铕异常在地质学中的应用  

Application of Eu anomaly in Geology

作　　者：郑方顺 宋国学 ZHENG FangShun;SONG GuoXue(College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China;Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]中国科学院大学地球与行星科学学院,北京100049 [2]中国科学院矿产资源研究重点实验室,中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2023年第9期2832-2856,共25页Acta Petrologica Sinica

基　　金：国家自然科学基金面上项目(42073040);中国科学院大学优秀青年教师科研能力提升项目联合资助.

摘　　要：Eu是地学界最受关注的变价稀土元素(Eu、Eu^(2+)和Eu^(3+))。长期以来,“Eu异常”(δEu)作为重要的地球化学参数而被广泛应用于刻画精细地质过程。一般认为,Eu^(2+)的存在及其在自然环境中的稳定性是造成“Eu异常”的主要原因,但查阅近期有关文献显示,很多关于“Eu异常”的成因解释存在模棱两可甚至相互矛盾的情形。本文在梳理“Eu异常”的定义以及计算公式的基础上,总结了不同地质过程中Eu异常的应用现状及相关的实验地球化学成果,包括岩浆演化、热液作用、沉积作用、变质作用、风化作用等。综合研究表明,造成不同地质过程中“Eu异常”差异的主要因素有:(1)母岩浆(熔/流体)的Eu含量;(2)不同矿物相的晶体化学效应(离子半径);(3)温度-压力条件;(4)氧化-还原条件(氧、硫逸度);(5)水-岩作用;(6)流体酸碱度(pH值);(7)络合作用;(8)吸附作用;(9)风化作用;以及(10)星云冷凝与挥发等。其中,“Eu异常”的定量表述和应用均涉及到岩浆、熔/流体或矿物中Eu^(2+)和Eu^(3+)的精确测定。但纵观当前应用于地球科学的众多测试手段,除X射线吸收光谱(XAS)可以原位测定Eu^(2+)、Eu^(3+)的相对含量外,尚缺乏精确测定研究对象中Eu^(2+)、Eu^(3+)含量的可行方法。由此提出,实验地球化学将是解决“Eu异常问题”的新途径,利用已知Eu^(2+)、Eu^(3+)含量的熔/流体开展重要地质过程的模拟,可有效揭示“Eu异常”变化的关键制约因素。Eu is the most concerned variable valence rare earth element(Eu,Eu^(2+)and Eu^(3+))in the geological science.For a long time,“Eu anomaly”(δEu)has been widely used as an important geochemical parameter to finely characterize geological processes.It is generally believed that the existence and stability of Eu^(2+)in natural environments are the main causes of“Eu anomaly”.However,recent literatures have shown that many explanations for the cause of“Eu anomaly”are vague and even contradictory.On the basis of the definition and calculation formula of“Eu anomaly”,this article summarizes the application status of Eu anomaly and related experimental geochemical results in different geological processes,including magma evolution,hydrothermal action,sedimentation,metamorphism and weathering.Comprehensive research shows that the main factors causing“Eu anomaly”in different geological processes are summarized as follows:(1)Eu content in parent magma(melt/fluid);(2)crystallization chemistry effects(ionic radius)of different mineral phases;(3)temperature-pressure;(4)oxidation-reduction conditions(oxygen,sulfur fugacity);(5)water-rock interaction;(6)fluid acidity and alkalinity(pH value);(7)complexation;(8)adsorption;(9)weathering;and(10)nebular condensation and volatilization,etc.Quantitative expression and application of“Eu anomaly”involves the precise determination of Eu^(2+)and Eu^(3+)in magma,melt/fluid or minerals.However,looking at many testing methods currently applied in Earth science,there is still a lack of feasible methods for accurately determining the content of Eu^(2+)and Eu^(3+)in the research objects,except for X-ray absorption spectroscopy(XAS)which can determine the relative content of Eu^(2+)and Eu^(3+)in situ.Therefore,experimental geochemistry presents a new method to solve the problem of“Eu anomaly”.Simulation of important geological processes using melt/fluid with known Eu^(2+)and Eu^(3+)contents can effectively reveal the key limiting factors of“Eu anomaly”changes.

关 键 词：铕异常 地质过程 氧化还原条件 晶体化学效应 实验地球化学 

分 类 号：P595[天文地球—地球化学] P599[天文地球—地质学]