Radionuclide behaviour in high temperature gases from Satsuma Iwojima volcano, Japan.

Radionuclide activities (210Pb, 210Bi, 210Po) were investigated in Mount Etna plume from 1983 to 1995. These parameters are related to magma dynamics and their determination allows the volcanic activity to be characterized from the plume composition, and the volume of degassing magma to be estimated. At the Sud Est crater the long-term observation of the 210Po/210Pb ratio shows that it behaves as a degassing vent not directly related to the main magma reservoir, depending on the magma level inside the volcano. Since 1992 the simultaneous determination of radionuclides and sulfur in the main plume enable us to identify two sources of 210Po. One is magmatic, while the other is issued from the decay of 210Pb in the shallow degassing cell and depends directly on the residence time of the gases before their emission. Estimation of the volume degassing magma, the residence time of gases and the proportion of undegassed magma renewing the shallow degassing cell were calculated for the period 1992-1995. During the 1992 eruption, the rate of degassing magma volume is estimated at 5 x 106 m3/day, and the volume of the shallow degassing magma reservoir at about 0.5 Km3. In 1994 and 1995 the rate of non-erupted degassing magma volume was estimated at about 0.18 Km3/year. During the entire 1983-1995 period only 15-20% of the degassed magma was erupted.