A systematic for oxygen isotopic variation in meteoritic chondrules

Abstract

Primitive meteorites are characteristically formed from an aggregation of sub-millimeter silicate spherules called chondrules. Chondrules are known to present large three-isotope oxygen variations, much larger than shown by any planetary body. We show here that the systematic of these oxygen isotopic variations results from open-system gas–melt exchanges during the formation of chondrules, a conclusion that has not been fully assessed up to now. We have considered Mg-rich porphyritic chondrules and have modeled the oxygen isotopic effects that would result from high-temperature interactions in the disk between precursor silicate dust and a gas enriched in SiO during the partial melting and evaporation of this dust. This formation process predicts: (i) a range of oxygen isotopic composition for bulk chondrules in agreement with that observed in Mg-rich porphyritic chondrules, and (ii) variable oxygen isotopic disequilibrium between chondrule pyroxene and olivine, which can be used as a proxy of the dust enrichment in the chondrule-forming region(s). Such enrichments are expected during shock waves that produce transient evaporation of dust concentrated in the mid-plane of the accretion disk or in the impact plumes generated during collisions between planetesimals. According to the present model, gas–melt interactions under high PSiO(gas) left strong imprints on the major petrographic, chemical and isotopic characteristics of Mg-rich porphyritic chondrules.