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Behavior of feldspars during the Giant Impact - L'état physique des feldspaths pendant l'Impact Géant

jeu 09 juil 2020



Soutenance de thèse de Mme Anaïs KOBSCH du Laboratoire de Géologie de Lyon: Terre, Planètes, Environnement, sous la direction de M. Razvan CARACAS

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Description générale

The impact of a planet in formation with the proto-Earth, also known as the Giant Impact, is now the main hypothesis for the Moon formation. Nevertheless, there are still discrepancies between the impact simulations and the observations of the current Earth-Moon system. To improve their models, geophysicists need better understanding of geological materials not only at high pressures and high temperatures, typical of impacts, but also at low pressures and high temperatures, typical of the debris disc that follows the impact. Since this latter region cannot be reached by experiments we use here ab-initio molecular dynamics simulations. 
We work on feldspars, with formula (Ca,K,Na)(Al,Si)4O8, as they represent the major mineral component of the crust of terrestrial bodies. Using the VASP® code for numerical experiments and the home-made UMD package for post-processing, we obtain structural, transport and thermodynamic data on a wide range of temperatures (2000-7000 K) and densities (0.5-6 g.cm3). 
The three feldspar end-members display a critical density between 0.4 and 0.9 g.cm3 and critical temperatures as follows: 5000 K < TK < 5500 K, 6000 K < TNa < 6500 K and 7000 K < TCa < 7500 K. At low densities and below the critical temperatures, we can identify the start of gas bubble nucleation. The vaporization is incongruent, the gas is mostly made of free Na or K and of SiO, SiO2 or O2 molecules.  There is an O2 degassing of the fluids above 4000 K at all densities. 
Our study at very high temperatures and pressures tells us that impacts in a cold crust would at most melt the crust, whereas impacts in a hot crust or in a magma ocean would completely bring the crust into supercritical state.

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