Evolution of High Temperature Early Atmosphere Under the Interaction of H2O-CO2 Super-critical Fluid With Minerals

The evolution of atmosphere-lithosphere system of the early Earth is controlled by mutual interaction of high temperature atmosphere with rocks and minerals. It is assumed that the total pressure of the early atmosphere and the surface temperature above initial magma ocean are 26MPa (H₂O 20MPa, CO₂ 6MPa) and 130-330° C, respectively. This composition, temperature 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° C. During the early period, hot rain of the Earth should be a supercritical acid rain. Cooling rate of the hot atmosphere is regulated by energy transportation capacity among the surface, atmosphere and radiation of the early Earth. In this study, we discuss evolution of the early atmosphere-lithosphere system based on the results of the alteration experiments of minerals simulated early crust with the H₂O-CO₂ fluid and the cooling rate estimation of the high temperature atmosphere. The H₂O-CO₂ fluid easily reacts with silicate minerals at around critical point of the fluid to produce carbonate and hydrous minerals. Consumption of CO₂ increases up to approximately 80% at around 250° C for olivine starting material. This means that most of 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 approximately 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° 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. Composition of the early atmosphere, or partial pressures of CO₂ and H₂O and temperature gradient of the atmosphere are essential factors controlling cooling history of the high temperature early atmosphere. We discuss evolution of the early atmosphere including effects of precipitation of super-critical H₂O-CO₂ fluid and atmosphere-rock interaction.