地学前缘 ›› 2020, Vol. 27 ›› Issue (3): 42-67.DOI: 10.13745/j.esf.sf.2020.5.56

• “非传统稳定同位素:分析方法、示踪机理和主要应用”主题专辑 • 上一篇    下一篇

氯同位素地球化学研究进展

周秋石,王瑞   

  1. 中国地质大学(北京) 科学研究院 地质过程与矿产资源国家重点实验室, 北京 100083
  • 收稿日期:2020-04-14 修回日期:2020-04-19 出版日期:2020-05-20 发布日期:2020-05-20
  • 作者简介:周秋石(1999—),男,本科生,研究方向为矿床地球化学。E-mail:zhouqiushi121@gmail.com
  • 基金资助:

    国家自然科学基金面上项目(41973037);教育部、国家外国专家高等学校学科创新引智基地项目(B18048);中组部第十四批千人计划青年项目

 Advances in chlorine isotope geochemistry

ZHOU Qiushi,WANG Rui   

  1. State Key Laboratory of Geological Processes and Mineral Resources, Institute of Earth Sciences, China University of Geosciences(Beijing), Beijing 100083, China
  • Received:2020-04-14 Revised:2020-04-19 Online:2020-05-20 Published:2020-05-20

摘要:

作为最具代表性的、地球丰度最高的卤素,氯元素因其独特的性质和各大储库中的可观分布而受到重视。氯元素具有显著的亲硫性和挥发性(不相容性),这些性质也影响着其地球化学行为以及在地球中的分布。氯含有两种稳定同位素,分别是35Cl和37Cl,同位素丰度分别为75.76%和24.24%,Cl同位素组成也常以δ37Cl值报道;目前对Cl同位素进行分析的最为传统也是最主要的方法是IRMS(同位素比值质谱法),尽管存在分析速度慢、对样品质量要求大等缺陷,但因其相对更高的精度而被广泛应用于现代氯稳定同位素研究。其他常见的分析方法还有TIMS,SIMS,LA-ICP-MS,在Cl同位素研究领域尚在实验探索阶段。Cl同位素标准在目前也已有广泛接受的统一国际标准,即Kaufmann所提出的平均海洋Cl同位素标准(SMOC),海水Cl同位素组成在长久演化中已经相对稳定,取样也简单方便,在实验室分析中也能产生优良的再现性。氯在地球上的储库可在宏观上分为地幔、陆壳与洋壳、海洋。地幔以其较大的体量无疑是氯元素主要储库之一,但受限于现有观测技术,各研究通过各种方法观测到的氯含量有较大出入;关于地幔的Cl同位素分布也并不明确,各种如俯冲带流体输入等深部过程都可能是同位素异质性的原因,许多研究观测到的地幔流体的Cl同位素有正也有负。陆壳可下分为沉积物及其孔隙水、蒸发岩和硅质岩石圈,氯因其亲水性和化学沉积性质则主要集中于沉积物孔隙水和蒸发岩中,而硅质岩石圈中的氯则相对较少;沉积物孔隙水常被观测到极低的δ37Cl值,扩散和离子淋滤都是对其可能的解释,但对其完全做出解释的机理还有待继续研究;蒸发岩中的δ37Cl值常因其氯盐种类而有所不同,也已有诸多实验给出制约;硅质岩石圈虽含氯较少,但出露于地表的岩株或岩脉中观测到的磷灰石中的Cl同位素有明显的随岩性变化的规律性,对于热液交代历史与热液成分都有很好的限制作用。洋壳中的氯也主要分布在洋壳沉积物及其孔隙水、蒸发岩,以及蚀变洋壳(以角闪石、蛇纹石等蚀变矿物为代表)中。蚀变矿物常被观察到较高的δ37Cl值,一些研究认为可能和与氯结合的金属阳离子氧化状态有关,该高δ37Cl值也常被用来解释俯冲板片输入地幔流体的高δ37Cl值。海洋也是巨大的氯元素储库,其同位素组成稳定性也已经得到证明。除以上储库以外,Cl同位素还应用于对陨石、月球和大气圈的研究。Cl同位素的分馏机制也同其他许多同位素一样可大体分为平衡分馏和动力学分馏,平衡分馏主要包括由与氯结合的金属阳离子的价态高低或氯自身价态的高低决定的分馏,动力学分馏则包括扩散和离子淋滤以及与地球岩浆去气、月球岩浆去气有关的动力学分馏。总体而言,氯元素及与其结合的金属离子的氧化状态、轻重同位素逃逸倾向、扩散系数的差异是决定分馏行为的两种机理。从氯的两种稳定同位素自大约100年前被发现起到现在,与氯稳定同位素相关的地球化学方法已经在各地质学科分支中得到广泛应用。从利用Cl同位素解释地层水来源的水文地质、示踪污染物来源的环境地质,到利用Cl同位素证据支撑矿床形成机制研究的矿床学以及参与解释地球演化过程的行星地质等领域,都有Cl同位素应用研究的尝试与探索。但受限于对实际地质过程的认知,仍然具有许多前沿问题阻碍Cl同位素研究的进展,如太阳系早期过程如何决定了地球现今的氯元素含量、地球中的挥发分经历了怎样的演化过程才达到至今状态、现今地球挥发分的循环过程又具体如何、俯冲过程中氯在俯冲带与地幔之间的流通过程具体如何等。

关键词: Cl同位素, 分析方法, 分馏机理, 地质应用, 磷灰石

Abstract:

 As the most representative and the most abundant halogen on the earth, chlorine has gained high attention due to its significant properties and remarkable distribution among reservoirs. Chlorines chalcophile and volatile (incompatible) nature are significant and affect its geochemical behavior and distribution. 35Cl and 37Cl are the two stable isotopes of chlorine, their isotope abundances are respectively 75.76% and 24.24%. The stable isotope composition is reported as δ37Cl. The commonest and the most traditional analytical method in modern chlorine isotope research is IRMS, it is highlighted by better precision compared to other analytical methods, though there are some defects such as the demand for a large sample mass and slow processing. Other analytical methods include TIMS, SIMS, LA-ICP-MS, their employment in Cl isotope analysis is still under development and not mature enough for geological application. A chlorine isotope standard commonly accepted by worldwide researchers is present and known as the Standard Mean Ocean Chloride (SMOC) proposed by Kaufmann. Its outstanding advantages include handy to collect, stable in analysis, and excellent in reproducibility. On the macroscale, the major reservoirs of chlorine on the earth can be divided into the mantle, continental crust, oceanic crust, and oceans. The mantle is undoubtedly a major reservoir due to its volume, yet we are still unable to acquire a clear result of the exact chlorine concentration constrained by our limited approaches to investigate it. The same situation applies to the Cl isotope composition of the mantle as well, in which all kinds of processes may bring about changes to Cl isotope composition. Continental crust can be further divided into sediments and its pore water, evaporites, and silicate lithosphere. Many low δ37Cl values were observed in pore water in previous research and were generally interpreted as the result of kinetic fractionation. Large variation of δ37Cl exists among evaporites depending on the different kinds of chloride species; although silicate lithosphere is light in chlorine contents, an observation is made suggesting the apatite Cl isotope composition varies as a function of the host rock lithology, which may be a good indicator of hydrothermal fluid activities. The oceanic crust can also be further divided into sediments and pore water, evaporite, and additionally altered oceanic crust (AOC). The altered hydrous minerals (amphibole, serpentine, etc.) are featured by a heavy δ37Cl value and may have something to do with the oxidation states of metal cations. The oceans are together a massive chlorine reservoir as well, the stability of their Cl isotope composition has already been approved by many studies. In addition to all the mentioned reservoirs, Cl isotope analysis is applied to extraterrestrial samples such as meteorites, lunar rocks, and atmosphere as well. Generally, the mechanisms of Cl fractionation can be divided into equilibrium fractionation and kinetic fractionation. Factors controlling equilibrium fractionation include the valence state of chlorine itself, the metal cation it bonds with, and the difference of chloride species in a solid-aqueous phase equilibrium system. Processes of kinetic fractionation include diffusion, ion filtration, and activities in the magma degassing system. Ever since the two stable isotopes of chlorine got discovered about 100 years ago, geochemical methods related to chlorine isotopes have been applied to all branches of geology. For example, chlorine isotopes can be used to trace the sources of formation water in hydrogeology, or contaminants in environmental geology; chlorine isotopes can also be used to track deposit formation in economic geology or interpret the evolution process of the earth in planetary geology. Attempts and explorations are continuously conducted using chlorine isotopes. However, constrained by our knowledge of the actual geological processes, there are still a bunch of frontier problems hindering the development of chlorine isotope research. For example, what is the key factor in early solar system processes determining the current chlorine content of the earth; what kind of evolution history caused the current volatile distribution in the earth; what is going on with respect of the volatile cycling; what is the flux process between the subducting plate and mantle and so on.

Key words: chlorine isotope, analytical method, fractionation mechanism, geological application, apatite

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