Fluid inclusion triple halogen (Cl-Br-I) systematics as tracers of fluid provenance in metamorphic and magmatic systems

Heavy halogens are exceptional tracers of fluid provenance due to their largely conservative behaviour during most fluid-rock interaction processes. In particular, Br/Cl ratios have long since been established as a tracer of fluid origin in certain geological settings (e.g. unconformity-related ore systems), where they have significantly contributed to our understanding of ore forming processes. However, mounting evidence suggests that similar Br/Cl signatures can, in fact, be produced by very different geological processes, significantly impacting the applicability of Br/Cl as a fluid source tracer to other crustal environments. Iodine provides an additional proxy and constraint that makes it possible to discern between these different processes and is thus crucial in utilizing the full potential of halogens as fluid provenance tracers. The generally low concentrations of iodine in crustal fluids have posed significant analytical challenges, and iodine analysis was until very recently only possible using bulk analytical techniques.

Recent advances in halogen fluid inclusion microanalysis have made it possible to determine Cl-Br-I systematics in individual fluid inclusions, providing novel opportunities for the study of complex ore forming systems with protracted and complex overprinting fluid signatures. We present a comprehensive triple-halogen fluid inclusion LA-ICP-MS dataset of various magmatic and metamorphic systems, showcasing the power of Cl-Br-I systematics in identifying and characterizing fluid sources, reconstructing processes such as fluid mixing, and the – rather surprising – temporal changes in metamorphic halogen signatures between the Archean and the Phanerozoic.