Rutile solubility in supercritical NaAlSi3O8–H2O fluids

Rutile solubility was measured in the supercritical NaAlSi3O8–H2O system using hydrothermal piston–cylinder methods at 900 °C. Solubility in intermediate albite–H2O fluids varies as a function of both pressure (P) and fluid composition (X). Rutile solubility increases with increased albite content of the fluid, and is on the order of ~1000 to ~5000 ppm for all P–X conditions investigated here. The relationship between solubility and pressure is complex, and solubility decreases with increasing pressure for intermediate fluid compositions (35–75 wt.% H2O). Ti contents of glasses indicate that, with rising pressure at 900 °C, the liquid–vapor miscibility gap in the albite–H2O system closes at 0.9–1.0 GPa. A single supercritical fluid is stable above this pressure, to 2.0–2.2 GPa when further compression induces a return to a subcritical state. Supercritical fluids could be plausible transfer agents for Ti and other HFSE in high-P metasomatic environments, but the results show that their stability may be more limited than previously recognized.