Hydrogen self-diffusion in single crystal olivine and electrical conductivity of the Earth's mantle

Nominally anhydrous minerals formed deep in the mantle and transported to the Earth's surface contain tens to hundreds of ppm wt H₂O, providing evidence for the presence of dissolved water in the Earth's interior. Even at these low concentrations, H₂O greatly affects the physico-chemical properties of mantle materials, governing planetary dynamics and evolution. The diffusion of hydrogen (H) controls the transport of H₂O in the Earth's upper mantle, but is not fully understood for olivine ((Mg, Fe)₂SiO₄) the most abundant mineral in this region. Here we present new hydrogen self-diffusion coefficients in natural olivine single crystals that were determined at upper mantle conditions (2 GPa and 750-900 °C). Hydrogen self-diffusion is highly anisotropic, with values at 900 °C of 10^-10.9, 10^-12.8 and 10^-11.9 m²/s along [100], [010] and [001] directions, respectively. Combined with the Nernst-Einstein relation, these diffusion results constrain the contribution of H to the electrical conductivity of olivine to be σ_H = 10^2.12S/m·C_H2O·exp^{-187kJ/mol/(RT)}. Comparisons between the model presented in this study and magnetotelluric measurements suggest that plausible H₂O concentrations in the upper mantle (≤250 ppm wt) can account for high electrical conductivity values (10^-2-10^-1 S/m) observed in the asthenosphere.