H2O SOLUBILITY IN HAPLOGRANITIC MELTS - COMPOSITIONAL, PRESSURE, AND TEMPERATURE-DEPENDENCE

H2O solubility has been determined in haplogranitic melts (system SiO2 -NaAlSi3O8-KAlSi3O8, Qz-Ab-Or) in the range 0.5-8 kbar and 800-1350 de grees C. Three types of starting materials were used: dry glass cylind ers, prehydrated glass pieces, or dry glass blocks surrounded by glass powder. All the starting materials gave consistent results. The H2O c ontents of the glasses were determined by Karl-Fischer titration. Diss olved H2O was demonstrated to be distributed homogeneously throughout the isobarically quenched melts (glasses) using infrared spectroscopy. The compositional dependence of H2O solubility was mainly determined at 0.5 kbar, 900 and 1000 degrees C; 1 kbar, 850 degrees C; and 4.8 kb ar, 800 degrees C. Seventeen compositions containing 25, 35, or 45 wt% normative Oz and with various Or/(Or + Ab) ratios (0.86-0.09, Ab and Or expressed as normative weight percent) were investigated. At 0.5 kb ar, H2O solubility was little affected by the anhydrous composition. B y contrast, molar H2O solubility in aluminosilicate melts was signific antly dependent upon anhydrous composition between 1 and 5 kbar. The h ighest solubility values were obtained for the most Ab-rich melts. Thi s alkali effect has important implications for the physical and chemic al properties ofgranitic melts. The effect of pressure (P) on H2O solu bility at P greater than or equal to 3 kbar is greater than that repor ted in previous studies. Between 3 and 8 kbar at 800 degrees C, there is a (nearly linear) positive correlation between P and H2O solubility . The effect of temperature (T) on H2O solubility was investigated for a composition Qz(28)Ab(38)Or(34) (normative weight percent) in the P- T range 0.5-8 kbar and 800-1350 degrees C. Water solubility ranged fro m retrograde (with increasing T) at P less than or equal to 4 kbar thr ough temperature independence at approximately 4.5 kbar to prograde at P = 5 kbar. Calculated H2O solubilities using the model of Burnham an d Nekvasil (1986) are slightly high at 0.5 kbar and significantly low at 5 kbar, compared with the experimental data. This implies that calc ulated H2O activities for haplogranitic systems using the H2O content of the melt may be overestimated at high pressure (P greater than or e qual to 5 kbar). Using the thermodynamic model of Silver and Stolper ( 1985) and assuming a proportion of molecular H2O and OH groups close t o that defined for albite melts by Silver and Stolper (1989), we found that the partial molar volume of H2O in a melt with a composition Qz( 28)Ab(38)Or(34) has to be close to 10-12 cm(3)/mol to obtain a good ag reement between the calculated and the experimentally determined H2O s olubility curves in the pressure range 1-8 kbar at 900 degrees C.