Effect of temperature, pressure and iron content on the electrical conductivity of orthopyroxene

Abstract

The electrical conductivity of (Mg1−x , Fe x )SiO3 orthopyroxene with various iron contents [X Fe = Fe/(Fe + Mg) = 0, 0.1, 0.3, 0.5, 0.7 and 1.0] was measured in a Kawai-type multianvil apparatus by impedance spectroscopy over a wide range of pressure (P) and temperature (T) covering the stability field of orthopyroxene. Impedance spectroscopy measurements indicated that the electrical conductivity of orthopyroxene systematically increased with increasing total iron content. The conductivity slightly decreased with increasing pressure at a constant temperature. For samples with lower Fe content, two conduction mechanisms were identified from the Arrhenius behavior. A change in the activation enthalpy indicated that the dominant conduction mechanism changed from small polaron to ionic conduction with increasing temperature. At temperature below 1373 K, relatively low activation enthalpies and small positive activation volumes suggest that the dominant mechanism of charge transport is Fe2+‒Fe3+ hopping (small polaron). At higher temperatures above 1473 K, ionic conduction (via Mg vacancy mobility) dominates, with higher activation enthalpy exceeding 2 eV and larger positive activation volume. All electrical conductivity data fit the formula for electrical conductivity $$\sigma = \sigma_{0}^{i} \exp \left[ { - \frac{{\left( {\Delta E_{0}^{i} + P\Delta V_{0}^{i} } \right)}}{{k_{\text{B}} T}}} \right] + \sigma_{0}^{p} X_{\text{Fe}} \exp \left\{ { - \frac{{\left[ {\Delta E_{0}^{p} - \alpha X_{\text{Fe}}^{1/3} + P\left( {\Delta V_{0}^{p} - \beta X_{\text{Fe}} } \right)} \right]}}{{k_{\text{B}} T}}} \right\},$$ where σ 0 is the pre-exponential term, ΔE 0 and ΔV 0 are the activation energy and the activation volume at very low total iron concentration, k B is the Boltzmann constant, T is the absolute temperature, and superscripts i and p denote the ionic and small polaron conductions, respectively. Electrical conductivity of Al-free orthopyroxene with X Fe = 0.1 is distinctly lower than that of olivine with X Fe = 0.1. Above 3 GPa Al content in orthopyroxene becomes smaller in association with garnet formation. Unless iron content in orthopyroxene is considerably high (X Fe > 0.2), orthopyroxene has little influence on the electrical structure of the upper mantle.