New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions

1,2,3Levke Kööp, 4,5Daisuke Nakashima, 1,2,3Philipp R. Heck, 4Noriko T. Kita, 4Travis J. Tenner, 6Alexander N. Krot, 6Kazuhide Nagashima, 6,7Changkun Park, 1,2,3,8Andrew M. Davis
1Department of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
2Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA
3Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, USA
4Department of Geoscience, University of Wisconsin, Madison, WI 53706, USA
5Division of Earth and Planetary Material Sciences, Faculty of Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578 Japan
6Hawai‘i Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, HI
7Korea Polar Research Institute, Incheon 406-840, Korea
8Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA.

We report oxygen, calcium, titanium and 26Al-26Mg isotope systematics for spinel-hibonite inclusions (SHIBs), a class of calcium-aluminum-rich inclusions (CAI) common in CM chondrites. In contrast to previous studies, our analyses of 33 SHIBs and four SHIB-related objects obtained with high spatial resolution demonstrate that these CAIs have a uniform Δ17O value of approximately –23‰, similar to many other mineralogically pristine CAIs from unmetamorphosed chondrites (e.g., CR, CV, and Acfer 094). Five SHIBs studied for calcium and titanium isotopes have no resolvable anomalies beyond 3σ uncertainties. This suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where SHIBs with uniform Δ17O formed. We established internal 26Al-26Mg isochrons for eight SHIBs and found that seven of these formed with uniformly high levels of 26Al (a multi-CAI mineral isochron yields an initial 26Al/27Al ratio of ∼4.8×10–5), but one SHIB has a smaller initial 26Al/27Al of ∼2.5×10–5, indicating variation in 26Al/27Al ratios when SHIBs formed. The uniform calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low initial 26Al/27Al ratios allow for two interpretations. (1) If subcanonical initial 26Al/27Al ratios in SHIBs are due to early formation, as suggested by Liu et al. (2012), our data would indicate that the CAI formation region had achieved a high degree of isotopic homogeneity in oxygen and refractory elements before a homogeneous distribution of 26Al was achieved. (2) Alternatively, if subcanonical ratios were the result of 26Al-26Mg system resetting, the clustering of SHIBs at a Δ17O value of ∼ –23‰ would imply that a 16O-rich gaseous reservoir existed in the nebula until at least ∼0.7 Ma after the formation of the majority of CAIs.

Reference
Kööp L, Nakashima D, Heck PR, Kita NT, Tenner TJ, Krot AN, Nagashima K, Park C, Davis AM (2016) New constraints on the relationship between 26Al and oxygen, calcium, and titanium isotopic variation in the early Solar System from a multielement isotopic study of spinel-hibonite inclusions. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.04.018]
Copyright Elsevier

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