Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration

Abstract

Abstract— The oxidized CV3 chondrites can be divided into two major subgroups or lithologies, Bali‐like (CV3oxB) and Allende‐like (CV3oxA), in which chondrules, calcium‐aluminum‐rich inclusions (CAIs) and matrices show characteristic alteration features (Weisberg et al, 1997; Krot et al, 1997d; Kimura and Ikeda, 1997). The CV3oxB lithology is present in Bali, Kaba, parts of the Mokoia breccia and, possibly, in Grosnaja and Allan Hills (ALH) 85006. It is characterized by the presence of the secondary low‐Ca phyllosilicates (saponite and sodium phlogopite), magnetite, Ni‐rich sulfides, fayalite (Fa>90), Ca‐Fe‐rich pyroxenes (Fs10–50Wo45–50) and andradite. Phyllosilicates replace primary Ca‐rich minerals in chondrules and CAIs, which suggests mobilization of Ca during aqueous alteration. Magnetite nodules are replaced to various degrees by fayalite, Ca‐Fe‐rich pyroxenes and minor andradite. Fayalite veins crosscut fine‐grained rims around chondrules and extend into the matrix. Thermodynamic analysis of the observed reactions indicates that they could have occurred at relatively low temperatures (<300 °C) in the presence of aqueous solutions. Oxygen isotopic compositions of the coexisting magnetite and fayalite plot close to the terrestrial fractionation line with large Δ18Ofayalite‐magnetite fractionation (∼20%). We infer that phyllosilicates, magnetite, fayalite, Ca‐Fe‐rich pyroxenes and andradite formed at relatively low temperatures (<300 °C) by fluid‐rock interaction in an asteroidal environment.Secondary fayalite and phyllosilicates are virtually absent in chondrules and CAIs in the CV3oxA lithology, which is present in Allende and its dark inclusions, Axtell, ALHA81258, ALH 84028, Lewis Cliff (LEW) 86006, and parts of the Mokoia and Vigarano breccias. Instead secondary nepheline, sodalite, and fayalitic olivine are common. Fayalitic olivine in chondrules replaces low‐Ca pyroxenes and rims and veins forsterite grains; it also forms coarse lath‐shaped grains in matrix. Secondary Ca‐Fe‐rich pyroxenes are abundant. We infer that the CV3oxA lithology experienced alteration at higher temperatures than the CV3oxB lithology. The presence of the reduced and CV3oxA lithologies in the Vigarano breccia and CV3oxA and CV3oXB lithologies in the Mokoia breccia indicates that all CV3 chondrites came from one heterogeneously altered asteroid. The metamorphosed clasts in Mokoia (Krot and Hutcheon, 1997) may be rare samples of the hotter interior of the CV asteroid. We conclude that the alteration features observed in the oxidized CV3 chondrites resulted from the fluid‐rock interaction in an asteroid during progressive metamorphism of a heterogeneous mixture of ices and anhydrous materials mineralogically similar to the reduced CV3 chondrites.