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 °C. Three types of starting materials were used: dry glass cylinders, prehydrated glass pieces, or dry glass blocks surrounded by glass powder. All the starting materials gave consistent results. The H2O contents of the glasses were determined by Karl-Fischer titration. Dissolved 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 °C; 1 kbar, 850 °C; and 4.8 kbar, 800 °C. Seventeen compositions containing 25, 35, or 45 wt% normative Qz and with various Or/(Or + Ab) ratios (0.86–0.09, Ab and Or expressed as normative weight percent) were investigated. At 0.5 kbar, H2O solubility was little affected by the anhydrous composition. By contrast, molar H2O solubility in aluminosilicate melts was significantly dependent upon anhydrous composition between 1 and 5 kbar. The highest solubility values were obtained for the most Ab-rich melts. This alkali effect has important implications for the physical and chemical properties of granitic melts.

The effect of pressure (P) on H2O solubility at P ≥ 3 kbar is greater than that reported in previous studies. Between 3 and 8 kbar at 800 °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 Qz28Ab38Or34 (normative weight percent) in the P-T range 0.5–8 kbar and 800–1350 °C. Water solubility ranged from retrograde (with increasing T) at P ≤ 4 kbar through temperature independence at approximately 4.5 kbar to prograde at P = 5 kbar.

Calculated H2O solubilities using the model of Burnham and Nekvasil (1986) are slightly high at 0.5 kbar and significantly low at 5 kbar, compared with the experimental data. This implies that calculated H2O activities for haplogranitic systems using the H2O content of the melt may be overestimated at high pressure (P ≥ 5 kbar). Using the thermodynamic model of Silver and Stolper (1985) and assuming a proportion of molecular H2O and OH groups close to 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 Qz28Ab38Or34 has to be close to 10–12 cm3/mol to obtain a good agreement between the calculated and the experimentally determined H2O solubility curves in the pressure range 1–8 kbar at 900 °C.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.