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

前寒武纪大气中的氧浓度在过去的十年里得到了广泛的研究，主要是因为它与地球表面的环境，特别是与生命的演化密切相关。古土壤是经过古代风化作用形成的土壤，它保留了风化过程中大气氧气的信息。然而，古土壤受到成岩作用和变质作用等后期地质事件的影响，实际上是变质岩。人们对前寒武纪的风化过程还没有很好的了解，古土壤所显示的大气氧气也不是明确的。我们在前寒武纪大气条件下进行了矿物溶解实验，并与古土壤实验结果进行了比较，试图阐明前寒武纪矿物-水-大气相互作用。富铁黑云母在手套箱中，二氧化碳分压为1 atm，氧分压为-lt；3×10(-5)atm(缺氧实验)。为了进行比较，在现代大气条件下进行了类似的实验(氧气实验)。在氧化实验中，Fe(II)瞬间被氧化成Fe(III)氧化物，溶液中几乎不存在Fe(II)。相反，在缺氧实验中，Fe(II)大量存在，并形成了含Fe(II)的蛭石。实验结果很好地解释了2.5Ga古土壤中绿泥石中Fe(II)和Fe(III)分布以及Fe/Mg比值随深度的变化。其他流动溶解的缺氧实验表明，矿物在缺氧条件下的溶解速度比在有氧条件下快3~4倍，且不受溶液中Fe(II)的影响。

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.