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).

Lithologies and Chronologic Opportunities of Materials to Be Returned From the Artemis Exploration Zone

“…The associated 40 K‐ 40 Ca chronology of felsic lunar materials (Shih et al., 1993) is possible for specialized applications. Analytical details of the 40 Ar‐ 39 Ar method are complex (Beard et al., 2022; Cohen et al., 2000, 2005; McDougall et al., 1999; Mercer & Hodges, 2016; Niihara et al., 2019; Schaen et al., 2020; Swindle et al., 2009; Weirich et al., 2010; Wittmann et al., 2011). Recent advancements in in situ analytical approaches (Mercer et al., 2015) make it easier for analyses of critical petrographic contexts.…”