Aqueous alteration in the Kaba CV3 carbonaceous chondrite

The Kaba CV3 carbonaceous chondrite has undergone pervasive aqueous alteration that produced Fe-bearing saponite in chondrules and matrix. Saponite in chondrules is coarse grained and shows a crystallographic orientation relationship such that c^* of saponite parallels a^* of enstatite. Matrix saponite exhibits a variety of textures including coarse- and fine-grained clusters, isolated packets, and sparse, lamellar replacements of Fe-rich olivine. The coarse-grained clusters were probably derived from altered chondrules that were fragmented during regolith gardening. The fine-grained clusters and isolated packets formed from material released during the partial dissolution of matrix silicates. Much saponite is associated with submicron Fe-Ni sulfides, suggesting that conditions were not oxidizing during aqueous alteration. We believe that the alteration occurred after accretion and on the Kaba parent body because saponite is present throughout the sample. By analogy to terrestrial occurrences, the saponite in Kaba probably formed at low temperatures (<100°C). The distribution of saponite in Kaba is determined by local bulk compositions; enstatite in chondrules has been altered more extensively than the fine-grained olivine in matrix. The alteration products in Kaba resemble those in other aqueously altered carbonaceous chondrites such as the Mokoia CV3 chondrite and in certain CI chondrites. There are, however, significant differences. Kaba lacks the abundant high-Al phyllosilicates in chondrules and in CAIs reported from Mokoia; also lacking in Kaba is the serpentine and ferrihydrite found in Orgueil. Alteration products in Kaba matrix are dissimilar from those in CO3 chondrites even though their prealteration mineralogies were similar, an observation that probably reflects different temperatures during alteration.