Completo_DEF.qxd 1471 ANNALS OF GEOPHYSICS, VOL. 47, N. 4, August 2004 Key words Somma-Vesuvius – magma chamber – crustal contamination 1. Introduction Vesuvius is one of the most dangerous vol- canoes in the world. At the present, half a mil- lion people live in a near-continuous belt of towns and villages around the area possibly ex- posed to the effects of future eruptions. Due to the high volcanic risk, Vesuvius is among the Mailing address: Dr. Lucia, Pappalardo, Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanolo- gia, Via Diocleziano 328, 80124 Napoli, Italy; e-mail: lucy@ov.ingv.it The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius: constraints on its behaviour and present state through a review of Sr-Nd isotope data Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, Napoli, Italy Abstract Vesuvius, dominating the densely-populated Neapolitan area, is one of the most dangerous volcanoes in the World. Its destructive power derives from energetic subplinian and plinian eruptions, such as the one which occurred in 79 A.D. Generally such large-scale events follow a long period of quiescence; a behaviour interpreted as the gradual build-up of magma volumes between periods of major activity. After the 1631 subplinian eruption until the last 1944 A.D. eruption, it experienced an almost continuous and less energetic explosive/effusive activity. The erupt- ed magmas are characterized by undersaturated potassic to ultrapotassic nature, and compositional and Sr-isotopic variability. Furthermore geobarometric studies indicate two different crystallization depths located at 4 and >11 km, respectively. According to most of the recent literature, the eruptions were triggered by the injection in a shallow- er magma chamber, of isotopically distinct magma batches derived from heterogeneous mantle source(s) and/or contamination processes occurred within the deep reservoir. In our review of petrochemical data, we consider the period between the 3550 years BP plinian eruption and the 472 A.D. sub-plinian eruption, which includes 79 A.D. event, and the most recent period of activity which started in 1631 A.D. and lasted up to the 1944 A.D. eruption, characterized by a near continuous effusive/explosive activity. For both periods we identify a correlation between Sr-isotopical features of magmas and their crystallization depth. In particular, we show that pyroxenes have Sr-iso- topic ratios lower than 0.7074 and an equilibrium crystallization depth of 22-11 km. Moreover feldspars have high- er 87Sr/86Sr values (0.7075-7) and an equilibrium crystallization depth of about 4 km. Therefore the most radiogenic magmas did not derive from a deeper reservoir but their higher Sr-isotopic ratios have been acquired at a shallow- er depth likely by crustal contamination during magma evolution. In contrast, the lower Sr-isotope compositions characterise the less contaminated magmas coming from deeper crustal levels. On the basis of this evidence, the temporal Sr-isotopical variation of magma which erupted in the 1631-1944 A.D. period probably derives from the progressive withdrawal of the shallow magma chamber, which was completely empty before the 1805-1944 A.D. period of volcanism. Therefore the effusive and explosive events of the most recent 1805-1944 A.D. period were fed directly by the deep reservoir located at a depth exceding 11 km. 1472 Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo best- studied and monitored active volcanoes. Much research (e.g., Santacroce, 1987; Civetta et al., 1991; Civetta and Santacroce, 1992; Mas- trolorenzo et al., 1993; Rolandi et al., 1993, 1998; Santacroce et al., 1993; Marianelli et al., 1995, 1999; Cioni et al., 1995, 1998; Arrighi et al., 2001; Peccerillo, 2001; Somma et al., 2001; Andronico and Cioni, 2002) has been devoted to reconstructing its volcanic and magmatic histo- ry with the aim of understanding the proceeding of geochemical and geophysical precursors and forecasting its future behaviour. In spite of the wealth of studies, many prob- lems related to magma genesis and evolution and to the present state of the Vesuvius plumb- ing system remain open: i) the intraplate versus subduction-related origin of potassic magma- tism (e.g., Beccaluva et al., 1991; Ayuso et al., 1998; Peccerillo, 2001); ii) the relative roles of source composition and low-pressure processes in determining the geochemical features of vol- canic products (e.g., Savelli, 1967, 1968; Turi and Taylor, 1976; Cortini and Hermes, 1981; Civetta et al., 1991; Civetta and Santacroce, 1992; Santacroce et al., 1993; Cioni et al., 1995; Fulignati et al., 1995; Ayuso et al., 1998; Fulignati et al., 1998; Del Moro et al., 2001; Gilg et al., 2001; Somma et al., 2001); and iii) the present state of the plumbing system, con- stituted or not by a shallower magma chamber (e.g., Santacroce, 1987; Zollo et al., 1996; De Natale et al., 2001). In this paper, petrological data are revisited to investigate the role of low-pressure evolutionary processes in magma genesis and their relation- ships with the style of eruptions (moderate ex- plosive/effusive or highly explosive) during the 3550 years BP-1944 A.D. period of activity for which a complete set of data have been collected. 2. Geological and petrological background Somma-Vesuvius (fig. 1a-c) is a strato-vol- cano consisting of a recent cone, the Vesuvius, which rises within the older Somma caldera and build ups on the Meso- and Cenozoic carbonatic succession cropping out in the nearest Apennine. The age of volcanic rocks ranges between not less than 300 ka (Brocchini et al., 2001 and ref- erences therein) and 1944 A.D. At present, Vesu- vius is in a quiescent state characterised by fu- marolic and low-magnitude seismic activities. Somma-Vesuvius has been characterized by both highly explosive subplinian and plinian eruptions, as well as intermediate and small scale explosive and explosive/effusive ones. The plinian eruptions, such as the well-known 79 A.D. event, were preceded by long periods of quiescence; a behaviour interpreted as the gradual build-up of the magma reservoir be- tween periods of major activity (e.g., Civetta et al., 1991; Civetta and Santacroce, 1992 and ref- erences therein; Mastrolorenzo et al., 2002). A ca. 300 years long period of continuous moder- ately explosive and effusive eruptions followed the 1631 A.D. subplinian event. However a quite moderate explosive activity can also be inferred for the periods between 79 A.D. and 1631 A.D. (Medioeval eruptions, Rolandi et al., 1998; Somma et al., 2001) and between the 79 A.D. and 472 A.D. (Ancient historic eruptions, Rolandi et al., 1998; Somma et al., 2001). A 300 km3 magma volume is inferred to have been erupted throughout the history of Somma-Vesuvius (Civetta and Santacroce, 1992). One to a few km3 DRE of magmas are generally erupted during sub-plinian and plin- ian eruptions (e.g., Santacroce, 1987; Civetta and Santacroce, 1992; Rolandi et al., 1993), while 0.01 to 0.1 km3 DRE of magmas are erupted during intermediate and small scale events characterizing the last 300 years (Scan- done et al., 1986; Mastrolorenzo et al., 1993; Arrighi et al., 2001). A near constant averaged output rate of about 0.3 m3/s has been calculat- ed for the whole volcano history (Scandone et al., 1986; Civetta and Santacroce, 1992). The magmatic system of Somma-Vesuvius produced silica undersaturated potassic (KS) to ultrapotassic (HKS) rocks and consisted of multi- depth reservoirs located at about 4 and 10 km as deduced by fluid and glass inclusion data (e.g., Belkin et al., 1985, 1998; Cortini et al., 1985; Belkin and De Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000). K-basalts to K-trachytes were produced in the oldest period of volcanism, before 11 500 years BP. Between 11.5 ka BP and 79 A.D., the eruptions produced K- phonolitic tephrites to K-phonolites. Finally, be- 1473 The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius Fig. 1a-c. Somma-Vesuvio map showing: a) Geological sketch map of Campanian Plain 1 = Quaternary and active terrigenous sediments; 2 = Somma-Vesuvius volcanic rocks; 3 = Phlegraean volcanic rocks; 4 = Roccamonfina vol- canic rocks; 5 = Pliocene and Miocene terrigenous sediments; 6 = Mesozoic carbonatic units; 7 = Faults; 8 = nappes. b) Reconstructed Somma-Vesuvius stratigraphic succession, modified after Santacroce (1987) on the basis of data from Rolandi et al. (1993, 1998) and Somma et al. (2001), red = high-energetic eruptions, yellow = moderate/low explosive eruptions, green = effusive eruptions, x-scale indicates the erupted volume. c) Geological sketch map of Somma-Vesuvius, after Santacroce (1987). c a b 1474 Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo tween 472 A.D. and 1944 A.D., leucitic tephrites to leucitic phonolites were erupted. The crys- tallinity of Somma-Vesuvius volcanic rocks is strongly variable, from nearly aphiric (mostly in the plinian eruptions) to highly porphiritic (in 472 A.D. eruption and in the products younger than 1631 A.D.) (e.g., Villemant et al., 1993). The Sr- isotope data of Vesuvius magmas have a large range of values between 0.7069 and 0.7081 (e.g., Cortini and Hermes, 1981; Civetta et al., 1991; Civetta and Santacroce, 1992; Caprarelli et al., 1993; Santacroce et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; Somma et al., 2001). The role of evolutionary processes and the nature of source(s) in magma composition is debated: compositional variability of the erupt- ed magma has been attributed to heterogeneous enriched mantle sources, to differentiation and magma mixing (e.g., Vollmer, 1976; Civetta et al., 1991; Civetta and Santacroce, 1992; San- tacroce et al., 1993; Cioni et al., 1995; Ayuso et al., 1998; Peccerillo, 2001; Somma et al., 2001; Piochi et al., 2004) or to crustal contamination (e.g., Rittman, 1933; Savelli, 1967, 1968; Turi and Taylor, 1976). Contamination of magmas by assimilation of carbonate material has been also suggested for the genesis of Ca-Mg-silicate rich ejecta (skarns) on the basis of mineral chemistry, fluid inclusion and isotope data (e.g., Savelli, 1968; Fulignati et al., 1995, 1998; Gilg et al., 1999; 2001; Del Moro et al., 2001). 3. Geochemistry and isotope geochemistry Volcanic products of Somma-Vesuvius have a potassic signature with different degrees of sil- ica undersaturation. The most mafic rocks (SiO2 less than 50 wt%) have been produced during the last 300 years of volcanic activity. They com- monly have low Mg numbers (100 × Mg/ /Mg + Fe = 40 – 20), and low Ni (less than 80 ppm) and Cr (less than 300 ppm) abundances in- dicating the non primary nature of these mafic rocks. Furthermore whole-rock compositions of these rocks do not always coincide with liquid compositions due to phenocrysts accumulation or depletion (Belkin et al., 1993; Trigila et al., 1993; Villemant et al., 1993). In general, as the fractionation index (CaO/Al2O3) decreases, SiO2, Al2O3, K2O, Na2O, Rb, Th, Zr and REE (except Eu) contents increase, while FeO and CaO, Co, Cr, Ni, Sc and V contents decrease; TiO2 and P2O5 contents are roughly constant up to CaO/Al2O3 of 0.5, then decrease; Sr and Ba contents slightly increase up to CaO/Al2O3 of 0.5, then decrease (fig. 2). The strong enrich- ment in K and Sr abundances in the 472 A.D. products could be related to their high por- phiricity (e.g., Rosi and Santacroce, 1983) that generates the nearly vertical trend such as that in fig. 2 and that could be the reason of their ultra- potassic character. These rocks are in fact con- stituted at 50% in volume of mineral phases (preferentially leucite crystals and subordinately clinopyroxene) in contrast to other Vesuvian plinian and sub-plinian rocks that are generally phenocryst poor. Chondrite-normalized REE patterns are generally light REE enriched, Eu anomaly is present in the most evolved rocks and increases according to the degree of frac- tionation (fig. 3a). Generally the described ma- jor and trace element variations range according to fractionation index indicating an origin for most of the evolved rocks by fractional crystalli- sation in magma reservoirs of the mineral phas- es observed on the liquidus. Constrains on pres- sure and temperature (P-T) conditions of these reservoirs come from melts and fluid inclusions trapped in phenocrysts which grow in magma chambers. FT-IR analyses on inclusions in phe- nocrysts (e.g., Belkin et al., 1985, 1998; Cortini et al., 1985; Belkin and De Vivo, 1993; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000) indicate two different crystallization depths lo- calized at 4 km and > 11 km, and temperatures around 850-900°C and 1200°C, respectively. Moreover isotopic variations observed in Vesuvius rocks suggest processes other than crystal fractionation and accumulation during magma petrogenesis. In fact 87Sr/86Sr ratios ranges between 0.7069 and 0.7080; 143Nd/144Nd ratio vary from 0.51261 to 0.51238 (fig. 3b); al- though the limited data, a variability is observed also for lead isotope ratios (206Pb/204Pb = 18.95- 19.12; 207Pb/204Pb = 15.62-15.72; 208Pb/204Pb = = 38.92-39.29) (e.g., Civetta et al., 1991; Civet- ta and Santacroce, 1992; Caprarelli et al., 1993; Santacroce et al., 1993; Cioni et al., 1995; Ayu- so et al., 1998; Somma et al., 2001). 0.7071 0.7073 0.7075 0.7077 0.7079 0.5124 0.5125 0.5126 143Nd/144Nd 87Sr/86Sr 1475 The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius 1 2 3 4 5 6 7 8 9 10 0 0.2 0.4 0.6 0.8 1 1.2 200 400 600 800 1000 1200 1400 1600 0.0 0.5 1.0 1.5 2.0 CaO/Al2O3 2 3 4 5 6 7 8 9 10 11 0.2 0.4 0.6 0.8 1 100 200 300 400 500 600 700 800 900 0.5 1.0 1.5 2.0 CaO/Al2O3 Na2O (wt%) K2O (wt%) P2O5 (wt%)TiO2 (wt%) Zr (ppm)Sr (ppm) Ol + cpx + pl removal C px + f el d+ lc r em ov al m t + b t r em ov al A pa tit e re m ov al Leucite accumulation? 1 10 100 1000 La Ce Pr NdPmSm Eu Gd Tb Dy Ho ErTmYb Lu R oc k /C h on d ri te s Fig. 2. Selected Harker diagrams. Symbols: open circles = high explosive eruptions of the last 3350 kyrs, full circles = moderate explosive/effusive eruptions erupted in the last 300 years. Data from Rosi and Santacroce (1983); Santacroce (1987); Civetta et al. (1991); Civetta and Santacroce (1992); Belkin et al. (1993); Santacroce et al. (1993); Trigila et al. (1993); Villemant et al. (1993); Cioni et al. (1995); Ayuso et al. (1998); Del Moro et al. (2001) and Somma et al. (2001). Fig. 3a-b. a) Rock/Chondrite spiderdiagram (Nakamura, 1974) for 1944 A.D. (closed circle) and 1631 A.D. (open circle) eruptions; b) Isotope versus Isotope diagrams. Sr-Nd isotope data from Cortini and Hermes (1981); Civetta et al. (1991); Civetta and Santacroce (1992); Santacroce et al. (1993); Caprarelli et al. (1993); Cioni et al. (1995); Ayuso et al. (1998); Del Moro et al. (2001) and Somma et al. (2001). Symbols as in fig. 2. a b 1476 Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo 4. Interpretation of petrochemical data and modelling of crustal evolutionary processes Figure 4 shows a relationship from the Sr- isotopic ratios of minerals from rocks repre- sentative of different eruptions and the crystal- lization depths. In particular, diopsides de- rived from magma erupted during the 79 and 1944 A.D. eruptions have Sr-isotopic ratios lower than 0.7074 (Civetta et al., 1991; Civet- ta and Santacroce, 1992; Ayuso et al., 1998) and an equilibrium crystallization depth of 12- 10 km (e.g., Belkin et al., 1985; Cortini et al., 1985; Belkin and De Vivo, 1993; Belkin et al., 1998; Marianelli et al., 1999). In addition, feldspars and salites from magma emitted dur- ing the 79 and 1906 A.D. eruptions have high- er 87Sr/86Sr values (0.7075-7) (Civetta and San- tacroce, 1992; Santacroce et al., 1993) and an equilibrium crystallization depth of 4 km (Cioni et al., 1998; Cioni, 2000), moreover leucites have high Sr ratios (0.7077) and crys- tallization depth lower than 1 km (Cioni, 2000) (fig. 4). This suggests that the higher Sr isotopic ratios are acquired in a late stage of the crystallisation coincident sialic minerals growth at shallow depth. In contrast, the low- er Sr isotopic ratios shown by diopsides and some whole-rocks could reflect that of the deeper reservoir, probably fed by more primi- tive magma. The high Sr-isotope ratios acquired at shal- low depth during crystallisation are consistent with the one inferred for crustal contamination processes, as also suggested by the systematic variations of Sr-isotopic ratios with differentia- tion index (fig. 5a,b). Evidence of open-system Fig. 5a,b. CaO/Al2O3 versus 87Sr/86Sr ratios for high-explosive eruptions of the last 3550 kyr (a) and mod- erate explosive/effusive eruptions of the last 1631 A.D. (b). Sr-isotope data as in fig. 2. Dashed lines = mix- ing trends. a b 0 2 4 6 8 10 12 0.7070 0.7074 0.7078 87Sr/86Sr D ep th - k m Fig. 4. Sr isotopic ratios of minerals versus equilib- rium crystallization depth. Symbols: squares = diop- side and salite, diamonds = feldspar, triangle = leucite. Isotopic data as in fig. 2, pressure data from Belkin et al. (1985; 1998), Cortini et al. (1985), Belkin and De Vivo (1993), Cioni et al. (1998), Marianelli et al. (1999), Cioni (2000). 1477 The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius magma processes, such as crustal contamina- tion and/or fluids exchange processes, derives from O-isotope data (Turi and Taylor, 1976; Ayuso et al., 1998) and U-disequilibria (Black et al., 1998). Plotting CaO/Al2O3 wt% as fractionation index versus Sr isotopic ratios for a complete set of eruptions (fig. 5a-b) shows three main trends. Two trends are represented by the evolved rocks (SiO2 > 50 wt%) generated dur- ing high explosive eruptions: the first shows at decreasing fractionation index against a quite constant Sr ratio around 0.7073; the second shows an overall increase in initial Sr-isotopic ratios with fractionation from 0.7073 to 0.7077. Isotopic disequilibrium between sepa- rated minerals and glass is a common feature for these evolved rocks. The third trend is rep- resented by mafic rocks (SiO2 < 50 wt%) pro- duced during the last 300 years of moderate explosive/effusive activity and is character- ized by an increase in radiogenic Sr from 0.7073 to 0.7078 at decreasing fractionation index. Isotopic equilibrium has been generally observed between separated minerals and glass. The scattering data observed in fig. 5a,b could be attributed to the existence of isotopic disequilibria, crystal accumulation, magma mixing (dashed lines in figure) different ana- lytical procedures, etc. Mesozoic carbonatic succession can be identified as the main source of magma con- tamination processes. This hypothesis can be inferred on the basis of geological, chemical and geophysical evidence: explosive erup- tions which occurred at Somma-Vesuvius produced volcanic deposits containing car- bonatic lithics clearly yielded from the wall- rocks in which magma rises and fragments. Recently Fulignati et al. (1998), Del Moro et al. (2001) and Gilg et al. (2001) recognise that some volcanic magma erupted at Vesu- vius shows chemico-isotopical evidence of interaction with carbonate country-rocks. Furthermore geophysical data identify the Mesozoic basement between 2 and not less than 8-11 km of depth (e.g., Imbò, 1949; Berrino et al., 1998; Zollo et al., 1996; 1998), where crystallization of sialic 87Sr-rich miner- als occurred. 4.1. Quantitative modelling of crustal contamination process In order to test the consistence of composi- tional variations of Somma-Vesuvius magmas with a crustal contamination model, we adopt the quantitative EC-AFC (Energy Conserva- tion-Assimilation Fractional Crystallisation) ap- proach that accounts for mass and energy con- servation (Bohrson and Spera, 2001; Spera and Bohrson, 2001). Less-evolved samples with highest MgO content and lowest Sr-isotopic ra- tio were selected as magmatic end members (close to the parental magma compositions). The isotopic compositions of these samples are assumed to be unaffected by contamination rel- ative to other samples with low MgO content. Carbonatic rocks were also used as end-mem- bers (representing the crustal contaminant). The obtained results (fig. 6a,b and table I) show that the isotopic variations are consistent with a crystal fractionation process controlled by the contamination of carbonatic rocks. Since melt- ing of limestone is an improbable event at the P- T condition of Vesuvius magmas, the assimila- tion was likely a selective process consisting in the inter-exchange between magmas and fluids derived by thermal decomposition (decarboni- sation process; Tracy and Frost, 1991) of the sedimentary wall-rocks. This process has been recently proposed for undersaturated volcanic products from the Albani Hills on the basis of petrological experiments (Freda et al., 2001). In our approach, the values of thermo-chem- ical parameters (table I) were calculated from the chemistry of rocks and from data on melt in- clusions (e.g., Belkin et al., 1985; Cortini et al., 1985; Belkin and De Vivo, 1993; Belkin et al., 1998; Cioni et al., 1998; Marianelli et al., 1999; Cioni, 2000). Calculations based on the above model indicate that the isotopic variations rela- tive to the magmas feeding explosive eruptions (curve b in fig. 6a) are explained by the carbon- atic contamination with a Ma* (mass of melted wall rock) = 0.10-0.25 and Mc (mass of cumu- la- tes) = 0.60-0.75. A fractionation mechanism involving sidewall crystallisation and liquid fractionation in the magma chamber (Chen and Turner, 1980; Mc Birney et al., 1985) can justi- fy the high Mc values despite the crystal content 1478 Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo Fig. 6a,b. EC-AFC model following Spera and Bohrson (2001) and Bohrson and Spera (2001). Lines represent sim- ulations made up using thermal and geochemical parameters listed in table I. Symbols and Sr-isotopic data as in fig. 2. In fig. 6a, light field indicate the white fallout layers, dark fields indicate the grey fallout layers. observed in most pumice samples. Moreover, calculation based on the above model indicates that Sr isotopic variations observed in rocks younger than 300 years (curve c in fig. 6b) are consistent with Ma* = 0.2-0.4 and Mc = 0.6- 0.7. The high mass of cumulates is compatible with the higher crystal content – from 40 to 60% (Trigila and De Benedetti, 1993; Villemant et al., 1993) – observed in lavas. The higher Ma*/Mo ratio observed in mafic magma erupt- ed in the last 300 years of activity (curve c in fig. 6b) is likely due to their higher T and reduced volume of involved magma that favours the contamination processes. a b 1479 The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius 4.2. Evolution of magma plumbing system in the last 300 years and its present state The rocks erupted during the last 300 years of activity show a correlation between Sr-isotopic ratios and age (fig. 7). After the 1631 A.D. and until the 1737 A.D. eruption, the Sr-isotopic ratio remained constant thus indicating that magma was isotopically and geochemically similar to those erupted during the previous sub-plinian event. From 1737 A.D. to 1805 A.D. more radi- ogenic (0.708) magma was erupted. Then Sr-ra- tio decreases defining two distinct trends, as evi- denced by Cortini and Hermes (1981), below 0.7074 during the 1906 A.D. eruption and below 0.7071 during the 1944 A.D. eruption. This was also evidenced by Somma et al. (2001) who re- ported time-dependent variations for Protohis- toric, Ancient Historic and Medioeval lavas and tephra also for Nd and Pb isotopic ratios that Table I. EC-AFC parameters, Somma-Vesuvius rocks. Thermal parameters Compositional parameters (curve b in figure 6a) tlm 1320 °C Element Sr Nd Th tm0 1200 °C Magma: conc. 1600 35 5 tla 1000 °C bulk D0 2.5 0.25 0.1 ta0 750 °C Assimilant: conc. 500 26 10.7 ts 800 °C bulk D0 3 0.25 0.1 cpm 1484 J/kg K Isotope 87Sr/86Sr 143Nd/144Nd cpa 1370 J/kg K ratio magma 0.7073 0.513 hcry 396 000 J/kg ratio assimilant 0.71 0.5118 hfus 250 000 J/kg (curve c in figure 6b) tlm 1320 °C Element Sr Nd Th tm0 1320 °C Magma: conc. 1600 35 5 tla 1000 °C bulk D0 0.8 0.25 0.1 ta0 750 °C Assimilant: conc. 500 26 10.7 ts 800 °C bulk D0 1 0.25 0.1 cpm 1484 J/kg K Isotope 87Sr/86Sr 143Nd/144Nd cpa 1370 J/kg K ratio magma 0.7073 0.513 hcry 396 000 J/kg ratio assimilant 0.71 0.5118 hfus 250 000 J/kg Terminology from Spera and Bohrson (2001). tlm = liquidus of magma; tm0 = initial temperature of magma; tla = liquidus of assimilant; ta0 = initial temperature of assimilant; ts = solidus temperature; cpm = magma iso- baric specific heat capacity; cpa = assimilant isoba 1480 Lucia Pappalardo, Monica Piochi and Giuseppe Mastrolorenzo show an opposite behaviour with respect to that of 87Sr/86Sr ratios. Consistently with the above in- terpretation of Sr-isotope data, a residual shal- lower magma chamber was still active beneath the volcano between the 1631 A.D. and 1737 A.D. eruptions. The complete emptying of such a chamber allows mafic magma to ascent directly from the deep reservoir. The increasing isotopic ratios observed in rocks erupted between 1737 and 1805 A.D. likely derives from fractionation and crustal contamination processes at shallow depth in the conduit, consistently with the pres- ence of high Sr-isotope ratio leucite (0.7077), crystallized at depth less than 1 km (Cioni, 2000). A progressive vanishing of crustal contamination processes occurred between 1805 A.D. and the last 1944 A.D. eruption, as indicated by the con- tinuous decreasing of Sr ratios of erupted mag- mas. This vanishing possibly derives from cool- ing-induced crystal mush growth along conduit walls or, alternatively, from the rise in the magma ascent rate from the deep reservoir, both reducing magma/rocks exchange. Although the two hy- potheses are both likely, the latter possibility is consistent with a general increase in eruption ex- plosivity (e.g., Arrighi et al., 2001 and references therein) and the emission of poorly crystalline magma (e.g., Villemant et al., 1993) between 1805 and 1944 A.D., both characteristics of a de- gassing in near closed-system conditions (fig. 7). 5. Discussion and conclusions The relationship between the depth of mag- ma crystallization and Sr-isotopic ratios, inter- preted and quantitatively modelled as deriving from crustal contamination processes, indi- cates that the condition and timing of magma storage in the crust is a major controlling fac- tor of the eruptive activity at Somma-Vesuvius. Less radiogenic (0.70730) mafic crystals have Fig. 7. Isotopic variations through time after the 1631 A.D. eruption. Symbols: open circles = high explosive erup- tions of the last 3550 years; full circles = moderate explosive/effusive eruptions of the last 300 years; squares = diop- side; triangles = leucite. Field defines the compositional range of the 1631 A.D. volcanic products. Data as in fig. 2. Fig. 8. Cartoon showing the supply system for Vesu- vius reconstructed on the basis of the geochemical and Sr-isotopical modeling. 1481 The 3550 year BP-1944 A.D. magma-plumbing system of Somma-Vesuvius equilibrium crystallization depth exceding 10 km, while more radiogenic (0.7075-0.7077) sialic minerals have shallower equilibrium crystallization depth of about 5 km. This sug- gests that the magma acquired higher Sr-ratios by a process which occurred subsequently to its genesis, when sialic minerals crystallized at shallow depth, in the chamber or in the con- duit. Magma storage at very shallow depth (likely in the upper conduit) being irrelevant, evidence of high Sr-isotope values of leucite at such depths (Cioni, 2000) implies that the con- tamination processes may occur even in a rela- tively short time. The change in Sr-isotopic ratios through the last 300 yr suggests that after the 1631 A.D. sub-plinian eruption and until the last 1944 A.D. eruption no significant shallower reservoir developed in the upper crust (fig. 8). This shallower reservoir probably grew during the long period of quiescence preceding the sub-plinian and plinian eruptions, by the peri- odical recharge of less contaminated magma rising from deeper storage zones (fig. 8). This shallow reservoir was partially or completely emptied during major explosive events. This magmatological evidence is strongly support- ed by the results of seismic tomography (e.g., Zollo et al., 1996; De Natale et al., 2001) which does not reveal seismic evidence of magma storage down to a depth of about 10 km. In contrast, the recurrence of the lower isotopic imprints in the products of most of the Somma-Vesuvius eruptions suggests that the deep reservoir can be considered a long-lived reservoir acting throughout the volcanic histo- ry and furnishing at least 300 km3 (Santacroce and Civetta, 1992) of magma. Consistently with results of the recent seismic tomography (Zollo et al., 1996, 1998; De Natale et al., 2001), a large (400 km2 wide) reservoir, locat- ed at a depth between 8 and 25 km is still pres- ent beneath the volcano. Acknowledgements Authors are grateful to Chris Kilburn for suggestions that strongly improved the manu- script. REFERENCES ANDRONICO, D. and R. 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