Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
Full equilibrium calculations of the sequence of condensation of the elements from cosmic gases made by total vaporization of dust-enriched systems were performed in order to investigate the oxidation state of the resulting condensates. The computations included 23 elements and 374 gas species, and were done over a range of Ptot from 10-3 to 10-6 bar and for enrichments up to 1000x in dust of C1 composition relative to a system of solar composition. Because liquids are stable condensates in dust-enriched systems, the MELTS non-ideal solution model for silicate liquids
(Ghiorso and Sack, 1995) was incorporated into the computer code.
Condensation at 10-3 bar and dust enrichments of 100x, 500x and 1000x occurs at oxygen fugacities of IW-3.1, IW-1.7 and IW-1.2, respectively, and, at the temperature of cessation of direct condensation of olivine from the vapor, yields XFa of 0.019, 0.088 and 0.164, respectively. Silicate liquid is a stable condensate at dust enrichments >~12.5x at 10-3bar and >~425x at 10-6 bar. At 500x, the liquid field is >1000K wide and accounts for a maximum of 48% of the silicon at 10-3 bar, and is 240K wide and accounts for 25% of the silicon at
10-6 bar. At the temperature of disappearance of liquid, XFa of coexisting olivine is 0.025, 0.14 and 0.31 at 100x, 500x and 1000x, respectively, almost independent of Ptot. At 1000x, the Na2O and K2O contents of the last liquid reach 10.1 and 1.3 wt%, respectively, at 10-3 bar but are both negligible at 10-6 bar. At 10-3 bar,
iron sulfide liquids are stable condensates at dust enrichments at least as low as 500x and coexist with silicate liquid at 1000x. No sulfide liquid is found at 10-6 bar. At 10-3 bar, the predicted distribution of Fe between metal, silicate and sulfide at 1310K and a dust enrichment of 560x matches that found in H-Group chondrites, and at 1330K and 675x matches that of L-Group chondrites prior to metal loss.
Only at combinations of high Ptot and high dust enrichment do the bulk chemical composition trends of condensates reach the FeO contents typical of Type IIA chondrules at temperatures where dust and gas could be expected to equilibrate, (> or =) 1200K. Even under these conditions, however, the composition trajectories of predicted condensates pass through compositions with much more CaO + Al2O3 relative to MgO + SiO2 than those of most Type IA chondrules. Furthermore, on a plot of wt% Na2O vs. wt% FeO, most chondrule compositions are too Na2O-rich to lie along trends predicted for the
bulk chemical compositions of the condensates at Ptot (> or =) 10-3 bar and dust enrichments (> or =) 1000x. Together, these chemical differences indicate that individual chondrules formed neither by quenching samples of the liquid + solid condensates that existed at various temperatures nor by quenching secondary liquids that formed from such samples. With the exception of very FeO-poor, Na2O-rich glasses in Type I chondrules and glasses with very high FeO and Na2O in Type II chondrules, however, many chondrule glass compositions fall along bulk composition trajectories
for liquids in equilibrium with cosmic gases at 10-3 bar and dust enrichments between 600x and 1000x. If these chondrules formed by secondary melting of mixtures of condensates that formed at different temperatures, nebular regions with characteristics such as these would have been necessary to prevent loss of Na2O by evaporation and FeO by reduction from the liquid precursors of their glasses, assuming that the liquids were hot for a long enough time to have equilibrated with the gas.