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

Abstract

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 NaAlSi3O8 and KAlSi3O8 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 5500–6000 K range for the Na‐feldspar, 0.5–0.9 g cm−3 and 5000–5500 K range for the K‐feldspar. The simulations suggest that the vaporization is incongruent, with a degassing of O2 starting at 4000 K and gas component made mostly of free Na and K cations, O2, SiO and SiO2 species for densities below 1.5 g cm−3. The Hugoniot equations of state imply that low‐velocity impactors (<8.3 km s−1) would at most melt a cold feldspathic crust, whereas large impacts in molten crust would see temperatures raise up to 30000 K.

The Extent, Nature, and Origin of K and Rb Depletions and Isotopic Fractionations in Earth, the Moon, and Other Planetary Bodies

“…Metal in the LMO could have segregated into a core, so we only plot the results for isotopic fractionation between vapor and silicate at 1000, 1250, and 1550 K. In the disk at 2500 K, metal could have been present, so we show the predictions for both metal (blue filled circles) and silicate (red filled circles). Kobsch & Caracas (2020) ran first-principles molecular dynamics simulations to calculate the critical temperature of Na and K-feldspar composition and to study the speciation of these elements in liquid and vapor. The critical temperatures that they calculate are in the range 5000-5500 K. Expectedly, at the high temperatures relevant to MVE depletion, the melts become depolymerized, and the species present are more similar to those encountered in the vapor.…”

The critical point and the supercritical regime of MgO

“…At high temperatures and at densities below 1.24 g/cm 3 , the O-O pair distribution function reveals the presence of a peak around 1.3 Å, which corresponds to the characteristic bond length of the O 2 molecule. As observed in various silicate systems, such as feldspars [24] or silica [38], oxygen molecules are present right below the critical temperature as well as in the supercritical fluid. FIG.…”

“…In temperature, it lies between the last isotherm whose pressure shows a local minimum and a local maximum, and the first isotherm that shows a monotonous decrease of pressure. This procedure was applied with success on various other material in the same phase space [3,24,26,42].…”

“…The area above the isotherm of the critical temperature is where the supercritical fluid is stable. Figure adapted from Kobsch and Caracas [24].…”

“…But these studies stop short at relatively low temperature, and they fail to reach the CP and to explore the supercritical regime. For the large majority of rock-forming minerals, such as feldspars [24], MgSiO 3 , and Mg 2 SiO 4 , the vaporization is incongruent [21,[24][25][26].…”