No cosmic‐ray precompaction exposure of chondrules in CR2.7 MIL 090657

Antoine S. G. Roth, Ingo Leya

Meteoritics & Planetary Science
First published: 16 August 2018


Cosmogenic nuclides are useful for reconstructing the cosmic‐ray precompaction exposure history of the chondritic components in the early solar system. Here, we present He and Ne isotope data for 22 chondrules and 4 matrix samples from Miller Range (MIL) 090657, which is the third most pristine Renazzo‐type (CR) carbonaceous chondrite (CR2.7). The studied samples contain variable mixtures of galactic cosmic ray (GCR)‐produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules and matrix samples have within analytical uncertainty almost identical nominal 21Ne cosmic‐ray exposure (CRE) ages in the range 3.45–6.20 Ma. The nominal 3He CRE ages are mostly discordant, possibly due to partial 3H and 3He losses and/or an inaccurate correction for trapped 3He. The absence of precompaction exposure effects longer than a few Ma in MIL 090657 is at variance with data for other CR chondrites. We conclude that most CR chondrules experienced the same precompaction exposure history—although the nature of the precompaction exposure differs between the chondrites—in regions of the disk that were either exposed to or shielded from GCRs but in which exposure effects due to solar wind and/or solar cosmic rays (SCRs) were negligible. The new data do not support the X‐wind model of Shu and coworkers. “