Cosmogenic He and Ne in chondrules from clastic matrix and a lithic clast of Murchison: No pre-irradiation by the early sun

My E.I. Riebe*, Liliane Huber, Knut Metzler, Henner Busemann, Stefanie M. Luginbuehl, Matthias M.M. Meier, Colin Maden, Rainer Wieler

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Whether or not some meteorites retain a record of irradiation by a large flux of energetic particles from the early sun in the form of excesses of cosmic-ray produced noble gases in individual crystals or single chondrules is a topic of ongoing debate. Here, we present He and Ne isotopic data for individual chondrules in Murchison, a chondritic regolith breccia of the CM group. We separated 27 chondrules from a clastic matrix portion and 26 chondrules from an adjacent single so-called “primary accretionary rock” (Metzler et al., 1992). All chondrules from the primary rock fragment are expected to share a common irradiation history, whereas chondrules from the clastic matrix were stirred in the regolith independently of each other. All “primary rock chondrules” and 23 of the “matrix chondrules” have very similar concentrations of cosmogenic 3He and 21Ne, corresponding to a cosmic-ray exposure age to galactic cosmic rays (GCR) of ∼1.3–1.9 Ma, in the range of Murchison's meteoroid exposure age determined with cosmogenic radionuclides. Four clastic matrix chondrules contain excesses of cosmogenic 3He and 21Ne, corresponding to nominal 4π exposure ages of ∼4–∼29 Ma, with a Ne isotopic composition as expected for production by GCR. If the fraction of excess cosmogenic gas bearing chondrules in the primary rock and clastic matrix were the same, we would expect this result with a statistical probability of only 0.5 – 2.7%. Therefore, the exposure age distributions for Murchison chondrules in primary rock and clastic matrix are very likely different. Such a difference is expected if the excess cosmogenic gas was acquired by some of the matrix chondrules in the regolith, but not if chondrules were irradiated in the solar nebula by the early sun before they accreted on the Murchison parent body. Therefore, Murchison does not provide evidence for irradiation by a high fluence of energetic particles from the early sun. By inference, this statement likely holds for the other regolithic meteorites for which large occasional excesses of cosmogenic noble gases have been reported. Considering pre-irradiation in a regolith (2π exposure), the pre-exposure times for these four chondrules are at least between some 4 and 40 Ma near the very surface of the parent body, and even longer if they were buried deeper in the regolith.

Original languageEnglish
Pages (from-to)618-634
Number of pages17
JournalGeochimica et Cosmochimica Acta
Volume213
DOIs
Publication statusPublished - 15 Sept 2017

Funding

We appreciate fruitful discussions with Antoine Roth and helpful assistance by Lukas Martin at the electron microprobe. Detailed and thoughtful reviews from Julia A. Cartwright and two anonymous reviewers, as well as comments by AE Gregory Herzog improved this manuscript. This work was supported by the Swiss National Science Foundation (SNSF), and in part (HB) carried out within the frame of the National Centre for Competence in Research ?PlanetS? supported by (SNSF).

FundersFunder number
National Centre for Competence in Research
Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung154874, 143950

    Keywords

    • Cosmic-ray exposure ages
    • Early Solar System
    • Murchison chondrules

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