부산대학교 도서관

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2006.09.013 Byline: Fernando Barra (a)(b), Timothy D. Swindle (a)(c), Randy L. Korotev (d), Bradley L. Jolliff (d), Ryan A. Zeigler (d), Eric Olson (c) Abstract: Twenty-one 2-4mm rock samples from the Apollo 12 regolith were analyzed by the.sup.40Ar/.sup.39Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700-800Ma, with minimum formation ages that range from 1.0 to 3.1Ga. We estimate that this degassing event occurred at 782[+ or -]21Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site..sup.40Ar/.sup.39Ar dating of two alkali anorthosite clasts yielded ages of 3.256[+ or -]0.022Ga and 3.107[+ or -]0.058Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing. Author Affiliation: (a) Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA (b) Instituto de Geologia Economica Aplicada, Universidad de Concepcion, Chile (c) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (d) Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA Article History: Received 27 January 2006; Accepted 20 September 2006 Article Note: (miscellaneous) Associate editor: Rainer Wieler