Intensive gas sampling of noble gases and carbon at Vulcano Island (southern Italy)

The helium isotopic ratio of crater, beach and submarine fumaroles, water w ells and soil gases at Vulcano Island has been, since 1987, repeatedly meas ured. The He-3/He-4 from crater fumaroles (F5 and FA) oscillates biennially between 4.9 and 6.0-6.2 R/R-a. The periodicity of the 3He/4He oscillation may be linked to pressure variations in a deep gas reservoir. Chemical and carbon isotope variations during the same period closely reflect trends sho wn by helium. These simultaneous variations, chemical and isotopic, suggest a close relationship between these species and reflect one dominant proces s, which belong to the routine activity of the volcano. The Ar-40/Ar-36 rat io up to 1200 is not consistent with the presence of an air-rich source and suggests the addition of radiogenic argon. Neon data (Ne-20/Ne-22 and Ne-2 1/Ne-22) indicates that a crustal component is present beneath the volcano. Heavy carbon, 0 parts per thousand similar to -2 parts per thousand, also supports the existence of a crustal component. The isotopic data here obtai ned do not indicate that the recorded variations (chemical and isotopic) in dicate a "period of unrest'' at Vulcano Island. The present phase of activi ty is explained with a two-state model of the feeding sources of the volcan o: (a) a gas release from intermediate and surficial sources and (b) a magm atic pulse, He-3-rich, from a deep gas reservoir (not migration of magma) w hich continuously mixes with more surficial fluids. Additions of surficial (air saturated waters) or atmospheric fluids are minor and may occur during the ascent of the gas phase to the surface or, more likely, be added at th e time of the gas collection.