Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
Several lines of evidence suggest that most chondrites formed at oxygen fugacities significantly higher than those of a solar gas (e.g., Fegley and Palme, 1985; Rubin et al., 1988; Palme and Fegley, 1990; Weinbruch et al., 1990).
The most compelling evidence is the high FeO content of chondritic olivine and pyroxene grains, many of which have molar FeO/(FeO+MgO) ratios greater than 0.15 (Wood, 1967; Van Schmus, 1969). Grossman (1972) showed that the first olivine and pyroxene
to condense from a cooling solar gas contain only trace amounts of FeO, because iron is more stable as co-condensing metallic nickel-iron. At equilibrium, olivine will not incorporate significant FeO until below 550 K, when iron metal reacts with
gaseous H2O to form FeO (Grossman, 1972) which must then diffuse into the crystal structure of previously condensed forsterite, replacing MgO. At these low temperatures, however, this mechanism
for producing the observed FeO content of olivine in chondrites encounters two fundamental problems: solid-gas equilibrium is unlikely, and diffusion in olivine is very slow. Enhancing the oxygen fugacity of the system in which chondritic matter formed
is one way FeO could have been stabilized at temperatures high enough that it was incorporated into ferromagnesian silicates when, or soon after, they first condensed.
The most reasonable mechanism proposed for producing the oxygen fugacity required to form fayalitic olivine at higher temperatures is enhancement of the dust/gas ratio (Wood, 1967; Rubin et al., 1988). In such a model, the initial nebula is a
cold cloud of interstellar gas and dust, whose overall composition is solar and in which ~30% of the oxygen is in the dust, and virtually all of the H and C are in the gas. If, before nebular temperatures reach their maximum, dust concentrates in certain
regions relative to the gas compared to solar composition, then total vaporization of such regions will produce a gas enriched in oxygen relative to hydrogen and carbon compared to solar composition. Subsequent condensation in such a region occurs in a
gas with a significantly higher oxygen fugacity than one of solar composition. Furthermore, the abundance ratios of condensable elements such as Mg and Si to H are increased much more than the O/H ratio, because the dust contains nearly 100% of each of
the condensable elements, compared to only 30% of the oxygen. The condensation temperature of any phase increases with increasing partial pressures of its gaseous constituents, which in turn increase with their abundances relative to hydrogen.
Dust enrichment therefore not only increases oxygen fugacity, but also increases condensation temperatures, possibly to temperatures at which partial melts are stable.
Wood and Hashimoto (1993) and Yoneda and Grossman (1995) performed full equilibrium calculations of condensation in dust-enriched systems, and both studies found stability fields of silicate liquids at relatively low total pressure.
Therefore, an accurate thermodynamic description of silicate liquids is a prerequisite for an accurate description of condensation in dust-enriched systems. Yoneda and Grossman (1995) were the first to assess the stability of non-ideal
CaO-MgO-Al2O3-SiO2 (CMAS) silicate liquid (Berman, 1983), but
were unable to address the stability of ferromagnesian liquids due to lack of an accurate thermodynamic model for silicate liquids containing Fe, Ti, Na and K.
The present work is the first to explore condensation in either solar composition or dust-enriched systems using a thermodynamic model for ferromagnesian liquids which has been tested against experimental data and natural assemblages.
An 11-component subset of the 15-component "MELTS" silicate liquid model, developed by Ghiorso and Sack (1995) to model crystallization of natural silicate liquids of peridotite to intermediate compositions, has been incorporated into condensation
calculations. In addition, this liquid model is shown here to describe accurately the crystallization of liquids in the FeO-CMAS system, similar to many of the liquids predicted in this work. Condensation sequences are computed at dust enrichments
of up to 1000x, and at Ptot of 10-3 and 10-6 bar, at temperatures from 1100 to 2400 K. Results indicate the composition
changes in solid, liquid and gas phases likely to occur during direct condensation, partial evaporation, or pre-accretion metasomatism of matter in dust-enriched systems at these temperatures and pressures. The idea that ferromagnesian chondrules
formed by direct condensation in the solar nebula has persisted since Sorby (1877) likened chondrules to solidified "drops of fiery rain", and Wood (1967) revived it by suggesting that liquids of forsterite composition might be stable at
low total pressures in gases enriched (by >5000x) in precondensed dust. Therefore, in this work, specific equilibrium assemblages are compared with specific chondrules, and the implications of dust enrichment for chondrule stability in the protoplanetary
nebula are explored. Preliminary versions of this work were presented by Ebel and Grossman (1996, 1997a,b, 1998).