Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia

We report in situ measurements of O-isotopic compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chon drites Kaba and Mokoia. In both meteorites, the magnetite and the secondary olivine (fayalite, Fa(90-100)) have O-isotopic compositions near the terre strial fractionation (TF) line; the mean Delta O-17 (= delta O-17-0.52 x de lta O-18) value is about -1 parts per thousand. In contrast, the compositio ns of nearby primary (chondrule), low-FeO olivines (Fa(1-2)) are well below the TF line; Delta O-17 values range fi om -3 to -9 parts per thousand. Kr ot et ad. (1998) summarized evidence indicating that the secondary phases i n these chondrites formed by aqueous alteration in an asteroidal setting. T he compositions of magnetite and fayalite in Kaba and Mokoia imply that the O-isotopic composition of the oxidant was near or somewhat above the TF li ne. In Mokoia the fayalite and magnetite differ in delta O-18 by similar to 20 parts per thousand, whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayali te may indicate formation in isotopic equilibrium in a water-rich environme nt at low temperatures, similar to 300 K. In contrast, the similar composit ions of these phases in Kaba may indicate formation of the fayalite by repl acement of preexisting magnetite in dry environment, with the O coming enti rely from the precursor magnetite and silica. The Delta O-17 of the oxidant incorporated into the CV parent body las phyllosilicates or H2O) appears t o have been much (7-8 parts per thousand) lower than that in that incorpora ted into the LL parent body (Choi et al., 1998), which suggests that the O- isotopic composition of the nebular gas was spatially or temporally variabl e.