Argon-40-argon-39 dating of Apollo sample 15555.

An age of 3.33 (plus or minus 0.05) b.y. was obtained for Apollo 15 sample 15555 by argon-40-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 500 m.y. Therefore, the mare basalts cannot be simple impact melts but rather must result from some igneous activity on the moon.