Granulites, CO2 and graphite

Abstract

Externally derived, pure CO2 that mixes with a carbon-(under)saturated C-O-H-fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (fO2rock) is below the upper fO2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO2 fluid into a dry granulite (C-O fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO2 infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO2 → CO + ½ O2. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a C-O-H fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a C-O-H fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO2 infiltration requires detailed information on the P-T-fO2rock conditions, the relative amount of CO2 that infiltrates into the rock, and whether H2O is present or not.