Fluorine in aluminosilicate systems: Phase relations in the system NaAlSi 3O 8-CaAl 2Si 2O 8-F 2O −1

Abstract

The liquidus phase equilibria of 0.9 NaAlSi 3O 8-0.1CaAl 2Si 2O 8-F 2O −1 was determined to 10 kbar for a composition with molar F (F + O) = 0.095 . The temperature of the liquidus is depressed by ~200°C at 1 bar, 2 kbar, and 10 kbar, relative to the volatile-free liquidus. Additional experiments were carried out at 2 kbar to examine the effects of increasing F content on phase equilibria. Increasing molar F (F + O) to 0.156 at fixed pressure decreases the temperature of the liquidus ~350°C from the temperature of the volatile-free liquidus. At constant F (F + O) (=0.095), X An of the plagioclase coexisting with the liquid decreased from An 30 at 1 atm to An 22 at 2 kbar and to An 17 at 10 kbar. Increasing fluorine content of the liquid at 2 kbar decreased the An content of the feldspar to a value of An 13 in equilibrium with the liquid with F (F + O) = 0.156 . Thermodynamic calculations demonstrate that the activity of NaAlSi 3O 8 in the liquid decreases with increasing fluorine content and concomitant decreasing temperature, but to a lesser extent than would be expected by the magnitude of the decrease in temperature. In contrast, the activity of CaAl 2Si 2O 8 decreases to a greater extent than is attributable solely to the effect of temperature. The effect of fluorine is in marked contrast to that of H 2O, which has little effect on the composition of the plagioclase in equilibrium with a melt of given (anhydrous) composition. Because of this, the degree of plagioclase fractionation in a fluorine-bearing, anhydrous system would be in considerable excess of estimates based solely on the fluorine-free anhydrous albite-anorthite system or the H 2O-saturated albite-anorthite-H 2O system. Rigorous application of these results to fractionation in late-stage, felsic systems will require additional experiments on the combined effects of H 2O and F in Ca-bearing granitic systems.