Hydrogen isotope behavior during rhyolite glass hydration under hydrothermal conditions

Abstract

The diffusion of molecular water (H 2 O 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-H 2 O 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 H 2 O 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 H 2 O vapor (10 3 lnα 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 H 2 O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H 2 O 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 10 3 lnα 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 10 3 lnα 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 H 2 O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface.