A. A. Tomilenko

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.

Is quartz a potential indicator of ultrahigh-pressure metamorphism? Laser Raman spectroscopy of quartz inclusions in ultrahigh-pressure garnets

Laser Raman microspectroscopy was applied to quartz inclusions in coesite- and diamond-grade metapelites from the Kokchetav ultrahigh-pressure metamorphic (UHPM) complex, Northern Kazakhstan, and diamond-grade eclogite xenoliths from the Mir kimberlite pipe, Yakutiya, Russia to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnets have a higher frequency shift than those in the matrix. Residual pressures retained by quartz inclusions depend on the metamorphic history of the garnet host. The Raman frequency shift of quartz inclusions in garnet from coesite-grade and diamond-grade metamorphic rocks shows no systematic change with increasing peak metamorphic pressures. The highest shifts of the main Raman bands of quartz were documented for monocrystalline quartz inclusions in garnets from a diamond-grade eclogite xenolith. Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1–0.6 GPa for quartz inclusions from coesite-grade metapelites from Kokchetav, 0.1–0.3 GPa for quartz inclusions from diamond-grade metapelites from Kokchetav, and 1.0–1.2 GPa for quartz inclusions from the diamond-grade eclogite xenoliths from the Mir kimberlite pipe. Normal stresses and internal (residual) pressures of quartz inclusions in garnet were numerically simulated with a 3-shell elastic model. Estimated values of residual pressures are inconsistent with the residual pressures estimated from the frequency shifts. Residual pressure slightly depends on P–T conditions at peak metamorphic stage. Laser Raman microspectroscopic analysis of quartz is a potentially powerful method for recovering an ultrahigh pressure metamorphic event. Monocrystalline quartz inclusions yielding a residual pressure greater than 2.5 GPa might indicate the presence of a former coesite.

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.

Synthesis of heavy hydrocarbons under P-T conditions of the Earth’s upper mantle

32 The Earth’s mantle, especially the upper mantle, is highly stratified according to the oxidation–reduction conditions [1]. The main trend of variations in physi cochemical conditions recorded in the mantle is a decrease in oxygen fugacity with depth. According to recent concepts, at a depth of 150 km the zone of sta bility of the oxidized form of carbon (carbonates) is followed by a stability zone of elemental carbon forms (graphite/diamond) [2]. At a depth of approximately 250 km, the oxida tion–reduction conditions of the upper mantle corre spond to the stability of metallic iron [3, 4]. Given these data, in the framework of the conceptual model of the global carbon cycle [5], the probability of decomposition of carbonates entering into the mantle in the subduction zones with formation of solid carbon phases (graphite/diamond) and volatile hydrocarbon compounds is of interest. The formation of the latter is especially acute for scientists considering the possibility of their abiotic origin. During experimental studies [6–9], attempts were made to synthesize hydrocarbons from СаСО3 in aqueous media under the P–T conditions of the Earth’s upper mantle. As reducing agents, FeO or metallic Fe were used. Experiments were carried out using different types of high pressure apparatuses. In all cases, СаСО3 was decomposed. In the gas phase, СН4 was recorded. In addition, a mixture of hydrocarbons corresponding to the hydrocarbon portion of natural gas in chemical composition was produced [7, 9]. However, heavy hydrocarbons (HHCs), components of crude oil, were not synthesized at the P–T conditions of the Earth’s upper mantle. This work provides the first results of HHC synthe sis from the carbonate material at high P–T parame ters. A series of three experiments was carried out using the multi anvil high pressure split sphere appa ratus (BARS) in a working cell made of ZrO2 and MgO with a tubular graphite heater under the following mode: pressure buildup, sample heating, and cooling and pressure release after holding under the given P–T conditions. The research procedure is given in works [9, 10]. Experiments were carried out in Pt ampoules sealed by welding with an electric arc in the open air. The first experiment (no. 4 6 13) was carried out for 17 hours in the following system: СаCO3 (140.3 mg)–Ca(OH)2 (10.1 mg)–Femetal (10.5 mg) at P = 4.5 GPa and T = 1600°C. The sample cooling was performed by interruption of the power supply to the high pressure apparatus. The second experiment (no. 4 8 13) was carried out for 5 hours in the system MgCO3 (15.4 mg)– Ca(OH)2 (14.3 mg)–Femetal (51.9 mg)–SiO2 (20.8 mg) at P = 4.0 GPa and T = 1400°C. The sample was cooled to room temperature under high pressure for 14 hours. In this experiment, the Pt ampoule was coated with 0.05 mm W foil (132.6 mg). The third experiment (no. 4 10 13) was carried out for 24 hours in the sys tem MgCO3 (7.7 mg)–Ca(OH)2 (7.0 mg)–Femetal (26.0 mg)–SiO2 (11.1 mg) at P = 3 GPa and T = 1400°C with subsequent cooling by quenching. For the third experiment, crushed and mixed com ponents were put into an ampoule (height of 6 mm, a diameter of 5.0/3.3 mm, weight of 328.4 mg), which, in turn, was inserted into the Pt ampoule. Since the experiment was conducted in the closed system (to preserve volatile products) titanium was used as an oxygen scavenger, forming as a result of reduction of СО2 and Н2О. After the experiments, volatile components in the samples were analyzed by combined gas chromatogra phy mass spectrometry. Pt ampoules were opened with a punch in a special device put into the gas chro matograph circuit ahead of the analytical column, which was heated in the carrier gas (He, purity 99.999%) flow at temperature of 140°С for 90 min utes. The gas mixture was analyzed using a Thermo Synthesis of Heavy Hydrocarbons under P–T Conditions of the Earth’s Upper Mantle

The composition of melt and fluid inclusions in spinel of peridotite xenoliths from Avacha volcano (Kamchatka)

This work considers the studies of melt and fluid inclusions in spinel of ultramafic rocks in the mantle wedge beneath Avacha volcano (Kamchatka). The generations of spinel were identified: 1 is spinel (Sp-I) of the “primary” peridotites, has the highest magnesium number (#0.69–0.71), highest contents of Al2O3 and lowest contents of Cr2O3 (26.2–27.1 and 37.5–38.5 wt %, respectively), and the absence in it of any fluid and melt inclusions; 2 is spinel (Sp-II) of the recrystallized peridotites, has lower magnesium number (Mg# 0.64–0.61) and the content of Al2O3 (18–19 wt %), a higher content of Cr2O3 (45.4–47.2 wt %) and the presence of primary fluid inclusions; 3 is spinel (Sp-III) that is characterized by the highest content of Cr2O3 (50.2–55.4 wt %), the lowest content of Al2O3 (13.6–16.6 wt %), and the presence of various types of primary melt inclusions. The data obtained indicate that metasomatic processing of “primary” peridotites occurred under the influence of high concentrated fluids of mainly carbonate-water-chloride composition with influx of the following petrogenic elements: Si, Al, Fe, Ca, Na, K, S, F, etc. This process was often accompanied by a local melting of the metasomatized substrate at a temperature above 1050°C with the formation of melts close to andesitic.

Unique Compositional Peculiarities of Olivine Phenocrysts from the Post Flood Basalt Diamondiferous Malokuonapskaya Kimberlite Pipe, Yakutia

ISSN 1028334X, Doklady Earth Sciences , 2015, Vol. 463, Part 2, pp. 828–832.

The composition of volatile components in olivines from Yakutian kimberlites of various ages: Evidence from gas chromatography–mass spectrometry

The composition of volatiles from fluid and melt inclusions in olivine phenocrysts from Yakutian kimberlite pipes of various ages (Olivinovaya, Malokuonapskaya, and Udachnaya-East) were studied for the first time by gas chromatography–mass spectrometry. It was shown that hydrocarbons and their derivatives, as well as nitrogen-, halogen-, and sulfur-bearing compounds, played a significant role in the mineral formation. The proportion of hydrocarbons and their derivatives in the composition of mantle fluids could reach 99%, including up to 4.9% of chlorineand fluorine-bearing compounds.

Problem of water in the upper mantle: Antigorite breakdown

The subducting oceanic crust has a heterogeneous composition but mainly is composed of a mixture of anhydrous dolerite and gabbro with mafic green schist (albite + epidote + chlorite + actinolite) and amphib� olite [1]. An increase in pressure and temperature results in rock dehydration in the subducting oceanic crust [2]. As this takes place, dehydration proceeds successively depending on phase transitions in hydrous phases (chlorite, lawsonite, amphibole, phengite, zoisite–clinozoisite, and others). The sub� ducting oceanic crust contains >5 wt % Н2О at the ini�

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.