Fingerprinting late accretion: Precise measurements of highly siderophile elements in planetary materials

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Excess abundances and proportions of the highly siderophile ( iron-loving ) elements (HSE; Ir, Rh, Ru; Pd, Pt, Os, Re, Au) in the Earth s mantle are commonly explained by late accretion of large planetesimals after formation of the Earth s core and the Moon. Recent analytical progress resulted in improved precision of abundance data for some of the HSE in planetary materials. These data hint that the Earth s mantle may be characterized by unique proportions of HSE that are unlike those in primitive meteorites that are commonly assumed to represent the building blocks of the terrestrial planets. Impact melt rocks from one of the large lunar basins formed during late accretion 4 billion years ago, and show HSE abundance patterns similar to those in the Earth s mantle. In contrast to Earth where recycling has returned early meteorite-contaminated crust back into the mantle, the lunar crust still maintains a record of processes since its formation 4.5 billion years ago. Lunar impact rocks are mixtures between meteoritic components derived from the impacting bolides and crust of the lunar highlands. This study will evaluate the detailed origin of the HSE signature in Earth s mantle and the lunar crust with the help of precise abundance data for Rh, Au and other HSE in primitive meteorites, samples of the Earth s mantle and lunar samples. We will study the origin of the different constituents of the lunar crust, in particular the origin and nature of meteoritic components, the HSE signature imposed on lunar highland rocks by the lunar magma ocean and implications for processes on the surface of the early Earth.