Thermodynamic properties of aqueous NaCl solutions to 1073K and 4.5GPa, and implications for dehydration reactions in subducting slabs

摘要

NaCl is the most common solute in aqueous fluids in many geological settings but the thermodynamic properties of binary NaCl-H_2O solutions have remained understudied at pressures above 0.5GPa. We report the first high-pressure (>0.5GPa) density data for NaCl-H_2O fluids derived from acoustic velocity measurements in 1m and 3m NaCl solutions in diamond anvil cells to 673K and 4.5GPa using Brillouin scattering spectroscopy. An empirical equation of state (EoS) for NaCl-H_2O fluids has been generated fitting the new density data together with literature data for water, and used for extrapolation of thermodynamic data to 1073K and 4.8m NaCl at 0.5-4.5GPa, spanning the conditions for fluids expelled by subducted slabs. The EoS yields densities for binary NaCl-H_2O solutions with uncertainty lower than 0.3-0.5% below 673K and lower than 2-4% in the extrapolation region. The EoS may allow extrapolations beyond 1073K although the resulting uncertainties are difficult to assess. Densities for NaCl-H_2O fluids computed from the EoS at 0.5GPa generally agree within 1% with published data below 673K, although differences of up to 5% are observed at 1073K. The experimentally-based EoS is more reliable in the calculation of pure water fugacities and phase equilibria involving fluid phases than previous formulations, particularly in the high pressure range. The presence of dissolved NaCl substantially decreases the activity of water, indicating large non-ideality in NaCl aqueous fluids at subduction zone conditions. This behavior has important implications for the localization of dehydration reactions in the slab, as the dehydration boundaries of hydrous phases shift toward shallower depths in the presence of dissolved NaCl in the fluid.