Oxygen isotope systematics of chondrule phenocrysts from the CO3.0 chondrite Yamato 81020: Evidence for two distinct oxygen isotope reservoirs

Abstract

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 33 chondrules in the Yamato 81020 CO3.0 chondrite by secondary ion mass spectrometry. In chondrules where oxygen isotopes were measured in both olivine and pyroxene, the majority of grains have similar values, indicating co-magmatic crystallization. However, many chondrules contain relict grains with unique oxygen isotope ratios. A striking feature of Yamato 81020 chondrules is a bimodal distribution of oxygen isotope ratios, as those with Mg# >97 phenocrysts range in Δ17O from −4.8‰ to −6.5‰ (“−5.5‰” group), and those with Mg# 96–36 phenocrysts have Δ17O values of −2.1‰ to −3.0‰ (“−2.5‰” group). A single Mg# 99.6 barred olivine chondrule has a Δ17O of −3.3‰. We discuss that Δ17O ∼−5.5‰ chondrules are derivative of a reservoir with limited dust enrichment (100× Solar System), which yielded a relatively reduced chondrule-forming environment. In contrast, the Δ17O ∼−2.5‰ chondrules may have been influenced by 16O-poor H2O ice that sublimed and then homogenized with precursor material. The addition of H2O, when combined with high dust enrichment (1000× Solar System) and greater bulk Fe content, could have induced an oxidized environment at high temperatures, forming Mg# 96–36 chondrules. Among the 33 chondrules studied, the Al–Mg relative ages of 20 had been obtained previously. Comparing the oxygen isotope ratios and the 26Al ages of these chondrules, it is likely that the “−5.5‰” and “−2.5‰” oxygen isotope reservoirs existed contemporaneously. This implies that the snow line was spatially fixed during chondrule formation, and separated the CO chondrite accretion region into two distinct volumes of precursors.