Times of impacts that deliver samples of Vesta to Earth derived from ultrasensitive 81Kr–Kr cosmic ray exposure age analysis of Eucrites

Abstract

Concentrations and isotopic compositions of krypton have been measured in Eucrite falls using a new resonance ionization mass-spectrometer (RIMSKI). The high sensitivity of the apparatus allowed sample sizes to be reduced to 1–4mg for 81Kr–Kr cosmic ray exposure age determination. 81Kr CRE ages have been determined for: Bereba, Bouvante, Ibitira, Juvinas, Millbillillie, Padvarninkai, Pasamonte and Stannern with precision comparable to those reported by other groups for analyses of >100 times larger samples. The 81Kr–Kr CRE ages of Lakangaon (25.3±1.1My) and Bunburra Rockhole (27.6±1.4My) have been determined for the first time.The 81Kr CRE ages of 16 out of 17 ordinary Eucrite falls can be grouped into clusters at 10.6±0.4Ma, 14.4±0.6My, 21.7±0.4My, 25.4±0.4My, 37.8±0.6My using an analysis based on the Akaike information criterion, which is well suited to addressing such issues. This shows that most of the meteorites originate from five collision events over the last ∼50My, which is hard to reconcile with models requiring multiple fragmentation events between Vesta and Earth.