Comparative study of anhydrous alteration of chondrules in reduced and oxidized CV chondrites

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Chondrules in three CV chondrites of the reduced subgroup (Leoville, Efremovka and Vigarano) were studied to explore the effect of anhydrous alteration, in comparison to those in Allende of the oxidized subgroup. The alteration consists of secondary olivine zonation, replacement of low-Ca pyroxene by ferroan olivine, and replacement of primary groundmass by nepheline and sodalite. The highest degree of alteration occurs in chondrules of Allende. Leoville chondrules show only olivine zonation and seem not to have experienced replacement of pyroxene and groundmass. Some chondrules in Efremovka have replacement textures of groundmass and show olivine zonation. On the other hand, many chondrules in Vigarano were subjected to all types of alteration. Thus, chondrules even in reduced CV chondrites do not completely preserve their primary features, although the degree of alteration for the reduced CVs varies, and is lower on the whole than that of Allende. The differences between CV chondrites of the oxidized and reduced subgroups appear to be determined by the degree of secondary alteration and oxidation of Fe. The alterations took place prior to the final consolidation with the matrix on the CV parent body.
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