The Solubility of H2O in Melts in the System SiO2-Al2O3-Na2O-K2O at 1 to 2 Kbars

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Abstract
The solubilities of water in six melts in the system were determined at 970-1630 bars and 800°C. Melts were synthesized hydrothermally in Pt capsules in cold-seal vessels and then quenched isobarically. The solubilities were determined by micromanometric measurement of evolved during vacuum fusion of vesicle-free glass wafers and are reproducible to ±2.8% (2 s.d.) of the concentration. The solubilities at 970 bars for the granitic and phonolitic minimum melts are wt % and wt %, respectively. Both peralkaline and peraluminous granitic melts have higher solubilities than the 1 kbar minimum melt and indicate the existence of minima in solubilities at molar (Na + K)/Al = 1 along joins of constant . Two melts at constant and (Na + K)/Al ratio, but varying in Na/(Na + K) (0.57, 0.69 molar), have the same solubility ( wt %). The compositional dependence of solubilities for peralkaline and peraluminous melts is not predicted by the solubility mechanism and calculation method of Burnham (1975, 1979, 1981) that was developed for metaluminous melts. The pressure dependence of solubility was investigated for a peralkaline melt with six determinations at four pressures from 970 to 1620 bars. The mole fraction of dissolved water (based on 8 moles of O) for this melt is proportional to the square root of the fugacity of water; the root mean square deviation from linearity is 1.4% and is equal to the analytical precision (1 s.d.). Our data for the haplogranite minimum are lower than some other determinations; however, the limitations of the various methods for determining solubilities in melts make detailed comparisons with our data difficult. The solubility of water in a Bishop Tuff melt was estimated from our measurements and used with direct measurements of in rhyolitic melt inclusions ( wt %, Druitt et al. 1982) to obtain the minimum pressure of water ( bars) and minimum depth ( km) of crystallization.