中国科学院地球化学研究所机构知识库

The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3 GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1 Hz to 1 MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723 K and for hydrogen-doped samples at relatively lower temperature range of 500–900 K to minimize dehydration of samples. For the hydrogen-undoped samples, the activation enthalpy is around 1.9 eV at the higher temperatures range (>1300 K) suggesting that the dominant charge transfer mechanism is Fe²⁺−Fe³⁺ hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900 K, the activation enthalpy decreases from 1.11 to 0.70 eV, 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=σ_0hexp(−H_h/kT)+σ_0pC_wexp[−(H_p⁰−αC_w^1/3)/kT], where σ₀ is pre-exponential factor, C_w is the water content in weight percent, H is the activation enthalpy, H_p⁰ 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 prepresent 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.