Hydrous albite magmas at lower crustal pressure: new results on liquidus H2O content, solubility, and H2O activity in the system NaAlSi3O8–H2O–NaCl at 1.0 GPa

Abstract

The system albite–H2O serves as an important model for the generation of granitic magmas, yet relatively few experimental investigations have focused on phase relations at high pressure. This study reports new experimental results, at 1.0 GPa and 690–1050 °C, on the temperature and liquid composition at vapor-saturated melting, the H2O content of undersaturated silicate liquids in equilibrium with albite, the solubility of albite in H2O–NaCl fluids immediately below the solidus, and the activity of H2O in hydrous NaAlSi3O8 liquids along the liquidus. Albite melts and dissolves congruently at all temperatures and salinities. In the NaCl-absent system, the temperature of vapor-saturated melting of low albite, confirmed by X-ray diffraction, is 695 ± 5 °C and the liquid composition is 18.14 ± 1.35 wt% H2O. The temperature dependence of the fluid-undersaturated liquidus curve in the system NaAlSi3O8–H2O varies with H2O wt% (\(w_{{{\text{H}}_{2} {\text{O}}}}\)) according to $$T = - 2.0331 \times 10^{ - 3} w_{{{\text{H}}_{2} {\text{O}}}}^{3} + 1.6497w_{{{\text{H}}_{2} {\text{O}}}}^{2} {-} 58.963w_{{{\text{H}}_{2} {\text{O}}}} + 1235.5\,^{ \circ } {\text{C}}$$ indicating positive curvature in temperature-composition coordinates and a dry melting temperature of ~1235 °C. At 690 °C, immediately below the solidus, albite solubility decreases drastically with NaCl content of the fluid phase, from 8.8 ± 0.6 wt% in the NaCl-free fluid to ˂2 % at NaCl concentration of only 10 mol%. Experiments determining the activity of H2O (\(a_{{{\text{H}}_{2} {\text{O}}}}\)) in liquids at vapor-saturated melting exploited low Cl solubility in liquids and low albite solubility in the presence of H2O–NaCl fluids. The maximum Cl content of quenched glasses, only 0.95 wt%, and very low albite solubility together make possible H2O activity measurement in melts equilibrated with NaCl–H2O solutions. When combined with activity data for H2O–NaCl fluids, experimentally determined \(a_{{{\text{H}}_{2} {\text{O}}}}\) along the liquidus is described by $$T = - {469.16a_{{H_{2} O}}^{L}}^{{\frac{1}{2}}} {-} 93.382a_{{H_{2} O}}^{L} + 1235.5\;^\circ C.$$ At 1 GPa, H2O activities in hydrous albitic melts in H2O–NaCl fluids agree with those in the presence of H2O–CO2 fluids, provided appropriate fluid mixing data are used. Our results constrain an asymmetric regular solution model for the mixing of Na1/8Al1/8Si3/8O and H2O, yielding Margules parameters of W Ab = 25.56 ± 0.54 and \(W_{{{\text{H}}_{2} {\text{O}}}} = \, 10.50\; \pm \;0.74{\text{ kJ}}/{\text{mol}}\). The results imply critical mixing of liquid and vapor at ~1100 °C. Using existing speciation models, our results imply that H2O dissolved in hydrous albitic liquids is almost entirely in the form of molecular H2O (OH− concentration is negligible) at temperatures near vapor-saturated melting. The results provide a more complete experimental foundation for modeling the system NaAlSi3O8–H2O at high pressure.