Chemical and isotopic variations in fumarolic discharge and thermal watersat Vulcano Island (Aeolian Islands, Italy) during 1996: evidence of resumed volcanic activity

Gas samples from some fumaroles at 'La Fossa' crater and Baia di Levante on Vulcano Island and from a diffuse soil gas emission were analysed during 1 995-1996, along with water samples from thermal wells in the area of Vulcan o Porto. During 1996, we observed a significant increase both in the gas/st eam ratio and in the CO2 concentration, as well as strong variations in del ta(13)C(CO2), delta D-H2O and delta(18)OH(2)O of fumarolic gases. These var iations are probably related to an increased inflow of deep fluids of magma tic origin. The temperatures of fumaroles did not show remarkable variation s except for fumarole F11. In this case, temperature increased by about 80 degrees C from February to August 1996. During the same period, remarkable variations in temperature, phreatic level and chemical and isotopic composi tion of water were also recorded in one of the geothermal wells in the Vulc ano Porto area (Camping Sicilia; T similar to 60 degrees C). The observed v ariations in this well are probably related to a pressure build-up, occurri ng at least in the surficial part of the system, because of increased gas f lux and/or decreased permeability of the fumarolic degassing system. Chemic al and isotopic composition of the water showed that during this evolutiona ry phase, the content of fumarolic condensate in this well was about 80 to 90%. Based on the observation of physical and chemical variables of the Cam ping Sicilia fluids, during this phase of activity, it is concluded that th is area is affected by a phreatic eruption hazard if a volcanic episode wit h high energy discharge in a limited time span occurs. It follows that this well may be considered as a preferential point for volcanic activity monit oring, both in the case of normal routine surveillance and in the case of i naccessibility to the crater area. (C) 1999 Elsevier Science B.V. All right s reserved.