Potassium chloride-bearing ice VII and ice planet dynamics

Abstract

Accurate modeling of planetary interiors requires that the pressure–volume–temperature (PVT) properties of phases present within the body be well understood. The high-pressure polymorphs of H2O have been studied extensively due to the abundance of ice phases in icy moons and, likely, vast number of extra-solar planetary bodies, with only select studies evaluating impurity-laden ices. In this study, ice formed from a 1.6mol percent KCl-bearing aqueous solution was studied up to 32.89±0.19GPa and 625K, and the incorporation of K+ and Cl− ionic impurities into the ice VII structure was documented. The compression data at 295K were fit with a third order Birch–Murnaghan equation of state and yielded a bulk modulus (KT0), its pressure derivative (KT0′), and zero pressure volume (V0) of 24.7±0.9GPa, 4.44±0.09, and 39.2±0.2Å3, respectively. The impurity-laden ice was found to be 6–8% denser than ice VII formed from pure H2O. Thermal expansion coefficients were also determined for several isothermal compression curves at elevated temperatures, and a PVT equation of state was obtained. The melting curve of ice VII with incorporated K+ and Cl− was estimated by fitting experimental data up to 10.2±0.4GPa, where melting occurred at 625K, to the Simon–Glatzel equation. The melting curve of this impurity-laden ice is systematically depressed relative to that of pure H2O by approximately 45K and 80K at 4 and 11GPa, respectively. A portion of the K+ and Cl− contained within the ice VII structure was observed to exsolve with increasing temperature. This suggests that an internal differentiating process could concentrate a K-rich phase deep within H2O-rich planets, and we speculate that this could supply an additional source of heat through the radioactive decay of 40K. Our data illustrate ice VII can incorporate significant concentrations of K+ and Cl− and increasing the possibility of deep-sourced and solute-rich plumes in moderate to large sized H2O-rich planetary bodies.