Ar-40 - Ar-39 Geochronology on Apollo 12 Regolith

The Apollo 12 landing site in Oceanus Procellarum is located about 400 km south of the rim of Copernicus. The geochemical and petrographical characterization of the Apollo 12 lunar samples have been extensively studied. The site is characterized by a high proportion of nonmare material with KREEP composition and mare basalts low in Th. Recent studies have focused on the geochemistry of the nonmare material and on inferring the origin of these materials. Previous geochronological investigations of Apollo 12 mare basalts have yielded ages ranging from 3.1 to 3.3 Ga. On the other hand, nonmare materials have yielded older ages between 3.8 to 4.0 Ga; with the sole exception of a single lunar granite fragment from sample 12033. This granite sample has an Ar-40 - Ar-39 age of 800 +/- 15 Ma. This resetting age has been interpreted as the age of the Copernicus crater since a ray from this crater crosses the landing site. Here we present new 40Ar-39Ar geochronological data on three samples of nonmare material from the Apollo 12 regolith in order to further constrain the age and source of this material.