Early cosmochemical fractionation by collisional erosion during the Earth's accretion

Wednesday, 17 December 2014
Asmaa Boujibar, Denis Andrault, Nathalie Bolfan-Casanova and Mohamed Ali Bouhifd, Laboratoire Magmas et Volcans, Clermont-Ferrand Cedex, France
Early in the Solar System history, planets grew through energetic collisions between planetesimals. Partial destruction of planetary embryos could have produce heterogeneous bodies, because of widespread internal chemical differentiation through core-segregation, partial melting of silicated mantles and fractional crystallization of magma oceans. On larger proto-planets, including the early Earth, meteoritic impacts have potentially induced erosion of part of the surface, which could have affected significantly the final chemical composition of terrestrial planets. Here we report that collisional erosion of 15-18% of the early Earth’s surface reconciles the major apparent compositional differences between the known enstatite chondrites (EC) and the bulk Earth. We base our arguments on experimental measurements of the melting properties of an EC body at pressures between 1 bar and 25 GPa. At low degree of partial melting, the silicate melts are found highly enriched in SiO2, Al2O3 and Na2O and strongly depleted in MgO. Loss through collisional erosion of the surface of such differentiated planetesimal should ultimately raise its Mg/Si ratio and decrease its concentration in incompatible elements. We elaborate auto-coherent models of the Earth formation. Vaporization into space of the protocrusts could be a source of the Na-rich SiO gas that interacted with chondrules and explain the enrichments of enstatite chondrites in SiO2 and volatile lithophile elements.