Old formation ages of igneous clasts on the L chondrite parent body reflect an early generation of planetesimals or chondrule formation

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The Barwell meteorite contains large, abundant clasts that are igneous in nature. We report iodine–xenon ages of five clasts and one sample of host chondrite material. The fragment of host chondrite material yielded the oldest age determined: 4567.8 ± 1.2 Ma. Two clasts produced old, well defined ages of 4564.96 ± 0.33 Ma and 4565.60 ± 0.33 Ma. These, and a third clast having a less precise old age of 4566.0 ± 3.2 Ma, are interpreted as recording the timing of crystallisation of the samples. They were incorporated into the Barwell parent body before it underwent thermal metamorphism, but the I–Xe ages survived secondary processing on the parent body and were not reset by metamorphism, metasomatism or shock. Two further clasts record younger ages of 4560.96 ± 0.45 Ma and 4554.22 ± 0.38 Ma. These samples contain a high abundance of albitic mesostasis, and the most likely explanation of the ages is that they record the timing of metasomatism on the parent body. We also analysed four host chondrite samples that do not give I–Xe ages: in these samples, the system appears to have been disturbed by shock. It has been suggested previously that the igneous clasts are derived from an early generation of partially melted asteroids. We do not have direct evidence that the clasts we examined were necessarily derived from a partially differentiated body, only that they were derived from cooling of a silicate melt; the clasts could thus be the products of any one of several proposed models for chondrule formation. Our results indicate that processes akin to chondrule formation, in that they involve rapid cooling of a silicate melt, were ongoing at the same time as CAI formation, lending support to the suggestion that Al–Mg chondrule ages indicate either heterogeneous distribution of 26Al or resetting of the Al–Mg system after chondrule formation.




Crowther, S. A., Filtness, M. J., Jones, R. H., & Gilmour, J. D. (2018). Old formation ages of igneous clasts on the L chondrite parent body reflect an early generation of planetesimals or chondrule formation. Earth and Planetary Science Letters, 481, 372–386. https://doi.org/10.1016/j.epsl.2017.10.047

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