Hydrogen diffusivity in wadsleyite and water distribution in the mantle transition zone

Abstract

The kinetics of the hydrogen diffusivity in synthesized polycrystalline wadsleyite was measured by IR spectroscopy in order to determine the diffusion coefficient of hydrogen in wadsleyite, the major constituent mineral in the mantle transition zone, with the composition of (Mg 0.89Fe 0.11) 2SiO 4. The hydration experiments were conducted at 15–16 GPa and temperature range from 900 to 1200 °C with Mg(OH) 2 as the water source by Kawai-type multi-anvil apparatus. The diffusion rate obtained here is considered to be an effective diffusion coefficient with the grain size of ∼9 μm involving contributions both from the lattice diffusion and the grain boundary diffusion. The temperature dependence of diffusion of hydrogen in polycrystalline wadsleyite was determined to be D H = 9.6 × 10 − 6 exp [− 123 (± 32) (kJ mol − 1 ) / RT] at 15 GPa. Hydrogen diffusion rate in wadsleyite is roughly consistent with the average diffusivity of hydrogen in olivine. In recent years, several authors have suggested a possibility of the hydrous transition zone, and the distribution and transport properties of water are still debated. On the basis of water-dependence of the electrical conductivity the water content in the mantle transition zone was estimated from the observed conductivity by using the Nernst–Einstein relation. The results obtained here show that the distribution of water should be quite heterogeneous throughout the mantle transition zone.