基于结构体的峨眉山玄武岩风化程度评价(Ⅰ):风化结构体地球化学

The assessment of the weathering intensity of Emeishan basalt based on rock blocks(Ⅰ):Geochemistry of weathered basalt blocks

峨眉山玄武岩属典型低渗透介质,其岩体风化是通过结构体风化实现的。风化玄武岩结构体具有一层或多层腐岩壳包围核心石形成的壳状结构;从表面向内,越靠近结构体几何中心,玄武岩风化程度越低。玄武岩风化可分为初期和中后期两个阶段,第一阶段仅存在二价铁向三价铁的转变而无明显组分流失与相对富集,第二阶段二价铁氧化与活动性组分(Si、Ca、Mg、Na、K)淋失和惰性组分(Al、Ti、∑Fe)相对富集同时发生。风化初期,随着风化程度的提高,FeO和Fe2O3相对含量此消彼长,但∑Fe相对含量变化不大;风化中后期,随着风化程度的提高,SiO2、CaO、MgO、Na2O、K2O、FeO相对含量单调降低,Al2O3、TiO2、Fe2O3、LOI单调升高。玄武岩风化过程中,FeO和Fe2O3相对含量对风化程度变化最为敏感。贯穿整个风化过程的含铁矿物氧化引起的铁种相对含量变化应成为峨眉山玄武岩风化程度评价需要考虑的关键因素。

Emeishan basalt is a typical medium with low permeability. The weathering of basalt rock mass results from basalt block weathering. The weathered basalt block has a shell-like structure formed by core stone which is surrounded by one or more layers of saprolite crusts. From the block surface inward, the nearer it approaches the geometrical center of the block, the lower the degree of weathering becomes. The weathering process of the basalt can be divided into two stages：the early period and the mid-late period. Bivalent iron turns into trivalent iron without significant loss and relative enrichment of components during the first stage. Bivalent iron is oxidized continuously with the leaching of mobile components （Si, Ca, Mg, Na, K） and the relative enrichment of immobile components （Al, Ti, ΣFe） during the second stage. As the intensity of weathering increases, the relative content of FeO decreases and that of Fe2O3 increases, but there is little change in the relative content of ΣFe during the early period of weathering. With the increase of weathering degree, the relative content of SiO2, CaO, MgO, Na2O, K2O and FeO decreases monotonically while that of Al2O3, TiO2, Fe2O3 and LOI increases monotonically during the mid-late stage of weathering. During the whole weathering process of the basalt, the relative content of FeO and Fe2O3 is most sensitive to the degree of weathering. Changes in the iron species caused by oxidation of iron-bearing minerals throughout the weathering process should serve as the key factor which can be used to evaluate the weathering degree of Emeishan basalt.