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 δ ³⁷Cl. 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 δ ³⁷Cl values were observed in pore water in previous research and were generally interpreted as the result of kinetic fractionation. Large variation of δ ³⁷Cl 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 δ ³⁷Cl 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