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

Abstract

The Sun is 16O-enriched (Δ17O = -28.4 ± 3.6‰) relative to the terrestrial planets, asteroids, and chondrules (-7‰ < Δ17O < 3‰). Ca,Al-rich inclusions (CAIs), the oldest Solar System solids, approach the Sun's Δ17O. 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 Δ17O. 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 Δ17O, 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 Δ17O, -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.