Ferric iron dependence of the electrical conductivity of the Earth´s lower mantle material

Abstract

We measured the DC electrical conductivity of samples of magnesiowustite-perovskite assemblage, prepared by laser heating at 400 kbar in a diamond-anvil cell, from San Carlos olivine equilibrated at various oxygen partial pressures. The conductivity was measured as a function of temperature between room temperature and 400 °C. The conductivity obeys the Arrhenius law, compatible with a conduction by small polarons. The activation energy is about 0.3 eV atm -1 , and decreases with increasing pressure and increasing oxygen fugacity of equilibration. The effect of the oxygen fugacity on the pre-exponential factor is negligible. We suggest that the effect of oxygen fugacity is essentially due to the compression of the lattice accompanying the increase in trivalent iron ions content. The effect of oxygen partial pressure is therefore comparable to the effect of pressure. The present results confirm our previous estimate that the electrical conductivity of the lower part of the mantle is of the order of 1-10 S m -1 .