Post-Imbrium Pb-Pb isochron ages for Apollo basaltic impact melt samples 14078 and 68415

The Apollo 14 and 16 missions returned several samples commonly interpreted as crystalline impact melt, with ages of approximately 3800-3850 Ma. Previous work has suggested that these rocks formed in one or more pre-Imbrium basin forming impact(s). By contrast, recent ages determined for a range of lunar breccias provide compelling evidence that the Imbrium basin was formed at approximately 3920 Ma. Using an approach previously demonstrated in lunar basalts, Pb-Pb isochron ages are determined for two of these proposed impact melt samples (14078: 3848 ± 4 Ma; and 68415: 3834 ± 11 Ma). In the case of 14078, the least radiogenic Pb isotopic compositions measured here are interpreted as representing the initial Pb isotopic composition of the sample. This value indicates derivation from a source (or sources) with high ²³⁸U/²⁰⁴Pb ratios (approx. 2400), similar to those predicted for the Apollo 14 high-Al and very high-K basalts. It was not possible to determine an equivalent initial Pb isotopic composition for 68415, but Pb isotope evolution models indicate that the sample would have been derived from lithologies with lower ²³⁸U/²⁰⁴Pb source ratios (approx. 1000). In both cases, the samples are interpreted as having been formed by impacts local to the Apollo 14 and Apollo 16 landing sites.