EXTREME 16O ENRICHMENT IN CALCIUM-ALUMINUM-RICH INCLUSIONS FROM THE ISHEYEVO (CH/CB) CHONDRITE

Calcium-aluminum-rich inclusions (CAIs) from the metal-rich (CH/CB-like) carbonaceous chondrite Isheyevo are mineralogically pristine and show no evidence for postcrystallization alteration. Many of them are composed of very refractory minerals, such as hibonite (CaAl12O19), grossite (CaAl4O7), aluminum-rich pyroxene, and perovskite (CaTiO3). Twenty-eight out of 35 studied CAIs from Isheyevo have oxygen isotopic compositions similar to those of CAIs from the CM and CR carbonaceous chondrites (Δ17O ∼ -20 per mille ). Five igneous CAIs are 16O-depleted to a level observed in Isheyevo chondrules (Δ17O ∼> -10 per mille ), suggesting remelting and isotope exchange in an 16O-poor gaseous reservoir. Two CAIs, WA9 and B1, show the highest enrichment in 16O (δ17O ∼ -68 per mille , δ18O ∼ -66 per mille , Δ17O ∼ -34 per mille ) ever observed among refractory inclusions. In the context of the self-shielding model for the evolution of oxygen isotopes in the solar accretion disk, these CAIs may have recorded the initial oxygen isotopic composition of the solar system, and hence of the Sun.