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

前寒武纪含铁硅酸盐矿物的溶解及风化过程和速率

基本信息

批准号：
14340159
负责人：
MURAKAMI Takashi
金额：
$ 10.75万
依托单位：
The University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2004
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-14340159/
关键词：
Precombrian Atmospheric evolution Paleosol Weathering Mineral-water-atmosphere interaction Dissolution rate 黒雲母

项目摘要

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.

前寒武纪大气氧浓度与地球表面环境，特别是生命演化密切相关，因此近十年来一直是研究的热点。古土壤是由古代风化作用形成的土壤，它保留了风化作用时大气中氧的信息。然而，古土壤受到后期地质事件的影响，如成岩作用和变质作用，实际上是变质岩。前寒武纪的风化过程还没有被很好地理解，古土壤所暗示的大气氧也不明确。在前寒武纪大气条件下进行了矿物溶解实验，并将实验结果与古土壤的结果进行了比较，试图阐明前寒武纪矿物-水-大气的相互作用。富铁黑云母的溶解实验在手套箱中进行，二氧化碳分压为1 atm，氧分压<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（有氧实验）。Fe（II）瞬间氧化形成Fe（III）氧化物，并且在好氧实验中溶液中存在少量Fe（II）。与此相反，Fe（II）的存在相当大，并在缺氧实验中形成含Fe（II）的蛭石。实验结果很好地解释了2.5Ga古土壤中Fe（II）和Fe（III）的分布以及Fe/Mg比值随深度的变化。其他缺氧实验的流通溶解表明，矿物溶解速率快3至4倍，在缺氧条件下比好氧的，不受Fe（II）的存在下，在溶液中。