High-precision Ruthenium Isotope Measurements for Constraining Late Accretion

served in the Earth's mantle are commonly explained by the addition of a late veneer of primitive meteoritic material to the mantle after core formation was complete [1-3]. The composition of the late veneer has been inferred from the comparision of Os isotope compositions and relative HSE abundances in Earth's primitive mantle to those of chondritic meteorites [4-11]. However, while the 187 Os/ 188 Os ratio of the Earth's primitive mantle overlaps with those of ordinary and enstatite chondrites, relative HSE abundances are inconsistent with a late veneer this composition. In particular the suprachondritic Ru/Ir of the primitive mantle cannot be accounted for by late accretion of known chondrites [e.g. 4,8]. The composition of the late veneer may also be assessed through the analysis of lunar impact melt rocks. These rocks bear testimony to the composition of bodies impacting the Earth and Moon since their formation about 4.4 Ga ago. Relative HSE abundances and 187 Os/ 188 Os ratios of some lunar impact rocks reveal the presence of both chondritic and differentiated me-teoritic impactor material [12-15]. On this basis it has been proposed that the HSE composition of the Earth's primitive mantle has been established by late accretion of predominantly carbonaceous chondrite material, mixed with a minor portion of differentiated iron meteorite like material [15]. The fractionated iron meteorite like component can account for the elevated Re/Os (and hence the observed 187 Os/ 188 Os), Ru/Ir and Pd/Ir of the Earth's mantle in comparison to carbonaceous chondrites. More recently, the relative abundances of Se, Te and S have also been used to link the late veneer to a specific type of chondrite [16]. The chalcogen systematics suggest that the late veneer predominantly consists of carbonaceous chondrite-like material, consistent with the composition inferred from the lunar impact melt rocks. The Ru isotope compositions of meteorites and the Earth's mantle provide new constraints on the composition and origin of the late veneer. Because almost all meteorites are characterized by nucleosynthetic Ru isotope anomalies, caused by a deficit in s-process nuclides relative to terrestrial Ru [17,18], Ru isotopes are a sensitive tracer for the composition of the late veneer and to assess whether known meteorites can be