Dissolution of Fe-bearing silicate minerals and weathering processes and rates in the Precambrian

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Atmospheric oxygen concentration in the Precambrian has been intensively studied for the past decade mainly because it is closely related to the environment of the Earth's surface, especially life evolution. Paleosols, soils fowled by ancient weathering, retain the information on atmospheric oxygen at the time of weathering. Paleosols, however, were affected by later geologic events such as diagenesis and metainoiphism, and are actually metamorphic rocks. Weathering processes in the Precambrian are not well understood, and atmospheric oxygen suggested by the paleosols is not unambiguous. We carried out dissolution experiments of minerals under Precambrian atmospheric conditions, compared the results to those obtained from paleosols, and tried to clarify the mineral-water-atmosphere interactions in the Precambrian.Dissolution experiments of Fe-rich biotite were carried out in a glove box at 1 atm of carbon dioxide partial pressure and < 3x10(-5) atm of oxygen partial pressure (anoxia experiments). For comparison, similar experiments were done under modern atmospheric conditions (oxic experiments). Fe(II) is instantaneously oxidized to form Fe(III) oxides and little Fe(II) is present in the solution in the oxic experiments. In contrast, Fe(II) is present considerably and Fe(II)-bearing vermiculite is formed in the anoxia experiments. The variations of Fe(II) and Fe(III) distributions and of Fe/Mg ratio in chlorite with depth for a 2.5 Ga paleosol are well explained by the present experimental results. Other anoxia experiments of flow-through dissolution revealed that mineral dissolution rates are faster by 3 to 4 times under anoxic conditions than oxic ones and are not affected by the presence of Fe(II) in the solution.