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103 times the observed atmospheric inventories. The CO2 fugacity determiend for the buffered reaction MgCO3 plus SiO2 going to MgSiO3 plus CO2 yields values which increase from 10−4 to 100 times the lithostatic pressure, going from 35 to 800 km depths in the earth. The major uncertainties in the fugacity calculations arise from lack of knowledge of effective activities. The calculated fugacity of CO2 and water brought to the surface from hypothetical mantle reservoirs on Earth and Venus indicate that, in the case of the Earth, the cool and dry atmosphere is strongly depleted in both CO2 and H2O compared to the low velocity zone. In contrast to the Earth, on Venus, the 750 K surface temperature and 90 bar CO2 surface pressure is in equilibrium and probably in communication with an assumed upper mantle CO2 reservoir.
doi:10.1016/0016-7037(81)90110-1 
Copyright © 1981 Published by Elsevier Ltd.
Planetary phase equilibria: application to formation of Earth, Venus and Mercury
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S.K Saxenaa
Received 25 June 1980;
Abstract
Calculations of phase equilibria in a solar mixture with variable hydrogen abundance show that the major element chemical composition of Earth and Venus can be simply explained by their formation in equilibrium at 800 and 1000 K respectively at a pressure of 1 × 10−3 atm, provided that there is an iron loss from the region of proto-Venus relative to the solar nebula. The calculated mineralogical chemical compositions of the two planets are in excellent agreement with the available chemical and physical data. Phase equilibrium calculations at 1500 K and 1 × 10−3 atm show that nearly 96% of the silicates and 81% of metal must have been lost from the region of proto-Mercury.
