Tracing Crustal Fluids: Applications of Natural ¹²⁹I and ³⁶Cl

The two cosmogenic isotopes ¹²⁹I and ³⁶Cl have half-lives and geochemical characteristics that make their application of interest in the tracing of crustal fluids, oil field brines, and geothermal fluids. The focus of this review is to compare ¹²⁹I data from volcanic fluids with those from mud volcanoes and gas hydrate locations associated with the same subduction zone in order to demonstrate that fundamentally different hydrologic systems are present in active margins. Whereas ¹²⁹I/I ratios in volcanic fluids are site dependent and show a relation to the ages of subducting marine sediments, ratios in fore arc fluids are similar in all sites investigated and are independent of the ages of the host sediments and the age of the subducting slab. Volcanic fluids contain iodine transported in sediments from the trench to the main volcanic zone, whereas iodine in fore arc fluids is derived from organic material stored in the upper plates of subduction zones.