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 CO3 chondrite Allan Hills (ALH) A77307 using secondary ion mass spectrometry in order to study the chondrule formation process and the origin of isolated olivine grains in unequilibrated chondrites. Oxygen-isotopic ratios of olivines 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 FeO-rich olivines are more depleted in O-16. Most ratios lie close to the carbonaceous 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 isolated forsterites, have similar O-isotopic 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 O-16 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 that appears to be a primitive nebular component.