Water and the viscosity of andesite melts

The viscosity of a synthetic andesite-like melt has been measured between 1010 and 1014 P for water contents in the range 0–3.5 wt%. The very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision. After a steep viscosity decrease of > 5 orders of magnitude for 1 wt% H2O, an additional 2.5 wt% H2O causes a further viscosity decrease of only 2 orders of magnitude. These viscosity decreases are qualitatively similar to those observed previously for more silica-rich compositions. The new data join smoothly with available high-temperature measurements made at high pressures on water-bearing andesite melts. Because the intrinsic effects of pressure are as small for water-bearing as for water-free samples, the depressing effect of water on the viscosity of natural andesite melts can be estimated. Changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity. Although quantitative applications are not yet possible, the configurational entropy theory accounts qualitatively for these features.