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)(2)SiO4) 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 degrees C). Hydrogen self-diffusion is highly anisotropic, with values at 900 degrees C of 10(-10.9), 10(-12.8) and 10(-11.9) m(2)/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 sigma(H) = 10(2.12)S/m.CH2O.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.