The effect of water on the viscosity of a haplogranitic melt under P-T-X conditions relevant to silicic volcanism

Water content has a strong effect on the viscosity of hydrous silicic melts. Viscometric studies on synthesized hydrous haplogranitic melts serving as a model for calcalkaline rhyolites show that viscosity drops nonlinearly with the addition of water. The results of the study are used to reveal the similarity in the effects of alkali oxides on the viscosity of silica melt and water on that of a rhyolite melt. The consequences for degassing subvolcanic silicic melts are detailed. Significance of the prevailing pressure-temperature-composition conditions during volcanism is emphasized.