Opaque minerals in Antarctic CO3 carbonaceous chondrites, Yamato-74135, -790992, -791717, -81020, -81025, -82050 and Allan Hills-77307
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Metamorphic sequence and mineralogy of opaque minerals of Antarctic seven CO3 chondrites, Yamato (Y)-74135,Y-790992,Y-791717,Y-81020,Y-81025,Y-82050 and Allan Hills (ALH)-77307 was studied by an optical, scanning electron microscope and electron microprobe technique. Based on olivine and chromite compositions, Y-74135,Y-81020 and Y-81025 are classified as least metamorphosed CO3 chondrites. Metamorphic grades of these three chondrites are the same as or less than that of ALH-77307. Whereas metamorphic grades of Y-790992,Y-79171 and Y-82050 are higher than that of ALH-77307. The three least metamorphosed chondrites include cohenite abundantly. Judging from the mode of occurrence of cohenite, cohenite would have formed not by the diffusion of carbon into solid Fe-Ni metal, but by crystallization from Fe-Ni-C melt. Fine-grained tetrataenite within kamacite suggest that precipitation of the tetrataenite occurred under the condition in which Ni movement in kamacite was sluggish extremely (probably below 450℃). In Y-81025,assemblage of spherical magnetite like as framboidal magnetite in CI chondrites was observed. It would be also one of evidences of the low-temperature condition which the least metamorphosed chondrites in this study were situated. In Y-81025 and Y-74135,there are fine-grained perryite and phosphide. Although previous workers have already pointed out that primitive chondrites often have carbides, this study make clear that the existence of cohenite is indeed one of the essential properties in least metamorphosed CO3 chondrites.
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