FERRIC IRON DEPENDENCE OF THE ELECTRICAL-CONDUCTIVITY OF THE EARTHS LOWER MANTLE MATERIAL

We measured the DC electrical conductivity of samples of magnesiowusti te-perovskite assemblage, prepared by laser heating at 400 kbar in a d iamond-anvil cell, from San Carlos olivine equilibrated at various oxy gen partial pressures. The conductivity was measured as a function of temperature between room temperature and 400 degrees C. The conductivi ty obeys the Arrhenius law, compatible with a conduction by small pola rons. The activation energy is about 0.3 eV atm(-1), and decreases wit h increasing pressure and increasing oxygen fugacity of equilibration. The effect of the oxygen fugacity on the pre-exponential factor is ne gligible. We suggest that the effect of oxygen fugacity is essentially due to the compression of the lattice accompanying the increase in tr ivalent iron ions content. The effect of oxygen partial pressure is th erefore comparable to the effect of pressure. The present results conf irm our previous estimate that the electrical conductivity of the lowe r part of the mantle is of the order of 1-10 S m(-1).