WEATHERING IN HADEAN AND ITS ROLE IN MATERIAL CYCLE

Atmosphere-ocean chemistry of Hadean (> 4Ga) is an important factor in order to discuss birth environment of life. Among diverse processes, which control the composition of the Hadean atmosphere and ocean, we focus on weathering and material circulation.

In the Phanerozoic material-circulation system known as the Urey cycle, alkali and bicarbonate ions eluted from the rock are transported to the ocean, then carbonate mineral precipitates in the weak-alkaline ocean. Thus, CO₂ in the atmosphere is fixed to the carbonate mineral. Although less information about the surface environment of the Hadean is known, it is expected that the following differences exist in Hadean compared with Phanerozoic; 1) very high atmospheric CO₂ concentration, 2) low pH of the ocean, and 3) no bioprocess is interposed weathering. In addition, we have less information about amount and composition (from TTG to anorthosite) of the Hadean landmass.

Due to the differences above, the weathering and material circulation model for Phanerozoic is not possible to directly apply to the Hadean. Hence, we employed the thermodynamics of the solution and the rock to extrapolate the Urey cycle into the Hadean environment, and discussed the weathering, temperature, pCO₂, and the temporal change in the atmosphere-ocean composition. The result shows that efficiency of the weathering in Hadean was lower than Phanerozoic because of saturation of CO₂ in the rain and no biologic acceleration. In addition, the acidic ocean prevented the precipitation of carbonate minerals, therefore the Urey reaction no longer had a role of CO₂ fixation. Therefore, the ocean floor hydrothermal alteration in mid-ocean ridge had an important role in modification of ocean composition in the Hadean.