THE EFFECT OF FLUORINE ON THE VISCOSITY OF DACITIC MELTS: PRELIMINARY RESULTS

Fluorine is a common volatile component in silicate melts of all compositions. Although less abundant than water, fluorine is known to reduce significantly the liquidus temperature and the viscosity of silicic liquids, as does water. In contrast, previous studies suggest that fluorine has little or no effect on the viscosity of basaltic liquids. We are studying the effect of fluorine on dacitic and rhyodacitic liquids, to see if fluorine has a strong or weak effect on their viscosity; the results may be important for a better understanding of the rheology of subduction zone magmas.

In our experiments, we synthesize iron-free dacitic liquids from oxide and carbonate powders, and incorporate fluorine by using varying amounts of calcium fluoride instead of calcium carbonate. The nominal fluorine contents of the liquids are 0.5, 1, 2 and 4 weight percent. After decarbonation, and repeated cycles of melting and grinding to ensure homogeneity, the samples are quenched to a glass. Cylindrical cores are drilled from the glass and the viscosity measured by parallel plate viscometry, over the range 10⁹ to 10^12.5 Pa.s. For the dacites, this corresponds to temperatures between about 700 and 840?C, i.e. relevant magmatic temperatures.

At 800°C, the viscosity of fluorine-absent dacite is 10^10.79 Pa.s. Our preliminary results indicate that addition of 0.5 wt.% and 1 wt.% fluorine decreases the viscosity by factors of about 3 and 8, to 10^10.34 and 10^9.91 Pa.s, respectively. This compares with a viscosity of about 10^5.5 Pa.s for the same dacite containing 1 wt.% H₂O. Even on a mole percent basis, water is far more efficient at reducing the viscosity of dacitic liquids than fluorine. This suggests that dissolution mechanisms other than simple depolymerization of the silicate network are operating. Ongoing experiments will test the effect of higher fluorine contents, and the effect of fluorine on more polymerized rhyodacitic melts.