Solubility and diffusivity of H2O in rhyolitic glass at hydrothermal temperatures

Water concentration and speciation in volcanic glasses and melts have numerous applications including constraining the initial magmatic H₂O content, estimating cooling and ascent rates, and differentiating between magmatic and secondary water. The diffusivity (D_H2O) and solubility (C_sat) of H₂O in glass or melt is well constrained at high temperatures, but more challenging to constrain at low temperatures because of the unknown Arrhenius behavior of D_H2O. Similarly, H₂O C_sat is highly variable in natural samples. We conduct isothermal, isobaric hydration experiments on anhydrous Newberry obsidian and rehydrated Yellowstone perlites at temperatures below the glass transition (<400°C) and above ambient surface conditions (room T) to constrain these parameters over a range of T that glasses invariably pass through in nature. Hydration profiles were measured by NanoSIMS, FTIR, and using the water-by-difference technique by EMPA. Precise Bulk H₂O and D/H measurements by TC/EA-MAT253 of spherical particles of known size and shape provide a constraint for mass balance calculations. Measured profiles by NanoSIMS and EMPA are lobate as expected for diffusivity that is a function of H₂O content. Challenges associated with measuring H₂O concentrations at the boundary of a spherical glass bead yield a range of solubility estimates. However, profile length and shape are consistent in all techniques at 225°C. Water D_H2O and C_sat in rhyolitic glass will be further constrained by reconciling diffusion modeling and mass balance calculations. The H₂O boundary condition in the glass affects the D_H2O in diffusion modeling. The minimum C_sat of 0.6 wt.% H₂O by FTIR and maximum C_sat of 3.5 wt.% by EPMA water by difference yield a D_H2O at 225°C that is 10-100x greater than predicted by the D_H2O equations calibrated by Zhang and Behrens (2000). Finally, the high resolution NanoSIMS data shows no discernable difference in the shape or length of ²H and ¹H (D and H) hydration profiles in glasses hydrated by a 1:1 mixture of H₂O and D₂O, suggesting that diffusion-driven kinetic fractionation of hydrogen isotopes in glass is negligible or currently analytically unresolvable. These results can be extrapolated to lower T for which experiments cannot be easily conducted and are of relevance for understanding rehydrated volcanic glasses and ashes.