Abstract

Abstract— We have characterized Ca‐Fe‐rich silicates (salite‐hedenbergite pyroxenes (Fs10–50Wo45–50), andradite (Ca3Fe2Si3O12), kirschsteinite (CaFeSiO4), and wollastonite (Ca3Si3O9)) in the type I chondrules and matrices in the Bali‐like and Allende‐like oxidized CV3 chondrites and Allende dark inclusions. In type I chondrules in the Bali‐like CV3 chondrites, metal is oxidized to magnetite; magnetite‐sulfide nodules are replaced by Ca‐Fe‐rich pyroxenes with minor andradite and pure fayalite. We infer that Ca‐Fe‐rich pyroxenes, andradite, fayalite, magnetite, and phyllosilicates (which occur in mesostases) formed at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Thermodynamic analysis of phase relations in the Si‐Fe‐Ca‐O‐H system and large O isotopic fractionation of the coexisting magnetite and fayalite (∼20%) (Krot et al., 1998) are consistent with this interpretation.In type I chondrules in the Allende‐like CV3 chondrites and dark inclusions, magnetite‐sulfide nodules are replaced by Ca‐Fe‐rich pyroxenes and ferrous olivine; low‐Ca pyroxene and forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca‐Fe‐rich pyroxenes predate formation of ferrous olivine; the latter postdates formation of talc and biopyriboles (Brearley, 1997). The Allende dark inclusions are crosscut by Ca‐Fe‐pyroxene‐andradite veins and surrounded by Ca‐rich rims that consist of Ca‐Fe‐rich pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium‐rich veins and rims formed after aggregation and lithification of the dark inclusions. The rimmed dark inclusions show zoned depletion in Ca, which is due to a lower abundance of Ca‐Fe‐rich pyroxenes close to the rim. Calcium was probably leached from the inclusions and redeposited along their edges. We infer that the Allende‐like chondrites and dark inclusions experienced similar aqueous alteration to the Bali‐like chondrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates.We conclude that Ca‐Fe‐rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid‐rock interactions during progressive metamorphism of a heterogeneous mixture of hydrous (ices?) and anhydrous materials; the latter were possibly mineralogically similar to the reduced CV3 chondrites.

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