Radiogenic 4He, 21Ne and 40Ar in fumarolic gases on Vulcano: implication for the presence of continental crust beneath the island

From December 1991 to January 1993, 15 gas samples were collected from different fumaroles on the island of Vulcano (Aeolian archippelago), one of the most active volcanoes in Italy, actually in a quiescent phase, and then analysed for noble gas composition. Data on the F5 fumarole showed a large fluctuation, with a peak around November 1992. The oscillation is explained with continuous degassing of different sources: deep and more superficial. Enhanced contribution of radiogenic21Ne to the crater and beach fumarolic gases were accompanied by an increase in both radiogenic 4He and 40Ar. Remarkably, constant 21Ne/4He and 4He/40Ar ratios appeared to have been unchanged for more than a year. These observations, especially concerning the radiogenic 21Ne, suggest a deep-seated old continental crust beneath the volcano as a source of these radiogenic noble gases. Two possibilities are taken into consideration: thermo-metamorphic sedimentary formations located at the base of the volcano and/or the subducted African continental plate beneath the European plate causing the so-called subduction-type volcanism in the Aeolian islands (southern Italy).