THE ELECTRICAL-CONDUCTIVITY OF AN ISOTROPIC OLIVINE MANTLE

In order to extend the useful temperature range of interpretation of olivine electrical conductivity-sigma we have used the nonlinear iterative Marquardt technique to fit experimental data over the range 720-degrees-1500-degrees-C to the parametric form sigma = sigma-1e(-A1/kT) + sigma-2e(-A2/kT), where k is Boltzmann's constant and T is absolute temperature. The model describes conduction by migration of two different thermally activated defect populations with activation energies A1 and A2, and preexponential terms sigma-1 and sigma-2 that depend on number of charge carriers and their mobility and that may be different for each crystallographic direction. A combined interpretation of recent high (San Carlos olivine) and low (Jackson County dunite) temperature measurements has been made that demonstrates that a single activation energy A1 for all three crystallographic directions adequately fits the data. The parametric fits show that the high-temperature conduction mechanism has far greater anisotropy than the low-temperature mechanism, consistent with previous assignments to ionic and electronic conduction, respectively. The geometric mean of the conductivity in the three directions is approximately sigma-BAR 10(2.402)e-1.60eV/kT + 10(9.17)e-4.25eV/kT S/m and is presented as a model for isotropic olivine, SO2, appropriate from 720-degrees-C to above 1500-degrees-C, at oxygen fugacities near the center of the olivine stability field. It is observed that the magnitudes of sigma-1 for the three crystal directions are similar to the ratios of the inter-ionic distances between the M1 magnesium sites in olivine, to within 5%, consistent with Fe3+ preferring the M1 site below 1200-degrees-C.