CHANGE OF ELECTRICAL-CONDUCTIVITY OF OLIVINE ASSOCIATED WITH THE OLIVINE SPINEL TRANSITION

The electrical conductivity sigma of (Mg1-x,Fe(x) 2SiO4 was measured under the stable conditions for olivine (alpha-phase) and spinel phases (beta- and gamma-phases). The present measurements cover much lower iron-content olivines (X = 0.28 and 0.10) and higher temperatures ( approximately 1000 to 1200-degrees-C) than previous works. Conductivity changes associated with the phase transitions (sigma-gamma/sigma-alpha and sigma-beta/sigma-alpha) were observed. They were approximately 10 for sigma-gamma/sigma-alpha and approximately 2 for sigma(beta)/sigma(alpha) at about 1200-degrees-C, independent of Fe content. These values are significantly smaller than those obtained so far for iron-rich samples at low temperatures in previous works. The activation energy E* of electrical conductivity of the spinel phase was slightly smaller by about 0.1 eV than that of the olivine phase of approximately 0.8 to 1.4 eV. It indicates that the conductivity change depends only slightly on temperature. These data can be applied to the condition at the base of the Earth's upper mantle. The measured conductivity change of one order of magnitude owing to the olivine-spinel transition is much smaller than the steep increase of conductivity of two orders of magnitude at the mantle transition zone estimated from geomagnetic observations.