Oxygen isotope heterogeneity in chondrules from the Mokoia CV3 carbonaceous chondrite

We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by delta(17)O similar to -2.5parts per thousand) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have delta(18)O values around 0parts per thousand, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher delta(18)O values than olivines in the same chondrules. Our bulk chondrule data spread to lower delta(18)O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk delta(18)O values (-7.5parts per thousand and -11.7parts per thousand) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to delta(17)O, delta(18)O around -50parts per thousand in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an O-16-rich precursor and an O-16-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had delta(17)O and delta(18)O values around -50parts per thousand, similar to those observed in refractory inclusions. Copyright (C) 2004 Elsevier Ltd.