Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets
Abstract
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. Here we show that a concentrated NaCl solution, when quenched in an amorphous state before compression, crystallises in a metastable NaCl-bearing ice VII phase. Synchrotron x-ray diffraction and neutron diffraction reveals an expanded and strongly disordered ice VII lattice with no detectable crystalline NaCl. Ab initio calculations reproduce lattice expansion by substituting water molecules for Na+ and Cl- ions and show that the structural response increases strongly with salt concentration. Upon decompression at 300 K, the phase expels crystalline NaCl at 5.1 ± 0.3 GPa and does not reform upon recompression, demonstrating that its formation is controlled by the thermodynamic pathway. 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 salt near the 5 GPa boundary, where the denser products could inhibit further upward transport. Pathway-dependent salt trapping may therefore provide a previously unrecognised control on chemical exchange between rocky interiors and subsurface oceans.
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BibTeX
@misc{nicholls2026pressureinduced,
title = {{Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets}},
author = {Alasdair Nicholls and Tomasz Poreba and Xi Zhu and Richard Gaal and Stefan Klotz and Gabriel Tobie and Henrik M. Ronnow and Razvan Caracas and Livia Eleonora Bove},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.03806},
eprint = {2610.03806},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.03806},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Pressure-induced desalting of NaCl-bearing ice VII: A compositional filter in water-rich planets
AU - Alasdair Nicholls
AU - Tomasz Poreba
AU - Xi Zhu
AU - Richard Gaal
AU - Stefan Klotz
AU - Gabriel Tobie
AU - Henrik M. Ronnow
AU - Razvan Caracas
AU - Livia Eleonora Bove
PY - 2026
DA - 2026/10/01
PB - arXiv
DO - 10.48550/arxiv.2610.03806
UR - https://arxiv.org/abs/2610.03806
AB - 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. Here we show that a concentrated NaCl solution, when quenched in an amorphous state before compression, crystallises in a metastable NaCl-bearing ice VII phase. Synchrotron x-ray diffraction and neutron diffraction reveals an expanded and strongly disordered ice VII lattice with no detectable crystalline NaCl. Ab initio calculations reproduce lattice expansion by substituting water molecules for Na^+ and Cl^- ions and show that the structural response increases strongly with salt concentration. Upon decompression at 300 K, the phase expels crystalline NaCl at 5.1 ± 0.3 GPa and does not reform upon recompression, demonstrating that its formation is controlled by the thermodynamic pathway. 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 salt near the 5 GPa boundary, where the denser products could inhibit further upward transport. Pathway-dependent salt trapping may therefore provide a previously unrecognised control on chemical exchange between rocky interiors and subsurface oceans.
ER -