OXYGEN ISOTOPES IN FORSTERITE GRAINS FROM JULESBURG AND ALLENDE - OXYGEN-16-RICH MATERIAL IN AN ORDINARY CHONDRITE

We have used the Manchester ISOLAB 54 ion microprobe to make in situ m easurements of the O-17/O-16 and O-18/O-16 ratios of olivine grains in the Julesburg (L3.6) and Allende (CV3) chondrites. We have discovered a population of olivines in Julesburg characterised by (1) the most O -16-rich compositions yet reported for olivine from an ordinary chondr ite; (2) cores of low-Fa olivine, which frequently shows blue cathodol uminesce; (3) thick coats of more Fa-rich (Fa similar to 20) olivine, which is also O-16-enriched. In an O isotopic plot, the Julesburg O-16 -rich grains form a roughly linear array that is offset from the Allen de mixing line. The presence of very low Fa olivine and, sometimes, we ll-defined Fa-rich coats indicates that these grains experienced signi ficantly less thermal metamorphism than most of the olivine in the met eorite. Some O-16-rich Julesburg grains are associated with minor feld spar or pyroxene and are probably chondrule fragments. They are isotop ically indistinguishable from forsterite in Allende; however, Allende forsterite grains do not have the thick Fa20 coats typical of those in Julesburg. These O-16-rich forsterite grains appear to be related to the ''blue olivine'' of Steele (1986). Both cores and coats of O-16-ri ch grains in Julesburg are isotopically distinct from olivine in Semar kona group A and group B chondrules.