Oxygen isotopic heterogeneity in the early Solar System inherited from the protosolar molecular cloud

Abstract

The Sun is 16O-enriched (17O = −28.4 ± 3.6) relative to the terrestrial planets, asteroids, and chondrules (− < 17O < 3). Ca,Al-rich inclusions (CAIs), the oldest Solar System solids, approach the Sun’s 17O. Ultraviolet CO self-shielding resulting in formation of 16O-rich CO and 17,18O-enriched water is the currently favored mechanism invoked to explain the observed range of 17O. However, the location of CO self-shielding (molecular cloud or protoplanetary disk) remains unknown. Here we show that CAIs with predominantly low (26Al/27Al)0, <5 × 10−6, exhibit a large inter-CAI range of 17O, from −40 to −5. In contrast, CAIs with the canonical (26Al/27Al)0 of ~5 × 10−5 from unmetamorphosed carbonaceous chondrites have a limited range of 17O, −24 ± 2. Because CAIs with low (26Al/27Al)0 are thought to have predated the canonical CAIs and formed within first 10,000–20,000 years of the Solar System evolution, these observations suggest oxygen isotopic heterogeneity in the early solar system was inherited from the protosolar molecular cloud.