Yury N. Palyanov

Fluid regime of diamond crystallisation in carbonate-carbon systems

The gaseous phase in experiments of diamond crystallisation in the carbonate-carbon systems Li 2 CO 3 -C, Na 2 CO 3 -C, K 2 CO 3 -C, CS 2 CO 3 -C, CaCO 3 -C, CaMg(CO 3 ) 2 -C at P = 7 GPa and T =1 700–1750°C (using the “split-sphere” high-pressure device) was studied by means of gas chromatography. Unlike in traditional metal-carbon systems, in which diamond forms under highly reducing conditions in the presence of a methane-hydrogen fluid, crystallisation of diamond in carbonate-carbon systems occurs in the presence of a H 2 O-CO 2 -rich fluid. The results provide experimental confirmation of the possibility for diamond to crystallise in nature in a wide range of redox conditions.

Conditions of diamond formation through carbonate-silicate interaction

The formation of elemental carbon (diamond or graphite) via the carbonate-silicate interaction in MgCO 3 -SiO 2 , CaMg(CO 3 ) 2 -SiO 2 , and MgCO 3 -(SiO 2 +Al 2 O 3 ) Systems has been experimentally studied at 5.2–7.5 GPa and 1200 to 1800°C. A special high-pressure cell in which a source of hydrogen (TiH 1.9 ) was placed outside the Pt-capsule containing a carbonate-silicate mixture, was used. As the result of the carbonate-silicate interaction, carbon of carbonates was converted into diamond in association with enstatite+coesite+magnesite, forsterite+enstatite+magnesite, coesite+diopsite+dolomite, and magnesite+coesite+pyrope. The main parameters governing the process of diamond formation via the carbonate-silicate interactions are the temperature, the pressure, and the oxygen fugacity. Depending on the temperature, diamond and graphite crystallized in either subsolidus fluid or melt. According to the gas chromatographic data, fluid composition during diamond crystallization varied from almost pure CO 2 to aqueous. Under the experimental conditions corresponding to the field of thermodynamic stability of diamond, the increase in the temperature caused evolution of the processes of carbon formation, from crystallization of metastable graphite to nucleation of diamond.

High-temperature calibration of a multi-anvil high pressure apparatus

Fusion and solidification of Al and Ag samples, as well as Fe93–Al3–C4, Fe56–Co37–Al3–C4, and Fe57.5–Co38–Al1–Pb0.5–C3 alloys (in wt%), have been investigated at 6.3 GPa. Heater power jumps due to heat consumption and release on metal fusion and solidification, respectively, were used to calibrate the thermal electromotive force of the thermocouple against the melting points (mp) for Ag and Al. Thus, obtained corrections are +100°C (for sample periphery) and +65°C (center) within the 1070–1320°C range. For small samples positioned randomly in the low-gradient zone of a high pressure cell, the corrections should be +80°C and +84°C at the temperatures 1070°C and 1320°C, respectively. The temperature contrast recorded in the low-gradient cell zone gives an error about ±17°C. The method has been applied to identify the mp of the systems, which is especially important for temperature-gradient growth of large type IIa synthetic diamonds.

An experimental demonstration of diamond formation in the dolomite-carbon and dolomite-fluid-carbon systems

The results of a study of diamond crystallisation in dry and fluid-rich dolomite-carbon systems are presented. For the dry system an induction period preceding spontaneous diamond nucleation was found of about 4 h at 7 GPa, 1700 °C. No diamonds were observed after 42 h of reaction at 5.7 GPa, 1420 °C. Adding H 2 O and H 2 O-CO 2 fluids to the dolomite-carbon system resulted in spontaneous diamond nucleation at 1420 °C, and growth of diamond on seed crystals at 1300–1420 °C. In the presence of H 2 O or H 2 O-CO 2 fluids, dolomite decomposes to dolomite solid solution + brucite + aragonite. Results of the experiments indicate that dolomitic melts in the mantle, enriched in H 2 O and CO 2 , promote the formation of natural diamond.

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.

Diamond crystallization from an Mg–C system under high pressure, high temperature conditions

Diamond nucleation and growth in the magnesium–carbon system were studied at a pressure of 7 GPa and temperatures in the range of 1500–1900 °C. To explore the effects of kinetics in diamond crystallization processes the duration of experiments was varied from 5 min to 20 h. It was established that the induction period preceding diamond nucleation decreased with increasing temperature from about 17.5 h at 1500 °C to almost zero at 1900 °C, while the rate of diamond growth increased by almost three orders of magnitude, from 10 μm h−1 (1500 °C) to 8.5 mm h−1 (1900 °C). The cubic morphology was found to be the stable growth form of diamond over the entire range of temperatures used in the study. Based on the data obtained it was suggested that diamond growth in the Mg–C system took place in the kinetically controlled regime. Spectroscopic characterization revealed that the synthesized diamond crystals contained boron and silicon impurities. A specific continuous absorption, giving rise to the abundant brown coloration of the produced crystals, and a band at about 1480 cm−1 found in the Raman spectra were tentatively assigned to defects involving π-bonded carbon atoms.

High-pressure synthesis and characterization of diamond from an Mg–Si–C system

Diamond crystallization in the Mg–Si–C system has been studied at high-pressure high-temperature conditions of 7 GPa and 1500–1900 °C. The features of nucleation and growth of diamond from the carbon solution in the Mg–Si melt are established. The degree of the graphite-to-diamond transformation is found to depend significantly on the crystallization temperature. As opposed to the pure Mg–C system where the cubic morphology dominates, the octahedron with the antiskeletal structure of faces is the dominant form of growth in the Mg–Si–C system over the entire temperature range. The possibility of epitaxial growth of silicon carbide tetrahedral crystals on diamond upon their co-crystallization was noted. Synthesized diamonds are found to contain optically active silicon-vacancy (Si-V) centers and inactive substitutional silicon defects, giving rise to the 1.68 eV system in the photoluminescence spectra and an absorption peak at 1338 cm−1 in the infrared absorption spectra, respectively.

The role of water in generation of group II kimberlite magmas: Constraints from multiple saturation experiments

Abstract Multiple saturation experiments have been performed in a multicomponent system at 6.3 to 7.5 GPa and 1400-1670 °C using a split-sphere multi-anvil apparatus to constrain the conditions of kimberlite magma generation. The starting bulk compositions of samples corresponded to the average group II kimberlite (orangeite), with water contents varying from 5 to 9 wt% H2O and the CO2/(CO2+H2O) molar ratio from 0.37 to 0.24. The charges were placed inside graphite liners sealed in Pt capsules to avoid Fe loss. Oxygen fugacity (fO2) during the experiment was buffered by the equilibrium between graphite and a hydrous carbonate-silicate melt about EMOG/D. As water in the starting kimberlite increased from 5 to 9 wt%, the temperature of its complete melting became ~100 °C lower (relative to 1670 °C), both in the 6.3 and 7.5 GPa runs. Orthopyroxene was stable just below the liquidus at all pressures and H2O concentrations applied in the experiments. An olivine + garnet + orthopyroxene assemblage was present at ≤100 °C below the liquidus when H2O was 5 wt%. At 7 and 9 wt% H2O, the same assemblage appeared at 100-150 and >200 °C below the liquidus, respectively. In no experiment was clinopyroxene observed as a run product. Olivine, garnet, and orthopyroxene stable in the multiply saturated melt were compositionally similar to mantle peridotite minerals found as xenoliths in kimberlites worldwide. Thus we infer that generation of group II kimberlite magma may occur by partial melting of carbonated (metasomatized) garnet harzburgite at pressures from 6.3 to 7.5 GPa, temperatures about 1500-1600 °C, and no more than 5 wt% H2O in the melt. Water, in the amounts required to produce this magma, may come from interaction of K-Ca-rich carbonatite melt, infiltrating from a deeper mantle source, with a peridotite protolith containing H2O in nominally anhydrous minerals and, possibly, also in phlogopite.

Carbon and Nitrogen Speciation in N-poor C-O-H-N Fluids at 6.3 GPa and 1100–1400 °C

Deep carbon and nitrogen cycles played a critical role in the evolution of the Earth. Here we report on successful studying of speciation in C-O-H-N systems with low nitrogen contents at 6.3 GPa and 1100 to 1400 °C. At fO2 near Fe–FeO (IW) equilibrium, the synthesised fluids contain more than thirty species. Among them, CH4, C2H6, C3H8 and C4H10 are main carbon species. All carbon species, except for C1-C4 alkanes and alcohols, occur in negligible amounts in the fluids generated in systems with low H2O, but С15-С18 alkanes are slightly higher and oxygenated hydrocarbons are more diverse at higher temperatures and H2O concentrations. At a higher oxygen fugacity of +2.5 Δlog fO2 (IW), the fluids almost lack methane and contain about 1 rel.% C2-C4 alkanes, as well as fractions of percent of C15–18 alkanes and notable contents of alcohols and carboxylic acids. Methanimine (CH3N) is inferred to be the main nitrogen species in N-poor reduced fluids. Therefore, the behaviour of CH3N may control the nitrogen cycle in N-poor peridotitic mantle. Oxidation of fluids strongly reduces the concentration of CH4 and bulk carbon. However, higher alkanes, alcohols, and carboxylic acids can resist oxidation and should remain stable in mantle hydrous magmas.

Distribution of light alkanes in the reaction of graphite hydrogenation at pressure of 0.1–7.8 GPa and temperatures of 1000–1350°C

ABSTRACT We studied the effect of pressure and temperature on the hydrocarbon (HC) chain length distribution and total amount of HCs in the reaction of direct graphite hydrogenation at pressures of 0.1–7.8 GPa and temperatures of 1000–1350°C. An increase in pressure was found to lead both to an increase in the absolute yield of HCs due to direct graphite hydrogenation and to chain elongation of HC products. Light alkanes predominate among HCs in the entire studied range of P–T parameters. However, their concentration in quenched fluids increases as pressure is elevated, from less than 10 rel.% at 0.1 GPa to more than 40–50 rel.% at P ≥ 3.8 GPa. Methane is actually the only light alkane among reaction products at 0.1 GPa and 1000°C, while it is a minor component at 7.8 GPa and 1350°C. The most stable alkane at pressures above 3.8 GPa is ethane (C2H6).