δ³⁷Cl: the geochemistry of chlorine isotopes

Abstract

In this thesis the geochemistry of the stable isotopes of chlorine will be examined. Chlorine is one of the halogens, the seventh group in the periodic system of elements. This group consists of five elements, fluorine, chlorine, bromine, iodine, and astatine. This thesis presents the first chlorine isotope measurements on carbonatites. Significant variations in δ³⁷Cl were observed, but since the samples came from different geological settings it is difficult to interpret the results in terms of fractionating processes. A correlation between chlorine, oxygen and carbon isotopes is suggested. If fractionation of chlorine isotopes is caused by the same processes as the fractionation of oxygen and carbon isotopes, then exchange with the country-rocks or Reyleigh fractionation may be important. The data suggest that δ³⁷Cl in primary carbonatites can be between -1 and 0‰. If these data represent mantle values, which is not proven, this is in agreement with data of volcanic water samples from Indonesia which indicate that chloride escaped from the mantle is enriched in ³⁷Cl, resulting in negative δ³⁷Cl values in the mantle. The results invite more research. It is clear that further research must be combined with an effort to understand the chloride geochemistry of carbonatites, since almost nothing is known on this subject. It is also important to obtain good fractionation knowledge of the HCI degassing system. It will be important to measure both the HN0₃-extractable and the total chloride.