Evidence in CO3.0 chondrules for a drift in the O isotopic composition of the solar nebula

Abstract

Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Delta-17O (=delta-16O - 0.52 x delta-18O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, ∆17O tends to increase as the FeO contents of the silicates increase. We report a comparison of the O isotopic composition of olivine phenocrysts in low-FeO (less than or equal to Fa1) type I and high-FeO (greater than or equal to Fa15) type II porphyritic chondrules in the highly primitive CO3.0 chondrite Yamato-81020. In agreement with a similar study of chondrules in CO3.0 ALH A77307 by Jones et al. (2000), Delta-17O tends to increase with increasing FeO. We find that ∆17O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high-FeO chondrules, the difference between Delta-17O of the late-formed, high-FeO phenocryst olivine and those in the low-FeO cores of relict grains is well-resolved (although one of the relicts is interpreted to be a partly melted amoeboid olivine inclusion by Yurimoto and Wasson [2002]). It appears that, during much of the chondrule-forming period, there was a small upward drift in the Delta-17O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.