Research Progress on Ion-Partitioning Pathways in High-Pressure Saline Water Systems

Phase-transition behavior in high-pressure saline water systems is fundamental to understanding the deep hydrospheres of icy moons, the ice mantles of water-rich planets, and high-pressure ice physics. Compared with pure H₂O systems, salt ions not only modify the formation pathways of high-pressure ice phases such as ice Ⅵ and ice Ⅶ, but may also be redistributed as salt hydrates, salt-affected ice Ⅶ, or residual brines. In this review, we address the question of where salt ions reside in high-pressure aqueous systems by establishing a comparative framework based on two aspects: phase-transition pathway controls and evidence-assessment criteria. Concentration, water activity, pressure-temperature path, precursor state, and hydrate stability mainly govern the partitioning of salt ions among ice phases, hydrates, and residual fluids, whereas evidence type and evidence strength define the reliability and wording boundaries of different conclusions. On this basis, we review recent progress in representative systems including LiCl, NaCl, KCl, KBr, CaCl₂, and MgCl₂. Existing studies show that the LiCl system provides representative evidence for salt-affected ice Ⅶ formed through a non-equilibrium pathway; the NaCl system exhibits strong condition dependence and remains controversial with respect to extensive lattice incorporation; KCl/KBr and some NaCl systems preferentially form salt hydrates; the CaCl₂ system reveals a stepwise mechanism involving hydrate formation, hydrate decomposition, and subsequent perturbation of ice Ⅶ; and the MgCl₂ system indicates that the relationship between multiple hydrates and potential MgCl₂-ice Ⅶ still requires confirmation by continuous in situ experiments. It should be noted that this review focuses on representative chloride and halide systems for which relatively systematic high-pressure experimental data are available, whereas sulfate, carbonate, and multicomponent salt systems are discussed only briefly in the context of planetary implications and future research directions.