Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets

Sodium chloride is a major component of extraterrestrial oceans, but it is generally assumed to be excluded from the high-pressure ice mantles of water-rich planets. This work tests that assumption directly by examining what happens to a concentrated NaCl solution under compression.

The experiments began by quenching a concentrated NaCl solution into an amorphous state before compression. The resulting samples were studied with synchrotron x-ray diffraction and neutron diffraction, and the observations were complemented by ab initio calculations.

The compressed material crystallized into a metastable NaCl-bearing ice VII phase. Diffraction revealed an expanded and strongly disordered ice VII lattice with no detectable crystalline NaCl. The ab initio calculations reproduced the lattice expansion by substituting water molecules with Na+ and Cl- ions, and showed that this structural response increases strongly with salt concentration.

On decompression at 300 K, the phase expelled crystalline NaCl at 5.1 ± 0.3 GPa and did not reform upon recompression — demonstrating that its formation is controlled by the thermodynamic pathway rather than by equilibrium alone.

Despite an 8.5% larger unit-cell volume, NaCl-bearing ice VII has a density close to that of pure ice VII and is substantially less dense than an equivalent mechanical mixture of ice and halite. These properties suggest that salty ice may be transported through deep convecting mantles but release its salt near the 5 GPa boundary, where the denser products could inhibit further upward transport. Pathway-dependent salt trapping may therefore provide a previously unrecognized control on chemical exchange between rocky interiors and subsurface oceans.