Oxygen isotope systematics of chondrules in the Murchison CM2 chondrite and implications for the CO-CM relationship

High-precision oxygen three-isotope measurements of olivine and pyroxene were performed on 29 chondrules in the Murchison CM2 chondrite by secondary ion mass spectrometry (SIMS). The oxygen isotope ratios of analyzed chondrules all plot very close to the primitive chondrule minerals (PCM) line. In each of 24 chondrules, the olivine and/or pyroxene grains analyzed show indistinguishable oxygen isotope ratios. Exceptions are minor occurrences of isotopically distinguished relict olivine grains, which were found in nine chondrules. The isotope homogeneity of these phenocrysts is consistent with a co-magmatic crystallization of olivine and pyroxene from the final chondrule melts and a significant oxygen isotope exchange between the ambient gas and the melts. Homogeneous type I chondrules with Mg#'s of 98.9-99.5 have host chondrule Delta O-17 values ranging from -6.0 parts per thousand to -4.1 parts per thousand, with one exception (Delta O-17: -1.2%; Mg#: 99.6). Homogeneous chondrules with Mg#'s <96, including four type II chondrules (Mg# -65-70), have Delta O-17 values of around -2.5%. Five type I chondrules (Mg# >= 99) have internally heterogeneous oxygen isotope ratios with (DO)-O-17 values ranging from -6.5 parts per thousand to -4.0 parts per thousand, similar to those of host chondrule values. These heterogeneous chondrules have granular or porphyritic textures, convoluted outlines, and contain numerous metal grains dispersed within fine-grained silicates. This is consistent with a low degree of melting of the chondrule precursors, possibly because of a low temperature of the melting event and/or a shorter duration of melting. The (DO)-O-17 values of relict olivine grains in nine chondrules range from -17.9% to -3.4%, while most of them overlap the range of the host chondrule values. Similar to those reported from multiple carbonaceous chondrites (Acfer 094, Y-82094, CO, and CV), the (DO)-O-17 similar to -5 parts per thousand and high Mg#(>= 99) chondrules, which might derive from a reduced reservoir with limited dust enrichments (similar to 50 x Solar System), dominate the population of chondrules in Murchison. Other chondrules in Murchison formed in more oxidizing environment (Mg# < 96) with higher (DO)-O-17 values of -2.5 parts per thousand, in agreement with the low Mg# chondrules in Acfer 094 and CO chondrites and some chondrules in CV and CR chondrites. They might form in environments containing the same anhydrous precursors as for the (DO)-O-17 similar to -5 parts per thousand and Mg# similar to 99 chondrules, but enriched in O-16-poor H2O ice (similar to 0.3-0.4 x the CI dust; (DO)-O-17 > 0 parts per thousand) and at dust enrichments of similar to 300-2000x. Regarding the Mg# and oxygen isotope ratios, the chondrule populations sampled by CM and CO chondrites are similar and indistinguishable. The similarity of these O-16-rich components in CO and CM chondrites is also supported by the common Fe/Mn ratio of olivine in type II chondrules. Although they accreted similar high-temperature silicates, CO chondrites are anhydrous compared to CM chondrites, suggesting they derived from different parent bodies formed inside and outside the snow line, respectively. If chondrules in CO and CM chondrites formed at the same disk locations but the CM parent body accreted later than the CO parent body, the snow line might have crossed the common chondrule-forming region towards the Sun between the time of the CO and CM parent bodies accretion. (C) 2018 Elsevier Ltd. All rights reserved.