川北韩家店组页岩风化过程的矿物学与元素地球化学研究

Mineral and Element Geochemical Characteristics of Shale Weathering in Hanchiatien Formation,North Sichuan

风化作为一种重要的地表作用,能造成岩石崩解或分解,并使岩石在结构或成分上不同于基岩。为研究页岩风化过程中矿物转变机制及主微量元素变化规律,以四川北部江油地区韩家店组页岩风化剖面为实例,采用薄片鉴定、全岩X射线衍射、X射线荧光光谱和等离子体质谱等方法,对不同风化层及基岩层的矿物成分及主微量元素进行研究。结果表明,与基岩相比,风化层中方解石和长石等矿物占比降低,石膏和黏土矿物等次生矿物生成;根据元素地球化学质量迁移系数计算得出:与基岩相比,风化层中Mg、Ca、Fe、Mn、Sr和U等元素明显亏损,Pb、Zn、Cu、Rb和Cs等元素富集。风化层中稀土元素含量增加,可能是风化生成的黏土矿物吸附稀土元素导致的。根据化学蚀变指数(CIA=69.51~76.21)判断该剖面属于中等风化程度,A-CN-K图解显示剖面处于脱Ca和Na早期风化阶段。根据垂向剖面上Si/Al、Ca/Al、Mg/Al、Na/Al、K/Al和P/Al等元素比值变化规律及样品中元素Pearson相关分析得出,方解石和磷灰石等矿物发生了不同程度的分解,次生矿物铁锰氧化物和伊利石等生成。因此,风化剖面矿物转变及元素活动耦合性研究对于揭示页岩风化过程及机制具有重要意义。

Weathering is an important surface process that can cause the disintegration or decomposition of rocks that differ from unweathered rocks in structure or composition.To study the mechanism of mineral transition and the variation of major and trace elements in the weathering process of shale,the field weathering profile of the Silurian Hanchiatien Formation shale in northern Sichuan,thin section identification,whole-rock Xray diffraction,X-ray fluorescence spectroscopy,inductively coupled plasma mass spectrometry,and other methods were used.The mineral composition and major and trace elements of different degrees of weathering of the rock samples were studied.The results show that the calcite and feldspar contents in the weathered samples noticeably decreased compared with the unweathered samples.Secondary minerals such as gypsum and clay were formed.According to the mass transfer coefficients of major and trace elements,it was concluded that Mg,Ca,Fe,Mn,Sr,and U were depleted.At the same time,Pb,Zn,Cu,Rb,and Cs were enriched simultaneously due to the weathering process.The content of rare earth elements increased in the regolith and resulted from absorption by clay minerals formed during the weathering process.According to the chemical alteration index(CIA=69.51~76.21),it can be judged that the weathering profile belongs to a moderate weathering degree.The A-CN-K diagram shows that the profile has reached the weak-to-moderate chemical weathering stage,the initial Ca and Na removal stage.According to the ratio variation diagram of Si/Al,Ca/Al,Mg/Al,Na/Al,K/Al,and P/Al in the vertical section and the element Pearson correlation analysis of samples,it can be found that calcite,apatite,and other minerals have undergone different degrees of decomposition.Secondary minerals such as iron-manganese oxides and illite are formed,and the elements occurring in the minerals also migrate.Therefore,the study of the coupling of mineral transition and element activity in weathering profiles is of great significance for revealing the weathering process and mechanism of shale.