Silicate-SiO reaction in a protoplanetary disk recorded by oxygen isotopes in chondrules

The formation of planetesimals and planetary embryos during the earliest stages of the solar protoplanetary disk largely determined the composition and structure of the terrestrial planets. Within a few million years of the birth of the Solar System, chondrule formation and the accretion of the parent bodies of differentiated achondrites and the terrestrial planets took place in the inner protoplanetary disk 1,2. Here we show that, for chondrules in unequilibrated enstatite chondrites, high-precision Î " 17 O values (where Î " 17 O is the deviation of the Î 17 O value from a terrestrial silicate fractionation line) vary significantly (ranging from â '0.49 to +0.84‰) and fall on an array with a steep slope of 1.27 on a three-oxygen-isotope plot. This array can be explained by the reaction between an olivine-rich chondrule melt and an SiO-rich gas derived from vaporized dust and nebular gas. Our study suggests that a large proportion of the building blocks of planetary embryos formed by successive silicate-gas interaction processes: Silicate-H 2 O followed by silicate-SiO interactions under more oxidized and reduced conditions, respectively, within a few million years of the formation of the Solar System.