U-series disequilibrium radionuclides in sulphur incrustations from the fumarolic field of Vulcano Island

U-series disequilibrium radionuclides (238U,234U,210Pb,210Po) in sulphur incrustations deposited at La Fossa cone (Vulcano Island, Italy) were measured over the period 1990–1995 to collect information about changes in the feeding system of the fumarolic field. The 210Po/210Pb and 210Pb /Pb ratios of the sulphur incrustations allowed the identification of a fumarolic vent which is free from magmatic contribution and remobilized sublimate contamination. In spite of the low temperature of the selected fumarole (280 –350°C), kinetic data show that 210Po is almost fully volatilized due to the relatively high velocity of the gas; as a result, the 210Po/222Rn activity ratio of the gas can be used to estimate fluid rising times (about 100 days). 234U/238U activity ratios of sulphur incrustations deposited in 1991–1993 showed a shift toward high values typical of nonmagmatic fluids. Both short-lived and long-lived U-series disequilibrium radionuclides contribute to volcanic forecasting and give information on fluid sources and rising times as well as the residence times of the aquifers involved in the degassing process of the fumarolic field.