A Simple Model for the Viscosity of Rhyolites as a Function of Temperature, Pressure and Water Content

In order to better constrain the viscosity (η) of high-silica rhyolite at low to moderate water contents (X), which represent water saturation at near-surface pressure–temperature (P–T) conditions, we made 211 viscosity measurements on Mono Craters rhyolites containing between 0.01 and 1.2 wt.% H2O, at temperatures between 796 and 1774 K using parallel plate and concentric cylinder methods at atmospheric pressure. We then developed and calibrated a new empirical model for the rhyolitic melt viscosity, where non-linear variations due to temperature (T), and water content (X) are nested within linear and exponential dependencies of log η on pressure (P). The model was fitted to a total of 691 data points including published data on rhyolites, granites and haplogranites. The significance of model parameters was evaluated at the 95% confidence level