EXPERIMENTAL MEASUREMENTS OF THE GRAPHITE C-O EQUILIBRIUM AND CO2 FUGACITIES AT HIGH-TEMPERATURE AND PRESSURE

The f(O2) of the equilibrium between graphite and C-O fluid has been determined from 15-30 kbar and 1100-1400 degrees C using a sliding redox sensor consisting of (Ni,Mn)O + Ni metal. The equilibrium composition of oxide coexisting with metal was approached from both directions in each experiment with convergence to within 1 mol% NiO. Since, in the P-T range of the experiments, C-O fluids are >90% CO2 our measurements of f(O2) translate into determinations of CO2 fugacity with an uncertainty of +/-0.1 log units. These new determinations of the P-T-f(O2) plane of GCO equilibrium are in excellent agreement with the mainly unreversed measurements of Ulmer and Luth (1991) using pure metal-metal oxide sensors and with the equation of state of Saxena and Fei (1987). Modified forms of the Redlich-Kwong (MRK) equation of state (Holloway 1977; Flowers 1979; Kerrick and Jacobs 1981) predict higher values of f(O2) for the GCO equilibrium than determined experimentally. This implies that CO2 is more compressible than the MRK predicts.