Impact history of the Apollo 17 landing site revealed by U-Pb SIMS ages

8 Secondary ion mass spectrometry (SIMS) U-Pb ages of Ca-phosphates from four texturally 9 distinct breccia samples (72255, 76055, 76015, 76215) collected at the Apollo 17 landing site 10 were obtained in an attempt to identify whether they represent a single or several impact 11 event(s). The determined ages, combined with inferences from petrologic relationships, may 12 indicate two or possibly three different impact events at 3920±3 Ma, 3922±5 Ma and 3930±5 13 Ma (all errors 2σ). Searching for possible sources of the breccias by calculating the 14 continuous ejecta radii of impact basins and large craters as well as their expected ejecta 15 thicknesses, we conclude that Nectaris, Crisium, Serenitatis and Imbrium are likely 16 candidates. If the previous interpretation that the micropoikilitic breccias collected at the 17 North Massif represent Serenitatis ejecta is correct, then the average 207 Pb/ 206 Pb age of 18 3930±5 Ma (2σ) dates the formation of the Serenitatis basin. The occurrence of zircon in the 19 breccias sampled at the South Massif, which contain Ca-phosphates yielding an age of 20 3922±5 Ma (2σ), may indicate that the breccia originated from within the Procellarum 21 KREEP terrane (PKT) and the Imbrium basin appears to be the only basin that could have 22 sourced them. However, this interpretation implies that all basins suggested to fall 23 stratigraphically between Serenitatis and Imbrium formed within a short (<11 Ma) time 24 interval, highlighting serious contradictions between global stratigraphic constraints, sample 25 interpretation and chronological data. Alternatively, the slightly older age of the two 26 micropoikilitic breccias may be a result of incomplete resetting of the U-Pb system preserved 27 in some phosphate grains. Based on the currently available dataset this possibility cannot be 28 excluded. 29