Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
Condensation of systems sufficiently enriched in dust of C1 composition to yield ferromagnesian silicates with molar FeO/FeO + MgO ratios of 0.1 to 0.4 at temperatures above 1200K also produces copious molten silicate. The distribution of Fe between metal, silicate and sulfide in specific classes of ordinary chondrites can be produced by high-temperature condensation at specific dust enrichments. While a rigorous thermodynamic model at last shows how direct condensation of silicate liquid can occur within the range of Ptot thought to have existed in the inner part of the solar nebula, the compositions of the partially molten condensates so formed do not match the compositions of Types I and II chondrules in important ways. Such chondrules are thus secondary objects that did not form by direct condensation. Nevertheless, if chondritic matter owes its oxidation state to condensation of dust-enriched systems, the present work shows the sequence of condensation, details of the condensation reactions and the evolution of the compositions of solid and liquid solution phases that may be relevant to the formation of chondrites and the precursors to chondrules. The present work also gives physico-chemical conditions capable of stabilizing against evaporation silicate liquids of specific compositions, some of which are similar to the compositions of some chondrule glasses, in cosmic gases at low nebular pressures.
Acknowledgements - The authors extend thanks to J. Valdes, S. Champion, D. Archer and G. Miller for technical assistance, and to M. S. Ghiorso and R.O. Sack for providing advice and the MELTS code. Critical reviews by J.R. Beckett and an anonymous reviewer were very helpful. Material support for this project was through NASA grants NAGW-3340 and NAG5-4476.