Abstract:
The composition-dependence of the solubility of H2O in silicate melts along the binary joins NaAlSi3O8-KAlSi3O8, NaAlSi3O8-LiAlSi3O8, and KAlSi3O8-LiAlSi3O8 has been determined at 2 kbar and 1040°C. The study involved 1 atm dry oxide fusion, hydrothermal saturation, isobaric rapid quench and macroscopic analysis for H2O using Karl Fischer titration (KFT) and microscopic investigation of homogeneity using infrared absorption spectroscopy, respectively. The solubility of H2O in these melts increases in the order KAlSi3O8 (5.12 wt%) < NaAlSi3O8 (6.03 wt%) < LiAlSi3O8 (7.32 wt%). The total relative weight percent variation in solubility is 43%, which corresponds to a relative mole percent variation of 23%. Along the joins, small but systematic deviations from additivity are observed at the 1σ uncertainty level. These deviations are positive on the NaAlSi3O8-KAlSi3O8 join but negative on the LiAlSi3O8-bearing joins. The present results confirm the early suggestion of Voigt et al. (1981) that solubility variation along the NaAlSi3O8-KAlSi3O8 join is nonlinear and disagree in detail with the equimolal approximation of Burnham (1975, 1981) and Burnham and Davis (1974). Models of H2O solubility must take into account not only the nonequimolal solubility of H2O in feldspathic melts but also nonlinear binary variations as well. We interpret the relative solubilities of the end-member compositions in terms of the relative stability of the tectosilicate melt structure and the nonlinearities along the joins in terms of the next-nearest neighbor distributions of Al and Si in these melts.