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

Abstract

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 (<250 μm) of plagioclase, pyroxene, matrix and melt rock from these meteorites yielded well-defined plateau and/or isochron ages for two samples of Lake Carnegie, four samples of Millbillillie and two samples of Camel Donga. Two concordant 40Ar/39Ar plateau ages were obtained for plagioclase separates from Lake Carnegie, resulting in a weighted mean age of combined plateau ages of 4507 ± 20 Ma. Millbillillie recorded three impact events at 3722 ± 55 Ma, 3579 ± 28 Ma and 3313 ± 174 Ma from plagioclase, matrix, and pyroxene respectively; this highlights the power of the K/Ar system to record impacts of different magnitudes experienced by a single meteorite. Two aliquots from Camel Donga returned concordant plateau ages interpreted to represent a single significant impact event at 3693 ± 51 Ma. The anomalous basaltic achondrite, Deakin 010, did not yield any plateau and resulted in a series of minimum apparent ages with an oldest minimum age of >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.