Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999

Results: Vapor of Solar Composition

Different thermodynamic data are employed for some crystalline phases, many more chemical species are included and a very different computational procedure was used in the present study than in our previous work on condensation (Yoneda and Grossman, 1995). It is therefore important to compare results from the two studies, and this is done for the case of a solar gas at P^tot = 10^-3 bar in Table 6. Appearance temperatures of phases refer to the highest temperature step at which a phase is part of the condensate assemblage in the 2K steps of the calculations. Our results are quite similar, but not identical, to those of Yoneda and Grossman (1995), referred to as the previous work in the following explanation of the differences which, in all cases, are due to differences in thermodynamic data. Note that, although some of the data used in our previous work may be more accurate, e.g. those for hibonite and grossite, we use those in Table 3 in the present study because the latter are more consistent with the MELTS liquid model. Hibonite forms from corundum 15K lower in the present calculations than in the previous work because hibonite is 2.5 kJ less stable and corundum 0.3 kJ more stable at 1700K in the present work. The lesser stability of hibonite in the present work allows it to be replaced by grossite and CaAl₂O₄, which are 16.7 and 8.7 kJ more stable, respectively, at 1700K in the present work. The gehlenite end-member of the melilite solid solution series is here 10.1 kJ less stable at 1600K than previously, and it forms from CaAl₂O₄, a phase more stable than hibonite in the present calculations. This causes the appearance temperature of melilite to be suppressed by nearly 50K and allows grossite and hibonite to partially replace it at lower temperature. Spinel condenses 13K lower in the present work than previously, primarily because the MgAl₂O₄ end-member is now 3.5 kJ less stable at 1500K. Plagioclase forms from spinel ~10K lower in the present work because the CaAl₂Si₂O₈ end-member is 6.5 kJ less stable at 1400K than previously. In the present work, Ti₃O₅ forms from Ti-bearing clinopyroxene 18K lower and Ti₄O₇ does not form at all because of gross differences in the way the Ti-bearing end-member components are treated in the two calculations. In the previous work, literature data were used for the Ti³⁺-bearing component, CaTiAlSiO₆, and estimated for the Ti⁴⁺-bearing component, CaTiAl₂O₆, while, in the present work, only data for the Ti⁴⁺-bearing components, CaTi_0.5Mg_0.5AlSiO₆ and CaTi_0.5Mg_0.5FeSiO₆, are used. Cordierite replaces plagioclase in the present work because it is 12.8 kJ/mole more stable, and plagioclase is 7.2 kJ less stable, at 1300K relative to the previous work. Sphene does not form above 1200K in the present work because it is 5.5 kJ/mole less stable than previously. No liquids were found to be stable in solar gas at P^tot=10^-3 bar by us or any previous workers, e.g. Wagner (1979), Wood and Hashimoto (1993) and Yoneda and Grossman (1995), despite a contrary claim by Wark (1987).

CONDENSATION in DUST-ENRICHED SYSTEMS Denton S. Ebel (1) Lawrence Grossman(1,2) (1) Department of The Geophysical Sciences The University of Chicago 5734 South Ellis Ave. Chicago, IL 60637 (2) Enrico Fermi Institute The University of Chicago 5640 South Ellis Ave. Chicago, IL 60637 Submitted December 22, 1998 to Geochimica et Cosmochimica Acta Revised version submitted June 30, 1999

Abstract Introduction Technique Bulk Composition Method of Calculation Data for Elements and Gas Species Data and Models for Solids Data and Models for Silicate Liquids Test of MELTS: Peridotite KLB-1 Transition Between Liquid Models Results Vapor of Solar Composition General Effects of Dust Enrichment and Total Pressure Oxygen Fugacity Condensation Temperatures and Liquid Stability Condensation at 100x Dust Enrichment, Ptot=10-3bar Condensation at 1000x Dust Enrichment, Ptot=10-3bar Condensation of Oxidized Iron at High Temperature Bulk Chemical Composition of Condensates Composition of Silicate Liquid Composition of Spinel Composition of Clinopyroxene Composition of Feldspar Composition of Metallic Nickel-Iron Metal-Sulfide Condensate Assemblages Discussion Stability of Silicate Liquid in Solar Gas Chondrules in Dust-enriched Systems Conclusions References