CARBON AND THE FORMATION OF REDUCED CHONDRULES

CHONDRULES are millimetre-sized spheroidal bodies composed mainly of o livine and orthopyroxene, which comprise the dominant fraction of most chondritic meteorites. They are the products of partial melting of ag gregates of fine-grained silicates with minor contributions from metal s, sulphides and oxides. Although the formation conditions of chondrul es are not well understood, these are thought to involve a transient m elting event in the solar nebula(1-3). The ubiquity of reduced carbon in interstellar clouds and primitive meteorites implies that it was al so present in the early solar nebula, and may thus have been a potenti al constituent of chondrule precursor material. We describe here exper iments in which carbon and magnesian silicate precursor material of pr imitive chondrule composition are 'flash-heated' together and then cry stallized. The resulting material shows many mineralogical features ch aracteristic of natural chondrules, which are not produced in the abse nce of carbon(4-12). Our results suggest not only that carbon was pres ent in the solar nebula, but also that it played a key role in chondru le formation by creating within the melt a reducing environment that w as decoupled from the nebula gas.