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Wiley, Meteoritics & Planetary Science, 11(43), p. 1861-1877, 2008

DOI: 10.1111/j.1945-5100.2008.tb00648.x

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A refractory inclusion returned by Stardust from comet 81P/Wild 2

This paper is available in a repository.
This paper is available in a repository.

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Data provided by SHERPA/RoMEO

Abstract

Among the samples returned from comet 81P/Wild 2 by the Stardust spacecraft is a suite of particles from one impact track (Track 25) that are Ca-, Al-rich and FeO-free. We studied three articles from this track that range in size from 5.3 × 3.2 µm to 15 × 10 µm. Scanning and transmission electron microscopy show that they consist of very fine-grained (typically from ~0.5 to ~2 µm) Al-rich, Ti-bearing and Ti-free clinopyroxene, Mg-Al spinel and anorthite, with trace amounts of fine perovskite, FeNi metal and osbornite (TiN) grains. In addition to these phases, the terminal particle, named ``Inti'', also contains melilite. All of these phases, with the exception of osbornite, are common in refractory inclusions and are predicted to condense at high temperature from a gas of solar composition. Osbornite, though very rare, has also been found in meteoritic refractory inclusions, and could have formed in a region of the nebula where carbon became enriched relative to oxygen compared to solar composition. Compositions of Ti-pyroxene in Inti are similar, but not identical, to those of fassaite from Allende inclusions. Electron energy loss spectroscopy shows that Ti-rich pyroxene in Inti has Ti3+/Ti4+ within the range of typical meteoritic fassaite, consistent with formation under reducing conditions comparable to those of a system of solar composition. Inti is 16O-rich, with delta18O≈delta17O≈ 400/00, like unaltered phases in refractory inclusions and refractory IDPs. With grain sizes, mineralogy, mineral chemistry, and an oxygen isotopic composition like those of refractory inclusions, we conclude that Inti is a refractory inclusion that formed in the inner solar nebula. Identification of a particle that formed in the inner solar system among the comet samples demonstrates that there was transport of materials from the inner to the outer nebula, probably either in a bipolar outflow or by turbulence.