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L. Schaefer, B. Fegley, Jr. (Planetary Chemistry Laboratory, Department of Earth and Planetary Sciences, Washington University)
Galileo NIMS observations indicate magmas with temperatures of 1700-2100K on Io. Vaporization of rock-forming oxides should occur at such temperatures. We calculated total vapor pressures and vapor compositions from 1700-3000K with the MAGMA code described in Fegley and Cameron (1987) for various magmas: chondritic, basaltic (OIBs, MORBs, tholeiites), molten CAIs, and komatiitic. Preliminary results were reported by Fegley and Kargel (2002).The major oxides included in the computations are SiO2, MgO, FeO, Al2O3, CaO, Na2O, K2O, and TiO2. The average total vapor pressures over each magma type at 2100 K are, respectively, 0.020, 0.013, 0.09, 0.006, 0.003, 0.005, and 0.002 bars. The temperature-dependent vapor compositions are initially dominated by Na gas, along with O2, O, and K, followed by Na2, SiO, FeO, Mg, and Fe. Abundances of SiO, Mg, O, Fe, and MgO increase at higher temperatures, whereas Na2 and FeO decrease and Na, O2, and K remain approximately the same. Fractional vaporization at constant temperature was also modeled. After alkali loss, the vapor pressure drops and the composition of the vapor is dominated by SiO, O2, and O~Fe. No single species is diagnostic of a specific magma, but simultaneous observations of several species could be diagnostic. Gaseous SiO should be observable above hot magmas by microwave, IR or UV spectroscopy. Very high O2 pressures may also be observable. Thermal ionization of monatomic alkali vapors should produce significant Na+ and K+ . Preliminary equilibrium calculations show that the magma vapors should react with other volatiles in volcanic gases.
Acknowledgements: Supported by NASA Grant NAG 5-11958 from the Planetary Atmospheres Program.
References: B. Fegley, Jr. and A.G.W. Cameron, (1987), EPSL, 82, p.207-222.
B. Fegley, Jr. and J.S. Kargel, (2002), "Vapor Pressure and Vapor Chemistry of Metals over High Temperature Magmas and Oxides", Workshop on Thermally Extreme Volcanism on Io.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.