The non-carbonaceous nature of Earth’s late-stage accretion

Abstract

Constraining the origin of Earth’s building blocks requires knowledge of the chemical and isotopic characteristics of the source region(s) where these materials accreted. The siderophile elements Mo and Ru are well suited to investigating the mass-independent nucleosynthetic (i.e., “genetic”) signatures of material that contributed to the latter stages of Earth’s formation. Studies contrasting the Mo and Ru isotopic compositions of the bulk silicate Earth (BSE) to genetic signatures of meteorites, however, have reported conflicting estimates of the proportions of the non-carbonaceous type or NC (presumptive inner Solar System origin) and carbonaceous chondrite type or CC (presumptive outer Solar System origin) materials delivered to Earth during late-stage accretion (likely including the Moon-forming event and onwards). The present study reports new mass-independent isotopic data for Mo, which are presumed to reflect the composition of the BSE. A comparison of the new estimate for the BSE composition with new data for a select suite of NC iron meteorites is used to constrain the genetic characteristics of materials accreted to Earth during late-stage accretion. Results indicate that the final 10 to 20 wt% of Earth’s accretion was dominated by NC materials that were likely sourced from the inner Solar System, although the addition of minor proportions of CC materials, as has been suggested to occur during accretion of the final 0.5 to 1 wt% of Earth’s mass, remains possible. If this interpretation is correct, it brings estimates of the genetic signatures of Mo and Ru during the final 10 to 20 wt% of Earth accretion into concordance.