Abstract
Several lines of evidence indicate a non-chondritic composition for bulk Earth. If Earth formed from the accretion of chondritic material, its non-chondritic composition, in particular the super-chondritic 142Nd/144Nd and low Mg/Fe ratios, might be explained by the collisional erosion of differentiated planetesimals during its formation. In this work we use an N-body code, that includes a state-of-the-art collision model, to follow the formation of protoplanets, similar to proto-Earth, from differentiated planetesimals (>100km) up to isolation mass (>0.16M⊕). Collisions between differentiated bodies have the potential to change the core–mantle ratio of the accreted protoplanets. We show that sufficient mantle material can be stripped from the colliding bodies during runaway and oligarchic growth, such that the final protoplanets could have Mg/Fe and Si/Fe ratios similar to that of bulk Earth, but only if Earth is an extreme case and the core is assumed to contain 10% silicon by mass. This may indicate an important role for collisional differentiation during the giant impact phase if Earth formed from chondritic material.
Highlights
A basic premise when estimating elemental budgets of planets is that they were constructed from material represented by our collection of undifferentiated, chondritic meteorites
Our work shows that it is possible for collisional erosion during runaway and oligarchic growth to change the composition of the chondrites, in terms of their Mg/Fe and Si/Fe into that of bulk Earth
We investigated whether the non-chondritic composition of Earth, in particular its low Mg/Fe ratio, could be explained due to the collisional stripping of mantle material during the formation of proto-Earth
Summary
A basic premise when estimating elemental budgets of planets is that they were constructed from material represented by our collection of undifferentiated, chondritic meteorites This assumption has recently come under close scrutiny due to the super-chondritic 142Nd/144Nd composition of the accessible Earth (Boyet and Carlson, 2005). Either the Earth has an untapped, hidden reservoir, most likely located at the bottom of the mantle (Boyet and Carlson, 2005; Labrosse et al, 2007), leaving the observable, outer silicate Earth super-chondritc, or the bulk composition of Earth is inherently non-chondritic (O’Neill and Palme, 2008; Caro et al, 2008).
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