Variations of Cl, F, and S in Mount Etna's plume, Italy, between 1992 and 1995

Abstract

This study investigates data on the Cl, F, and S concentrations of plume‐filter samples collected between 1992 and 1995 from Mount Etna. The samples were collected during eruptive and noneruptive activity and show fairly constant Cl/F ratios but wide variation in the Cl/S ratios. The determination of the Cl/F ratio (between 5 and 14) allows an estimate of F degassing to be derived that ranges from 10 to 30% of the initial F content of the magma. The variation in the Cl/S ratio fits an empirical two‐stage degassing model where gas with high Cl/S ratio is discharged during quiescient degassing and gas with low Cl/S ratios is discharged during eruptive degassing. The typical permanent degassing occurs from a depth at which chlorine exsolution exceeds sulfur exsolution (Cl/S > 1). The opposite of this behavior is observed during eruptive periods when significant sulfur degassing occurs because magma degasses at near atmospheric pressure (Cl/S ∼ 0.1). The Cl/S ratio in the plume, characterizing the type of degassing, has the potential to be a useful geochemical tool for monitoring the volcano.