The presolar grain inventory of fine‐grained chondrule rims in the Mighei‐type (CM) chondrites

1Jan Leitner,2Knut Metzler,3Christian Vollmer,4Christine Floss,4Pierre Haenecour,1János Kodolányi,5Dennis Harries,1Peter Hoppe
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13412]
1Max Planck Institute for Chemistry, Particle Chemistry Department, Hahn‐Meitner‐Weg 1, 55128 Mainz, Germany
2Institute for Planetology, University of Münster, 48149 Münster, Germany
3Institute for Mineralogy, University of Münster, 48149 Münster, Germany
4Laboratory for Space Sciences, Physics Department and McDonnell Center for Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri, 63130 USA
5Institute of Geoscience, Friedrich Schiller University Jena, Carl‐Zeiss‐Promenade 10, 07745 Jena, Germany
Published by arrangement with John Wiley & Sons

We investigated the inventory of presolar silicate, oxide, and silicon carbide (SiC) grains of fine‐grained chondrule rims in six Mighei‐type (CM) carbonaceous chondrites (Banten, Jbilet Winselwan, Maribo, Murchison, Murray and Yamato 791198), and the CM‐related carbonaceous chondrite Sutter’s Mill. Sixteen O‐anomalous grains (nine silicates, six oxides) were detected, corresponding to a combined matrix‐normalized abundance of ~18 ppm, together with 21 presolar SiC grains (~42 ppm). Twelve of the O‐rich grains are enriched in 17O, and could originate from low‐mass asymptotic giant branch stars. One grain is enriched in 17O and significantly depleted in 18O, indicative of additional cool bottom processing or hot bottom burning in its stellar parent, and three grains are of likely core‐collapse supernova origin showing enhanced 18O/16O ratios relative to the solar system ratio. We find a presolar silicate/oxide ratio of 1.5, significantly lower than the ratios typically observed for chondritic meteorites. This may indicate a higher degree of aqueous alteration in the studied meteorites, or hint at a heterogeneous distribution of presolar silicates and oxides in the solar nebula. Nevertheless, the low O‐anomalous grain abundance is consistent with aqueous alteration occurring in the protosolar nebula and/or on the respective parent bodies. Six O‐rich presolar grains were studied by Auger Electron Spectroscopy, revealing two Fe‐rich silicates, one forsterite‐like Mg‐rich silicate, two Al‐oxides with spinel‐like compositions, and one Fe‐(Mg‐)oxide. Scanning electron and transmission electron microscopic investigation of a relatively large silicate grain (490 nm × 735 nm) revealed that it was crystalline åkermanite (Ca2Mg[Si2O7]) or a an åkermanite‐diopside (MgCaSi2O6) intergrowth.

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