Hydrogen diffusion in hydrous minerals from Raman and NanoSIMS mapping – implications for isotopic characterization of fluid–rock interactions

Abstract. Hydrogen diffusion in major hydrous minerals determines the closure temperatures of isotopic exchange used to track fluid–rock interactions. Deuterium–hydrogen (D–H) exchange was experimentally investigated between minerals and deuterated gas (D2) in an ambient-pressure furnace over the temperature range of 400–650 °C (tremolite, vesuvianite) and between minerals and D2O at high pressure (1.5–3 GPa) and temperature (315–500 °C) in a belt press (glaucophane, epidote). D / (D + H) ratios in exchanged mineral grains were mapped using Raman spectroscopy calibrated by comparison with NanoSIMS analyses. Diffusion coefficients constrained by isotopic profiles were fitted to the Arrhenius equation DD/H=D0e(-Ha/RT), where Ha is the activation enthalpy, and D0 is the diffusion coefficient at infinite temperature T. The validity of intracrystalline diffusion laws from the literature is discussed with respect to the mechanical properties of hydrous minerals. Diffusion in tremolite is affected by intense cleavage, which reduces the effective grain size relative. High pressure appears to suppress cleavage opening in glaucophane. Results suggest that plasticity counteracts grain size reductions along cleavage planes in phyllosilicates. For vesuvianite, which lacks cleavage planes, intracrystalline diffusion is a valid assumption. In epidote, diffusivities are scattered over several orders of magnitude. Closure temperatures for hydrogen isotope diffusion were calculated and indicate that vesuvianite and phyllosilicates can record fluid–rock interactions under regional metamorphic conditions. Amphiboles may retain information about relatively short-lived eruptive events. Spatially resolved measurements of hydrogen isotopic compositions in those minerals may reveal low-temperature (100 < T < 400 °C) fluid–rock interactions associated with slip along major faults and metamorphic terrane exhumation.