Experimental measurements of the fugacity of CO₂ and graphite/diamond stability from 35 to 77 kbar at 925 to 1650°C

The f_O2 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 f_CO2 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 f_CO2 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.