CHEMICAL VARIATIONS IN FUMAROLIC GASES AT VULCANO ISLAND (SOUTHERN ITALY) - SEASONAL AND VOLCANIC EFFECTS

Because of increasing in fumarolic activity at Vulcano since September 1987, regular sampling and analysis is conducted on the well-known F5 crater fumarole (T = 330 +/- 5-degrees-C). Significant chemical variations have been recorded over the period 1987-1989. Variations of water vapour in the fumarolic fluid follow seasonal pattern and can be related to both external (seasonal) and internal (volcanic) influences suggesting a control by a shallow water aquifer. Significant variations of H2O and of some species in the anhydrous gas phase (H2, SO2, HCl, HF, N2, He) probably result from chemical or dynamic changes in the feeding system at depth. Several mechanism are considered for explaining the chemical trends results in relationship to the recent increase in activity. A growing input of deep magmatic fluids is not supported by the low CO content and the apparent constancy of the He-3/He-4 ratio. Apparent equilibrium temperatures close to the fumarolic outlet temperature (350-400-degrees-C) were calculated, suggesting isothermal expansion of the gas from a shallow equilibration zone. Comparison with the fumaroles at sea level (Porto di Levante) suggests that these latter may be fed a different hydrothermal systems. Variable interaction between both systems may account for some of the chemical variations observed at F5 fumarole.