Gas-retention and cosmic-ray exposure ages of lunar rock 15555

The last lava flow in the Hadley Rille area of Mare Imbrium, as inferred from an argon-40-argon-39 experiment on a plagioclase separate from the lunar basalt 15555, occurred 3.31 ± 0.03 × 10 9 years ago. An argon-40-argon-39 experiment on a whole rock sample shows significant loss of radiogenic argon-40 and yields a well-defined, high-temperature plateau indicating a lower age of 3.22 ± 0.03 × 10 9 years. A cosmic-ray exposure age of 90 ± 10x10 6 years is determined from the ratio of spallogenic argon-38 to calcium.