Near surface halogen (F, Cl, Br, I) cycle

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Halogens are omnipresent in nature; the heavier halogens (Cl, Br and I) are commonly regarded as inert tracers in fluid-involving processes in soils and upper crustal rocks. There is, however, increasing evidence that this may not be true and that the halogens record various fluid sources, reactions and precipitation processes in low-temperature environments. These processes are recorded by pyromorphite-group minerals (PyGM), which form by the supergene weathering of Pb ores (e. g., galena) incorporating halogens at the wt.% level. These minerals still form today during near-surface fluid-mediated processes and record the halogen characteristics (e.g., F/Cl, Br/Cl, I/C ratios) of near surface fluids that are involved in their formation at ambient conditions.During the first funding period of the project, the halogen incorporation into PyGM, the behavior of halogens (F, Cl, Br, I) in different depth layers of two soil profiles and in surface waters (precipitation, soil solutions, creek waters and adit waters) was investigated. Furthermore, since adsorption has a major influence on the near surface halogen cycle, we already started to investigate adsorption processes of halogens on different clay mineral fractions. To finish up this project we aim to characterize halogen partitioning between PyGM and fluids of different composition. It is therefore planned 1. to perform a series of experiments and to determine halogen concentrations in freshly precipitated PyGM crystals and in the corresponding fluid from which the crystals precipitated and 2. to finish the determination of halogen concentrations in separated clay fractions from the soil profiles. With this comprehensive data set we will be able to develop a detailed model for halogen distribution, mobilization and important retention processes close to the surface, with a possible application of PyGM as low-temperature fluid monitor.