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Project Summary

Lunar impact melt rocks have been examined for absolute and relative abundances of the highly siderophile elements. This suite of iron-loving elements can potentially be used to fingerprint the large impactors that struck the Earth and Moon during late stages of bombardment. Results for a variety of Apollo and meteoritic impact melt rocks suggest that some impactors were similar to chondritic meteorites but others were not, suggesting an origin via a type of impactor that is no longer sampled by the Earth. Synthesis of these data, as well as data for terrestrial and martian mantle suggests that late accretion was involved in the establishment of the abundances of these elements in their planetary mantles, but could not have been the only process. Pristine lunar crust has very low abundances of all HSE and cannot account for any of the inter-element variations that have been recorded in some lunar impact melt rocks.