Secondary calcium-iron-rich minerals in the Bali-like and Allende-like oxidized CV3 chondrites and Allende dark inclusions

Abstract— We have characterized Ca-Fe-rich silicates (salite-hedenbergite pyroxenes (Fs_10–50Wo_45–50), 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.