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1 2 3 4 Chapter 9 5 6 The magma feeding system of Somma-Vesuvius (Italy) 7 strato-volcano: new inferences from a review of geochemical 8 and Sr, Nd, Pb and O isotope data 9 10 1 Monica Piochia,∗, Benedetto De Vivob and Robert A. Ayusoc 2 aIstituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy 3 bDipartimento di Geofisica e Vulcanologia, Università Federico II, Napoli, Italy 4 cU.S. Geological Survey, MS 954 National Center, Reston, VA, USA 5 6 Abstract 7 A large database of major, trace and isotope (Sr, Nd, Pb, O) data exists for rocks produced by the volcanic activity 8 of Somma-Vesuvius volcano. Variation diagrams strongly suggest a major role for evolutionary processes such 9 as fractional crystallization, contamination, crystal trapping and magma mixing, occurring after magma genesis 20 in the mantle. Most mafic magmas are enriched in LILE (K, Rb, Ba), REE (Ce, Sm) and Y, show small Nb–Ta AQ1 1 negative anomalies, and have values of Nb/Zr at about 0.15. Enrichments in LILE, REE, Nb and Ta do not 2 correlate with Sr isotope values or degree of both K enrichment and silica undersaturation. The results indicate mantle source heterogeneity produced by slab-derived components beneath the volcano. However, the Sr isotope 3 values of Somma-Vesuvius increase from 0.7071 up to 0.7081 with transport through the uppermost 11–12 km 4 of the crust. The Sr isotope variation suggests that the crustal component affected the magmas during ascent 5 through the lithosphere to the surface. Our new geochemical assessment based on chemical, isotopic and fluid 6 inclusion data points to the existence of three main levels of magma storage. Two of the levels are deep and may 7 represent long-lived reservoirs, and an uppermost crustal level that probably coincides with the volcanic conduit. The deeper level of magma storage is deeper than 12 km and fed the 1944 AD eruption. The intermediate level 8 coincides with the seismic discontinuity detected by Zollo et al. (1996) at about 8 km. This intermediate level 9 supplies magmas with 87Sr/86Sr values between 0.7071 and 0.7074, and δO18 Ͻ8‰ that typically erupted both 30 during interplinian (i.e. 1906 AD) and sub-plinian (472 AD, 1631 AD) events. The shallowest level of magma stor- 1 age at about 5 km was the site of magma chambers for the Pompei and Avellino eruptions. New investigations 2 are necessary to verify the proposed magma feeding system. 3 4 5 1. Introduction 6 7 Somma-Vesuvius (Fig. 1a) has long attracted intense scrutiny because of its recent activity, 8 enormous hazard potential to the Campanian region and immediate proximity to the city of 9 Naples. Plinian eruptions from the Somma-Vesuvius volcano were first described during 40 the eruption of 79 AD. The erupted silica-undersaturated potassium-rich rocks have been the 41 object of petrological studies (Rittmann, 1933; Savelli, 1967; Cortini and Hermes, 1981; 42 Joron et al., 1987; Civetta and Santacroce, 1992; Belkin et al., 1993; Cioni et al., 1995, 43 1998; Ayuso et al., 1998; Cioni, 2000; Peccerillo, 2001; Paone, 2005; Piochi et al., 2005; 44 45 46 *Corresponding author. Fax: 139-81-6100811. E-mail address: [email protected] (M. Piochi). Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 184

184 M. Piochi, B. De Vivo, R.A. Ayuso

1 and references therein) aimed at evaluating how the erupted magmas reflect the contribu- 2 tions of mantle sources, how their compositions have been affected during transport, and to 3 what extent they can be used to deduce their geodynamic setting. Recently, a large major, 4 trace and isotope (Sr, Nd, Pb, O) database has been published (De Vivo et al., 2003) and can 5 be downloaded at the Internet site http://www.dgv.unina.it/ricerca/de_vivo.htm. The 6 summary of results shows that rocks produced during major plinian and sub-plinian 7 eruptions, and during the last interplinian period of activity which started in 1631 AD, are 8 relatively well characterized on the basis of mineralogy, chemistry and isotopes. Adequate 9 data also exist for some rocks from interplinian periods of volcanism occurring before the 10 last sub-plinian eruption in 1631 AD. 1 In this paper, we briefly present a description of the chemical and isotopic database and 2 a synthesis of previous petrological studies in order to summarize the main evidence for 3 mantle source heterogeneity associated with the Somma-Vesuvius magmas, and highlight 4 the results supporting the importance of shallow-level evolution. Particularly, our brief 5 review of existing data points to a magma feeding system formed by multi-depth storage 6 levels; the magma reservoir at 8 km imaged by seismic tomography (Zollo et al., 1996) fed 7 both low- and large-magnitude eruptions. Significant progress has been made in the last 8 20 years of research focused on Somma-Vesuvius volcano (Civetta and Santacroce, 1992; 9 Belkin et al., 1993; Villemant et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; Del Moro 20 et al., 2001; Peccerillo, 2001; Fulignati et al., 2004, 2005; Pappalardo et al., 2004; Piochi 1 et al., 2005), and it is now possible to combine the results of previous studies to produce a 2 framework for more detailed investigations of the behaviour of magma and the magma 3 feeding system in Somma-Vesuvius volcano. 4 5 6 2. Volcanological and magmatological background 7 8 Somma-Vesuvius is a strato-volcano (Fig. 1a) that consists of an older collapsed edifice 9 (Somma), and a younger cone (Vesuvius). The volcano has been active at least since 300 ky 30 bp (Brocchini et al., 2001 and references therein) up to the major eruption of 1944 AD. 1 Presently, the volcano is the site of fumaroles, diffuse degassing (Chiodini et al., 2001; 2 Federico et al., 2002; Frondini et al., 2004) and low-magnitude seismicity (Bianco et al., 3 1999; Vilardo et al., 1999). Volcanism has been characterized by high explosive sub-plinian 4 and plinian eruptions that followed long periods of quiescence, and by intermediate and 5 small-scale explosive and explosive/effusive eruptions that occurred during continuous 6 periods of activity (interplinian period) (Fig. 1b) (Arnò et al., 1987; Civetta and Santacroce, 7 1992; Rolandi et al., 1998; Principe et al., 2004). Sub-plinian and plinian eruptions have 8 always produced larger volumes of rocks (one to a few cubic kilometres DRE, i.e. Dense 9 Rock Equivalent) (Rosi and Santacroce, 1983; Arnò et al., 1987; Civetta and Santacroce, 40 1992; Rolandi et al., 1993; Cioni et al., 1995; Landi et al., 1999) than the intermediate and 41 small-scale events (0.01–0.1 km3 DRE) (Scandone et al., 1986; Mastrolorenzo et al., 1993; 42 Rolandi et al., 1998; Arrighi et al., 2001). 43 The volcano rests on a sequence of Mesozoic and Cenozoic carbonates overlain by 44 Miocene sediments outcropping in the surrounding Apennine chain (D’Argenio et al., 45 1973; Ippolito et al., 1975) and encountered at a depth of around 2 km (Brocchini et al., 46 2001). The Moho discontinuity has been detected at about 30 km of depth (Corrado and Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 185

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1 2 Plinian Activity Inter-Plinian Activity 3 Repose time ?? 18th (1907-1944) 9th (1783-1794) 17th (1874-1906) 4 A.D.1944 8th (1770-1779) 16th (1870-1872) 7th (1764-1767) 15th (1864-1868) 5 Recent 5th (1712-1737) 14th (1854-1861) 4th (1700-1707) 13th (1841-1850) 6 A.D.1631 3rd (1696-1698) 12th (1835-1839) 2nd (1685-1694) 11th (1825-1834) 7 Repose time 1st (1638-1682) 10th (1700-1707) 8 III cycle A.D.1139 Medieval 2nd ( A.D.~635) 4th(~A.D.1095.) 9 A.D.472 1st (>A.D. 512) 3rd (>A.D.893.) 10 (Pollena) Repose time 1 A.D.303 2 Ancient Historic A.D.79 No geochronologic determinations 3 Pompei Repose time 4 Transitional 800 years B.C.700 5 Protohistoric 1st (~1758B.C.) 2nd (~1414 B.P) 3rd (~832 B.C.) 6 3.5 ky.B.P. Avellino 7 II cycle 8.0 ky.B.P. 8 Ottaviano (Mercato) Repose time 9 6000 years 20 16-14 ky.B.P. 1 Novelle (Verdoline) 2 18.6 ky.B.P. Somma

I cycle Sarno (Pomici di Base) 3 25.0 ky.B.P. 4 Codola Older Vesuvius 5 6 Somma activity 7 8 9 30 1 b) a) 2 3 Figure 1. (a) DTM of the Somma-Vesuvius strato-volcano; (b) Reconstructed stratigraphy of volcanic activity 4 during the last 25 ka. Source: Arnò et al. (1987); Arrighi et al. (2001); Ayuso et al. (1998); Landi et al. (1999); 5 Rolandi et al. (1993, 1998); Rosi and Santacroce (1983). Symbols as used in the following figures. Names of eruptions in parenthesis are from Arnò et al. (1987). 6 7 8 Rapolla, 1981; Ferrucci et al., 1989; Chiarabba et al., 2005). A high-velocity body dipping 9 westward from 65 km down to 285 km was interpreted as a plate within the mantle 40 (De Natale et al., 2001). Furthermore, an active, large magma chamber is located at about 41 8–10 km (Zollo et al., 1996; Di Maio et al., 1998) and has been proposed to extend up to 42 30 km (De Natale et al., 2001). However, based on fluid and melt inclusion evidence, 43 magma storage is indicated at 3.5–5, 8–10 and Ͼ 12 km (Belkin et al., 1985; Belkin and 44 De Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000; Lima et al., 2003). 45 At present, no geophysical evidence for magma chambers of significant lateral extension 46 has been found at Ͻ 8 km (Zollo et al., 1996; Di Maio et al., 1998). This may be due to Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 186

186 M. Piochi, B. De Vivo, R.A. Ayuso

1 the fact that resolution for the method used in tomography investigations is “blind” for 2 magma chambers with lateral extension Ͻ 1 km. 3 4 5 3. Mineral, chemical and isotopic data: description and previous interpretations 6 7 3.1. Mineralogy and classification 8 9 Somma-Vesuvius volcanic rocks are poorly (lava) to highly (scoria to pumice) vesiculated, 10 and nearly aphyric (mostly in the plinian eruptions) to strongly porphyritic (up to 50%; in 1 472 AD eruption and in the products younger than 1631 AD) (Joron et al., 1987; Villemant 2 et al., 1993). Two rock types are generally distinguishable on the basis of occurrence of 3 leucite minerals. In leucite-free rocks, olivine and Mg-rich diopside, plagioclase, Fe-rich 4 diopside, K-feldspar, magnetite and biotite can also occur, depending on the degree of 5 evolution. Leucite-bearing rocks contain olivine, Fe-rich and Mg-poor diopside, plagioclase 6 and oxide, also depending on the degree of evolution. Apatite, amphibole, garnet, phlogo- 7 pite and forsterite are present as accessory phases. Nepheline, as the only feldspathoid, and 8 scapolite have been occasionally recovered (e.g. 472 AD and Avellino rocks). 9 Feldspar (both K-feldspar and plagioclase) is the most abundant mineral phase in 20 leucite-free rocks, such as Avellino and Sarno (Pomici di Base) (Joron et al., 1987; Landi 1 et al., 1999), as well as in 79 AD leucite-bearing pumices (Cioni et al., 1998). Instead, diop- 2 side is the most common mineral in the products younger than 1631 AD, whose abundance 3 changes as function of the degree of vesicularity of rocks (Villemant et al., 1993). 4 Clinopyroxenes have compositions indicative of multiple stages of crystallization in the 5 upper (Ͻ 10 km) crust (Trigila and De Benedetti, 1993; Marianelli et al., 1995). Olivines 6 from 1944 and 1906 AD eruptions show compositions similar to olivine from peridotite 7 (Marianelli et al., 1995) and high pressure (Ͼ 400 MPa) of volatile entrapment (Marianelli 8 et al., 1999) indicative of very early stage of magma crystallization. 9 Metamorphosed carbonates, skarns, lavas, cumulates, hornfels, sub-volcanic igneous 30 rocks have been generally recovered as xenolith ejecta within pyroclastic deposits (Savelli, 1 1967; Barberi and Leoni, 1980; Hermes and Cornel, 1981; Belkin et al., 1985; Del Moro 2 et al., 2001; Gilg et al., 2001; Fulignati et al., 2004, 2005). Metamorphosed carbonate 3 ejecta are considered to be representative of the carbonate basement modified during 4 contact metamorphism under the pressure of 1500–2000 bars. Skarn xenoliths consist of

5 calc-silicate and carbonatic components and contain fassaitic pyroxene, forsterite (FoϾ90), 6 spinel, calcite, phlogopite, nepheline, garnet, periclase, brucite, calcite, and dolomite. They 7 are considered as representative of the crystallizing margins of the magma chamber (Del 8 Moro et al., 2001; Gilg et al., 2001; Fulignati et al., 2004, 2005). However, these xenoliths 9 were also interpreted to represent highly metasomatized blocks of stopped carbonates 40 incorporated into the magma (Hermes and Cornell, 1981). Silicate melt inclusions from 41 skarns show homogenization temperatures (Th) of 1000 Ϯ 50°C and trapping pressures 42 between 925 and 3550 bars (Belkin et al., 1985; Fulignati et al., 2004). Hornfels are 43 characterized by rhyolitic vesiculated glass and minerals of wollastonite, anorthite, calcite, 44 pyroxene and quartz, and have been considered the products of high-grade thermometa- 45 morphism from marly siltite rocks (Del Moro et al., 2001; Fulignati et al., 2005). 46 Cumulates are dunites, wherlites and biotite-bearing pyroxenites (Joron et al., 1987; Belkin Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 187

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1 and De Vivo, 1993). The cumulus phases are clinopyroxene, phlogopite, biotite, apatite,

2 plagioclase and olivine with Fo80–90; glass also occurs between individual crystal grains or 3 within cavities. Leucite is rare in cumulate nodules. Spinel and chromite can occur as 4 accessory phases. Th and trapping pressure of silicate melt inclusions in cumulates are 5 1200 Ϯ 50°C and in the range 1200–3050 bars, respectively (Belkin et al., 1985). 6 7 8 3.2. Major and trace elements 9 10 It is well known that rocks from Somma-Vesuvius are characterized by large compositional 1 variations. These rocks show variable alkali contents (Fig. 2a), and, in particular, show

2 variable degree of K2O enrichment. These rocks are slightly, mildly and highly silica 3 undersaturated, following Peccerillo (2003). Slightly silica-undersaturated volcanic rocks 4 are leucite-free and range in composition from shoshonites to trachy-phonolites; mildly to 5 highly undersaturated, nepheline- or, more commonly, leucite-bearing rocks, range from 6 alkali-basalt to phonolite. 7 Plinian and sub-plinian deposits are generally characterized by the most evolved com- 8 positions and chemical gradients through the stratigraphic sequence. The basal part of 9 deposits (white pumices) always shows the more sialic compositions, and the evolution 20 degree decreases upwards (grey pumices) (Arnò et al., 1987; Civetta et al., 1991; Civetta 1 and Santacroce, 1992; Rolandi et al., 1993; Cioni et al., 1995; Landi et al., 1999). These 2 features possibly reflect the progressive withdrawal of a chemically (and density) stratified 3 magma chamber located at shallow depth beneath the volcano. The variable layers can be 4 linked through simple chemical differentiation of unique parental magma (Landi et al., 5 1999) or can be generated due to the arrival of diverse magma batches from deeper 6 reservoirs (e.g. Civetta et al., 1991; Cioni, 2000). Sometimes, the occurrence of products 7 with compositions intermediate between that of the different layers indicates syn-eruptive 8 mingling of magmas or the existence of a double-diffusive interface between the two mag- 9 matic layers within the magma chamber (Landi et al., 1999). 30 Because of the occurrence of carbonate and metamorphic ejecta (see previous section), 1 it has been suggested that plinian and sub-plinian chambers formed within the carbonate 2 basement, between 5 and 8 km depth (Barberi and Leoni, 1980; Belkin and De Vivo, 1993; 3 Landi et al., 1999; Cioni, 2000) during the long time of quiescence that precedes the 4 eruption (Fig. 1b) and that allows reaching the high evolution degree of these rocks. 5 Magmas erupted during interplinian periods are characterized by low degree of evolution 6 (Fig. 2) and depths of storage at Ͻ 5 km, 8–10 km and Ͼ 12 km (Belkin et al., 1985; Belkin 7 and De Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000; Lima et al., 8 2003). Owing to the occurrence of deeply crystallized olivines (see previous section), the

9 existence of CO2-bearing melt inclusions and the brief repose time between two eruptions 40 (not more than 7 years) (Arnò et al., 1987), the various authors indicate that during 41 interplinian periods magmas can rapidly rise to the surface in open-conduit conditions. The 42 last 1944 AD eruption was fed by a magma directly rising from a depth of Ͼ 12 km. After 43 61 years of volcanic quiet, this latter eruption probably closes the third, last mega-cycle of 44 volcanism (Ayuso et al., 1998) and marks the transition to the closed-conduit condition 45 (Rosi et al., 1987). This situation of repose might last for centuries, heading towards the 46 starting of new, fourth, mega-cycle of volcanism, with a new plinian–sub-plinian eruption Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 188

188 M. Piochi, B. De Vivo, R.A. Ayuso

1 16 2 14 Phonolite 3 Tephri- a) phonolite 4 12 Trachyte 5 Phono- 10 Tephrite Latite Trachy- 6 Trachy- Trachydacite

O (wt%) basalt Rhyolite 2 8 andesite 7 Foidite Tephrite Shoshonite

8 O+K 6 Basanite 2 9 Dacite Na Andesite 4 Basaltic 10 andesite 1 2 Picro- Basalt basalt SiO (wt%) 2 2 0 3 35 40 45 50 55 60 65 70 75 4 5 1600 Sr (ppm) 6 b) 7 8 1200 9 20 800 1 2 3 400 4 SiO2 (wt%) 5 0 6 45 50 55 60 65 7 120 8 La (ppm) c) 9 100 30 1 80 2 3 4 60 5 6 40 7 MgO (wt%) 8 20 9 0369 40 Figure 2. (a) T.A.S. (Le Bas et al., 1986); (b) Sr versus SiO contents; and (c) La versus MgO for SV rocks. 41 2 Symbols as in Figure 1: bold crosses are dykes from Somma activity; closed symbols are rocks from plinian and 42 sub-plinian events; and open symbols rocks from interplinian periods. Circles, first magmatic cycle; rhombus, 43 second magmatic cycle; triangles, transitional magmatic cycle; squares, third magmatic cycle. Source: Cioni 44 et al. (1995); Civetta et al. (1991); Civetta and Santacroce (1992); De Vivo et al. (2003); Marianelli et al. (1999); 45 Santacroce et al. (1993). 46 Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 189

The magma feeding system of Somma-Vesuvius (Italy) strato-volcano 189

1 (Lima et al., 2003). In this context, to predict the behaviour of the volcano, acquiring a bet- 2 ter understanding of magma evolution processes, of the magma feeding system, and of the 3 precise role of the volatiles are crucial (Raia et al., 2000; Webster et al., 2001, 2003, 2005).

4 In many studies, SiO2 or MgO have been utilized as differentiation indices. Variations 5 in SiO2 seem to adequately illustrate the evolution of intermediate-to-most fractionated 6 rocks (Fig. 2b), but not the least-fractionated rocks. In contrast, MgO, appears more 7 adequate for the least-fractionated rocks (Fig. 2c). In any case, based on major and trace 8 elements variations (Fig. 2a–c), diverse evolutionary trends characterized by variable K, P, 9 Ti, some trace elements (i.e. Th, U, Sr) and LREE enrichment have been found (Joron 10 et al., 1987; Ayuso et al., 1998; Piochi et al., 2005). Within each trend, the role of crystal 1 fractionation processes in magma evolution has been widely accepted (Joron et al., 1987; 2 Civetta et al., 1991; Ayuso et al., 1998; Piochi et al., 2005 and references therein). 3 Feldspar and clinopyroxene are the main crystallizing minerals, in agreement with

4 petrographic data reported in previous section. Chemical trends in Sr and CaO/Al2O3 versus 5 K2O diagrams (Fig. 3a,b) suggest clinopyroxene associated with feldspar (mostly plagio- 6 clase and subordinately K-feldspar) crystallization during evolution of magmas older than 7 472 AD eruption (Piochi et al., 2005). The Sr versus Th diagram (Fig. 3c) highlights plagio- 8 clase fractionation. In contrast, clinopyroxene crystallization dominated during evolution of 9 highly undersaturated magmas of the post-1631 AD interplinian period, as also suggested by 20 Belkin et al. (1993), Villemant et al. (1993) and Trigila and De Benedetti (1993). In these 1 younger rocks, the variable abundance of clinopyroxene affects major- and REE-elements 2 variation (Belkin et al., 1993; Villemant et al., 1993). REE showing fairly homogeneous 3 patterns and variable LREE enrichment support the above data. In particular, the Eu anom- 4 aly is not a typical feature of primary magmas from Somma-Vesuvius. It seems to be 5 correlated with the degree of evolution; it is mostly present in highly evolved rocks, such as 6 79 AD, Avellino, and probably reflects feldspar fractionation (Joron et al., 1987). 7 In the MORB- and OIB-multi-elements normalized diagrams (Fig. 4a,b) rocks from 8 Somma-Vesuvius show similar trace elements distribution, regardless of the degree of silica 9 undersaturation and K enrichment. The least evolved rocks (MgO Ͼ 3 wt%) are character- 30 ized by high LILE (Rb, Ba, Th, K) and slight HFSE (Zr, Nb) enrichment, and slight Nb and 1 Ta trough with respect to MORB (Fig. 4a), similarly to other potassic magmas (Peccerillo 2 and Manetti, 1985; Peccerillo, 2001, 2003). Furthermore, these rocks have higher Cs, K, Pb, 3 Rb, Th, Ba and lower Nb and Ti contents compared to OIB (Fig. 4b). A heterogeneous 4 mantle source(s) has been therefore proposed to explain the variable undersaturation degree 5 of the rocks and, in particular, the occurrence of different parental magmas and different 6 evolutionary trends as shown in Figure 2 (Civetta et al., 1991; Civetta and Santacroce, 1992; 7 Ayuso et al., 1998; Piochi et al., 2005). Other authors (Rittmann, 1933; Pappalardo et al., 8 2004; Piochi et al., 2005) have also speculated that crustal contamination processes 9 contributed to the enrichment in K and in various other trace elements. 40 41 42 3.3. Sr, Nd, Pb, Hf, O and He isotope ratios 43 44 The variable silica-undersaturated Somma-Vesuvius volcanic rocks show similar range of Sr, 45 Nd, Pb and O isotopic compositions, with large variability within each cycle. 87Sr/86Sr 46 isotopic values span from 0.706283 to 0.708070 (Cortini and Hermes, 1981; Civetta and Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 190

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1 2 1600 a) 3 4 cpx 5 1200 6 7 800 8 9 400 10 feld+cpx

1 CaO/Al2O3 2 0 3 0.0 0.4 0.8 1.2 1.6 2.0 4 5 1600 6 cpx b) 7 1200 8 feld+cpx 9 20 800 1 Sr (ppm) 2 400 3 4 K2O (wt%) 5 0 6 246810 7 1600 8 c) 9 cpx 30 1200 1 feld+cpx 2 3 800 4 5 400 6 7 Th (ppm) 0 8 0 20 40 60 80 100 9

40 Figure 3. (a) Sr versus Al2O3; (b) Sr versus K2O; and (c) Sr versus Th contents for Somma-Vesuvius rocks. 41 Symbols and source of data as in Figure 2. 42 43 Santacroce, 1992; Caprarelli et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; De Vivo 44 et al., 2003; Piochi et al., 2005). The 143Nd/144Nd values range from 0.51225 to 0.51226 45 (Fig. 5a). Pb isotopic compositions have a moderate variation (Fig. 5b): 206Pb/204Pb values 46 vary from 18.94 to 19.09, 208Pb/204Pb from 38.7 to 39.3 and 207Pb/204Pb from 15.61 to 15.71 Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 191

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1 2 100 3 4 5 6 7 10 8 9 10 1 Rock/MORB 2 1 3 4 5 6 7 .1 8 Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb 9 100 20 1 2 3 4 10 5 6 7

8 Rock/OIB 9 1 30 1 2 3 4 .1 5 Cs Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu 6 Figure 4. Spider diagrams for selected Somma-Vesuvius rocks with MgO Ͼ 3 wt%. Source of data and 7 symbols as in Figure 2. 8 9 40 (Somma et al., 2001; De Vivo et al., 2003; Cortini et al., 2004). Pb isotope variations are not 41 correlated to Sr and Nd isotope variations. δO18 values obtained on whole-rocks range from 42 7.5% to 10‰, showing no correlation with Nd and Pb isotopic compositions, and defines no 43 typical correlation with the 87Sr/86Sr ratio (Fig. 5c) (Wilson, 1989). Among the isotopes, only 44 δO18 correlates (positively) with degree of chemical evolution (Fig. 6a,b). He isotope com- 45 position is about 2.4 Ra (where Ra is the 3He/4He of the atmosphere equal to 1.40 ϫ 10−6) 46 (Graham et al., 1993) for 1944 AD olivines and pyroxenes, indicating a source within the Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 192

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1 0.5126 143Nd/144Nd 2 a) 3 4 0.5125 5 6 0.5124 7 8 9 0.5123 87Sr/86Sr 10 1 0.5122 2 0.7060 0.7064 0.7068 0.7072 0.7076 0.7080 3 39.4 208Pb/204Pb 4 b) 5 39.2 6 7 8 39.0 9 20 38.8 1 2 206Pb/204Pb 3 38.6 4 18.90 18.94 18.98 19.02 19.06 19.10 5 11 6 δO18 c) 7 10 8 9 9 30 1 8 2 7 3 87Sr/86Sr 4 5 6 0.7060 0.7064 0.7068 0.7072 0.7076 0.7080 6 7 Figure 5. Isotopic diagrams for Somma-Vesuvius rocks: (a) 87Sr/86Sr versus 143Nd/144Nd ratios; (b) 208Pb/204Pb 8 versus 206Pb/204Pb; and (c) δO18 versus 87Sr/86Sr ratio. Symbols and source of data as in Figure 2. 9 40 41 lithospheric or in a slab-enriched mantle source. Similar He-isotopic values have been meas- 42 ured in fumarole gases suggesting a magmatic contribution to the degassing observed at the 43 surface (Graham et al., 1993). 176Hf/177Hf ratios determined on two Somma-Vesuvius rocks 44 characterized by Sr isotopic values lower than 0.7072 are 0.282784 and 0.282786, suggest- 45 ing a pelagic component added to HIMU and DM mantle sources (Gasperini et al., 2002). 46 Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 193

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1 The Sr isotope compositions of products from plinian and sub-plinian eruptions follow 2 a systematic trend through the stratigraphic sequence, consistent with the previously 3 recognized chemostratigraphy (see previous section) though to represent magmas residing 4 in a shallow and chemically stratified chamber (Civetta et al., 1991). For example, the 5 Avellino and the 79 AD pyroclastic sequences consist of white pumices, at the base, over- 6 lain by grey pumice deposits. White and grey pumices have different chemical and Sr iso- 7 tope compositions. However, both pumice types contain feldspars with a constant Sr 8 isotopic composition, similar to that of white pumices, suggesting Sr isotopic disequilib- 9 rium in rocks upwards in the sequence and mingling of magmas during eruption. 10 Moreover, the lowermost part of the 79 AD eruption and the uppermost part of Avellino 1 have similar 87Sr/86Sr values, suggesting that magma remnants can be left behind within 2 the chamber after large magnitude events (Civetta et al., 1991; Civetta and Santacroce, 3 1992). Such a type of incomplete magma removal has also been suggested by evidence 4 showing that events following plinian or sub-plinian eruptions produced magmas that have 5 isotopic characteristics comparable to those of previous eruptions (Civetta and Santacroce, 6 1992; Piochi et al., 2005) (Fig. 7). 7 8 9 δ 18 20 11 O a) 11 b) 1 2 3 9 4 9 5 6 7 7 7 8 0.0 0.2 0.4 0.6 0.8 1.0 0 400 800 1200 Sr (ppm) 9 CaO/Al2O3 30 Figure 6. δO18 versus CaO/Al O ratio (a) and Sr (b) for Somma-Vesuvius rocks. Lines indicate trend of magma 1 2 3 evolution. Symbols and source of data as in Figure 2. 2 3 4 0.7080 5 87Sr/86Sr b) 6 0.7076 7 8 0.7072 9 40 0.7068 41 42 0.7064 43 0.7060 44 10 100 1000 10000 45 46 Figure 7. 87Sr/86Sr versus age of rocks from Somma-Vesuvius. Symbols and source of data as in Figure 2. Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 194

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1 The 87Sr/86Sr isotopic variations have been attributed to the arrival of isotopically 2 diverse magma batches generated in a variable mantle source(s) (Cortini and Hermes, 3 1981; Civetta and Santacroce, 1992; Caprarelli et al., 1993; Cioni et al., 1995; Ayuso et al., 4 1998; Piochi et al., 2005). Recently, as first recognized by Rittmann (1933), various 5 authors (Civetta et al., 2004; Pappalardo et al., 2004; Paone, 2005; Piochi et al., 2005) 6 suggested the fundamental role of crustal contamination in modifying the isotopic 7 composition of erupted magmas at Somma-Vesuvius. Civetta et al. (2004) and Paone 8 (2005) proposed that contamination occurred within a Hercynian-like basement, similarly 9 to what happens at the Campi Flegrei (Pappalardo et al., 2002). Pappalardo et al. (2004) 10 and Piochi et al. (2005) suggested that carbonate was the main contaminant. In particular, 1 based on Sr isotope variations through time, Pappalardo et al. (2004) suggested that 2 between 1631 and 1944 AD the degree of magma contamination decreased owing to 3 magma rising from a deep reservoir in open-conduit conditions. 4 5 6 4. Discussion 7 8 The relationship between magma compositions and tectonic setting depends on reliably 9 distinguishing among geochemical features that image the source region and those that 20 resulted from magma evolution during transport. Processes affecting magmas after their 1 genesis are important in characterizing the behaviour of the magmatic supply system. Such 2 processes, for example, fractional crystallization, can produce highly evolved magmas, 3 which when associated with long-lived magma storage in the crust can generate high- 4 magnitude explosive events. Recharge of distinct magma batches from deeper levels within 5 the feeding reservoir may be required to trigger volcanic eruptions. Crustal contamination 6 requires chemical exchange between magma and wall rocks that can lead to fluid enrich- 7 ment, increasing the possibility of highly explosive eruptions, or that can induce quick 8 cooling and/or crystallization of magma limiting its further mobility. Properly identifying 9 the exact mechanism of magma evolution, i.e. magma mixing or crustal contamination, can 30 be a useful tool for hazard assessment studies. For the Somma-Vesuvius volcano, it would 1 be important to determine to what extent the evolution of the magmas depend on involve- 2 ment of the crust during magma genesis (with heterogeneously slab-enriched mantle 3 sources) or during magma evolution (Rittman, 1933; Savelli, 1967, 1968; Turi and Taylor, 4 1976; Vollmer, 1976; Civetta and Santacroce, 1992; Santacroce et al., 1993; Cioni et al., 5 1995; Ayuso et al., 1998; Peccerillo, 2001; Pappalardo et al., 2004; Piochi et al., 2005), and 6 how the geochemical evolution exactly triggers sub-plinian and plinian eruptions. 7 8 9 4.1. The role of crustal component on magma composition 40 41 The role of the crust on magma composition at the Somma-Vesuvius volcano is suggested 42 from both mineralogical and compositional data. For example, phlogopite occurs among 43 mineral phases. Th/Yb is always higher than 2 (Peccerillo and Manetti, 1985; Peccerillo, 44 2001). Ce/Pb ratios, being significantly lower than those of mantle sources free of subduc- 45 tion influences (Ϸ 25; Hofmann et al., 1986), tend towards the upper crustal value (Ϸ 3.5; 46 Taylor and Mc Lennan, 1985). Similarly, Nb/U value mostly falls within the continental crustal range (Ͻ 12; Rudnick and Fountain, 1995) (Fig. 8). Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 195

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1 2 0.7080 87Sr/86Sr a) 3 4 0.7076 5 0.7072 6 7 0.7068 8 9 0.7064 10 Ce/Pb 1 0.7060 2 1 2 3 4 5 678 3 0.7080 4 b) 5 0.7076 6 7 0.7072 8 9 0.7068 20 1 0.7064 Nb/U 2 0.7060 3 2345678910 4

5 11.5 18 δO c) 6 7 10.5 8 9 9.5 30 8.5 1 2 7.5 3 Ce/Pb 4 6.5 5 12345678 6 87 86 87 86 δ 18 7 Figure 8. (a) Sr/ Sr versus Ce/Pb ratios; (b) Sr/ Sr versus Nb/U ratios; and (c) O versus Ce/Pb ratios for Somma-Vesuvius rocks. Symbols and source of data as in Figure 2. 8 9 40 In addition, the role of the crust is also suggested from Sr, Pb and O (as well as Hf) 41 isotope ratios. In fact, these isotope ratios, although highly scattered, show rough correla- 42 tions with the above chemical ratios: Ce/Pb negatively correlates with 87Sr/86Sr and δO18, 43 Nb/U positively correlates with Sr isotope composition (Fig. 8a–c). These ratios do not 44 depend on the stage of evolution of the rocks because Ce and Pb, as well as Nb and U, show 45 almost comparable behaviour with respect to SiO2 or MgO, suggesting a similar partition 46 coefficient in the melt. The observed correlations can be attributed to the variable contri- butions of the crustal component to the magma. Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 196

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1 One important problem is to establish if the crustal component was involved at the time 2 of melting of the source or subsequently during ascent. Generally, radiogenic and stable 3 isotopes can be used to define the site at which contamination occurs. Nevertheless, 4 available O and Sr isotopic data do not conclusively provide information about input of 5 crustal materials/fluids to the magma (either in the mantle source or during shallow-levels 6 differentiation processes), although we know that higher O isotope compositions are found 7 in plinian-type rocks. Below we report some evidence that can be helpful to deal with this 8 fundamental question. 9 The generally low Mg, Ni and Cr (most values are Ͻ 40 and 100 ppm, respectively) 10 contents, and high crystallinity suggest the importance of processes occurring in magmas 1 during crustal storage and ascent. Chemical exchange processes between magmas and 2 carbonate wall rocks are indicated by garnet and phlogopite (Belkin et al., 1985; Joron 3 et al., 1987) and by Ca–Mg-silicate-rich ejecta (skarns) (Savelli, 1968; Fulignati et al., 4 1995, 1998, 2005; Gilg et al., 1999, 2001; Del Moro et al., 2001). Oxygen isotope studies 5 (Turi and Taylor, 1976; Ayuso et al., 1998), U-disequilibria (Black et al., 1998) and Pb iso- 6 tope data (Cortini et al., 2004) document shallow-level evolution of Somma-Vesuvius 7 magmas as open systems. Nevertheless, the strongest evidence for the dominating role of 8 shallow-level (crust) processes subsequent to high-pressure (mantle) processes derives 9 from a synthesis of Sr isotope and fluid inclusion data that suggests a positive correlation 20 between 87Sr/86Sr values and the estimated depths of mineral crystallization (Fig. 9). The 1 suggestion is that products enriched in radiogenic Sr formed during later stages of magma 2 evolution (Pappalardo et al., 2004). 3 The lower 87Sr/86Sr ratios (mostly around 0.7071–0.7072 with few spikes at 4 0.7062–0.7068) are associated with the highly silica-undersaturated rocks from the 1944 AD 5 eruption containing primitive olivine compositions (Marianelli et al., 1995). These ratios par- 6 tially overlap the Campi Flegrei Sr-isotope range (0.7068–0.7086) (Pappalardo et al., 2002), 7 8 0 9 30 2 1 4 2 6 Depth - km 3 4 8 5 10 6 12 7 8 14 9 16 40 18 41 87Sr/86Sr 42 20 43 0.7070 0.7074 0.7078 44 Figure 9. 87Sr/86Sr versus depth of crystallizing phases from SV rocks. Squares, clinopyroxene; rhombus, 45 feldspar; and triangles, leucite. Source of data as in Figure 2 (modified from Pappalardo et al. (2004). Grey areas 46 indicate probably levels of magma storage, based on fluid inclusion, volcanological and seismic data (see text). Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 197

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1 differ from values recovered at the nearby Procida (0.70523–0.70678) (De Astis 2 et al., 2004) and are higher than the Tyrrhenian Sea basalts (0.70733–0.7056) (Beccaluva 3 et al., 1990). In addition, they are associated with 176Hf/177Hf ratios of 0.282785 (two 1944 AD 4 samples reported in Gasperini et al., 2002) and He isotope ratio lower than MORB-like 5 magmas (Graham et al., 1993). Moreover, the 1944 AD eruption, and other rocks that are 6 generally poorly evolved (MgO Ͼ 3 wt%), are enriched in LILE, LREE and other incompat- 7 ible trace elements (e.g. Th, Nb, Ta), as well as in more compatible elements such as HREE 8 and Y (Fig. 4a). These geochemical features are usually related to magmas erupted along 9 subduction zones, implying the involvement of a crustal component in the mantle source 10 beneath Somma-Vesuvius. 1 2 4.2. The mantle source 3 4 The least-evolved Somma-Vesuvius rocks (MgO Ͼ 3 wt%) belong to the within-plate 5 type in term of Zr (Ͼ 100 ppm) and Zr/Y (Ͼ 4) (Pearce and Norry, 1979) (Fig. 10), in 6 agreement with evidence from the multi-element normalized diagram (Fig. 4b) showing 7 a certain similarity to the OIB basalts. The positive correlation in Figure 10 points to a 8 decrease in degree of partial melting or (fluid-controlled) source heterogeneity. Based on 9 the Cs–Pb enrichment in Figure 4b, the LILE enrichment and the slight Nb–Ta negative 20 anomalies in Figure 4a, and Nb/Zr at about 0.15, as well as on the isotope features dis- 1 cussed in the previous section, we suggest that the mantle source of Somma-Vesuvius 2 contains a slab-derived component. This conclusion is consistent with the general idea 3 that enriched potassium-rich magmas are generated by partial melting of phlogopite-rich 4 garnet peridotite (Gupta and Fyfe, 2003). 5 Poorly evolved rocks (MgO Ͼ 3 wt%) with a high degree of silica undersaturation 6 show significant constancy of Th/Zr (0.05–0.08), Ta/Yb (0.7) and Cs/Rb (Ͻ 0.06), as well 7 as Th/Yb, Th/Ta and other ratios, that are independent of fractional crystallization and/or 8 partial melting. These relatively unevolved rocks, as well as the slightly and mildly silica- 9 undersaturated rocks, have comparable trace elements distributions, showing similar 30 1 2 3 Zr/Y 4 5 Phlegraean area 10 6 Vesuvius 7 8 9 Tyrrhenian sea 40 41 Zr (ppm) 42 1 43 100 1000 44 Figure 10. Zr and Zr/Y for SV rocks with MgO Ͼ3 wt%. Source of data and symbols as in Figure 2. Data from 45 Phlegraean Fields (D’Antonio et al., 1999; Pappalardo et al., 1999; Piochi et al., 1999) and Tyrrhyenian Sea 46 (Beccaluva et al., 1990) are also reported for comparison. Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 198

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1 enrichment in LILE, Ce and other incompatible trace elements (e.g. Th, Nb, Ta), as well 2 as in more compatible elements such as Sm and Y (Fig. 4) independent of their Sr isotope 3 values and K-enrichment degree. Therefore, in a general sense, these data suggest the 4 existence of an invariable mantle source during the life of the Somma-Vesuvius volcano. 5 In agreement with Peccerillo and Manetti (1985), we suggest that diverse degrees of silica 6 undersaturation in potassic “mafic” rocks was linked to small degrees of partial melting 7 at different pressures in a phlogopite-bearing potassium-rich peridotitic mantle source

8 containing CO2 and small amounts of water. Sr, Nd, Pb, O, He and Hf isotopes were likely 9 affected by processes in the mantle source. However, with our hypothesis, the absence of 10 relationships between Sr–Nd isotope compositions and degree of both alkali enrichment 1 and silica undersaturation of “mafic” rocks suggests that mantle source processes mostly 2 influence the chemical composition of parental magmas, but it cannot be the main cause 3 of the large isotopic variability of Somma-Vesuvius rocks with 87Sr/86Sr ratios higher than 4 0.7071. 5 6 7 4.3. The behaviour of the magmatic feeding system 8 9 Based on the variation of the 87Sr/86Sr values, contamination of Somma-Vesuvius magmas 20 was attributed to a Hercynian-like basement (Civetta et al., 2004; Paone, 2005) or to rocks 1 in the overlying sedimentary series (Rittmann, 1933; Pappalardo et al., 2004; Piochi et al., 2 2005). However, on the basis of data in Figure 9 we suggest that the increase in Sr isotope 3 values from 0.7071-3 to 0.7081 mostly occurs within the uppermost 11–12 km of the crust 4 and points to these sedimentary rocks as the main crustal contaminant. However, we can- 5 not exclude that magma contamination could have occurred in crustal rocks underlying the 6 carbonate basement. We stress the fact that no xenolith of possible Hercynian origin has 7 been found at Somma-Vesuvius, contrary to what happened at the nearby Campi Flegrei 8 (Pappalardo et al., 2002; Paone, 2005). 9 Contamination of magma (87Sr/86Sr Ϸ 0.7071) by carbonate rocks (87Sr/86Sr Ϸ 30 0.7073–00709; Sr ϭ 700–1000 ppm) (Civetta et al., 1991; Iannace, 1991) at Somma- 1 Vesuvius has been quantitatively modelled by Pappalardo et al. (2004) and Piochi et al. 2 (2005) who suggested that crustal contamination was a selective process involving thermal 3 decomposition (decarbonation reactions) of the sedimentary wall rocks and exchange 4 between magmas and fluids. Fulignati et al. (2004, 2005) also suggested similar conclu- 5 sions on the basis of geochemical and mineralogical data collected on 79 and 1944 AD 6 skarn ejecta. We recognize, however, that magma evolution was likely more complicated 7 than as stated previously because no correlation has been found for δO18 and 87Sr/86Sr 8 values, and because of the negative correlation between phenocryst abundance and values 9 of 87Sr/86Sr (Figs. 5c and 11). Moreover, hornfels rhyolitic pumices characterized by 40 87Sr/86Sr higher than 0.711 and δO18 at around 15‰ have been found among ejecta in var- 41 ious pyroclastic deposits and have been interpreted as the result of the partial melting of 42 the pelitic sediments during thermometamorphic event (Del Moro et al., 2001; Fulignati et 43 al., 2005). This fact suggests the possible involvement of Miocene sediments in addition 44 to carbonate during the evolution of magmas at the Somma-Vesuvius. 45 Fluid exchange between magmas and wall rocks could be more pervasive on magmas 46 associated with high-explosive eruptions. Available data reveal relatively high values and Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 199

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1 0.7081 2 87Sr/86Sr 3 0.7079 a) 4 5 0.7077 6 7 0.7075 8 9 0.7073 10 % phenocrysts 1 0.7071 2 10 20 30 40 50 3 Figure 11. (a) 87Sr/86Sr versus phenocryst content in rocks from recent interplinian period of volcanism; 4 (b) 87Sr/86Sr versus age of rocks. Symbols and source of data as in Figure 2. 5 6 7 a large range of δO18 for pumices from plinian and sub-plinian eruptions, and relatively 8 low δO18 values and a smaller range for highly silica-undersaturated volcanic rocks from 9 interplinian events (Figs. 5c, and 6a,b). The correlation for δO18 and chemical differentia- 20 tion indices (better defined for rocks from high-explosive eruptions), together with numer- 1 ical considerations reported in Ayuso et al. (1998), data from Cortini et al. (2004) and the 2 observed enrichment in some incompatible trace elements (La, Nb, Zr) of pumices from 3 plinian eruptions (Fig. 2c), also support the effects of fluid exchange, rather than isotope 4 fractionation determined by exsolution of gas from magma. 5 Magmas erupted during the post-1631 AD interplinian period are characterized by the 6 decrease of the 87Sr/86Sr ratio with increasing phenocryst content down to typical values of 7 clinopyroxenite (Ͻ 0.7071) (Del Moro et al., 2001). This relation can be attributed to (1) the 8 entrapment of crystal mush generated during previous magma storage in the crust by rising 9 magmas and/or (2) the accumulation/depletion of phenocrysts during magma movements 30 through the crust towards the surface. In the first case, magmatic melts should be charac- 1 terized by higher 87Sr/86Sr ratios. Otherwise, phenocrysts can be accumulated or be depleted 2 in magma as a function of the ascent rate of magma towards the surface (see also Villemant 3 et al., 1993). In particular, low ascent rate can result in crystal segregation and in longer time 4 during which melt stay within wall rocks, thus producing rocks with lower crystal content 5 and possibly higher crustal contamination. This second hypothesis is in agreement with evi- 6 dence from Villemant et al. (1993) indicating that lavas derived from magmas experiencing 7 volatile degassing generally contain lower crystal abundance than vesiculated fragments 8 generated by gas overpressure. This idea is supported by evidence that magmas with the 9 lowermost Sr isotope ratios erupted during the 1944 AD rose to the surface from 11–22 km 40 depth (Marianelli et al., 1999). However, the repetitive and regular variation of 87Sr/86Sr 41 values through time (Fig. 7) is consistent with the idea that residual magma or crystal mush 42 remaining in the magmatic system after the end of the plinian (or sub-plinian) eruptive 43 event, can be involved in subsequent eruptions (Civetta et al., 1991; Civetta and Santacroce, 44 1992; Cioni et al., 1995; Lima et al., 2003; Piochi et al., 2005). 45 87Sr/86Sr, δO18 and fluid inclusion data strongly suggest polybaric evolutionary 46 processes of diverse parental magmas at Somma-Vesuvius. Evolutionary processes were Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 200

200 M. Piochi, B. De Vivo, R.A. Ayuso

1 dominated by crustal contamination and crystal entrapment, in addition to crystal frac- 2 tionation and magma mixing. Evidence presented in this paper, in particular data shown in 3 Figure 9, allows us to speculate that magmas with 87Sr/86Sr ratios of around 0.7071-3 and 4 of 0.7074-5 derive from reservoirs probably located at different depths, i.e. Ͼ 12 km and 5 at around 8–12 km, respectively. Magmas with higher Sr isotope compositions, for exam- 6 ple those from Pompei and Avellino eruptions, evolved during storage in shallower magma 7 chambers or, for example those from some of post-1631 AD interplinian eruptions, during 8 the ascent through the conduit. 9 10 1 5. Conclusions 2 3 Available data in the literature furnish the possibility to preliminarily define the magma 4 feeding system beneath the Somma-Vesuvius strato-volcano. It consists of three main 5 levels of magma storage, the two deepest probably being long-lived reservoirs, and an 6 uppermost crustal level that probably includes the volcanic conduit and hosted magmas 7 during interplinian period of volcanism. The deeper level is located at depths exceeding 8 15 km and should furnish magma with 87Sr/86Sr ratios of Ͻ 0.7072 and δO18 Ͻ 8‰. The 9 intermediate level occurs at around 8–12 km depth and supplies magmas with 87Sr/86Sr 20 ratios between 0.7071 and 0.7074, and δO18 Ͻ 8‰ typically erupted both during 1 interplinian (i.e. 1906 AD) and sub-plinian (472 AD, 1631 AD) events. The shallow level at 2 around 5 km depth was the site of plinian magma chambers such as those of Pompei and 3 Avellino eruptions. This type of magma feeding system fits with fluid and melt inclusions 4 data (Belkin et al., 1985; Belkin and De Vivo, 1993; Marianelli et al., 1999; Cioni, 2000; 5 Lima et al., 2003) indicating magma storage at 3.5–5 km, 8–10 km and Ͼ 12 km, with 6 results of seismic (Zollo et al., 1996) and magnetotelluric (Di Maio et al., 1998) investiga- 7 tions indicating a discontinuity at 8–10 km depth, with seismic evidence of deeper magma 8 storage extending up to 30 km depth (De Natale et al., 2001), and with the magnetized 9 character of a narrow shallow crustal volume (Fedi et al., 1998). However, geophysical data 30 do not indicate the occurrence of current magma storage at a depth of Ͻ 5 km, 1 as vice versa is indicated by fluid and melt inclusion studies (Belkin et al., 1985; Belkin 2 and De Vivo, 1993; Marianelli et al., 1999; Cioni, 2000; Lima et al., 2003). 3 4 5 Acknowledgements 6 7 The authors are thankful to A. Peccerillo for his constructive review, which helped to 8 improve the final version of the manuscript. The paper has benefited from MIUR-PRIN 9 funds to B. De Vivo (2003–2004). 40 41 42 References 43 Arnò, V., Principe, C., Rosi, M., Santacroce, R., Sbrana, A., Sheridan, M.F., 1987. Eruptive history. In: 44 Santacroce, R. (Ed.), Somma-Vesuvius, Quaderni de “La Ricerca Scientifica”, CNR, Italy, 251 pp. 45 Arrighi, S., Principe, C., Rosi, M., 2001. Violent strombolian and subplinian eruptions at Vesuvius during post- 46 1631 activity. Bull. Volcanol. 63, 126–150. Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 201

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