EFFECT OF NaCl on SOLUBILITY AND MELTING IN THE SYSTEM SiO2-H2O AT 15-20 KBAR AND 900-1100°C

Crustal fluids are rich in dissolved salts such as NaCl, and in SiO₂, an abundant rock-forming solute. Studying the behavior of the system SiO₂-NaCl-H₂O, therefore, gives first-order insights applicable to natural geologic fluids at depth. We investigated quartz solubility and melting in the presence of NaCl-H₂O fluids at 15-20 kbar and 900-1100°C using hydrothermal piston-cylinder methods. High-purity quartz crystals and/or powder, reagent-grade NaCl, and millipure H₂O were sealed in a platinum capsule by arc-welding. Experiments were held at pressure and temperature for 1-3 hr. After rapid quench (<30 sec), solubility was determined either from the weight loss of quartz, or by bracketing the presence or absence of quartz and/or melt to determine compositional limits of a phase field. Additionally, pH measurements were made on bracketing-run quench fluids, and electron microprobe analyses were performed on retrieved cohesive phases. Solubility results show exponential decrease of SiO₂ molality (m_SiO2) with increasing NaCl for any constant P and T. Where m_SiO2<2, data are consistent with Si dissolution as monomers and dimers with two H₂O molecules of hydration per Si, as at lower P and T; however, at higher Si concentrations, this relationship breaks down, suggesting the presence of more polymerized SiO₂ species. Bracketing-run results show the suppression of critical melt-vapor mixing (i.e., enlarged P-T stability of the melt-vapor miscibility gap) at salinities as low as X_NaCl=0.05. Runs inferred to have 2 fluid phases, melt and vapor (based on the presence of both glass and colloidal silica “roe” in quenched charges), were coincident with a highly acidic quench pH of ~1. Electron microprobe analyses indicate substantial Na in glass, with Cl below detection. Thus, the low quench pH is consistent with Na and Cl fractionation into the melt and vapor, respectively. Our results show that acid fluids can be generated by partitioning of alkalis and metals even when the silicate melt is derived from pure SiO₂, and that such behavior is possible at deep-crust/upper-mantle P and T. Furthermore, these results suggest that the ternary critical curve of the system follows a steep P/T trajectory, from the upper critical endpoint of pure SiO₂-H₂O, i.e., X_NaCl=0, at ~10 kbar, 1080°C to above 15 kbar, and 1100°C at X_NaCl=0.05.