TY - JOUR
T1 - Variations in the O-isotope composition of gas during the formation of chondrules from the CR chondrites
AU - Schrader, Devin L.
AU - Nagashima, Kazuhide
AU - Krot, Alexander N.
AU - Ogliore, Ryan C.
AU - Hellebrand, Eric
N1 - Funding Information:
The authors are grateful to the ANSMET, JSC, MWG, NIPR, Smithsonian Institution, M. Farmer, J. Schrader, and E. Olson for samples. We thank M. Chaussidon, who kindly provided the 313 orthopyroxene standard, and J. Valley and N. Kita, who kindly provided the UWQ-1 quartz standard. We also thank G. Huss and J. Davidson for helpful comments and discussions. Constructive comments by J. T. Wasson, an anonymous reviewer, and the Associate Editor T. R. Ireland significantly improved the quality of the manuscript. This research was funded in part by NASA grants, NNX11AJ51G (Q.-Z. Yin, PI), NNX08AH77G (K. Nagashima, PI), NNX10AH76G (A. N. Krot, PI), and NNX07AM62G (R. C. Ogliore).
PY - 2014/5/1
Y1 - 2014/5/1
N2 - 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 Shişr 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 ~4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (δ17O vs. δ18O), olivine phenocrysts in type I and type II BO chondrules plot along ~slope-1 line; with the exception of a type II BO chondrule that plots along ~slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of δ17O values (~-3.8‰ to ~-1.3‰ and ~-0.8‰ to ~+1.4‰, respectively) than those in type I and type II porphyritic chondrules (~-4.6‰ to ~-0.3‰ and ~-1.8‰ to ~+0.9‰, 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 16O-enriched solids in the presence of relatively 16O-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 (δ17O ranges from ~-3‰ to +3‰) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy.
AB - 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 Shişr 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 ~4% of the total CR chondrule population by volume. On a three-isotope oxygen diagram (δ17O vs. δ18O), olivine phenocrysts in type I and type II BO chondrules plot along ~slope-1 line; with the exception of a type II BO chondrule that plots along ~slope-0.5 line. Olivine phenocrysts in type I and type II BO chondrules have similar but more restricted ranges of δ17O values (~-3.8‰ to ~-1.3‰ and ~-0.8‰ to ~+1.4‰, respectively) than those in type I and type II porphyritic chondrules (~-4.6‰ to ~-0.3‰ and ~-1.8‰ to ~+0.9‰, 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 16O-enriched solids in the presence of relatively 16O-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 (δ17O ranges from ~-3‰ to +3‰) is inconsistent with a high abundance of water from the outer Solar System, which has been predicted to be isotopically heavy.
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U2 - 10.1016/j.gca.2014.01.034
DO - 10.1016/j.gca.2014.01.034
M3 - Article
AN - SCOPUS:84896790976
SN - 0016-7037
VL - 132
SP - 50
EP - 74
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -