Viscosities of hydrous leucogranitic melts: a non-Arrhenian model

An equation is presented for the calculation of the shear viscosity of hydrous (0-12.5 wt.% H₂O) leucogranitic melts from 10² to 10¹³ Pa.s. The equation is a multiple nonlinear least-squares regression of a data set of 111 viscosity determinations in the literature. It is based on the Vogel-Fulcher-Tammann (VFT) form and thus accounts for the very important non-Arrhenian temperature dependence of the viscosity. This is possible because of the inclusion of data obtained recently in the high-viscosity region. The equation includes a logarithmic dependence of the three adjustable parameters of the VFT equation on the H₂O content of the melt. The root-mean-standard deviation for the 111 data points included is 0.46 log₁₀ units. In comparison with earlier methods based on the Arrhenian approximation of the temperature dependence of viscosity, the present method provides significant improvement in the accurate prediction of viscosity.