Grain growth of ice doped with soluble impurities

Abstract. The grain size of polycrystalline ice affects key parameters related to the dynamics of ice masses, such as the rheological and dielectric properties of terrestrial ice flow as well as the ice shells of icy satellites. To investigate the effect of soluble impurities on the grain-growth kinetics of polycrystalline ice, we conducted annealing experiments on polycrystalline ice samples doped with different concentrations of KCl (10⁻², 10⁻³, 10⁻⁴ and 10⁻⁵ mol/L) or MgSO₄ (10⁻² and 10⁻⁵ mol/L). Samples were annealed for a maximum of 100 h at a hydrostatic confining pressure of 20 MPa (corresponding to a depth of about 2 km ) and different constant temperatures of 268, 263, 258 and 253 K. After each experiment, images of a polished sample surface were obtained using an optical microscope equipped with a cold stage. With grain boundaries detected, grains were reconstructed from the images and an average grain size was determined for each sample. Normal grain growth occurred in all samples. Grain-size data are interpreted using the grain-growth model, dⁿ − d₀ⁿ kt (d: grain size; d₀: starting grain size; n: grain-growth exponent; k: growth constant; t: duration). Values of the best-fit grain growth exponent, n, for all samples range from 2.6 to 6.2, with an average value of 4.7. Pure ice exhibits 3.1 ≤ n ≤ 4.6, with an average value of 3.8. Above the eutectic point, soluble impurities enhance grain growth, as a melt phase is formed and it could provide a fast diffusion pathway. Below the eutectic point, soluble impurities impede grain growth probably via the formation of salt hydrates that could pin the grain boundaries. Close to the eutectic point, the grain growth of doped ice is similar to pure ice. The effect of soluble impurities on grain growth can be articulated as the application of a factor on the growth constant, k. We found this factor is a function of temperature and eutectic temperature. Natural ice is impure, often containing air bubbles and soluble impurities, and is usually subjected to a hydrostatic pressure. Our data set and the established impurity factor will provide new insights to the evolution of grain size within and the dynamics of natural ice masses.