Impact history of the HED parent body(ies) clarified by new 40Ar/39Ar analyses of four HED meteorites and one anomalous basaltic achondrite

We investigate the thermal/impact histories of four HED meteorites: one cumulate eucrite (Lake Carnegie) two brecciated basaltic eucrites (Camel Donga, Millbillillie), one howardite (Old Homestead 003) and an anomalous basaltic achondrite (Deakin 010). We have measured eight convincing new 40Ar/39Ar ages for three HED meteorites. Laser incremental 40Ar/39Ar analyses of carefully separated small grains (3.66 Ga for the largest impact recorded. Cosmic ray exposure ages yielded apparent 38Arc ages ranging from 6 to >40 Ma.Two clusters of ages that represent significant impacts into the HED parent body may have been identified. The first high-temperature event is recorded by Lake Carnegie and other unbrecciated eucrites at ~4.5 Ga and is interpreted as a mega-impact, although it is also possible that this age is recording magmatic crystallisation due to the similarity of published Sm–Nd and Pb–Pb ages. Additionally, from statistically reliable isotopic ages, provided in this study, it appears that the age range of major impact heating events post 4.5 Ga might be more tightly restricted (~3.8–3.5 Ga) than previously suggested (~3.4–4.1 Ga), however a larger dataset is required before comparison with the range of the lunar heavy bombardment is justified. If HED are indeed from 4 Vesta, we raise the possibility that the impact at ~4.5 Ga might be associated with the formation of the south polar Veneneia basin, whereas an age at ~3.7 Ga may represent the formation age of either Veneneia, or the younger Rheasilvia impact structure