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Ar/Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials |
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| Authors: | Fernando Barra Timothy D Swindle Randy L Korotev Bradley L Jolliff Ryan A Zeigler Eric Olson |
| Institution: | aDepartment of Geosciences, University of Arizona, Tucson, AZ 85721, USA bInstituto de Geologia Economica Aplicada, Universidad de Concepcion, Chile cLunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA dDepartment of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA |
| Abstract: | Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the 40Ar/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 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. 40Ar/39Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. 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. |
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