Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.262m&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 262
Other
Scientific paper
Five polymict eucrites recovered from Yamato in Antarctica (Y-790020, -82049, -82052, -82066 and -82082) were investigated based on noble gases, major and trace element compositions. Cosmogenic radionuclide ^81Kr concentrations were also measured for these samples to determine the ^81Kr terrestrial ages [1]. Using these data obtained in this work, we considered characteristics and grouping of the five eucrites. Regarding Yamato eucrites, it has been reported that there are at least five groups corresponding to different falls based on noble gas compositions [2]. In addition to these falls, another fall was discovered in our previous study [3]. Y-790020 has a long exposure age, 70 my, and short terrestrial age, <0.03 my, identical with other eight Yamato eucrites belonging to a single fall. Besides, this eucrite shows high abundances of REE and Eu anomaly, which are common characteristics for this group. Identical noble gas compositions, major and trace element compositions, the exposure age and the terrestrial age were obtained for Y-82049 and Y-82052. The bulk concentrations of Mg and Cr in the two eucrites are (1.5-2) times higher than the other Yamato eucrites. On the other hand, Ca contents in these two eucrites are lower, which lead to 40% and 30% lower production rates of cosmogenic ^38Ar than the values for typical eucrites. These two eucrites are plotted within an area for howardite in CaO-Al(sub)2O(sub)3 diagram. As to eucrite only Binda was plotted in such area. Y-82049 and Y82052 form a group to which another fall can be assigned. For Y-82066, the concentrations of cosmogenic ^3He and ^38Ar are similar to those in Y-82049 and -82052 within 30%; however, the concentrations of cosmogenic ^21Ne and ^83Kr in Y-82066 are about 40% lower than those in Y82049 and Y-82052. The concentration of ^4He in Y-82066 is an order of magnitude higher than those in the latter two eucrites, and the major element compositions are also different between these eucrites. These differences imply the independent fall for Y-82066. The exposure age of Y-82082 is the same as those of Y-790020 group. However, the terrestrial age using ^81Kr-method is obviously longer than the short age obtained for Y-790020 group. The large Ce anomaly, which may be generated by terrestrial weathering [3,4,5], also supports the long terrestrial age for Y-82082. Our present study on noble gases and bulk chemical compositions for Yamato eucrites suggests that there are at least three new groups corresponding to independent falls for Y-82 eucrites in addition to the groups already reported for Y-79 eucrites. As a result, the number of eucrite falls in Yamato area became totally nine. Reference: [1] Miura Y., Nagao K., and Fujitani T. (1991) 16th Symposium on Antarctic Meteorites, 129-130 (abstract), Natl. Inst. Polar Res., Tokyo. [2] Nagao K. and Ogata A. (1989) Mass Spectroscopy 37, 313-324. [3] Miura Y., Nagao K., and Fujitani T. (1991) submitted to G.C.A. [4] Mittlefehldt D.W. and Lindstrom M.M. (1991) G.C.A. 55, 77-87. [5] Shimizu H., Masuda A., and Tanaka T. (1983) Natl. Inst. Polar Res. Spec. Iss. 30, 341-348.
Fujitani Tatsuya
Miura Yas.
Nagao Keiichi
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