The Efficient Delivery of Highly Siderophile Elements to the Core Creates a Mass Accretion Catastrophe for the Earth

Abstract

Abstract The excess abundance of highly siderophile elements (HSEs), as inferred for the terrestrial planets and the Moon, is thought to record a “late veneer” of impacts after the giant impact phase of planet formation. Estimates for total mass accretion during this period typically assume all HSEs delivered remain entrained in the mantle. Here, we present an analytical discussion of the fate of liquid metal diapirs in both a magma pond and a solid mantle, and show that metals from impactors larger than approximately 1 km will sink to Earth's core, leaving no HSE signature in the mantle. However, by considering a collisional size distribution, we show that to deliver sufficient mass in small impactors to account for Earth's HSEs, there will be an implausibly large mass delivered by larger bodies, the metallic fraction of which lost to Earth's core. There is therefore a contradiction between observed concentrations of HSEs, the geodynamics of metal entrainment, and estimates of total mass accretion during the late veneer. To resolve this paradox, and avoid such a mass accretion catastrophe, our results suggest that large impactors must contribute to observed HSE signatures. For these HSEs to be entrained in the mantle, either some mechanism(s) must efficiently disrupt impactor core material into 0.01 mm fragments, or alternatively Earth accreted a significant mass fraction of oxidized (carbonaceous chondrite‐like) material during the late veneer. Estimates of total mass accretion accordingly remain unconstrained, given uncertainty in both the efficiency of impactor core fragmentation, and the chemical composition of the late veneer. Plain Language Summary Highly siderophile elements (HSEs) have a very strong tendency to partition into planetary cores, rather than mantles. If Earth's mantle and core were chemically equilibrated, these elements should be almost non‐existent in the mantle. Yet, these elements are much more abundant in the mantle than expected, and are present in roughly the same relative abundance as in chondritic meteorites. A widely admitted hypothesis is that these elements were delivered as a “late veneer”of chondritic material, carrying about % of Earth's mass after core formation was complete. This estimate assumes that all HSEs delivered during the late veneer remained suspended in Earth's mantle. In this work, we show that it is very challenging for these elements, delivered by planetesimals larger than approximately 1 km, to avoid sinking to Earth's core, due to the large density of these metals relative to Earth's silicate mantle. Our calculations further show, by considering a realistic planetesimal size distribution, that there is insufficient mass in small planetesimals to account for Earth's HSEs. These results therefore highlight a contradiction between estimates of mass accretion during the late veneer, and our understanding of metal delivery to Earth's mantle. Key Points The entrainment of metal, and its HSE content, in a magma pond is possible only for ≤0.01 mm droplets Metal delivered by ≥1 km impactors is lost to Earth's core, yet constraints on total mass accretion prevents HSE delivery by ≤1 km impactors Thus, either an oxidized late veneer or the disruption of large impactors into ≤0.01 mm droplets is required to account for observed HSEs