Experimental study of the NaCl–H2O system up to 28 GPa: Implications for ice-rich planetary bodies

Recent studies have hypothesized that high-pressure H2O polymorphs, specifically Ice VI and Ice VII, make up a significant portion of the interiors of select outer planets and their moons; most notably the Galilean satellites, Saturn's Titan and possibly Neptune's moon Triton as well as potential H2O-rich extra-solar bodies. Several of these bodies have been conjectured to contain subsurface salty H2O waters; therefore, any potential ice phases in the interior of these satellites could have interacted extensively with the salty oceans. Raman spectroscopy and synchrotron radiation have been used previously to study the bonding structure and unit cell parameters of pure Ice VII. However, no data exist on the effect of salts on the unit cell parameters and volume of solid H2O at high pressure. To obtain pertinent data for use in planetary physics, it is important to understand the effect of impurities on H2O at high pressure. The NaCl–H2O system was chosen as a first order approximation of H2O-rich planetary bodies. The unit cell parameters and OH stretching frequencies of Ice VII formed from 5 and 10 wt.% NaCl–H2O solutions were studied in detail up to 27 GPa at 298 K by using a diamond anvil cell, synchrotron X-ray radiation and Raman spectroscopy. The data indicate that, over the range in pressure and temperature of this study, the maximum solubility of solutes in Ice VII was not pressure dependent. Our data suggest that the maximum concentration of NaCl that can be incorporated into Ice VII at 298 K is 7.5 ± 2.5 wt.% (or 2.4 ± 0.8 mol% NaCl). Ice VII formed from a 5 wt.% NaCl–H2O solution has a density that is up to 5% greater at any given pressure relative to the density of Ice VII formed from pure H2O. Additionally, the bulk modulus, 26.2 ± 1.4 GPa, was found to be approximately 10–20% greater relative to Ice VII formed from pure H2O. Relative OH stretching frequency shifts from Ice VII formed from the NaCl–H2O solutions were compared to Ice VII formed from pure H2O. Ice VII formed from a 5 wt.% NaCl–H2O solution shows a systematic increase of approximately 15 rel cm−1 in the OH stretching frequency relative to pure Ice VII for any given pressure. We hypothesize that the incorporation of Na+ and Cl− into the body centered cubic structure of Ice VII results in systematic variations in the intensive thermodynamic properties of Ice VII formed from low salinity solutions. These new data will provide an improved first order approximation of high-pressure H2O phases found within H2O-rich bodies and should be used to model the density profiles of H2O-rich bodies.