Abstract
Vesuvius, dominating the densely-populated Neapolitan area, is one of the most dangerous volcanoes in the World. Its destructive power derives from energetic subplinian and plinian eruptions, such as the one which occurred in 79 A.D. Generally such large-scale events follow a long period of quiescence; a behaviour interpreted as the gradual build-up of magma volumes between periods of major activity. After the 1631 subplinian eruption until the last 1944 A.D. eruption, it experienced an almost continuous and less energetic explosive/effusive activity. The erupted magmas are characterized by undersaturated potassic to ultrapotassic nature, and compositional and Sr-isotopic variability. Furthermore geobarometric studies indicate two different crystallization depths located at 4 and >11 km, respectively. According to most of the recent literature, the eruptions were triggered by the injection in a shallower magma chamber, of isotopically distinct magma batches derived from heterogeneous mantle source(s) and/or contamination processes occurred within the deep reservoir. In our review of petrochemical data, we consider the period between the 3550 years BP plinian eruption and the 472 A.D. sub-plinian eruption, which includes 79 A.D. event, and the most recent period of activity which started in 1631 A.D. and lasted up to the 1944 A.D. eruption, characterized by a near continuous effusive/explosive activity. For both periods we identify a correlation between Sr-isotopical features of magmas and their crystallization depth. In particular, we show that pyroxenes have Sr-isotopic ratios lower than 0.7074 and an equilibrium crystallization depth of 22-11 km. Moreover feldspars have higher 87Sr/86Sr values (0.7075-7) and an equilibrium crystallization depth of about 4 km. Therefore the most radiogenic magmas did not derive from a deeper reservoir but their higher Sr-isotopic ratios have been acquired at a shallower depth likely by crustal contamination during magma evolution. In contrast, the lower Sr-isotope compositions characterise the less contaminated magmas coming from deeper crustal levels. On the basis of this evidence, the temporal Sr-isotopical variation of magma which erupted in the 1631-1944 A.D. period probably derives from the progressive withdrawal of the shallow magma chamber, which was completely empty before the 1805-1944 A.D. period of volcanism. Therefore the effusive and explosive events of the most recent 1805-1944 A.D. period were fed directly by the deep reservoir located at a depth exceding 11 km.
Highlights
Vesuvius is one of the most dangerous volcanoes in the world
In order to test the consistence of compositional variations of Somma-Vesuvius magmas with a crustal contamination model, we adopt the quantitative EC-AFC (Energy Conservation-Assimilation Fractional Crystallisation) approach that accounts for mass and energy conservation (Bohrson and Spera, 2001; Spera and Bohrson, 2001)
Less radiogenic (0.70730) mafic crystals have equilibrium crystallization depth exceding 10 km, while more radiogenic (0.7075-0.7077) sialic minerals have shallower equilibrium crystallization depth of about 5 km. This suggests that the magma acquired higher Sr-ratios by a process which occurred subsequently to its genesis, when sialic minerals crystallized at shallow depth, in the chamber or in the conduit
Summary
Vesuvius is one of the most dangerous volcanoes in the world. At the present, half a million people live in a near-continuous belt of towns and villages around the area possibly exposed to the effects of future eruptions. In spite of the wealth of studies, many problems related to magma genesis and evolution and to the present state of the Vesuvius plumbing system remain open: i) the intraplate versus subduction-related origin of potassic magmatism (e.g., Beccaluva et al, 1991; Ayuso et al, 1998; Peccerillo, 2001); ii) the relative roles of source composition and low-pressure processes in determining the geochemical features of volcanic products (e.g., Savelli, 1967, 1968; Turi and Taylor, 1976; Cortini and Hermes, 1981; Civetta et al, 1991; Civetta and Santacroce, 1992; Santacroce et al, 1993; Cioni et al, 1995; Fulignati et al, 1995; Ayuso et al, 1998; Fulignati et al, 1998; Del Moro et al, 2001; Gilg et al, 2001; Somma et al, 2001); and iii) the present state of the plumbing system, constituted or not by a shallower magma chamber (e.g., Santacroce, 1987; Zollo et al, 1996; De Natale et al, 2001). Petrological data are revisited to investigate the role of low-pressure evolutionary processes in magma genesis and their relationships with the style of eruptions (moderate explosive/effusive or highly explosive) during the 3550 years BP-1944 A.D. period of activity for which a complete set of data have been collected
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