Condensation in dust-enriched systems

Abstract 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 1000× in dust of Cl composition relative to a system of solar composition. Because liquids are stable condensates in dust-enriched systems, the MELTS nonideal solution model for silicate liquids (Ghiorso and Sack, 1995) was incorporated into the computer code. Condensation at 10−3 bar and dust enrichments of 100×, 500×, and 1000× occur 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 >∼12.5× at 10−3 bar and >∼425× at 10−6 bar. At 500×, the liquid field is >1000 K wide and accounts for a maximum of 48% of the silicon at 10−3 bar, and is 240 K 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 100×, 500×, and 1000×, respectively, almost independent of Ptot. At 1000×, 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 500× and coexist with silicate liquid at 1000×. No sulfide liquid is found at 10−6 bar. At 10−3 bar, the predicted distribution of Fe between metal, silicate and sulfide at 1310 K and a dust enrichment of 560× matches that found in H-group chondrites, and at 1330 K and 675× 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, ≥1200 K. 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 ≤ 10−3 bar and dust enrichments ≤1000×. 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 600× and 1000×. 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.