著者
Kazushige Tomeoka Hideyasu Kojima Keizo Yanai
雑誌
Proceedings of the NIPR Symposium on Antarctic Meteorites
巻号頁・発行日
vol.2, pp.55-74, 1989-10

A petrographic and mineralogical study of Yamato-86720 (Y-86720) shows that it may be a CM carbonaceous chondrite that has experienced a considerably different alteration history from most CM chondrites. Y-86720 has an unusually high abundance of troilite (~9 vol%); most troilite occurs in submicron to micron grains dispersed throughout the meteorite. Ca-Mg carbonates and minor Fe-Ni metal, mostly taenite, are present, but magnetite and PCP are absent. This meteorite contains chondrules and aggregates that were completely replaced by optically translucent materials, presumably phyllosilicates. Thus Y-86720 was probably extensively affected by aqueous alteration; the degree of alteration may be the highest of the CM chondrites. Matrix consists largely of fine grains of Mg-Fe olivine, a nearly amorphous Si-Mg-Fe-rich material, and an Fe-rich material; the latter may be ferrihydrite. Phyllosilicates are rare. The textures suggest that the olivine and the Si-Mg-Fe-rich material were produced by alteration of phyllosilicates by heating. The replacement products of chondrules and aggregates show lower (Mg+Fe)/(Si+Al) ratios than serpentine and relatively high Na contents (up to 2.0wt% as Na_2O), suggesting the presence of not only serpentine but another type of phyllosilicate. However, the replacement products show consistently high analytical totals; thus they may have been dehydrated and partially altered to anhydrous phases. These results suggest that Y-86720 has experienced mild thermal metamorphism after the aqueous alteration; it appears to have been heated above 500℃ in a reduced condition. Most fine-grained troilite probably segregated from matrix during the metamorphic process. Many of the troilites are replaced by a ferrihydrite-like material; thus, Y-86720 may have been affected by additional mild aqueous alteration after the thermal metamorphism.
著者
Keizo Yanai
雑誌
Proceedings of the NIPR Symposium on Antarctic Meteorites
巻号頁・発行日
vol.7, pp.30-41, 1994-03

A new angrite-type achondrite was discovered at a new site of meteorite concentrations in Antarctica by the Japanese expedition party in December 1988. Antarctic meteorite Asuka-881371 (previously Asuka-9) is an angrite-type achondrite, which is a rounded individual stone, almost completely covered with a dull-black fusion crust. Pale green, relatively large porphyritic olivine crystals can be seen on the exposed face of the interior. Asuka-881371 is an unbrecciated, igneous rock with typically ophitic (doleritic) texture, consisting mainly of olivine, pyroxene (fassaite) and plagioclase with spinel. From its texture, mineral assemblages and chemical composition, Asuka-881371 is identified and classified as an angrite. However, based on several characteristics, especially bulk chemical composition, Asuka-881371 is different from Angra dos Reis, LEW86010 and LEW87051 angrites which are the three previously known angrites collected from non-Antarctic and Antarctic regions.