Electrical conductivity of wadsleyite as a function of temperature and water content

Electrical conductivity of wadsleyite was measured at 16 GPa using a KAWAI-type multi-anvil press under controlled oxygen fugacity. Water-doped and -undoped samples were used to examine the effect of water on conductivity. Two water-doped wadsleyite samples used for the conductivity measurements contained 0.3 ± 0.01 and 1.2 ± 0.02 wt.% water. The temperature ranges for the conductivity measurements were 500–2000 K for water-undoped samples and 500–1000 K for water-doped samples. Above 1500 K, the electrical conductivity values are essentially the same among different runs with an average activation enthalpy of 1.5 eV, suggesting that small polaron conduction should dominate. Below 1000 K, the conductivity systematically increases with increasing water content, suggesting proton conduction as a dominant conduction mechanism. Electrical conductivity of anhydrous wadsleyite in the mantle transition zone should be about 3 × 10−2 S/m in the normal geotherm. Hydration enhances the conductivity of wadsleyite; by containing 0.1 wt. % water, the conductivity of wadsleyite increases by 0.3 log units. The conductivity jump associated with the dry olivine–wadsleyite transition is only 0.7 log units. A dry olivine–wadsleyite conductivity model well agrees with the current semi-global conductivity-depth profiles, thus the concept of a globally hydrated mantle transition zone is not necessary in view of electrical conductivity. The high conductivity observed in the wedge mantle cannot be explained by hydrous wadsleyite. Alternatively, the presence of either phase E or supercritical fluids in the wedge mantle is proposed.