Halogen concentrations and stable Cl isotopes in apatite as a fluid probe: mapping regional-scale fluid pulses by Cl-isotopes

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Fluid-rock interaction causes element mobilisation and fractionation. It is related to the formation of economic mineral deposits and has consequences for the petrophysical properties of the affected rocks and the geodynamics of the related large-scale systems. One fundamental question associated with fluid-rock interaction is how fluids flow in lowpermeability crystalline rocks – how are fluids channelled and what is the geochemical signature of fluid flow on a regional scale? To better understand flow pattern formation and reaction front propagation, we need chemical tracers for the fluid (e.g., halogens – ratios and stable Cl isotopes) and fluid-composition indicative minerals that can be used as fluid probes. Apatite occurs within all kinds of rock types and is a typical accessory vein-mineral found in hydrothermal systems over a wide range of temperatures. Apatite reacts rather sensitively to changes of fluid compositions by dissolution-reprecipitation, making it a reasonable candidate as a fluid probe. Another advantage of apatite is that, unlike many other minerals, it routinely incorporates measurable concentrations of halogens. This project aims to develop an apatite-based fluid probe by applying field data combined with experimentally derived partition coefficients and isotopic fractionation data. Finally, textural controlled in situ measurements of halogen concentrations and the stable Cl isotopic compositions of apatites will allow to decipher quantitatively the chemical evolution of a reacting fluid, to detect distinct fluid pulses, the main fluid pathways, and the direction and the extent of fluid flow in a combination with data about the metasomatic mass transfer. This could be a powerful tool for understanding fluid flow systematics in crystalline rocks as well as for the research on the formation of economic mineral deposits.