Phyllosilicate-rich chondrule rims in the Vigarano CV3 chondrite: Evidence for parent-body processes

A petrographic and scanning electron microscopic study of the Vigarano CV3 carbonaceous chondrite reveals that a small proportion (similar to 5%) of the fine-grained rims surrounding chondrules consist mainly of phyllosilicates (saponite and serpentine) and minor but variable amounts of ferrihydrite. Most of the rims consist mainly of Fe-rich olivine. The phyllosilicate-rich rims and the chondrules that they enclose show abundant evidence of aqueous alteration. Low-Ca pyroxene and olivine at chondrule margins have been partially replaced by phyllosilicate-rich materials, producing embayments on chondrule surfaces. Fe-Ni metal, Fe sulfide and magnetite in the chondrules have also been partially replaced by Fe hydroxide. Due to preferential replacement of low-Ca pyroxene and olivine, sulfide, magnetite and Fe hydroxide remain exposed at chondrule margins. We discovered a clast (0.7 X 0.9 mm in size) containing several chondrules embedded in a phyllosilicate-rich matrix that is almost identical in mineralogy, texture and composition to the rims. We also found numerous small clasts (20-400 mu m in size) consisting of a similar phyllosilicate-rich material in the host Vigarano meteorite; many of the clasts include fragments of chondrules, amoeboid olivine aggregates and Ca-Al-rich inclusions. These clasts also show abundant evidence of aqueous alteration, which is similar to that observed in the rims and their host chondrules. From these observations, we conclude that the chondrule/rim assemblages and the clasts are a part of the same precursor chondrite, which was fragmented during brecciation on the meteorite parent body. We interpret that a portion (or portions) of the Vigarano parent body was aqueously altered, and the interchondrule matrix was largely converted to phyllosilicates and ferrihydrite. This hydrated portion was later fragmented and brecciated by impacts. As fragmentation occurred preferentially along the phyllosilicate-rich porous matrix, most of the ejected clasts consisted of a chondrule rimmed by remnant matrix. The clasts were rounded by abrasion during brecciation and transportation and finally mixed into an anhydrous portion in the regolith, from which the Vigarano meteorite was derived. Some of the phyllosilicate-rich rims show evidence for partial dehydration during heating before fnal incorporation into the regolith. These results, taken together, suggest that the Vigarano parent body was spatially heterogeneous in degree of aqueous alteration and thermal metamorphism and the host Vigarano meteorite is a breccia consisting of numerous clasts from many different locations. Copyright (C) 2000 Elsevier Science Ltd.