The rheology of peralkaline rhyolites from Pantelleria Island

The viscosity of pantelleritic melts from the Khaggiar lava flow (5.5 ka - Pantelleria Island) was investigated as a function of temperature and water content. High (1673–1323 K) and low (973–613 K) temperature dry and hydrous liquid viscosity was determined by a combination of concentric cylinder (102.44 to 104.56 Pa s) and micropenetration (108.67 to 1011.37) viscometric techniques. The effect of water, from 0.02 to 3.55 wt% H2O, was explored in the temperature range of 973–613 K. Liquid viscosities have been parameterized by means of a modified Vogel-Fulcher-Tammann equation (VFT) which describes viscosities and derivative properties (glass transition temperature Tg, fragility m) of silicic liquids as a function of T-X (H2O). The results yield the expected strong decrease of viscosity with temperature and water content. Fragility, as a measure of the deviation from Arrhenian behavior, increases with H2O content from m = 23 to m = 28. The peralkalinity (molar alkali excess over alumina), which characterizes the pantelleritic magmas, exerts a primary control over the rheological behavior of these melts. The excess of alkalies over alumina content is responsible for the peculiarly low viscosities of pantelleritic liquids compared to their metaluminous counterpart in the high temperature range, and leads to an even more dramatic decrease in the viscosities of these rhyolites in the low temperature range. Comparison with available models shows substantial differences between the measured and calculated viscosities. Based on our new viscosity results combined with existing literature data, we propose a new parameterization based on a modified Vogel-Fulcher-Tammann equation, accounting for the effect of water and composition, which can be used to determine the viscosity of pantelleritic melts. The derived relationship reproduces the experimental data (58 in total) in the viscosity range from 102.44 to 1011.37 Pa s and in the temperature range from 613 to 1673 K within a root-mean-square-error (RMSE) of 0.12 log units. Application of this calculation model for high temperature low viscosity hydrous melt is limited by the lack of experimental data and needs verification by additional measurements. Volcanological implications for welding and rheomorphic processes as well as conduit dynamic modelling are also discussed in light of the results presented in this work.