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The case for an Imbrium origin of the Apollo thorium‐rich impact‐melt breccias

Abstract— Mafic, Th‐rich impact‐melt breccias, most of which are identified with the composition known as low‐K Fra Mauro (LKFM), are the most common rock type in the nonmare regoliths of the Apollo lunar landing sites. The origin of mafic impact‐melt breccias bears on many lunar problems: the nature of the late meteoroid bombardment (cataclysm); the spatial distribution of KREEP, both near the surface and at depth; the ages of the major basins; and the composition of the early crust of the nearside lunar highlands. Thus, it is crucial that the origin of mafic impact‐melt breccias be accurately understood. Because of both intra‐ and intersite differences in compositions of mafic impact‐melt breccia samples, apparent differences in crystallization age, and differences in siderophile‐element ratios, previous studies have argued that either (1) most mafic impact‐melt breccias are the products of several large craters local to the site at which they were found but that some are of basin origin or that (2) they are all from the Imbrium (Apollos 14 and 15), Nectaris (Apollo 16), and Serenitatis (Apollo 17) basins. Here, we reconsider the hypothesis that virtually all of the Th‐rich, mafic impact‐melt breccias from the Apollo missions are products of the Imbrium impact.