Behavior of halogens during the degassing of felsic magmas

Residual concentrations of halogens (F, Cl, Br, I) and H2O in glass (matrix glass and melt inclusions) have been determined in a series of volcanic clasts (pumice and lava‐dome fragments) of plinian, vulcanian and lava dome‐forming eruptions. Felsic magmas from calc‐alkaline, trachytic and phonolitic systems have been investigated: Montagne Pelée and Soufrière Hills of Montserrat (Lesser Antilles), Santa Maria‐Santiaguito (Guatemala), Fogo (Azores) and Vesuvius (Italy). The behavior of halogens during shallow H2O degassing primarily depends on their incompatible character and their partitioning between melt and exsolved H2O vapor. However, variations in pre‐eruptive conditions, degassing kinetics, and syn‐eruptive melt crystallization induce large variations in the efficiency of halogen extraction. In all systems studied, Cl, Br and I are not fractionated from each other by differentiation or by degassing processes. Cl/Br/I ratios in melt remain almost constant from the magma reservoir to the surface. The ratios measured in erupted clasts are thus characteristic of pre‐eruptive magma compositions and may be used to trace deep magmatic processes. F behaves as an incompatible element and, unlike the other halogens, is never significantly extracted by degassing. Cl, Br and I are efficiently extracted from melts at high pressure by H2O‐rich fluids exsolved from magmas or during slow effusive magma degassing, but not during rapid explosive degassing. Because H2O and halogen mobility depends on their speciation, which strongly varies with pressure in both silicate melts and exsolved fluids, we suggest that the rapid pressure decrease during highly explosive eruptions prevents complete equilibrium between the diverse species of the volatiles and consequently limits their degassing. Conversely, degassing in effusive eruptions is an equilibrium process and leads to significant halogen output in volcanic plumes.