M. Kimura^1,5, J. A. Barrat², M. K. Weisberg^3,4, N. Imae⁵, A. Yamaguchi⁵, H. Kojima⁵

¹Faculty of Science, Ibaraki University, Mito, Japan
²Université Européenne de Bretagne, 2CNRS UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Plouzané Cedex, France
³Department of Physical Sciences, Kingsborough College and Graduate School of the City University of New York, Brooklyn, New York, USA
⁴Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁵National Institute of Polar Research, Tokyo, Japan

Carbonaceous chondrites are classified into several groups. However, some are ungrouped. We studied one such ungrouped chondrite, Y-82094, previously classified as a CO. In this chondrite, chondrules occupy 78 vol%, and the matrix is distinctly poor in abundance (11 vol%), compared with CO and other C chondrites. The average chondrule size is 0.33 mm, different from that in C chondrites. Although these features are similar to those in ordinary chondrites, Y-82094 contains 3 vol% Ca-Al-rich inclusions and 5% amoeboid olivine aggregates (AOAs). Also, the bulk composition resembles that of CO chondrites, except for the volatile elements, which are highly depleted. The oxygen isotopic composition of Y-82094 is within the range of CO and CV chondrites. Therefore, Y-82094 is an ungrouped C chondrite, not similar to any other C chondrite previously reported. Thin FeO-rich rims on AOA olivine and the mode of occurrence of Ni-rich metal in the chondrules indicate that Y-82094 is petrologic type 3.2. The extremely low abundance of type II chondrules and high abundance of Fe-Ni metal in the chondrules suggest reducing condition during chondrule formation. The depletion of volatile elements indicates that the components formed under high-temperature conditions, and accreted to the parent body of Y-82094. Our study suggests a wider range of formation conditions than currently recorded by the major C chondrite groups. Additionally, Y-82094 may represent a new, previously unsampled, asteroidal body.

Reference
Kimura M, Barrat JA, Weisberg MK, Imae N, Yamaguchi A and Kojima H (in press) Petrology and bulk chemistry of Yamato-82094, a new type of carbonaceous chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12254]
Published by arrangement with John Wiley & Sons

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