New insights into magma dynamics during last two eruptionsof Mount Etna as inferred by geochemical monitoring from2002 to 2005

A. Rizzo, A. Caracausi, R. Favara, M. Martelli, A. Paonita, and M. PaternosterIstituto Nazionale di Geofisica e Vulcanologia—Sezione di Palermo, Via Ugo La Malfa 153, Palermo I-90146, Italy(a.rizzo@pa.ingv.it)

P. M. Nuccio and A. RosciglioneDipartimento CFTA, Universita di Palermo, Via Archirafi 36, Palermo I-90123, Italy

[1] Two distinct eruptive events characterize the volcanic activity at Mount Etna during the 2002 to 2005period. We identified signals of magma ascent preceding these eruptions by geochemical monitoring of bothchemical composition and He-isotope ratio of gas emissions from five locations in the peripheral area of thevolcano. The geochemical signals are interpreted using the models proposed by Caracausi et al. (2003a,2003b) and allow identification of episodes of magma ascent and estimation of the pressures of degassingmagma. As observed for the 2001 eruption (Caracausi et al., 2003b), magma ascent probably triggered theonset of the 2002–2003 eruption, and minor events of magma ascent were observed between May andDecember 2003. In contrast to the previous two eruptions, the 2004–2005 eruption was not preceded bysignificant geochemical signals of volcanic unrest, suggesting that this eruption was mainly triggered by thefailure of the upper portion of the volcanic edifice under the magmatic hydrostatic pressure in the conduits.High 3He/4He ratio revealed new volatile-rich magma accumulation. The 2002–2003 eruption was precededby a much shorter period of new magma accumulation from deep levels of the feeding system. Few minorsignals of magma migration were detected at some of the sites during the months preceding the 2004–2005eruption, suggesting that the degassed 3He-depleted magma resident in the volcanic conduits was notreplaced by new volatile-rich magma. This is in agreement with the lack of explosive activity during the2004–2005 eruption and with petrologic observations that the parent magma probably erupted in 2000 and2001. New geochemical signals of magma ascent from the deep reservoir have been identified since June2005, indicating that the volcanic activity of Mount Etna is evolving toward new pre-eruptive conditions.

Components: 6783 words, 5 figures, 2 tables.

Keywords: chemical composition; geochemical monitoring; He isotope; magma migration.

Index Terms: 1009 Geochemistry: Geochemical modeling (3610, 8410); 8434 Volcanology: Magma migration and

fragmentation.

Received 3 November 2005; Revised 7 March 2006; Accepted 17 March 2006; Published 8 June 2006.

Rizzo, A., A. Caracausi, R. Favara, M. Martelli, A. Paonita, M. Paternoster, P. M. Nuccio, and A. Rosciglione (2006), New

insights into magma dynamics during last two eruptions of Mount Etna as inferred by geochemical monitoring from 2002 to

2005, Geochem. Geophys. Geosyst., 7, Q06008, doi:10.1029/2005GC001175.

1. Introduction

[2] Mount Etna has been characterized in the last5–6 years by frequent and sudden eruptions with

many new and unusual geochemical and petrologicfeatures [Clocchiatti et al., 2004; Andronico et al.,2005; Corsaro and Miraglia, 2005] that havesignificantly modified the morphology of the upper

G3G3GeochemistryGeophysics

Geosystems

Published by AGU and the Geochemical Society

AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES

GeochemistryGeophysics

Geosystems

Research Letter

Volume 7, Number 6

8 June 2006

Q06008, doi:10.1029/2005GC001175

ISSN: 1525-2027

Copyright 2006 by the American Geophysical Union 1 of 12

part of the volcanic edifice. Regular geochemicalmonitoring of the Mount Etna volcano has beenaimed at relating the variations observed in plumechemistry [Bruno et al., 2001; Aiuppa et al., 2004a,2004b, 2005; Allard et al., 2004], soil CO2 flux[Giammanco et al., 1998a, 1998b; Alparone et al.,2004], and both the chemical and the He-isotopecomposition of gas emitted [Caracausi et al.,2003a, 2003b], with the degassing mechanismand eruption dynamics. Caracausi et al. [2003a]investigated the 3He/4He ratio in gas emissionsmeasured twice per month from five locations inthe peripheral area of the volcano. They identifiedsynchronous variations in the compositions of gassampled at sites located 60 km apart. They attrib-uted these variations to magma degassing process,and suggested that the Etnean plumbing system ismuch more extensive than previously thought.They also detected pulses of ascending magmathat probably provided the engine for the onset ofthe 2001 eruption. Caracausi et al. [2003b] inves-tigated the chemical composition of the sameperipheral gas emissions, and found that gas dis-charged during magma degassing interacts withlocal and shallow hydrothermal aquifers and causesthe selective dissolution of CO2. They proposed aquantitative model for determining the pristinemagmatic gas composition. Briefly, they consid-ered that each species in the magmatic gasesdissolves into an infinitesimal parcel of gas-freehot water up to saturation, with the process beingincrementally repeated until the results are consis-tent with the chemical composition measured in thecollected gases. The recalculated He/CO2 andHe/Ne ratios displayed variations over time thatwere synchronous at all the monitored sites. On thebasis of numerical simulations of the Etnean mag-ma degassing, these variations have been attributedto volatile outgassing following the ascent anddepressurization of magma [Caracausi et al.,2003b]. This allowed the authors to determine thedepth of magma batches feeding the sampledemissions, with their results being in good agree-ment with the two main magma bodies at depths ofabout 10 and 3 km b.s.l., as identified by geophys-ical investigations [e.g., Murru et al., 1999; DeGori et al., 2005].

[3] In this paper, we describe data from geochem-ical monitoring of peripheral gas discharges in theperiod 2002–2005, and explain the observed var-iations on the basis of the above mentioned modelsof chemical composition as well as 3He/4He ratiosof the gases, as already monitored by Caracausi etal. [2003a, 2003b] up to the end of 2001. We

identify the signals indicative of magma migrationstoward the shallowest levels of the Etnean plumb-ing system during the investigated period, whichshow that the 2002–2003 eruption was precededby magma injections into the shallow storagevolume, and that minor signals were detectablebefore the onset of the 2004–2005 eruption. Fi-nally, we show that a new magma migration towarda depth of about 4 km b.s.l. occurred between Juneand August 2005.

2. Eruptive Activity During 2002–2005

[4] Following the 2001 eruption, Etna was almostquiescent until March 2002, when the summitcraters reactivated [Patane et al., 2003; Andronicoet al., 2005]. Strombolian activity (at the BoccaNuova and Northeast Craters) together with shal-low seismicity and deformation then preceded the2002–2003 eruption [Andronico et al., 2005; Neriet al., 2005]. This eruption began on 26 October2002 and was preceded by a seismic swarm lasting2 hours, while a 1-km-long eruptive fractureopened on the southern flank, at 2850–2600 ma.s.l. [Acocella et al., 2003]. Early in the morningof 27 October 2002, a second eruptive fractureopened on the northeastern flank of the volcano ata similar altitude as the southern flank fracture,showing lava fountains coupled with violentStrombolian activity [Andronico et al., 2005].The lava effusion followed explosive activity andproduced a 1-km-long flow. The petrography andgeochemistry of the erupted products indicate thatthe lava on the northeastern flank was partiallydegassed during its storage at shallow depths of theplumbing system. The 2002–2003 volcanic prod-ucts show features similar in petrography as the2001 eruptive products emitted from the ventslocated on the upper portion of the volcano (higherthan 2700 m a.s.l.). In particular, they are minera-logically similar to trachybasalts erupted duringthe past decades by the summit craters. Theyare porphyritic, showing 30–40% phenocrysts[Clocchiatti et al., 2004]. Conversely, the magmaerupted from the southern fissure is relatively un-degassed, and can be linked to the deeper portionof the plumbing system. Its petrographic featureswere close to those of the magma feeding the 2001eruptive fissure located along the lower southernflank (2600–2100 m a.s.l. [Andronico et al.,2005]). These lavas are more basic and show amuch smaller amount of phenocrysts (5–10%),with the occasional occurrence of amphibole andorthopyroxene, and abundant sandstone inclusions

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

2 of 12

[Clocchiatti et al., 2004]. It is noteworthy that theexceptional explosive activity characterizing the2002–2003 eruption (among the most intense inrecent times) significantly modified the morpholo-gy of the upper parts of the volcano with thegrowing of two distinct cones [Patane et al.,2003; Andronico et al., 2005]. Furthermore, havingdifferent magmas feeding the same eruptive activ-ity is extraordinary, with the last such identifiedevent (apart from the 2001 eruption) occurring300 years ago [Andronico et al., 2005].

[5] The 2002–2003 eruption on 28 January 2003was followed by a long period of quiescence andon 7 September 2004 a new effusive eruptionstarted. This event was not preceded by seismicactivity or ground deformations [Burton et al.,2005; Patane et al., 2005]. The lava flow fed afissure system that opened on the lower easternflank of the Southeast Crater, and maintainedalmost steady conditions until the eruption endedon 8 March 2005 [Corsaro and Miraglia, 2005].Combined geophysical, petrological and geochem-ical studies [e.g., Burton et al., 2005; Corsaro andMiraglia, 2005; Patane et al., 2005] suggest thatthis eruption stored in the shallow part of theplumbing system since the volcanic activity in2000 and 2001. Moreover, the absence of signifi-cant explosive activity during this eruption indi-cates that the erupted magma was extremelydegassed [Corsaro and Miraglia, 2005].

3. Analytical Techniques

[6] The determination of the helium isotopic com-position was carried out on a static vacuum massspectrometer (VG-5400TFT, VG Isotopes) modi-fied for the simultaneous detection of 3He and 4Heion beam, in order to reduce the error of the3He/4He measurements in 3He-poor gases (e.g.,Etnean volcanic gases) down to very low values(an average of ±0.05 Ra). The 3He/4He ratio wasdetermined by measuring 3He on the Daly detectorand 4He in the Faraday cup. The 3He/4He ratioshave been corrected for the atmospheric contami-nation [Sano and Wakita, 1985]. The correctionwas generally negligible and the corrected valuesare reported as R/Ra values (where Ra is equal to1.39 � 10�6). The neon content was determined bythe comparison of peak intensity of the 20Ne of airand that of the gas sample.

[7] Chemical abundances of He, O2, N2, CH4 andCO2 were measured with a Perkin Elmer Autosys-tem XL gas-chromatograph equipped with a 4 m

Carboxen 1000 column and double detector (HotWire Detector followed by a methanizer and FlameIonisation Detector). Analytical errors were ±5%for He and ±3% for the other species.

4. Results and Discussion

[8] We have investigated the volcanic activity ofMount Etna from the end of the 2001 eruption untilOctober 2005. Gas samples were collected fromfour sites (Stadio, P39, Vallone Salato, and Fonda-chello) located around Mount Etna and from one(Naftia) that is 40 km south of the Mount Etnaborder at the limit with the Hyblean volcanics(Figure 1), with a sampling frequency ranging fromtwice a month during quiescence and up to twice aweek during periods of volcanic unrest. The chem-ical composition as well as the helium isotoperatios are listed in Table 1.

4.1. He Isotopes

[9] The long-term record of 3He/4He ratios (asR/Ra) measured in the sampled gases are shown inFigure 2. Most of the analyzed gases have 4He/20Neratios in excess of 300, well above that in the air(4He/20Ne = 0.318) and hence ruling out any sig-nificant air contamination. The measured 3He/4Heratios showed a large variability, well outside theoverall error (±0.05 Ra). Most of the He-isotopesvariations are coherent between the sites. Similarcoherence was observed in the time period asobserved between 1996 and 2001 [Caracausi etal., 2003a], even if the monitored emissions havedifferences in the average 3He/4He ratios. Theseauthors used the He-isotope variations at constantCH4 concentrations as evidence for the lack ofinteraction with crustal fluids. Instead the spatialand temporal variations in 3He/4He may beexplained by magma degassing, which would causekinetic fractionation of helium isotopes during theexsolution of the gas phase. During bubble growth,He isotopes can undergo kinetic fractionation asthe lighter 3He diffuses into the growing bubblesat a higher rate than 4He. The residual meltsare consequently depleted of 3He, relative to 4He[Nuccio and Valenza, 1998; Caracausi et al.,2003a]. This process can be modeled by the follow-ing equation, which is similar to that proposed byHoefs [1987] for Rayleigh distillation:

R=R0 ¼ F k�1ð Þ; ð1Þ

where R is the 3He/4He ratio in the melt, F is theresidual fraction of helium after a degassing event,

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

3 of 12

R0 refers to initial conditions (namely 3He/4He inmelt before the degassing event), and k is thekinetic fractionation factor (k = [4/3]1/2). Equation(1) predicts that degassing will decrease the3He/4He ratio of the melt.

[10] The lower 3He/4He ratio of the emissions atthe Stadio and Fondachello sites indicates these arefed by more-degassed portions of the plumbingsystem, especially compared to site P39. Thedifferent extent of evolution of the magma batchesinside the same plumbing system may be explainedby a system of fractures and dykes (and not simplemagma chambers). This plumbing system is feddirectly from the asthenospheric mantle. This is inagreement with geophysical observations [Hirn etal., 1997; Chiarabba et al., 2004; De Gori et al.,2005]. Thus gas discharges (separated by severalkilometers) originate from magma bodies that areonly partially in communication and hence experi-encing differing extents of degassing.

[11] In this respect, lava of the 2002–2003 eruption,for which the 3He/4He ratio in the fluid inclusions ofolivine phenocrysts was 6.6 ± 0.16 Ra (average ofN = 16 samples; NatalieMacLean and Finlay Stuart,personal communication), would be fed by magmabatches that were more degassed than that contem-poraneously feeding site P39 that show highervalues.

[12] Kinetic fractionation could also be the keyprocess underlying the temporal 3He/4He varia-tions observed at the monitored sites. Decreasing3He/4He values are due to progressive magmadegassing driven by magma ascent and decompres-sion toward the surface, while high 3He/4He ratioin the record are either discharge of bubbleskinetically enriched in 3He during magma vesicu-lation or outgassing of new volatile-rich magmacoming from deep levels of the plumbing system[Nuccio and Valenza, 1998; Caracausi et al.,2003a]. The contemporaneous occurrence of theisotopic variations at all the monitored sites wouldbe indicative of an extensive involvement of theplumbing system during the degassing and/orrefilling events. As inferred by Caracausi et al.[2003a], the Etnean feeding system appears to bemuch wider than previously reported [Sharp et al.,1980; Hirn et al., 1997; Murru et al., 1999], withan alignment in a northeast-southwest direction[Sharp et al., 1980] that would support the syn-chronous variations observed at the Naftia andFondachello sites, which are separated by 70 km.Conversely, minor magma input from depth mayonly partially involve the plumbing system, caus-ing synchronous variations at only two or three ofthe monitored sites. In this respect, the correlationof the R/Ra variations among the sites can be usedto qualitatively estimate the magnitude of magmadynamics events in the plumbing system.

Figure 1. Satellite image of Sicily and the Mount Etna ash plume on 28 October 2002 just after the onset of the2002–2003 eruption (modified from the MODIS Rapid Response System, NASA). Sketch map of the location ofsampling sites. The characteristics of the sampling sites are detailed by Caracausi et al. [2003a].

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

4 of 12

Table 1 (Representative Sample). Chemical and Helium Isotopic Composition of the Sampled Gasesa [The fullTable 1 is available in the HTML version of this article at http://www.g-cubed.org]

Date He Ne N2 CH4 CO2 R/Ra

P39 Site01/09/02 75 0.065 1.1 12.6 86.1 7.0101/22/02 93 - 0.9 12.9 86.6 -02/05/02 96 0.049 0.7 12.5 86.4 7.0702/14/02 88 0.050 1.1 12.5 86.6 7.0302/27/02 84 0.084 1.0 12.8 86.4 7.0403/12/02 72 0.218 0.7 12.8 86.6 7.1203/26/02 77 0.175 0.0 12.4 89.4 7.0104/09/02 72 0.018 0.6 12.2 87.9 7.0004/23/02 67 0.063 0.9 12.5 86.5 6.9705/07/02 74 0.035 0.4 12.5 87.6 6.9105/21/02 75 0.078 0.6 11.9 87.6 6.8906/04/02 83 0.039 0.6 12.3 87.2 6.9406/17/02 59 0.071 0.8 11.9 87.3 6.9007/01/02 90 0.108 0.7 12.1 87.2 6.9007/16/02 69 0.084 0.7 11.9 87.3 6.8307/31/02 78 0.093 0.8 12.2 87.1 6.7008/13/02 91 0.171 1.1 12.3 86.4 6.6508/28/02 67 0.277 1.3 12.2 86.5 6.7309/10/02 73 0.139 1.0 12.5 86.8 6.8309/24/02 85 0.188 1.3 12.3 86.7 6.9110/10/02 78 0.139 0.7 12.4 87.0 6.9910/22/02 86 0.093 0.8 12.3 87.0 7.1710/27/02 105 0.081 1.1 13.1 85.8 7.0210/31/02 76 0.090 0.8 12.5 86.6 6.9111/03/02 82 0.117 0.8 12.5 86.7 6.8511/08/02 68 0.089 0.7 12.8 87.8 6.9311/11/02 93 0.114 1.0 13.3 85.5 6.9411/14/02 63 0.220 0.5 12.3 87.3 6.9011/18/02 - 0.118 - - - 6.8911/21/02 73 0.056 0.8 12.8 86.9 6.9711/25/02 77 0.038 6.6 12.3 80.5 7.0012/02/02 71 0.035 0.6 12.4 87.2 6.9512/11/02 79 0.102 0.6 13.0 86.7 6.9812/17/02 70 0.107 0.6 12.0 87.6 6.9412/24/02 74 0.082 0.5 12.2 87.8 7.0212/27/02 72 0.054 0.5 12.4 87.3 7.0901/08/03 74 0.077 0.5 12.3 87.4 7.2301/16/03 75 0.058 0.6 12.0 87.6 6.8901/23/03 59 0.131 0.5 12.1 88.1 7.0201/30/03 72 0.040 0.5 12.3 87.4 7.0002/05/03 75 0.030 0.5 12.4 87.6 7.0602/22/03 66 0.078 0.5 12.5 87.2 6.9503/12/03 61 0.050 0.6 12.6 87.6 7.0604/02/03 60 0.021 0.5 13.0 87.1 7.1204/16/03 66 0.047 0.7 13.4 86.3 6.9805/06/03 66 0.061 0.7 12.8 86.7 6.9505/30/03 55 0.084 0.6 13.5 86.2 6.6306/13/03 56 0.082 0.6 14.5 85.0 6.8306/24/03 60 0.068 0.6 14.4 85.2 6.9607/03/03 57 0.044 0.6 14.1 86.1 7.0107/17/03 57 0.054 0.7 10.7 89.3 6.7407/28/03 58 0.105 0.8 11.0 88.9 6.7408/07/03 60 0.047 0.9 13.0 86.3 6.9608/19/03 56 0.085 0.7 13.1 87.0 6.9809/03/03 56 0.267 4.2 11.1 84.5 6.9109/30/03 56 0.045 0.5 13.6 85.6 6.9810/16/03 54 1.045 0.5 13.9 85.7 7.0110/28/03 50 2.045 0.3 13.9 85.8 7.00

aOxygen is not reported because it is generally negligible, except for a few neon-rich samples that were air contaminated. Helium and neon are

expressed in ppmVol, while N2, CH4, and CO2 are in %vol. R/Ra values are reported already corrected for the small amount of air contamination.

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

5 of 12

[13] The above considerations enabled us to iden-tify magma migrations toward the surface. Follow-ing the major variations recorded before and duringthe July 2001 eruption [Caracausi et al., 2003a],new and significant variations of the 3He/4Hevalues were observed during the last 4 months of2001 (Figure 2), and these have been interpreted asepisodes of magma ascent within the plumbingsystem. The 3He/4He variations also indicateda magma migration toward the surface and associ-ated degassing from February to August 2002(Figure 2), which finished just before the onset ofthe 2002–2003 eruption (27 October 2002). Afurther input of magma probably occurred closeto the end of the eruptive period (January 2003).The asynchronous He-isotope record from thedifferent sites would suggest that only a part ofthe plumbing system experienced a new magmainput. Following the eruption, new episodes ofmagma recharge from May to September 2003were evident in the gases collected from all thesites, with a behavior similar to that followingthe 2001 eruption; no significant variations weresubsequently observed. The most-recent variationof the He-isotope ratio was recorded from June toAugust 2005, when it reached its highest value in2 years, suggesting a new episode of magmamigration toward the surface.

[14] To elucidate the long-term behavior of theEtnean plumbing system, we considered 9 years

of 3He/4He ratio data recorded at the Naftia andVallone Salato sites beginning from 1997, includ-ing data from Caracausi et al. [2003a]. Trend linesfor the data using moving averages of 12 points(equating to about 6 months) are plotted inFigure 3; note that similar results were observedfor the gas discharges from the other sites (Stadio,Fondachello, and P39). These trend lines clearlyshow that the main variations occurred almostsimultaneously at both sampling sites. Decreasing3He/4He with time is attributed to progressivemagma degassing, while the increasing 3He/4Hewith time a gradual replenishment of new volatile-rich melts with a relatively high 3He/4He ratio.

[15] The variations of the average 3He/4He valuesobserved between 1997 and the first few months of1999 (Figure 3) suggest a recharge of new volatile-rich magma (followed by its progressive degass-ing) involving the entire plumbing system, whichprobably fed the explosive and effusive activitythat took place at Mount Etna during that period[Alparone et al., 2003, and references therein]. Theaverage 3He/4He ratios increased from the first halfof 1999 till August 2001 (Figure 3), suggesting anew massive recharge of the plumbing system bynondegassed magma in accordance with the con-clusions reached by Caracausi et al. [2003a] fromlong-term monitoring at site P39. The new magmaexhibited a progressive degassing from the end of2001 until September 2002, after which a new

Figure 2. Variations of the 3He/4He ratio (as R/Ra) from June 2001 to October 2005 at all five sampling sites. R/Ravalues are corrected for the small amount of air contamination on the basis of the 4He/20Ne ratio [Sano and Wakita,1985]. Average error bar (±0.05 Ra) is also shown. Vertical lines include the last three eruptive periods (2001, 2002–2003, and 2004–2005). Data of 2001 are from Caracausi et al. [2003a]. The sampling procedures are detailed byCaracausi et al. [2003b].

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

6 of 12

replenishment of volatile-rich magma occurred thatboth preceded and accompanied the 2002–2003eruption. Subsequent small and localized variationsin a general degassing phase were recorded untilthe first half of 2005, crossing the 2004–2005effusive eruption, with a new and ongoing magmarecharge apparently starting in June 2005.

[16] Figure 3 suggests decoupling of the deepmagma supply to the plumbing system of thevolcano and the superficial occurrence of an erup-tion. The 2001 and 2002–2003 eruptions occurredduring a deep recharging phase of the plumbingsystem but ended while magma supply to the deeplevels of the plumbing system was still ongoing.The 2004–2005 eruption appears to have lackedmagma recharging at a shallow depth, with theeruption probably being due to failure of the upperparts of the volcanic edifice [Burton et al., 2005;Corsaro and Miraglia, 2005].

4.2. Chemical Composition

[17] In addition to the He isotopic composition, wealso monitored the chemical composition of thedischarged gases (Table 1). As already observed byCaracausi et al. [2003a, 2003b] for the pre-2001gasses, the gases are of magmatic origin, and arenormally dominated by CO2, and have a variableCH4 content. The CH4 content progressively in-creased from 0.4% at Naftia site up to 90% at theFondachello site. Oxygen was generally present atonly trace level in all the collected samples. He

ranged from 30 ppmV at the Stadio site to morethan 700 ppmV at the Fondachello site, while Nevaried from a few ppbV to 0.7 ppmV. The gasreleased from the magma interacts with hot, shal-low hydrothermal aquifers during its rise towardthe surface, causing significant changes in itschemical composition [D’Alessandro et al., 1997;Caracausi et al., 2003b]. Thus the variability ofmethane content, as well as that of He and Necontent, is due to the selective dissolution of CO2

in water that induces a relative enrichment of theless-soluble species in the residual gas phase.Caracausi et al. [2003b] modeled the interactionof the ascending Etnean magmatic gas through anaquifer, and their quantitatively evaluation allowedthem to calculate the pristine magmatic gas com-position prior to interaction. Briefly, they consid-ered that each species in the magmatic gasesdissolves into an infinitesimal parcel of gas-freehot water up to saturation, with the process beingincrementally repeated until the results are consis-tent with the chemical composition measured in thecollected gases.

[18] As observed by Caracausi et al. [2003b], thecorrected He/Ne and He/CO2 ratios display syn-chronous variations over time, which can be attrib-uted to magma degassing caused by ascent anddecompression of the magma body. The volatilesdissolved exhibit different solubilities in silicatemelt, and thus the composition of the exsolvedvapor varies as a function of the extent of degassing

Figure 3. Twelve-point moving average (equating to about 6 months) of R/Ra from 1996 to 2005. Only the data forthe Naftia and Vallone Salato sites are shown, since similar behavior was observed at all the sampling sites. Yellowbar represents an intense phase when explosive and eruptive activity occurred at all the craters. Vertical lines are as inFigure 2.

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

7 of 12

Table 2. Main Variations of He/Ne and He/CO2 Ratios Over the Investigated Period and Related Initial and FinalDegassing Pressures of the Magmaa

Date of Variations Restored DHe/CO2b Measured DHe/Ne Computed Pressure in MPa

P39 SiteStart 08/03/01 474End 08/16/01 0.471 0.699 150Start 10/09/01 471End 10/29/01 0.349 0.635 198Start 08/28/02 453End 10/27/02 0.319 0.600 208Start 10/09/02 491End 10/27/02 0.214 0.577 276Start 05/30/03 401End 07/17/03 0.281 0.498 212

Vallone Salato SiteStartc 06/28/01 444Endc 07/27/01 0.389 0.500 176Startc 08/16/01 443Endc 09/18/01 0.387 0.626 177Start 07/05/05 447End 08/03/05 0.235 0.529 249

Fondachello SiteStartc 07/27/01 381Endc 08/03/01 0.277 0.472 206Startc 09/07/01 388Endc 09/14/01 0.194 0.400 249Startc 10/02/01 332Endc 10/23/01 0.354 0.449 161Startc 01/22/02 354Endc 02/27/02 0.244 0.402 210Start 07/16/02 314End 08/13/02 0.387 0.438 147Start 08/28/02 407End 09/24/02 0.543 0.646 119Start 10/10/02 342End 10/22/02 0.336 0.451 171Start 10/28/02 305End 10/31/02 0.355 0.404 154Start 01/23/03 503End 01/30/03 0.371 0.690 200Start 09/08/04 364End 10/12/04 0.149 0.314 261Start 07/05/05 342End 08/25/05 0.315 0.438 179

Stadio SiteStartc 07/27/01 398Endc 08/13/01 0.404 0.571 162Startc 09/07/01 356Endc 10/02/01 0.478 0.549 129Startc 10/23/01 382Endc 11/07/01 0.310 0.493 193Start 07/01/02 394End 10/27/02 0.758 0.694 64Start 08/19/03 452End 09/03/03 0.244 0.543 245Start 12/01/03 294End 12/30/03 0.429 0.429 130Start 09/22/04 466End 10/12/04 0.182 0.500 288

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

8 of 12

and of the magma pressure [Nuccio and Paonita,2001]. Caracausi et al. [2003b] calculated that bothHe/Ne and He/CO2 ratios increase with increasingextent of degassing because CO2 and Ne are bothless soluble than He. By simultaneously incorporat-ing the temporal variation of both He/Ne andHe/CO2 in the degassing model, we were able toestimate the initial and final degassing pressures ofthe magma body (Table 2).

[19] During the investigated period, we detectedseveral synchronous increases in the He/Ne andHe/CO2 ratios (Figure 4), most of which occurredaround the 2002–2003 eruption. On the basis ofthe above interpretation, we consider that the He/Ne and He/CO2 covariation indicates events ofmagma migration in the Etnean plumbing system,and we have quantified the related degassingpressures using the method of Caracausi et al.[2003b]. The calculations suggest the occurrenceof magma-ascent events from the deep reservoirtoward the shallowest one (Table 2, Figure 5). Inthe periods preceding and following the onset ofthe 2002–2003 eruption, several inputs of magmafrom deep levels of the plumbing system wereevident from the synchronous increases in chemi-cal ratios. In agreement with the data on He

isotopes, several magma-ascent events toward adepth of 3–5 km were evident in the monthsfollowing the 2001 eruption until the end of thatyear (Figure 5). No subsequent magma input wasobserved until July 2002, except for a tail eventduring January and February 2002 that wasrecorded only at the Fondachello site. New magmainputs from about 10-km deep to about 2-km deepoccurred from July to October 2002 (Figure 5). It isnoteworthy that these migrations were recordedfrom a few months to a few days before the onsetof the 2002–2003 eruption, suggesting a cause-and-effect relation related to the high eruptivepotential of the volcano. During this eruptiveperiod, magma ascent was only detected at theFondachello site at the end of the eruption. Simi-larly to the July 2001 eruption [Caracausi et al.,2003b], the 2002–2003 eruption also lasted only afew months, most likely because (after the onset)limited magma inputs occurred at shallow depthuntil January 2003 (Figure 4). Few episodes ofmagma ascent have been identified since the endof the eruption, and only during the period fromMayto December 2003. Particularly, and consistent withinferences from measurements of He isotopes, twomagma migrations were recorded at the Stadio site,indicating a magma batch rising from 12 km b.s.l.

Notes to Table 2.aPressures were computed following the method proposed by Caracausi et al. [2003b], which uses each covariation of He/Ne and He/CO2 ratio

into a proper degassing model for the Etnean basalt. Isothermal, open system degassing was considered, starting from a CO2-dominated (CO2 85–95 mol%, H2O 15–5 mol%) gas phase at 700 MPa and 1200�C [Caracausi et al., 2003b]. Errors due to uncertainties in the initial conditions aregenerally within ±15%.

bDisplayed values of restored DHe/CO2 have been computed after the restoring of pristine ratios corrected for dissolution process in shallow

aquifers. For further details, see the text and also Caracausi et al. [2003b].cData from Caracausi et al. [2003b].

Figure 4. Synchronous increases in the He/Ne and He/CO2 ratios at the Fondachello site around the 2002–2003eruption (shown by the vertical red lines). The variations recorded from July 2002 to January 2003 enabled magmamigration in the plumbing system to be identified (see text). Both ratios are corrected for shallow dissolutionprocesses [Caracausi et al., 2003b].

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

9 of 12

toward about 2 km b.s.l. (see arrows in Figure 5). Nosubsequent events were recorded until the onset ofthe 2004–2005 eruption (on 7 September 2004),when magma depressurization probably occurredfrom 12 to 6 km b.s.l. Magma ascent during thisperiod probably did not directly trigger the effusionactivity, as it was not accompanied by new inputs ofmagma at shallow levels. This possibly induced adestabilization of the plumbing system at shallowlevels. Finally, in agreement with our He-isotopedata, a new magma-migration episode was recordedbetween July and August 2005.

4.3. Comparison With Volcanological andGeophysical Evidence

[20] The combination of our geochemical data withvolcanological observations of eruptive activity inrecent years at Mount Etna allow us to detectforerunner signals of magma migration towardthe shallowest levels of the plumbing system. Inparticular, both isotope (3He/4He ratio) and chem-ical (He/Ne and He/CO2 ratios) data suggested thata new recharge phase of the shallowest levels ofthe plumbing system (at 3–5 km b.s.l.) occurredafter the end of the 2001 eruption. This inference isstrongly supported by geophysical data. In fact,Patane et al. [2003] found that the seismicity rateremained high after the eruption and that a renewalarea dilatation by means of geodetic data was

observed, which they interpreted as a new magmaaccumulation at shallow depths (3 to 5 km). Thereactivation of summit craters which started inMarch 2002 [Andronico et al., 2005] may furthersupport this interpretation. Our geochemical sig-nals obtained from July to October 2002 suggestthat new batches of volatile-rich magma migratedtoward the surface, forecasting the onset of the2002–2003 eruption of a few months (Figures 2and 5). Even though it was preceded by very fewgeophysical precursors [Monaco et al., 2005],ground deformations [Aloisi et al., 2003; Pataneet al., 2005] and seismic activity [Gambino et al.,2004; Monaco et al., 2005] indicated that a con-tinuous magma refilling occurred from February2002. Nevertheless, the most evident geophysicalsignals were tilt data and seismic tremors recordedfor a few hours before the onset of the eruption[Aloisi et al., 2003; Gambino et al., 2004; Pataneet al., 2005]. From May 2003 until the end of2003, new phases of magma migrations toward2 km depth b.s.l. have been identified from geo-chemical data (Figures 2 and 5). Only minor varia-tions of the geochemical parameters weresubsequently observed, while Patane et al. [2005]recorded a recharging phase from June 2003through GPS and seismic data, which they consid-ered a possible cause of the last eruption. Never-theless, several authors [e.g., Burton et al., 2005;

Figure 5. Assessment of the degassing pressures of magma feeding the monitored gas emissions. Naftia data areexcluded because this location is fed by magma intruding into the lateral part of the plumbing system. Solid and opensymbols represent the computed initial and final pressures, respectively. Red arrows, in example, indicate twoepisodes of magma migration. Two gray bars indicate the location of magma reservoirs identified by recentgeophysical investigations [e.g., Murru et al., 1999; De Gori et al., 2005]. Yellow bars represent the main importantphases of 3He/4He ratio (as R/Ra) variations. Further details about the calculation of degassing pressures are given byCaracausi et al. [2003b]. Vertical lines are as in Figure 2.

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

10 of 12

Corsaro and Miraglia, 2005] agree that the 2004–2005 eruption was notable for the lack of clearsignals associated with magmatic processes such asseismic swarms, volcanic tremors, and sustainedmagmatic gas emissions. More likely, it was trig-gered by the failure of the upper portion of thevolcanic edifice under magmatic hydrostatic pres-sure in the conduits. Such pressure at a shallowdepth might be induced by the magma still residentat shallow levels since the last eruption (2002–2003). The absence of explosive activity at thevents is evidence against the intrusion of newmagma from depth. In fact, the petrography andglass composition of samples collected during theeffusion suggest that the magma feeding the erup-tion was stored at the shallow levels of the plumb-ing system during the 2000 and 2001 activity[Corsaro and Miraglia, 2005]. This is stronglysupported by our long-term monitoring of R/Raaverage values, which has not revealed a newrecharge of volatile-rich magma before the 2004–2005 eruption (Figure 3), in contrast to what hap-pened during the previous two eruptions. Ourgeochemical monitoring from June to August2005 identified a new magma migration towardabout 3 km b.s.l. This episode was followed by anincrease of the seismic activity that was still ongo-ing as at 31 October 2005 (see http://www.ingv.it/terremoti/terremoti.html for the latest status). Webelieve that Mount Etna could rapidly evolve intonew pre-eruptive conditions.

5. Conclusions

[21] Geochemical monitoring of Mount Etna dur-ing the period 2002–2005 combined with previ-ously developed degassing models of the Etneansystem [Caracausi et al., 2003a, 2003b] hasallowed us to identify several episodes of magmamigrations inside the plumbing system from thedeep reservoir toward the shallowest one. The mainimportant evidence is summarized as follows:

[22] 1. After the July 2001 eruption, several inputsof magma from deep levels of the plumbing systemwere recorded until the end of 2001. Subsequentnew events of magma recharge were detected for afew months before the onset of the 2002–2003eruption, in agreement with geophysical investiga-tions that found a high seismicity just after the2001 eruption and a progressive inflation of thevolcanic edifice beginning from February 2002.We hypothesize that this phase of magma recharg-ing induced a high pressure at shallow depths inthe system and triggered the eruption.

[23] 2. Following the 2002–2003 eruption, newepisodes of magma migrations toward the surfacewere observed between May and December 2003.Subsequent minor geochemical signals wererecorded until the onset of the 2004–2005 erup-tion, when a small magma-ascent event occurred.We infer that the 2004–2005 eruption was prob-ably due to an edifice collapse along the alreadyfractured sector of the volcano induced by thestill-high hydrostatic pressure of magma in theconduits, as also confirmed by other authors[Burton et al., 2005; Corsaro and Miraglia,2005].

[24] 3. Long-term monitoring of R/Ra averagevalues has allowed us to elucidate the main phasesof magma recharging in the period 1997–2005. Inparticular, we infer that the 2001 and 2002–2003eruptions were preceded by recharge of new vol-atile-rich magma, while the 2004–2005 eruptionwas not supplied by undegassed magma, as con-firmed by the absence of explosive activity. Thepetrologic features of the emitted lava stronglysupport our inferences, considering that the magmaerupted during the last eruption was a degassedportion of the magma that erupted during thesummit activity in 2000 and 2001.

[25] Finally, we point out that a new magma migra-tion inside the feeding system was observed fromJune to August 2005, followed by an increase of theseismicity that was still ongoing as at 31 October2005. We therefore consider that close attentionshould be paid to the recharging phase of the Etneanplumbing system, which is evolving toward newpre-eruptive conditions.

Acknowledgments

[26] We wish to thank F. Salerno and M. Tantillo for technical

support during the analyses in the gas chromatographic and

noble gas spectrometric laboratories. We are also grateful to

F. Grassa for his support during fieldwork. We finally thank

Vincent Salters, David R. Hilton, Finlay Stuart, and an

anonymous reviewer for providing significant suggestions that

improved the clearness of the manuscript. English text was

revised two times by English Science Editing.

References

Acocella, V., B. Behncke, M. Neri, and S. D’Amico (2003),Link between major flank slip and 2002–2003 eruption atMt. Etna (Italy), Geophys. Res. Lett., 30(24), 2286,doi:10.1029/2003GL018642.

Aiuppa, A., M. Burton, F. Mure, and S. Inguaggiato (2004a),Intercomparison of volcanic gas monitoring methodologiesperformed on Vulcano Island, Italy, Geophys. Res. Lett., 31,L02610, doi:10.1029/2003GL018651.

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

11 of 12

Aiuppa, A., C. Federico, G. Giudice, S. Gurrieri, A. Paonita,and M. Valenza (2004b), Plume chemistry provides insightsinto the mechanisms of sulfur and halogen degassing at ba-saltic volcanoes, Earth Planet. Sci. Lett., 222, 469–483.

Aiuppa, A., S. Inguaggiato, A. J. S. McGonigle, M. O’Dwyer,C. Oppenheimer, M. J. Padgett, D. Rouwet, and M. Valenza(2005), H2S fluxes from Mt. Etna, Stromboli, and Vulcano(Italy) and implications for the sulfur budget at volcanoes,Geochim. Cosmochim. Acta, 69(7), 1861–1871.

Allard, P., M. Burton, and F. Mure (2004), Spectroscopic evi-dence for a lava fountain driven by previously accumulatedmagmatic gases, Nature, 433, 407–410.

Aloisi, M., A. Bonaccorso, S. Gambino, M. Mattia, andG. Puglisi (2003), Etna 2002 eruption imaged from contin-uous tilt and GPS data, Geophys. Res. Lett., 30(23), 2214,doi:10.1029/2003GL018896.

Alparone, S., D. Andronico, L. Lodato, and T. Sgroi (2003),Relationship between tremor and volcanic activity during theSoutheast Crater eruption on Mount Etna in early 2000,J. Geophys. Res., 108(B5), 2241, doi:10.1029/2002JB001866.

Alparone, S., D. Andronico, S. Giammanco, and L. Lodato(2004), A multidisciplinary approach to detect active path-ways for magma migration and eruption at Mt. Etna (Sicily,Italy) before the 2001 and 2002–2003 eruptions, J. Volca-nol. Geotherm. Res., 136, 121–140.

Andronico, D., et al. (2005), A multi-disciplinary study of the2002–03 Etna eruption: Insight into a complex plumbingsystem, Bull. Volcanol., 67, 314–330.

Bruno, N., T. Caltabiano, S. Giammanco, and R. Romano(2001), Degassing of SO2 and CO2 at Mount Etna (Sicily)as an indicator of pre-eruptive ascent and shallow emplace-ment of magma, J. Volcanol. Geotherm. Res., 110, 137–153.

Burton, M. R., et al. (2005), Etna 2004–2005: An archetypefor geodynamically-controlled effusive eruptions, Geophys.Res. Lett., 32, L09303, doi:10.1029/2005GL022527.

Caracausi, A., R. Favara, S. Giammanco, F. Italiano, A. Paonita,G. Pecoraino, A. Rizzo, and P. M. Nuccio (2003a), MountEtna: Geochemical signals of magma ascent and unusuallyextensive plumbing system, Geophys. Res. Lett., 30(2),1057, doi:10.1029/2002GL015463.

Caracausi, A., F. Italiano, A. Paonita, A. Rizzo, and P. M.Nuccio (2003b), Evidence of deep magma degassing andascent by geochemistry of peripheral gas emissions at MountEtna (Italy): Assessment of the magmatic reservoir pressure,J. Geophys. Res., 108(B10), 2463, doi:10.1029/2002JB002095.

Chiarabba, C., P. De Gori, and D. Patane (2004), The MountEtna plumbing system: The contribution of seismic tomogra-phy, in Mount Etna: Volcano Laboratory, Geophys. Monogr.Ser., vol. 143, edited by S. Calvari et al., pp. 191–204,Washington, D. C.

Clocchiatti, R., M. Condomines, N. Guenot, and J. C. Tanguy(2004), Magma changes at Mount Etna: The 2001 and2002–2003 eruptions, Earth Planet. Sci. Lett., 226, 397–414.

Corsaro, R. A., and L. Miraglia (2005), Dynamics of 2004–2005 Mt. Etna effusive eruption as inferred from petrologicmonitoring, Geophys. Res. Lett., 32, L13302, doi:10.1029/2005GL022347.

D’Alessandro, W., S. De Gregorio, G. Dongarra, S. Guerrieri,F. Parello, and B. Parisi (1997), Chemical and isotopic char-acterization of the gases of Mount Etna (Italy), J. Volcanol.Geotherm. Res., 78, 65–76.

De Gori, P., C. Chiarabba, and D. Patane (2005), Qp structureof Mount Etna: Constraints for the physics of the plumbingsystem, J. Geophys. Res., 110, B05303, doi:10.1029/2003JB002875.

Gambino, S., A. Mostaccio, D. Patane, L. Scarfı, and A. Ursino(2004), High-precision locations of the microseismicity pre-ceding the 2002–2003Mt. Etna eruption,Geophys. Res. Lett.,31, L18604, doi:10.1029/2004GL020499.

Giammanco, S., S. Inguaggiato, and M. Valenza (1998a), Soiland fumarole gases of Mount Etna: Geochemistry and rela-tions with volcanic activity, J. Volcanol. Geotherm. Res., 81,297–310.

Giammanco, S., S. Gurrieri, and M. Valenza (1998b), Anom-alous soil CO2 degassing in relation to faults and eruptivefissures on Mount Etna (Sicily, Italy), Bull. Volcanol., 60,252–259.

Hirn, A., R. Nicolich, J. Gallart, M. Laigle, and L. Cernobori(1997), Roots of Etna volcano in faults of great earthquakes,Earth Planet. Sci. Lett., 148, 171–191.

Hoefs, J. (1987), Stable Isotope Geochemistry, 241 pp.,Springer, New York.

Monaco, C., S. Catalano, O. Cocina, G. De Guidi, C. Ferlito,S. Gresta, C. Musumeci, and L. Tortorici (2005), Tectoniccontrol on the eruptive dynamics at Mt. Etna Volcano (Si-cily) during the 2001 and the 2002–2003 eruptions, J. Vol-canol. Geotherm. Res., 144, 211–233.

Murru, M., C. Montuori, M. Wyss, and E. Privitera (1999),The locations of magma chambers at Mt. Etna, Italy, mappedby b-values, Geophys. Res. Lett., 16, 2553–2556.

Neri, M., V. Acocella, B. Behncke, V. Maiolino, A. Ursino,and R. Velardita (2005), Contrasting triggering of the 2001and 2002–2003 eruptions of Mount Etna (Italy), J. Volcanol.Geotherm. Res., 144, 235–255.

Nuccio, P. M., and A. Paonita (2001), Magmatic degassing ofmulticomponent vapors and assessment of magma depth:Application to Vulcano Island (Italy), Earth Planet. Sci.Lett., 193, 467–481.

Nuccio, P. M., and V. Valenza (1998), Magma degassing andgeochemical detection of its ascent, Water-Rock Interaction,edited by G. B. Arehart and J. R. Hulston, pp. 475–478,A. A. Balkema, Brookfield, Vt.

Patane, D., P. De Gori, C. Chiarabba, and A. Bonaccorso(2003), Magma ascent and the pressurization of the MountEtna’s volcanic system, Science, 299, 2061–2063.

Patane, D., M. Mattia, and M. Aloisi (2005), Shallow intrusiveprocesses during 2002–2004 and current volcanic activityon Mt. Etna, Geophys. Res. Lett., 32, L06302, doi:10.1029/2004GL021773.

Sano, Y., and H. Wakita (1985), Geographical distribution of3He/4He in Japan: Implications for arc tectonics and incipientmagmatism, J. Geophys. Res., 90, 8729–8741.

Sharp, A. D. L., P. M. Davis, and F. Gray (1980), A lowvelocity zone beneath Mt. Etna and magma storage, Nature,287, 587–591.

GeochemistryGeophysicsGeosystems G3G3

rizzo et al.: mount etna magma dynamics 10.1029/2005GC001175

12 of 12