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

Results: Composition of Metallic Nickel-Iron

------------------- Figure 14: Mole per cent of (a) Ni; (b) Co; and (c) Cr in metallic nickel-iron alloy as a function of temperature at the stated conditions. Mole fractions of albite (Ab) and orthoclase (Or) in feldspar as a function of temperature at Ptot= 10-3 bar and a dust enrichment of 100x and 1000x. (enlarge a) (enlarge b) (enlarge c)

The concentrations of Ni, Co and Cr in the metallic nickel-iron alloy at various combinations of Ptot and dust enrichment are shown in Figs. 14a, b and c, respectively. Under all conditions shown, Ni and Co are slightly more refractory and have slightly steeper condensation curves than Fe. This leads to high concentrations of Ni and Co in the first-condensing alloys, steadily declining concentrations of Ni and Co with falling temperature as condensation of slightly less refractory Fe dilutes the previously condensed Ni and Co, and finally a leveling off of the Ni and Co contents when all three elements are totally condensed.

At still lower temperatures in the more oxidizing cases, 1000x at 10-3 and 10-6 bar, Ni and Co contents begin to rise very gradually with falling temperature due to oxidation of the Fe component of the alloy. At 1380K at 1000x and 10-3 bar, the Ni and Co contents of the alloy begin to rise very sharply with falling temperature due to reaction of gaseous sulfur with the Fe component of the alloy to form pyrrhotite. Under oxidizing conditions, Cr is slightly more refractory than Fe and, like Ni and Co, falls steadily in concentration with falling temperature. Under more reducing conditions, however, the behavior of Cr is completely different. At 100x and 10-3 bar, Cr is slightly less refractory than Fe, its concentration in the metal increases sharply with falling temperature in the high-temperature alloys, and only reverses itself below the formation temperature of Cr-spinel, which extracts Cr from the metal alloy. The increase in Cr content with falling temperature is not seen at 100x and 10-6 bar because most of the Cr has already condensed as Cr-spinel at a higher temperature than that where the metal alloy begins to condense. While high Si concentrations in metallic nickel-iron alloys can result from condensation from gases more reducing than a gas of solar composition, XSi is always < 10-4 in the systems considered in this work.

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