Protracted storage of CR chondrules in a region of the disk transparent to galactic cosmic rays

1,2Antoine S. G. Roth,3Knut Metzler,4Lukas P. Baumgartner,5Beda A. Hofmann,6Ingo Leya
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12923]
1Institute of Physics, University of Bern, Bern, Switzerland:
2Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
3Institute for Planetology, University of Münster, Münster, Germany
4Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland
5Department of Earth Sciences, Natural History Museum Bern, Bern, Switzerland
6Institute of Physics, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

Renazzo-type carbonaceous (CR) chondrites are accretionary breccias that formed last. As such they are ideal samples to study precompaction exposures to cosmic rays. Here, we present noble gas data for 24 chondrules and 3 dark inclusion samples (DIs) from Shişr 033 (CR2). The meteorite was selected based on the absence of implanted solar wind noble gases and an anomalous oxygen isotopic composition of the DIs; the oxygen isotopes match those in CV3 and CO3 chondrites. Our samples contain variable mixtures of galactic cosmic ray (GCR)-produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules have cosmogenic 3He and 21Ne concentrations up to 4.3 and 7.1 times higher than the DIs, respectively. We derived an average 3He-21Ne cosmic ray exposure (CRE) age for Shişr 033 of 2.03 ± 0.20 Ma (2 SD) and excesses in cosmogenic 3He and 21Ne in chondrules (relative to the DIs) in the range (in 10−8 cm3STP/g) 3.99–7.76 and 0.94–1.71, respectively. Assuming present-day GCR flux density, the excesses translate into average precompaction 3He-21Ne CRE ages of 3.1–27.3 Ma depending on the exposure geometry. The data can be interpreted assuming a protracted storage of a single chondrule generation prior to the final assembly of the Shişr 033 parent body in a region of the disk transparent to GCRs.

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