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 Δ17O values ranging from −6.0‰ to −4.1‰, with one exception (Δ17O: −1.2‰; Mg#: 99.6). Homogeneous chondrules with Mg#’s <96, including four type II chondrules (Mg# ∼65–70), have Δ17O values of around −2.5‰. Five type I chondrules (Mg# ≥99) have internally heterogeneous oxygen isotope ratios with Δ17O values ranging from −6.5‰ to −4.0‰, 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 Δ17O 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.