The graphite-COH fluid equilibrium in P, T, $f_{O_2 }$ space

Abstract

The oxygen fugacity ($f_{O_2 }$) of a C-O-H fluid in equilibrium with graphite has been determined in the range 10–30 kbar by equilibrating solid $f_{O_2 }$-buffer assemblages in graphite capsules containing C-O-H fluid. By using different buffers (Fe_xO-Fe₃O₄, Ni-NiO, Co-CoO, Mo-MoO₂), the $f_{O_2 }$ of the graphite-saturated fluid is bracketed within a narrow range. This technique produces a calibration for the $f_{O_2 }$ imposed on a sample contained within a graphite capsule. To achieve a thermodynamically-invariant system at fixed P and T, the $f_{H_2 }$ was imposed on the system with an external buffer and the double-capsule technique. The experiments were performed in solid-media, high pressure apparatus with 19 mm tale-pyrex assemblies. A series of experiments at 10, 15, 20, 25, and 30 kbar, 800–1600° C, with $f_{H_2 }$ imposed by the Fe₂O₃-Fe₃O₄-H₂O equilibrium were conducted. The experimental results have been fitted to the following equation:

$$\log f_{O_2 } = \frac{{ - 22324 + 189 \cdot P - 1.41 \cdot P^2 }}{T} + 4.62$$

(T in K, P in kbar). This equation reproduces the experimental results to ±0.1 log $f_{O_2 }$. The results show that the experimentally-determined $f_{O_2 }$ of the graphite-COH fluid equilibrium is close to the equilibrium calculated by combining thermodynamic data with a modified Redlich-Kwong (MRK) or virial-type equation of state for the derivation of the fugacity coefficients. At P≥20 kbar, the virial-type equation (Saxena and Fei 1987, 1988) reproduces the experimental results better than does the MRK, which produces a difference of 0.8 log $f_{O_2 }$ at 800° C and 30 kbar.