Existence of an ¹⁶O-Rich Gaseous Reservoir in the Solar Nebula

Carbonaceous chondrite condensate olivine grains from two distinct petrographic settings, calcium-aluminum–rich inclusion (CAI) accretionary rims and amoeboid olivine aggregates (AOAs), are oxygen-16 (¹⁶O) enriched at the level previously observed inside CAIs. This requires that the gas in the nebular region where these grains condensed was ¹⁶O-rich. This contrasts with an ¹⁶O-poor gas present during the formation of chondrules, suggesting that CAIs and AOAs formed in a spatially restricted region of the solar nebula containing ¹⁶O-rich gas. The ¹⁶O-rich gas composition may have resulted either from mass-independent isotopic chemistry or from evaporation of regions with enhanced dust/gas ratios, possibly in an X-wind environment near the young Sun.