1. INTRODUCTION

Mount Etna, located in eastern Sicily, is the largest stra-to-volcano in Europe (3.3 km high a.s.l. with an ellipticalbase whose axes are about 60 and 40 km) and one of themost active in the world. It grew in proximity to the colli-sion boundary of the African and Eurasian continentalplates (Figure 1), from repeated eruptions of alkali basalts-

hawaiites over the last 200 ka [Chester et al., 1985]. Aprominent feature of Etna’s activity is the persistent emis-sion of a huge volcanic plume arising from summit cratersduring both quiescent and eruptive magma degassing. Othertypes of gas manifestations (mofettes, “mud volcanoes”, dif-fuse soil degassing, bubbling gases in surface and ground-water) also occur in peripheral sectors of the volcano.

Etna’s gas emissions were investigated for the first timein the second half of the 19th century [Silvestri, 1880;Ponte, 1914], but they began to be studied quantitativelyafter 1970 and, more intensively, since 1987. The firstchemical analyses of Etna eruptive gases, collected fromnear summit crater vents, were by Elskens et al. [1970],Huntingdon [1973] and Le Guern [1973]. Samples werefrequently polluted with air and partly altered by secondaryreactions upon sampling, but their pristine compositioncould be restored by Gerlach [1979], using chemical mod-eling. The first isotope data on Etna’s gases, obtained by

1

Magmatic Gas Leakage at Mount Etna (Sicily, Italy):Relationships With the Volcano-Tectonic Structures, the

Hydrological Pattern and the Eruptive Activity.

Alessandro Aiuppa1, Patrick Allard2, Walter D'Alessandro3, Salvatore Giammanco3,Francesco Parello1, Mariano Valenza1

In this paper we provide a review of chemical and isotopic data gathered over the lastthree decades on Etna volcano's fluid emissions and we present a synthetic frameworkof their spatial and temporal relationships with the volcano-tectonic structures, ground-water circulation and eruptive activity. We show that the chemistry, intensity and spa-tial distribution of gas exhalations are strongly controlled by the main volcano-tectonicfault systems. The emission of mantle-derived magmatic volatiles, supplied by deep toshallow degassing of alkali-hawaiitic basalts, persistently occurs through the centralconduits, producing a huge volcanic plume. The magmatic derivation of the hot gasesis verified by their He, C and S isotopic ratios. Colder but widespread emanations ofmagma-derived CO2 and He also occur through the flanks of the volcano and throughaquifers, mainly concentrated within two sectors of the south-southwest (Paternò-Belpasso) and eastern (Zafferana) flanks. In these two peripheral areas, characterized byintense local seismicity and gravity highs, magma-derived CO2 and helium are variablydiluted by shallower crustal-derived fluids (organically-derived carbon, radiogenic heli-um). Thermal and geochemical anomalies recorded in groundwaters and soil gaseswithin these two areas prior to the 1991-1993 eruption are consistent with an input ofhot fluids released by ascending magma. Magmatic fluids interacted with the shallowaquifers, modifying their physico-chemical conditions, and led to strong variations ofthe soil CO2 flux. In addition to routine survey of the crater plume emissions, geo-chemical monitoring of remote soil gases and groundwaters may thus contribute to fore-casting Etna's eruptions.

1Dipartimento di Chimica e Fisica della Terra ed Applicazioni, Università di

Palermo, Palermo, Italy.2 Laboratoire Pierre Süe, CNRS- CEA, CE-Saclay, Gif/Yvette, France.3 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Palermo, Italy.

2 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

Allard [1983, 1986], provided insights on the origin ofwater, carbon, and sulfur. Since the middle of the seventiesof the last century, new methodologies of both remote(COSPEC) and in situ measurements were used at Etna todetermine the chemistry and SO2 flux of the volcanic plumeissuing from the summit craters [Haulet et al., 1977;Malinconico, 1979; Allard et al., 1991; Caltabiano et al.,1994; Bruno et al., 1996]. More recently, other remote sens-ing techniques (FTIR, MIVIS, Lidar and DOAS) wereapplied to estimate the volatile emission rates from the vol-cano [Francis et al., 1995, 1998; Edner et al., 1994].

Since the beginning of the 1980’s, the attention of geo-chemists also focused on the study of cold gases emanating

diffusely through the flanks of the volcano [Allard et al.,1991, 1997; Anzà et al., 1993; Giammanco et al., 1995,1997, 1998c; Baubron, 1996]. These emanations, recog-nized earlier by Platania [1916], were shown to be exten-sive even during inter-eruptive periods.

Geothermal surveys were also carried out since the begin-ning of the 1960’s around the base of the volcano.Evaluations of the heat flux excluded the presence ofexploitable geothermal reservoirs [Facca, 1964]. However,the overall heat flux from Etna region was shown to be sig-nificant and strongly controlled by the regional structuralframework [Minett and Scott, 1985]. Groundwaters at Etnahave long been the target of hydrological and geochemicalstudies [Silvestri, 1873; 1882; Ogniben, 1966; Aureli,1973; Ferrara, 1975], but only recently did they becomeinvestigated and monitored in a more systematic way forvolcanological purposes.

Since 1987, different kinds of natural fluids on Mt. Etna(fumaroles, soil gas emanations, bubbling and dissolvedgases, thermal and cold groundwaters) were investigatedand monitored by the staff of Istituto Geochimica deiFluidi-C.N.R. (now INGV, Sezione di Palermo),University of Palermo and co-workers [Aiuppa et al.,2000, 2002, 2003; Anzà et al., 1989, 1993; Badalamenti etal, 1994; Brusca et al., 2001; Chiodini et al., 1996;D’Alessandro et al., 1992, 1994, 1996, 1997a;Giammanco et al., 1995, 1996, 1997, 1998a,b, 1999;Allard et al., 1997].

In the present work we put together all geochemicaldata till now obtained on Etna’s volatile emissions and,combined with volcanological and geological data, weintegrated them into a broad interpretative model of thevolcano’s behavior. We show that the abundant chemicaland isotopic data set acquired on Etna’s volatile emis-sions over the last three decades relates to the volcano-tectonic structures, the hydrological system and the erup-tive activity.

2. VOLCANO-TECTONIC STRUCTURE ANDHYDROGEOLOGICAL SYSTEM OF MT. ETNA

2.1. Tectonic and Structural Setting

Mt. Etna developed at the intersection of three regionalfaults systems: a) the Tindari-Giardini system and the con-nected Maltese fault escarpment, both oriented NNW-SSEand elongated discontinuously from the Aeolian Islands tothe Island of Malta [Carbone et al., 1987]. b) the Comiso-Etna system and its inferred prolongation, the Messina -Capo Vaticano system [Lo Giudice et al., 1982; Bousquetet al., 1988]. This fault system trends NE-SW and extendsdiscontinuously from the southwestern limit of theHyblean foreland to the Catania plain and further north-east from the northeastern boundary of Etna to southwest-ern Calabria. The southwesternmost portion of the Comiso

Figure 1. Simplified tectonic setting of the Etnean area and loca-tion of the sampling sites. Pale grey area = sedimentary outcrops.Circles represent the sampled gas manifestations, numbers as intable 2. Crosses represent groundwater sampling points. Blacksquares are those groundwater sampling points in which periodicalsampling is also performed. Dark grey areas are those on whichperiodical CO2 flux measurements are performed. The three mainhydrogeological basins, after Ferrara [1990], are delimited by thethick lines and indicated with the letters a, b and c. Volcanotectonicstructures include the Summit craters (S) Valle del Bove depres-sion (VB), the North-east Rift (RNE) and the main seismogeneticfaults: Pernicana fault (PF), Ragalna fault system (RFS) and Timpefault system (TFS). CF = Chiancone volcanoclastic formation;Zaf. = Zafferana.

AIUPPA ET AL.. 3

– Etna system marks the boundary between the Hybleanforeland and the Gela-Catania foredeep. c) the MonteKumeta-Alcantara system, oriented E-W [Ghisetti andVezzani, 1984], which runs through central Sicily andbounds the northern side of Mt. Etna. Both the Maltesefault and the Comiso – Etna – Messina systems are subjectto tensional strains and seem to guide the upraise of Etna’smagmas from depth, as the majority of the tectonic struc-tures and eruptive fissures in the Etna area follow either ofthese two structural directions [Chester et al., 1985; Rasàet al., 1995]. At a regional scale the NE-SW and the NW-SE fault systems separate two main structural units: theMaghrebian-Appenninic chain to the north, which is partof the Euroasian plate, and the Hyblean platform to thesouth, which instead belongs to the African plate [Lentini,1982]. The former is reduced into several south-vergingsheets that progressively thrusted over each other duringthe collision with the African plate, from Eocene to theQuaternary [Lentini, 1982].

The sedimentary basement of Mt. Etna is made of fly-sch and clayey deposits that belong to the Maghrebian-Appenninic chain [Lentini, 1982] and overlay 10 km-thickcarbonate sequences. The lowermost part of the crustbeneath Etna is likely to be crystalline.

The magmas erupted at Mt. Etna arise from a mantlediapir, whose top lies at the base of this crust (about 16 kmdepth in its shallower portion, according to Hirn et al.[1997]). The first erupted lavas were tholeiitic basalts, butmost of the edifice formed through eruptions of alkalibasalts – hawaiites (trachybasalts) of quite monotonouscomposition [Romano, 1982; Joron and Treuil, 1984;Chester et al., 1985; Tanguy et al., 1997].

2.2. Hydrogeological System

The surface of Etna’s edifice lacks a real hydrographicnetwork and waters mostly tend to seep and feed theunderground circulation, the run-off coefficient beingonly 5% [Ferrara, 1975]. The importance of the effectiveinfiltration is highlighted by high outflows at the springsalong the perimeter of the volcano, at the contact with thesedimentary rocks of the basement, and especially alongthe coastline, where considerable amounts of water aredischarged into the sea [Ogniben, 1966; Ferrara, 1975].

The volume of water that accumulates every year intothe aquifers of Etna is huge: about 0.7 km3, according toOgniben [1966]. This is due both to the high amount ofrain and snow falls (about 0.86 km3 [Ogniben, 1966]) andto the high permeability of Etna’s volcanic rocks (perme-ability coefficients ranging from 10-5 to 10-7 cm/s[Ferrara, 1975; Schilirò, 1988]). The whole volcanic edi-fice can thus be considered as a highly porous medium,with a permeability coefficient that varies as a function ofboth the lithology and the volcano-tectonic structures. Ingeneral, Etna presents a hydrogeological situation com-

mon to many other basaltic volcanoes: fissured and high-ly permeable lava layers are interbedded with discontinu-ous layers of scarcely permeable pyroclastics. These vol-canic products are superimposed on an impermeable sed-imentary substratum, with permeability coefficients rang-ing from 10-7 to 10-13 cm/s [Schilirò, 1988]. The base ofthe main aquifers of Etna thus lies at the contact betweenthe volcanic rocks and these sediments [Ogniben, 1966].

The movement of Etna’s groundwater is controlled bythree main factors: 1) the morphology of the sedimentarybasement; 2) the structure of the volcano; 3) the topogra-phy that influences the piezometric level of water.Because Etna’s basement reaches its highest elevation(about 1300 m a.s.l.) a few kilometers NW of the volcanosummit and it generally slopes to SE, the south-easternaquifers of Etna are partly recharged by waters comingfrom its north-western flanks [Ogniben, 1966]. This com-plex hydrogeologic framework includes undergroundwatersheds such as to delimit main and secondary hydro-geologic structures (i.e. basins). On the basis of structur-al, geological and geophysical data, three main hydroge-ological basins were identified [Ogniben, 1966; Ferrara,1990] (Figure 1). They roughly correspond to three sec-tors of the volcano: a) the eastern one, tributary of theIonian Sea; b) the southwestern one, tributary of theSimeto River; c) the northern one, whose waters feed theAlcantara River. The eastern basin is the most importantone as far as the exploitation of the conspicuous waterresources of Etna is concerned.

Preliminary data on tritium content in rain- and ground-water [D’Alessandro et al., 2001] indicate that waters cir-culating in the eastern and northern hydrogeologic basinshave shorter travel times (1 – 20 years) with respect tothose of the south-western one (> 50 years).

The presence of an impermeable sedimentary basementbeneath Etna’s volcanics and the limited thickness ofthese latter prevent Etna’s groundwaters from reachingconsiderable depths and thus limits their thermalization.Temperatures measured in Etna’s groundwaters are gen-erally lower than 25 °C [Aiuppa et al., 2003]. The onlyexception is the aquifer feeding the mud volcanoesknown as “Salinelle di Paternò”, on the lower south-west-ern flank. The waters emitted in this area are character-ized by an abundant free gaseous phase and show typicalfeatures of waters linked to hydrocarbon reservoirs.Geothermometric estimates carried out on both the liquidand the gaseous phases emitted at the “Salinelle” gavetemperatures in the range 100 - 150 °C for their last equi-libration [Chiodini et al., 1996].

3. CHEMISTRY OF ETNA’S FLUIDS

3.1. Magmatic Gases and Plume Emissions

The magmatic gases emitted on Etna were collected on

4 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

quite rare occasions at eruptive vents and/or lava flows[Elskens et al., 1970; Huntingdon, 1973; Le Guern, 1973,1983; Allard, 1983, 1986]. Air contamination in the sampledgases was generally high, due to the high permeability of theloose pyroclastic materials that make up Etna’s summit andto the preferential drainage of the emitted gases through theopen summit craters. Reliable chemical and isotope datacould thus be obtained for only a few gas samples, taken attemperatures of 800 to 1100°C and representative of differ-ent stages of eruptive activity. Table 1 reports the average orindividual chemical composition of these samples.

Chemically, Etna eruptive gases do not differ much fromother basaltic gases, except for their tendency to be richer inwater and to display high C/S ratio [e.g., Le Guern, 1973;Allard, 1983, 1986]). A noticeable feature is the measureddifference in C/S ratio between samples collected from thesummit craters (C/S mainly between 2 and 5) and those col-lected from lateral vents or lava flows (C/S always << 1).Such a trend is consistent with a preferential loss of less sol-uble carbon dioxide during central conduit venting (summitcraters), leaving lateral eruptive emissions depleted in CO2and hence with a lower C/S ratio [Allard, 1986; Burton etal., 2003]. This pattern is identical to that inferred byGerlach and Graeber [1985] for the Type I and Type IIgases from Kilauea volcano, in Hawaii

A great mass of information on the chemistry and budgetof magmatic volatiles emitted through the summit craters of

Etna was obtained from repeated investigations of the vol-canic plume, using both ground-based and remote sensingmethodologies. Based on COSPEC measurements [Haulet etal., 1977; Malinconico, 1979; Allard et al., 1991; Caltabianoet al., 1994; Bruno et al., 1996; Allard et al., 1998], the time-averaged SO2 emission rate over the last two decades wasfound to be about 2 Mt/a [Allard, 1997], which correspondsto about 10% of the annual global volcanic flux [Stoiber etal., 1987]. By coupling this averaged SO2 plume flux withthe mean C/S plume molar ratio of 3 to 5 for steady-state cen-tral conduit magmatic [Allard et al., 1998] degassing yields amean CO2 output of ~4 to 7 Mt/a from the volcano conduits[Allard et al., 1997]. A higher CO2 output of 13 ± 3 Mt/a waspreviously estimated for the period 1975-1986 [Allard et al.,1991]. These figures indicate that the CO2 production of Etnamay vary with time, but that this volcano alone emitsbetween 5 and 15 % of the estimated global volcanic emis-sions of carbon dioxide [Gerlach, 1991]. Taken together, thefluxes of S and CO2 demonstrate that Etna is one greatestemitter of volcanic volatiles on Earth.

The same conclusion is drawn for halogens. Halogenfluxes from the summit crater plume, computed by cross-correlating SO2 emission rates with mean S/Cl, S/F andS/Br ratios, are 0.26-1.5 Mt/a (Cl), 0.04-0.18 Mt/a (F) and0.002-0.015 Mt/a (Br) [Andres et al., 1993; Francis et al.,1998; Pennisi and Le Cloarec, 1998]. Pennisi and LeCloarec [1998] reported a high variability of S/Cl ratio in

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the volcanic plume (0.25-10), attributed to changes invapor-melt partitioning during basalt degassing. Recentmeasurements with FTIR remote sensing [Burton et al.,2003] and filter pack sampling [Aiuppa et al., 2002a]however show smaller ranges of S/Cl and S/F ratios dur-ing steady-state non-eruptive plume degassing at Etna,respectively 1.5-3 and 5-12. Higher (>3) to much higher(>15) S/Cl ratios recently measured during violentStrombolian and lava fountain phases imply the fasteruprise of S-rich, deeper-equilibrated melts and vapors[Burton et al., 2003].

Trace metal fluxes in Mt. Etna summit crater plumevary from about 36 kt/a for major species (K, Na, Al) to10-2 t/a for ultra-trace elements [Bùat-Menard andArnold, 1978; Andres et al., 1993; Gauthier and Le

Cloarec, 1998; Aiuppa, 1999]. Again, Mt. Etna appearsas one largest volcanic emitter of several metals andtrace elements.

3.3. Low Temperature Gas Manifestations

Several low temperature (< 100°C) gas manifestationsoccur on the Etna area. They include bubbling gases (i.e.,sites 5 and 6; Figure 1), mud volcanoes (sites 1, 2 and 3) andCO2-rich soil gases. In recent years, these manifestationswere analyzed repeatedly [Allard et al, 1991, 1997;D’Alessandro et al., 1996a, 1997a; Giammanco et al., 1995,1998b], Table 2 showing some representative analyses.

Plotting our data and other published results[Giammanco et al., 1998b; D’Alessandro et al., 1996,1997a] in a He-N2-CO2 diagram (Figure 2a) highlights theexistence of two clear-cut trends. Dissolved and soil gasesplot on a straight line linking a CO2-rich component andair. By contrast, free gases are characterized by a preferen-tial enrichment in non-atmospheric He, and plot along theHe-CO2 side of the diagram. Considering dissolved gasesin more detail (Figure 2b), it emerges that both He and CO2are often in excess with respect to the air ratio. Both speciesare probably added through dissolution of deep magmaticgas rising across the aquifers. The wide range of He/CO2ratios can be ascribed to the contrasting solubilities of CO2and He and to the variable influx of magmatic gas intogroundwater [Giammanco et al., 1998b]. In areas with lowgas flow, CO2 preferentially dissolves in deep water levelswhile poorly soluble He becomes strongly enriched in theresidual gas phase and migrates further toward the surface.In contrast, in areas with high CO2 fluxes groundwaterbecomes rapidly saturated in this gas and, so, the deepvolatile phase can flow through the aquifer without signif-icant interactions. In that case, the gas phase approximate-ly keeps its original He/CO2 ratio. In support to this inter-pretation, we outline that higher CO2 contents and lowerHe/CO2 values are found in dissolved gases from areaswith intense soil degassing (Paternò - Belpasso andZafferana - S.Venerina; Figure 1).

The free gases also display variable He/CO2 ratios, asexpected from the above discussion. Some samples (5, 8,and 9), in which methane is the dominant gaseous com-pound (Figure 3), plot near the helium vertex. There areseveral methane-rich manifestations around the base of Mt.Etna and several gas fields were (Catania, Cisina andRizzo) or are currently commercially exploited (Bronte,S.Nicola). Preliminary isotopic data (δ13C of -44 to -64 vs.PDB and δD of -140 to -150 vs. SMOW) indicate a ther-mogenic origin of their methane, best accounted for by thehigher-than-normal geothermal gradient affecting the sedi-mentary basement of the volcano. Samples from salinehydrothermal manifestations (“Salinelle”, sites 1, 2 and 3 inFigure 1) show distinct He/CH4 ratios (Figure 3) that remainbroadly constant over time [D’Alessandro et al., 1997a].

Figure 2. (a) He-N2-CO2 ternary diagram for the Etnean gases.The line represents the He/N2 ratio of the atmosphere. (b) He-N2-CO2 ternary diagram for the dissolved gases only. Note that Hehas been multiplied by 10 with respect to Figure 2a. The arrowsindicate different mixing trends between atmospheric air and gasesof deep origin whose He/CO2 ratio has been modified by the inter-action with the aquifers.

6 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

According to Chiodini et al. [1996], the chemical composi-tion of these Salinelle gases is controlled by variable separa-tion of a CO2-rich gas phase from hydrothermal aquiferswith boiling temperature of 100 to 150°C. Due to the com-parable solubility of He and CH4 in hot water, the He/CH4ratio is little dependent on the boiling temperature and theextent of gas partitioning, in sharp contrast to He/CO2 andthe CH4/CO2 ratios. The observed spatial variations inHe/CO2 therefore depend on the boundary conditions, whichregulate each of the hydrothermal subsystems feeding theSalinelle, while variations over time may reflect changes inthe stress field related to the geodynamic context[D’Alessandro et al., 1994].

3.4. Diffuse Soil Degassing

The first scientific study on diffuse CO2 emissions fromsoils on Mt. Etna dates back to the early 20th century, whenPlatania [1916] described several areas of anomalous soilCO2 degassing in the eastern flank of the volcano and alsoreported of temporal variations in the emission rate. Insome places, the release of CO2 from the soil was so highas to kill vegetation as well as small animals, and to causedifficulty in breathing to inhabitants. The temporal varia-tions of soil CO2 emissions were found to be negativelycorrelated with barometric pressure, but an increase in CO2degassing was observed after the 1911 eruption. The first

large-scale measurements of soil CO2 efflux and soil CO2concentration on Etna were carried out in 1987 [Anzà etal., 1993]. Since then, several hundred measurements ofboth parameters were made [Allard et al., 1991; Baubron,1996; D’Alessandro et al., 1992; Badalamenti et al., 1994;Giammanco et al., 1995, 1997, 1998c, 1999].

During September 1993 we conducted a broad, multi-parametric survey to determine CO2 and He concentra-tions, as well as 222Rn activity, in the soils of Etna. Thesethree gases are of major significance in the geochemicalstudy of volcanic soil degassing [Baubron et al. 1991].Measurements were carried out at a depth of 50 cm, on 124sampling points distributed over most of Etna’s surface(about 1100 km2) with a regular grid. The measured CO2values ranged from 0.01 to 3 % by volume, with an aver-age of 0.35 %, thus confirming a broadly high CO2 emis-sion rate from Etna’s edifice. A large variability was alsoobserved for 222Rn activity. The spatial distribution of bothCO2 (Figure 4) and radon revealed two areas of anomaloussoil degassing (Zafferana - S. Venerina on the E flank andPaternò - Belpasso on the SSW one), which exactly corre-spond to the areas where local groundwaters have thehighest values of PCO2. He contents in excess of air con-centration were low (from 0 to 210 ppb vol) but are indica-tive of additional helium of likely magmatic derivationaccording to the high 3He/4He ratio measured for free anddissolved gases in the same areas.

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Notwithstanding the low density of sampling points, theresults of the 1993 survey indicate that diffuse emanationsof magmatic gas on Mt. Etna occur not only in its upperparts, as reported in early studies [Allard et al., 1991], butalso in peripheral areas far from the volcano’s summit,thus confirming the findings of Platania [1916]. Moregenerally, all soil gas measurements made since 1987 arecoherent in indicating the existence of preferential diffusedegassing in two wide areas of the volcano, i.e. the centralpart of the eastern flank and the southern and south-west-ern flanks, where diffuse soil degassing of CO2 is particu-larly high. Anomalous soil CO2 emissions are oftenaccompanied by anomalies in the emission of reduced gasspecies such as H2 and CO, whose origin is likely to bealso magmatic [D’Alessandro et al., 1992; Giammanco etal., 1998b]. The location and the geometry of the areaswith the largest diffuse anomalies of soil degassing are ingood agreement with recent geophysical [Loddo et al.,1989] and geological [Rasà et al., 1995] data indicatingthinning of the underlying impermeable sedimentarydeposits in these areas. The structural control in openingconnections between the deep-seated source of magmaticdegassing and the surface is more evident in the Zafferana– S.Venerina area, where the highest soil gas emissions aremostly found along faults belonging to theNNW–SSE–trending regional system of the Maltese faultor to its conjugate systems (TFS - Figure 1).

Anomalous soil CO2 degassing was also found in otherparts of the volcano’s flanks, but almost exclusively alongactive volcano-tectonic faults [Giammanco et al., 1998c].In particular, detailed studies were conducted in the south-

eastern flank of Etna along the fracture system that openedduring the 1989 eruption down to an altitude of 1500 ma.s.l. [Badalamenti et al., 1994; Giammanco et al., 1995].Anomalous CO2 emissions were found along the axis ofthis volcano-tectonic structure, with important temporalvariations that will be discussed later. Other investigationswere carried out in the north-eastern flank of the volcanoalong the NE rift system and Pernicana fault, one of themost important active faults of Etna [D’Alessandro et al.,1992; Giammanco et al., 1997, 1999; Azzaro et al., 1998].The spatial distribution of soil CO2 anomalies in this areaallowed us to track some sections of the Pernicana fault thatare invisible at the surface, as well as other sub-parallelfaults. Similar results were obtained in other sectors of thevolcano where the density of tectonic structures is particu-larly high [Giammanco et al., 1998c].

Figure 3. He-CH4-CO2 ternary diagram of the Etnean gases. Datafrom D'Alessandro et al [1997a], from Giammanco et al. [1998b]and present work. The lines represent the He/CH4 ratio measuredat Simeto, Stadio and Vallone Salato sampling sites. Note in thiscase that although this ratio shows remarkably different valuesamong the sampled sites, it is the same at all the pools present ineach site and it also remains constant in time.

Figure 4. Distribution map of the CO2 concentrations (in ppmV)in the soils of Etna at 50 cm depth. Circles represent the samplingpoints.

8 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

3.5. Groundwater Chemistry

The very first hydrogeochemical surveys on Etna’sgroundwaters (springs, wells and drainage galleries) werecarried out in 1988 (89 samples [Anzà et al., 1989]), and1991-92 (43 samples [Dall’Aglio et al., 1994]). Morerecently, Brusca et al. [2001] and Aiuppa et al. [2003]undertook extensive investigations, collecting a total of 277water samples distributed almost uniformly around the vol-canic edifice.

Table 3 shows the compositional range of Etna ground-waters, their compositional features being described by thetriangular plots of Figure 5. According to Brusca et al.[2001] and Aiuppa et al. [2003], three main processes con-cur in determining the chemical composition of Etneangroundwaters:

a) the water composition is strongly controlled by theinput of magma-derived CO2, which in turn promotesintense acid leaching of the host basalts. This process isresponsible for (i) the prevailing acidic pH values ofgroundwaters; (ii) their high TDC (total dissolved carbon)values; (iii) the general correlation between bicarbonatecontent and major cations, giving rise to the typical bicar-bonate alkaline to earth-alkaline composition; (iv) theoccurrence of groundwaters with anomalous concentra-tions of dissolved gases in the most fractured and seismi-cally active zones of the volcano (i.e., Paternò - Belpassoto the SW and Zafferana - S.Venerina to the E)[D’Alessandro et al., 1999].

b) A second influence is due to mixing with thermalbrines rising from the sedimentary basement. These brines,displaying an NaCl-composition (Figure 5), are known torise throughout the whole western flank of the volcano,emerging undiluted only at the Paternò mud volcanoes(“Salinelle”). They probably represent connate fluids

linked to hydrocarbon reservoirs hosted in the sedimenta-ry basement below Etna [Chiodini et al., 1996]. Due to itsvery high salt content (~70 g/l), even limited (often <1%)contribution from this sedimentary component has a cleareffect on the composition of Etnean groundwaters. This isreflected by the fact that several groundwaters from theSW hydrogeological basins are systematically more salinethan the HCO3-TDS relation for “common Etnean ground-waters” would foresee (Figure 6a), pointing toward thecompositional field of chloride-alkaline waters (Figure 5).Also, water temperature, Na, Li, Cl and B do not fit with asimple gas-water-basalt interaction process. Rather theirspatial distribution points to anomalous contents in thePaternò-Adrano area. Brusca et al. [2001] highlighted thatthe prolonged residence time of groundwaters in the SWsector, apart from being responsible for enhanced basaltweathering, favors the mixing process with sedimentaryfluids. In the eastern sector, the rise of these saline“Salinelle”-type waters – if any – is probably masked bythe different hydrogeological conditions (greater precipita-tion, greater steepness of the volcano’s slope and loweraltitude of the recharge zone) that allow a considerablyhigher water discharge.

c) Finally, it can be observed that a few waters from theeastern sector also depart from the main trend of Figure 6b,and have an anomalous chloride-sulfate earth-alkaline com-position (Figure 5). The remarkable correlation betweensulfate and nitrate contents displayed by these groundwatersamples supports the hypothesis of localized pollution ofthe aquifer by NH4-Ca-SO4 wastewaters produced by agri-cultural activities. The use of ammonium-sulfate and/or cal-cium-sulfate fertilizers is indeed widely diffused in theEtnean area.

As expected, also trace element’s content was found to behigher where the total amount of solids dissolved into the

AIUPPA ET AL.. 9

water was greater, that is in the south-western basin and sec-ondarily in the eastern one [Giammanco et al., 1998a;Aiuppa et al., 2000]. Moreover, the mean concentration val-ues of Rb, V, Fe, Sr and Pb calculated for each of the threemain hydrogeological basins show a high positive correla-tion with the mean calculated values of PCO2 (R = 0.89).This suggests that, on average, the dissolution of the aboveelements is mainly controlled by the amount of dissolvedmagmatic gas that increases the original acidity and thus thechemical aggressiveness of groundwater towards the hostrocks. A huge fractionation of trace metals was demonstrat-ed to exist during basalt weathering at Etna, the relative par-titioning between the altering solution and the mineral

residuum being strongly dependent on environmental con-ditions and specific aquatic chemistry of the elements[Aiuppa et al., 2000].

4. STABLE ISOTOPES4.1. δD and δ18O of Rain- and Ground-waters

In the period October 1997 – October 2000 a network of11 raingauges was deployed on Etna, collecting monthlybulk rainwater samples. Raingauges were distributed at dif-ferent altitudes (from sea level up to 2900 m a.s.l.) allaround the volcanic edifice. Samples were analyzed bothfor δD (47 samples) and for δ18O (270 samples) and displaya rather large range of values from -84 to –12 ‰ and from–13.6 to +1.9 ‰ respectively [D’Alessandro et al., 2001].When plotted in a δD- δ18O diagram (Figure 7), most rain-water samples define a mean straight line given by the rela-tion δD = 8 δ18O + 19 [D’Alessandro et al., 2001], inter-

Figure 5. Major anion (a) and cation (b) triangular plots. Watersare subdivided according to the hydrogeologic basin they belongto. Sea water (S.W.) and sedimentary basement waters (B.W.) arealso plotted.

Figure 6. Alkalinity-TDS (a) and NO3-SO4 (b) scatter diagramsfor Etna groundwaters.

10 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

mediate between the worldwide meteoric water domain (D= 8 δ18O + 10 [Craig, 1961]) and that for precipitations ineastern Mediterranea (δD = 8 δ18O + 22 [Gat and Carmi,1970]). This is consistent with the fact that the highest con-tribution to precipitation in the Etnean area is due to wet airmasses coming from the Ionian sea (easternMediterranean). Few samples deviate from this local mete-oric line, probably due to evaporation processes in dryerand hotter periods or to the contribution of air masses com-ing from western Mediterranean. The latter process isapparent only in a few samples of the northernmost rain-gauges [D’Alessandro et al., 2001]. However, these twoprocesses are not very effective on infiltrating waters, giventhat almost all Etnean groundwaters plot on the local mete-oric water line (Figure 7) [Anzà et al., 1989; Allard et al.,1997; D’Alessandro et al., 2001]. Etna’s groundwaters donot display isotopic shifts due to high-temperature water-rock interactions [Anzà et al., 1989; Allard et al., 1997]confirming their shallow circulation.

Rainwaters collected below about 1000 m (the mainrecharge area) define an oxygen isotope gradient of about 2.7δ ‰ km-1 [D’Alessandro et al., 2001] that is quite standard[Poage and Chamberlain, 2001]. Samples from higher alti-tude raingauges define an unusually smooth gradient of 0.6 δ‰ km-1. Two complementary processes could explain such agreat difference: i) contribution of isotopically enriched vaporemitted from the summit craters (δ18O = +6 ‰ [Allard,1986]) and ii) influence of heat flux through the emittedplume on precipitation’s condensation temperatures that inturn influence the isotope composition of rainwater.

Mean recharge altitudes derived from the above isotopegradient are much lower for the waters circulating in the east-

ern hydrogeologic basin (480 ± 210 m a.s.l.) than those cir-culating in the northern (880 ± 220 m a.s.l.) and the south-western ones (990 ± 260 m a.s.l.) [D’Alessandro et al, 2001].Seasonal variations of δ18O values in most Etna groundwatersfall within a narrow range (± 0.2 ‰ [D’Alessandro et al.,2001]), thus suggesting that underground circulation involvesa well-mixed reservoir.

4.2. Helium

3He/4He isotope data were measured in soil gases [Allard etal., 1991], mud pools [Allard et al., 1991, 1997; Marty et al.,1994] and groundwaters [Allard et al., 1997]. The 3He/4Heratio was found to range from 1.0 to 6.4 Ra. Samples of bub-bling gases collected in the peripheral areas of Paternò (# 1, 2,3, 4, 6 in Figure 1) and Fondachello (# 5 in Figure 1) displayratios from 5.2 to 6.4 Ra [Allard et al., 1997; Marty et al.,1994]. The isotopic composition of dissolved He in most ofEtna’s groundwaters results from simple air dilution of a dis-solved mantle-derived magmatic component, whose ultimate3He/4He ratio is 6.9±0.2 Ra [Allard et al., 1997]. Also in thiscase, the water samples with the highest contribution of deephelium are those from the areas of Paternò (SSW flank) andZafferana - S.Venerina (E flank). The 3He/4He ratio of the heli-um magmatic end-member in all fluids matches the highestvalues (mean range: 6.7±0.4 Ra) measured in olivine crystalsof Etna basalts [Marty et al., 1994], therefore pointing to itsminor dilution by radiogenic He from the crustal basement[Allard et al., 1997]. By contrast, samples collected from themethane gas field near Bronte show a distinct crustal signature(0.08-0.74 R/Ra), indicating no magmatic contribution in thisarea.

4.3. Carbon

The overall δ13C range displayed by CO2 in Etna fluids isfrom -25 to +1 ‰ vs. PDB [Allard, 1986; Allard et al., 1991,1997; Anzà et al., 1993; Giammanco et al., 1998b;D’Alessandro et al., 1996, 1997a]. A narrow range from -5 to-2 ‰ was measured in both high-temperature (800-1090°C)eruptive gases and intense soil emanations on the upperslopes of the volcano [Allard, 1986; Allard et al., 1991; Anzàet al., 1993; D’Alessandro et al., 1997a; Giammanco et al.,1998b]. Together with He isotopic evidence, this isotopicrange is typical for magmatic carbon at Mt. Etna. Theobserved variability in the measured δ13C can be ascribed toseveral processes, which may operate together or separately:i) 13C fractionation during magma degassing; ii) shallowinteraction between magmatic gas and water (moisture)and/or vapor in the soil; iii) variable crustal contamination ofthe magmatic gas, at the time scale of some years, due to dif-ferences in the geometry of emplacement and to the residencetime of magma dykes into the sedimentary basement.

Carbon isotopic data for the peripheral CO2 emissions pro-vide evidence of a magmatic contribution in both soil gases

Figure 7. d18O vs. dD diagram of natural waters collected in theEtnean area. Crosses refer to rainwater samples while circles togroundwater samples. The global meteoric water line [Craig,1961], the east Mediterranean meteoric water line [Gat & Carmi,1970] and the local meteoric water line are also reported.

AIUPPA ET AL.. 11

and groundwaters, that is variably mixed with shallowercrustal-derived (mainly organic) carbon. The isotopic compo-sition of total dissolved carbon (TDC) in groundwatersenables one to compute the δ13C of the gaseous CO2 in equi-librium with water under the measured temperature, pH andalkalinity conditions [Allard et al., 1997; D’Alessandro et al.,1997a]. Computed δ13C(CO2) values from -22 to -5 ‰ resultfor the samples with low TDC, with the more negative valuesclearly indicating almost purely organic carbon (e.g., derivedfrom organic activity in the soils). In contrast, a narrow rangeof -5 to -3 ‰, similar to that for the high temperature erup-tive CO2, characterizes the waters with high TDC, indicatingthe dominant contribution of magma-derived carbon dioxide.

Paternò samples 1, 2 and 4, whose δ13C values rangebetween -1 and +1 ‰, may reflect gas-water interactions atrelatively high temperature (> 120 °C [Chiodini et al., 1996]).The equilibrium between HCO3

- and CO2 in water at tem-peratures above 120 °C, in fact, increases 13C content in thegaseous CO2 [Mook et al., 1974].

4.4. Sulphur

Allard [1986] determined sulfur isotope composition ofEtna volatiles, providing δ34S of SO2 in summit craterfumaroles ranging from +0.8 to +2.6 ‰ vs. CDT. This sug-gests a mantle derivation of sulfur. As regards Etna’sgroundwaters, the only δ34S data (+25.6 ‰) are for dis-solved sulfate in thermal waters issuing close to the villageof Acireale, along the eastern coast of the volcano[Dongarrà and Hauser, 1982]. The same authors measuredalso the isotope composition of sulfur from both hydrogensulfide and native sulfur (respectively +27.8 and +24.5 ‰)in the incrustations at discharge point. The above valueswere interpreted as a result of the interaction betweengroundwater and gypsum-bearing evaporite deposits.

5. TEMPORAL CHANGES AND RELATIONSHIPSWITH VOLCANIC ACTIVITY

Since 1987 regular surveys of groundwaters and gasemissions were carried out on Mt. Etna, with attention beingfocused on the areas affected by more extensive degassing.

Figure 8 shows the temporal variations of mean tempera-ture and PCO2 in 12 groundwaters monitored since 1989[Bonfanti et al., 1996a, 1996b]. Figure 8a highlights thehuge temperature changes that occurred prior to and duringthe great 1991-1993 eruption, which emitted about 0.23 km3

of lava [Stevens et al., 1997]. A sharp temperature increasestarted six months before the onset of the eruption, followedby a decrease down to a minimum at the beginning of theeruption. Not only the amplitude of this change exceeded bya factor 2-3 the normal seasonal variation, but conductivity,pH and the bicarbonate content of the waters also displayedsynchronous variations that cannot be accounted for by sea-sonal effects [Bonfanti et al., 1996b]. The beginning of the1991-1993 eruption was also preceded by a sudden peak inthe partial pressure of CO2, immediately followed by asharp fall (Figure 8b). This phenomenon was observed to benearly synchronous, but with a different intensity, in bothsurveyed areas though distant from each other by 30 km!

Bonfanti et al. [1996a] attributed these anomalies to aninput of magmatic gas into the aquifers before the 1991-1993 eruption. They estimated a heat input of 2·1015 J tobe responsible for the water temperature increase prior tothe eruption, which is equivalent to only 0.2% of the totalthermal energy subsequently released during the eruption(about 9·1017 J). The amount of gas required to carrysuch a heat amount into the aquifers was estimated atabout 7% of the total amount of erupted gas. This meansthat, before erupting, the intruding magma looses a sig-nificant fraction of its original volatiles through volcano-tectonic fractures of the basement and that the storedmagma volume was wide or/and deep enough to be ableto influence widely distant sectors of the volcano.Otherwise, the associated geochemical signals would

Figure 8. Variations in time of temperature (a) and PCO2 (b) val-ues of Etnean groundwaters during the period 1989-99. Each pointon the plot represents the average of 12 sampling points. Eruptiveactivity is also indicated: grey lines are periods with effusive activ-ity while crosses mark major explosive activity at the summitcraters (strong strombolian activity or lava fountaining).

12 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

have been diluted or even lost during transfer and, due tothe different circulation times between the eastern andthe south-western basins, they would not have appearedsynchronously in the two surveyed areas.

Although numerous and important eruptive events tookplace on Etna since July 1995, Figure 8 shows that no geo-chemical signals comparable to those observed prior toand during the 1991-1993 eruption were detected ingroundwaters. A phase of increasing PCO2 recorded at theend of 1997, particularly in the area of Belpasso-Paternò,could be attributed to the onset of magma accumulation atshallow level beneath and within the edifice which subse-quently fed intense eruptive activity in Feb. – Nov. 1999and Jul. - Aug 2001. In fact, the PCO2 anomaly reached itsmaximum in January 1998 in coincidence with a majorseismic swarm, interpreted by seismologists as due to theintrusive process that led to the February– November 1999eruption [Bonaccorso and Patanè, 2001]. The reasons forthe lack of temperature anomaly in groundwater prior tothis 1999 eruption however remain to be elucidated.

Figure 9 shows the temporal evolution of the average soilCO2 degassing in the areas of Zafferana - S. Venerina and ofPaternò. An anomalous increase in the CO2 flux from thesoils was simultaneously recorded in both areas about ninemonths before the 1991-1993 eruption, followed by a sud-den decrease a few weeks before the onset of the eruption.These CO2 anomalies marked the progressive ascent of adeep magma batch toward the upper portions of the crust[Giammanco et al., 1995]. Very low CO2 fluxes were meas-ured in the same areas during the 1991-1993 eruption,which is consistent with magma degassing taking placemostly in the central conduit system.

Since 1993, data indicate a decoupling of the Zafferana -S.Venerina and Paternò sectors: in the former, CO2 soil

degassing increased since the end of 1993, while it remainedbroadly steady and significantly lower in the latter. The onlyexception to this behaviour was the first half of 1998, when CO2emissions in the Paternò area had a marked increase and werecomparable to those of the other area. Short-term pulses of highemissions of CO2, CH4 and He, sometimes accompanied byanomalous emissions of other minor gas species such as H2 andCO, took occasionally place in a soil site near Paternò (P39)[Giammanco et al., 1998b; Bruno et al., 2001]. All of the anom-alous changes in soil gas emissions observed since 1994 couldbe related to gas released from ascending magma that causedthe renewal of eruptive activity since July 1995.

6. DISCUSSION: A GEOCHEMICAL MODEL FORMOUNT ETNA VOLCANO

The extensive geochemical data set collected at Mt. Etnaduring the last three decades was briefly summarized above.The data emphasize that a huge mass and energy transfer fromthe mantle reservoirs to the surface persistently takes place atthis very active basaltic volcano. Degassing of magma-derived volatiles occurs both through the central conduits,generating a voluminous volcanic plume, and in peripheralareas. Along the flanks of the volcano, the upraise of magmat-ic gases is mainly governed by the regional and local tectonicstructures and by the geometry of the underlying sedimentarybasement, giving rise to soil manifestations and gas-chargedgroundwaters.

The main chemical and isotopic features of Etna summitand peripheral manifestations are summarized in Figure 10.The Figure schematically sketches the geometry of mantle andshallow magma reservoirs, the vertical gradient of rock per-meability and the consequent flow paths of fluids (gases andgroundwaters) inside the volcano.

Figure 9. Variations in time of CO2 dynamic concentration (in ppmV, that is proportional to the gas flux according to the method pro-posed by Gurrieri and Valenza [1988]) measured in the areas of Paternò and Zafferana. Each point represents the average of 69 and 65sampling points, respectively. Eruptive activity as in Figure 8.

AIUPPA ET AL.. 13

The open, deep central feeding conduits of the volcano aremajor pathways for the migration of volatiles, as illustrated bythe persistency of a voluminous plume issuing from the sum-mit craters. Water, carbon dioxide and sulfur dioxide are themain species emitted at the summit craters. The estimated CO2output from the summit craters ranges between 4 and 13 Mt/a,and exceeds by a factor of ~100 the rate of transport throughthe aquifers (Figure 11). The amount of CO2 diffusely emittedfrom the soils, deduced from measurements of the diffuse soilCO2 flux, leads to a conservative value of 1 Mt/a referred to asurface of 1000 km2 [D’Alessandro et al., 1997b].Considering the above figures, the minimum volume ofmagma supplying the total CO2 budget can be tentativelyassessed at about 0.7 km3/a [D’Alessandro et al., 1997b],assuming a uniform CO2 content of 0.6 wt % in Etna magmas[Clocchiatti et al., 1992]. Allard [1997] estimated a volume of0.3 - 0.6 km3/a of CO2-degassed magma. Both figures are 15to 20 times greater than the average volume of lava annuallyerupted during the period 1971-1995. Such a relevant massdefect, also evidenced by the sulfur budget [Allard, 1997],points to the existence of large volume convecting cells bothin the crust and in a deep mantle diapir [Allard et al. 1997;Tanguy et al., 1997]. Continuous turnover of magma supply-ing fresh, gas–rich melts to the topmost parts of the upwellingastenosphere and deep crust [D’Alessandro et al., 1997b] is a

practically infinite source of volatiles, thus allowing Etna’sprodigious output [Allard et al., 1991].

The 3He/4He ratio for Etna magma (~7 Ra) ranks amongthe highest ratios yet found on an active volcano in conti-nental Europe. A slightly higher 3He/4He ratio (7.3 Ra) wasmeasured only at Pantelleria volcanic island, further south[Parello et al., 2000]. Being lower than the typical-MORBvalue of 8 R/Ra, these 3He/4He ratios at Etna and Pantelleriamost likely reflect a relatively radiogenic upper mantlezone, with slight HIMU affinity, that is upwelling beneaththe whole Western-Central Europe [Allard et al., 1997;D’Alessandro et al., 1997b; Parello et al., 2000] and is acommon parent source to volcanism in this region [Hoernleet al., 1995].

The diffusive leakage of magma-derived gases (mainlyCO2) through the flanks of the volcano is widespread andsignificant [Allard et al., 1991; Baubron, 1996; Giammancoet al., 1995, 1997], reflecting a wide horizontal extension ofthe mantle diapir and of the plutonic bodies underlying thisvolcano [Hirn et al., 1997; Allard et al, 1997; D’Alessandroet al, 1997b]. The volcano-tectonic structures of Mt. Etnadetermine both the distribution and the extent of the variousgas exhalations. Compared with magmatic volatiles emittedat the summit (δ13C –3±1 ‰; 3He/4He ~7 Ra), data relativeto peripheral manifestations (free and dissolved gases)

Figure 10. Simplified fluid circulation model of Mt. Etna illustrating the main differences between gas emissions andgroundwater compositions between the eastern and the southwestern sectors of the volcano.

14 MAGMATIC GAS LEAKAGE AT MOUNT ETNA:

reveal a variable mixing of the deep magmatic componentwith shallow fluids, including air, organic CO2 and amethane-enriched sedimentary component. The relativeabundance of the deep and shallow components depends onthe geological and structural settings, on the thickness of theaquifers, and on the flow rate of the deep gas. Free and dis-solved gases with a typical magmatic fingerprint issue fromtwo wide sectors with more intense degassing, namely theareas of Paternò and Zafferana. These are among the mostseismically active areas of Etna and are crossed by regionaltectonic faults oriented NNW-SSE and NE-SW [Lo Giudiceet al., 1982]. These faults are in turn intersected by shal-lower structures oriented E-W. We believe that these tecton-ic structures are preferential pathways for deep magmaticgas upraise [D’Alessandro et al., 1992; Anzà et al., 1993].Furthermore, gravimetric investigations [Loddo et al.,1989] highlighted two relative gravity maxima in corre-spondence with these areas of anomalous degassing which,according to geoelectric data, may reflect a thinning of theless dense, impermeable sediments underlying Etna’s vol-canic rocks. Such a tectonically induced thinning of sedi-ments could therefore favor the upraise of deep gases to thesurface. Atmospheric and organic components, in turn, pre-vail in areas where the gas influx from depth is the lowest.Mixing with methane-rich fluids seems to be favored in thewestern sector of Etna by the presence of hydrocarbon andbrine reservoirs trapped in shallow sedimentary rocks.These sedimentary fluids are discharged almost unmixed atthe surface at the “Salinelle” (Figure 10). It should be noted,finally, that even in the absence of mixing processes, thepristine composition of magma-derived volatiles can be

altered by chemical and isotope fractionation processesupon dissolution in the aquifers, carbon dioxide and itsheaviest isotope (13C) being preferentially partitioned in theaqueous phase.

Etna is, on long-term average, the strongest volcanicemitter of volatiles to the atmosphere [Allard et al., 1991;Allard, 1997], but also hosts a vast hydrological system.Figure 11 compares the yearly averaged elemental plumeemission rates with mean annual discharges from the vol-cano’s aquifer system. The figure demonstrates that thecrater plume emissions and aqueous transport contributecomparable amounts of many elements, but allows to dis-tinguish between hydrophile and atmophile elements. Theformer ones (K, Na, Mg, Ca, Sr, Li, V and U) are more effi-ciently transported by groundwaters than discharged to theatmosphere at the craters. This relies on their high abun-dances in the host rocks and on the lack of significant sinksin the aquatic media. Plume emissions, in contrast, are amuch greater source of elements with high volatility or/andaffinity for the vapor phase (C, S, Cl, F and Br), as well asof elements hich have a limited mobility in the aqueousphase (Fe, Ni, Th, Mn), or both (Pb, Cu, Cd, Zn). The emis-sion rates of most volatile elements in the volcanic plumeare 100-10,000 times higher than their corresponding aque-ous transport rates. Among the halogens, fluorine displaysthe most atmophile behavior; its less efficient transport bygroundwater is related to its preferential uptake in soils. Thespecific chemical behavior of each element or species deter-mines the modalities and pathways of the flows taking placeat Mount Etna. Better quantification of these flows andexchanges is definitely a central aim for future geochemicalresearch and monitoring at the largest and most activebasaltic volcano in Europe.

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Alessandro Aiuppa, Francesco Parello, and Mariano ValenzaDipartimento di Chimica e Fisica della Terra ed Applicazioni,Università di Palermo, via Archirafi 36, 90123 Palermo, Italy,aiuppa@unipa.it

Patrick Allard, Laboratoire Pierre Süe, CNRS-CEA, CEN-Saclay, 91191 Gif/Yvette, Franc,. allard@lsce.saclay.cea.fr

Walter D’Alessandro, and Salvatore Giammanco, IstitutoNazionale di Geofisica e Vulcanologia, Sezione di Palermo, viaUgo La Malfa 153, 90146 Palermo, Italy, walter@pa.ingv.it

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