Premium
Oxygen isotopes in magnetite and fayalite in CV chondrites Kaba and Mokoia
Author(s) -
CHOI ByeonGak,
KROT Alexander N.,
WASSON John T.
Publication year - 2000
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.2000.tb01512.x
Subject(s) - fayalite , magnetite , chondrite , mineral redox buffer , olivine , meteorite , mineralogy , geology , chondrule , chemistry , fractionation , isotopes of oxygen , parent body , geochemistry , analytical chemistry (journal) , astrobiology , environmental chemistry , chromatography , mantle (geology) , paleontology , physics
— We report in situ measurements of O‐isotopic 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, Fa 90–100 ) have O‐isotopic compositions near the terrestrial fractionation (TF) line; the mean Δ 17 O (= δ 17 O‐0.52 × δ 18 O) value is about −1%‰. In contrast, the compositions of nearby primary (chondrule), low‐FeO olivines (Fa 1–2 ) are well below the TF line; Δ 17 O 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 O‐isotopic composition of the oxidant was near or somewhat above the TF line. In Mokoia the fayalite and magnetite differ in δ 18 O 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 Δ 17 O of the oxidant incorporated into the CV parent body (as phyllosilicates or H 2 O) appears to have been much (7–8%‰) lower than that in that incorporated into the LL parent body (Choi et al , 1998), which suggests that the O‐isotopic composition of the nebular gas was spatially or temporally variable.