PHASE-RELATIONS AND COMPOSITIONAL DEPENDENCE OF H2O SOLUBILITY IN QUARTZ-FELDSPAR MELTS

Liquidus phase relations are presented for the systems Qz-Ab and Qz-Or at 2 and 5 kbar under both H2O-saturated and H2O-undersaturated conditions. The data allow to specify the effects on phase equilibria of. (1) varying the water content of the melt under isobaric conditions; and (2) varying pressure under H2O-saturated conditions. The effects are different from each other and they depend on the system investigated. Increasing the water content of the melt at 2 kbar lowers liquidus and eutectic temperatures more in the Qz-Ab than in the Qz-Or system, while eutectic compositions remain unchanged. In contrast, the effect of increasing P (= P(H2O)) from 2 to 5 kbar is mainly to shift the eutectic composition in the Qz-Ab system towards more albite-rich compositions, whereas no compositional change is observed for the Qz-Or eutectic. The contrast between the two systems when isobarically varying the water content of the melt can be attributed to higher water solubilities in Na- than in K-bearing melt compositions. Liquidus phase relations with varying P (= P(H2O)) reflect the combination of the effects of pressure and water content of the melt. The compositional range of water contents in the melt used in this study implies that molecular H2O is the water species involved in the observed changes in phase relations. Current models and recent spectroscopic data for water solubility mechanisms in aluminosilicate glasses and melts are critically examined. The variation in water solubility between aluminosilicate melts is due to the differential incorporation of molecular H2O.