Complex exposure histories for meteorites with “short” exposure ages

Abstract

Abstract— We report measurements of 26 Al and 10 Be activities in nine ordinary chondrites and of the light noble gas concentrations and 36 Cl and 41 Ca activities in subsets of those meteorites. All but Murray have low 21 Ne concentrations (<1.0 × 10 −8 cm 3 STP/g) and have previously been used to estimate 21 Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21 Ne production rates that are consistent with the standard L‐chondrite value of 0.33 × 10 −8 cm 3 STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower 3 He/ 21 Ne ratios than expected based on their 22 Ne/ 21 Ne ratios. A general association between low 21 Ne contents and 3 He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low 21 Ne contents, one‐stage exposure histories, and losses of cosmogenic 3 He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia.