Viscosity of magmatic melts: Improved structural - chemical model

Abstract

Viscosity of magmatic melts is a key physical property that controls a variety of processes such as magma crystallization and differentiation, ascent and eruptions dynamics. Therefore, the physical-chemical model of forecasts and calculations of viscosity of magmatic melts is an actual problem of petrology and geochemistry. Here we present a unique model that for the first time allows reliable prediction of the viscosity of near-liquid magmatic melts in a full range of composition and conditions with high accuracy of the prediction (±30% rel. for viscosity, and ±1.0% rel. for activation energy). The new model is an advanced version of the model proposed earlier. The basic equation used in this model to calculate the composition, temperature, pressure, and phase dependences of the viscosity of magmatic melts is a partly modified Arrhenian equation:ηPT=ηoexpEXP/RTwhere ηo is the pre-exponential factor for the viscosity of melts at T → ∞, (ηo = 10–4.5 ± 10–0.1 Pa s); ηTP is the melt viscosity at given temperature, pressure and a volume content of crystals and bubbles in the melt (Pa s); T is the absolute temperature in K; EXP is the activation energy of viscous flow (J/mol) depending on melt composition, including volatile components, and pressure; R = 8.3192 (J/mol K) is the gas constant.This model predicts the viscosity of near-liquid silicate and magmatic melts over nine orders of magnitude of viscosity (10−1-108 Pa s) and transforms about 3 decades of experimental study of silicate and magmatic melt viscosities.