Yurimoto et al.: Origin and Evolution of Oxygen-Isotopic Compositions 849 849 Origin and Evolution of Oxygen-Isotopic Compositions of the Solar System Hisayoshi Yurimoto and Kiyoshi Kuramoto Hokkaido University Alexander N. Krot and Edward R. D. Scott University of Hawaii at Manoa Jeffrey N. Cuzzi NASA Ames Research Center Mark H. Thiemens University of California San Diego James R. Lyons University of California, Los Angeles On a three-isotope diagram oxygen-isotopic compositions of most primitive meteorites (chon- drites), chondritic components (chondrules, refractory inclusions, and matrix), and differenti- ated meteorites from asteroids and Mars deviate from the line along which nearly all terrestrial samples plot. Three alternative mechanisms have been proposed to explain this oxygen-isoto- pic anomaly: nucleosynthetic effects, chemical mass-independent fractionation effects, and pho- tochemical self-shielding effects. Presently, the latter two are the most likely candidates for production of the isotopic anomalies. Recent data on solar wind oxygen isotopes lends support to the photochemical self-shielding scenario, but additional solar-isotopic data are needed. Ob- servations, experiments, and modeling are described that will advance our understanding of the complex history of oxygen in the solar system. 1. INTRODUCTION Oxygen is the third most abundant element in the solar system and the most abundant element of the terrestrial planets. The presence of oxygen in both gaseous and solid phases makes O isotopes (the terrestrial abundance: 16 O = 99.757%, 17 O = 0.038%, and 18 O = 0.205%) important trac- ers of various fractionation processes in the solar nebula, which are essential for understanding the evolution of gas- eous and solid phases in the early solar system. Oxygen-isotopic compositions are normally expressed in δ units, which are deviations in part per thousand (per- mil, ‰) in the 17 O/ 16 O and 18 O/ 16 O ratios from standard mean ocean water (SMOW) with 17 O/ 16 O = 0.0003829 and 18 O/ 16 O = 0.0020052 ( McKeegan and Leshin, 2001): δ 17,18 O SMOW = [( 17,18 O/ 16 O) sample /( 17,18 O/ 16 O) SMOW – 1] × 1000. On a three-isotope diagram of δ 18 O vs. δ 17 O, com- positions of nearly all terrestrial samples plot along a single line of slope 0.52 called the terrestrial fractionation line. This line reflects mass-dependent fractionation from a sin- gle homogeneous source during chemical and physical proc- esses that results from differences in the masses of the O isotopes. The slope 0.52 results from changes in 17 O/ 16 O that are nearly half those in 18 O/ 16 O because of isotopic mass differences; the precise value of the slope depends on the nature of the isotopic species or isotopologues (e.g., Thie- mens, 2006). In contrast, O-isotopic compositions of the vast majority of extraterrestrial samples, including prim- itive (chondrites) and differentiated (achondrites) meteor- ites, deviate from the terrestrial fractionation line (Fig. 1; see section 4 for details), reflecting mass-independent frac- tionation processes that preceded accretion of these bodies in the protoplanetary disk. Samples from bodies that were largely molten and homogenized such as Mars and Vesta lie on lines that are parallel to the terrestrial fractionation line. Lunar samples show no detectable deviations from the ter- restrial fractionation line, for reasons that are still debated (see section 8.4.3 for details). The deviation from the terres- trial fractionation line is commonly expressed as Δ 17 O SMOW = δ 17 O SMOW – 0.52 × δ 18 O SMOW . The origin of the O-isotopic variations or anomalies in solar system materials has been a major puzzle for plane- tary scientists since they were discovered over 30 years ago (Clayton et al., 1973). The interpretation of the mass-inde- pendent O-isotopic variations or anomalies is one of the most important outstanding problems in cosmochemistry (McKeegan and Leshin, 2001). Here we discuss the nature of the O-isotopic anomalies in the solar system, the evolu-