Oxygen isotope systematics of chondrules in the Paris CM2 chondrite: Indication for a single large formation region across snow line

Abstract

Abstract In-situ oxygen three-isotope analyses of chondrules and isolated olivine grains in the Paris (CM) chondrite were conducted by secondary ion mass spectrometry (SIMS). Multiple analyses of olivine and/or pyroxene in each chondrule show indistinguishable Δ17O values, except for minor occurrences of relict olivine grains (and one low-Ca pyroxene). A mean Δ17O value of these homogeneous multiple analyses was obtained for each chondrule, which represent oxygen isotope ratios of the chondrule melt. The Δ17O values of individual chondrules range from –7‰ to –2‰ and generally increase with decreasing Mg# of olivine and pyroxene in individual chondrules. Most type I (FeO-poor) chondrules have high Mg# (∼99) and variable Δ17O values from –7.0‰ to –3.3‰. Other type I chondrules (Mg# ≤97), type II (FeO-rich) chondrules, and two isolated FeO-rich olivine grains have host Δ17O values from –3‰ to –2‰. Eight chondrules contain relict grains that are either 16O-rich or 16O-poor relative to their host chondrule and show a wide range of Δ17O values from –13‰ to 0‰. The results from chondrules in the Paris meteorite are similar to those in Murchison (CM). Collectively, the Δ17O values of chondrules in CM chondrites continuously increase from –7‰ to –2‰ with decreasing Mg# from 99 to 37. The majority of type I chondrules (Mg# >98) show Δ17O values from –6‰ to –4‰, while the majority of and type II chondrules (Mg# 60-70) show Δ17O values of –2.5‰. The covariation of Δ17O versus Mg# observed among chondrules in CM chondrites may suggest that most chondrules in carbonaceous chondrites formed in a single large region across the snow line where the contribution of 16O-poor ice to chondrule precursors and dust enrichment factors varied significantly.