The magma feeding system of Somma-Vesuvius (Italy) strato ... · feeding system in Somma-Vesuvius volcano. 2. Volcanological and magmatological background Somma-Vesuvius is a strato-volcano

Chapter 9

The magma feeding system of Somma-Vesuvius (Italy) strato-volcano: new inferences from a review of geochemicaland Sr, Nd, Pb and O isotope data

Monica Piochia,∗, Benedetto De Vivob and Robert A. Ayusoc

aIstituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, ItalybDipartimento di Geofisica e Vulcanologia, Università Federico II, Napoli, ItalycU.S. Geological Survey, MS 954 National Center, Reston, VA, USA

Abstract

A large database of major, trace and isotope (Sr, Nd, Pb, O) data exists for rocks produced by the volcanic activityof Somma-Vesuvius volcano. Variation diagrams strongly suggest a major role for evolutionary processes suchas fractional crystallization, contamination, crystal trapping and magma mixing, occurring after magma genesisin the mantle. Most mafic magmas are enriched in LILE (K, Rb, Ba), REE (Ce, Sm) and Y, show small Nb–Tanegative anomalies, and have values of Nb/Zr at about 0.15. Enrichments in LILE, REE, Nb and Ta do notcorrelate with Sr isotope values or degree of both K enrichment and silica undersaturation. The results indicatemantle source heterogeneity produced by slab-derived components beneath the volcano. However, the Sr isotopevalues of Somma-Vesuvius increase from 0.7071 up to 0.7081 with transport through the uppermost 11–12 kmof the crust. The Sr isotope variation suggests that the crustal component affected the magmas during ascentthrough the lithosphere to the surface. Our new geochemical assessment based on chemical, isotopic and fluidinclusion data points to the existence of three main levels of magma storage. Two of the levels are deep and mayrepresent 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 levelcoincides with the seismic discontinuity detected by Zollo et al. (1996) at about 8 km. This intermediate levelsupplies magmas with 87Sr/86Sr values between 0.7071 and 0.7074, and δO18 �8‰ that typically erupted bothduring interplinian (i.e. 1906 AD) and sub-plinian (472 AD, 1631 AD) events. The shallowest level of magma stor-age at about 5 km was the site of magma chambers for the Pompei and Avellino eruptions. New investigationsare necessary to verify the proposed magma feeding system.

1. Introduction

Somma-Vesuvius (Fig. 1a) has long attracted intense scrutiny because of its recent activity,enormous hazard potential to the Campanian region and immediate proximity to the city ofNaples. Plinian eruptions from the Somma-Vesuvius volcano were first described duringthe eruption of 79 AD. The erupted silica-undersaturated potassium-rich rocks have been theobject of petrological studies (Rittmann, 1933; Savelli, 1967; Cortini and Hermes, 1981;Joron et al., 1987; Civetta and Santacroce, 1992; Belkin et al., 1993; Cioni et al., 1995,1998; Ayuso et al., 1998; Cioni, 2000; Peccerillo, 2001; Paone, 2005; Piochi et al., 2005;

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Volcanism in the Campania Plain:Vesuvius, Campi Flegrei and Ignimbritesedited by B. De Vivo© 2006 Elsevier B.V. All rights reserved. 183

*Corresponding author. Fax: 139-81-6100811. E-mail address: [email protected] (M. Piochi).

AQ1

Else_DV-DEVIVO_ch009.qxd 3/2/2006 2:53 PM Page 183

and references therein) aimed at evaluating how the erupted magmas reflect the contribu-tions of mantle sources, how their compositions have been affected during transport, and towhat extent they can be used to deduce their geodynamic setting. Recently, a large major,trace and isotope (Sr, Nd, Pb, O) database has been published (De Vivo et al., 2003) and canbe downloaded at the Internet site http://www.dgv.unina.it/ricerca/de_vivo.htm. Thesummary of results shows that rocks produced during major plinian and sub-plinianeruptions, and during the last interplinian period of activity which started in 1631 AD, arerelatively well characterized on the basis of mineralogy, chemistry and isotopes. Adequatedata also exist for some rocks from interplinian periods of volcanism occurring before thelast sub-plinian eruption in 1631 AD.

In this paper, we briefly present a description of the chemical and isotopic database anda synthesis of previous petrological studies in order to summarize the main evidence formantle source heterogeneity associated with the Somma-Vesuvius magmas, and highlightthe results supporting the importance of shallow-level evolution. Particularly, our briefreview of existing data points to a magma feeding system formed by multi-depth storagelevels; the magma reservoir at 8 km imaged by seismic tomography (Zollo et al., 1996) fedboth low- and large-magnitude eruptions. Significant progress has been made in the last20 years of research focused on Somma-Vesuvius volcano (Civetta and Santacroce, 1992;Belkin et al., 1993; Villemant et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; Del Moroet al., 2001; Peccerillo, 2001; Fulignati et al., 2004, 2005; Pappalardo et al., 2004; Piochiet al., 2005), and it is now possible to combine the results of previous studies to produce aframework for more detailed investigations of the behaviour of magma and the magmafeeding system in Somma-Vesuvius volcano.

2. Volcanological and magmatological background

Somma-Vesuvius is a strato-volcano (Fig. 1a) that consists of an older collapsed edifice(Somma), and a younger cone (Vesuvius). The volcano has been active at least since 300 kybp (Brocchini et al., 2001 and references therein) up to the major eruption of 1944 AD.Presently, the volcano is the site of fumaroles, diffuse degassing (Chiodini et al., 2001;Federico et al., 2002; Frondini et al., 2004) and low-magnitude seismicity (Bianco et al.,1999; Vilardo et al., 1999). Volcanism has been characterized by high explosive sub-plinianand plinian eruptions that followed long periods of quiescence, and by intermediate andsmall-scale explosive and explosive/effusive eruptions that occurred during continuousperiods of activity (interplinian period) (Fig. 1b) (Arnò et al., 1987; Civetta and Santacroce,1992; Rolandi et al., 1998; Principe et al., 2004). Sub-plinian and plinian eruptions havealways produced larger volumes of rocks (one to a few cubic kilometres DRE, i.e. DenseRock Equivalent) (Rosi and Santacroce, 1983; Arnò et al., 1987; Civetta and Santacroce,1992; Rolandi et al., 1993; Cioni et al., 1995; Landi et al., 1999) than the intermediate andsmall-scale events (0.01–0.1 km3 DRE) (Scandone et al., 1986; Mastrolorenzo et al., 1993;Rolandi et al., 1998; Arrighi et al., 2001).

The volcano rests on a sequence of Mesozoic and Cenozoic carbonates overlain byMiocene sediments outcropping in the surrounding Apennine chain (D’Argenio et al.,1973; Ippolito et al., 1975) and encountered at a depth of around 2 km (Brocchini et al.,2001). The Moho discontinuity has been detected at about 30 km of depth (Corrado and

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

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Rapolla, 1981; Ferrucci et al., 1989; Chiarabba et al., 2005). A high-velocity body dippingwestward from 65 km down to 285 km was interpreted as a plate within the mantle (De Natale et al., 2001). Furthermore, an active, large magma chamber is located at about8–10 km (Zollo et al., 1996; Di Maio et al., 1998) and has been proposed to extend up to30 km (De Natale et al., 2001). However, based on fluid and melt inclusion evidence,magma storage is indicated at 3.5–5, 8–10 and � 12 km (Belkin et al., 1985; Belkin andDe Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000; Lima et al., 2003).At present, no geophysical evidence for magma chambers of significant lateral extensionhas been found at � 8 km (Zollo et al., 1996; Di Maio et al., 1998). This may be due to

The magma feeding system of Somma-Vesuvius (Italy) strato-volcano 185

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Figure 1. (a) DTM of the Somma-Vesuvius strato-volcano; (b) Reconstructed stratigraphy of volcanic activityduring the last 25 ka. Source: Arnò et al. (1987); Arrighi et al. (2001); Ayuso et al. (1998); Landi et al. (1999);Rolandi et al. (1993, 1998); Rosi and Santacroce (1983). Symbols as used in the following figures. Names oferuptions in parenthesis are from Arnò et al. (1987).

25.0 ky.B.P.Codola

18.6 ky.B.P.Sarno (Pomici di Base)

16-14 ky.B.P.Novelle (Verdoline)

3.5 ky.B.P.Avellino

8.0 ky.B.P.Ottaviano (Mercato)

A.D.79Pompei

Ancient Historic

A.D.1139

A.D.1631

Protohistoric

Medieval

Recent

A.D.194418th (1907-1944)17th (1874-1906)16th (1870-1872)15th (1864-1868)14th (1854-1861)13th (1841-1850)12th (1835-1839)11th (1825-1834)10th (1700-1707)

9th (1783-1794)8th (1770-1779)7th (1764-1767)5th (1712-1737)4th (1700-1707)3rd (1696-1698)2nd (1685-1694)1st (1638-1682)

1st (~1758B.C.) 2nd (~1414 B.P) 3rd (~832 B.C.)

Repose time6000 years

Repose time ??

Repose time800 years

Repose time

B.C.700

A.D.472(Pollena)

III

cycl

eI

cycl

e

Repose timeA.D.303

II c

ycle

Plinian Activity Inter-Plinian ActivityT

rans

ition

al

Somma activity

Older

a)b)

Somma

Vesuvius

No geochronologic determinations

2nd ( A.D.~635)1st (>A.D. 512)

4th(~A.D.1095.)3rd (>A.D.893.)


the fact that resolution for the method used in tomography investigations is “blind” formagma chambers with lateral extension � 1 km.

3. Mineral, chemical and isotopic data: description and previous interpretations

3.1. Mineralogy and classification

Somma-Vesuvius volcanic rocks are poorly (lava) to highly (scoria to pumice) vesiculated,and nearly aphyric (mostly in the plinian eruptions) to strongly porphyritic (up to 50%; in472 AD eruption and in the products younger than 1631 AD) (Joron et al., 1987; Villemantet al., 1993). Two rock types are generally distinguishable on the basis of occurrence ofleucite minerals. In leucite-free rocks, olivine and Mg-rich diopside, plagioclase, Fe-richdiopside, K-feldspar, magnetite and biotite can also occur, depending on the degree ofevolution. Leucite-bearing rocks contain olivine, Fe-rich and Mg-poor diopside, plagioclaseand oxide, also depending on the degree of evolution. Apatite, amphibole, garnet, phlogo-pite and forsterite are present as accessory phases. Nepheline, as the only feldspathoid, andscapolite have been occasionally recovered (e.g. 472 AD and Avellino rocks).

Feldspar (both K-feldspar and plagioclase) is the most abundant mineral phase inleucite-free rocks, such as Avellino and Sarno (Pomici di Base) (Joron et al., 1987; Landiet al., 1999), as well as in 79 AD leucite-bearing pumices (Cioni et al., 1998). Instead, diop-side is the most common mineral in the products younger than 1631 AD, whose abundancechanges as function of the degree of vesicularity of rocks (Villemant et al., 1993).Clinopyroxenes have compositions indicative of multiple stages of crystallization in theupper (� 10 km) crust (Trigila and De Benedetti, 1993; Marianelli et al., 1995). Olivinesfrom 1944 and 1906 AD eruptions show compositions similar to olivine from peridotite(Marianelli et al., 1995) and high pressure (� 400 MPa) of volatile entrapment (Marianelliet al., 1999) indicative of very early stage of magma crystallization.

Metamorphosed carbonates, skarns, lavas, cumulates, hornfels, sub-volcanic igneousrocks have been generally recovered as xenolith ejecta within pyroclastic deposits (Savelli,1967; Barberi and Leoni, 1980; Hermes and Cornel, 1981; Belkin et al., 1985; Del Moroet al., 2001; Gilg et al., 2001; Fulignati et al., 2004, 2005). Metamorphosed carbonateejecta are considered to be representative of the carbonate basement modified duringcontact metamorphism under the pressure of 1500–2000 bars. Skarn xenoliths consist ofcalc-silicate and carbonatic components and contain fassaitic pyroxene, forsterite (Fo�90),spinel, calcite, phlogopite, nepheline, garnet, periclase, brucite, calcite, and dolomite. Theyare considered as representative of the crystallizing margins of the magma chamber (DelMoro et al., 2001; Gilg et al., 2001; Fulignati et al., 2004, 2005). However, these xenolithswere also interpreted to represent highly metasomatized blocks of stopped carbonatesincorporated into the magma (Hermes and Cornell, 1981). Silicate melt inclusions fromskarns show homogenization temperatures (Th) of 1000 � 50°C and trapping pressuresbetween 925 and 3550 bars (Belkin et al., 1985; Fulignati et al., 2004). Hornfels arecharacterized by rhyolitic vesiculated glass and minerals of wollastonite, anorthite, calcite,pyroxene and quartz, and have been considered the products of high-grade thermometa-morphism from marly siltite rocks (Del Moro et al., 2001; Fulignati et al., 2005).Cumulates are dunites, wherlites and biotite-bearing pyroxenites (Joron et al., 1987; Belkin


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and De Vivo, 1993). The cumulus phases are clinopyroxene, phlogopite, biotite, apatite,plagioclase and olivine with Fo80–90; glass also occurs between individual crystal grains orwithin cavities. Leucite is rare in cumulate nodules. Spinel and chromite can occur asaccessory phases. Th and trapping pressure of silicate melt inclusions in cumulates are1200 � 50°C and in the range 1200–3050 bars, respectively (Belkin et al., 1985).

3.2. Major and trace elements

It is well known that rocks from Somma-Vesuvius are characterized by large compositionalvariations. These rocks show variable alkali contents (Fig. 2a), and, in particular, showvariable degree of K2O enrichment. These rocks are slightly, mildly and highly silicaundersaturated, following Peccerillo (2003). Slightly silica-undersaturated volcanic rocksare leucite-free and range in composition from shoshonites to trachy-phonolites; mildly tohighly undersaturated, nepheline- or, more commonly, leucite-bearing rocks, range fromalkali-basalt to phonolite.

Plinian and sub-plinian deposits are generally characterized by the most evolved com-positions and chemical gradients through the stratigraphic sequence. The basal part ofdeposits (white pumices) always shows the more sialic compositions, and the evolutiondegree decreases upwards (grey pumices) (Arnò et al., 1987; Civetta et al., 1991; Civettaand Santacroce, 1992; Rolandi et al., 1993; Cioni et al., 1995; Landi et al., 1999). Thesefeatures possibly reflect the progressive withdrawal of a chemically (and density) stratifiedmagma chamber located at shallow depth beneath the volcano. The variable layers can belinked through simple chemical differentiation of unique parental magma (Landi et al.,1999) or can be generated due to the arrival of diverse magma batches from deeperreservoirs (e.g. Civetta et al., 1991; Cioni, 2000). Sometimes, the occurrence of productswith compositions intermediate between that of the different layers indicates syn-eruptivemingling of magmas or the existence of a double-diffusive interface between the two mag-matic layers within the magma chamber (Landi et al., 1999).

Because of the occurrence of carbonate and metamorphic ejecta (see previous section),it has been suggested that plinian and sub-plinian chambers formed within the carbonatebasement, between 5 and 8 km depth (Barberi and Leoni, 1980; Belkin and De Vivo, 1993;Landi et al., 1999; Cioni, 2000) during the long time of quiescence that precedes theeruption (Fig. 1b) and that allows reaching the high evolution degree of these rocks.

Magmas erupted during interplinian periods are characterized by low degree of evolution(Fig. 2) and depths of storage at � 5 km, 8–10 km and � 12 km (Belkin et al., 1985; Belkinand De Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000; Lima et al.,2003). Owing to the occurrence of deeply crystallized olivines (see previous section), theexistence of CO2-bearing melt inclusions and the brief repose time between two eruptions(not more than 7 years) (Arnò et al., 1987), the various authors indicate that duringinterplinian periods magmas can rapidly rise to the surface in open-conduit conditions. Thelast 1944 AD eruption was fed by a magma directly rising from a depth of � 12 km. After61 years of volcanic quiet, this latter eruption probably closes the third, last mega-cycle ofvolcanism (Ayuso et al., 1998) and marks the transition to the closed-conduit condition(Rosi et al., 1987). This situation of repose might last for centuries, heading towards thestarting of new, fourth, mega-cycle of volcanism, with a new plinian–sub-plinian eruption


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Figure 2. (a) T.A.S. (Le Bas et al., 1986); (b) Sr versus SiO2 contents; and (c) La versus MgO for SV rocks.Symbols as in Figure 1: bold crosses are dykes from Somma activity; closed symbols are rocks from plinian andsub-plinian events; and open symbols rocks from interplinian periods. Circles, first magmatic cycle; rhombus,second magmatic cycle; triangles, transitional magmatic cycle; squares, third magmatic cycle. Source: Cioni et al. (1995); Civetta et al. (1991); Civetta and Santacroce (1992); De Vivo et al. (2003); Marianelli et al. (1999);Santacroce et al. (1993).

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Picro-basalt

Basalt

Basalticandesite

AndesiteDacite

Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Latite

Shoshonite

Trachy-basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

FoiditeN

a 2O

+K2O

(w

t%)

SiO2 (wt%)

SiO2 (wt%)

a)

45 50 55 60 650

400

800

1200

1600

La (ppm)

b)

0 3 6 920

40

60

80

100

120

MgO (wt%)

c)

Sr (ppm)


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

In many studies, SiO2 or MgO have been utilized as differentiation indices. Variationsin SiO2 seem to adequately illustrate the evolution of intermediate-to-most fractionatedrocks (Fig. 2b), but not the least-fractionated rocks. In contrast, MgO, appears moreadequate for the least-fractionated rocks (Fig. 2c). In any case, based on major and traceelements variations (Fig. 2a–c), diverse evolutionary trends characterized by variable K, P,Ti, some trace elements (i.e. Th, U, Sr) and LREE enrichment have been found (Joron et al., 1987; Ayuso et al., 1998; Piochi et al., 2005). Within each trend, the role of crystalfractionation processes in magma evolution has been widely accepted (Joron et al., 1987;Civetta et al., 1991; Ayuso et al., 1998; Piochi et al., 2005 and references therein).

Feldspar and clinopyroxene are the main crystallizing minerals, in agreement withpetrographic data reported in previous section. Chemical trends in Sr and CaO/Al2O3 versusK2O diagrams (Fig. 3a,b) suggest clinopyroxene associated with feldspar (mostly plagio-clase and subordinately K-feldspar) crystallization during evolution of magmas older than472 AD eruption (Piochi et al., 2005). The Sr versus Th diagram (Fig. 3c) highlights plagio-clase fractionation. In contrast, clinopyroxene crystallization dominated during evolution ofhighly undersaturated magmas of the post-1631 AD interplinian period, as also suggested byBelkin et al. (1993), Villemant et al. (1993) and Trigila and De Benedetti (1993). In theseyounger rocks, the variable abundance of clinopyroxene affects major- and REE-elementsvariation (Belkin et al., 1993; Villemant et al., 1993). REE showing fairly homogeneouspatterns and variable LREE enrichment support the above data. In particular, the Eu anom-aly is not a typical feature of primary magmas from Somma-Vesuvius. It seems to becorrelated with the degree of evolution; it is mostly present in highly evolved rocks, such as79 AD, Avellino, and probably reflects feldspar fractionation (Joron et al., 1987).

In the MORB- and OIB-multi-elements normalized diagrams (Fig. 4a,b) rocks fromSomma-Vesuvius show similar trace elements distribution, regardless of the degree of silicaundersaturation and K enrichment. The least evolved rocks (MgO � 3 wt%) are character-ized by high LILE (Rb, Ba, Th, K) and slight HFSE (Zr, Nb) enrichment, and slight Nb andTa trough with respect to MORB (Fig. 4a), similarly to other potassic magmas (Peccerilloand Manetti, 1985; Peccerillo, 2001, 2003). Furthermore, these rocks have higher Cs, K, Pb,Rb, Th, Ba and lower Nb and Ti contents compared to OIB (Fig. 4b). A heterogeneousmantle source(s) has been therefore proposed to explain the variable undersaturation degreeof the rocks and, in particular, the occurrence of different parental magmas and differentevolutionary trends as shown in Figure 2 (Civetta et al., 1991; Civetta and Santacroce, 1992;Ayuso et al., 1998; Piochi et al., 2005). Other authors (Rittmann, 1933; Pappalardo et al.,2004; Piochi et al., 2005) have also speculated that crustal contamination processescontributed to the enrichment in K and in various other trace elements.

3.3. Sr, Nd, Pb, Hf, O and He isotope ratios

The variable silica-undersaturated Somma-Vesuvius volcanic rocks show similar range of Sr,Nd, Pb and O isotopic compositions, with large variability within each cycle. 87Sr/86Srisotopic values span from 0.706283 to 0.708070 (Cortini and Hermes, 1981; Civetta and


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Santacroce, 1992; Caprarelli et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; De Vivo et al., 2003; Piochi et al., 2005). The 143Nd/144Nd values range from 0.51225 to 0.51226 (Fig. 5a). Pb isotopic compositions have a moderate variation (Fig. 5b): 206Pb/204Pb valuesvary from 18.94 to 19.09, 208Pb/204Pb from 38.7 to 39.3 and 207Pb/204Pb from 15.61 to 15.71


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Figure 3. (a) Sr versus Al2O3; (b) Sr versus K2O; and (c) Sr versus Th contents for Somma-Vesuvius rocks.Symbols and source of data as in Figure 2.

0.0 0.4 0.8 1.2 1.6 2.00

400

800

1200

1600

2 4 6 8 100

400

800

1200

1600

CaO/Al2O3

K2O (wt%)

a)

b)

feld

+cpx

cpx

cpx

feld+cpxSr

(pp

m)

0 20 40 60 80 1000

400

800

1200

1600

Th (ppm)

c)

feld+cpx

cpx


(Somma et al., 2001; De Vivo et al., 2003; Cortini et al., 2004). Pb isotope variations are notcorrelated to Sr and Nd isotope variations. δO18 values obtained on whole-rocks range from7.5% to 10‰, showing no correlation with Nd and Pb isotopic compositions, and defines notypical correlation with the 87Sr/86Sr ratio (Fig. 5c) (Wilson, 1989). Among the isotopes, onlyδO18 correlates (positively) with degree of chemical evolution (Fig. 6a,b). He isotope com-position is about 2.4 Ra (where Ra is the 3He/4He of the atmosphere equal to 1.40 � 10−6)(Graham et al., 1993) for 1944 AD olivines and pyroxenes, indicating a source within the


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Figure 4. Spider diagrams for selected Somma-Vesuvius rocks with MgO � 3 wt%. Source of data andsymbols as in Figure 2.

.1

1

10

100

Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb

.1

1

10

100

Cs Rb Ba Th U Nb K La Ce Pb Pr Sr P Nd Zr Sm Eu Ti Dy Y Yb Lu

Roc

k/M

OR

BR

ock/

OIB


lithospheric or in a slab-enriched mantle source. Similar He-isotopic values have been meas-ured in fumarole gases suggesting a magmatic contribution to the degassing observed at thesurface (Graham et al., 1993). 176Hf/177Hf ratios determined on two Somma-Vesuvius rockscharacterized by Sr isotopic values lower than 0.7072 are 0.282784 and 0.282786, suggest-ing a pelagic component added to HIMU and DM mantle sources (Gasperini et al., 2002).


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Figure 5. Isotopic diagrams for Somma-Vesuvius rocks: (a) 87Sr/86Sr versus 143Nd/144Nd ratios; (b) 208Pb/204Pbversus 206Pb/204Pb; and (c) δO18 versus 87Sr/86Sr ratio. Symbols and source of data as in Figure 2.

0.7060 0.7064 0.7068 0.7072 0.7076 0.70800.5122

0.5123

0.5124

0.5125

0.5126

87Sr/86Sr

18.90 18.94 18.98 19.02 19.06 19.1038.6

38.8

39.0

39.2

39.4

206Pb/204Pb

208Pb/204Pb

0.7060 0.7064 0.7068 0.7072 0.7076 0.70806

7

8

9

10

11

143Nd/144Nd

87Sr/86Sr

δO18

a)

b)

c)


The Sr isotope compositions of products from plinian and sub-plinian eruptions followa systematic trend through the stratigraphic sequence, consistent with the previouslyrecognized chemostratigraphy (see previous section) though to represent magmas residingin a shallow and chemically stratified chamber (Civetta et al., 1991). For example, theAvellino and the 79 AD pyroclastic sequences consist of white pumices, at the base, over-lain by grey pumice deposits. White and grey pumices have different chemical and Sr iso-tope compositions. However, both pumice types contain feldspars with a constant Srisotopic composition, similar to that of white pumices, suggesting Sr isotopic disequilib-rium in rocks upwards in the sequence and mingling of magmas during eruption.Moreover, the lowermost part of the 79 AD eruption and the uppermost part of Avellinohave similar 87Sr/86Sr values, suggesting that magma remnants can be left behind withinthe chamber after large magnitude events (Civetta et al., 1991; Civetta and Santacroce,1992). Such a type of incomplete magma removal has also been suggested by evidenceshowing that events following plinian or sub-plinian eruptions produced magmas that haveisotopic characteristics comparable to those of previous eruptions (Civetta and Santacroce,1992; Piochi et al., 2005) (Fig. 7).


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Figure 6. δO18 versus CaO/Al2O3 ratio (a) and Sr (b) for Somma-Vesuvius rocks. Lines indicate trend of magmaevolution. Symbols and source of data as in Figure 2.

0.0 0.2 0.4 0.6 0.8 1.07

9

11 δO18

CaO/Al2O3

0 400 800 12007

9

11

Sr (ppm)

a) b)

Figure 7. 87Sr/86Sr versus age of rocks from Somma-Vesuvius. Symbols and source of data as in Figure 2.

10 100 1000 100000.7060

0.7064

0.7068

0.7072

0.7076

0.7080 87Sr/86Sr b)


The 87Sr/86Sr isotopic variations have been attributed to the arrival of isotopicallydiverse magma batches generated in a variable mantle source(s) (Cortini and Hermes,1981; Civetta and Santacroce, 1992; Caprarelli et al., 1993; Cioni et al., 1995; Ayuso et al.,1998; Piochi et al., 2005). Recently, as first recognized by Rittmann (1933), variousauthors (Civetta et al., 2004; Pappalardo et al., 2004; Paone, 2005; Piochi et al., 2005)suggested the fundamental role of crustal contamination in modifying the isotopiccomposition of erupted magmas at Somma-Vesuvius. Civetta et al. (2004) and Paone(2005) proposed that contamination occurred within a Hercynian-like basement, similarlyto what happens at the Campi Flegrei (Pappalardo et al., 2002). Pappalardo et al. (2004)and Piochi et al. (2005) suggested that carbonate was the main contaminant. In particular,based on Sr isotope variations through time, Pappalardo et al. (2004) suggested thatbetween 1631 and 1944 AD the degree of magma contamination decreased owing tomagma rising from a deep reservoir in open-conduit conditions.

4. Discussion

The relationship between magma compositions and tectonic setting depends on reliablydistinguishing among geochemical features that image the source region and those thatresulted from magma evolution during transport. Processes affecting magmas after theirgenesis are important in characterizing the behaviour of the magmatic supply system. Suchprocesses, for example, fractional crystallization, can produce highly evolved magmas,which when associated with long-lived magma storage in the crust can generate high-magnitude explosive events. Recharge of distinct magma batches from deeper levels withinthe feeding reservoir may be required to trigger volcanic eruptions. Crustal contaminationrequires chemical exchange between magma and wall rocks that can lead to fluid enrich-ment, increasing the possibility of highly explosive eruptions, or that can induce quickcooling and/or crystallization of magma limiting its further mobility. Properly identifyingthe exact mechanism of magma evolution, i.e. magma mixing or crustal contamination, canbe a useful tool for hazard assessment studies. For the Somma-Vesuvius volcano, it wouldbe important to determine to what extent the evolution of the magmas depend on involve-ment of the crust during magma genesis (with heterogeneously slab-enriched mantlesources) or during magma evolution (Rittman, 1933; Savelli, 1967, 1968; Turi and Taylor,1976; Vollmer, 1976; Civetta and Santacroce, 1992; Santacroce et al., 1993; Cioni et al.,1995; Ayuso et al., 1998; Peccerillo, 2001; Pappalardo et al., 2004; Piochi et al., 2005), andhow the geochemical evolution exactly triggers sub-plinian and plinian eruptions.

4.1. The role of crustal component on magma composition

The role of the crust on magma composition at the Somma-Vesuvius volcano is suggestedfrom both mineralogical and compositional data. For example, phlogopite occurs amongmineral phases. Th/Yb is always higher than 2 (Peccerillo and Manetti, 1985; Peccerillo,2001). Ce/Pb ratios, being significantly lower than those of mantle sources free of subduc-tion influences (� 25; Hofmann et al., 1986), tend towards the upper crustal value (� 3.5;Taylor and Mc Lennan, 1985). Similarly, Nb/U value mostly falls within the continentalcrustal range (� 12; Rudnick and Fountain, 1995) (Fig. 8).


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In addition, the role of the crust is also suggested from Sr, Pb and O (as well as Hf)isotope ratios. In fact, these isotope ratios, although highly scattered, show rough correla-tions with the above chemical ratios: Ce/Pb negatively correlates with 87Sr/86Sr and δO18,Nb/U positively correlates with Sr isotope composition (Fig. 8a–c). These ratios do notdepend on the stage of evolution of the rocks because Ce and Pb, as well as Nb and U, showalmost comparable behaviour with respect to SiO2 or MgO, suggesting a similar partitioncoefficient in the melt. The observed correlations can be attributed to the variable contri-butions of the crustal component to the magma.


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Figure 8. (a) 87Sr/86Sr versus Ce/Pb ratios; (b) 87Sr/86Sr versus Nb/U ratios; and (c) δO18 versus Ce/Pb ratios forSomma-Vesuvius rocks. Symbols and source of data as in Figure 2.

1 2 3 4 5 6 7 80.7060

0.7064

0.7068

0.7072

0.7076

0.708087Sr/86Sr

Nb/U

2 3 4 5 6 7 8 9 100.7060

0.7064

0.7068

0.7072

0.7076

0.7080

Ce/Pb

Ce/Pb

δO18

1 2 3 4 5 6 7 86.5

7.5

8.5

9.5

10.5

11.5

a)

b)

c)


One important problem is to establish if the crustal component was involved at the timeof melting of the source or subsequently during ascent. Generally, radiogenic and stableisotopes can be used to define the site at which contamination occurs. Nevertheless,available O and Sr isotopic data do not conclusively provide information about input ofcrustal materials/fluids to the magma (either in the mantle source or during shallow-levelsdifferentiation processes), although we know that higher O isotope compositions are foundin plinian-type rocks. Below we report some evidence that can be helpful to deal with thisfundamental question.

The generally low Mg, Ni and Cr (most values are � 40 and 100 ppm, respectively)contents, and high crystallinity suggest the importance of processes occurring in magmasduring crustal storage and ascent. Chemical exchange processes between magmas andcarbonate wall rocks are indicated by garnet and phlogopite (Belkin et al., 1985; Joron et al., 1987) and by Ca–Mg-silicate-rich ejecta (skarns) (Savelli, 1968; Fulignati et al.,1995, 1998, 2005; Gilg et al., 1999, 2001; Del Moro et al., 2001). Oxygen isotope studies(Turi and Taylor, 1976; Ayuso et al., 1998), U-disequilibria (Black et al., 1998) and Pb iso-tope data (Cortini et al., 2004) document shallow-level evolution of Somma-Vesuviusmagmas as open systems. Nevertheless, the strongest evidence for the dominating role ofshallow-level (crust) processes subsequent to high-pressure (mantle) processes derivesfrom a synthesis of Sr isotope and fluid inclusion data that suggests a positive correlationbetween 87Sr/86Sr values and the estimated depths of mineral crystallization (Fig. 9). Thesuggestion is that products enriched in radiogenic Sr formed during later stages of magmaevolution (Pappalardo et al., 2004).

The lower 87Sr/86Sr ratios (mostly around 0.7071–0.7072 with few spikes at0.7062–0.7068) are associated with the highly silica-undersaturated rocks from the 1944 AD

eruption containing primitive olivine compositions (Marianelli et al., 1995). These ratios par-tially overlap the Campi Flegrei Sr-isotope range (0.7068–0.7086) (Pappalardo et al., 2002),


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Figure 9. 87Sr/86Sr versus depth of crystallizing phases from SV rocks. Squares, clinopyroxene; rhombus,feldspar; and triangles, leucite. Source of data as in Figure 2 (modified from Pappalardo et al. (2004). Grey areasindicate probably levels of magma storage, based on fluid inclusion, volcanological and seismic data (see text).

0

2

4

6

8

10

12

0.7070 0.7074 0.7078

87Sr/86Sr

Dep

th -

km

16

14

18

20


differ from values recovered at the nearby Procida (0.70523–0.70678) (De Astis et al., 2004) and are higher than the Tyrrhenian Sea basalts (0.70733–0.7056) (Beccaluva et al., 1990). In addition, they are associated with 176Hf/177Hf ratios of 0.282785 (two 1944 AD

samples reported in Gasperini et al., 2002) and He isotope ratio lower than MORB-likemagmas (Graham et al., 1993). Moreover, the 1944 AD eruption, and other rocks that aregenerally poorly evolved (MgO � 3 wt%), are enriched in LILE, LREE and other incompat-ible trace elements (e.g. Th, Nb, Ta), as well as in more compatible elements such as HREEand Y (Fig. 4a). These geochemical features are usually related to magmas erupted alongsubduction zones, implying the involvement of a crustal component in the mantle sourcebeneath Somma-Vesuvius.

4.2. The mantle source

The least-evolved Somma-Vesuvius rocks (MgO � 3 wt%) belong to the within-platetype in term of Zr (� 100 ppm) and Zr/Y (� 4) (Pearce and Norry, 1979) (Fig. 10), inagreement with evidence from the multi-element normalized diagram (Fig. 4b) showinga certain similarity to the OIB basalts. The positive correlation in Figure 10 points to adecrease in degree of partial melting or (fluid-controlled) source heterogeneity. Based onthe Cs–Pb enrichment in Figure 4b, the LILE enrichment and the slight Nb–Ta negativeanomalies in Figure 4a, and Nb/Zr at about 0.15, as well as on the isotope features dis-cussed in the previous section, we suggest that the mantle source of Somma-Vesuviuscontains a slab-derived component. This conclusion is consistent with the general ideathat enriched potassium-rich magmas are generated by partial melting of phlogopite-richgarnet peridotite (Gupta and Fyfe, 2003).

Poorly evolved rocks (MgO � 3 wt%) with a high degree of silica undersaturationshow significant constancy of Th/Zr (0.05–0.08), Ta/Yb (0.7) and Cs/Rb (� 0.06), as wellas Th/Yb, Th/Ta and other ratios, that are independent of fractional crystallization and/orpartial melting. These relatively unevolved rocks, as well as the slightly and mildly silica-undersaturated rocks, have comparable trace elements distributions, showing similar


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Figure 10. Zr and Zr/Y for SV rocks with MgO �3 wt%. Source of data and symbols as in Figure 2. Data fromPhlegraean Fields (D’Antonio et al., 1999; Pappalardo et al., 1999; Piochi et al., 1999) and Tyrrhyenian Sea(Beccaluva et al., 1990) are also reported for comparison.

Zr (ppm)

Zr/Y

Vesuvius

Phlegraean area

Tyrrhenian sea

100 10001

10


enrichment in LILE, Ce and other incompatible trace elements (e.g. Th, Nb, Ta), as wellas in more compatible elements such as Sm and Y (Fig. 4) independent of their Sr isotopevalues and K-enrichment degree. Therefore, in a general sense, these data suggest theexistence of an invariable mantle source during the life of the Somma-Vesuvius volcano.In agreement with Peccerillo and Manetti (1985), we suggest that diverse degrees of silicaundersaturation in potassic “mafic” rocks was linked to small degrees of partial meltingat different pressures in a phlogopite-bearing potassium-rich peridotitic mantle sourcecontaining CO2 and small amounts of water. Sr, Nd, Pb, O, He and Hf isotopes were likelyaffected by processes in the mantle source. However, with our hypothesis, the absence ofrelationships between Sr–Nd isotope compositions and degree of both alkali enrichmentand silica undersaturation of “mafic” rocks suggests that mantle source processes mostlyinfluence the chemical composition of parental magmas, but it cannot be the main causeof the large isotopic variability of Somma-Vesuvius rocks with 87Sr/86Sr ratios higher than0.7071.

4.3. The behaviour of the magmatic feeding system

Based on the variation of the 87Sr/86Sr values, contamination of Somma-Vesuvius magmaswas attributed to a Hercynian-like basement (Civetta et al., 2004; Paone, 2005) or to rocksin the overlying sedimentary series (Rittmann, 1933; Pappalardo et al., 2004; Piochi et al.,2005). However, on the basis of data in Figure 9 we suggest that the increase in Sr isotopevalues from 0.7071-3 to 0.7081 mostly occurs within the uppermost 11–12 km of the crustand points to these sedimentary rocks as the main crustal contaminant. However, we can-not exclude that magma contamination could have occurred in crustal rocks underlying thecarbonate basement. We stress the fact that no xenolith of possible Hercynian origin hasbeen found at Somma-Vesuvius, contrary to what happened at the nearby Campi Flegrei(Pappalardo et al., 2002; Paone, 2005).

Contamination of magma (87Sr/86Sr � 0.7071) by carbonate rocks (87Sr/86Sr �0.7073–00709; Sr � 700–1000 ppm) (Civetta et al., 1991; Iannace, 1991) at Somma-Vesuvius has been quantitatively modelled by Pappalardo et al. (2004) and Piochi et al.(2005) who suggested that crustal contamination was a selective process involving thermaldecomposition (decarbonation reactions) of the sedimentary wall rocks and exchangebetween magmas and fluids. Fulignati et al. (2004, 2005) also suggested similar conclu-sions on the basis of geochemical and mineralogical data collected on 79 and 1944 AD

skarn ejecta. We recognize, however, that magma evolution was likely more complicatedthan as stated previously because no correlation has been found for δO18 and 87Sr/86Srvalues, and because of the negative correlation between phenocryst abundance and valuesof 87Sr/86Sr (Figs. 5c and 11). Moreover, hornfels rhyolitic pumices characterized by87Sr/86Sr higher than 0.711 and δO18 at around 15‰ have been found among ejecta in var-ious pyroclastic deposits and have been interpreted as the result of the partial melting ofthe pelitic sediments during thermometamorphic event (Del Moro et al., 2001; Fulignati etal., 2005). This fact suggests the possible involvement of Miocene sediments in additionto carbonate during the evolution of magmas at the Somma-Vesuvius.

Fluid exchange between magmas and wall rocks could be more pervasive on magmasassociated with high-explosive eruptions. Available data reveal relatively high values and


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a large range of δO18 for pumices from plinian and sub-plinian eruptions, and relativelylow δO18 values and a smaller range for highly silica-undersaturated volcanic rocks frominterplinian events (Figs. 5c, and 6a,b). The correlation for δO18 and chemical differentia-tion indices (better defined for rocks from high-explosive eruptions), together with numer-ical considerations reported in Ayuso et al. (1998), data from Cortini et al. (2004) and theobserved enrichment in some incompatible trace elements (La, Nb, Zr) of pumices fromplinian eruptions (Fig. 2c), also support the effects of fluid exchange, rather than isotopefractionation determined by exsolution of gas from magma.

Magmas erupted during the post-1631 AD interplinian period are characterized by thedecrease of the 87Sr/86Sr ratio with increasing phenocryst content down to typical values ofclinopyroxenite (� 0.7071) (Del Moro et al., 2001). This relation can be attributed to (1) theentrapment of crystal mush generated during previous magma storage in the crust by risingmagmas and/or (2) the accumulation/depletion of phenocrysts during magma movementsthrough the crust towards the surface. In the first case, magmatic melts should be charac-terized by higher 87Sr/86Sr ratios. Otherwise, phenocrysts can be accumulated or be depletedin magma as a function of the ascent rate of magma towards the surface (see also Villemantet al., 1993). In particular, low ascent rate can result in crystal segregation and in longer timeduring which melt stay within wall rocks, thus producing rocks with lower crystal contentand possibly higher crustal contamination. This second hypothesis is in agreement with evi-dence from Villemant et al. (1993) indicating that lavas derived from magmas experiencingvolatile degassing generally contain lower crystal abundance than vesiculated fragmentsgenerated by gas overpressure. This idea is supported by evidence that magmas with thelowermost Sr isotope ratios erupted during the 1944 AD rose to the surface from 11–22 kmdepth (Marianelli et al., 1999). However, the repetitive and regular variation of 87Sr/86Srvalues through time (Fig. 7) is consistent with the idea that residual magma or crystal mushremaining in the magmatic system after the end of the plinian (or sub-plinian) eruptiveevent, can be involved in subsequent eruptions (Civetta et al., 1991; Civetta and Santacroce,1992; Cioni et al., 1995; Lima et al., 2003; Piochi et al., 2005).

87Sr/86Sr, δO18 and fluid inclusion data strongly suggest polybaric evolutionaryprocesses of diverse parental magmas at Somma-Vesuvius. Evolutionary processes were


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Figure 11. (a) 87Sr/86Sr versus phenocryst content in rocks from recent interplinian period of volcanism; (b) 87Sr/86Sr versus age of rocks. Symbols and source of data as in Figure 2.

10 20 30 40 500.7071

0.7073

0.7075

0.7077

0.7079

0.708187Sr/86Sr

% phenocrysts

a)


dominated by crustal contamination and crystal entrapment, in addition to crystal frac-tionation and magma mixing. Evidence presented in this paper, in particular data shown inFigure 9, allows us to speculate that magmas with 87Sr/86Sr ratios of around 0.7071-3 andof 0.7074-5 derive from reservoirs probably located at different depths, i.e. � 12 km andat around 8–12 km, respectively. Magmas with higher Sr isotope compositions, for exam-ple those from Pompei and Avellino eruptions, evolved during storage in shallower magmachambers or, for example those from some of post-1631 AD interplinian eruptions, duringthe ascent through the conduit.

5. Conclusions

Available data in the literature furnish the possibility to preliminarily define the magmafeeding system beneath the Somma-Vesuvius strato-volcano. It consists of three mainlevels of magma storage, the two deepest probably being long-lived reservoirs, and anuppermost crustal level that probably includes the volcanic conduit and hosted magmasduring interplinian period of volcanism. The deeper level is located at depths exceeding 15 km and should furnish magma with 87Sr/86Sr ratios of � 0.7072 and δO18 � 8‰. Theintermediate level occurs at around 8–12 km depth and supplies magmas with 87Sr/86Srratios between 0.7071 and 0.7074, and δO18 � 8‰ typically erupted both duringinterplinian (i.e. 1906 AD) and sub-plinian (472 AD, 1631 AD) events. The shallow level ataround 5 km depth was the site of plinian magma chambers such as those of Pompei andAvellino eruptions. This type of magma feeding system fits with fluid and melt inclusionsdata (Belkin et al., 1985; Belkin and De Vivo, 1993; Marianelli et al., 1999; Cioni, 2000;Lima et al., 2003) indicating magma storage at 3.5–5 km, 8–10 km and � 12 km, withresults of seismic (Zollo et al., 1996) and magnetotelluric (Di Maio et al., 1998) investiga-tions indicating a discontinuity at 8–10 km depth, with seismic evidence of deeper magmastorage extending up to 30 km depth (De Natale et al., 2001), and with the magnetizedcharacter of a narrow shallow crustal volume (Fedi et al., 1998). However, geophysical datado not indicate the occurrence of current magma storage at a depth of � 5 km, as vice versa is indicated by fluid and melt inclusion studies (Belkin et al., 1985; Belkinand De Vivo, 1993; Marianelli et al., 1999; Cioni, 2000; Lima et al., 2003).

Acknowledgements

The authors are thankful to A. Peccerillo for his constructive review, which helped toimprove the final version of the manuscript. The paper has benefited from MIUR-PRINfunds to B. De Vivo (2003–2004).

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