The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism
Abstract
Ages were determined by the ^(40)Ar-^(39)Ar method on two metaclastic rocks returned from the lunar highlands north of Mare Fecunditatis by the Luna 20 probe. Both samples gave very well-defined argon retention ages of 3.90 ± 0.04 AE which are indistinguishable from each other within a resolution of 0.02 AE. Both fragments, 22006 and 22007, are highly recrystallized polymict breccias; there is no evidence for loss of radiogenic ^(40)Ar, and the age almost surely dates the time of recrystallization. The cosmic ray exposure ages of these fragments are similar and high: 900 million years for 22006, 1300 million years for 22007. 22007 also contains substantial trapped argon with a high ^(40)Ar^(36)Ar ratio. The Luna 20 results greatly extend the area of the Moon's surface exhibiting a well-defined record of metamorphism at 3.9 AE. So far, lunar history in the interval 4.6−3.9 AE is not preserved in the ages of surface rocks. This obliteration suggests lunar-wide metamorphic conditions occurring or terminating at this time as a result of major impacts.
Additional Information
© 1973 Published by Elsevier Ltd. Received 30 January 1973, Accepted 8 February 1973. We are pleased to acknowledge the role of the Lunar Sample Analysis Planning Team in the design of a series of experiments which seeks to maximize the scientific information obtained from extremely limited material. We are grateful for the friendly cooperation of J.C. LAUL and R. A. SCHMITT in coordinating cascaded experiments. The continuing interest and friendly criticism of A. L. ALBEE was greatly appreciated. We also thank T. J. GAY for flux-wire counting. This work was supported by NASA under grant [NGL 05-002-188.)Additional details
- Eprint ID
- 85326
- DOI
- 10.1016/0016-7037(73)90186-5
- Resolver ID
- CaltechAUTHORS:20180315-083426044
- NGL 05-002-188
- NASA
- Created
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2018-03-30Created from EPrint's datestamp field
- Updated
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2021-11-15Created from EPrint's last_modified field
- Other Numbering System Name
- Caltech Division of Geological and Planetary Sciences
- Other Numbering System Identifier
- 2315