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

Abstract

Abstract A spinel-rich, layered calcium- aluminum-rich spherule from the MIL 090019 CO3 chondrite contains a spinel core with a relatively 16O-rich (Δ17O ∼ −18‰) and mass-fractionated oxygen isotopic composition surrounded by minerals, including spinel, that are relatively 16O-poor (Δ17O ∼ −7‰), which are in turn surrounded by layers of 16O-enriched silicates (Δ17O ∼ −17‰). 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 16O-poor gas at the beginning of solar system formation, 105 yr earlier than it can be produced by photochemical self-shielding in the solar nebula and introduced to the inner protoplanetary disk.