U-Th-Pb and Rb-Sr systematics of Apollo 17 boulder 7 from the North Massif of the Taurus-Littrow Valley

Abstract

Portions of highland breccia boulder 7 collected during the Apollo 17 mission were studied using U Th Pb and Rb Sr systematics. A Rb Sr internal isochron age of 3.89 ± 0.08b.y. with an initial 87 Sr/ 86 Sr of0.69926 ± 0.00008 was obtained for clast 1 (77135,57) (a troctolitic microbreccia). A troctolitic portion of microbreccia clast 77215,37 yielded a U Pb internal isochron of 3.8 ± 0.2b.y. and an initial 206 Pb/ 207 Pb of 0.69. These internal isochron age are interpreted as reflecting metamorphic events, probably related to impacts, which reset Rb Sr and U Pb mineral systems of older rocks. Six portions of boulder 7 were analyzed for U, Th, and Pb as whole rocks. Two chemical groups appear to be defined by the U, Th, and Pb concentration data. Chemical group A is characterized by U, Th, and Pb concentrations and 238 U/ 204 Pb values which are higher than those of group B. Group A rocks have typical 232 Th/ 238 U ratios of ∼ 3.85, whereas-group B rocks have unusually high Th/U values of ∼ 4.1. Whole-rock U Pb and Pb Pb ages are nearly concordant. Two events appear to be reflected in these data — one at ∼ 4.4 b.y. and one at ∼ 4.5 b.y. The chemical groupings show no correlation with documented ages. The old ages of ∼ 4.4 b.y. and ∼ 4.5 b.y. may, like the younger ∼ 4.0 b.y. ages, be related to basin excavation events.