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

We have measured O-isotopic ratios in a variety of olivine grains in the CO 3 chondrite Allan Hills (ALH) A77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolate d olivine grains in unequilibrated chondrites. Oxygen-isotopic ratios of ol ivines in this chondrite are variable from delta(17)O = -15.5 to +4.5 parts per thousand and delta(18)O = -11.5 to +3.9 parts per thousand, with Delta (17)O varying from -10.4 to +3.5 parts per thousand. Forsteritic olivines, Fa(<1), are enriched in O-16 relative to the bulk chondrite, whereas more F eO-rich olivines are more depleted in O-16. Most ratios lie close to the ca rbonaceous chondrite anhydrous minerals (CCAM) line with negative values of Delta(17)O, although one grain of composition Fa(4) has a mean Delta(17)O of +1.6 parts per thousand. Marked O-isotopic heterogeneity within one FeO- rich chondrule is the result of incorporation of relic, O-16-rich, Mg-rich grains into a more O-16-depleted host. Isolated olivine grains, including i solated forsterites, have similar O-isotopic ratios to olivine in chondrule s of corresponding chemical composition. This is consistent with derivation of isolated olivine from chondrules, as well as the possibility that isola ted grains are chondrule precursors. The high O-16 in forsteritic olivine i s similar to that observed in forsterite in CV and CI chondrites and the or dinary chondrite Julesburg and suggests nebula-wide processes for the origi n of forsterite that appears to be a primitive nebular component.