A. I. Chepurov

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.

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 action of iron particles at catalyzed hydrogenation of {100} and {110} faces of synthetic diamond

Abstract Results of treatment of {100} and {110} faces of synthetic diamond with dispersed iron at catalyzed hydrogenation are presented. The experiments were held at a temperature of 900 °C in flow hydrogen medium. The regular behavior of iron particles on the surface of diamond crystals was established. It was found that breaking of –C–C- bonds in diamond during its dissolution in the metal-catalyst is the limiting stage of the process.

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�

Volatile compounds of sulfur in the Fe-C-S system at 5.3 GPa and 1300°C

This report presents the results of experimental studies of the fluid phase in the Fe-C-S system at high P and T values (5.3 GPa and 1300°C) conforming to diamond synthesis. The samples for experiments were mounted on air; therefore, the volatile compounds detected after the experiments are characterized by a wide variety and complicated composition involving both inorganic and organic components. Among the inorganic compounds, CO2, H2O, N2, SO2, CS2, and COS were detected. The GC/MS analysis revealed hydrocarbons (paraffins, olefins, and arenes), including high-molecular compounds. The formation of heavy hydrocarbons confirms their thermodynamic stability under high pressure. Oxygenated hydrocarbons (alcohols, aldehydes, ketones, carboxylic acids, and ethers) were also detected.

Effect of Sulfur Concentration on Diamond Crystallization in the Fe-C-S System at 5.3-5.5 GPa and 1300-1370°C

The paper presents results of experiments aimed at diamond synthesis in the Fe–C–S system at 5.3–5.5 GPa and temperatures of 1300–1370°C and detailed data on the microtextures of the experimental samples and the composition of the accompanying phases (Fe3C and Fe7C3 carbides, graphite, and FeS). It is demonstrated that diamond can be synthesized after temperatures at which carbides are formed are overcome and can crystallize within the temperature range of 1300°C (temperature of the peritectic reaction melt + diamond = Fe7C3) to 1370°C (of thermodynamically stable graphite) under the appearance experimental pressure. The possible involvement of natural metal- and sulfur-bearing compounds in the origin of natural diamond is discussed.

Nitrogen Incorporation in Octahedral Diamonds Grown in the Fe-Ni-C System

The extension of the experimental database on the formation and transformation of nitrogen defects in diamond [1–7] provides a basis for the interpretation of the real features of this widespread structural impurity in natural diamonds. At the same time, a number of questions about nitrogen and conditions of crystal growth remain poorly understood even in simple model systems. In particular, the influence of growth rate, which depends on supersaturation, on the intensity of incorporation of single nitrogen atoms in octahedral diamonds synthesized in the Fe–Ni–C system is not still determined. Based on the analysis of differences in the content of nitrogen defects in various growth sec tors, Satoh et al. [3] suggested that low growth rates are favorable for the entrapment of nitrogen. Kiflawi et al. [8] performed IR spectroscopy and also observed a decrease in nitrogen content in response to a short term increase in growth rate, which was interpreted as the result of the mutual influence of admixtures (N and Ni) on the growth surface. On the other hand, a comparison of the total contents of nitrogen defects in adjoining octahedral growth sectors formed at different growth rates did not reveal any clear tendency [9]. Additional capability for the elucidation of this problem was pro vided by the detection of positively charged single sub stitutional nitrogen N+ in the diamond structure [10] and information that nitrogen in such a state can be used as a peculiar qualitative indicator of supersatura tion and, correspondingly, growth rate [11]. This is pos sible, because part of nitrogen transformed into N+ serves as a charge compensator for substitutional nickel in the diamond structure [12, 13], the intensity of entrapment of which is known to depend on growth rate [8, 14, 15]. It is obvious that, in the case of the existence of any relation between the intensity of nitrogen incor poration in the diamond structure and the growth rate, a similar (positive or negative) relation must be observed between bulk nitrogen and N+ contents. Therefore, in order to establish the relation between the incorpora tion of single nitrogen and growth rate, we used IR microspectroscopy to evaluate the correlation between total nitrogen and N+ contents for synthetic diamonds grown in the nickel bearing metal–carbon (Fe–Ni–C) system.

Interaction of diamond with ultrafine Fe powders prepared by different procedures

We have studied the interaction of synthetic diamond crystals with ultrafine Fe powders during catalytic diamond gasification in a hydrogen atmosphere at 900°C. The Fe powders were prepared by three procedures: reduction of Fe2O3 nanopowder; evaporation using an ELV-6 electron accelerator, followed by condensation; and reduction of ferric chloride. The surface-processed diamond crystals were examined by electron microscopy. The results indicate that ultrafine powders produced by the first two procedures cause predominantly lateral etching of diamond. The Fe particles prepared by the third procedure penetrate into the bulk of diamond crystals and produce etch pits and “tunnels,” thereby markedly increasing the specific surface area of the crystals.

On the formation of element carbon during decomposition of CaCO3 at High P-T parameters under reducing conditions

This work presents data on the experimental study of CaCO3 stability (3.0–5.5 GPa; 1300–1400°C) under reducing conditions modeling the presence of metallic iron. It is established that CaCO3 is stable at the above P-T parameters under reducing conditions (in the presence of metallic Ti). CaCO3 decomposed only when it chemically interacted with iron, forming Ca-ferrites and releasing solid carbon in the form of graphite in the closed system (in sealed Pt-ampoules).

Migration of molten iron through an olivine matrix in the presence of carbon at high P–T parameters (experimental data)

This report presents the primary results of experiments at 5.5 GPa and 1600°C on the migration of molten iron through a solid silicate matrix consisting of olivine crystals with graphite-filled interstices. It was shown that the migration rate was quite high at the given P and T values (up to 2.5 mm/h). It was shown that carbon in its elemental forms might affect the migration capability of molten iron. This effect must be included in creating a correct model of the formation of the inner parts of the Earth, the terrestrial planets, and planetoids.