National Institute of Polar Research

New finding: an ungrouped carbonaceous chondrite collected from the bare ice field around the Yamato Mountains

The classification of Antarctic Meteorites has been carried out at Antarctic Meteorite Research Center in National Institute of Polar Research. Reexamination of Yamato-82094 (Y-82094; Fig. 1) suggested that it is an ungrouped carbonaceous chondrite, different from any known chondrites as well as any known carbonaceous chondrites. The new results will shed light on the evolution of the early solar system. It will be soon published in a journal of Meteoritics and Planetary Science.

Background of the research:

The carbonaceous chondrite, Y-82094 (Fig. 1), has been classified into a CO3 chondrite*. During the classification of NIPR meteorite collection, we clarified that this meteorite is not classified into a CO3 but an ungrouped carbonaceous chondrite.

* Carbonaceous chondrites are one of the most primitive meteorites preserving the information of the primordinal solar nebula. They are classified into CI, CM, CV, CR, CK, CO, CB, and CH.

Analytical methods:

The observation of the textures and the identification of the main constituting phases were carried out using a scanning electron microscope (JSM-5900 LV) and an electron probe microanalyzer (JEOL JXA-8200) in National Institute of Polar Research. The bulk composition was measured by Prof. Barrat using the ICP mass spectrometer in Bretagne University.

Research results:

In this chondrite, chondrules occupy 78 vol.%, and the matrix is distinctly poor in abundance (11 vol.%), compared to CO and other C chondrites. The average chondrule size is 0.33 mm, different from those in C chondrites. Although these features are similar to those in ordinary chondrites, Y-82094 contains 3 vol.% CAIs and 5 % AOAs. The bulk composition also resembles those 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.

Perspect of the coming study:

Antarctic Meteorite Research Center handles 18000 meteorite samples, and the primary classification is continuously conducted. The result of the classification is annually published in Meteorite Newsletter, and many classified meteorites are studied in detail by researchers. The discovery of new types of meteorites would be expected during the classification and the detailed research. Especially, carbonecaous chondrite is very important for clarifying the formation and evolution of the solar system.

Title of the paper and the authors:

Petrology and bulk chemistry of Yamato-82094, a new type of carbonaceous chondrite
Makoto Kimura (Ibaraki University & National Institute of Polar Research)
Jean-Alix Barrat (Bretagne University)
Mike K. Weisberg (Kingsborough College and American Museum of Natural History)
Naoya Imae (National Institute of Polar Research and Graduate University for Advanced Studies)
Akira Yamaguchi (National Institute of Polar Research and Graduate University for Advanced Studies)
Hideyasu Kojima (National Institute of Polar Research and Graduate University for Advanced Studies)

Figure 1.
(a) Photograph of the Y-82094 ungrouped carbonaceous chondrite. The side of the cube is 1 cm.
(b) An image of the polished thin section under an optical microscope.