Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

The diffusion of molecular water (H₂O_m) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δD_H2O) to the glass (δD_gl) offset by a glass-H₂O fractionation factor (ΔD_gl-H2O = δD_gl – δD_H2O) which is approximately −33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H₂O during eruption, transport, and emplacement. To aid in the interpretation of δD_gl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 °C for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 °C and below 250 °C. Lower temperature experiments yield raw ΔD_gl-H2O values in the range of −33 ± 11‰. Experiments at 225 °C using both positive and negative initial ΔD_gl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H₂O vapor (10³lnα_gl-H2O) below 250 °C. Variation in ΔD_gl-H2O (−33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 °C yield unexpectedly low δD_gl values with ΔD_gl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H₂O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H₂O_m and 1.0 wt% hydroxl (OH⁻) in the highest P-T experiment at 375 °C and 21.1 MPa. Therefore, we employ a 1D isotope diffusion–reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 10³lnα_gl-H2O values of −33‰ ± 5‰ and −25‰ ± 5‰ at 175 °C and 225 °C, respectively. At higher temperatures, ΔD_gl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δD_gl value is highly responsive to changes in the δD_gl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δD_gl fractionation with progressive glass hydration. These results indicate that the observed ΔD_gl-H2O of ∼−33 ± 11‰ can be cautiously applied as an equilibrium 10³lnα_gl-H2O value to natural silicic glasses hydrated below 250 °C to identify hydration sources. This approximate ΔD_gl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H₂O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface.