This project will investigate the accretion of the Earth and the seperation of the Earth's core and crust from the mantle by analyzing geochemically well-characterized samples from different geochemical reservoirs. Neutron activation analysis will be used for the moderately siderophile and chalcophile trace elements Mo, W, As, and Sb. The abundances of highly siderophile elements in the mantle are not consistent with equilibrium between the core and mantle, as would be expected assuming completely homogenous accretion of the Earth. The varying depletions relative to elements with different affinities for Fe-metal or sulfide provide clues to the events involving accretion and metal segregation in the mantle, including the possibility of late veneers that accreted on the Earth after core formation, or the possibility of metal retention in the mantle during core formation. Determining the abundances and depletion of these elements in samples of different geological ages will also address the timing of core formation in the Earth, beacuse the possibility of continued core formation through a substantial portion of geologic time has been invoked to explain some of the recent lead isotope results for samples from the mantle.