Abstract
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 σH = 102.12S/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.
Highlights
Earth’s hydrosphere is a distinctive feature of our planet where massive oceans affect its climate and support its ecosystem
The Nernst-Einstein theory has been used previously to simulate the influence of hydrogen on upper mantle electrical conductivity with models subsequently compared to magnetotelluric measurements of electrical conductivity anomalies in the asthenosphere13, 30
Utilizing H chemical diffusion coefficients for the [100], Karato13 concluded that the asthenospheric conductivity anomalies were consistent with olivine containing only limited amounts of H2O (
Summary
Earth’s hydrosphere is a distinctive feature of our planet where massive oceans affect its climate and support its ecosystem. Minerals found in mantle xenoliths, including olivine, that are brought to surface by alkaline or kimberlitic magma eruptions, contain up to hundreds of ppm wt (parts per million by weight) H2O5, providing direct evidence of H2O incorporated in NAMs from the Earth’s interior. A crystal of terrestrial ringwoodite (a high-pressure polymorph of olivine that forms below ~410 km depth), discovered as an inclusion in a natural diamond, was found to contain ~1.4 wt% H2O6. This confirms that at least some regions within Earth’s transition zone contain large concentrations of dissolved. Knowledge of the electrical conductivity of mantle minerals is critical for interpreting magnetotelluric sounding measurements used to interrogate the structure and composition of the Earth’s interior. Despite substantial follow-on work devoted to determining the influence of H on olivine and mantle conductivity, the disparate interpretations remain unresolved
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