The origin of the moon and the early history of the earth—A chemical model. Part 2: The earth

doi:10.1016/0016-7037(91)90169-6

Geochimica et Cosmochimica Acta

Volume 55, Issue 4, April 1991, Pages 1159-1172

https://doi.org/10.1016/0016-7037(91)90169-6 Get rights and content

Abstract

The geochemical implications for the Earth of a “giant impact” model for the origin of the Earth-Moon system (O'Neill, 1991) are discussed, using a mass balance between three components: the proto-Earth, the Impactor, and a late veneer. It is argued that the proto-Earth (nearly 90% of the present Earth) accretes from material resembling a high temperature condensate from the solar nebula. Core formation takes place under very reducing conditions, resulting in the mantle of the proto-Earth being completely stripped of all elements more siderophile than Fe, and partly depleted in the barely siderophile elements V, Cr, and perhaps Si. The Impactor (11% of the present Earth), which is constrained to be an oxidised body of approximately chondritic (CI) composition, then collides with the proto-Earth, causing vaporisation of both the Impactor and a substantial portion of the Earth's mantle. Most of this material recondenses to the Earth, but some forms the Moon. The Impactor adds most of the complement of the siderophile elements of the present mantle (e.g., Ni, Co) in an oxidised form. The oxidation state of the mantle is set near to its present, oxidised level. The Impactor also adds volatile elements, including a small amount of S. This S forms a separate Ni-rich sulfide phase, which segregates to the core, taking with it all the highly siderophile elements (e.g., the noble metals), and depleting the mantle in the moderately siderophile elements according to their sulfide/silicate partition coefficients. Finally, the addition of a late veneer, of composition similar to that of the H-group ordinary chondrites, accounts for the complement of the highly siderophile elements of the present mantle.

The model accounts at least semi-quantitatively for the siderophile element abundances of the present mantle; a more quantitative test would be possible if suitable partition coefficients, between silicate and Ni-rich sulfide under relatively oxidising conditions, were available. Implications for the composition of the Earth's core are discussed; the model predicts that neither S, O, nor Si should be present in sufficient quantities to provide the required light element in the core, whose identity, therefore, remains enigmatic.

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The origin of the moon and the early history of the earth—A chemical model. Part 2: The earth

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