Yukio Ikeda

PETROLOGIC STUDY OF THE BELGICA 7904 CARBONACEOUS CHONDRITE : HYDROUS ALTERATION, OXYGEN ISOTOPES, AND RELATIONSHIP TO CM AND CI CHONDRITES

Abstract Seven components were separated from the Belgica 7904 (B7904) carbonaceous chondrite in order to learn more about the genetic relationships between the petrology, hydration reactions, and the oxygen isotopic compositions. These components include two partially altered chondrules, two phyllosilicate clasts, two olivine fragments, and one matrix sample. The partially altered chondrules have a core-rim structure; cores have igneous textures and are unaltered, and rims consist mainly of talc component-bearing phyllosilicates and unaltered olivine. Clear textural evidence indicates that the phyllosilicates in the rims were produced by replacing pyroxene and plagioclase, while the olivine remained unaltered during the process. On the other hand, the phyllosilicate clasts consist mainly of fine-grained serpentine produced from olivine, as well as the pyroxene and plagioclase. The olivine fragments, derived by disaggregation of large porphyritic olivine chondrules, are unaltered. The matrix sample consists mainly of fine-grained phyllosilicates. If the hydration reactions took place with liquid water in the parent body, olivine would have produced talc component-free phyllosilicates before pyroxene and plagioclase reacted. This is not the case for the phyllosilicates in the chondrule rims. Therefore, we suggest that the chondrule rims formed by hydration in the nebula prior to accretion rather than with liquid water in the parent body. Thermodynamic calculations show that the reactions of pyroxene and plagioclase with a gas would produce the talc component-bearing phyllosilicates at temperatures higher than that at which olivine reacts to produce serpentine. Assuming that total gas pressure was 10−3 bars, the chondrule rims may have been produced by reactions with a nebular gas having an H 2 O H 2 ratio of 0.052 at temperatures ranging from 560 to 330 K. The phyllosilicate clasts, where olivine also reacted, may have been produced by reactions with this nebular gas at temperatures near 300 K, and the matrix phyllosilicates may have been in equilibrium with the gas at temperatures higher than 250 K. There is no evidence for liquid water interacting with the chondrite. The CM mixing line in the three-oxygen isotope plot ( mayeda et al., 1991), which includes the separated components and whole-rock composition of B7904, is explained by hydration in nebular gas. CI chondrites may have formed from materials similar to the B7904 matrix by reactions with liquid water in a parent body. CM chondrites have experienced two-stage alteration, including both hydrous reactions with a gas in the nebula and aqueous alteration with liquid water in their parent body.