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Extreme oxygen-isotope compositions in magnetite from unequilibrated ordinary chondrites

Choi, Byeon-Gak and McKeegan, Kevin D. and Krot, Alexander N. and Wasson, John T. (1998) Extreme oxygen-isotope compositions in magnetite from unequilibrated ordinary chondrites. Nature, 392 (6676). pp. 577-579. ISSN 0028-0836. http://resolver.caltech.edu/CaltechAUTHORS:20150617-083558470

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Abstract

Primitive meteorites (such as the unequilibrated ordinary chondrites) have undergone only minor thermal processing on their parent asteroids, and thus provide relatively unaltered isotopic records from the early Solar System. For terrestrial materials, oxygen isotope compositions form a linear array called the terrestrial fractionation line. In meteorites the oxygen isotopic composition commonly deviates from this line, the magnitude of the deviation being expressed by the quantity Δ^(17)O. Such deviations, which cannot be explained by mass-dependent fractionation processes, are probably caused by the mixing of two or more nebular components having different nucleosynthetic histories, for example, solids and gas. But no direct evidence for the oxygen isotopic composition of the latter (which is the dominant oxygen reservoir) has hitherto been available. Here we report in situ oxygen-isotope measurements of magnetite grains in unequilibrated ordinary chondrites. Magnetite (which formed by aqueous alteration of metal in the parent asteroid) may serve as a proxy for nebular H_2O. We measured a value of Δ^(17)O ≈5‰, much higher than typical values of 0–2‰ in ordinary-chondrite silicate grains. Our results imply that a nebular component of high- Δ^(17)O H_2O was incorporated into the parent asteroid of the unequilibrated ordinary chondrites.


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http://dx.doi.org/10.1038/33356 DOIArticle
http://www.nature.com/nature/journal/v392/n6676/full/392577a0.htmlPublisherArticle
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Additional Information:© 1998 Macmillan Publishers Ltd. Received 30 April; accepted 15 December 1997. We thank C. Coath, G. Jarzebinski and L. Leshin for technical assistance with the ion microprobe, A. Rubin for petrological advice and S. Savin and R. N. Clayton for discussions. Reviews by G. Huss and B. Giletti led to improvements in the manuscript. The Semarkona and Ngawi thin sections were provided by G. MacPherson (Smithsonian Instn) and A. Brearley (Univ. New Mexico).
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Deposited By: Tony Diaz
Deposited On:17 Jun 2015 16:36
Last Modified:17 Jun 2015 16:36

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