Diamond formation in the system MgO-SiO2-H2O-C at 7.5 GPa and 1,600 degrees C

Diamond crystallization has been studied in the SiO2-H2O-C, Mg2SiO4-H2O-C and H2O-C subsystems at 7.5 GPa and 1,600 degrees C. We found that dissolution of initial graphite is followed by spontaneous nucleation of diamond and growth of diamond on seed crystals. In 15-h runs, the degree of graphite to diamond transformation [alpha = M-Dm/(M-Dm + M-Gr) 100, where M-Dm is mass of obtained diamond and MGr mass of residual graphite] reached 100% in H2O-rich fluids but was only 35-50% in water-saturated silicate melts. In 40-h runs, an abrupt decrease of a has been established at the weight ratio H2O/(H2O + SiO2) <= 0.16 or H2O/(H2O + Mg2SiO(4)) <= 0.15. Our results indicate that a is a function of the concentration of water, which controls both the kinetics of diamond nucleation and the intensity of carbon mass transfer in the systems. The most favorable conditions for diamond crystallization in the mantle silicate environment at reliable PT-parameters occur in the fluid phase with low concentration of silicates solute. In H2O-poor silicate melts diamond formation is questionable.