The age and petrography of two Luna 20 fragments and inferences for widespread lunar metamorphism

Abstract

Ages were determined by the 40Ar- 39Ar 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 40Ar, 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 40Ar 36Ar 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.