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

风化岩石具有与地球表面的矿物质和水相互作用的大气信息。这种相互作用发生在前寒武纪到现在。如果大气成分在前寒武纪不同，矿物学和化学上不同的次生矿物形成。然而，关于前寒武纪大气的信息是模糊的，因为前寒武纪的风化岩石受到后期地质事件，如成岩作用。除了传统的岩石整体分析外，我们还分析了前寒武纪风化岩石中的矿物，从微米到纳米级，以阐明前寒武纪矿物-水-大气的相互作用，并估计大气中的氧气和二氧化碳。主要结果和结论如下：（1）发现了富Ce（3+）的红柱石，其形成于2.5Ga的花岗岩风化过程中。结果表明，Ce（3+）在前寒武纪地下水中的行为类似于La和Nd。这表明，2.5 Ga风化发生在缺氧条件下。(2)2.5Ga风化剖面中Fe（2+）和Fe（3+）的变化表明，含铁矿物释放的Fe（2+）流出风化剖面。这也表明，2.5 Ga风化发生在缺氧条件下。(3)我们的实验表明，蛭石是在缺氧条件下形成的黑云母风化过程中的铁含量无关，并在埋藏条件下转化为黑云母。(4)在2.5 Ga风化剖面中的绿泥石的显微结构表明，在后期的成岩作用中，绿泥石被风化成蛭石，然后转化为黑云母。

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.