THE INFLUENCE OF H2O ON THE VISCOSITY OF A HAPLOGRANITIC MELT

The viscosity eta of dry and hydrous haplogranitic melts (anhydrous no rmative composition: Qz(28)Ab(34)Or(38)) has been determined between 3 and 10 kbar and 800 and 1400 degrees C using the falling-sphere metho d. The H2O content of the melt ranged from 0.03 to 8.21 wt%. Experimen ts were performed in internally heated pressure vessels (T = 900-1400 degrees C) and cold-seal pressure vessels (T = 800 degrees C). The vis cosity decreases with increasing H2O content of the melt. The stronges t decrease is observed at low H2O concentrations. The effect of H2O is smaller at high H2O concentrations in the melt, with an almost linear behavior between log eta and H2O content expressed as weight percent H2O (decrease of 0.26 log units per weight percent H2O for H2O content s greater than or equal to 4 wt% H2O). No pressure effect could be obs erved between 3 and 10 kbar at 900 degrees C for a melt containing 5.9 0 wt% H2O. In the investigated range the activation energy of the visc ous flow decreases from 195 to 133 kJ/mol for melts with 1.05 to 8.21 wt% H2O. The effect of T and H2O content of the melt on viscosity can be calculated with a precision of +/-0.2 log units with the use of the following expression: log eta = -1.57 + [23.398 - 13.197(c(H2O))(0.11 )] x 10(3)(1/T). Viscosities calculated using the model of Shaw (1972) show that, for the investigated composition, the model underestimates the influence of H2O for low H2O concentrations (less than or equal t o 4 wt% H2O, difference up to two orders of magnitude at 800 degrees C ) and overestimates slightly the influence of H2O for high H2O concent rations (greater than or equal to 5 wt% H2O). In comparison with the m odel of Persikov et al. (1990), which takes into account the OH- and m olecular H2O proportions, the experimental data at 800 degrees C are i n good agreement with the calculated viscosities (deviation less than or equal to 1 log unit) assuming that the proportions of OH- groups an d molecular H2O are those found in an in situ spectroscopic investigat ion of the melt. However, at higher temperatures (1000-1300 degrees C) the viscosity is overestimated for the OH- and H2O proportions recalc ulated for the appropriate temperatures.