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

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 ²H was exchanged into hydrogen ¹H saturated single crystals of San Carlos olivine between 750 and 900°C at 2 GPa. We measured and fit the resulting ²H profiles to obtain D_H,[100] = 10^(-4.9±1.4)*e^{(-140±30kJ/mol)/(RT)} m²/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 % H₂O) 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.