On the significance of oxygen-isotope variations in chondrules from carbonaceous chondrites

Oxygen-isotope studies of carbonaceous chondrite chondrules are of pivotal importance for understanding of the evolution of the solar protoplanetary disk. It has been previously concluded that the observed variations in A17O (=d17O - 80.52 x d18O) of chondrule olivines and pyroxenes are intimately tied to their Mg# (=MgO/(MgO + FeO) x 100, mol%) and the inferred oxygen fugacity (fO2) of gaseous reservoirs produced by evaporation of disk regions with different (silicate dust +/- water ice)/gas ratios. Using high resolution cathodoluminescence and secondary ion mass spectrometry, we show, in contrast to host chondrule data, that A17O of chondrule olivines from the Yamato 81020 CO3.05 carbonaceous chondrite is independent of their Mg# and of the imposed fO2. Instead, there is a A17O bimodal distribution of Mgrich olivines that gradually turns into a unimodal distribution as Mg# decreases. We suggest that chondrules recorded an evolution of an isotopically heterogeneous vapor plume resulting from a high temperature mixing of the 16O-enriched (A17O -6 +/- 2%w) and 16O-depleted (A17O -2.5 +/- 1%w) reservoirs. Drop in the vapor plume temperature under unbuffered redox conditions favors the dissolution of Fe, Ni-metal of chondrules and the subsequent crystallization of FeO-rich olivines at saturation; the 16O-depleted signature being the most resilient at lower temperature. Chondrules are thus inferred to have formed in a same turbulent heterogeneous environment in which locally high temperatures and reducing conditions prevailed (Type I), adjacent in space and/or in time to areas submitted to lower temperatures and more oxidizing conditions (Type II). Subtracting of chondrules at different stages of the gaseous plume evolution by fast cooling rates give rise to the chemical and isotopic diversity of chondrules. No extrinsic oxidizing agents (e.g., water ice) are needed in this scenario. (c) 2023 Elsevier Ltd. All rights reserved.