The Somma-Vesuvius Magma Chamber: a Petrological and Volcanological Approach F. BARBERI H. BIZOUARD R. CLOCCHIATTI N. METRICH R. SANTACROCE A. SBRANA [stituto di Mineralogia e Petrografia, University of Pisa, Italy. Laboratoire de Petrographie et Volcanologie, E.R. 45 CNRS, Universit~ Paris XI, Orsay, France. Istituto di Mineralogia e Petrografia, University of Pisa, Italy. AGIP S.p.A., Servizio Esplorazione Geotermica, S. Donato Milanese, Italy. ABSTRACT The volcanic history of Somma-Vesuvius indicates that salic products compatible with an origin by fractionation within a shallow magma chamber have been repeatedly erupted (~(Plinian~ pumice deposits). The last two of these eruptions, (79 A.D. and 3500 B.P.) were carefully studied. Interaction with calcareous country rocks had limited imp0~ance, and all data indicate that differentiated magmas were produced by crystal-liquid fractionation within the undersaturated part of petrogeny's residua system at about 1 kb water pressure. The solid-liquid trend indicates that the derivative magmas originated by fractionation of slightly but signfficantly different parental liquids. Some lavas of appropriate composition were selected as parental liquids to compute the entity of the fractionatien. Results suggest that in both bases a fractionation of about 70 weight % was needed to produce liquids with the composition of the pumice. The combination of all data indicates that the two Plinian eruptions were fed by a magma chamber (3-4 km deep) having a volume of approx. 2.0-2.5 km3. The temperature of the magma that initially entered the chamber was about ll00°C, whereas the temperature of the residual liquids erupted was Plinian pumice was 800 ° and 850°C respectively. There is no evidence that such a magma chamber existed at Vesuvius after the 79 A.D. eruption. These results have relevant practical implications for volcanic hazard and monitoring and for geothermal energy. Bull. Volcanol., Vol. 44-3, 198I INTRODUCTION Somma-Vesuvius volcano is character- ized by a dominant association of silica undersaturated and potassic basic products, ranging in composition from tephrites to leucitites, that were erupted during long periods of prevalently effusive activity with only minor associated pyro- clastic rocks. This relatively monotonous sequence of mostly basic lavas was repeti- tively interrupted by highly explosive eruptions of Plinian type, that produced pumice-fall deposits, very often associated with pyroclastic surges and flows. Seven eruptions of this kind occorred in the last 17.000 years (DELIBRIAS et al., 1979), the most famous of which being that of 79 A.D. which destroyed Pompei, Hercula- neum and Stabiae. These Plinian eruptions correspond almost always (6 times in 7 cases) to the beginning of a new eruptive cycle, follow- ing a long period of quiescence, as indicat- ed by the presence of paleosofls underly- ing the pumice deposits. Furthermore Plinian pumice is characterized by highly evolved (mostly phonolitic) composition and correspond to the most salic magmas erupted by the volcano. Because of this highly evolved nature and the long period of quiescence that preceded their erup-