Deuterium‑hydrogen inter-diffusion in chlorite
Résumé
Interactions between aqueous fluids and rocks occur in a broad range of contexts ranging from hydrothermal alteration veins to regional metamorphism. Tracking these processes and understanding their reaction kinetics require a precise knowledge of the diffusion of water in rocks, and of isotope fractionation in major minerals, such as chlorite. Deuterium‑hydrogen exchange between Mg-rich chlorite and water (D2O) was experimentally investigated using a belt press over the temperature range of 315–650 °C at pressures of 1.5 GPa and 3 GPa. Both chloritite chip and chlorite powder were used. D/(D + H) ratios in chlorite grains were mapped using Raman spectroscopy. Deuterium‑hydrogen exchange proceeded by deuterium‑hydrogen inter-diffusion in chlorite (lattice diffusion). As chlorite is a phyllosilicate, i.e. an anisotropic mineral, diffusion coefficients were determined for crystallographic directions parallel and perpendicular to the silicate layers (perpendicular and parallel to the c* axis, respectively). Arrhenius relations for deuterium‑hydrogen inter-diffusion coefficients were derived from the new dataset. Activation energy (E) is comprised between 215 ± 21 (1σ) kJ/mol (perpendicular to the c* axis) and 194 ± 21 kJ/mol (parallel to the c* axis). Log10(D0) is −2.2 ± 1.3 m2/s for diffusion in the direction perpendicular to the c* axis and −4.5 ± 1.3 m2/s for diffusion parallel to it. The bulk diffusion law derived from a combination of our data with data from Graham et al. (1987) yields E = 176 ± 8 kJ/mol and log10(D0) = −5.1 ± 0.4 m2/s.