Deuterium‐hydrogen exchange in olivine: Implications for point defects and electrical conductivity

Abstract

Knowledge about hydrogen self diffusion (D H ) is critical for determining mantle hydrogen distribution and understanding point defects. Also, chemical diffusion of hydrogen in olivine, such as redox exchange with polarons (D Redox ), depends on D H . In this study deuterium 2 H was exchanged into hydrogen 1 H saturated single crystals of San Carlos olivine between 750 and 900°C at 2 GPa. We measured and fit the resulting 2 H profiles to obtain D H,[100] = 10 (−4.9±1.4) *e (−140±30kJ/mol)/(RT) m 2 /s, which is ∼1 log unit lower than D Redox,[100] , with similar activation enthalpy H a . By comparing these two diffusion coefficients, we estimate the small polaron diffusion coefficient. Additionally, we estimate D H in the [010] and [001] orientations, demonstrating that D H is highly anisotropic in olivine. These D H values were used with the Nernst‐Einstein relation to estimate the electrical conductivity by hydrogen in olivine ( σ H = 10 1.1 *e (−130kJ/mol)/(RT) S/m for 10 −2 wt % H 2 O) that is lower in magnitude than previous measurements. Our results suggest that hydrogen alone cannot account for high electrical conductivity anomalies observed at asthenospheric depths (∼10 −2 to ∼10 −1 S/m). The maximum anisotropic variation of D H and σ H in olivine is ∼2 log units between 750 and 900°C and increases when extrapolated to higher temperature (∼3.3 at 1400°C). Anisotropy observed in the mantle may indicate substantial amounts of hydrogen in olivine with lattice‐preferred orientation.