Oxygen Isotopes in the Minerals of Unequilibrated Chondrites

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9418520 Wasson The P.I.s propose to use the UCLA Cameca 1270 ion-microprobe and petrographic techniques to determine the three O-isotopes in the conducting phase magnetite present in unequilibrated chondritic meteorites in order to obtain detailed compositional data that relate to: (1) the nebular processes that led to the formation of the chondrites; (2) the compositions of major O reservoirs that contributed to the solar nebula; and (3) low-grade metamorphic processes that occurred in parent bodies. During the next year the P.I.s will study the formation of the magnetite in unequilibrated carbonaceous (CV) and ordinary (H, L, LL) chondrites. The P.I.s examination of many thin sections of CV chondrites shows that magnetite is relatively common in chondrules. The P.I.s will use the ion microprobe to determine the compositions of these magnetites on the oxygen 3-isotope diagram relative to other CV minerals that describe a mixing line extending from 18O values -40% along a slope of 0.94 almost to the intersection of this line with the terrestrial fractionation line. A key question the P.I.s hope to answer is whether this magnetite formed in the solar nebula or during the oxidation of the CV parent body. The P.I.s' second project will be to examine the three O isotopes in magnetite present in magnetite-carbide assemblages in some unequilibrated ordinary chondrites. Krot discovered a number of these assemblages while at UCLA. The origin of these oxidized/reduced assemblages is not known; again, the possible locales are the solar nebula and the parent body. Precise O-isotope data will provide important constraints both on the locale and on the nature of the agent that oxidized the Fe of the magnetite.