Rare gas constraints on the history of Boulder 1, Station 2, Apollo 17

Rare gas isotopic analyses have been performed on both pile-irradiated and unirradiated samples from Boulder 1, Station 2. Two samples from rock 72255, the Civet Cat clast and a sample of adjacent breccia, have concordant40Ar-39 Ar ages of 3.99±0.03 b.y. and 4.01±0.03 b.y., respectively. Several samples from rock 72275 have complex thermal release patterns with no datable features, but an intermediate-temperature plateau from the dark rim material of the Marble Cake clast yields an age of 3.99±0.03 b.y. - indistinguishable from the age of rock 72255. We regard these ages as upper limits on the time of the Serenitatis basin-forming event.