Geochimica et Cosmochimica Acta, 1999
Condensation in Dust-enriched Systems, by D.S. Ebel and L. Grossman,
Geochimica et Cosmochimica Acta, 1999
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------------------- Figure 4: Variation of oxygen fugacity with temperature for gas in equilibrium with condensates at the stated conditions of total pressure and dust enrichment, with the iron-wüstite buffer curve (dash-dot pattern) shown for reference. (enlarge) |
| Shown in Fig. 4 is the temperature dependence of the oxygen fugacity of the gas in equilibrium with condensate assemblages computed at dust enrichments of 100x, 500x and 1000x at Ptot=10-3 bar, and at dust enrichments of 100x and 1000x at 10-6 bar, along with that of the iron-wüstite buffer (log fO2 = IW) and that of a gas of solar composition at 10-3 bar (log fO2~IW-6) for reference. The curves for Ptot = 10-3 bar are nearly concentric with one another and show the expected increase of fO2 with increasing dust enrichment. The curves for dust enrichments of 100x, 500x and 1000x lie at about IW-3.1, IW-1.7 and IW-1.2, respectively. Exceptions to this concentric behavior are seen as subtle changes in curvature, particularly noticeable at high dust enrichments where the onset of olivine condensation removes significant fractions of the oxygen from the vapor. Comparison of the two curves for a constant dust enrichment of 100x shows a slight increase in fO2 by as much as 0.4 log units as Ptot drops from 10-3 to 10-6 bar below 2000K. The smallness of the variation with Ptot is due to the fact that fO2 in oxygen-rich cosmic gases is largely controlled by the equilibrium H2 + 1/2O2 = H2O and therefore depends on the P(H2O)/P(H2) ratio which is almost independent of Ptot at a given temperature, as discussed by Yoneda and Grossman (1995). Above 2000K, however, Fig. 4 shows that, at dust enrichments of 100x and 1000x, the fO2 values at 10-6 bar drop below their respective values at 10-3 bar and the difference in fO2 between the two total pressures at constant dust enrichment increases with increasing temperature, reaching nearly 4 log units at 2400K. This exceptionally large variation in fO2with Ptot is due to the fact that, at 10-6 bar, almost all of the H2 and H2O are dissociated into monatomic species at these high temperatures, making the above equilibrium irrelevant to the fO2 while, at 10-3 bar, this dissociation occurs above 2400K because the higher pressure favors polyatomic over monatomic species. |
