ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
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 (FexO-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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00324556
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