/Ar-40/-/Ar-39/ dating of Apollo 16 and 17 rocks

The reported study is a continuation of an investigation begun by Alexander and Kahl (1974), with primary emphasis on (Ar-40)-(Ar-39) dating of lunar breccias. A primary objective of the study was related to the resolution of small plateau-age differences between Apollo 16 and 17 samples which have been irradiated in the same irradiation. This objective was not realized because (1) neither of the considered Apollo 16 samples has a well-defined plateau, and (2) the release patterns of the two Apollo 17 samples with plateaux (73235 and 77017) are sufficiently complex to suggest caution in interpretation of fine-scale differences among apparent ages. The plateau age of 73235 agrees well with plateau ages measured for a nearby boulder at the South Massif. The plateau age of 77017, on the other hand, agrees well with plateau ages measured in an nearby boulder at the base of the North Massif. These rock and boulder ages are tightly clustered about 3.99 b.y., and this clustering suggests that the North and South Massifs were made in the same basin-forming event, presumably Serenitatis.