Planetesimal compositions governed by aerodynamic sorting from the onset of Solar System formation

Abstract

Carbonaceous (CC) chondrites from the outer Solar System are mixtures of fine-grained, volatile-rich matrix and thermally sintered, volatile-poor chondrules. Among CC chondrite parent bodies that accreted ~2–4 million years after calcium–aluminium-rich inclusions (CAIs), matrix fraction increases with accretion time, consistent with aerodynamic sorting in the protoplanetary disk. Whether this process operated during the earliest planetesimal accretion (<1–2 Ma) remains unknown. Here we use two independent chemical proxies, bulk sulfur content and iron valence state, which track moderately volatile element inventory and the combined abundance of water–ice and pre-accretionary oxidized silicate precursors, respectively, to constrain matrix mass fractions in CC iron meteorite parent bodies—the earliest outer Solar System planetesimals. Both proxies show that CC iron meteorite parent bodies accreted systematically less matrix than CC chondrite parent bodies, corresponding to matrix mass fractions of only 0.08–0.17. These results extend the matrix-accretion time correlation to the Solar System’s earliest epoch, establishing aerodynamic sorting as a fundamental control on planetesimal compositions and requiring that chondrule formation was widespread from the onset of Solar System formation.