Experiments have been performed that calibrate the stoichiometry and thermodynamics of the iron oxidation reaction in natural silicate melts. A series of experiments was carried out on six melt compositions covering a far larger range of oxygen fugacities than had been examined previously. Oxygen fugacities between air and 5.2 log10 units below those defined by the nickel-nickel oxide assemblage were investigated at 1360 °C and 1460 °C. Results of these experiments confirm that ln(XFe2O3/XFeO) is a linear function of In fO2 over this entire range, and that this linear behavior is independent of composition over the range considered. These results are inconsistent with an ideal mixing between FeO and Fe2O3 components. They are, however, entirely consistent with ideal mixing between FeO and FeO1.464±0.003 (FeO·-6Fe2O3) components. A second series of experiments was performed on a single mid-ocean ridge basalt composition (JDFD2) in order to better constrain the temperature dependence of the iron oxidation reaction in this simplified two-component subsystem. This series was carried out at temperatures between 1299 °C and 1636 °C in air, CO2, and 0.2 log10 units below the fayalite-magnetite-quartz buffer assemblage. Results of both series of experiments were combined with the Sack et al. (1980) and Kilinc et al. (1983) databases to estimate thermodynamic parameters for the iron oxidation reaction expressed in terms of FeO and FeO1.464 components. These coefficients offer the most precise method available for estimation of iron oxidation state in natural silicate melts as a function of ln fO2, temperature, and composition. Our results support the conclusion (Christie et al., 1986) that mid-ocean ridge basalts (MORBs) equilibrate as much as 3 log10 units below that defined by the nickel-nickel oxide assemblage.

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