Electrochemical measurements and thermodynamic calculations of redox equilibria in pallasite meteorites: Implications for the eucrite parent body

The intrinsic oxygen fugacity (IOF) of olivine separates from the Salta, Springwater, and Eagle Station pallasites was measured between 850 and 1150°C using oxygen-specific solid zirconia electrolytes at 10 ⁵ Pa. The experimental apparatus consisted of a double-opposed electrolyte configuration with a CO-CO ₂ gas-mix bridging atmosphere. Four initial IOF measurements on Salta olivines revealed the effects of grain size and cell memory on experimental results; experiments with an IW cell memory and fine grain size (<45 μm) gave results most consistent with thermodynamic calculations. The IOF experimental results for olivine separates from Salta, Springwater, and Eagle Station fall within one log unit of the iron-wüstite oxygen buffer (IW). While these measurements may provide only an upper limit to the IOF of the pallasites, they correlate with the compositions of the olivine and metal in these samples: Salta, the most reduced pallasite studied (IW - 0.5 logfO₂), contains Fa _12.5 and Fe _90.5Ni _9.0; Springwater, of intermediate redox state (~IW), contains Fa _18.0 and Fe _87.5Ni _12.1; Eagle Station, the most oxidized sample ( IW + 0.5 logfO₂), contains Fa _20.5 and Fe _85.0Ni _14.6. Electron microbeam characterization of the starting materials and run products from these experiments have shown that olivine is the only phase present. Thermodynamic calculations of redox equilibria involving equilibrium pallasite assemblages are in good agreement with our experimental results and provide a lower limit to pallasite redox stability; others involving disequilibrium assemblages, suggest that pallasites experienced localized, late-stage oxidation and reduction effects. Consideration of the redox buffer metal-olivine-orthopyroxene utilizing calculated Eucrite Parent Body (EPB) mantle phase compositions indicates that small redox gradients may have existed in the EPB. Such gradients may have produced strong compositional variation within the EPB. In addition, there is apparently significant redox heterogeneity in the source area of Eagle Station Trio (EST) pallasites and Bocaiuva iron meteorites.