Condensation in dust-enriched systems

Full equilibrium calculations of the sequence of condensation of the elemen ts from cosmic gases made by total vaporization of dust-enriched systems we re performed in order to investigate the oxidation state of the resulting c ondensates. The computations included 23 elements and 374 gas species, and were done over a range of P-tot from 10(-3) to 10(-6) bar and for enrichmen ts up to 1000x in dust of Cl composition relative to a system of solar comp osition. Because liquids are stable condensates in dust-enriched systems, t he MELTS nonideal solution model for silicate liquids (Ghiorso and Sack, 19 95) was incorporated into the computer code. Condensation at 10(-3) bar and dust enrichments of 100x, 500x, and 1000x occur at oxygen fugacities of IW -3.1, IW-1.7, and IW-1.2, respectively, and, at the temperature of cessatio n of direct condensation of olivine from the vapor, yields X-Fa of 0.019, 0 .088, and 0.164, respectively. Silicate liquid is a stable condensate at du st enrichments >similar to 12.5x at 10(-3) bar and >similar to 425x at 10(- 6) bar. At 500x, the Liquid field is >1000 K wide and accounts for a maximu m of 48% of the silicon at 10(-3) bar, and is 240 K wide and accounts for 2 5% of the silicon at 10(-6) bar. At the temperature of disappearance of liq uid, X-Fa of coexisting olivine is 0.025, 0.14, and 0.31 at 100x, 500x, and 1000x, respectively, almost independent of P-tot. At 1000x, the Na2O and K 2O 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 1 0(-6) bar. At 10(-3) bar, the predicted distribution of Fe between metal, s ilicate and sulfide at 1310 K and a dust enrichment of 560x matches that fo und in H-group chondrites, and at 1330 K and 675x matches that of L-group c hondrites prior to metal loss. Only at combinations of high P-tot and high dust enrichment do the bulk che mical composition trends of condensates reach the FeO contents typical of t ype IIA chondrules at temperatures where dust and gas could be expected to equilibrate, greater than or equal to 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.% Fe O, most chondrule compositions are too Na2O-rich to lie along trends predic ted for the bulk chemical compositions of the condensates at P-tot less tha n or equal to 10(-3) bar and dust enrichments less than or equal to 1000x. Together, these chemical differences indicate that individual chondrules fo rmed neither by quenching samples of the liquid + solid condensates that ex isted at various temperatures nor by quenching secondary liquids that forme d 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 ch ondrules, however, many chondrule glass compositions fall along bulk compos ition trajectories for liquids in equilibrium with cosmic gases at 10(-3) b ar and dust enrichments between 600x and 1000x. If these chondrules formed by secondary melting of mixtures of condensates that formed at different te mperatures, nebular regions with characteristics such as these would have b een necessary to prevent loss of Na2O by evaporation and FeO by reduction f rom the liquid precursors of their glasses, assuming that the liquids were hot for a long enough time to have equilibrated with the gas. Copyright (C) 2000 Elsevier Science Ltd.