The influence of H₂O on the viscosity of a haplogranitic melt

Abstract

The viscosity T/of dry and hydrous haplogranitic melts (anhydrous normative composition: QZ28Ab340r38)has been determined between 3 and 10 kbar and 800 and 1400 DC using the falling-sphere method. The H20 content of the melt ranged from 0.03 to 8.21 wt%. Experiments were performed in internally heated pressure vessels (T = 900-1400 DC)and cold-seal pressure vessels (T = 800 DC). The viscosity decreases with increasing H20 content of the melt. The strongest decrease is observed at low H20 concentrations. The effect of H20 is smaller at high H20 concentrations in the melt, with an almost linear behavior between log T/and H20 content expressed as weight percent H20 (decrease of 0.26 log units per weight percent H20 for H20 contents ~4 wt% H20). No pressure effect could be observed between 3 and 10 kbar at 900 DCfor a melt containing 5.90 wt% H20. In the investigated range the activation energy of the viscous flow decreases from 195 to 133 kJ/mol for melts with 1.05 to 8.21 wt% H20. The effect of T and H20 content of the melt on viscosity can be calculated with a precision of :to.2 log units with the use of the following expression: log T/= -1.57 + [23.398 - 13.197(cH2o)o.1l]x 103 (1/1). Viscosities calculated using the model of Shaw (1972) show that, for the investigated composition, the model underestimates the influence of H20 for low H20 concentrations (:::;4 wt% H20, difference up to two orders of magnitude at 800 DC)and overestimates slightly the influence of H20 for high H20 concentrations (~5 wt% H20). In comparison with the model ofPersikov et al. (1990), which takes into account the OH- and molecular H20 proportions, the experimental data at 800 DCare in good agreement with the calculated viscosities (deviation:::; I log unit) assuming that the proportions ofOH- groups and molecular H20 are those found in an in situ spectroscopic investigation of the melt. However, at higher temperatures (1000-1300 DC)the viscosity is overestimated for the OH- and H20 proportions recalculated for the appropriate temperatures.