Constraining the timing of volatile arrival on Earth using Ru and Mo isotopes in the Archean mantle

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Volatile elements represent a crucial prerequisite for the birth and evolution of life on Earth. While it is largely accepted that water and other volatiles were brought to the Earth by the accretion of carbonaceous chondrite-like materials originating from the outer solar system, the timing of this accretion remains highly debated. It has long been suggested that volatile elements were added to the Earth by late accretion of a volatile-rich late veneer after core formation had ceased. However, this hypothesis was recently refuted by a Ru isotope study, which shows that the late veneer was made of reduced and volatile-poor material originating from the inner solar system. Hence, the Earth’s volatiles must have been added during the main accretion, i.e. before the cessation of core formation, but exactly when remains unconstrained. This proposal aims at providing new constraints on the timing of volatile arrival on Earth during its main accretion stage. This will be tackled by the investigation of the Ru and Mo isotopic signature of early Archean komatiites suggested to have preserved the pre-late veneer isotopic composition of the mantle. The approach builds on the results of recent nucleosynthetic isotope anomaly studies on meteorites, which show that volatile-rich carbonaceous chondrites exhibit Ru and Mo isotope signatures clearly distinct from volatile-poor meteorites. Hence, the particular genetic fingerprints of these meteorites can be used to assess whether significant amounts of volatile-rich material was present in the source of early Archean komatiites. This would be visible in their Ru and Mo isotope composition, which would be different from that of the modern mantle. The results of this study should ultimately provide critical insights for the discrimination between the two main models of the Earth’s accretion: heterogeneous and homogeneous.