Argon-40-Argon-39 Dating of Apollo Sample 15555

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Science  28 Jan 1972:
Vol. 175, Issue 4020, pp. 417-419
DOI: 10.1126/science.175.4020.417


An age of 3.33±0.05x109 years was obtained for Apollo 15 sample 15555 by argon40-argon-39 dating. The age of rock 15555, a basalt from the rim of Hadley Rille, establishes an upper limit to the age of the rille. The basalt flows filling the Hadley Rille section of the Imbrium basin postdate the formation of the basin—as measured by the Apollo 14 samples of the Fra Mauro formation—by at least 500x106 years. Therefore, the mare basalts cannot be simple impact melts but rather must result from some igneous activity on the moon.