Viscosity of andesitic melts—new experimental data and a revised calculation model

The viscosity of a synthetic andesite-like melt was measured in the low viscosity range (101–106 Pa s) using the falling sphere(s) method and in the high viscosity range (108–1013 Pa s) using parallel-plate viscometry. Falling sphere experiments with melts containing 2.3 and 5.6 wt.% H2O were carried out in an internally heated gas pressure vessel (IHPV) at 500 MPa confining pressure. The sinking velocity of Pt and Pd spheres and in one case of a corundum sphere was used to measure the melt viscosity. In addition, a creep experiment was performed at ambient pressure using a glass containing 2.73 wt.% H2O . A more water-rich glass (5.6 wt.% H2O ) was investigated with a high pressure parallel-plate viscometer at 400 MPa confining pressure in an IPHV. By combining our new data with previous results for a similar melt composition we derived the following expression to describe the viscosity η (in Pa s) as a function of temperature T (in K) and water content w (in wt.%)log⁡η=−4.86+8198(T−530)−6060(T−573)·w(w1.1673−2.724+0.0056·T).This expression reproduces the experimental data (191 in total) in the viscosity range from 101 to 1013 Pa s with a root mean squared deviation of 0.15 log units.