The metamorphism of granulites and devolatilization of the lithosphere

Abstract

Fluids in the lithosphere are treated as mixtures of H2O, CO2, CH4, CO, H2, 02. The following three conditions have been utilised to determine the partial pressures of these constituents: (1) a fugacity of H2 as indicated by biotite feldspar magnetite fluid assemblages with mineral compositions representative of granulites, (2) oxygen fugacity as specified by the quartz fayalite magnetite fluid reference assemblage, and (3) graphite saturation. Fluid pressure equal to lithostatic pressure has not been assumed in the calculation in order to derive the relationship between fluid pressure and lithostatic pressure on the basis of mineral compositions. For oxidation levels along the quartz fayalite magnetite fluid reference assemblage, there are significant segments above 500oC along pressure temperature time paths where calculated fluid pressure exceeds lithostatic pressure, and devolatilization is predicted for graphite-saturated rocks containing typical biotite solidsolutions. At lower oxygen fugacities, the condition for devolatilization requires higher temperature for a given depth; at higher oxygen fugacities expected in some granulite-facies rocks (since orthopyroxene is more stable under these conditions than fayalite), the region of devolatilization increases by expanding to lower temperatures for a given depth. Devolatilization of the lower crust is predicted by these calculations, with the hydrogen reservoir (held in hydrous silicate minerals) and the carbon reservoir (excess graphite) being depleted in the process. The commonly observed CO2-rich fluid inclusions in granulites are consistent with the type of residual fluids expected from the process described above.