Variations in the O-isotope composition of gas during the formation of chondrules from the CR chondrites

To better understand the environment of chondrule formation and constrain the O-isotope composition of the ambient gas in the Renazzo-like carbonaceous (CR) chondrite chondrule-forming region, we studied the mineralogy, petrology, and in situ O-isotope compositions of olivine in 11 barred olivine (BO) chondrules and pyroxene and silica in three type I porphyritic chondrules from the CR chondrites Gao-Guenie (b), Graves Nunataks (GRA) 95229, Pecora Escarpment (PCA) 91082, and Shisr 033. BO chondrules experienced a higher degree of melting than porphyritic chondrules, and therefore, it has been hypothesized that they more accurately recorded the O-isotope composition of the gas in chondrule-forming regions. We studied the O-isotope composition of silica as it has been hypothesized to have formed via direct condensation from the gas. BO chondrules constitute similar to 4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (delta O-17 vs. delta O-18), olivine phenocrysts in type I and type II BO chondrules plot along similar to slope-1 line; with the exception of a type II BO chondrule that plots along similar to slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of Delta O-17 values (similar to 3.8 parts per thousand to similar to 1.3 parts per thousand and similar to-0.8 parts per thousand to similar to+1.4 parts per thousand, respectively) than those in type I and type II porphyritic chondrules (similar to-4.6 parts per thousand to similar to 0.3 parts per thousand and similar to 1.8 parts per thousand to similar to+0.9 parts per thousand, respectively). The observation that olivine grains in type I BO chondrules have similar chemical and O-isotope compositions to those of olivine in their porphyritic counterparts argues against the hypothesis that olivine grains in type I porphyritic chondrules are xenocrysts and represent relict fragments of early formed planetesimals. The compositional and O-isotope data suggest that BO chondrules experienced more extensive, but incomplete exchange with the ambient gas than porphyritic chondrules. We suggest that CR chondrules formed from relatively O-16-enriched solids in the presence of relatively O-16-depleted gaseous H2O. The O-isotope compositions of chondrule olivine likely result from differences in the O-isotope composition of both the chondrule precursors and the ambient gas during chondrule formation. The inferred O-isotope composition of this gas (Delta O-17 ranges from similar to 3 parts per thousand to +3 parts per thousand) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy. (C) 2014 Elsevier Ltd. All rights reserved.