Halogens In Mafic And Intermediate-Silica Content Magmas

As volatile and variably mobile components, halogens play key roles in magmatic, metasomatic, mineralizing, and volcanic processes by influencing the physical and chemical properties of melts, fluids, and minerals. Volcanic emission of halogens to the atmosphere leads to long(-) and short-term impacts on atmospheric chemistry that range from global perturbation of the stratospheric O-3 budget to more localized life-threatening contamination of soils and fresh water. The concentrations of F, Cl, Br, and I in melts, fluids, and minerals provide crucial geochemical information and insights into magmatic processes ranging from partial melting to volcanic eruptions. Halogen research is useful for evaluating global-scale recycling processes involving the atmosphere, hydrosphere, lithosphere, and mantle. This study reviews halogens in basaltic to andesitic magmas and in their alkaline equivalents (basanites to phonolites). We examine and apply the results of hydrothermal experiments and thermodynamic modeling to the impacts of halogens on melting and crystallization behavior, exsolution of vapor and/or hydrosaline liquids, thermal stability of hydrous minerals, viscosity and diffusion in melts, and on melt-fluid(s) partitioning as they bear on volcanic degassing and the generation of halogen-bearing mineralizing fluids. A recurrent observation is that data for F and Cl in many of these magmas are available, but Br and I data are insufficient and much needed. It is also apparent that additional experiments on halogen-bearing mafic to intermediate-silica content melts and fluids are required and that thermodynamic models of magmatic and hydrothermal processes should include the role of Cl in melt-vapor-hydrosaline equilibria. We apply data on halogen behavior in magmatic systems to processes of assimilation and partial melting in magmatic seafloor environments to interpret observed glass compositions, and to processes involving halogens erupted via volcanic plumes to the atmosphere.