Oxygen Isotopic Variations in the Calcium, Aluminum-rich Inclusion–forming Region Recorded by a Single Refractory Inclusion from the CO3.1 Carbonaceous Chondrite Dar al Gani 083

Abstract Calcium, aluminum-rich inclusions (CAIs) are the oldest solids dated that formed in the solar system. Most CAIs in unmetamorphosed chondritic meteorites (chondrites; petrologic type ≤3.0) have uniform solar-like 16 O-rich compositions (Δ 17 O ∼ −24‰) and a high initial 26 Al/ 27 Al ratio ( 26 Al/ 27 Al) 0 of ∼(4–5) × 10 −5 , consistent with their origin in a gas of approximately solar composition during a brief (3.0) remains an open issue. This heterogeneity could have recorded fluctuations of O-isotope composition of nebular gas in the CAI-forming region and/or postcrystallization O-isotope exchange of CAI minerals with aqueous fluids on the chondrite parent asteroids. To obtain insights into possible processes resulting in this heterogeneity, we investigated the mineralogy, rare-earth element abundances, and O- and Mg-isotope compositions of a CAI from the CO3.1 chondrite Dar al Gani 083. This concentrically zoned inclusion has a Zn-hercynite core surrounded by layers of (from core to edge) grossite, spinel, melilite, and Al-diopside. The various phases have heterogeneous Δ 17 O (from core to edge): −2.2 ± 0.6‰, −0.9 ± 2.1‰, −13.7 ± 2.1‰, −2.6 ± 2.3‰, and −22.6 ± 2.1‰, respectively. Magnesium-isotope compositions of grossite, spinel, melilite, and Al-diopside define an undisturbed internal Al–Mg isochron with ( 26 Al/ 27 Al) 0 of (2.60 ± 0.29) × 10 −6 . We conclude that the variations in Δ 17 O of spinel and diopside recorded fluctuations in O-isotope composition of nebular gas in the CAI-forming region prior to injection and/or homogenization of 26 Al at the canonical level. The 16 O depletion of grossite and melilite resulted from O-isotope exchange with asteroidal fluid, which did not disturb Al–Mg isotope systematics of the CAI primary minerals.