Petrology and rheology of high pressure planetary ices

H2O ice is found in a variety of planetary environments, notably in the form of high pressure polymorphs inside icy moons and extrasolar ocean planets. The great diversity of thermodynamic conditions predicted inside such planetary bodies, reveals the need for new experimental and computational data to allow modeling of their internal structure and dynamics.Structural and spectral properties of H2O pure ices have been intensively studied, but surprisingly there is a lack of petrological data on impurities rich ice solid solutions. This Ph.D. thesis work focused on the study of ice VI and ice VII fusion curves in the H2O-NaCl binary, using diamond anvil cell and Raman spectroscopy. We later determined the partitioning of the NaCl analog salt, RbI, between ice VI and VII and the aqueous fluid using X- ray fluorescence and X-ray diffraction techniques at the European Synchrotron Research Facility (Grenoble). Our results enable us to observe a density inversion between ice VI and the salty fluid, and to measure a strong difference in salt partitioning between ice VI and ice VII with a partition coefficient of Kd(VI-VII)=4.5(±2.7)10-2. Inside the largest H2O rich planetary bodies, called ocean planets, the icy mantle, putatively more than 1000 km thick, shelters an ultra high pressure ice form called ice X. This H2O ice phase is unique because of its ionic crystallographic structure, in contrast with lower pressure ices polymorphs, all being molecular solids. This characteristic coupled with the fact that no data are available yet on its mechanical properties, encouraged us to study its elastic and plastic properties. Using ab initio calculations and the Peierls Nabarro model, I showed the strong variation of elastic anisotropy with increasing pressure and determined the dominant slip system inside the structure of ice X over its entire pressure stability range from 100 to 350 GPa. Our calculations suggest that plasticity in ice X is dominated by displacement always occurring on the {110} glide plane. Also, it reveals that the <110>{110} glide system is dominant below 250 GPa and that the <100>{110} slip system controls the plasticity of ice X. Our results also show that, if elastic anisotropy of ice X is strongly increasing with increasing pressure, the plasticity becomes almost isotropic at 350 GPa.