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 (10(1)-10(6) Pa s) using the falling sphere(s) method and in the high viscosity range (10(8)-10(13) Pa s) using parallel-plate viscometry. Failing 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 eta (in Pa s) as a function of temperature T (in K) and water content w (in wt.%) log eta = -4.86 + 8198/(T-530) - 6060/(T-573) . w/(w(1.1673)-2.724+0.0056.T). This expression reproduces the experimental data (191 in total) in the viscosity range from 10(1) to 10(13) Pa s with a root mean squared deviation of 0.15 log units. (c) 2005 Elsevier B.V. All rights reserved.