A melt viscosity scale for preeruptive magmas

Abstract

A simplified method to estimate preeruptive melt viscosity by using only melt SiO2 content (groundmass SiO2 content) is proposed for sub-alkaline magmas. Melt viscosity is controlled by many magmatic properties (e.g., melt composition, melt water content, and temperature); however, these properties are linked by phase equilibrium in preeruptive magmas. In this study, the magmatic properties were investigated by compiling data of phase equilibria experiments performed under preeruptive conditions. Negative correlations are found between melt SiO2 contents and liquidus temperatures, and between liquidus temperatures and melt water contents. Both increasing melt SiO2 content and decreasing liquidus temperature have the effect of increasing the melt viscosity, producing a linear positive correlation between logarithmic values of melt viscosity and linear values of melt SiO2 content. For a specific melt SiO2 content, an increase in liquidus temperature causes a decrease in melt viscosity, whereas a decrease in water content causes an increase in melt viscosity. As a result of this opposing effect, the melt viscosity is strongly correlated with the melt SiO2 content (the correlation coefficient of ∼1). Based on this relationship, an empirical equation predicting logarithmic values of preeruptive melt viscosity is proposed as a linear function of melt SiO2 content, referred to as the melt viscosity scale. The equation reproduces melt viscosities for compiled experimental melts and natural melts with root-mean-square deviation of ∼0.4 and ∼0.5 log units, respectively. This method provides order-of-magnitude estimates for preeruptive melt viscosity. Its strength is in being applicable to examples for which a full dataset of preeruptive magmatic properties is lacking or has large uncertainties (e.g., slowly cooled lavas). The simplicity of this method enables us to easily and promptly estimate preeruptive melt viscosity. Combined with rheological models for multiphase magmas, the present method can be applied widely and thus greatly increase the number of case studies which include evidence-based estimates of preeruptive magma viscosity.