WATER AND THE VISCOSITY OF ANDESITE MELTS

The viscosity of a synthetic andesite-like melt has been measured betw een 10(10) and 10(14) P for water contents in the range 0-3.5 wt%. The very slow kinetics of water exsolution over this viscosity range allo wed 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, a n additional 2.5 wt% H2O causes a further viscosity decrease of only 2 orders of magnitude. These viscosity decreases are qualitatively simi lar to those observed previously for more silica-rich compositions, Th e new data join smoothly with available high-temperature measurements made at high pressures on water-bearing andesite melts. Because the in trinsic effects of pressure are as small for water-bearing as for wate r-free samples, the depressing effect of water on the viscosity of nat ural 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 qualitativel y for these features.