Carbon and the formation of reduced chondrules

CHONDRULES are millimetre-sized spheroidal bodies composed mainly of olivine and orthopyroxene, which comprise the dominant fraction of most chondritic meteorites. They are the products of partial melting of aggregates of fine-grained silicates with minor contributions from metals, sulphides and oxides. Although the formation conditions of chondrules are not well understood, these are thought to involve a transient melting event in the solar nebula1–3. The ubiquity of reduced carbon in interstellar clouds and primitive meteorites implies that it was also present in the early solar nebula, and may thus have been a potential constituent of chondrule pre-cursor material. We describe here experiments in which carbon and magnesian silicate precursor material of primitive chondrule composition are 'flash-heated' together and then crystallized. The resulting material shows many mineralogical features character-istic of natural chondrules, which are not produced in the absence of carbon4–12. Our results suggest not only that carbon was present in the solar nebula, but also that it played a key role in chondrule formation by creating within the melt a reducing environment that was decoupled from the nebula gas.