THE COMBINED EFFECTS OF WATER AND FLUORINE ON THE VISCOSITY OF SILICIC MAGMAS

Abstract

The Newtonian viscosity of water-plus-fluorine-bearing silicate melt of haplogranitic composition (HPG8) has been determined. Viscosities of HPG8 melt with addition of 3.11 and 4.25 wt.% of F and up to 3 wt.% H2O have been obtained using a micropenetration technique in the interval 109.74 to 1011.84 Pa · s and temperatures varying from 370 to 700°C, at ambient pressure. Determination of the temperature dependence of viscosity from this and previous studies permits the parameterization of the viscosity of melts containing water and fluorine, having similar composition, within a 0.3 log units standard error. The viscosity of water-bearing, F-rich haplogranitic samples is represented by a modified Vogel-Fulcher-Tammann (VFT) equation which provides a non-Arrhenian description of the temperature dependence of the viscosity. The results of this study indicate that, taken individually or together, both H2O and F− have a strong and similar effect on the viscosity of SiO2-rich compositions. This similarity between F2O−1 and H2O greatly simplifies the task of predicting viscosity for volatile-rich, highly silicic magmas. The low viscosities of hydrous fluorine-bearing granitic melts favour efficient crystallization-fractionaction paths for these liquids, controlling degassing paths and consequently the eruptive behaviour. Numerical simulations of eruptive events normally do not take into account the contribution of fluorine; this may introduce a significant error in the description of the fluid-dynamic properties of magma and, therefore, in the accurate prediction of eruptive scenarios, as well as in hazard assessment studies. Fluorine, unlike water, remains dissolved in the melt at high concentrations and low confining pressures. The incorporation of fluorine data and the modelling of fluorine-bearing viscosity data are therefore of fundamental importance for simulations of magma dynamics and prediction of eruptive scenarios.