Fluid regime and diamond formation in the reduced mantle: Experimental constraints

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 degrees 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 >> H-2 > CH4 (at fO(2) somewhat lower than the water maximum) to H-2 > CH4 > (C2H4 + C2H6)>C3H8 (in C-H system). In the C-H system the maximum concentrations of major species in the synthesized fluid were: H-2 = 79 mol.% and CH4 = 21 mol.%. The composition of the H-2-rich fluids, which were synthesized at 6.3 GPa and 1400-1600 degrees 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 degrees C. Thermodynamic calculations of the reduced C-O-H fluids at the P-T conditions of the experiments revealed CH4-rich compositions (CH4 >> H-2 > (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 H-2-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. (C) 2009 Elsevier Ltd. All rights reserved.