Magmatic Processes : Physicochemical Principles
نویسنده
چکیده
The viscosities of eight melt compositions in the System albite-H20-F20_1 have been determined at high pressure and temperature. Measurements were performed using the falling sphere method at 1000-1600°C and 2.5 to 22.5 kbar. Fluorine and water strongly decrease the viscosity of albite melt at all pressures and temperatures 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 pressure the efFects of fluorine and water, compared on an equimolar basis of added F20_i and H 2 0 (or F and OH), are broadly similar and both F20_i and H 2 0 are less effective than Na20 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.
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