Pressure effect on electrical conductivity of mantle olivine

We report new measurements of electrical conductivity of olivine that were made in a multi-anvil press using solid buffers (Mo-MoO2) to stabilize oxygen partial pressure. The temperature range was 1000 degrees C-1400 degrees C and the pressure range was 4-10 GPa. When pressure effects are interpreted as activation volumes in the Arrhenius equation, they yield values of order 0.6 +/- 0.6 cm(3)/mole. Values of the pre-exponential factor sigma(0) are of the order of hundreds of S/m, and the zero-pressure activation energy is about 1.5 eV, in agreement with laboratory measurements at lower pressure conditions. In addition, we analyzed older literature data, which produced a similar result but with larger variation. The common practice of assuming that the pressure effect on electrical conductivity of olivine in the Earth's upper mantle depends weakly on pressure has rested on uncertain grounds. These new observations suggest that for upper mantle conditions (depths of 80-200 km) neglecting pressure effects on olivine conductivity is justified. A weak pressure effect also supports the small polaron (Fe3+) model as the dominant conduction mechanism, although the mainly positive activation volumes argue for a component of lattice deformation. When applied to the depth range 200-400 km in the Earth, the laboratory data yield conductivities of order 10(-2) S/m, slightly lower than the range of geophysical conductivity profiles but well within this range after allowing for probable effects of oxygen activity and temperature uncertainty. (C) 2000 Elsevier Science B.V. All rights reserved.