U-Th-Pb systematics of selected samples from Apollo 17, Boulder 1, Station 2 |
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Authors: | P. D. Nunes M. Tatsumoto |
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Affiliation: | 1. U.S. Geological Survey, Denver, Colo., USA
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Abstract: | Nine U-Th-Pb whole-rock analyses of selected brecciated materials from sample 72215 and one analysis of a pigeonite basalt clast from 72275 are presented. Both samples are from Boulder 1, Apollo 17. These data supplement previous Boulder 1 U-Th-Pb analyses of samples 72275 and 72255. U and Th concentrations indicate that most of the samples contain a moderate to large KREEP component. Samples containing the least KREEP are a noritic clast (72255,49; Civet Cat clast) and an anorthositic clast (72275,117). Evidence for the migration of Pb from Pb-rich matrix material into relatively Pb-poor clasts is presented for two clasts. Most of the Boulder 1 data define a linear trend that intersects concordia at ~ 3.9 and 4.4 b.y. when plotted on a U-Pb concordia diagram. The presence of one anorthositic clast distinctly off this trend indicates that a simple two-stage U-Pb evolution history is inadequate to explain all the data. Accordingly physical significance is only attached to the lower concordia intercept age of 3.9–4.0 b.y. The older concordia intercept age of ~ 4.4 b.y. is interpreted to reflect an averaging of events both older and younger than 4.4 b.y. The data suggest that significant differentiation and/or metamorphism occurred ~ 4.2 b.y. ago. The age of this event, however, is not accurately defined by these data. |
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