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

• DOI: 10.1016/j.gca.2021.02.012
• Publication Date: 2021-02-18T07:36:41Z
• Authors: Noël Chaumard; Céline Defouilloy; Andreas T. Hertwig; Noriko T. Kita
• Keywords: Chondrule; Pyroxene; Carbonaceous chondrite; Murchison meteorite

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.