Classification of the Apollo-11 mare basalts according to /Ar-39/-/Ar-40/ ages and petrological properties

Previously unpublished (Ar-39)-(Ar-40) ages and exposure ages are presented for Apollo 11 whole rocks and mineral separates. The age data for the Apollo 11 mare basalts in general are discussed in relation to their textural and petrological properties. The oldest rocks (10003, 10029) belong to the low-K group and crystallized at 3.90 + or - 0.03 AE. Three other low-K groups are defined in relation to the petrological classification of Beaty and Albee (1978) and to information on the rock crystallization ages, exposure ages and normative quartz contents. These groups have ages of 3.84 + or - 0.04 AE (10062), 3.75 + or - 0.03 AE (10050) and 3.72 + or - 0.04 AE (10044, 10047, 10058). The best defined (Ar-39)-(Ar-40) age for the high-K basalts is 3.62 + or - 0.05 AE (for mineral separates from 10072). At least four distinct, fairly thin flows of low-K basalt are believed to have been erupted over a 200-my period, followed by either one or several, as yet indistinguishable, very thin flows of high-K basalt. It is not possible to decide whether the earliest dated of the mare basalt eruptions at Mare Tranquillitatis pre-dated, post-dated, or were contemporary with the Imbrian impact event