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

Anaïs Kobsch,Razvan Caracas

doi:10.1029/2020je006412

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

Highlights

For more than 20 years, the simulations of formation of the Moon from an impact-generated disk have made huge progress and went through many different models, from the canonical impact (Canup & Esposito, 1996) to the high-energy high-angular-momentum impact (Canup, 2012; Cuk & Stewart, 2012) and more recently to the formation of a synestia (Lock et al, 2018)
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
We perform the calculations along several isotherms, ranging from 2000 K, corresponding to a hot magma, up to 7000 K, corresponding to the supercritical fluid

Summary

Introduction

For more than 20 years, the simulations of formation of the Moon from an impact-generated disk have made huge progress and went through many different models, from the canonical impact (Canup & Esposito, 1996) to the high-energy high-angular-momentum impact (Canup, 2012; Cuk & Stewart, 2012) and more recently to the formation of a synestia (Lock et al, 2018). For many years shock experiments improved equations of state, which are major parameters of the hydrodynamics simulations, on a variety of major geological materials, like MgSiO3 glass, enstatite and olivine (Luo et al, 2004), silica (Kraus et al, 2012), and MgO (Root et al, 2015). These experiments sample points along the shock Hugoniot equations of state, at high temperatures and pressures, typical for the peak conditions attained during the shock. We study the behavior of two feldspar end-members terms in this regime

Methods

Results

Conclusion