Exploring the Complexities of the Serenitatis Basin: Breccia Clasts from Apollo 17

Thirteen new breccia clasts of highland lithologies are described from the Apollo 17 site. The clasts were extracted from three breccias: two from sample 73215 (aphanitic impact melt breccia), six from 73216 (impact melt breccia), and five from 77035 (micropoikilitic impact melt breccia). Breccias 73215 and 73216 were collected from the rim of a 10 m crater at Station 3 and breccia 77035 was collected at Station 7 at the base of the North Massif. The clasts are subdivided into melt-free and melt-rich groups. The melt-free clasts include pristine and brecciated lithologies, with one pristine gabbronoritic anorthosite containing 15 ppb Ir. The melt-rich clasts have compositions that are similar to LKFM, rather than to Apollo 17 KREEP basalts described as clasts from breccia 72275 by Salpas et al. (1987). Three main conclusions are drawn from this study: (1) the presence of 15 ppb Ir in a pristine gabbronoritic anorthosite further demonstrates the complexities associated with estimating the highly siderophile element content of the lunar crust and siderophile element abundances cannot be used in isolation to demonstrate pristinity (or lack thereof); (2) the KREEP basalts from breccia 72275 probably are not indigenous to the Serenitatis Basin and were transported to the Apollo 17 site by impact processes. Apollo 17 KREEP is in reality LKFM in composition; and (3) the “cryptic” component required to define KREEPy LKFM impact compositions probably will never be defined mineralogically. This component was derived from the final liquid to crystallize from fractional crystallization of either an Mg-suite pluton or the lunar magma ocean. Consequently, this component will easily melt when disrupted by large impacts and now is present as glass in breccia matrixes and/or as grain boundary films.