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

We report oxygen, calcium, titanium and Al-26-Mg-26 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 Delta O-17 value of approximately -23 parts per thousand, 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 3 sigma uncertainties. This suggests that nucleosynthetic anomalies in the refractory elements had been significantly diluted in the environment where SHIBs with uniform Delta O-17 formed. We established internal Al-26-Mg-26 isochrons for eight SHIBs and found that seven of these formed with uniformly high levels of Al-26 (a multi-CAI mineral isochron yields an initial Al-26/Al-27 ratio of similar to 4.8 x 10(-5)), but one SHIB has a smaller initial Al-26/Al-27 of similar to 2.5 x 10(-5), indicating variation in Al-26/Al-27 ratios when SHIBs formed. The uniform calcium, titanium and oxygen isotopic characteristics found in SHIBs with both high and low initial Al-26/Al-27 ratios allow for two interpretations. (1) If subcanonical initial Al-26/Al-27 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 Al-26 was achieved. (2) Alternatively, if subcanonical ratios were the result of Al-26-Mg-26 system resetting, the clustering of SHIBs at a Delta O-17 value of similar to-23 parts per thousand would imply that a O-16-rich gaseous reservoir existed in the nebula until at least similar to 0.7 Ma after the formation of the majority of CAIs. (C) 2016 Elsevier Ltd. All rights reserved.