Oxygen isotopic diversity of chondrule precursors and the nebular origin of chondrules

FeO-poor (type 1) porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet elusive mechanism. Two settings are generally considered for their formation: (i) a nebular setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca-Al-Ti-poor relative to host olivine crystals. Their variable Delta O-17, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120 degrees triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions. We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Delta O-17 values and Ca-Al-Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca-Al-Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca-Al-Ti-rich host olivine crystals via epitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i) the dilution of CAT-melts, as attested by the positive Al-Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Delta O-17 values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Delta O-17 values, possibly due to variable solid/gas mixing ratios. (C) 2018 Elsevier B.V. All rights reserved.