Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
The calculations show that no liquids are stable in a gas of solar composition,
even at a total pressure as high as 10-3 bar. Any liquids formed by
the partial or complete melting of agglomerated solids (Whipple, 1966; Lofgren, 1996) would therefore
be highly unstable with respect to partial evaporation in a solar nebula of canonical composition, and
would become even more unstable with decreasing pressure. Such liquids would lose FeO and alkali metals
most readily, followed by Mg and Si, then Ca and Al with increasing temperature.
Experimentally determined evaporation rates of Na2O from
liquids of chondrule composition (Radomsky and Hewins, 1990; Tsuchiyama et al., 1981; Yu and Hewins, 1998)
show that Na loss is faster at lower total pressures and lower fO2.
However, Lewis et al. (1993) showed that sodium loss in alkali olivine basalt melt droplets (3.05 wt%
Na2O) was nearly attenuated upon heating above 1600K
at Ptot=1 bar, at the iron-wüstite buffer in a
CO/CO2 + NaCl vapor with PNa >
4x10-6 atm. The present calculations show that these conditions are roughly
equivalent to a C1 dust enrichment factor well in excess of 1000x at 10-3 bar.
Lewis et al. (1993), based on their experiments and following Wood (1984), suggested chondrule formation
in 'clumps' where the local partial pressures of condensable elements and oxygen were enhanced by volatilization
of chondrule precursor material. The phase diagrams presented above provide the rigorous thermochemical basis for
concluding that such a mechanism would indeed stabilize liquids in the solar nebula and reduce or eliminate the
driving force for volatilization of Na and other elements from chondrule-like liquids.