The effect of water on the viscosity of a synthetic calc-alkaline basaltic andesite

Abstract

The viscosity of a series of 6 Fe-free, synthetic basaltic andesite liquids, containing up to 3.76wt.% dissolved water, was measured in the range of the glass transition (108–1013Pas) by parallel-plate viscometry. Concentric-cylinder and falling-sphere viscometry provided high-temperature measurements (10–103Pas) on basaltic andesite liquids containing up to 2wt.% dissolved water. The viscosity (η in Pas) of Fe-free basaltic andesite can be described as a function of temperature (T in Kelvin) and water content (w in wt.%) by the expression log(η)=−4.81+6940.7/(T−{491.9−272.5 log[w+0.49]}).This parameterization reproduces 55 viscosity data with a root-mean-square-deviation (RMSD) of 0.24 log units in viscosity. The results of this viscometry study suggest that basaltic andesite liquids should remain very fluid, even while undergoing equilibrium degassing, to pressures as low as 50MPa (i.e., less than 2km depth). Only a modest increase in viscosity of at most a factor of 100 would occur in the last 2km of ascent. Furthermore, our results show that water affects the viscosity of a wide range of depolymerized melts to a similar degree. For example, the addition of 2wt.% dissolved H2O reduces the viscosity of andesite, basaltic andesite, basalt and their alkalic counterpart liquids by a factor of ~15–50.