Extreme oxygen-isotope compositions in magnetite from unequilibrated ordinary chondrites

Byeon-Gak Choi (), Kevin D. McKeegan, Alexander N. Krot and John T. Wasson
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Byeon-Gak Choi: Department of Earth and Space Sciences
Kevin D. McKeegan: Department of Earth and Space Sciences
Alexander N. Krot: University of California
John T. Wasson: Department of Earth and Space Sciences

Nature, 1998, vol. 392, issue 6676, 577-579

Abstract: 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 line1. In meteorites the oxygen isotopic composition commonly deviates from this line2, the magnitude of the deviation being expressed by the quantity Δ17O. 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 H2O. We measured a value of Δ17O ≈ 5‰, much higher than typical values of 0–2‰ in ordinary-chondrite silicate grains. Our results imply that a nebular component of high-Δ17O H2O was incorporated into the parent asteroid of the unequilibrated ordinary chondrites.

Date: 1998
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DOI: 10.1038/33356

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