Gas-Retention and Cosmic-Ray Exposure Ages of Lunar Rock 15555

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Science  28 Jan 1972:
Vol. 175, Issue 4020, pp. 423-425
DOI: 10.1126/science.175.4020.423


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±0.03x109 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±0.03x109 years. A cosmic-ray exposure age of 90±10x106 years is determined from the ratio of spallogenic argon-38 to calcium.