摘要：

Equilibrium calculations and published phase equilibria are used to show that at temperatures below 400 °C and pressures above 300 bar, the fluids coexisting with graphite in the C-O-H system consist either of CO-HO mixtures or CH-HO mixtures. Bulk fluid compositions consisting of CH-HO will unmix to methane-rich fluid and HO-rich liquid at temperatures below about 325 °C. Compositions on the CO-HO join will unmix only at temperatures below about 275 °C. Oxygen fugacity in the CH-HO-graphite three-phase region is fixed (at constant P and T) and approximately equal to that of the quartz-magnetite-fayalite assemblage. In the CO-HO-graphite three-phase region, oxygen fugacity is about three orders of magnitude greater. From the end stages of diagenesis to temperatures of 300 °C (and possibly 400 °C in salt-rich systems), many metamorphic rocks may contain CH-HO fluids rather than CO-HO mixtures. Thus, metamorphic reactions involving carbonate minerals would involve CH, graphite, and HO rather than CO. The immiscibility between CHand HO could result in the common occurrence of methane (natural gas) in low-grade metamorphic terrains.

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