Re-evaluation of electrical conductivity of anhydrous and hydrous wadsleyite

Abstract

Recent laboratory electrical conductivity measurements of the main mantle constituent minerals have represented considerable efforts to determine the effects of water content on electrical conductivity. However, there are large discrepancies between the results of Yoshino et al. (2008a) and those of Dai and Karato (2009a) on hopping conduction and the effects of water on the electrical conductivity of wadsleyite. To investigate the cause of these discrepancies, the electrical conductivity of anhydrous and hydrous wadsleyite were newly measured under low and high temperature conditions by impedance spectroscopy. The conductivity values of dry wadsleyite aggregates with less than 2ppm H2O by weight were similar to those for hopping conduction reported by Yoshino et al. (2008a) and distinctly higher than those of Dai and Karato (2009a). For hydrous wadsleyite, at temperatures below 1000K, the electrical conductivity in an Arrhenius plot was repeatable along the heating–cooling paths and was similar to the results of Yoshino et al. (2008a). The impedance spectrum in the complex impedance plane of hydrous wadsleyite showed a semicircular shape, and the infrared spectrum did not show any shape change after the conductivity measurements. In contrast, when the temperature exceeds 1000K, the electrical conductivity in an Arrhenius plot showed higher activation enthalpy. The impedance spectra were greatly distorted and the impedance arc contained at least two relaxation processes. This shape is similar to those reported by Dai and Karato (2009a) who measured the conductivity above 1000K. The infrared spectra showed a large contribution from molecular water after conductivity measurements, suggesting significant dehydration during the conductivity measurements. In summary, the results obtained from conductivity measurements at higher temperatures (>1000K) do not represent the proton conduction in the grain interior.