EXPERIMENTAL MEASUREMENTS OF THE FUGACITY OF CO2 AND GRAPHITE/DIAMOND STABILITY FROM 35 TO 77 KBAR AT 925 TO 1650°C

Abstract

The fO2 of the equilibrium between graphite and C-O fluid has been determined at 35–77 kbar and 925-1650°C using metal-metal oxide reactions as redox sensors and by employing an assemblage of Co-Pd alloy and CoO as a sliding redox sensor. In the latter case, the equilibrium composition of the Co-Pd alloy was approached from two starting compositions of initially higher and lower Co contents. The measurements translate into measurements of the fugacity of CO2 with uncertainties in the range 0.13–0.20 log units.Our determinations of fCO2 show reasonable, but not perfect agreement, with the equations of state of Saxena and Fei (1987) and Sterner and Pitzer (1994). Modified forms of the Redlich-Kwong equation of state (Holloway, 1977; Flowers, 1979; Kerrick and Jacobs, 1981) predict values of fCO2 that, in some parts of P, T space, are over 1 log unit higher than those determined experimentally. An equation of state for CO2 has been parameterised using our new measurements and existing low pressure data (Kennedy and Holser, 1966; Shmonov and Shmulovich, 1974). The equation of state is suitable for the calculation of CO2 properties in the range 1 bar to 77 kbar and 925–1650°C.