CO Self-Shielding as a Mechanism to Make 16 O-Enriched Solids in the Solar Nebula

Photochemical self-shielding of CO has been proposed as a mechanism to produce solids observed in the modern, 16 O-depleted solar system. This is distinct from the relatively 16 O-enriched composition of the solar nebula, as demonstrated by the oxygen isotopic composition of the contemporary sun. While supporting the idea that self-shielding can produce local enhancements in 16 O-depleted solids, we argue that complementary enhancements of 16 O-enriched solids can also be produced via C 16 O-based, Fischer-Tropsch type (FTT) catalytic processes that could produce much of the carbonaceous feedstock incorporated into accreting planetesimals. Local enhancements could explain observed 16 O enrichment in calcium-aluminum-rich inclusions (CAIs), such as those from the meteorite, Isheyevo (CH/CHb), as well as in chondrules from the meteorite, Acfer 214 (CH3). CO self-shielding results in an overall increase in the 17 O and 18 O content of nebular solids only to the extent that there is a net loss of C 16 O from the solar nebula. In contrast, if C 16 O reacts in the nebula to produce organics and water then the net effect of the self-shielding process will be negligible for the average oxygen isotopic content of nebular solids and other mechanisms must be sought to produce the observed dichotomy between oxygen in the Sun and that in meteorites and the terrestrial planets. This illustrates that the formation and metamorphism of rocks and organics need to be considered in tandem rather than as isolated reaction networks.