Mineral-water-atmosphere interactions in the Precambrian : Estimation of atmospheric evolution

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Weathered rocks have information on the atmosphere that is interacted with mineral and water in the Earth's surface. The interaction has occurred through the Precambrian to the present time. If the atmospheric composition was different in the Precambrian, mineralogically and chemically different secondary minerals were formed. However, the information on the Precambrian atmosphere is ambiguous because the weathered rocks in the Precambrian were subjected to later geological events such as diagenesis. In addition to traditional bulk analysis of rocks, we have analyzed minerals in the Precambrian weathered rocks in the scale of micro- to nanometer to elucidate the mineral-water-atmosphere interactions in the Precambrian and to estimate atmospheric oxygen and carbon dioxide. Our main results and conclusions are as follows :(1) We found Ce(3+)-rich rabdophane that was formed during 2.5 Ga weathering of granite. The finding indicates that Ce(3+) behaved like La and Nd in the Precambrian ground water. This suggests that 2.5 Ga weathering occurred under an anoxic condition. (2) The variations of Fe(2+) and Fe(3+) in the weathering profile formed 2.5 Ga indicated that Fe(2+) released from Fe-bearing minerals flowed out of the weathering profile. This also suggests that 2.5 Ga weathering occurred under an anoxic condition. (3) We experimentally showed that vermiculite is formed under an anoxic condition during biotite weathering independent of Fe content, and that converted to biotite under burial conditions. (4) The microstructures of chlorites in the 2.5 Ga weathering profile indicated that chlorite was weathered to vermiculite that was then converted to biotite during later diagenesis.