Molecular Cloud Origin for Oxygen Isotopic Heterogeneity Recorded by a Primordial Spinel-rich Refractory Inclusion

A spinel-rich, layered calcium- aluminum-rich spherule from the MIL 090019 CO3 chondrite contains a spinel core with a relatively O-16-rich (Delta O-17 similar to -18 parts per thousand) and mass-fractionated oxygen isotopic composition surrounded by minerals, including spinel, that are relatively O-16-poor (Delta O-17 similar to -7 parts per thousand), which are in turn surrounded by layers of O-16-enriched silicates (Delta O-17 similar to -17 parts per thousand). Inclusions with refractory mineral assemblages such as this one are proposed to record inner nebula processes during the earliest epoch of solar nebula evolution. Mineralogical and textural analyses indicate that this primordial particle formed by high-temperature gas-solid reactions, partial melting, evaporation, and condensation. The radially distributed oxygen isotopic heterogeneity measured among multiple occurrences of several minerals, including spinel, requires the existence of O-16-poor gas at the beginning of solar system formation, 10(5) yr earlier than it can be produced by photochemical self-shielding in the solar nebula and introduced to the inner protoplanetary disk.