ID-TIMS zircon U–Pb geochronology of the Camel Donga eucrite

Abstract

Eucrite meteorites are considered to originate from the upper crust of asteroid 4-Vesta. These differentiated basaltic meteorites commonly contain accessory zircons with grain sizes smaller than 20–30 μm in diameter. U–Pb zircon dating using secondary ion mass spectrometry (SIMS) revealed that initial magmatism and global thermal metamorphism occurred in the Vestan crust ~4550–4555 million year ago (Ma). To improve the temporal resolution in order to distinguish zircons grown during different thermal events in the early stage of crustal evolution of Vesta, we have applied a chemical abrasion-based method for extracting small zircon crystals from bulk meteorite samples and obtained high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb dates for zircons from the Camel Donga eucrite. Some of the Camel Donga zircons occur in contact with ilmenite, implying that they have formed by exsolution of ilmenite followed by reaction with silica during high temperature metamorphism. Raman measurements indicate that the zircons are moderately radiation damaged and that high temperature annealing (900 °C for 48 h) is likely to improve their crystallinities. Zircons separated from acid residues after dissolving other minerals in a mixture of hydrofluoric and nitric acid experienced U–Pb and Pb isotope fractionation during the acid treatment. High temperature annealing prior to acid extraction efficiently suppresses Pb isotope fractionation resulting in accurate and precise 207Pb–206Pb dates. The weighted mean 207Pb–206Pb date of the annealed zircons is 4552.56 ± 0.69 Ma (2σ), which is significantly older than that of Camel Donga zircons previously obtained by SIMS (4531 ± 10 Ma) but is consistent with 207Pb–206Pb dates of zircons in the other eucrites. Thus, the Camel Donga eucrite zircons formed during initial magmatism or global thermal metamorphism in the early stage of Vestan crustal formation.