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

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 O-16-rich compositions (Delta O-17 similar to -24 parts per thousand) and a high initial 26Al/27Al ratio [(Al-26/Al-27)0] of similar to(4-5) x 10(-5), consistent with their origin in a gas of approximately solar composition during a brief (<0.3 Ma) epoch at the earliest stage of our solar system. The nature of O-isotope heterogeneity in CAIs (Delta O-17 range from similar to-24 up to similar to+5 parts per thousand) from weakly metamorphosed chondrites (petrologic type >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 Delta O-17 (from core to edge): -2.2 +/- 0.6 parts per thousand, -0.9 +/- 2.1 parts per thousand, -13.7 +/- 2.1 parts per thousand, -2.6 +/- 2.3 parts per thousand, and -22.6 +/- 2.1 parts per thousand, respectively. Magnesium-isotope compositions of grossite, spinel, melilite, and Al-diopside define an undisturbed internal Al-Mg isochron with (Al-26/Al-27)(0) of (2.60 +/- 0.29) x 10(-6). We conclude that the variations in Delta O-17 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 Al-26 at the canonical level. The O-16 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.