur as layers and lenses in biotite gneisses of the Kumdy-Kol microdiamond deposit, northern Kazakhstan. Mineralogical and petrographical data demonstrate that pyroxene of diamondiferous rocks differs in composition from pyroxene of nondiamondiferous rocks. The pyroxene in diamondiferous garnet-pyroxene rocks and dolomite marbles is characterized by the presence of potassium and by occurrences of lamellae of K-feldspar and phengite as well as quartz needles. No potassium is found in the pyroxene from associated nondiamondiferous rocks. Starting materials in experiments were diamondiferous marble and garnet-pyroxene rock. Experiments were carried out at P = 5.7 GPa and T = 1420° C, and at P = 7.0 GPa and 1700° C using a multi-anvil apparatus with a 300 mm outer diameter of the multi-anvil sphere. The following conclusions can be inferred from the data obtained. Unlike pyroxene in the starting specimens, the newly formed pyroxene is K-depleted, which indicates that the rocks used in the experiments differ in composition from the natural medium of diamond crystallization. The garnets synthesized in experiments with dolomite marble contain up to 4% majorite component, whereas the garnet from the initial rock contains no majorite. These data clearly show that the pressure under which dolomite marbles formed did not exceed 50 kbar. The experimental diamonds are all octahedra, whereas the diamonds in the starting samples were cubes. We believe that the main factor governing the morphology of diamond crystals is the composition of the medium of crystallization. The obtained data suggest that in dolomite marbles and garnet-pyroxene rocks, diamond crystallized from a carbonatite melt in equilibrium with a K-rich fluid.

Observation and interpretation of paramagnetic defects in Brazilian and Central African carbonados

Abstract Electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopies have been used to study Brazilian and Central African carbonados. We report the first observation of the N2V+ and N2 + nitrogen defect centers in carbonados. The fact that Brazilian carbonados contain the N3V (previously reported), N2V+, and N2+ defects, the formation of which requires complicated aggregation of nitrogen, indicates histories at elevated temperature and pressure. The observation of both the radiation-induced A1 defect, which has an annealing temperature of 700 K, and radiation defects containing two and/or five vacancies, suggest a continuous process of the formation and transformation of radiation defects in the carbonado under variable pressure and temperature conditions while in the upper mantle. The temperature at which these defects are formed varies from 300 to 1500 K. In addition we report a hydrogen-containing defect labeled H1, previously only observed in synthetic diamonds prepared by chemical vapor deposition. Together with the previously reported light carbon isotope ratio, the results of this study confirm the hypothesis that carbonados are derived from hydrocarbon source material.