A reexamination of the RB-SR isotopic systematics of lunar breccia 12013

Lunar breccia 12013 is one of the few rocks for which isotopic evidence has been interpreted as indicating a 4.5-AE age, and it is the only lunar rock which contains abundant granitic material that may date back to the initial lunar differentiation. An investigation of 12013 conducted by Quick et al. (1981) has led to the conclusion that 12013 is an extremely complex mixture of impact-generated melt and clastic components of uncertain affinities. It was found that the granitic component in 12013 need not be any older than 4.16-4.17 AE. In the present study a model is presented for the Rb-Sr evolution of 12013 which explains the isotopic data without resorting to a 4.5-AE age for any component. Attention is given to petrographic constraints, isotopic investigations, and an evaluation of the Rb-Sr isotopic data. It is concluded that lunar rock 12013 is an extremely complex polymict breccia in which the Rb-Sr isotopic systematics have been greatly influenced by mixing during breccia formation and by subsequent reequilibration.