Diamond Crystallization in Fluid and Carbonate-Fluid Systems under Mantle P-T Conditions:2.An Analytical Review of Experimental Data

摘要

This paper discusses recent experimental data on diamond crystallization in fluid(CO_2-C,H_2O-CO_2-C,H_2O-C,CH_4-H_2O-C,and CH_4-H_2-C)and carbonate-fluid(K_2CO_3-H_2O-CO_2-C,Na_2O-H_2-CO_2-C,and CaMg(CO_3)_2-H_2O-CO_2-C)systems under the thermodynamic conditions of diamond stability.The intensity of diamond crystallization under 5.7-7.7 GPa and 1150-2000 deg C is a function of the temperature,the composition,and probably the pressure of the system.Spontaneous diamond nucleation and growth on seed crystals is preceded by an inductive period,whose duration increases from several minutes to hundreds of hours as the temperature decreases from 2000 to 1150 deg C.The stable habit of the crystallizing diamond is octahedral.In terms of the experimental intensity of diamond crystallization,these fluid and fluid-carbonate systems are set in the following order:K_2CO_3(Na_2CO_3)-H_2O-CO_2-C > CO_2-C = H_2O-CO_2-C = H_2O-C = CaMg(CO_3)_2-H_2O-CO_2-C > CH_4-H_2O-C > CH_4-H_2-C.Metastable graphite can crystallize in them under 5.7 GPa and 1150-1420 deg C and under 7.7 GPa and 1400-1600 deg C.Our data show that,under upper mantle P-T conditions,C-O-H fluids and fluids with a more complex composition(with alkali and alkali-earth carbonates)can serve both as a carbon source and as a natural medium of diamond crystallization.