REACTIONS OF MINERALS WITH H2O-CO2 FLUID AND EVOLUTION OF THE EARLY CRUST - ATMOSPHERE SYSTEM

The composition, conditions and evolution of the early atmosphere are one of the hottest arguments on history of the Earth. Degassing theory of the early atmosphere suggests that thick and hot atmosphere of H₂O and CO₂ covers magma ocean at the final stage of the Earth formation. Partial pressures of the H₂O and CO₂ would be 20MPa and 6MPa, respectively. This composition and pressure are very close to an azeotropic critical point of the H₂O-CO₂ system. Cooling of the hot H₂O-CO₂ atmosphere brings the first precipitation of liquid phase at above 300 degrees C. The first, hot rain of the Earth should be a supercritical acid rain. Interaction between the first rain and the primitive crust involves the early evolution of the atmosphere and the crust. In this study, we discuss evolution of the early atmosphere based on the results of the alteration experiments of minerals with the H₂O-CO₂ fluid. The H₂O-CO₂ fluid easily reacts with silicate minerals at around critical point of the fluid. A mass of CO₂, which reacted with minerals to form carbonate minerals, can be measured by mass of the remaining CO₂ gas in the capsule after each run. Consumption of CO₂ increases up to approximately 40% and 80% at around 250 degrees C, for the mixture starting materials and olivine, respectively. The consumption of 80% of CO₂ means that almost all Mg and Fe in the olivine starting material react with CO₂. The Formation of carbonate minerals reduces the CO₂ composition of fluid in the capsule to almost one fifth. The fixation of CO₂ by carbonate formation should be very effective to reduce CO₂ pressure from the early atmosphere in cooling through 250 degrees C. The first sediment of the primitive ocean should contain dolomite and hydrous silicate. The CO₂ and H₂O fixed in the first sediment should take an important role in the evolution of the early crust.