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Meteorites and their components have anomalous oxygen isotopic compositions characterized by large variations in ^sup 18^O/^sup 16^O and ^sup 17^O/^sup 16^O ratios. On the basis of recent observations of star-forming regions and models of accreting protoplanetary disks, we suggest that these variations may originate in a parent molecular cloud by ultraviolet photodissociation processes. Materials with anomalous isotopic compositions were then transported into the solar nebula by icy dust grains during the collapse of the cloud. The icy dust grains drifted toward the Sun in the disk, and their subsequent evaporation resulted in the ^sup 17^O- and ^sup 18^O-enrichment of the inner disk gas.
Oxygen is the most abundant element in the solid phases that formed early in the solar system, and it has three stable isotopes of mass numbers 16, 17, and 18. On a threeoxygen isotope diagram, ^sup 18^O/^sup 16^O and ^sup 17^O/ ^sup 16^O abundance ratios of most terrestrial material constitute a line with slope of ~0.5, called the terrestrial fractionation (TF) line. This slope is due to isotope fractionation processes that depend on the mass difference between each pair of isotopes. In contrast, most meteorites have oxygen isotopic compositions that diverge from the TF line (1). Refractory inclusions and some chondrules in primitive meteorites have the most ^sup 16^O-enriched isotope compositions, shifted from the TF line with magnitudes of several percent in ^sup 17^O/^sup 16^O and ^sup 18^O/^sup 16^O ratios (1, 2). Nonradiogenic effects in the other major elements (e.g., Mg and Si) in these meteorite constituents have isotope compositions close to the terrestrial compositions, and their small deviations can be explained by isotope fractionation due to thermal processes, e.g., evaporation, condensation, aqueous alteration, and low-temperature chemical reaction (3).
The origin of mass-independent fractionation of oxygen isotopes and the lack of such fractionation in other major elements in meteorites remains poorly understood. It cannot be due to nucleosynthetic processes or nuclear reactions that involve energetic particles from the Sun or from Galactic cosmic rays, because these processes would also change the isotopic compositions of the other elements (1). In addition, presolar grains enriched in ^sup 16^O are rare in meteorites (4). Although some types of molecular reactions in gaseous phases have been found to induce such mass-independent isotope fractionation in oxygen...