Viscosity of Palmas-type magmas of the Paraná Magmatic Province (Rio Grande do Sul State, Brazil): Implications for high-temperature silicic volcanism

This paper provides a new parameterization of the temperature and H2O content dependence of the pure liquid viscosities of rhyolitic and dacitic magmas associated with representative volcanic products of the Caxias do Sul and Santa Maria eruptive sequences. The viscosities of silicic volcanic products from the Santa Maria rhyolite (SMr), Caxias do Sul (CSd) and Barros Cassal (BCd) eruptive sequences (Lower Cretaceous volcanism of Parana-Etendeka Large Igneous Province) were measured in the temperature range from ca. 1600 °C to the glass transition (Tg). Anhydrous melt viscosities of representative samples from the main eruptive sequences were determined via concentric cylinder viscometry in the super liquidus regime. The quench products of SMr and CSd were then hydrated using an Internally Heated Pressure Vessel to generate two suites of samples with variable water content of up to 4.41 (CSd) and 5.27 (SMr) wt% as determined by Karl Fischer Titration (KFT). Finally, both anhydrous and hydrous samples were used for micropenetration viscosity measurements near Tg. Both types of samples show a minor amount of Fe-Ti-oxide nanolites identified via Raman spectroscopy, which presence did not substantially interfere with viscosity determinations. Based on the results of the viscosity measurements we parameterized the viscosity dependence as a function of water content using the following Vogel Fulcher Tammann (VFT) expressions accounting for the water and temperature dependence of the viscosity: logη = −4.55 + (10065–176*H2O)/[T-(34.6 + 375.3/(1 + H2O))] for SMr. and logη = −4.55 + (9213–338.1*H2O)/[T-(148.5 + 301.3/(1 + H2O))] for CSd. where η is the viscosity in Pa s, T the absolute temperature and H2O the dissolved water content in wt%. This novel parameterization appears to solve a few inconsistencies associated with the variation of the main descriptive parameters of the effect of H2O, improving the performance of some previous parameterizations. These results are useful for scaling to the conditions extant during ascent and eruption and during flow, emplacement and welding, at temperatures above Tg, for the dacitic and rhyolitic products investigated here.