Broad bounds on Earths accretion and core formation constrained by geochemical models

Abstract

The Earth formed through the accretion of numerous planetary embryos that were already differentiated into a metallic core and silicate mantle. Prevailing models of Earths formation, constrained by the observed abundances of metal-loving siderophile elements in Earths mantle, assume full metal-silicate equilibrium, whereby all memory of the planetary embryos earlier differentiation is lost. Using the hafnium-tungsten (Hf-W) and uranium-lead (U-Pb) isotopic dating systems, these models suggest rapid accretion of Earths main mass within about 10 million years (Myr) of the formation of the Solar System. Accretion terminated about 30 or 100 Myr after formation of the Solar System, owing to a giant impact that formed the Moon. Here we present geochemical models of Earths accretion that preserve some memory of the embryos original differentiation. These disequilibrium models allow some fraction of the embryos metallic cores to directly enter the Earths core, without equilibrating with Earths mantle. We show that disequilibrium models are as compatible with the geochemical observations as equilibrium models, yet still provide bounds on Earths accretion and core formation. We find that the Hf-W data mainly constrain the degree of equilibration rather than the timing, whereas the U-Pb data confirm that the end of accretion is consistent with recent estimates of the age of the Moon. Our results indicate that only 36% of the Earths core must have formed in equilibrium with Earths mantle. This low degree of equilibration is consistent with the siderophile element abundances in Earths mantle. © 2010 Macmillan Publishers Limited. All rights reserved.