|Title:||Oxygen‐isotopic compositions in relict and host grains in chondrules in the Yamato‐81020 CO3.0 chondrite|
|Authors:||T. Kunihiro, A.E. Rubin, K.D. McKeegan, and J.T. Wasson|
|Publication:||Geochim. Cosmochim. Acta., v. 68, p. 3599‐3606.|
|Publish Date:||Sep 2004|
We report the oxygen‐isotope compositions of relict and host olivine grains in six high‐FeO porphyritic olivine chondrules in one of the most primitive carbonaceous chondrites, CO3.0 Yamato 81020. Because the relict grains predate the host phenocrysts, microscale in situ analyses of O‐isotope compositions can help assess the degree of heterogeneity among chondrule precursors and constrain the nebular processes that caused these isotopic differences. In five of six chondrules studied, the Δ17O (=δ17O −0.52 × δ18O) compositions of host phenocrysts are higher than those in low‐FeO relict grains; the one exception is for a chondrule with a moderately high‐FeO relict. Both the fayalite compositions as well as the O‐isotope data support the view that the low‐FeO relict grains formed in a previous generation of low‐FeO porphyritic chondrules that were subsequently fragmented. It appears that most low‐FeO porphyritic chondrules formed earlier than most high‐FeO porphyritic chondrules, although there were probably some low‐FeO chondrules that formed during the period when most high‐FeO chondrules were forming.