The case for a younger Imbrium basin: New 40Ar- 39Ar ages of Apollo 14 rocks

Abstract

Petrographically and chemically well-characterized rocks of the Apollo 14 landing site (12 impact melt breccias, 3 clast-free impact melt rocks, 2 granulitic rocks, 1 cataclastic noritic anorthosite, 1 shocked gabbronorite, 1 mare basalt, and 1 plagioclase single crystal) have been dated by the 40Ar- 39Ar technique. The samples represent two different formations at the Apollo 14 site: (1) small lithic clasts (10 specimens) from fragmental breccia 14063 ejected from the 26 Ma old Cone crater and one clast (mare basalt) from Cone crater soil 14140, and (2) large rock samples (10 specimens) collected throughout the regolith of the landing site. The first group of samples has exposure ages close to 26 Ma and 40Ar- 39Ar ages ranging from 3.86 to 4.09 Ga, which indicates that breccia 14063 was assembled 3.86 Ga ago. The second group of samples displays lower 40Ar- 39Ar ages ranging from 3.73 to 3.85 Ga. We interpret breccia 14063 as part of a Nectarian megabreccia unit which was incorporated into the Fra Mauro Formation as a megablock by secondary mass wasting of the ejecta blanket of a local pre-Imbrian crater. The remaining samples are thought to represent clasts of the Apollo 14 subregolith basement megabreccia defined as the Fra Mauro Formation. Consequently, the reported ages of these clasts confirm previous suggestions that the Fra Mauro Formation and therefore the Imbrium basin is only 3.75 Ga old and not 3.85 Ga as commonly assumed.