Volatile content and degassing processes in the AD 79 magma chamber at Vesuvius (Italy)

The evolution of volatiles in the AD 79 magma chamber at Vesuvius (Italy) w as investigated through the study of melt inclusions (MI) in crystals of di fferent origins. FTIR spectroscopy and EMPA were used to measure H2O, CO2, S and Cl of the different melts. This allowed us to define the volatile con tent of the most evolved, phonolitic portion of the magma chamber and of th e mafic melts feeding the chamber. MI in sanidine from phonolitic and tephr i-phonolitic pumices show systematic differences in composition and volatil e content, which can be explained by resorption of the host mineral during syn-eruptive mixing. The preeruption content of phonolitic magma appears to have been dominated by H2O and Cl (respectively 6.0 to 6.5 wt% and 6700 pp m), while magma chamber refilling occurred through the repeated injection o f H2O, CO2 and S-rich tephritic magmas (respectively 3%, 1500 ppm and 1400 ppm). Strong CO2 degassing probably occurred during the decompressional pat h of mafic batches towards the magma chamber, while sulphur was probably re leased by the magma following crystallization and mixing processes. Water a nd chlorine strongly accumulated in the magma and reached their solubility limits only during the eruption. Chlorine solubility appears to have been s trongly compositionally controlled, and Cl release was inhibited by groundm ass crystallization of leucite, which shifted the composition of the residu al liquid towards higher Cl solubilities.