Increasing fumarolic activity and the rise of outlet fumarole temperatures has occurred at Vulcano island (southern Italy) since September 1987. Regular sampling and analysis have been conducted on the well-known F5 fumarole and later on F5HT and FA fumaroles on the Fossa crater plus other gas emanations from different sites on the island. Significant chemical and isotopical oscillations have been recorded over the period 1987--1991. Variations of water vapor in the fumarolic fluid follow a seasonal pattern and can be related to both external (seasonal) and magmatic influences but they suggest a control by a source other than magmatic. Significant variations of H2O, and of some species in the anhydrous gas phase (H2, CO2, SO2, N2, and He), have been related in the past to chemical or dynamic changes in the feeding system at depth. Oscillations of the isotopic composition of helium and carbon suggest possible mixing with a nonsurficial source, possibly crustal in origin. Argon and neon isotopic data support this hypothesis. Several mechanisms are considered for explaining the chemical and isotopic data, in relation to the recent increase in activity. Not all of the chemical and isotopic variations recorded are related to geophysical parameters such as seismicity and deformation. The low CO content of the F5 furmarole and the apparent constancy of the 3He/4He ratio do not suggest an increase in the input of deep magmatic fluids.

The high CO content of the high temperature (650 ¿C) FA fumarole, located at the interior of the crater, along with helium and carbon isotopic ratios similar to other fumaroles is probably due to an easier ascent of these fluids to the surface. Apparent equilibrium temperatures, close to the fumarolic outlet temperature, indicate isothermal expansion of the gas from a shallow equilibration zone for both high- and lower-temperature gases. In particular, the chemical and isotopic features of fluids belonging to both crater and beach furamole fields are consistent with the existence of two intermediate system, independently feeding the two fields. High 3He/4He ratios for the beach fluids, similar to those of the crater fluids, suggest a large magmatic reservoir at depth, feeding both the crater and the beach intermediate reservoirs. ¿ American Geophysical Union 1995