Ar-Ar and U-Pb ages of Chelyabinsk and a re-evaluation of its impact chronology

Abstract

The LL5 chondrite Chelyabinsk has had numerous isotopic studies since its fall in 2013. These data have been used to suggest ~8 impact events recorded from multiple isotopic systems (e.g., Ar-Ar, U–Pb, Sm-Nd, Rb-Sr, among others). We report details of Ar-Ar and U-Pb results and re-evaluate the geochronology of Chelyabinsk. Argon has the youngest Ar-Ar age recorded in meteorites, 25 ± 11 Ma, and an older resetting event at ~2550 Ma. The U-Pb analysis has an upper concordia age of 4456 ± 23 Ma and a lower concordia age of 184 ± 200 Ma. The lower concordia intercept represents a later thermal event (e.g., an impact), the most recent time that lead loss occurred, and could represent resetting by the youngest event recorded by Ar-Ar. Combining our data with literature results, we find strong evidence of at least four impact events (~4450, 2550, 1700, 25 Ma), with some evidence for two additional impacts (~3700, 1000 Ma).