Melt viscosities in the system NaAlSi₃O₈-H₂O- F₂O_-1.

The viscosities of eight melt compositions in the system albite-H₂O-F₂O_-1 were determined at high P and T. Measurements were performed using the falling sphere method at 1000-1600oC and 2.5 to 22.5 kbar. Fluorine and water strongly decrease the viscosity of albite melt at all P and T investigated. The viscosity of a fluorine-bearing albitic melt decreases from 1 bar to 22.5 kbar, whereas there is a viscosity maximum near 7.5 kbar for a water-bearing albite melt. At constant P the effects of fluorine and water, compared on an equimolar basis of added F₂O_-1 and H₂O (or F and OH), are broadly similar, and both F₂O_-1 and H₂O are less effective than Na₂O in reducing the viscosity of albite melt. Log viscosity exhibits a positive curvature as a function of mole fraction of added fluorine or water. The effect of HF on albite melt viscosity is greater than that predicted from a linear interpolation of the effects of water and fluorine. This negative deviation from additivity may result in extremely low viscosities for granitic magmas enriched in both fluorine and water. (Author's abstract)-J.M.H.