Collisional erosion during accretion and Earth's non-chondritic Sm/Nd ratio

Abstract : Superchondritic Sm/Nd (Samarium/Neodymium) ratio has been evidenced in Earth's mantle rocks [1]. This observation is at odds with the canonical view of planetary accretion from chondritic building blocks, leading to chondritic ratios of lithophile (i.e. rock loving elements) and refractory elements in the silicate Earth. Here we evaluate the influence of collisional erosion during accretion on the budget of lithophile and refractory elements such as Sm and Nd. During the late stages of planetary accretion, planets grow by colliding with other embryos and a remnant planetesimals population. It has been suggested that the superchondritic Sm/Nd could have been produced by collisional striping of the Earth's early crust during its accretion. We quantitatively constrain the amount of eroded mass during the late-stage growth of Earth analogs using a combination of analytical modeling [2] and N-body numerical simulations [3]. Our model takes into account the distribution of impactors and their evolving compositions during accretion. The effect of changing accretion scenarios is also studied (e.g. with or without a Grand-Tack). The final Sm/Nd ratio of the bulk silicate Earth is determined under 3 sets of assumptions: (1) The mantle and crust fully reequilibrate after each impact; (2) The accreted material merges with the crust only and the mantle does not reequilibrate with the crust (3) an intermediate case that depends on the size of the impactors. We show that planetesimal impacts play a dominant role in determining Earth's Sm/Nd ratio due to the efficiency of crustal erosion. This work has implications for understanding Earth's accretion history. [1] Boyet and Carlson, 2005. Science 309, 576-581 [2] Svetsov, 2011. Icarus 214, 316-326 [3] Raymond, O'Brien, Morbidelli, Kaib, 2009. Icarus 203, 644-662
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Laetitia Allibert, Sébastien Charnoz, Julien Siebert, Sean N. Raymond. Collisional erosion during accretion and Earth's non-chondritic Sm/Nd ratio. American Astronomical Society, DPS meeting #50, id.113.08 held 21-26 October 2018 - Knoxville, Tenessee US, Oct 2018, Knoxville, Tenessee, United States. ⟨hal-01904807⟩



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