Some geological implications of equilibrium between graphite and a C‐H‐O gas phase at high temperatures and pressures

Abstract

The occurrence of graphite as a common accessory mineral in meteorites and in terrestrial metamorphic and igneous rocks gives particular importance to the study of equilibrium between graphite and a coexisting gas phase. By using a simplified model in which T, Pgas, and fO2 are independently specified for the system C‐H‐O, values of PCO2, PCO, PH2O, PH2 and PCH4 in a gas phase in equilibrium with graphite have been calculated for a wide range of geologically possible conditions by means of a high‐speed computer. The numerical results support the following general conclusions: (1) The assumption that Pgas = PH2O + PCO2 is significantly in error for many graphite‐bearing mineral assemblages. (2) Methane, CH4, may be a significant to dominant constituent of the gas phase in many possible geological environments involving moderate reduction; in particular, the occurrence of graphite with reduced minerals such as fayalite, wüstite, and iron is indicative of a methane‐rich gas phase. (3) Under metamorphic conditions, pure water is not stable with graphite, but graphite can coexist with a gas phase rich in CO2. (4) Original graphite in a sediment will stabilize increasingly reduced mineral assemblages during progressive thermal metamorphism. (5) The presence or absence of even small amounts of graphite can explain PO2 gradients observed over short distances or between adjacent layers in metamorphic rocks. (6) It is possible that the terrestrial atmosphere could have evolved by conversion of original methane to water and CO2 by reaction with graphite and other accessory minerals within the primordial earth at temperatures of 600° to 1000°C. Material requirements for such a conversion are not unreasonable, and the process itself is consistent with many proposed models for the origin of the earth.