Ages of Crystalline Rocks from Fra Mauro

Abstract
Crystallization ages for six rocks from Fra Mauro have been measured by the argon-40-argon-39 method. All six rocks give an age of 3.77 ± 0.15 x 109 years, which is the same as for fragmental rocks from this site. It is concluded that the Imbrium event and the crystallization of a significant portion of the pre-Imbrian basalts were essentially contemporaneous.