Abstract
We modified the MAGMA chemical equilibrium code developed by Fegley
and Cameron (1987, Earth Planet. Sci. Lett. 82, 207-222) and used
it to model vaporization of high temperature silicate lavas on Io.
The MAGMA code computes chemical equilibria in a melt, between melt
and its equilibrium vapor, and in the gas phase. The good agreement
of MAGMA code results with experimental data and with other computer
codes is demonstrated. The temperature-dependent pressure and composition
of vapor in equilibrium with lava is calculated from 1700 to 2400
K for 109 different silicate lavas in the O-Na-K-Fe-Si-Mg-Ca-Al-Ti
system. Results for five lavas (tholeiitic basalt, alkali basalt,
Barberton komatiite, dunite, and a molten type B I Ca, AT-rich inclusion)
are discussed in detail. The effects of continuous fractional vaporization
on chemistry of these lavas and their equilibrium vapor are presented.
The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and
Ti in the vapor equilibrated with lavas at 1900 K are lower than
published upper limits for To's atmosphere (which do not include
Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic
eruptions. Sodium is more volatile than K, and the Na/K ratio in
the gas is decreased by fractional vaporization. This process can
match To's atmospheric Na/K ratio of 10 3 reported by Brown (2001,
Icarus 15 1, 190-195). Silicon monoxide is an abundant species in
the vapor above lavas. Spectroscopic searches are recommended for
SiO at IR and mm wavelengths. Reactions of metallic vapors with S-
and Cl-bearing volcanic gases may form other unusual gases including
MgCl2, MgS, MgCl, FeCl2, FeS, FeCl, and SiS. (C) 2003 Elsevier Inc.
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