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

The LL5 chondrite Chelyabinsk has had numerous isotopic studies since its fall in 2013. These data have been used to suggest similar to 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 similar to 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 (similar to 4450, 2550, 1700, 25 Ma), with some evidence for two additional impacts (similar to 3700, 1000 Ma).