Oxygen isotopes in chondrule olivine and isolated olivine grains from the CO3 chondrite, Allan Hills A77307

We have measured oxygen isotope ratios in a variety of olivine grains in the CO3 chondrite, ALHA77307, using secondary ion mass spectrometry, in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites. Oxygen isotope ratios of olivines in this chondrite are variable, from (17O = -15.5 deg to +4.5 deg and (18O = -11.5 deg to + 3.9 deg with (17O varying from -10.4 to +3.5 deg Forsteritic olivines, Fa<1, are enriched in 16O relative to the bulk chondrite, while more FeO-rich olivines are more depleted in 16O. Most ratios lie close to the CCAM line with negative values of (17O, although one grain of composition Fa4 has a mean (17O of +1.6 deg. Marked oxygen isotopic heterogeneity within one FeO-rich chondrule is the result of incorporation of relict, 16O-rich, Mg-rich grains into a more 16O-depleted host. Isolated olivine grains, including isolated forsterites, have similar O isotope ratios to olivine in chondrules of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isolated grains are chondrule precursors. The high 16O in forsteritic olivine is similar to that observed in forsterite in CV and CI chondrites and the ordinary chondrite Julesburg, and suggests nebula-wide processes for the origin of forsterite which appears to be a primitive nebular component.