REACTIONS GOVERNING THE CHEMISTRY OF CRATER FUMAROLES FROM VULCANO-ISLAND, ITALY, AND IMPLICATIONS FOR VOLCANIC SURVEILLANCE

More than 200 chemical and isotope analyses of fumarolic fluids collec ted at the Fossa Grande crater, Vulcano Island, during the 1980s show that the main process controlling these fluids is mixing between the g as released by a magma body and the vapour produced through evaporatio n of brines of marine origin. Large variations in the relative contrib ution of these two sources have been observed during the last 10 a. Th e main species (H2O and CO2), the inert gases (He and N2), and the D c ontent of steam are fixed by the mixing processes; they are therefore the best tracers of the fraction of the deep magmatic component in the fumarolic fluids discharged at the surface. In contrast, the ''fast'' species (H-2 and CO) equilibrate at T,P values close to the outlet te mperature and atmospheric pressure, and under redox conditions governe d by the SO2-H2S buffer, as indicated by thermodynamic calculations. A cid gases (HCl, HF, H2S and SO2) are partly contributed by the magmati c component and partly produced by the reactions between hot rocks, st eam and salts which take place in the ''dry'' zones surrounding the ce ntral magmatic gas column, as suggested by the good agreement between their analytical and theoretical contents.