The dry and hydrous viscosities of alkaline melts from Vesuvius and Phlegrean Fields

Abstract Sophisticated models of volcanic scenarios are increasingly sensitive to the accuracy of their input parameters and constitutive equations for magma properties. Viscosity is certainly one of the most important magma properties, but only recently systematic investigations on silicate liquids with natural compositions have started. We investigated the Newtonian viscosity of dry and hydrous phonolitic and trachytic melts from Vesuvius and Phlegrean Fields volcanic complexes, respectively. The analysed samples come from the deposits of the AD 1631 (Vesuvius) and ca. 4400 BP Agnano Monte Spina (AMS) (Phlegrean Fields) eruptions that are commonly taken as reference events for the most hazardous scenarios in case of reactivation of the two volcanoes. Samples were hydrated via piston cylinder synthesis at P= 10 kbar and T = 1600 jC. The dry high temperature and dry or hydrous low temperature viscosities were measured by a combination of micropenetration and concentric cylinder techniques, covering a total temperature range from about 400 to 1500 jC, water content range from virtually dry to 3.8 wt.%, and viscosity range from 102 to 1012 Pa s. The viscosity data for each composition were fitted by a modified Tamman–Vogel–Fulcher equation, allowing viscosity calculations at eruption temperatures and for dissolved water contents in the range of those examined. The viscosity data and model calculations were used for a comparison with other natural or synthetic phonolitic and trachytic melts, as well as with rhyolitic melts, for which viscosities had been measured. At water contents less than 1 wt.%, a trend of increasing viscosity from phonolitic to trachytic to rhyolitic melts is found. At water contents larger than 1 wt.%, the viscosity of trachytic melts is close to that of rhyolitic melts, while the viscosity of phonolitic melts is one to two orders of magnitude lower. A compositional parameter given by the (Na +K+ H)/(Si + Al) molar ratio is found to be linearly related to the low-T hydrous viscosity of the trachytic and phonolitic melts considered, either analysed in this work or taken from literature. Differently, the rhyolitic melt shows significant variations from the trend found for phonolitic and trachytic melts.