The formation and alteration of the Renazzo-like carbonaceous chondrites II: Linking O-isotope composition and oxidation state of chondrule olivine

Abstract

To better understand the formation conditions of type-I and type-II chondrules in the Renazzo-like carbonaceous (CR) chondrites, an in situ major- and minor-element and O-isotope study was conducted. Twenty-one ferromagnesian chondrules from three CR chondrites (GRA 95229, GRA 06100, and QUE 99177) were analyzed to establish an internally-consistent data set. From this study we infer that type-II chondrule precursors contained enhanced S-bearing dust and ice abundances relative to type-I chondrules. There is a relationship between the O-isotope composition and oxidation state of olivine, which may be related to the amount of 16O-poor ice and reduced carbon accreted by chondrule precursors before melting. Type-II chondrules formed under H2O/H2 ratios of ∼230–740 times solar. In contrast, type-I chondrules formed under more reducing conditions with lower H2O/H2 ratios of ∼10–100 times solar. We find a relationship between type-II chondrule petrology (relict free vs. relict grain-bearing) and O-isotope composition, which is due to degree of melting and exchange with a 16O-poor gas reservoir. The 16O-poor gas that interacted with both type-I and type-II chondrules is estimated to have an isotopic composition between ∼δ18Og=13–27‰ and δ17Og=10–22‰, different from the O-isotope composition of the water accreted by the CR chondrite parent body. Due to partial melting, type-I chondrules and relict grain-bearing type-II chondrules exchanged with the 16O-poor gas to a lower degree than relict-free type-II chondrules.