FLUID GEOCHEMISTRY AT VULCANO ISLAND - A CHANGE IN THE VOLCANIC REGIME OR CONTINUOUS FLUCTUATIONS IN THE MIXING OF DIFFERENT SYSTEMS

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. Several mechanisms are considered for explaining the chemical and isotopic data, in relation to the recent increase in activity. 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. 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. -from Author