Bull Volcanol (1999) 61 : 48–63 Q Springer-Verlag 1999 ORIGINAL PAPER P. Marianelli 7 N. Métrich 7 A. Sbrana Shallow and deep reservoirs involved in magma supply of the 1944 eruption of Vesuvius Received: 14 September 1998 / Accepted: 10 January 1999 Editorial responsibility: D. A. Swanson Paola Marianelli (Y) 7 Alessandro Sbrana Dipartimento di Scienze della Terra, Università degli Studi di Pisa, via Santa Maria, 53, I-56126 Italy Nicole Métrich Laboratoire Pierre Sue CEA-CNRS, CE-Saclay, Gif sur Yvette, France Abstract During the 1944 eruption of Vesuvius a sud- den change occurred in the dynamics of the eruptive events, linked to variations in magma composition. K- phonotephritic magmas were erupted during the effu- sive phase and the first lava fountain, whereas the emis- sion of strongly porphyritic K-tephrites took place dur- ing the more intense fountain. Melt inclusion composi- tions (major and volatile elements) highlight that the magmas feeding the eruption underwent differentiation at different pressures. The K-tephritic volatile-rich melts (up to 3 wt.% H 2 O, 3000 ppm CO 2 , and 0.55 wt.% Cl) evolved to reach K-phonotephritic com- positions by crystallization of diopside and forsteritic olivine at total fluid pressure higher than 300 MPa. These magmas fed a very shallow reservoir. The low- pressure differentiation of the volatile-poor K-phono- tephritic magmas (H 2 O~1 wt.%) involved mixing, open-system degassing, and crystallization of leucite, salite, and plagioclase. The eruption was triggered by intrusion of a volatile-rich magma batch that rose from a depth of 11–22 km into the shallow magma chamber. The first phase of the eruption represents the partial emptying of the shallow reservoir, the top of which is within the volcanic edifice. The newly arrived magma mixed with that resident in the shallow reservoir and forced the transition from the effusive to the lava foun- tain phase of the eruption. Key words Vesuvius 7 Magma chamber 7 Melt inclusions 7 Volatiles Introduction In March 1944 a composite eruption, characterized by lava flows followed by violent lava fountains, ended a 300-year period of semipersistent activity of Vesuvius (Imbò 1949; Arnò et al. 1987; Scandone et al. 1993) and marked a transition to the current quiescent status of the volcano. It was preceded by a 31-year period during which the crater, created during the 1906 eruption, was filled by quiet effusions of tephritic and phonotephritic lava (Scherillo 1953). The products of the recent cycles of Vesuvius have been described in terms of mineralo- gy and petrology (Joron et al. 1987; Belkin et al. 1993; Santacroce et al. 1993; Villemant et al. 1993), but crys- tallization pressures, depth of the magma chamber(s) and volatile behavior have not been investigated. These aspects represent the main objective of this paper in re- lation to the products of the 1944 eruption. Further- more, a comparison with the products of the 1906 erup- tion allows some speculation as to the supply mode for the last two cycles of activity of Vesuvius. This work is based on the chemistry, mineralogy, and melt inclusion (MI) studies of samples of the pyro- clastic sequences that characterize the paroxysmal phase of the eruption. Melt inclusions allow the investi- gation of the evolution of major and volatile (CO 2 , H 2 O, S, and Cl) elements, the assessment of pre-erup- tive volatile abundances, and reconstruction of the mul- ti-stage crystallization and differentiation history of the 1944 magmas. The 1944 eruption The eruption scenario We describe the main 1944 eruption phases, based on the chronicles of Imbò (1949) and Parascandola (1945) and on the reconstruction of Scandone et al. (1986). The eruption started on 18 March with lava effusions