Electrical conductivity of enstatite as a function of water content: Implications for the electrical structure in the upper mantle

The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1Hz to 1MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723K and for hydrogen-doped samples at relatively lower temperature range of 500–900K to minimize dehydration of samples. For the hydrogen-undoped samples, the activation enthalpy is around 1.9eV at the higher temperatures range (>1300K) suggesting that the dominant charge transfer mechanism is Fe²⁺−Fe³⁺ hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900K, the activation enthalpy decreases from 1.11 to 0.70eV, and the conductivity values systematically increase with increasing water content, suggesting that proton conduction is the dominant conduction mechanism. Taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account, all electrical conductivity data were fitted to the formula σ=σ₀ₕexp(−Hₕ/kT)+σ₀ₚCwexp[−(Hₚ ⁰−αCw ¹/³)/kT], where σ₀ is pre-exponential factor, Cw is the water content in weight percent, H is the activation enthalpy, Hₚ ⁰ is the activation enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolution temperature and subscripts h and p represent hopping and proton conductions, respectively. Using the present results, a laboratory-based conductivity-depth profile in the Earth's upper mantle has been constructed as a function of water content. Comparison of our model with the currently available geophysical observations beneath the Eastern Pacific Rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine.