Anhydrous alteration of Allende chondrules in the solar nebura I: Description and alteration of chondrules with known oxygen-isotopic compositions

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Anhydrous alteration of eleven Allende chondrules with already-known oxygen isotopic compositions were studied in detail with reference to a dark inclusion and the matrix from Allende. Anhydrous alteration of chondrules includes three processes different from each other, formation of secondary olivine zonation, replacement of groundmassic plagioclase or glass by nepheline and/or sodalite, and replacement of phenocrystic enstatite by ferroan olivine. The degrees of each process are different among chondrules. A positive correlation in degrees between replacement of groundmassic plagioclase or glass and phenocrystic enstatite is recognized, suggesting that these two processes took place at the same time by reactions with a nebular gas at temperatures probably lower than 800K. The secondary olivine zonation may have formed prior to and in the early stage of the replacement of groundmassic plagioclase and phenocrystic enstatite. The anhydrous alteration of Allende dark inclusions was caused in a nebular gas having an oxygen isotopic composition along the Allende inclusion mixing line, but Allende chondrules have experienced the anhydrous alteration in a gas reservoir having an oxygen isotopic composition along the Allende chondrule mixing line. Therefore, the anhydrous alteration of chondules and dark inclusions took place in different nebular gas reservoirs prior to the accretion of the Allende parent body.
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