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Acoustic compressional and shear wave velocities ( V  P ,  V  S ) of anhydrous (AHRG) and hydrous rhyolitic glasses (HRG) containing 3.28 wt% (HRG-3) and 5.90 wt% (HRG-6) total water concentration (H 2 O t ) have been measured using Brillouin light scattering (BLS) spectroscopy up to 3 GPa in a diamond-anvil cell at ambient temperature. In addition, Fourier-transform infrared (FTIR) spectroscopy was used to measure the speciation of H 2 O in the glasses up to 3 GPa. At ambient pressure, HRG-3 contains 1.58 (6) wt% hydroxyl groups (OH – ) and 1.70 (7) wt% molecular water (H 2 O m ) while HRG-6 contains 1.67 (10) wt% OH –  and 4.23 (17) wt% H 2 O m  where the numbers in parentheses are ±1σ. With increasing pressure, very little H 2 O m , if any, converts to OH –  within uncertainties in hydrous rhyolitic glasses such that HRG-6 contains much more H 2 O m  than HRG-3 at all experimental pressures. We observe a nonlinear relationship between high-pressure sound velocities and H 2 O t , which is attributed to the distinct effects of each water species on acoustic velocities and elastic moduli of hydrous glasses. Near ambient pressure, depolymerization due to OH –  reduces  V  S  and  G  more than  V  P  and  K  S .  V  P  and  K  S  in both anhydrous and hydrous glasses decrease with increasing pressure up to ~1–2 GPa before increasing with pressure. Above ~1–2 GPa,  V  P  and  K  S  in both hydrous glasses converge with those in AHRG. In particular,  V  P  in HRG-6 crosses over and becomes higher than  V  P  in AHRG. HRG-6 displays lower  V  S  and  G  than HRG-3 near ambient pressure, but  V  S  and  G  in these glasses converge above ~2 GPa. Our results show that hydrous rhyolitic glasses with ~2–4 wt% H 2 O m  can be as incompressible as their anhydrous counterpart above ~1.5 GPa. The nonlinear effects of hydration on high-pressure acoustic velocities and elastic moduli of rhyolitic glasses observed here may provide some insight into the behavior of hydrous silicate melts in felsic magma chambers at depth.

Nonlinear effects of hydration on high-pressure sound velocities of rhyolitic glasses