40Ar/39Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials

Fernando Barra, Timothy Swindle, Randy L. Korotev, Bradley L. Jolliff, Ryan A. Zeigler, Eric Olson

Research output: Contribution to journalArticle

20 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)6016-6031
Number of pages16
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number24
DOIs
StatePublished - Dec 15 2006

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Alkalies
Bearings (structural)
regolith
Degassing
anorthosite
Rocks
mare basalt
degassing
Crystallization
Heating
vertical mixing
breccia
clast
rock
dating
material
olivine
crystallization
melt
heating

ASJC Scopus subject areas

  • Geochemistry and Petrology

Cite this

40Ar/39Ar dating of Apollo 12 regolith : Implications for the age of Copernicus and the source of nonmare materials. / Barra, Fernando; Swindle, Timothy; Korotev, Randy L.; Jolliff, Bradley L.; Zeigler, Ryan A.; Olson, Eric.

In: Geochimica et Cosmochimica Acta, Vol. 70, No. 24, 15.12.2006, p. 6016-6031.

Research output: Contribution to journalArticle

Barra, Fernando ; Swindle, Timothy ; Korotev, Randy L. ; Jolliff, Bradley L. ; Zeigler, Ryan A. ; Olson, Eric. / 40Ar/39Ar dating of Apollo 12 regolith : Implications for the age of Copernicus and the source of nonmare materials. In: Geochimica et Cosmochimica Acta. 2006 ; Vol. 70, No. 24. pp. 6016-6031.
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