Halogens (F, Cl, Br, I) are important volatile components in magmas in addition to H₂O, CO₂ and S. Their extremely high chemical activity of halogens in melts and liquids leads to a significant influence on (a) magmatic properties, (b) the degassing of magma, (c) the extraction, transport and deposition of metals, (d) the chemistry of volcanic emissions and last but not least (e) the composition of the atmosphere.

Since halogens can be inherited from different sources (mantle, crust, sediments, etc.) and by different processes (melting, assimilation, mixing) during magma genesis, their geochemical behaviour can be used as a key indicator of genetic conditions and evolution of the magma. While chlorine (and partly fluorine) is routinely measured in different magmatic phases including volcanic gases, the data for other halogens is quite scarce, unsystematic and inconsistent. However, also chlorine and fluorine are rarely measured in parallel including all different compartments (volcanic gas plume, melt inclusions, glasses). Thus, our understanding on how halogen concentrations, speciation and ratios are linked to magmatic processes and conditions remains elusive.

Therefore, several key questions still remain unsolved and require further intensive investigations to understand the interplay between volcanic eruptions, the related halogen load of the atmosphere and environment and the consequential hazards.

This presentation will start with an introduction of the state of the art knowledge on halogen degassing. It then will focus on recent field investigations on volcanic plumes with a special focus on Etna, Italy and discuss open questions rather than answers pointing out possible future research directions.