The relationship between light (F, Cl) and heavy halogens (Br, I) during the magmatic-hydrothermal transition in silicic magmatic systems

• Applicant: Professorin Dr. Juliana Troch
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 565703372

Silicic magmatic systems play an important role in the generation of continental crust, the generation of up-to-super-sized volcanic eruptions and the formation of granite-related ore deposits. In these systems, halogen accumulation is essential for transporting metals from melt-dominated to fluid-dominated environments at the magmatic-hydrothermal transition. For example, chlorine (Cl) influences the fluid-melt partitioning behavior of many metals, and fluorine (F) increases melt fractions and water solubility in the melt, thereby controlling the temperature threshold at which fluid exsolution occurs. Because of their unreactive behavior during fluid-rock interaction, the heavy halogens bromine (Br) and iodine (I) have garnered attention as fingerprints for reconstructing crustal fluid sources. However, despite their potential as geochemical tracers, their wide-spread application to natural systems remains limited. This lack in data is caused both by the analytical challenges associated with in situ analysis of low concentrations of multiple halogens, and our limited understanding of the processes controlling halogen ratios. If Cl, Br, and I contents are primarily controlled by fractional crystallization, fluid exsolution and later phase separation, Br/Cl and I/Cl ratios should remain unaffected, as long as partitioning between fluid, melt and bulk mineral assemblage remains constant. However, the variations in Br/Cl and I/Cl in natural systems suggests that additional processes fractionate heavy from light halogens. To address the missing link between light and heavy halogens, this PhD project links experimental data on the influence of F on the granitic solidus to the compositional evolution of halogens during crystallization of a natural Li-rich pegmatite system. The project integrates in situ analysis with fieldwork and experiments in three independent work packages: (1) Optimization of fluid inclusion LA-ICPMS analysis through the synthesis of a triple-halogen (Cl, I, Br) standard reference glass, (2) assessment of the effect of F on the solidus of water-saturated rhyolite through crystallization and melting experiments, and (3) characterization of halogens during differentiation of a zoned Li-pegmatite system (Rubicon in Namibia, one of Africa’s largest Li projects). The proposed research represents a step towards quantifying the complex interactions between multiple volatile components (halogens, Li) and towards identifying the optimal compositional and temperature conditions for Li-mineral precipitation in silicic systems. These advances will improve our understanding of the behavior of halogens as a function of crystallization, fluid exsolution and chemical disequilibrium. This link is essential for future application of halogen fingerprinting to other granite-related ore deposits, Sn-W greisen, and pegmatites in order to identify the locus and timing of the magmatic-hydrothermal transition in silicic systems.

• DFG Programme: Research Grants
• Co-Investigators: Dr. Tobias Fusswinkel; Professor Dr. Thomas Wagner