title: Gas-retention and cosmic-ray exposure ages of lunar rock 15555.
abstract: 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+/-0.03x10(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+/-0.03x10(9) years. A cosmic-ray exposure age of 90+/-10x10(6) years is determined from the ratio of spallogenic argon-38 to calcium.
authors.forename: F A
authors.initials: FA
authors.lastname: Podosek
authors.forename: J C
authors.initials: JC
authors.lastname: Huneke
authors.forename: G J
authors.initials: GJ
authors.lastname: Wasserburg