Thermal history of Apollo 12 granite and KREEP-rich rock: Clues from Pb/Pb ages of zircon in lunar breccia 12013

Abstract

With the mafic to ultramafic petrology of the Moon, the presence of granite appears anomalous. However, fragments of rocks with a definite granitic textures and mineralogy have been recovered from the maria, where their ages point to an ultimate origin in the highlands. Apollo 12 mare breccia 12013 is one of these unique lunar rocks, containing abundant granitic components. It consists of a black portion, composed mainly of fragments of plagioclase and noritic rock, with minor fragments of granitic rock, and a gray portion that is dominated by an overall granitic component. Zircon grains from this breccia were studied by combining cathodoluminescence (CL) imaging and micron-scale, ion-microprobe dating techniques. Zircon grains from the black portion have a large age variation (4.0–4.3Ga). A few of them exhibit complex age zoning within individual grains, corresponding to CL zoning. Zircon grains from the gray portion show no CL zoning features but also have a large age variation between different grains (4.2–4.3Ga).In this study, we suggest that the large age variations of zircon in 12013, among different grains and within individual grains were caused by post-crystallization impact events, and most of them are not original crystallization ages. The 4.3-Ga zircon age probably represents the minimum crystallization age of the granitic component in 12013, similar to lunar granites from the Apollo 14 and 17 landing sites. This old crystallization age also implies that various REE patterns of lunar granites might not be entirely related to their crystallization ages. Based on comparison with the 207Pb/206Pb ages for zircon from the Apollo 12, 14, and 17 landing sites, we suggest that there was at least one pre-3.9Ga major impact event on the Moon, at ∼4.2Ga.