Potassium-Rich Ices at High Pressures and Temperatures

Abstract:

Accurate modeling of planetary interiors requires that the pressure-volume-temperature 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 1.6 mole percent KCl-bearing aqueous solution was studied up to 33 GPa and 650 K, and the incorporation of K⁺ and Cl^- into the ice VII structure was documented. The compression data at 300 K were fit with a third order Birch-Murnaghan equation of state and yielded K, K^/, and V₀ of 24.7±0.9 GPa, 4.44±0.09, and 39.17±0.15 ų, respectively. Thermal expansion coefficients were also determined for several isothermal compression curves at elevated temperatures, and a P-V-T equation of state will be presented. The melting of ice VII with incorporated K⁺ and Cl^- was determined up to 625 K and 10.6 GPa and was fit by using a Simon-Glatzel equation. The melting curve is systematically depressed relative to the melting curve of pure H₂O by approximately 45 K and 80 K at 4 and 11 GPa, respectively. Interestingly, a portion of the K⁺ and Cl^- contained within the ice VII structure was observed to exsolve with increasing temperature and pressure. 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. Birch (1951; JGR, 56, 107-126) has estimated that ⁴⁰K contributes 2.7 µcal/g.year for each wt.% of K, and our results suggest at least 3.33 wt.% can be incorporated into the structure of ice VII, thus making it a source of heat rather than just a conductive layer. In conclusion, our data illustrate a mechanism that may concentrate K at depth and impact the supposed pressure and temperature within moderate to large sized H₂O-rich planetary bodies.