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


Several recent studies have shown that materials such as magnetite that formed in asteroids tend to have higher Delta(17)O (=delta(17)O - 0.52 x delta(18)O) values than those recorded in unaltered chondrules. Other recent studies have shown that, in sets of chondrules from carbonaceous chondrites, Delta(17)O tends to increase as the FeO contents of the silicates increase. We report a comparison of the I isotopic composition of olivine phenocrysts in low-FeO (less than or equal toFa(1)) type I and high-FeO ( greater than or equal toFa(15) ) 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 a]. (2000), Delta(17)O tends to increase with increasing FeO. We find that Delta(17)O values are resolved (but only marginally) between the two sets of olivine phenocrysts. In two of the high-FeO chondrules, the difference between Delta(17)O 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(17)O of nebular solids and that relict cores preserve the record of a different (and earlier) nebular environment.

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