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    Solid earth sciences

    201603201603

    Impact-induced melting during accretion of the Earth

    de Vries J, Nimmo F, Melosh H J, Jacobson S A, Morbidelli A, Rubie D C

    Accretion, Impacts, Melting, Core formation, Metal-silicate equilibration

    Equilibration pressures calculated for the Earth forming impacts. Large symbols are embryo - embryo collisions - these giant impacts create enough melting for a core-formation event to occur and material equilibrates at the bottom of the melt pool, created by the impact. Small symbols are planetesimal impacts into a leftover magma ocean (MO) that equilibrate at the bottom of this magma ocean (e.g. black ellipse). A horizontal line indicates planetesimal collisions that occur on a solid surface and equilibrate at the time of the next giant impact (e.g. magenta ellipse). Impacts in the red ellipse are planetesimal impacts on a solid surface after the last giant impact has occurred.

    Because of the high energies involved, giant impacts that occur during planetary accretion cause large degrees of melting. The depth of melting in the target body after each collision determines the pressure and temperature conditions of metal-silicate equilibration and thus geochemical fractionation that results from core-mantle differentiation. The accretional collisions involved in forming the terrestrial planets of the inner Solar System have been calculated by previous studies using N-body accretion simulations. Here we use the output from such simulations to determine the volumes of melt produced and thus the pressure and temperature conditions of metal-silicate equilibration, after each impact, as Earth-like planets accrete. For these calculations a parameterised melting model is used that takes impact velocity, impact angle and the respective masses of the impacting bodies into account. The evolution of metal-silicate equilibration pressures (as defined by evolving magma ocean depths) during Earth’s accretion depends strongly on the lifetime of impact-generated magma oceans compared to the time interval between large impacts. In addition, such results depend on starting parameters in the N-body simulations, such as the number and initial mass of embryos. Thus, there is the potential for combining the results, such as those presented here, with multistage core formation models to better constrain the accretional history of the Earth.