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

The f(O2) of the equilibrium between graphite and C-O fluid has been determined at 35-77 kbar and 925-1650 degrees 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 letter 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 Fitter (1994). Modified forms of the Redlich-Kwong equation of state (Holloway, 1977; Flowers, 1979; Kenick 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 degrees C. Copyright (C) 1997 Elsevier Science Ltd.