Determining the noble gas cosmic ray exposure ages of 23 meteorites (8 chondrites and 15 achondrites) from modeling and empirical methods. (30th June 2022) - Bekaert, David V.
Abstract: We present He‐Ne‐Ar isotope data for 23 meteorite samples mainly recovered in Antarctica (six ordinary chondrites [OC], two CV chondrites, eight eucrites, one diogenite, and six ureilites), which are used to compute radiogenic gas retention ages and cosmic ray exposure (CRE) ages using both empirical and modeling approaches. For all samples where both 40 K‐ 40 Ar and U, Th‐ 4 He retention ages could be derived, we find that U, Th‐ 4 He ages are systematically lower than 40 K‐ 40 Ar ages, likely reflecting preferential diffusive loss of He relative to Ar. There is good agreement between empirically derived CRE ages calculated by ( 22 Ne/ 21 Ne)cos ‐ 3 Hecos and ( 22 Ne/ 21 Ne)cos ‐ 21 Necos approaches; where discrepancies occur, the ( 22 Ne/ 21 Ne)cos ‐ 3 Hecos approach systematically yields lower CRE ages, also likely due to 3 He loss. Overall, CRE ages derived from the empirical and modeling approaches show excellent agreement, within ∼10%. CRE ages derived for OC (4–24 Myr), CV chondrites (12–26 Myr), eucrites (4–45 Myr), the diogenite (30 Myr), and ureilites (<10 Myr) are in line with previous investigations of these meteorite groups. Some ureilites and one eucrite exhibit remarkably high cosmogenic 22 Ne/ 21 Ne > 1.24, as previously observed in various other rare achondrites. These samples likely contain solar cosmic ray‐produced Ne (SCR‐Ne) in addition to the commonly found galactic cosmic ray‐produced Ne (GCR‐Ne), implying low pre‐atmospheric shielding and limited ablation upon atmospheric entry. The presence of SCR‐Ne complicates the determination of the pure GCR‐ 22 Ne/ 21 Ne, hampering its use as a shielding indicator. Nonetheless, we suggest that a first‐order correction for SCR‐Ne contribution can be used to derive a range of potential CRE ages for each sample.