¹J. Michael Brown, ²Olivier Bollengier, ¹Ula Jones, ³Steven D. Vance, ¹Baptiste Journaux
Geochimica et Cosmochimica Acta (in Press)
Open Access Link to Article [DOI: 10.1016/j.gca.2026.10.019]
¹Earth and Space Sciences, University of Washington, Seattle, 98195, WA, USA
²Chemistry Department, University of Nantes, Nantes, France
³Astrobiology Group 322A, MS183-301, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, 91109, CA, USA

Copyright Elsevier

We present a new Gibbs energy representation for aqueous NaCl, based on new sound speed and phase equilibrium measurements, using local basis functions. Throughout, NaCl(aq) denotes the bulk aqueous component—the sum of ${\text{NaCl}}^{\text{0}}$, Na${}^{+}$ and Cl${}^{-}$—rather than the neutral complex, with the degree of dissociation implicit in the measurements rather than imposed. This comprehensive equation of state, valid for pressures to $10000MPa$, temperatures from $230K\text{to}2000K$, and molality to $7mol{kg}^{-1}$, allows us to address chemical processes within Earth and interior structure and evolution of large icy moons with possible oceans. The region below $500MPa$ at temperatures above $500K$, which contains water’s saturation curve and critical point, is excluded for want of constraining measurements; within the domain represented the fluid is a single phase. In regimes unconstrained by data, it assumes near-gas behavior at high temperatures and volume mixing of water and salt at extreme pressures. Differentiation of the representation yields all equilibrium properties, including volumes, specific heat capacities, and chemical potentials. The model is validated by its accurate prediction of ice Ih and VI melting and brucite dehydration in saline solutions, and by its reproduction of antigorite dehydration in pure water through two changes in reaction product to beyond $6000MPa$. We describe three applications of the Gibbs energy representation. (1) The depth during subduction of the antigorite dehydration reaction in the presence of saline geofluids is estimated to be reduced by $2km\text{to}4km$ per mole of NaCl in solution, substantially less than the up to $10km$ per mole implied by previously inferred pressure-dependent water activities. (2) A ${\text{H}}_{\text{2}}\text{O}-\text{NaCl}$ phase diagram to $1000MPa$ and $230K\text{to}400K$ identifies the pressure-variant eutectics for hydrohalite and disodium chloride decaheptahydrate, and places the cenotectic—the absolute limit of liquid stability—at $205MPa$, $235K$ and $3.90mol{kg}^{-1}$, where the liquid coexists with ice Ih, ice II and disodium chloride decaheptahydrate rather than with hydrohalite. (3) Icy moons: structural models of Ganymede, Callisto, and two Titan cases with $1.90mol{kg}^{-1}$ aqueous NaCl show that melting point suppression stabilizes ices II, III, V, and VI, and that ices II and III can be buoyant in the coexisting fluid, with the sense of the ice II contrast reversing between 50 and $100g{kg}^{-1}$. The resulting co-occurrence of ices II and III, not previously reported, admits multiphase overturning and implies that salinity governs the geodynamics of these $>100km$ lithospheres. Finally, we set out how a bulk Gibbs energy surface interfaces with species-based frameworks such as HKF and the Deep Earth Water model, and propose that the pressure derivative of the surface can be used to constrain the degree of association at high pressure.