Oxygen isotopic composition of the Sun, primordial dust and nebular gas: Evidence from refractory inclusions
Preliminary analysis of O-isotopic composition of the solar wind returned by the Genesis spacecraft suggests that the Sun is 16O-rich compared to most chondrules, fine-grained chondrite matrices, and bulk compositions of chondrites, achondrites, and terrestrial planets (Δ17O = −26±6‰ vs. ~ ±5‰). The inferred composition of the Sun matches the 16O-rich compositions of typical refractory inclusions and micron-sized corundum grains from primitive chondrites, which are believed to have condensed from and/or been melted in a gas of solar composition (dust/gas ratio ~ 0.01 by weight) within the first 20−100 Kyr of the solar system formation. Based on solar system abundances, 26% of solar system oxygen is tied up in dust and 74% in gas. Because solar oxygen is dominated by the gas component, these observations suggest that O-isotopic composition of the solar nebula gas was initially 16O-rich. The mean O-isotopic composition of primordial dust is not known.
In the currently favored CO self-shielding models, it is assumed that primordial dust and solar nebula gas had initially identical, 16O-rich compositions, similar to that of the Sun, and solids subsequently evolved towards the terrestrial value (Δ17O = 0). We argue that this assumption is incorrect and suggest primordial dust was 16O-depleted (Δ17O ~ 0) and nebular gas was initially 16O-enriched relative the Sun’s value. This O-isotopic disequilibrium could have resulted from Galactic chemical evolution or be a result of pollution of the protosolar molecular cloud by massive star (>50 times the mass of our sun) ejecta. The 16O-depleted compositions of chondrules, fine-grained matrices, chondrites, and achondrites compared to the Sun reflect their formation in the protoplanetary disk with enhanced dust/gas ratio (up to 105× solar). CO self-shielding may have played only a minor role in O-isotopic evolution of the solar system dust.