Electrical conductivity of wadsleyite at high temperatures and high pressures

Abstract

The electrical conductivity of wadsleyite aggregates has been determined under the broad range of thermodynamic conditions using the impedance spectroscopy for a frequency range of 10 − 2 to 10 6 Hz. Two branches are observed in the complex impedance, one (at high frequency range) showing a half circle originated at Z′ (real part of impedance) = Z″ (imaginary part of impedance) = 0 in the Z′– Z″ plot, and another branch in the low frequency range. The results from high frequency semi-circles correspond to the electric properties of a sample, whereas the results from a low frequency branch correspond to the electrode effects. From the analysis of the results from the semi-circles, we have identified two distinct mechanisms of electrical conduction having different activation enthalpies and different sensitivity to oxygen fugacity and water content. One mechanism dominating at water-poor condition has a high activation enthalpy (~ 147 kJ/mol) and the conductivity increases with oxygen fugacity. We suggest that electrical conduction in this regime is due to charge transfer involving ferric iron (“polaron” conduction). Under water-rich conditions, electrical conductivity increases with water content but decreases with oxygen fugacity, and the activation enthalpy is smaller (~ 88 kJ/mol). We infer that electrical conduction in this regime is due to protons. The activation enthalpy in this regime is insensitive to water content and the conductivity is proportional to water content, C W, as σ ∝ C w r with r~ 0.72. The value of r is smaller than one suggests that minority defects such as H M′ or H ● are responsible for electrical conduction. Our results show that a completely dry transition zone is incompatible with most of the geophysical observations on the mantle transition zone, and some water (~ 0.1–0.3 wt.% in the Pacific) is required to explain the observed electrical conductivity.