The purpose of this work is to fingerprint the late additions to the Moon using the relative abundances of the highly-siderophile elements (HSE) that occur in generally high abundance in likely impactors, but extremely low abundance in the indigenous lunar crust. Towards this end, approximately 2g of several Apollo 14 and 17 melt breccias were obtained from the Johnson Space Center curatorial facilities. In collaboration with Dr. Odette James (USGS retired), a longstanding expert on these rocks, we have cleanly separated chips from 73215, 73255 and 72395, and analyzed them for Os isotopes and highly siderophile element abundances (Pt, Pd, Ir, Ru, Re and Os).

The Apollo 17 rocks likely sample the Serenitatis basin impactor. All of the rocks analyzed have ¹⁸⁷Os/¹⁸⁸Os ratios of approximately 0.129 to 0.133. These ratios are consistent with the Serenitatis impactor having long term Re/Os similar to enstatite or ordinary chondrites, rather than carbonaceous chondrites. Results for each rock show considerable variation in absolute abundances of the highly siderophile elements. Previous studies have utilized such variations to extrapolate to indigenous abundances by assuming very low concentrations of Ir in the target rocks. Our results for "aphanitic" melt rocks (containing relatively fine-grained melt fraction but substantial clasts) show non-linear relations between Ir and some other HSE (Fig 1a). General trends indicate that both Pd and Ru were present in the impact rocks at levels >1 ppb, considerably higher than previously assumed. This means that the abundances of these elements in the impactor were probably lower than suggested by previous studies. Results for Apollo 17 "poikilitic" melt rocks (coarser-grained melt fraction and fewer clasts than the aphanitic rocks), have much more linear relations between Ir and other HSE (Fig. 1b). Although these rocks presumably contain similar impactor materials to the aphanitic rocks, there is less indication of significant indigenous HSE. In the coming year we will attempt to deconvolute the indigeneous signal from the impactor and obtain a better understanding of the composition of the impactor.

Figure 1. Plot of Ir (in ng/g) versus Re, Os, Ru, Pt and Pd for Apollo 17 impact melt rocks (likely formed by the Oriental impact). If it is assumed that the lunar target rocks contained very low Ir, the near 0 y-axis intercepts of this plot for Re, Os and Pt suggest that these elements were also present in very low abundance in the lunar taget rocks. Non-zero intercepts for Pd and Ru, however, suggest that these two elements were present in significant abundance in the target rocks, and the indigenous abundances must be subtracted from the estimate for the impactor.

Figure 2. Plot of Ir (in ng/g) versus Re, Os, Ru, Pt and Pd for Apollo 17 poikilitic melt rocks. Non-zero intercepts for Pd and Ru, are suggested, however, the results are somewhat different from those of the aphanitic melt rocks.