EFFECTS OF F, B2O3 AND P2O5 ON THE SOLUBILITY OF WATER IN HAPLOGRANITE MELTS COMPARED TO NATURAL SILICATE MELTS

The effects of F, B2O3 and P2O5 on the H2O solubility in a haplogranite liquid (36 wt. % SiO2, 39 wt. % NaAlSi3O8, 25 wt. % KAlSi3O8) have been determined at 0.5, 1, 2, and 3 kb and 800, 850, and 900-degrees-C. The H2O solubility increases with increasing F and B content of the melt. The H2O solubility increase is more important at high pressure (2 and 3 kb) than at low pressure (0.5 kb). At 2 kb and 800-degrees-C, the H2O solubility increases from 5.94 to 8.22 wt. % H2O with increasing F content in the melt from 0 to 4.55 wt. %, corresponding to a linear H2O solubility increase of 0.53 mol H2O/mol F. With addition of 4.35 wt. % B2O3, the H2O solubility increases up to 6.86 wt. % H2O at 2 kb and 800-degrees-C, corresponding to a linear increase of 1.05 mol H2O/mol B2O3. The results allow to define the individual effects of fluorine and boron on H2O solubility in haplogranitic melts with compositions close to that of H2O-saturated thermal minima (at 0.5-3 kb). Although P has a dramatic effect on the phase relations in the haplogranite system, its effect on the H2O solubility was found to be negligible in natural melt compositions. The concomitant increase in H2O solubility and F can not be interpreted on the basis of the available spectroscopic data (existence of hydrated aluminofluoride complexes or not). In contrast, hydrated borates or more probably boroxol complexes have been demonstrated in B-bearing hydrous melts.