Carbon and the formation of reduced chondrules

Abstract

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 nebula^1–3. 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 carbon^4–12. 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.