{40}$Ar-$^{39}$Ar Dating of Luna 16 and Luna 20 Samples

• Published in: Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences physical, and engineering sciences, 1977-01, Vol.284 (1319), p.167
• Main Authors: P. H. Cadogan, G. Turner
• Format: Article
• Language: English
• ISSN: 1364-503X; 1471-2962
• DOI: 10.1098/rsta.1977.0007

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 $\pm $ 0.10 Ga and a minimum age of 4.30 $\pm $ 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.