The solubility of H₂O in melts in the system: SiO₂- Al₂O₃-Na₂O-K₂O at 1 to 2 Kbars.

The solubilities of water in six melts in this system were determined at 970-1630 bars and 800oC. Melts were synthesized hydrothermally in Pt capsules in cold-seal vessels and quenched isobarically. The solubilities at 970 bars for granitic and phonolitic minimum melts were 2.88 + or - 0.10 wt % and 5.01 + or - 0.14 wt %, respectively. Both peralkaline and peraluminous granitic melts have higher H₂O solubilities than the 1 kbar PH₂O minimum melt and indicate the existence of minima in solubilities at molar (Na + K)/Al = 1 along joins of constant SiO₂. Two melts at constant SiO₂ and (Na + K)/Al ratio, but varying in Na/(Na + K), 0.57, 0.69 M, have the same solubility, 2.88 + or - 0.10, 2.74 + or - 0.10 wt %. The pressure dependence of solubility was investigated for a peralkaline melt with six determinations at four pressures in the range 970-1620 bars. The mole fraction of dissolved water for this melt is proportional to the sq root fH₂O. The data for the haplogranite minimum are relatively low; however, detailed comparisons with other results are difficult. The solubility of water in a Bishop Tuff melt was investigated and used with direct measurements of H₂O in rhyolitic melt inclusions to obtain the minimum PH₂O (2100 + or - 300 bars), and minimum depth (8 + or - 1 km) of crystallization. (Authors' abstract) -C.N.