A refractory inclusion with solar oxygen isotopes and the rarity of such objects in the meteorite record

1,2Levke Kööp,3Kazuhide Nagashima,1,2,4Andrew M. Davis,1Alexander N. Krot
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13434]
1Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, 60637 USA
2Chicago Center for Cosmochemistry, The University of Chicago, Chicago, Illinois, 60637 USA
3Hawai’i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai’i at Mānoa, Honolulu, Hawai’i, 96822 USA
4Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, 60637 USA
Published by arrangement with John Wiley & Sons

NASA’s Genesis mission revealed that the Sun is enriched in 16O compared to the Earth and Mars (the Sun’s Δ17O, defined as δ17O–0.52×δ18O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as 16O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as 16O‐enriched as the Sun (Δ17O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in 48Ca and 50Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live 26Al; (26Al/27Al)0 = (0.03 ± 0.11) × 10–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ17O value near –23.5‰. Combined with 26Al‐26Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ17O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.

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