Hydrogen and deuterium diffusion in non-stoichiometric spinel

High pressure/temperature annealing experiments are used to determine diffusivities of H and D in non-stoichiometric spinel, a low-pressure analogue for nominally anhydrous minerals in Earth’s mantle. Data are fitted to the following Arrhenius law: Diffusivity (m/s) = 4 ± 1 × 10 exp(−54 ± 2 kJ mol/RT). At low temperatures, H and D diffusion in non-stoichiometric spinel is charge balanced by flux of O vacancies, with infrared data consistent with protonation of both octahedral and tetrahedral O–O edges in non-stoichiometric spinel, and additional fine structure due to Mg–Al mixing and/or coupling of structurally incorporated H with cation vacancies. Absence of changes in the fine structure of O–H absorption bands indicates that H can become locally coupled and uncoupled to other defects during bulk diffusion. As such, proton conductivity in spinel group minerals, arising from faster flux of uncoupled H, can only be calculated from H mobility data if the extent of defect coupling is constrained. ARTICLE HISTORY Received 10 May 2017 Accepted 5 July 2017