Electrical conductivity of San Carlos Olivine along [100] under oxygen‐ and pyroxene‐buffered conditions and implications for defect equilibria

Abstract

The electrical conductivity along [100] of single crystal San Carlos olivine was measured as a function of temperature between 1100° and 1200°C and oxygen fugacity between 10−6 and 10+0.5 Pa (at 1200°C), and either with (“pyroxene‐buffered”) or without (“self‐buffered”) an added natural pyroxene buffer from a San Carlos lherzolite. Under these temperature and ƒO2 conditions, electrical conduction in the self‐buffered sample is attributed to polarons (Fe*) and electrons (e′) and in the pyroxene‐buffered sample is attributed to polarons(Fe*) and magnesium vacancies(V″Mg). Over the range of temperature and ƒO2 investigated, the electrical conductivity of the self‐buffered sample is given by σsb[100] = 2.27(S/m)e−0.55(eV)/kT ƒO20.18+306.3(S/m)e−2.25(eV)/kT ƒO2−0.18 and for the pyroxene‐buffered sample by σpb[100] = 0.18(S/m)e−0.34(eV)/kT ƒO20.17+15.2(S/m)e−1.3(eV)/kT where k is Boltzmann's constant, T is in Kelvin, and ƒO2 is in atmospheres. The conductivity of the pyroxene‐buffered sample is lower than that of the self‐buffered sample, primarily as a result of a decrease in the polaron concentration. the electrical conductivity of both samples was found to decrease irreversibly once the samples experienced an oxygen fugacity more reducing than approximately the wüstite‐magnetite buffer. Electron microprobe analyses indicate that this effect results from loss of iron from the olivine samples to the iridium electrodes. A series conduction model based on the observed compositional gradient adequately accounts for the magnitude of the irreversible conductivity decrease and limits the thickness of any surficial pyroxene phase to <0.1 μm. Mantle temperature profiles based on laboratory measurements of self‐buffered samples predict temperatures of the order of between 25° and 150°C colder, depending on the ambient oxygen fugacity, than those based on measurements of pyroxene‐buffered samples.