|Title:||Oxygen‐isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia|
|Authors:||B.G. Choi, A.N. Krot, and J.T. Wasson|
|Publication:||Meteorit. Planet. Sci., v. 35, p. 1239‐1248.|
|Publish Date:||Nov 2000|
We report in situ measurements of oxygen‐isotope compositions of magnetite and primary and secondary olivine in the highly unequilibrated oxidized CV chondrites Kaba and Mokoia. In both meteorites the magnetite and the secondary olivine (fayalite, Fa90−100) have O‐isotope compositions near the terrestrial fractionation (TF) line; the mean Δ17O (= δ17O−0.52 × 18O) value is −1‰ In contrast, the compositions of nearby primary (chondrule), low−FeO olivines (Fa1‐2) are well below the TF line; 17O values range from ‐3 to ‐9‰ Krot et al. (1998) summarized evidence indicating that the secondary phases in these chondrites formed by aqueous alteration in an asteroidal setting. The compositions of magnetite and fayalite in Kaba and Mokoia imply that the oxygen isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ18O by 20‰ whereas these same materials in Kaba have virtually identical compositions. The difference between Mokoia magnetite and fayalite may indicate formation in isotopic equilibrium in a water−rich environment at low temperatures, ⁓300 K. In contrast, the similar compositions of these phases in Kaba may indicate formation of the fayalite by replacement of preexisting magnetite in dry environment, with the O coming entirely from the precursor magnetite and silica. The Δ17O of the oxidant incorporated into the CV parent body (as phyllosilicates or H2O) appears to have been much (7−8‰ lower than that in that incorporated into the LL parent body (Choi et al., 1998), suggesting that the O‐isotopic composition of the nebular gas was spatially or temporally variable.