The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts

摘要

The position of the vapor-liquid dome and of the critical point determine the evolution of the outermost parts of the protolunar disk during cooling and condensation after the Giant Impact. The parts of the disk in supercritical or liquid state evolve as a single thermodynamic phase; when the thermal trajectory of the disk reaches the liquid-vapor dome, gas and melt separate leading to heterogeneous convection and phase separation due to friction. Different layers of the proto-Earth behaved differently during the Giant Impact depending on their constituent materials and initial thermodynamic conditions. Here we use first-principles molecular dynamics to determine the position of the critical point for NaAlSi_3O_8 and KAlSi_3O_8 feldspars, major minerals of the Earth and Moon crusts. The variations of the pressure calculated at various volumes along isotherms yield the position of the critical points: 0.5-0.8 g cm~(-3) and 5500-6000K range for the Na-feldspar, 0.5-0.9 g cm~(-3) and 5000-5500K range for the K-feldspar. The simulations suggest that the vaporization is incongruent, with a degassing of O_2 starting at 4000K and gas component made mostly of free Na and K cations, O_2, SiO and SiO_2 species for densities below 1.5 g cm~(-3). The Hugoniot equations of state imply that low-velocity impactors (<8.3kms~(-1)) would at most melt a cold feldspathic crust, whereas large impacts in molten crust would see temperatures raise up to 30000 K.