基于结构体的峨眉山玄武岩风化程度评价(Ⅳ):风化指数FF

The assessment of the weathering intensity of Emeishan basalt based on rock blocks(Ⅳ):A proposed weathering index(FF)

岩石化学风化程度评价指标应该同时满足以下三个基本条件,即与风化程度之间的关系简单明确、对风化程度变化具有足够的敏感性和易于确定和不易受到人为因素影响。作为铁镁质岩石的主要代表,(峨眉山)玄武岩风化程度评价除考虑可引起组分淋失与富集的水解反应外,还应充分考虑二价铁的氧化反应。新鲜峨眉山玄武岩中并存的二价铁和三价铁的含量总体稳定,910个样品的FeO、Fe2O3平均含量分别为8.45%和5.15%,以均匀、随机的方式赋存于辉石、橄榄石、杏仁体中的绿泥石、磁铁矿及火山玻璃中。风化玄武岩、玄武岩斜坡地下水及新鲜玄武岩浸泡液的地球化学研究结果表明,FeO及Fe2O3含量对峨眉山玄武岩风化程度的敏感性明显高于其他组分,同时铁又是玄武岩风化过程中活动性最差的元素之一。三价铁和二价铁的摩尔数比值(FF)适合于峨眉山玄武岩整个风化过程的风化程度判别,比既有风化指数具有更高的分辨率,尤其是对风化初期玄武岩。

The index used to evaluate chemical weathering degree of rock should have simple, specific relationship with the weathering degree and sufficient sensitivity to rock weathering. In addition, it should be easily determined and should not be subject to human factors. Emeishan basalt is a typical representative of mafic rocks. Besides the hydrolysis reaction that may cause leaching and enrichment of components, the oxidation of divalent iron should also be taken into account in the evaluating process for the weathering intensity of Emeishan basalt. Bivalent and trivalent iron content in fresh Emeishan basalts is stable in general. Average values of FeO and Fe203 of 910 samples are 8.45% and 5.15% respectively. They occur uniformly and randomly in pyroxene, olivine, chlorite in almond-shaped structure, magnetite, titanium magnetite and volcanic glass. The geochemistry and hydrogeochemistry of the weathered basalt, the groundwater on a basalt slope and the soaking solution of the fresh basalt show that FeO and Fe2O3 are more sensitive to the weathering degree of Emeishan basalt than other components, while iron is one of the most immobile major elements in basalt weathering process. Trivalent iron to divalent iron molar ratio （FF） is suitable for discriminating the weathering degree in the whole weathering process of Emeishan basalt and has higher resolution than the existing weathering indices, especially in the early weathering stage of basalt.