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The oxygen isotopic composition of olivine and pyroxene from CI chondrites

Leshin, Laurie A. and Rubin, Alan E. and McKeegan, Kevin D. (1997) The oxygen isotopic composition of olivine and pyroxene from CI chondrites. Geochimica et Cosmochimica Acta, 61 (4). pp. 835-845. ISSN 0016-7037. doi:10.1016/s0016-7037(96)00374-2.

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The CI chondrites are taken to represent average solar system material based on the similarity of their elemental compositions to that of the solar photosphere. However, their oxygen isotope geochemistry is dominated by secondary minerals that formed during aqueous alteration on the CI parent body. Precursors to this alteration, namely olivine and pyroxene, are extremely rare in CI chondrites, precluding previous measurements of their oxygen isotopic composition. We report ion microprobe analyses of oxygen isotopes in single olivine and pyroxene grains separated from CI chondrites Orgueil and Ivuna. The CI chondrite olivine and pyroxene grains most likely represent liberated chondrule phenocrysts, based on petrographic, chemical, and isotopic evidence consistent with crystallization from a melt. The oxygen isotope data form an array that falls nearly along the carbonaceous chondrite ¹⁶O mixing line with δ¹⁸O values ranging from −9.3%, to + 12.3%o and δ¹⁷O from −11.3%o to +7.8%o, consistent with nebular processes being the source of the oxygen isotopic compositions. The degree of ¹⁶O-enrichment in Orgueil olivines is negatively correlated with FeO content, but the exact nature and timing of the process that introduced this variation remains unknown. The pyroxene oxygen isotopic compositions are similar to those of olivines with >5 mol% fayalite. The oxygen isotopic analyses of the olivine and pyroxene in CI chondrites have been used to revise previous models for the isotopic evolution of CI materials. Our data require more complete gas-solid equilibration in the nebula and constrain the initial aqueous fluids on the CI parent body to have lower Δ¹⁷O values than previously postulated. The refined model indicates that the temperature of aqueous activity on the CI parent body was no higher than ∼50°C, and the fluid:rock ratio was significantly less than previously estimated. Even prior to alteration and formation of secondary minerals, the CI chondrites were the most ¹⁶O-depleted carbonaceous chondrites and thus the solids originally contained in the CI chondrites are the most equilibrated nebular materials represented in the carbonaceous chondrites. The data suggest the oxygen isotopic composition of average solar system to be approximately equivalent to average terrestrial oxygen as recorded in the compositions of terrestrial and lunar basalts.

Item Type:Article
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URLURL TypeDescription
Rubin, Alan E.0000-0002-4515-4527
McKeegan, Kevin D.0000-0002-1827-729X
Additional Information:We thank the Museum National D’Histoire Naturelle, Paris, B. Zanda, C. Perron, and R. Hewins for the allocation of the Orgueil sample; J. Goswami for providing the Ivuna pyroxenes; J. Eiler and E. Stolper for providing the fayalite standard. Laboratory assistance and advice from C. Coath, F. Kyte, M. Grove, G. Jarzebinski, G. Srinivasan, and J. Wang are gratefully acknowledged. We thank B.-G. Choi, R. Clayton, T. Mayeda, H. McSween, and J. Wasson for helpful discussions and T. M. Harrison for his support of meteoritic studies on the UCLA ion microprobe. The dedication of CAMECA scientists, especially E. DeChambost and M. Schuhmacher, to the development of in situ oxygen isotope measurements is gratefully acknowledged. Reviews from R. Clayton, R. Harvey, H. McSween, and I. Steele were constructive and improved the manuscript. This work was supported by a University of California Postdoctoral Fellowship, NASA grant NAGW-4112, and NSF grant EAR 94-18520.
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University of CaliforniaUNSPECIFIED
NSFEAR 94-18520
Issue or Number:4
Record Number:CaltechAUTHORS:20230307-650576000.71
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ID Code:119784
Deposited By: George Porter
Deposited On:08 Mar 2023 16:36
Last Modified:08 Mar 2023 16:36

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