Magmatic feeding system and crustal magma accumulation

beneath Vulcano Island (Italy):

Evidence from CO2 fluid inclusions in quartz xenoliths

Vittorio ZanonDipartimento di Scienze della Terra, Perugia University, Perugia, Italy

Maria-Luce Frezzotti1

Dipartimento di Scienze della Terra, Universita di Siena, Siena, Italy

Angelo PeccerilloDipartimento di Scienze della Terra, Perugia University, Perugia, Italy

Received 6 August 2002; revised 20 January 2003; accepted 13 February 2003; published 13 June 2003.

[1] Pure CO2 fluid inclusions are observed in quartz xenoliths from four lava flows in theIsland of Vulcano, corresponding to distinct activity stages during the last 120 kyr.Xenoliths, which consist of aggregates of quartz grains, are present in lavas of contrastingcomposition ranging from basaltic-andesites to rhyolites. Two main generations of CO2

inclusions are observed: early (type I) inclusions were trapped prior to the ascent ofthe host xenoliths, while type II inclusions were trapped during the ascent into thehost magma. Fluid inclusions show a bimodal distribution of homogenisationtemperatures, corresponding to two distinct density intervals: 0.89–0.52 g/cm3(type I) and0.42–0.13 g/cm3(type II). Type I inclusions indicate pressures of 0.56–0.33 GPa (21–13 km), relating to the levels of xenolith entrapment in the host lavas. Type II fluidinclusions show considerably lower pressures ranging from 0.14 to 0.03 GPa (5.5–3 km).Present data suggest ponding of mantle-derived magmas in at least two distinct reservoirs,located at lower crustal depths and at shallow levels, respectively. Combined fluidinclusion and petrological data suggest that the deep reservoirs were the sites of extensivefractional crystallization, mixing with source-derived magmas, and various degrees ofcrustal assimilation. Evolutionary processes also occurred inside shallow magmachambers, in which deep magma mixed with residing melts and rested for short periods oftime before being erupted to the surface. INDEX TERMS: 5480 Planetology: Solid Surface Planets:

Volcanism (8450); 8414 Volcanology: Eruption mechanisms; 8434 Volcanology: Magma migration;

KEYWORDS: fluid inclusions, xenoliths, Vulcano Island, petrology, magma plumbing system

Citation: Zanon, V., M.-L. Frezzotti, and A. Peccerillo, Magmatic feeding system and crustal magma accumulation beneath Vulcano

Island (Italy): Evidence from CO2 fluid inclusions in quartz xenoliths, J. Geophys. Res., 108(B6), 2298, doi:10.1029/2002JB002140,

2003.

1. Introduction

[2] The Island of Vulcano is an arc-volcanic center grownon continental crust, dominated by mantle-derived magmaswhich underwent complex low-pressure evolution in thecrust [Keller, 1974; De Astis et al., 1997, 2000; Del Moroet al., 1998]. Previous studies indicate that Vulcano is madeof different magma types with a variable degree of evolution,ranging in composition from basalts to rhyolites, becominggradually richer in K, from early high-K calc-alkaline(HKCA), to shoshonites (SHO), to late leucite-tephrite of

the potassic series (KS) [Keller, 1980]. Castellet y Ballara etal. [1982] proposed that a single parent basaltic magmaprobably generated the whole series, through various gradesof fractional crystallization, involving several magma cham-bers at different depths (0.1 GPa for Vulcanello lavas, 0.3–0.7 GPa for La Fossa, 0.1 GPa for Lentia Dome Complex).More recently, De Astis et al. [2000] proposed that theobserved chemical heterogeneity might result from primarymantle sources, although magma mixing, crustal contami-nation of mafic melts, and fractional crystallization pro-cesses in the crust played a major role. Furthermore, theseauthors proposed a polybaric evolution of the magmas with aprogressive upward migration of the magma chambers.[3] In spite of the many studies carried out in the last

decades, reliable models for the internal structure of Vul-cano and the entire system are still lacking. Such a goal is of

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B6, 2298, doi:10.1029/2002JB002140, 2003

1Also at CNR–Centro Studio per il Quaternario e l’EvoluzioneAmbientale, Piazzale A. Moro, Rome, Italy.

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JB002140$09.00

ECV 7 - 1

paramount interest for whoever wants to get an insight intothe petrology and volcanology of this active volcano. Tobetter understand the magmatic evolution and the eruptivebehavior of Vulcano, it is essential to obtain more informa-tion on the location of the magma reservoirs. Fluid inclusionstudies in magmatic minerals and/or in xenoliths entrainedin volcanic rocks have been shown to represent a valuabletechnique to reach this objective [e.g., Clocchiatti et al.,1994; Andersen and Neumann, 2001]. Quartz-rich xenolithsare widespread in several lavas and pyroclastic deposits atVulcano; in most cases, they contain abundant fluid andmelt inclusions, thus representing suitable material for geo-barometric investigation.[4] In the present study, fluid inclusions in quartz xen-

oliths from older HKCA-SHO and from the younger SHO-

KS volcanics have been analyzed in order to determine thephysico-chemical conditions of fluid inclusion formation,which can indicate the depths of possible ponding sites ofmagmas during ascent to the surface. Present results shedlight on the structure of the magma plumbing system underVulcano through time and make it possible to relate it to itsmagmatic evolution.

2. Geological Setting and Volcanic Outline

[5] The Aeolian magmatic arc consists of seven islands(Figure 1) and nine seamounts located about 25 km north ofthe coast of Sicily [Beccaluva et al., 1985; Selli, 1985;Wezel, 1985]. Volcanic activity here started about 1 millionyears ago and was generated by subduction of the Ionian

Figure 1. Geological sketch of the Vulcano Island, showing the volcanic products belonging to the sixdifferent volcanic cycles, simplified from De Astis et al. [1997]. Numbers refer to distinct eruptive cycles:(1) Primordial Volcano (120 ka); (2) Piano Caldera (99–20 ka); (3) Lentia – Mastro Minico domecomplex; (4) La Fossa Caldera (15–8 ka); (5) La Fossa Cone (6 ka to present); (6) Vulcanello (183 B.C.to 1550 A.D.). Circles indicate sampling localities.

ECV 7 - 2 ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND

plate under the Calabro-Peloritani continental margin [Bec-caluva et al., 1982; Ellam et al., 1988; De Astis et al.,2000]. Nowadays the central and eastern islands lie on the18–25 km thick Calabro-Peloritano basement [Finetti,1982; Beccaluva et al., 1985; Falsaperla et al., 1999],consisting of Hercynian and pre-Hercynian metamorphicand granitic rocks, covered by Mesozoic and Cenozoicsediments [e.g., Rottura et al., 1991].[6] Vulcano is a composite volcanic edifice situated in the

central part of the archipelago, on the southern end of aNNW-SSE graben which extends as far as Lipari and Salina(Figure 1). De Astis et al. [1997, and references therein]indicated that subaerial volcanism initiated 120 ka andmigrated from S-SE to N-NW, forming four main eruptivecenters (Primordial Vulcano, Lentia-Mastro Minico, LaFossa Cone, and Vulcanello) and two multicollapse calderas(Piano Caldera and La Fossa Caldera).[7] The ‘‘Primordial Vulcano’’ stage (first cycle) began its

subaerial activity 120 ± 3.0 ka with the formation of acomposite stratocone. Volcanics fall in the field of HKCAbasalts and basaltic andesites (Sponda Lena basaltic ande-sites) and only part of them in the SHO field. Between 99and 20 ka, intracaldera activity resumed during the ‘‘PianoCaldera’’ stage (second cycle), emplacing thick lava flowpiles, pyroclastic flow units and forming small scoria cones,with the same chemical composition of the former strato-cone [De Astis et al., 1997]. Some parasitic centers built upon the southern and western flanks of the older stratocone(Gelso, Quadrara, and Spiaggia Lunga), during the final partof this period.[8] The third ‘‘Lentia-Mastro Minico’’ cycle (24–15 ka)

consists of several extrusive domes intersected by the ringfaults of La Fossa Caldera. The erupted lavas are among themost evolved products within the whole archipelago, fallingprevalently in the field of HKCA rhyolites, and to a lesserextent, in the field of latites and HKCA dacites [De Astis etal., 1989, 1997]. Between 15 and 8 ka, after the collapse ofthe Lentia-Mastro Minico lava dome complex, the mag-matic activity resumed during ‘‘La Fossa Caldera’’ stage,(fourth cycle), with the emission of pyroclastic and lavaflows which displayed a high compositional variation,covering all the fields from shoshonites to trachytes, withconsiderable enrichments in K2O up to the KS series. About5.5 ka, ‘‘La Fossa Cone’’ stage formed a central rhyoliticedifice in La Fossa Caldera (fifth cycle). Lavas and pyro-clasts consist of acid shoshonitic rhyolites, displaying strongevidence of mingling with mafic melts.[9] ‘‘Vulcanello’’ (sixth cycle) apparently formed as a

separate island, probably in 183 B.C.; its products aremainly leucite-tephritic lava flows, forming the platformaround the cones. A limited pyroclastic activity took placeduring the latest activity period. The last eruption of Vulcan-ello dates back to the 16th century, with the Valle delRoveto trachytic flow. The lavas from Vulcanello areshoshonites and latites (leucite-bearing tephrites [Keller,1980]) and, with the volcanics from La Fossa Cone, areamong the most alkaline in the whole archipelago.

3. Quartz Xenoliths

[10] A common character of most Aeolian lava flows andpyroclastics is the ubiquitous presence of crustal xenoliths.

These xenoliths are mostly composed of quartz aggregates,with minor metapelites, vesuviane-grossular-bearing skarns,and cordierite-sillimanite-bearing gneisses [Barker, 1987;Honnorez and Keller, 1968; Peccerillo and Wu, 1992]. AtVulcano the studied xenoliths consist mainly of quartzaggregates, although minor metapelites are also observed.These latter, however, are devoid of fluid inclusions.[11] Quartz xenoliths are cm-sized and show granular

textures (Figures 2a and 2b); contacts with the host lavasare sharp, but locally, they appear to disaggregate within thehost melt. They consist of quartz grains (>98% modal), withsubordinate refractory phases such as zircon, apatite andsphene. Rare K-feldspar and/or pyroxene and plagioclasegrains are at times observed. All xenolith samples containsignificant quantities of internal clear glass that are presentboth as intergranular films and silicate-melt inclusionswithin single quartz grains (Figure 2). Internal clear glassdiffers petrographically, from glass from the host lava on thebasis of the brown-yellowish color of the latter.[12] Depending on the microstructural character, it is

possible to distinguish two types of quartz grains: the firstone ‘‘Q1’’ is transparent, and it generally has a roundedborder that makes it resemble a relic phase (Figure 2b). Thesecond quartz type ‘‘Q2’’ is milky, from white to grey incolor, highly fractured, and with cracks filled with trails ofsecondary fluid inclusions (Figure 2a). The morphology ofQ2 quartz grains ranges from subhedral to anhedral, with astrong tendency to form subtextures of polygonal aggre-gates (i.e., recrystallization).[13] Clear glass occurs as intergranular veinlets up to

100–200 mm across, mostly along Q1 grain boundaries(Figure 2b), and as thin films (up to 50 mm) filling smallfractures forming short trails or clusters of silicate-meltinclusions in Q1 grains (Figure 2f). The veinlets are clearlyconnected to each other, suggesting that the melt may havemigrated along grain boundaries and microfractures. Over-all internal glass structures indicate rapid quenching, sug-gesting that veins represent an open system whichreequilibrated to very low-pressure conditions. Glass com-position obtained from silicate-melt inclusions resembles ahigh-K2O rhyolite (high silica (SiO2 > 73 wt%) andpotassium (K2O = 3–6.3 wt%) and low MgO (<1 wt%)and alumina (Al2O3 = 6–12 wt%) [Zanon, 2000] and issimilar to interstitial glass lining Q1 grains.[14] Quartz-rich xenoliths are extremely common in the

lavas and pyroclasts of the Aeolian arc, although quartzitesare rare in the outcropping portion of the Calabro-Peloritanibasement. Detailed geochemical studies of quartz-rich xen-oliths in the lavas of the island of Alicudi [Peccerillo andWu, 1992; Peccerillo et al., 1993] showed variable compo-sitions with respect to both major and trace elements, exceptfor Rb, Cs, and K, which were invariably present at very lowconcentration levels. Such compositions suggested thatquartz xenoliths were derived from gneisses and or otherquartz-rich metamorphic rocks that had undergone partialmelting with extraction of various amounts of acid liquid. AtVulcano, extensive melting evidence, coupled with thetextural character of quartz, lead to the conclusion that quartzxenoliths were not fragments of undisturbed metamorphicquartzites sampled by ascending magmas. On the contrary,their petrographic features lead to restitic rocks originatedby different degrees of partial melting of an original fertile

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 3

metamorphic rock such as a quartz-feldspatic gneiss and/ora mica-schist.

4. Sampling and Analytical Techniques

[15] Xenoliths were collected from lavas and pyroclasticunits of well-known chemistry and stratigraphy, correspond-ing to four different volcanic cycles, which are considered

to be representative for the overall petrologic evolution ofthe volcano (sample location in Figure 1 and Table 1):[16] The quartz xenoliths in Sponda Lena basaltic-andesite

lavas are representative of the initial phases of ‘‘PrimordialVulcano’’ activity (120 ± 0.5 ka), first cycle.[17] The xenoliths from the basaltic-shoshonite scoria and

ash flow outcropping at Spiaggia Lunga represent late stagesof the ‘‘Piano Caldera’’ activity (24 ± 0.5 ka), second cycle.

Figure 2. Photomicrographs of studied quartz xenoliths and fluid inclusions. (a) Microphotograph of aquartz xenolith in plane polarized light showing the presence of Q1 rounded grains and Q2 polygonal grains(sample Vu.Le 10a). (b) Q1 grain subtextures in a quartz xenolith from Vulcanello (sample Vu.Vl 2). RelicQ1 grains are surrounded by rims of rhyolitic glass (G). Plane polarized light. (c) Cluster of texturally earlytype I CO2 fluid inclusions in a Q1 grain from Lentia quartz xenolith (sample Vu.Le 10a). Most inclusionsare diphase (L + V; arrows) at room temperature. (d) Short intragranular trail of early type I inclusions in Q1

grain from Vulcanello quartz xenolith (sample Eo.98.22). Note the decrepitation features (arrow)consisting in short fractures extending from single inclusions. (e) Texturally early, partly decrepitedisolated type I fluid inclusion (arrow) in Q1 grain from Vulcanello quartz xenolith (sample Vu.Vl.2). (f)Typical association of Early type I CO2 inclusions with silicate melt inclusions of rhyolitic composition in aQ1 grain from Sponda Lena quartz xenolith (sample Eo.98.23). Inclusions consist of glass, CO2 and mixedCO2 + glass inclusions, and indicate trapping of a CO2 oversaturated silicate melt. Note the distribution ofinclusions close to grain boundaries, which are lined and separated by a film rhyolitic glass. The inclusionsprobably resulted from infiltration of rhyolitic melt in the relic quartz crystals. (g) Late trails of type II CO2

inclusions distributed along two main directions in a Q2 grain from Vulcanello quartz xenolith (sampleEo.98.22). (h) Texturally, late type II inclusions within a trail in a Q2 grain from Sponda Lena quartzxenolith (sample Vu.Ge 1). Most inclusions are vapor dominated at room temperature. (i) Particular of atrail of late type II CO2 inclusions in a Q2 grain from Sponda Lena quartz xenolith (sample Vu.Ge 2). Asshown here and in Figure 2h almost all inclusions do not give textural evidence for partial decrepitation.

ECV 7 - 4 ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND

[18] The xenoliths in rhyolites and dacites outcroppingalong the western slope of Monte Lentia and at MastroMinico are representative of ‘‘Lentia-Mastro Minico’’ stage(24–15 ± 0.5 ka), third cylce.[19] The xenoliths in shoshonite lavas from the north-

eastern cliffs of Vulcanello are representative of the whole‘‘Vulcanello’’ activity, dated 183 B.C. to 1550 A.D., sixthcycle.[20] The xenoliths from La Fossa Cone and La Fossa

Caldera (fourth and fifth cycles) have not been sampled,since fluid inclusion data are available from Clocchiatti etal. [1994] and from Gioncada et al. [1998].[21] Although more than 50 samples of crustal xenoliths

were collected, only 15 contained fluid inclusions, becauseof various degrees of remelting of some xenoliths or ofweathering of the host rocks. Double polished, 100–200 mmthick wafers were prepared for microthermometric andRaman microspectroscopic investigation. Microthermome-try was performed at the Centro per lo Studio del Quater-nario e dell’Evoluzione Ambientale (CNR) in Rome with aChaixmeca

1

[Poty et al., 1976] and a Linkam1

THM 600heating/freezing stages [Werre et al., 1979]. Calibration ofthe two stages was carried out by means of SYNFLINC

1

standard synthetic fluid inclusions, checking the temper-ature at the CO2 and H2O triple points (�56.6�C and 0.1�Crespectively), and the CO2 critical homogenisation temper-ature at 31.1�C. Instrumental error at the reference standardpoint was ± 0.5�C and ± 0.1�C, respectively. For all the runsthe heating rate was in the range 0.2–0.5�C/min. Isochoresfor fluid inclusions were calculated using the Mac Flincor

1

software package [Brown, 1989], using the equation of stateprovided by Holloway [1981] for CO2. Density of CO2 wasderived from Angus et al. [1976].[22] Raman analyses were performed at the Universita di

Siena with a confocal Labram multichannel spectrometer ofthe Jobin-Yvon LTD. The excitation line at 514.5 nm wasproduced by a Ar+ laser. Raman intensity was collected witha Peltier-cooled CCD detector. The beam was focused to aspot size of about 1–2 mm using an Olympus 100 � lens.The scattered light was analyzed using a Notch holographicfilter with a spectral resolution of 1.5 cm�1 and a grating of1800 grooves/mm.

5. Fluid Inclusions

[23] In all investigated xenoliths, the fluid inside inclu-sions is pure carbon dioxide, melting instantaneously (Tm)within a narrow temperature interval between �56.8 and�56.2 �C ± 0.1, with most data at �56.6�C. Raman micro-spectrometric measurements confirm the absence of othergaseous species besides CO2, at least above the detectionlimit of the instrument (0.1 mole% for CO and SO2; < 0.1mole% for H2S, CH4, and N2). Because of the identificationof aqueous fluid inclusions in quartz xenoliths from La FossaCone rhyolites [Clocchiatti et al., 1994], detailed Raman andmicrothermometric investigations were performed in order tocheck the presence of possible films of H2O and/or clathratesinside the inclusions. So far, no evidence has been found.

5.1. Petrography

[24] Carbonic fluid inclusions show striking texturalsimilarities in all studied xenoliths. Based on their texturalT

able

1.SummaryofResultsAchieved

ThroughFluid

InclusionsMicrothermometricInvestigationforCollectedXenolithsa

Sam

ple

Location

Age

HostRock

ThL,�C

ThV,�C

Trapping

T,�C

TypeId,

g/cm

3P,GPa

h,km

TypeIId,

g/cm

3P,GPa

h,km

Eo98.23(113)

SpondaLena

120ka

HKCA

Basalticandesite

22.5–31.0

–1090

0.52–0.75

0.20–0.38

7.7–14.2

––

VuGe1

(114)

SpondaLena

120ka

HKCA

Basalticandesite

–18.3–30.9

1090

––

0.18–0.40

0.05–0.14

2.1–5.4

VuGe2

(25)

SpondaLena

120ka

HKCA

Basalticandesite

24.5–30.1

21.2–30.5

1090

0.72–0.59

0.25–0.35

9.4–13.2

0.20–0.37

0.06–0.12

2.4–4.8

VuGe9

(40)

SpondaLena

120ka

HKCA

Basalticandesite

20.8–23.7

19.5–30.0

1090

0.73–0.77

0.36–0.40

13.6�15.0

0.19–0.34

0.06–0.11

2.2–4.4

VuGe14(14)

SpondaLena

120ka

HKCA

Basalticandesite

18.1–26.6

29.4–30.5

1090

0.69–0.79

0.32–0.43

12.1–16.2

0.32–0.37

0.10–0.12

4.1–4.8

VuSrSp1(147)

Spiaggia

Lunga

24.0

±5.0

ka

SHO

basalt

11.6–30.2

20.8–30.7

1090

0.59–0.85

0.24–0.50

9.3–18.9

0.20–0.38

0.06–0.13

2.4–5.0

VuSrSp4(26)

Spiaggia

Lunga

24.0

±5.0

ka

SHO

basalt

–24.6–30.8

1090

––

0.24–0.39

0.07–0.13

2.9–5.1

VuSrSp5(37)

Spiaggia

Lunga

24.0

±5.0

ka

SHO

basalt

27.5–28.7

27.1–31.1

1090

0.64–0.67

0.28–0.30

10.7–11.5

0.27–0.42

0.08–0.14

3.3–5.6

VuSrSp8(2)

Spiaggia

Lunga

24.0

±5.0

ka

SHO

basalt

25.8

27.7–27.8

1090

0.70

0.33

12.6

0.28–0.29

0.09

3.5

VuLe10(126)

LentiaM.Minico

24–15ka

HKCA/Dacite

26.0–30.5

9.30–30.2

980

0.57–0.70

0.22–0.31

8.2–11.9

0.13–0.35

0.03–0.11

1.4–4.1

VuLe14(26)

LentiaM.Minico

24–15ka

HKCA/Dacite

23.3–30.7

23.5–30.0

980

0.55–0.74

0.20–0.33

7.7–12.6

0.23–0.33

0.06–0.10

2.5–4.0

Eo98.21(53)

Vulcanello

183B.C.–

1550

Shoshonite

6.30–30.4

17.0–30.6

1090

0.58–0.89

0.24–0.56

9.0–21.0

0.17–0.38

0.05–0.13

2.0–4.9

Eo98.22(46)

Vulcanello

183B.C.–

1550

Shoshonite

13.9–30.8

17.5–31.0

1090

0.54–0.83

0.22–0.52

8.2–19.4

0.18–0.38

0.05–0.14

2.1–5.6

VuVl2

(75)

Vulcanello

183B.C.–

1550

Shoshonite

8.90–29.3

25.6–30.1

1090

0.62–0.87

0.27–0.53

10.2–19.9

0.25�0.35

0.08–0.11

3.1–4.4

VuVlx

(5)

Vulcanello

183B.C.–

1550

Shoshonite

20.8–22.7

–1090

0.74–0.77

0.37–0.40

14.1–15.0

––

–aValues

inparentheses

arethenumber

ofmeasurementsperform

ed.Ageofhostrock

isreported

byDeAstiset

al.[1997].Stratigraphic

datafrom

Falsaperla

etal.[1984]wereutilizedto

calculate

crustal

depths

correspondingto

trappingpressures.

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 5

characters, it is possible to distinguish two main generationsof fluid inclusions:5.1.1. Early CO2 Inclusions (Type I)[25] Texturally early type I carbonic inclusions occur

isolated or in small irregularly oriented clusters and arepresent only within relict Q1 quartz grains (Figure 2c). Afew inclusions are distributed along short intragranulartrails, both in the internal part of the grains and at grainboundaries (Figure 2d). At room temperature, type I inclu-sions are single-phase (L) or may contain a vapor bubble(L + V) with a df � 0.5 (df = Vvapor/(Vliquid + Vvapor)). Theyoften show textures that are characteristic for partial decrep-itation, such as haloes of tiny fluid inclusions (diameter <0.1 mm) surrounding the inclusion cavity, and/or shortcracks radiating from the microcavity (Figure 2e) [Andersenet al., 1984]. The size of the inclusions in the range 3–10 mmand their shape is generally regular, with subordinate neg-ative-crystal forms. No solid phases have been observedwithin these cavities. Type I fluid inclusions, occurring inclusters, may be associated with some of the rhyoliticsilicate-melt inclusions (Figure 2f), and mixed inclusionscontaining both CO2 and glass inclusions are observed.Inclusions containing both glass and CO2 show extremelyvariable fluid/glass ratios. This is taken as evidence that, atthe time of trapping, type I CO2 inclusions coexisted with animmiscible silicate melt of rhyolitic composition.5.1.2. Late CO2 Inclusions (Type II)[26] Texturally late CO2 inclusions of variable shape

and size (3–30 mm) occur mainly in Q2 quartz grains.They always line completely healed fractures, reaching orcrosscutting grain boundaries (Figure 2g). At room tem-peratures, type II inclusions are single-phase (V) or morecommonly, two-phase vapor-dominated (V + L), with adf > 0.7 (Figure 2h). They are never found associated withsilicate-melt inclusions. Most late type II inclusions appearto be undisturbed, showing little evidence of partial decrep-itation (Figures 2h and 2i). In some Q1 grains, late trail-bound type II inclusions cut the clusters of early type Iinclusions.[27] Both type I and type II CO2 fluid inclusions are

commonly observed within the same xenoliths, unevenlydistributed in the two different generations of quartz. Insome xenoliths, however, early type I inclusions dominate,whereas in others, only late type II inclusions are present.

5.2. Homogenisation Temperatures andDensity Calculations

[28] Fluid inclusions in quartz xenoliths from differentvolcanic cycles always show trapping of both high-densityand low-density carbonic fluids, corresponding to type I andtype II pure CO2 inclusions, respectively. Although thisbimodal density distribution is always observed, singlehomogenisation data sets vary within the different volcaniccycles (Figure 3):[29] Early type I inclusions homogenize to liquid phase

(ThL; L + V ! L) in a wide range of temperatures, whichare different for the various volcanic stages: 18.1 and31�C (d = 0.79–0.52 g/cm3; Figure 3) for Sponda Lenabasaltic andesites (first cycle), 11.6 and 30.2�C (d = 0.85–0.59 g/cm3) for The Spiaggia Lunga shoshonitic basalts(second cycle), 23.3 and 30.7�C (d = 0.74–0.55 g/cm3) forLentia-Mastro Minico dacites and rhyolites (third cycle), and

6.3 and 30.8�C (d = 0.84–0.54 g/cm3) from Vulcanello KSleucite-tephrites (sixth cycle).[30] Late type II vapor-rich inclusions show homogeni-

zation temperatures (ThV; L + V ! V) in a more restrictedrange (Figure 3): 18.3 and 30.9�C, with a maximum at 29�C(d = 0.40–0.18 g/cm3; Figure 3) for Sponda Lena basalticandesites (first cycle), 20.8–31.1�C (d = 0.42–0.2 g/cm3)for The Spiaggia Lunga shoshonitic basalts (second cycle),9.3 and 30.2�C (d = 0.35–0.13 g/cm3) for Lentia-MastroMinico dacites and rhyolites (third cycle), and 31 and17.0�C (d = 0.38–0.17 g/cm3) from Vulcanello KS leu-cite-tephrites (sixth cycle).[31] From density distribution data, it arises that, while

most type II fluid inclusions have apparently retained theiroriginal densities, type I inclusions often show texturalevidence for partial decrepitation, as is also indicated bytheir extremely scattered ThL distribution (Figure 3) [Vitykand Bodnar, 1998]. In xenoliths, decrepitation processes ofinclusions may occur in response to the changing pressureduring magma ascent, reducing the density of the fluidinclusion fluid [Andersen and Neumann, 2001]. Alterna-tively, density resetting of fluid inclusions may take placeduring periods of magma stagnation in the crust, reequili-brating the fluid pressure to a new value corresponding tothe confining pressure. Since xenoliths may trap new fluidsand magma as inclusions while ponding in magma cham-bers, the densities of partly decrepited inclusions should re-equilibrate to those of newly trapped fluids [Frezzotti et al.,1991].[32] Therefore only observed maximum densities in type

I fluid inclusions may be considered to have a petrogeneticsignificance (Figure 3). Since the density values ofdecrepited type I inclusions never overlap those of latestage type II fluid inclusions (compare in Figure 3), weexclude that density resetting of early type I fluid inclusionscoincided with levels of trapping of late type II inclusions.Decrepitation of type I fluid inclusions most likely occurredduring xenoliths ascent, due to internal fluid overpressure.

6. Discussion

6.1. Temperatures of Fluid Inclusion Formation

[33] In order to achieve thermobarometric information onthe preeruptive magma evolution, it is necessary to calculatethe trapping temperature of fluids contained within inclu-sions in quartz xenoliths. Trapping temperatures have beenobtained through two independent methods, namely, meltinclusion microthermometry [Gioncada et al., 1998; Zanon,2000; Frezzotti et al., 2003] and mineral geothermometry.Temperatures deduced from the homogenisation of silicate-melt inclusions refer to the conditions at the time of entrap-ment of the melts, which may have occurred at variousdepths within the volcanic system, while the latter methodprovides information on the temperatures of the magmasystem at shallow levels, before the eruption.[34] Zanon [2000] and Frezzotti et al. [2003] report

homogenisation temperatures of silicate-melts present asinclusions in quartz xenoliths in a restricted temperaturerange, which varies according to host lavas compositions.Measured values are between 1065�C and 1105�C (±10�C)for the xenolith present in basic lavas, and around 980�C(±10�C) for the xenoliths contained in Lentia-Mastro Min-

ECV 7 - 6 ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND

ico rhyolitic lavas. Homogenisation temperatures of meltinclusions present in phenocrysts of basic lavas eruptedduring the same magmatic cycles fall in similar temperatureranges [Gioncada et al., 1998]. Obtained temperaturesdecrease progressively from 1250 to 1040�C for meltinclusions in olivine, and from 1210 to 1040�C for meltinclusions in clinopyroxene.[35] Trapping temperatures were further estimated via the

application of several geothermometric algorithm toselected host lava compositions. Chemical compositionsof minerals and rocks were chosen among the volcaniccycles reported in the work of De Astis et al. [1997]. Thegeothermometer calibrated by Sisson and Grove [1993] forisland arc liquids coexisting with clinopyroxene, olivine andplagioclase was applied to whole rock compositions both intransitional and alkaline rocks with SiO2 content <55 wt%,as higher silica compositions are out of the range of

experimental calibration. Resulting temperatures indicate avalue of 1132 and 1124�C (10% trimmed mean) for basalticmagmas of the first and second cycles, and 1105�C forVulcanello lavas. Reported error is ± 7�C. A secondalgorithm by Sisson and Grove [1993], considering theequilibrium olivine-liquid, was applied. The chemical com-position restrictions resulting from the original calibrationdataset are: SiO2 � 55%, TiO2 < 3%, and FeO < 13%.Resulting temperature data indicate mean values of 1150�Cfor the first cycle olivine and 1065–1174�C for olivines ofthe second cycle. Reported error is ± 7–11�C.[36] The empirical geothermometer calibrated by Albar-

ede [1992] for MOR and island arc basaltic liquids coex-isting with clinopyroxene olivine and plagioclase wasapplied to the same database. Since this algorithm has nochemical restrictions, it was applied also to evolved lavacompositions. Mean temperatures trimmed by 10% of

Figure 3. Histograms of homogenisation temperatures for fluid inclusions present in xenoliths of thefour studied volcanic cycles, ordered from the oldest to the youngest. Histograms to the left side reporthomogenisation temperatures to the liquid phase (ThL) for early type I fluid inclusions. Histograms to theright side indicate homogenisation temperatures to the vapour phase (ThV) for late type II fluidinclusions. Corresponding density values are reported for comparison on the top frame.

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 7

extreme values indicate similar intervals: between 1129�Cand 1138�C for the first and the second basaltic cycles,990�C for the third cycle rhyolites and 1080�C for Vulcan-ello, sixth cycle lavas. The reported error is ± 40�C.[37] In summary, all data resulting from the calculation of

equilibrium temperatures of host lavas and xenoliths, bothfrom mineral-melt equilibrium and from melt-inclusionmicrothermometry, are on average consistent, althoughvalues obtained for quartz xenoliths which are presentwithin basaltic lavas can be 100 to 50�C lower than temper-atures measured in the host lavas. Such a discrepancypossibly indicates that complete reequilibration of thetemperatures between entrained xenoliths and host basalticlavas was not attained, due to a fast rise of magma from itssource-zone. For this reason, the averaged value of 1090�C,resulting from the homogenisation temperature of silicatemelt inclusions in xenoliths present in basic lavas isconsidered representative for the trapping temperature inevery volcanic cycle, with the exception of the Lentia-Mastro Minico rhyolitic compositions, where trapping tem-peratures of 980�C result both in quartz xenoliths and inrhyolitic host lavas. It is worth noting that the estimatederror of ± 10�C does not affect the resulting pressure values,which vary accordingly only by ±0.002 GPa.

6.2. Pressures of CO2 Fluids Entrapment

[38] The results of the microthermometric measurements(i.e., fluid composition and density) permit us to calculatetrapping pressures for the CO2 fluids inside the xenoliths atthe estimated trapping temperatures for every volcanic cyclebeneath the Vulcano Island. The knowledge of the strati-graphic sequence beneath the Aeolian archipelago furtherallows us to calculate for each pressure value the corre-sponding crustal depths as summarized in Table 1.[39] It has been shown above that type I pure CO2

inclusions are found only in restitic Q1 quartz grains andare often associated with silicate-melt inclusions, whosecomposition is very similar to interstitial glasses present inxenoliths. Formation of type I inclusions appears in factrelated to melting processes of crustal rocks and formationof host quartzites. This evidence indicates that type Icarbonic inclusions are strictly related to the pre-magmatichistory of the crustal country rocks, providing informationon the depth of origin of the xenoliths.[40] Figure 4 schematically reports pressures obtained by

fluid inclusions. Undisturbed early type I CO2-inclusionsfrom Primordial Vulcano and Sponda Lena caldera givepressure estimates of 0.5–0.43 GPa at 1090�C. Thesepressures correspond to depths of 18–16 km and aresuggestive for middle-to lower-crustal depths of origin formost quartz xenoliths, since the Moho beneath Vulcano hasbeen estimated at 21–25 km [Falsaperla et al., 1984]. Earlytype I inclusions in quartz xenoliths from the Lentia DomeComplex have densities that correspond to shallower con-ditions (Figure 4): 0.33–0.2 GPa (13–8 km) for magmatemperatures of 980�C (Figure 4). This may indicate eitherthat quartz xenoliths originated at shallower depths, or thatall measured CO2 densities at Lentia Dome Complex arereset to lower density values (Figure 4). For the recentactivity at Vulcanello, fluid inclusions record the deepestentrapment zone of all volcanic activity, located in the lowercrust at minimum depths of 20 km (0.56 GPa).

[41] In quartz xenoliths, a second episode of pure CO2

trapping without melt occurred at a later stage as clearlyshown by the trail-bound distribution of low-density type IIfluid inclusions in both Q1 and Q2 quartz grains. Lowdensities of type II inclusions systematically fall within arestricted interval corresponding to pressures comprisedbetween 0.14 and 0.07 GPa (5.6–3 km; Figure 4) at theinferred temperatures. In addition, there is a remarkableoverlap between the pressure ranges for the different xen-oliths from all studied volcanic cycles (Figure 4).[42] It has been proposed by many authors for the

Aeolian Islands and elsewhere, that low-density peaks offluid inclusions in histograms like these in Figure 3 mayreflect a period of residence in shallow magma chambers[Vaggelli et al., 1993; Clocchiatti et al., 1994; Lima, 1996;Hansteen et al., 1998]. Since low-density peaks are present

Figure 4. Calculated pressure ranges for trapped fluids(dark grey boxes) versus age of volcanic activity, asindicated by type I and II fluid inclusions in studiedlocalities. For each volcanic cycle, two distinct fluid-trapping episodes are present: the first at high pressure,corresponding to early type I fluid inclusion data, and thelatter, at shallower pressures, corresponding to late type IIfluid inclusion data (see text). Isothermal section at 1080�Cfor all volcanic cycles, with exception of M. Minico(980�C). Complementary data for La Fossa Cone activitywere taken from Clocchiatti et al. [1994].

ECV 7 - 8 ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND

in all investigated xenoliths, a shallow magma chamberlocated in the upper crust beneath Vulcano can be inferred,if the assumption is accepted that trapping of fluid inclu-sions are related to ponding sites for magmas during theirway to the surface (Figure 4).[43] The data reported by Clocchiatti et al. [1994] on CO2

inclusions present in quartz crustal xenoliths from La FossaCone (fourth cycle, 6 ka-Recent), provided similar evidenceof trapping of two generations of CO2-rich fluid inclusionscorresponding to high-density and low-density fluids. Thefirst group of inclusions had densities of 0.7 to 0.5 g/cm3

corresponding to pressures of 0.35 to 0.2 GPa at 1080�C(homogenisation temperature from melt inclusions) similarto the values currently obtained for the Lentia-MastroMinico activity (Figure 4). The second group of inclusionshad density of 0.025 g/cm3 that was interpreted to suggestthe presence of a magma chamber at about 1600 m depth (P= 0.015 GPa). As shown in Figure 4, this depth value ismuch shallower than that inferred from low-density type IIinclusions in the present study. It is noteworthy that at LaFossa Cone some of these low-density fluid inclusionscontain water along with the CO2 and have been suggestedto reflect the interaction of the shallow magma reservoirwith a superficial hydrothermal system [Clocchiatti et al.,1994].

6.3. Volcanological Implications

6.3.1. Presence of a Deep Magma AccumulationZone in the Crust Beneath Vulcano[44] It was shown above that the presence of early type I

CO2 inclusions associated with silicate-melt inclusions inquartz xenoliths trapped from 21 to 12 km provides infor-mation on the depths at which quartz xenoliths formed.Since similar quartz xenoliths are extremely common atVulcano and in the other Aeolian Islands, this suggests thata level of fertile metamorphic rocks should be present inmid- to lower-crustal conditions.[45] We thus propose a model where partial melting of

crustal rocks occurred in the middle-lower-crust formingrhyolitic melt + CO2 fluids (in part trapped as type I fluidand melt inclusions) and leaving restitic quartz agglomer-ates, which where sampled by the ascending lavas. Timingrelationships between generation of rhyolitic anatectic-meltsand trapping of host xenoliths in the ascending magma aredifficult to constrain. This overall lower crustal evolution,however, may involve two possible scenarios: (1) Meltingof the crust occurred earlier than magma intrusion, and thequartz-rich xenoliths represent the residual rocks, whichwere mechanically sampled by rising magmas. (2) Magmasascending from the upper mantle provided the necessaryheat for melting of crustal rocks.[46] These two possible figures bring along distinct

petrological consequences. In the first case, melting ofcrustal rocks and entrapment of xenoliths in host lavas aretemporally distinct events and do not necessarily imply aninteractions between newly generated crustal melts and hostmagma. In the latter, chemical and physical interactionsbetween melting crustal rocks and deep magmas may haveoccurred during crustal melting processes.[47] Texturally, early type I CO2 fluids are in equilibrium

with rhyolitic silicate melt present within melt inclusions atthe estimated temperatures of 980 and 1080�C, correspond-

ing to those of the host lavas. If melting processes in thecrust had occurred before incorporation of xenoliths in theascending magmas, then these events should have occurredat lower temperatures (i.e. close to the local geotherm), and,most of all, liquidus temperatures should not vary accordingto host-lavas compositions.[48] Petrological studies [De Astis et al., 1997, 2000]

clearly indicate that assimilation, contamination and frac-tionation processes occur in most mafic lavas from VulcanoIsland, especially the Primordial Vulcano and Piano cal-dera. These pieces of evidence suggest that crust meltingmay have been induced by a rise of temperature duringunderplating and intrusion of the continental crust bymantle-derived basaltic magmas [Huppert and Sparks,1988; Bergantz, 1989]. Heat transfer and partial meltingof the crust might have been enhanced by release of CO2

from the emplacing basaltic melts [Touret, 1992].[49] The massive presence of CO2 fluids associated with

rhyolitic melts in the xenoliths additionally support the ideathat part of the CO2 is of external origin (i.e., fluid releasedby a basaltic magma); the amount of CO2 released fromdehydration melting of metapelites is in fact little andcannot justify the high amount of carbonic inclusionsobserved in the study rocks, especially in the intermediateto the acid host lavas. Extensive carbon and oxygen isotopicinvestigation on CO2 inclusions in quartz xenoliths arenecessary to shed light on this issue.[50] Texturally, early type I CO2 inclusions may have

been trapped during melting of host rocks in a deep crustalpartial melting zone which has been active during theoverall evolution of Vulcano Island. With time, such apartial-melting zone may have acted as a density barrier,becoming a storage zone for the ascending mantle derivedmelts. Here magmas underwent fractionation, assimilationand homogenisation (MASH [Hildreth and Moorbath,1988] or RACF [Albarede, 1992]) giving a range of isotopi-cally different mafic melts, such as those forming Primor-dial Vulcano [De Astis et al., 1997].6.3.2. Magma Rise and Residence inMagma Chambers[51] The proposed magma plumbing model is illustrated

in Figure 5. Overall, fluid inclusion studies reveal theoccurrence of two major accumulation zones for the risingmagma that have been active during the entire history ofVulcano.[52] The Primordial Vulcano mafic HKCA magmas (first

cycle; 120–100 ka) evolved at high-pressures with a deepstorage level located in the mid to lower crust, at aminimum depth of 16 km (Figure 5a). In this reservoirprocesses of assimilation and fractional crystallization ofmantle derived magmas probably occurred, giving a rangeof mafic liquids that display variable isotopic signatures. Ashallow level magma reservoir was also present beneathVulcano during this phase, as suggested by the occurrenceof low-density fluid inclusions in quartz xenoliths, by thelarge caldera collapse, and by the low-pressure crystalliza-tion of clinopyroxene suggested by Nazzareni et al. [2001].During the successive Piano Caldera activity (second cycle)between 90 and 20 ka, the magma storage system beneaththe Island does not change significantly (Figure 5b); this isin keeping with the similar composition of Piano Calderaand Primordial Vulcano suites [De Astis et al., 1997].

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 9

[53] During the formation of the Lentia rhyolitic–daciticdome complex (24–15 ka), xenoliths apparently originatedat shallower depths (13–8 km; Figure 5c). Although wecannot exclude that these values may represent reset inclu-sion densities, the presence of high-silica compositions(Lentia rhyolites) would be in accordance with a period ofhigher-level magma ponding, with consequent loweramounts of mafic magma input and higher rates of frac-tional crystallization, explaining, this time, dependentchange of the dominant magmatic expression at the surface

from basic-intermediate to intermediate acidic. Petrologicaldata [De Astis et al., 1997] indicate that mixing betweenmafic and felsic magmas occurred at this stage in distinctreservoirs located at different depths in the crust. Thissuggests that the plumbing system may have consisted ofa deep reservoir of mafic magma that fed shallower reser-voirs of acid melts located in the upper crust (Figure 5c).[54] The recent activity from Vulcanello (183 B.C.) bears

evidence for the deepest fluid inclusion trapping depths andsuggests a reservoir for the island of Vulcano (Figure 5d) at

Figure 5. Schematic geological cross-section model of the magma plumbing system beneath theVulcano Island through time as inferred from fluid inclusion study and petrological evidences [De Astis etal., 1997]. Lithological boundaries are from Falsaperla et al. [1984]. (a. The Primordial Vulcano maficHKCA magmas (120–100 ka) evolved in a deep storage level located in the mid to lower crust, at aminimum depth of 16 km (solid white), by mixing plus assimilation and fractional crystallization(MAFC). Conditions of neutral buoyancy for ascending magmas are also met at about 5–3 km andindicate the presence of a shallower magma chamber (dark grey filling), which appears to be located atthe boundary between the intrusive complex and the underlying metamorphic basement. (b) Thefollowing activity between 90 and 20 ka (Piano Caldera) does not change significantly with respect tothe previous stage, which produced compositionally similar rocks as Primordial Vulcano. (c) During theformation of the Lentia–Mastro Minico dome complex (24–5 ka), magmas evolved at shallower depths(white magma chamber; 13–8 km), with higher rates of fractional crystallization and minor mixing(FCM), reaching rhyolitic compositions. On the contrary, the location of the shallow magma chamber(solid grey) does not vary, being estimated at 4–3 km. D) Vulcanello (183 B.C.) corresponds to thedeepest magma accumulation zone at 21–18 km (solid white), close the Moho, tapping a deep sourcezone of mafic potassic liquids. The high-level magma accumulation is located at 5.5–3 km indicating thatthis zone does not vary through time. At La Fossa Cone, Clocchiatti et al. [1994] report a very shallowmagma chamber at 1.6 km depth in which magma mixing and interaction with seawater occur. Such avery shallow reservoir is absent beneath Vulcanello, where the shallow accumulation level is at about5 km. For further explanations see text.

ECV 7 - 10 ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND

a minimum depth of 21 km. Fluid inclusion evidence fordepths close to the Moho (20–25 km) indicates thatresidence of magmas at midcrustal levels of young Lentiaand La Fossa Cone cycles is absent during Vulcanelloactivity. Since La Fossa Cone is situated at the center ofLa Fossa Caldera, while Vulcanello is located at a marginalposition with respect to La Fossa Caldera, we cannotexclude that a deep magma reservoir also fed La FossaCone activity. The absence of any evidence from fluidinclusion data might be because of resetting of fluidinclusions, or most likely because a rhyolitic magma cham-ber might act as a density barrier for deep rising magmas,preventing the rise of deep xenoliths.[55] One of the most important petrological problems at

Vulcano is to understand whether the younger products atVulcanello can be related to older (HKCA-SHO) magma-tism via processes such as fractional crystallization, mixing,and wall rock assimilation, or whether HK young rocksrepresent distinct magma batches generated from distinctmantle sources. Present results support the hypothesis thatthe transition from HKCA to SHO and KS volcanism atVulcano Island is not related to low-pressure evolution ofmantle magma.[56] Beneath the Vulcano Island, conditions of neutral

buoyancy for ascending magmas are also met at about0.15–0.07 GPa (5–3 km; Figure 5) and indicate thepresence of a permanent shallower accumulation level. Sucha high-level magma accumulation appears to be confined atthe same depth indicating that this zone does not varythrough time (Figures 5a–5d). Such a level may correspondto a boundary between crustal layers with different mechan-ical characteristics, as the contact between the intrusivecomplex and the underlying metamorphic basement.[57] The absence of high-silica melt inclusions associated

with type II fluid inclusions testifies that partial meltingprocesses did not affect quartz xenoliths at these lowpressures. This piece of evidence, along with the presenceof well-preserved fluid inclusions in a narrow density range,strongly suggests that this shallow accumulation levelcorresponds to a magma chamber in which xenoliths restedbefore eruption. Observations from fluid inclusions furthersuggest that the latter was a short-term reservoir. Prolongedresidence would have enhanced the probability of decrep-itation for high-density type I inclusions, while resetting ofdensity at these pressure is absent in investigated type Ifluids.[58] At La Fossa Cone, Clocchiatti et al. [1994] found

texturally late inclusions (mixed H2O + CO2 fluids) corre-sponding to pressure of about 0.015 GPa, which is consid-erably shallower than those found in late type II inclusionsin our xenoliths. Clocchiatti et al. [1994], in fact, indicatethat a very shallow magma chamber is located at about 1600m depth beneath the active crater of La Fossa (Figure 5d).This shallow depth evolution constrains models of theplumbing system that fed the recent activity.[59] As discussed by the De Astis et al. [1997], Vulcan-

ello mainly erupted KS mafic lavas, whereas La Fossa wasdominated by felsic magmas which contain mafic inclu-sions and xenocrysts, revealing mixing-mingling withmafic melts. These data, coupled with present study,strongly suggest that two independent shallow reservoirsfeed these two volcanic systems. Beneath La Fossa, the

plumbing system probably consists of small dykes andmagma pockets cutting through intrusive rocks at 1.6 kmdepth, forming a storage network in which complexevolutionary processes occur, including magma mixingand interaction with seawater. Such a very shallow reser-voir is absent beneath Vulcanello where the shallowaccumulation level is at about 5 km (Figure 5d). We haveno data that can constrain the provenance of mafic magmaresponsible for mixing-mingling in the products of LaFossa. Further investigations on mafic inclusions andxenocrysts are necessary to understand whether it camefrom the same intermediate and deep chambers that fedVulcanello volcanism.

7. Concluding Remarks

[60] The magma feeding system beneath Vulcano Islandis suggested to be characterized by multiple magma cham-bers that persisted throughout the last 120 kyr. Fluidinclusion data suggest that two spatially well-defined levelsof magma accumulation are present beneath Vulcano, one inthe upper crust, and in the other lowermost crust, near theboundary with upper mantle. Ponding of mafic magmas inthe deep reservoirs releases heat and fluids into the crust andmay have given rise to partial melting processes in the hostrocks.[61] The location of the deep reservoirs apparently

migrated toward slightly shallower level at the transitionfrom the Primordial Vulcano-Piano Caldera to Lentia domecomplex, which represent the two main systems that madeup the island. This transition was accompanied by a strongmodification in the degree of evolution of erupted magmas;basaltic and basaltic-andesitic magmas were erupted duringthe Primordial Vulcano and Piano Caldera activity (SpondaLena and Spiaggia Lunga), whereas rhyolites and trachytescharacterized the activity at Lentia. The young alkalinelavas of Vulcanello are fed by the deepest accumulationreservoir of the whole complex, at about 21–18 km.[62] A second shallower accumulation level is also

observed in all studied volcanic stages (Primordial Vulcano,Piano Caldera, Lentia dome complex and Vulcanello) andcorresponds to superficial magma accumulation zones.These are always confined at the same depth of about5 km, suggesting the existence of a boundary betweencrustal layers with different mechanical characteristics thatconstitutes a physical discontinuity, which favored magmastorage. Data on fluid inclusions in quartz xenoliths fromthe active La Fossa Cone, revealed an additional (?) veryshallow magma chamber at a depth of about 1600 m[Clocchiatti et al., 1994]. This may indicate a distinct veryshallow accumulation zone beneath La Fossa, which isabsent at Vulcanello.[63] The proposed model suggests that this shallow level

chamber hosts acidic magma, which undergoes mixing withmafic melt that could rise from deep reservoirs. This speaksfor two independent magma-feeding systems at the shal-lower level, but does not exclude that mafic magma at LaFossa could rise from the same reservoirs as at Vulcanello.[64] The shallow magma chambers at Vulcano were fed

by deep-sited reservoirs and were the site of evolutionaryprocesses such as fractional crystallization and mixing. Onthe other hand, crustal assimilation was dominant in the

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 11

deep reservoirs. Here, partial melting of wall rocks gener-ated acid liquids which are still preserved as rhyolitic glassinclusions in quartz-rich xenoliths. Interaction betweenthese crustal anatectic melts and mantle-derived magmamay have generated the observed considerable isotopicvariations observed especially at Primordial Vulcano andPiano caldera [De Astis et al., 1997; Del Moro et al., 1998].[65] Fluid inclusion studies also suggest that the residing

time of magmas in the shallow reservoir was very short, sincethere is little evidence of resetting of high-density fluidinclusions in quartz-rich xenoliths. This suggests that magmaupraise from deep reservoirs immediately preceded and mayactually have triggered volcanic eruption at Vulcano. Thishas important bearings onmonitoring the volcanic activity, inas much as recording by geophysical methods of deepmagma movements may represent an important hint ofsignificant physical modification of the shallow reservoir.

[66] Acknowledgments. The authors are grateful to G. Cavarretta forhosting V.Z. and for providing analytical facilities at the Centro per loStudio del Quaternario e dell’Evoluzione Ambientale - C.N.R. Thanks aredue to C. Ghezzo and E.A.J. Burke for helpful discussions throughout thestudy. Constructive reviews by B. De Vivo, T. Hansteen, E.R. Neumann,J.L.R. Touret, and the Associate Editor, L. Mastin, have greatly improvedthis manuscript. Researches on Italian recent and active volcanism arefinancially supported by C.N.R. - G.N.V. (Italian Organisation for ScientificResearch and National Volcanology Group), and by local projects from theUniversities of Perugia (A.P.) and Siena (M.L.F.). Raman microprobefacilities were provided by P.N.R.A., the Italian Organisation for ScientificResearch in Antarctica.

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�����������������������M. L. Frezzotti, Dipartimento di Scienze della Terra, Siena University,

I-53100 Siena, Italy. (frezzottiml@unisi.it)A. Peccerillo and V. Zanon, Dipartimento di Scienze della Terra, Perugia

University, I-06100 Perugia, Italy. (viczan@unipg.it; pecceang@unipg.it)

ZANON ET AL.: MAGMATIC FEEDING SYSTEM BENEATH VULCANO ISLAND ECV 7 - 13