The electrical conductivity of lherzolite

Abstract

Electrical conductivity as a function of oxygen fugacity (ƒO2) between 10−5 Pa and 1 Pa, temperature between 700°C and 1200°C, and time over a period of 1700 hours are reported for a lherzolite nodule from Mount Porndon, Australia. Analysis of complex impedance collected at 100 Hz, 1 kHz, and 10 kHz indicates that there is significant frequency dispersion in this rock at temperatures below 900°C–1000°C. By choosing the resistance of an equivalent parallel RC network at the frequency having minimum phase, the effect of dispersion is largely avoided. Conductivity as a function of time, collected following changes in ƒO2 at 1010 and 1200°C, has been analyzed to determine the diffusivity of defects relating ƒO2 to the electrical conduction mechanism in olivine. The diffusivities so obtained are in remarkably good agreement with those determined from strain measurements during creep tests, which implies that magnesium vacancies are the rate‐limiting step for conductivity reequilibration after ƒO2 changes. A longer‐term process is observed in this rock in which the conductivity drifts upward or downward with a time constant of hundreds of hours after initial reequilibration to gas mix ( ƒO2) changes. We speculate that this long‐term drift could be related to equilibration of the iron distribution between coexisting olivine and pyroxene as a function of ƒO2. The sense is an increase in conductivity (olivine gains iron) at low ƒO2 and vice versa. The lherzolite conductivity data are not significantly different from measurements made on olivine single crystals and polycrystals, even though the rock contains about 34 modal % pyroxene. This consistency of laboratory measurements of electrical conductivity of olivines from many localities and geological settings supports the use of recent models relating mantle temperature with electrical conductivity in the interpretation of mantle geo/electromagnetic soundings.