Degassing processes at Stromboli volcano inferred from short-lived disequilibria (Pb-210-Bi-210-Po-210) in volcanic gases

Volcanic aerosols and gases released by three active craters at Stromboli v olcano have been regularly collected since 1985. In this paper, we present new evidence of the high volatility of some nuclides among radon daughters (Pb-210, Bi-210 and Po-210), which are strongly fractionated, leading to si gnificant radioactive disequilibria in volcanic exhalations. The very low v olcanic activity in October 1996 allowed a separate sampling of each crater plume for the first time; remote sampling of the bulk plume were also perf ormed. These data show that the chemical composition of volcanic aerosols r emains constant within the first few hundred meters from their source vents , ensuring the validity of remote sampling when the activity does not allow one to approach the active craters. Moreover, it appears that there is no differentiation of gases from one crater to another suggesting that the geo metry of the upper plumbing system of the volcano is rather simple. gases b eing directly emitted from a shallow magma chamber without significant cool ing inside the edifice. On the basis of the assumption of a continuously re plenished shallow magma reservoir in steady state, we propose a dynamic mod el of degassing accounting for the variations of radionuclide contents and ratios observed in the gas phase since 1985. This model allows us to relate these variations to changes in the magma chamber dynamics, namely the magm a residence time inside the chamber and the escape time of gases from it, b oth parameters being closely linked to the volcanic activity. While gases a re always emitted within a few hours after bubble nucleation, suggesting th at the chamber is no deeper than a few hundred meters, magma residence time varies from less than 20 days during eruptive periods (highly explosive or effusive periods) to more than 200 days before the 1985 eruption. The latt er figure is explained by the storage at shallow depth of a poorly renewed magma batch that fed the eruption. The variations of the residence time obs erved over the studied period suggest that the steady-state dynamics of the Stromboli magma chamber is sustained for rather short periods of a few mon ths at most. On basis of the knowledge of the deep undegassed magma supply and the magma residence time, we estimate the volume of the shallow magma c hamber at about 7 +/- 2 x 10(5) m(3). The good agreement of our results wit h previous estimates based on both geochemical and geophysical studies sugg ests that the measurement of radioactive disequilibria in the gas phase pro vides a robust tool for a deeper understanding of the volcanic activity and the magma dynamics beneath active volcanoes. (C) 2000 Elsevier Science B.V . All rights reserved.