Re-Os geochronology and abundances of highly siderophile elements in ancient lunar impact rocks

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The excess abundances of highly siderophile elements (HSE) in the silicate earth are most commonly attributed to a late veneer of primitive planetesimals that were accreted after formation of the earths core. The HSE composition of the silicate earth, however, is unlike known primitive meteorites, and seems to require either a residual fractionated component in the mantle or late accretion of material that differs from the composition of primitive meteorites. In contrast to earth, the lunar surface retains a compositional and chronological record of late accretion, 4.5 to 3.8 billion years ago. Recent work on lunar impact rocks that are related to the large basin forming impacts has provided new constraints on the detailed composition and timing of early lunar impact events. These, still very limited, data suggest that a mixture of some of the major impactor compositions identified from ancient impact melt rocks can explain the HSE composition of the silicate earth. The proximity of Earth and Moon make this hypothesis attractive, however, it needs to be substantiated with additional data. These impacts may predate the period of the presumed late heavy bombardment 3.9-3.8 billion years ago. This project will use ancient lunar impact rocks to test hypotheses that address the main questions of composition and timing of late accretion in the early Earth-Moon system.