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+0.5Pa (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 fO2 conditions, electrical conduction in the self-buffered samples 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 fO2 investigated, the electrical conductivity of the self-buffered sample is given by &sgr;sb<100> =2.27(S/m)e-0.55(eV)/kT fO2 0.18 +306.3(S/m)e-2.25(eV)/kT fO2-0.18 and for the pyroxene-buffered sample by &sgr;pb<100> =0.18(S/m)e-0.34(eV)/kT fO20.17 +15.2(S/m)e-1.3(eV)/kT where k is Boltzmann's constant, T is in Kelvin, and fO2 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 w¿stite-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 thosebased on measurements of pyroxene-buffered samples. ¿ American Geophysical Union 1993