Oxygen isotopic compositions of chondrules: Implications for evolution of oxygen isotopic reservoirs in the inner solar nebula

We review the oxygen isotopic compositions of minerals in chondrules and compound objects composed of a chondrule and a refractory inclusion, and bulk oxygen isotopic compositions of chondrules in unequilibrated ordinary, carbonaceous, enstatite, and Kakangari-like chondrites, focusing on data acquired using secondary ion mass-spectrometry and laser fluorination coupled with mass-spectrometry over the last decade. Most ferromagnesian chondrules from primitive (unmetamorphosed) chondrites are isotopically uniform (within 3-4 parts per thousand in Delta O-17) and depleted in O-16 (Delta O-17 > -7 parts per thousand) relative to amoeboid olivine aggregates (AOAs) and most calcium-aluminum-rich inclusions (CAIs) (Delta O-17 < -20 parts per thousand), suggesting that these classes of objects formed in isotopically distinct gaseous reservoirs, O-16-poor and O-16-rich, respectively. Chondrules uniformly enriched in O-16 (Delta O-17 < -15 parts per thousand) are exceptionally rare and have been reported only in CH chondrites. Oxygen isotopic heterogeneity in chondrules is mainly due to the presence of relict grains. These appear to consist of chondrules of earlier generations and rare refractory inclusions; with rare exceptions, the relict grains are O-16-enriched relative to chondrule phenocrysts and mesostasis. Within a chondrite group, the magnesium-rich (Type I) chondrules tend to be O-16-enriched relative to the ferrous (Type II) chondrules. Aluminum-rich chondrules in ordinary, enstatite, CR, and CV chondrites are generally O-16-enriched relative to ferromagnesian chondrules. No systematic differences in oxygen isotopic compositions have been found among these chondrule types in CB chondrites. Aluminum-rich chondrules in carbonaceous chondrites often contain relict refractory inclusions. Aluminum-rich chondrules with relict CAIs have heterogeneous oxygen isotopic compositions (Delta O-17 ranges from -20 parts per thousand to 0 parts per thousand). Aluminum-rich chondrules without relict CAIs are isotopically uniform and have oxygen isotopic compositions similar to, or approaching, those of ferromagnesian chondrules. Phenocrysts and mesostases of the CAI-bearing chondrules show no clear evidence for O-16-enrichment compared to the CAI-free chondrules. Spinel, hibonite, and forsterite of the relict refractory inclusions largely retained their original oxygen isotopic compositions. In contrast, plagioclase and melilite of the relict CAIs experienced melting and O-16-depletion to various degrees, probably due to isotopic exchange with an O-16-poor nebular gas. Several igneous CAIs experienced isotopic exchange with an O-16-poor nebular gas during late-stage remelting in the chondrule-forming region. On a three-isotope diagram, bulk oxygen isotopic compositions of most chondrules in ordinary, enstatite, and carbonaceous chondrites plot above, along, and below the terrestrial fractionation line, respectively. Bulk oxygen isotopic compositions of chondrules in altered and/or metamorphosed chondrites show evidence for mass-dependent fractionation, reflecting either interaction with a gaseous/fluid reservoir on parent asteroids or open-system thermal metamorphism. Bulk oxygen isotopic compositions of chondrules and oxygen isotopic compositions of individual minerals in chondrules and refractory inclusions from primitive chondrites plot along a common line of slope of similar to 1, suggesting that only two major reservoirs (ga and solids) are needed to explain the observed variations. However, there is no requirement that each had a permanently fixed isotopic composition. The absolute (Pb-207-Pb-206) and relative (Al-27-Mg-26) chronologies of CAIs and chondrules and the differences in oxygen isotopic compositions of most chondrules (O-16-poor) and most refractory inclusions (O-16-rich) can be interpreted in terms of isotopic self-shielding during UV photolysis of CO in the initially O-16-rich (Delta O-17 similar to-25 parts per thousand) parent molecular cloud or protoplanetary disk. According to these models, the UV photolysis preferentially dissociates (CO)-O-17 and (CO)-O-18 in the parent molecular cloud and in the peripheral zones of the protoplanetary disk. If this process occurs in the stability field of water ice, the released atomic O-17 and O-18 are incorporated into water ice, while the residual CO gas becomes enriched in O-16. During the earliest stages of evolution of the protoplanetary disk, the inner solar nebula had a solar H2O/CO ratio and was O-16-rich. During this time, AOAs and the O-16-rich CAIs and chondrules formed. Subsequently, the inner solar nebula became H2O- and O-16-depleted, because ice-rich dust particles, which were depleted in O-16, agglomerated outside the snowline (similar to 5AU), drifted rapidly towards the Sun and evaporated. During this time, which may have lasted for similar to 3 Myr, most chondrules and the O-16-depleted igneous CAIs formed. We infer that most chondrules formed from isotopically heterogeneous, but O-16-depleted precursors, and experienced isotopic exchange with an O-16-poor nebular gas during melting. Although the relative roles of the chondrule precursor materials and gas-melt isotopic exchange in establishing oxygen isotopic compositions of chondrules have not been quantified yet, mineralogical, chemical, and isotopic evidence indicate that Type I chondrules may have formed in chemical and isotopic equilibrium with nebular gas of variable isotopic composition. Whether these variations were spatial or temporal are not known yet. (C) 2006 Elsevier GmbH. 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