Water solubility in aluminosilicate melts of haplogranite composition at 2 kbar

The compositional dependence of H₂O solubility was investigated at 2 kbar and 800°C in haplogranite melts (system SiO₂1bNaAlSi₃O₈1bKAlSi₃O₈ or Qz1bAb1bOr). The sixteen investigated compositions contained 25, 35 or 45 wt.% normative Qz and various Ab/(Ab+Or) ratios (0.15-0.92). Starting solid materials were anhydrous bubble-free glasses to which 10 wt.% H₂O was added. The H₂O contents of the isobarically quenched melts (glasses) were measured by Karl-Fischer titration. The results show that H₂O solubility in aluminosilicate melts depends significantly upon anhydrous composition. The highest solubility values are obtained for the most Ab-rich melts. At a constant normative quartz content, the solubility of water decreases from 6.49 ± 0.20 wt.% H₂O for a composition Qz₃₅Ab₆₀Or₀₅ (normative composition expressed in wt.%) to 5.50 ± 0.15 wt.% H₂O for a composition Qz₃₅Ab₁₀Or₅₅. Along this join, the most significant changes are observed for Ab-rich melts whereas H₂O solubility in Or-rich melts remains almost constant. The H₂O solubility data imply that H₂O is preferentially associated with the Ab component in aluminosilicate melts. Application of the results to natural granitic melts suggests that Na-rich, H₂O-saturated melts may be significantly less viscous than H₂O-saturated, K-rich melts. The temperature dependence of H₂O solubility, investigated for composition Qz₂₈Ab₃₈Or₃₄ at 2 kbar, is low. Increasing temperature from 750° to 1150°C only causes a decrease in H₂O solubility from 6.00 to 5.41 wt.% H₂O. These data are in agreement with previous data obtained for albite melts.