Experimental investigation of phase relations in the system NaAlSi₃O₈–H₂O at high temperatures and pressures: liquidus relations, liquid–vapor mixing, and critical phenomena at deep crust–upper mantle conditions

Abstract

Supercritical fluids in rock–H₂O systems have been proposed to be important agents of mass transfer in subduction zone environments. New experimental studies were conducted on the simple model granite system NaAlSi₃O₈ (Ab)–H₂O in order to investigate phase relations and to develop the thermodynamic mixing properties between aqueous fluid (a.k.a. vapor, V) and silicate melt (a.k.a. liquid, L) at pressures (P) and temperatures (T) approaching those of critical mixing. We established liquidus and solvus phase relations by analyzing the quenched run products from piston–cylinder experiments over a range of $P - T - X_{{{\text{H}}_{2} {\text{O}}}}$ conditions from 1.0 to 1.7 GPa, 630–1060 °C and 4–92 wt% H₂O. Equations for the critical curve, solidus temperatures, albite solubility at the solidus, and vapor-saturated solidus H₂O content were formulated as functions of $P - T - X_{{{\text{H}}_{2} {\text{O}}}}$. We constructed a subregular solution model to describe the solvus curves using P and T dependent Margules coefficients ($W_{{{\text{H}}_{2} {\text{O}}}}$ and $W_{\text{Ab}}$). Activities of H₂O and Ab ($a_{{{\text{H}}_{2} {\text{O}}}}$ and $a_{\text{Ab}}$) could be formulated using only the input at the solidus and the critical point at each pressure because of the nearly linear dependence of the parameters on T. The solvus curves were confirmed independently by means of criteria established for classification of quenched products as L, L + V, or V and are in excellent agreement with the compositions that can be calculated using the Margules coefficients. At 1.6 GPa, the H₂O content at the vapor-saturated solidus is 44.5 ± 5.5 wt% and the solubility of albite at the solidus is 42.95 ± 0.99 wt%, indicating the imminent intersection of the two curves and thus a stable critical endpoint at some slightly higher pressure. We constrain the critical endpoint at 1.63 ± 0.02 GPa, 659 ± 5 °C, and a composition of ~ 44.7 wt% H₂O based on the intersections of the pressure dependent solidus curves with the critical curve, the pressure dependent albite solubility curve with the vapor-saturated solidus curve. The 1.7 GPa experiments showed no evidence for liquid–vapor immiscibility across a wide range of compositions and temperatures (4–80 wt% H₂O and 630–1050 °C, and furthermore, that low albite is stable in the presence of the supercritical fluid near the breakdown of albite to jadeite and quartz. These results provide a comprehensive account of the solution properties of subcritical and supercritical fluids in this model granite system at temperatures and pressures corresponding to the deep-crust regions of granite magma generation.