Diamond Crystallization in Fluid and Carbonate-Fluid Systems under Mantle P-T Conditions:1.Fluid Composition

摘要

Graphite and a fluid-generating substance(silver oxalate,oxalic acid,or anthracene)were sealed in a platinum or a gold capsule and run for a long time at elevated pressure(5.7 GPa)and temperature(1200 or 1420 deg C).Both the starting materials and gases released from the capsule after the experiment were analyzed by gas chromatography.The results of the thermodynamic modeling of C-O-H fluid at 5.7 GPa and 1200-1500 deg C and at 7 GPa and 1300-1900 deg C were evaluated.The composition of the fluid generated by the decomposition of real substances was calculated.The experimental and calculated data show that the composition of CO_2 and H_2O-CO_2 fluids changed considerably during the long-term diamond crystallization experiments in the C-O-H system.The compositions of water and methane-hydrogen fluids are more stable.The compositional changes of the H_2O-CO_2 and H_2O fluids with increasing temperature from 1200 to 1420 deg C indicate a decrease in hydrogen fugacity in the material of the high-pressure cell surrounding the experimental capsules.