第2回極域科学シンポジウム/第34回南極隕石シンポジウム 11月17日（木） 国立国語研究所 2階講

Bérczi Sz.

Gucsik A.

Gyollai I.

Nagy Sz.

National Institute of Polar Research Repository

Comparison of aqueous alteration of two CV3 (Kaba and Yamato-86751) chondrites. I. Gyollai 1,2 (gyildi@gmail.com), Sz. Nagy1, Sz. Bérczi1 , A. Gucsik3 1Eötvös University, Faculty of Science, Institute of Physics, Dept. Material Physics, H-1117  Budapest, Pázmány P. s. 1/a, Hungary, 2Department of Lithospheric Research Center for Earth Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, 3Konkoly Observatory of the Hungarian Academy of Sciences, H-1121 Budapest, Konkoly Thege Miklós út 15-17., Hungary  Introduction The Y-86751 meteorite has been classified which is a transition of CVoxA and CVoxB groups, and it contains CAI’s, dark inclusions, and amoeboid olivine assemblages [1]. However, the Kaba sample has been studied earlier [2,3,4], we studied some textural characteristics of the aqueous alteration of chondrules in our samples.  Experimental procedure The mineral assemblages and textures were characterized with a Nikon Eclipse LV100POL optical microscope using transmitted (plane- and cross- polarized) and reflected light (at Eötvös Loránd University of Budapest, Hungary).  Kaba meteorite  The Kaba meteorite contains altered chondrules with different extent, which are embedded in a carbon enrichment matrix and they contain altered chondrule fragments at different stages. Each chondrule can be divided into various altered shales, and their size depends on the timescale of the interaction with the aqueous fluid. Phyllosilicates and troilit droplets were observed at the rim of the chondrules and in strongly altered parts of them by using reflectance microscope. Most of the inner part is best preserved, and the rate of the alteration is growing to the direction of outer shales of the chondrules.  Yamato-86751 meteorite In the carbonaceous matrix, there are fine-grained chondrules with the grain-boundaries being smooth inside of the chondrules, as well. Three processes were considered in our interpretation: thermal metamorphism, brecciation, and aqueous alteration. The sizes of the chondrule-forming phenocrysts are approximately between 40-50 µm, whereas the phenocrysts in the matrix are up to 100 µm in size. This sample contains predominantly porphyritic chondrules. The minerals are zoned, possibly due to non- equilibrium reactions. Both the chondrules and larger phenocrysts show signs of aqueous alteration. The reddish-brown carbonaceous matrices are probably the result of the presence of altered hematite, which shows a flow-textured appearance. The groundmass has porphyritic texture, the phenocrysts are clino- and orthopyroxenes, olivines, and feldspar. The chondrules have porphyritic, barred, and granular texture. Some chondules are elliptical in shape. The xenoliths with porphyritic and granular texture are dark brown and black in color, probably due to carbon content. The granular and porphyritic chondrules commonly contain mechanically-twinned pyroxenes. The boundaries of the chondrules are smooth and often penetrated. The groundmass around the chondrules change from light reddish brown to dark brown showing flow texture due to aqueous alteration.  Patchy extinction and fine grained textures are common in the chondrule minerals. The minerals are of inhomogenous composition, as it is indicated by the interference color differences of the feldspar and pyroxene.  Porphyritic chondrules show not only aqueously altered boundaries, but also that phenocrysts inside the chondrule show fine-grained rims around the mineral grains. If the aqueous alteration had been pervasive, the chondrules are completely altered to fine-grained minerals. Comparison of Y-86751 and Kaba meteorites Whereas the Kaba meteorite has more pervasive aqueous alteration inside their chondrules, the Y-86751 CV3 chondrite has more altered matrix with flow structure, but much better preserved chondrules. In Kaba sample, no shock features have been observed, but in Y-86751 sample mechanical twinned pyroxenes were found. The chondrules in Kaba sample consist of between 30-50 µm size crystalls. On the other hand, the chondrules in Y-86751 have coarser grains (between 80-100 µm).  Acknowledgments We are grateful to the NIPR Antarctic Meteorite Research Center (Tokyo), for the loan of the Antarctic meteorite set and to LRG (Eötvös University) for assistance with the analytical instruments. We are grateful to Professor C. Koeberl for supervising the results of Y-86751 sample. References [1] Komatsu et al. (2007): LPSC XXXVIII, 1987.pdf; [2] Nakamura et al. (2007): Workshop on parent-body and nebular modification of chondritic materials; [3] Lukács and Bérczi (1999) LPSC XXX. 1011.pdf; [4] Kubovics et al (1998) LPSC XXIX. 1120.pdf  

Comparison of aqueous alteration of two CV3 (Kaba and Yamato-86751) chondrites.

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Comparison of aqueous alteration of two CV3 (Kaba and Yamato-86751) chondrites.

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