Geochemistry and 40Ar-39Ar geochronology of impact-melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history

Abstract

We studied 42 impact-melt clasts from lunar feldspathic regolith breccias MacAlpine Hills (MAC) 88105, Queen Alexandra Range (QUE) 93069, Dar al Gani (DaG) 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to the impactmelt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from them, having lower Fe, Ti, K, and P, thus representing previously unsampled impacts. The 40Ar- 39Ar ages on 31 of the impact melts, the first ages on impact-melt samples from outside the region of the Apollo and Luna sampling sites, range from ~4 to ~2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact-melt rock age cluster at 3.9 Ga, but the meteorite impact-melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact-melt clast ages is a lunar-wide phenomenon. No meteorite impact melts have ages more than 1 older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.