Fluid regime and diamond formation in the reduced mantle: Experimental constraints |
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| Authors: | Alexander G Sokol Galina A Palyanova Yury N Palyanov Anatoly A Tomilenko Vasiliy N Melenevsky |
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| Institution: | aInstitute of Geology and Mineralogy, Russian Academy of Sciences, Siberian Branch, Koptyug Pr., 3, Novosibirsk 630090, Russian Federation |
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| Abstract: | The composition and potential diamond productivity of C–O–H fluids that could exist in the reduced regions of the Earth’s upper mantle and in the mantles of Uranus and Neptune were studied in experiments at 6.3 GPa and 1400–1600 °C and durations of 15–48 h. Hydrogen fugacity in the fluid phase was controlled by the Mo–MoO2 or Fe–FeO buffers, using a specially modified double-capsule method. The oxygen fugacity in the samples was controlled by adding different amounts of water, stearic acid, anthracene, and docosane to a graphite charge. At high P–T conditions, the degree of decomposition of the heavy hydrocarbons added to the charge was  99.9%. The composition of the fluids coexisting with graphite/diamond in the buffered experiments varied from H2O  H2 > CH4 (at fO2 somewhat lower than the “water maximum”) to H2 > CH4 > (C2H4 + C2H6)>C3H8 (in C–H system). In the C–H system the maximum concentrations of major species in the synthesized fluid were: H2 = 79 mol.% and CH4 = 21 mol.%. The composition of the H2-rich fluids, which were synthesized at 6.3 GPa and 1400–1600 °C for the first time, differs considerably from that of the ultra-reduced CH4-rich fluids stable at 2.0–3.5 GPa and 1000–1300 °C. Thermodynamic calculations of the reduced C–O–H fluids at the P–T conditions of the experiments revealed CH4-rich compositions (CH4 H2 > (C2H4 + C2H6)>C3H8), which however drastically differed from the synthesized compositions. The rates of diamond nucleation and growth in the experiments depended on the fluid composition. Diamond crystallization had a maximum intensity in the pure aqueous fluids, while in the H2-rich fluids no diamond formation was observed. Only metastable graphite precipitated from the ultra-reduced fluids. The type of the initial hydrocarbon used for the fluid generation did not affect this process. |
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