Carbon dioxide within Venus and the earth

The maximum inventories of CO2 and H2O in the terrestrial and Venus mantles are calculated on the basis of shock wave data for magnesite and dunite to be approximately 1,000 times the observed atmospheric inventories. The CO2 fugacity determined for the buffered reaction MgCO3 plus SiO2 going to MgSiO3 plus CO2 yields values which increase from 0.0001 to 1.0 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 are in equilibrium and probably in communication with an assumed upper mantle CO2 reservoir.