Halogen geochemistry and isotope geochemistry in magmatic systems

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The current project represents a systematic study on halogen distribution, halogen fractionation and halogen isotope fractionation in magmatic systems, focussing on Cl and Br. The study will investigate peralkaline rocks as these are halogen-rich and contain abundant sodalite and eudialyte-group minerals (EGM), the major Cl and Br carriers in peralkaline magmatic systems. Our preliminary data indicate that (i) Cl and Br may be fractionated from each other during magmatic differentiation and (ii) the amount and timing of sodalite fractionation has a major impact on the Cl/Br evolution of residual melts. Therefore, four otherwise well-studied peralkaline complexes that contain (i) abundant orthomagmatic sodalite-EGM assemblages (Ilímaussaq), (ii) minor orthomagmatic but abundant late-magmatic sodalite-EGM assemblages (Khibina), (iii) rare late-magmatic sodalite-EGM assemblages (Tamazeght) and (iv) sodalite-free assemblages with EGM as the only major Cl-rich phase (Norra Kärr), will be investigated and compared with each other with respect to halogen abundances (F, Cl, Br, I) and halogen isotope compositions (δ37Cl and δ81Br) of sodalite and EGM.This project is a first systematic study on the behaviour of Cl (and Br) stable isotopes in magmatic systems and will investigate the suitability of these two isotope systems for deciphering magmatic and hydrothermal processes, including fractional crystallization and fluid exsolution/degassing. The combination of the halogen concentration data and the halogen isotope data from all four localities, combined with mass balance and Rayleigh-type calculations accounting for the relative effects of sodalite and EGM fractionation will also allow for constraining compositional heterogeneities in the respective mantle sources for these rocks with respect to halogen ratios and halogen isotope compositions. This will help to constrain the scales and amplitudes of mantle heterogeneities concerning the halogens, for which already evidence was presented, both with respect to halogen concentrations and Cl-isotope compositions.