40 Ar— 39 Ar DATING OF LUNA 16 AND LUNA 20 SAMPLES

Abstract

Two fragments of Luna 16 mare basalt and two Luna 20 samples (one metaclastic fragment and a group of five anorthositic particles) from the Royal Society allocation have been dated by using the 40 Ar— 39 Ar technique. The results of this work effectively double the chronological information available concerning these sites. The Luna 16 ages confirm that mare lavas were extruded in Mare Fecunditatis 3.4—3.5 Ga ago. Similarly, the age of the Luna 20 metaclastic fragment provides further support for a 3.9 Ga age for the Crisium basin and for the widespread cataclysmic bombardment of the moon between 4.05 and 3.85 Ga. The presence of at least two isotopically distinct, non-radiogenic argon components in the Luna 20 anorthositic sample compromises an unequivocal age assignment but the simplest model leads to a plateau age of 4.40 ±0.10 Ga and a minimum age of 4.30 ± 0.10 Ga. There are now several lunar highland samples which are significantly older than 4.0 Ga and which give rise to a secondary age grouping in the interval 4.2-4.3 Ga. Depending on whether highland ages were primarily reset during the excavation of a large number of medium-sized craters or a small number of large basins, this secondary grouping implies either a significant peak in the cratering rate at 4.0 Ga or statistical fluctuations in a monotonically decreasing cratering rate.