Temperature induced immiscibility in the NaCl–H2O system at high pressure

High-pressure polymorphs of H2O are a major component in many outer planets, extra solar bodies, and icy satellites. This study sought to examine the influence of ionic impurities on the phase stability, thermal expansion, and melting curve of ice VII. Powder diffraction patterns of ice VII formed from pure H2O and 5 wt.% NaCl aqueous solutions were taken at room temperature up to 11.1 ± 0.3 and 26.6 ± 0.4 GPa, respectively. Thermal expansions, α, of all ice VII samples were recorded and modeled up to the melting point of the samples. Ice VII formed from a NaCl-bearing aqueous solution at pressures greater than 2.2 GPa and less than 500 K can be indexed by ice VII only, whereas at temperatures greater than 500 K, diffraction lines indicative of halite (NaCl) are observed and become more intense with increasing temperature and only disappear at the melting point of the high-pressure ice. This phenomenon was observed in all NaCl-bearing ice samples that were heated to greater than 500 K. The melting curves of ice VII formed from pure H2O and a 5 wt.% NaCl aqueous solution suggest that the presence of Na+ and Cl− in the ice VII structure results in a depression of the melting curve by approximately 40 K. The exsolution of halite from the NaCl-doped ice VII and the depression of the ice VII melting curve suggest that the presence of ionic impurities in ice VII may promote the formation of a self-segregating zone deep within ice-rich bodies. This zone could initiate the formation of solute-rich melt pockets that may ascend toward the surface and result in surface manifestations such as solute-bearing aqueous vents, unexplained domes/diapirism, and/or salt-rich regions.