Fast accretion of the Earth with a late Moon-forming giant impact

Abstract

Constraints on the formation history of the Earth are critical for understanding of planet formation processes. 182Hf- 182W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and 182Hf- 182W and 87Rb- 87Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to >100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the 182W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (>52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in <10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.