Diffusion of hydrogen and intrinsic point defects in olivine

We have analyzed the kinetics of hydrogen diffusion in ferromagnesian olivine by measuring the O-H distribution in single crystals annealed under hydrothermal conditions. The hydrogen concentration evolves toward an equilibrium value in two stages. First, hydrogen diffuses rapidly (D₁₁ ≃ 10^-10 m²/s at 1000 °C) into olivine probably charge compensated by a flux of polarons or of electrons. The resultant metastable equilibrium concentration of hydrogen is directly related to the imposed hydrogen fugacity and to the preexisting concentration of intrinsic point defects; except for electronic species, intrinsic defects diffuse significantly more slowly than hydrogen. Second, hydrogen and intrinsic point defects such as metal vacancies diffuse more slowly (D_VMe ≃ 10^-12 m²/s at 1000 °C) into or out of the olivine as the concentrations of all defect species evolve toward equilibrium with the imposed thermochemical environment.