A model for the viscosity of rhyolite as a function of H2O -content and pressure: a calibration based on centrifuge piston cylinder experiments

The Newtonian viscosity of synthetic rhyolitic liquids with 0.15–5.24 wt% dissolved water was determined in the interval between 580 and 1640 °C and pressures of 1 atm and 5–25 kbar. Measurements were performed by combining static and accelerated (up to 1000g) falling sphere experiments on water-bearing samples, with high temperature concentric cylinder experiments on 0.15 wt% H2O melts. These methods allowed viscosity determinations between 102 and 107 Pa s, and cover the complete range of naturally occurring magmatic temperatures, pressures, and H2O-contents for rhyolites. Our viscosity data, combined with those from previous studies, were modeled by an expression based on the empirical Vogel–Fulcher–Tammann equation, which describes viscosities and derivative properties (glass transition temperature Tg, fragility m, and activation volume of viscous flow Va) of silicic liquids as a function of P-T-X(H2O). The fitted expressions do not account for composition-dependent parameters other than X(H2O) and reproduce the entire viscosity database for silicic liquids to within 3.0% average relative error on log g (i.e. std. error of estimate of 0.26 log units). The results yield the expected strong decrease of viscosity with temperature and water content, but show variable pressure dependencies. Viscosity results to be strongly affected by pressure at low pressures; an effect amplified at low temperatures and water contents. Fragility, as a measure for the deviation from Arrhenian behavior, decreases with H2O-content but is insensitive to pressure. Activation volumes are always largely negative (e.g., less than -10 cm3/mol) and increase strongly with H2O-content. Variations in melt structure that may account for the observed property variations are discussed.