Chlorine Isotope Geochemistry

Chlorine played a prominent role in the discovery of isotopes. The famous Cavendish Laboratory scientists were fascinated with the atomic mass of Cl. Most elements have a mass that is a close approximation of the multiple of hydrogen (e.g., Aston 1927). By 1920, it was recognized that the atomic weight of Cl was ~35.5, which appeared to violate Francis Aston’s whole number rule. Sir Joseph J. Thomson started the famous “Discussion on Isotopes” (Thomson et al. 1921) with the following: “I will plunge at once into the most dramatic case of the isotopes—the case of chlorine”. The discussion that followed between three Nobel Prize winners pitted Thomson against Aston and Frederick Soddy, the latter two in defense of multiple isotopes of a single element. And so the game began. Aston (1919, 1920) argued that the mass spectra of Cl-bearing compounds (e.g., HCl, COCl) supported the existence of at least two isotopes of Cl, 35Cl and 37Cl. However, Thomson contended that the spectra may be the result of different compounds of Cl and not necessarily different isotopes of Cl (Thomson et al. 1921). Ultimately, Aston was proven correct (e.g., Harkins and Hayes 1921; Harkins and Liggett 1923) and is now credited with the discovery of the two stable isotopes of Cl, which is notable for the unusually large abundance of its “rare” isotope. The relative abundances of 35Cl and 37Cl are currently accepted to be 75.76% and 24.24%, respectively (Berglund and Wieser 2011). It was not until ~75 years after the discovery of the stable isotopes of Cl that they become more “routinely” analyzed and the chlorine isotope compositions of various chlorine reservoirs were beginning to be determined. Here we summarize the current state of chlorine isotope standards, analytical methods, and fractionation, as well …