SECONDARY CALCIUM-IRON-RICH MINERALS IN THE BALI-LIKE AND ALLENDE-LIKE OXIDIZED CV3 CHONDRITES AND ALLENDE DARK INCLUSIONS

We have characterized Ca-Fe-rich silicates (salite-hedenbergite pyroxe nes (Fs(10-50)Wo(45-50)), andradite (Ca3Fe2Si3O12), kirschsteinite (Ca FeSiO4), and wollastonite (Ca3Si3O9)) in the type I chondrules and mat rices in the Ball-like and Allende-like oxidized CV3 chondrites and Al lende dark inclusions. In type I chondrules in the Ball-like CV3 chond rites, metal is oxidized to magnetite; magnetite-sulfide nodules are r eplaced 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 l ow temperatures (<300 degrees 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 fay alite (similar to 20 parts per thousand) (Krot et al., 1998) are consi stent with this interpretation. In type I chondrules in the Allende-li ke CV3 chondrites and dark inclusions, magnetite-sulfide nodules are r eplaced by Ca-Fe-rich pyroxenes and ferrous olivine; low-Ca pyroxene a nd forsterite phenocrysts are rimmed and veined by ferrous olivine. It appear that the Ca-Fe-rich pyroxenes predate formation of ferrous oli vine; the latter postdates formation of talc and biopyriboles (Brearle y, 1997). The Allende dark inclusions are crosscut by Ca-Fe-pyroxene-a ndradite veins and surrounded by Ca-rich rims that consist of Ca-Fe-ri ch pyroxenes, andradite, wollastonite, and kirschsteinite. Calcium-ric h veins and rims formed after aggregation and lithification of the dar k inclusions. The rimmed dark inclusions show zoned depletion in Ca, w hich 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 Ball-like cho ndrites and were metamorphosed subsequently, which resulted in loss of aqueous solutions and dehydration of phyllosilicates. We conclude tha t Ca-Fe-rich silicates in the oxidized CV3 chondrites and Allende dark inclusions are secondary and resulted from aqueous fluid-rock interac tions during progressive metamorphism of a heterogeneous mixture of hy drous (ices?) and anhydrous materials; the latter were possibly minera logically similar to the reduced CV3 chondrites.