Water solubility in silica and quartzofeldspathic melts

Water solubility in silica melts was determined at 100-600 MPa, 1200-1350 d egrees C, and at each temperature (T) was found to increase with pressure ( P). At P less-than-or-equal-to 250 MPa, the effect of T on water solubility in silica melts is small and within analytical precision (+/-0.15 wt% H2O) . A positive correlation with T was observed at 400 MPa. Increasing solubil ity of water with increasing T was observed when large amounts of water are dissolved in silica and quartzofeldspathic melts (i.e., when molecular wat er is the dominant species in the glasses at room temperature), as already observed for feldspar melts. Change in water solubility (expressed in mol%) with decreasing SiO2 content of the melt is nonlinear along the silica-alb ite join. In the compositional range Ab(100) to Ab(25) (100 to 25 mol% albi te, respectively, compositions calculated on an eight-oxygen basis), the so lubility of water at 200 MPa decreases only slightly with decreasing Ab con tent (-0.1 +/- 0.01 mol% H2O per mol% albite). However, at Ab contents less than 25 mol%, water solubility decreases sharply with increasing Qz conten t. Similar behaviour was observed at 500 MPa. These results suggest that tw o different incorporation mechanisms of water in quartzofeldspathic melts m ust be considered: one corresponding to an NaAlSi3O8-H2O mechanism, the oth er to an SiO2-H2O mechanism.