plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: the 2001–2006...

Earth-Science Reviews xxx (2014) xxx–xxx

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Plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: The2001–2006 eruptive period at Mt. Etna

P.P. Giacomoni a,⁎, C. Ferlito b, M. Coltorti a, C. Bonadiman a, G. Lanzafame b

a Department of Physics and Earth Sciences, University of Ferrara, Ferrara, Italyb Department of Biology and Geological Sciences, University of Catania, Catania, Italy

⁎ Corresponding author. Tel.: +39 0532 974670.E-mail address: [email protected] (P.P. Giacomoni).

http://dx.doi.org/10.1016/j.earscirev.2014.06.0090012-8252/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: Giacomoni, P.P., eteruptive period at Mt. Etna, Earth-Sci. Rev. (

a b s t r a c t
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Article history:Received 16 July 2013Accepted 29 June 2014Available online xxxx

Keywords:PlagioclaseEtnaFeeding systemIntensive variablesCrystal growthCrystal textures

Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in mostlavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of themelt and, consequently, it can be used as a tool to record the processes occurring within the feeding system.With this aim, a detailed textural and compositional study of plagioclase was performed on the products emittedduring the 2001, 2002–2003, 2004–2005 and 2006 eruptions.Four distinct textureswere recognized at the crystal cores: (1) clear and rounded (An73–85), (2) dusty and round-ed (An73–85), (3) sieved (An82–88) and (4) patchy (An60–81), while two distinct textures are commonly observedat the crystal rim: (1) dusty (An73–90) and (2)withmelt inclusion alignments (An70–76).Moreover all plagioclasespresent a thin (10–20 μm) outermost less calcic (An53–76) rim. For each crystal a complex evolutionary path wasreconstructed, and several growth and resorption episodes were identified.The fO2 was estimated using Plag–Cpx/liquid equilibrium in order to calculate the Fe+3/Fe2+ ratio in the meltand, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the leastevolved lava of the four eruptive episodes.MELTS modeling was then developed using this primary magma composition, as well as a trachybasaltic lava.Calculations were performed at variable pressures (400–50 MPa, step of 0.50 MPa) and H2O contents(3.5–0 wt.%, step 0.5 wt.%) in order to estimate the crystallization temperature of olivine, clinopyroxene, plagio-clase and spinel, decreasing T from the liquidus down to 1000 °C at steps of 20 °C. P–T and water contents werealso determined using geothermobarometers and plagioclase–melt hygrometers respectively, aiming at verifyingthe parameters used in the MELTS modeling. At this point plagioclase textural features and compositions wererelated to specific P–T–fO2–H2O conditions.Plagioclase stability models indicate that: (1) H2O strongly influences the plagioclase–melt equilibrium allowingthe crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure(up to 350 MPa) and low water content (b1.7 wt.%); (3) clear dissolved cores form at lower pressure (150MPa) and higher water content (1.5–2.8 wt.%); (4) dusty rims form at even lower pressure straddling theH2O-saturation curve and, (5) melt alignments form during degassing. According to experimental works eachof these textures can be related to a different process within the feeding system, such as multiple magma inputs(patchy core), volatile addition or increase in T (clear core), mixing (dusty rims) and rapid decompression anddegassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptiveevolution of each event as deduced from direct observations, and geophysical and petrological data. The overallpicture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuousfeeding system in which at least two magma crystallization levels were identified. Plagioclase stability alsoindicates that a large variability in water content characterizes the magma within the feeding system.

al., Plagioclase as archive of magma ascent dyn2014), http://dx.doi.org/10.1016/j.earscirev.201

© 2014 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02. Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03. Volcanological evolution of 2001–2006 eruptive events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

amics on “open conduit” volcanoes: The 2001–20064.06.009

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2 P.P. Giacomoni et al. / Earth-Science Reviews xxx (2014) xxx–xxx

3.1. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.2. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.3. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.4. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

4. Intensive variables and parental magma composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.1. Oxygen fugacity and primary magma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.2. Temperature and pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.3. Water content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 04.4. MELTS phase stability modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

5. Plagioclase textures, compositional profiles and water estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.1. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.2. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.3. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05.4. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.1. Relationships between magmatic processes and plagioclase petrological features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.2. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.3. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.4. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.5. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06.6. Volcano feeding system and magma storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

1. Introduction

Plagioclase is the most common mineral phase crystallizing in basal-tic magmas. Several authors have shown that its abundance, chemistryand texture reflect the composition of themagma and the crystallizationconditions (T, P, XH2O, fO2) (Lofgren, 1980; Tsuchiyama, 1985; Cashmanand Marsh, 1988; Nelson and Montana, 1992; Landi et al., 2004; Viccaroet al., 2010; Iezzi et al., 2011). Such variables may change during magmapermanence in intratelluric conditions or ascent to the surface thus caus-ing remarkable variations in the growth of plagioclases and therefore intheir final morphology and composition. In particular, temperature de-crease and loss of H2O during degassing will promote crystal nucleationand growth, while temperature and/or H2O increase, due to magmamixing and/or volatiles flushing (Ferlito et al., 2014), will induce dissolu-tion of crystals. Thus phenocrysts in the lava are the result of a complexand articulated history (e.g., Lofgren, 1980; Smith and Lofgren, 1983;Tsuchiyama, 1985; Pearce et al., 1987; Nelson and Montana, 1992;Singer et al., 1995; Nakamura and Shimakita, 1998). The efforts of thescientific community have been devoted in disentangling this story toreconstructmagma dynamics as well as the geometry of the feeding sys-tem. Usually, such reconstruction is biased by toomany assumptions; es-peciallywhen attempts are done for past eruptions or for eruptive eventsof remote volcanoes. In fact, only a few volcanoes are heavily monitoredto have an array of seismological and grounddeformation data that allowconstraining the timing of the eruptive events and magma ascent. In thelast 15 yrs the Istituto Nazionale di Geofisica e Vulcanologia (INGV) hasdeveloped an instrumental network on and around Mt. Etna that allowsan efficient monitoring during the eruptive events as well as the quies-cent periods. Together with these favorable conditions Mt. Etna hasbeen recently characterized by important eruptive events (2001, 2002–2003, 2004–2005 and 2006), which have provided a variable spectrumof activity, from purely effusive to strongly explosive, and different dura-tion times, fromdays to severalmonths. Given these premiseswe chooseto study the plagioclase phenocrysts produced during these eruptiveevents and to relate their compositions and morphological features tomagma conditionswithin the feeding system. Plagioclases have been an-alyzed in 150 samples from theabove-mentioned eruptions.Morpholog-ical textures have been studied by high contrasting imagery with a

Please cite this article as: Giacomoni, P.P., et al., Plagioclase as archive of meruptive period at Mt. Etna, Earth-Sci. Rev. (2014), http://dx.doi.org/10.10

scanning electron microscope (SEM) and mineral compositions havebeen determined through core to rim profiles with an electron micro-probe (EMPA). This very detailed study allowed us to recognize severalgrowthmodalities within each individual crystal that in turn were relat-ed to different magmatic processes.

Even though Mt. Etna is one of the most studied volcanoes in theworld, a large consensus is not yet reached on physical–chemical crys-tallization conditions. Most of the uncertainties come from the lack ofprimitive lava composition and in the determination of oxygen fugacity(fO2) conditions. The latter was determined by using Plg–Cpx/liquidequilibrium (France et al., 2010) allowing the reconstruction of the orig-inal Fe3+/Fe2+ ratio in the melt and in turn the primitive magma com-position by backward fractionation until equilibration with peridotiticmantle paragenesis. Then MELTS simulations (Ghiorso and Sack,1995) were performed to constrain the mineral phase relationshipsand composition, particularly regarding plagioclase, during magmadifferentiation at different pressures and dissolved water contents.

Results were compared with estimates obtained by means of theclinopyroxene–liquid geothermometer of Putirka et al. (2003) onclinopyroxenes. T estimated on clinopyroxene rims ormicrophenocrystscoexisting with plagioclase were then used to predict the melt–watercontent by using the hygrometers of Putirka (2005) and Lange et al.(2009), assuming that the most basic magma erupted during eachevent was in equilibrium with the plagioclase core.

2. Geological setting

Mt. Etna is a 3340-m-high stratovolcano located on the eastern coastof Sicily, covering an area of over about 1418 km2 (Tanguy et al., 1997),and with its continuous activity represents the most important activevolcano in Europe. The complex volcanic edifice is grown at the intersec-tion of (1) twomajor fault belts trending NNW–SSE (Tindari–Letojanni–Malta) and NNE–SSW (Messina–Giardini) and (2) three structuraldomains: (1) to the north, the Peloritani mountain range correspondingto the Apennine–Maghrebian belt which extends westward; (2) to thesouth, the undeformed northern margin of the African plate constitutedby theHyblean Plateau and represents the forelandwhich plunges under

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the Catania–Gela foredeep and (3) to the east, the Ionian lithosphere,considered to be a remnant of the Mesozoic Tethys (Fig. 1a).

After a tholeiitic period (500 to 220 Ka ago) the erupted productsshift toward amarked Na-alkalic character (cf. Tanguy et al., 1997). Sev-eral distinct stratovolcanoes formed ~130 ka BP (Catalano et al., 2004)(Ancient Alkaline Centers, AAC, cf. Romano, 1982), which evolved intoedifices of considerable dimension, whose remains are still visible inthe Valle del Bove area. Finally, all ancient edifices were buried by the

Fig. 1. (a) Regional geologicalmap of Sicily showingposition ofMt. Etna andmain structural featdel Lago, LAG: Laghetto vent, C-L: Calcarazzi vent; (c)mapof 2002–2003Northeast Rift eruptiveand lava flows; (e) 2004 eruptive fissures and lava flows; (f) 2006 eruptive fissures and lava fl


lava flows and tephra originated by the activity of the Ellittico, the big-gest volcanic center that appeared on the Etnean region. The activitycontinuedwith the “RecentMongibello” (Romano, 1982),which started~15 ka BP, displaying a wide range of eruptive styles from effusive andmildly strombolian to sub-plinian.

The volcanic pile, resting on the top of the sedimentary substratumwith an elevation of about 1300 m a.s.l., reaches the thickness ofabout 2000 m in the central portion of the edifice. A reconstruction of

ures; (b)mapof 2001 eruptivefissures, vents and lavaflows. SE-Pl: South East Crater-Pianofissures, vents and lavaflows; (d)map of 2002–2003 Southeast Rift eruptivefissures, ventsows.




the hiddenmorphology allowed the estimation that the total bulk of theEtnean volcanics sums up to 373 km3 (Neri and Rossi, 2002). Most ofthis volume (at least two/third) can be referred to the activity occurredin the last 100 ky (Catalano et al., 2004).

The periodic eruptions of poorly evolved magma, the persistentdegassing from summit craters together with geophysical data (i.e.deformation of the edifice, persistent shallow seismic tremor) suggestthat the feeding system can be considered as an “open conduit” persis-tently filled with magma. Geological, geophysical and geochemical evi-dences support the hypothesis that the structure of the plumbingsystem consists of a plexus of dikes and sills, that can constitute a seriesof magma batches at a depth of 3–5 km b.s.l, not connected to the sur-face (Cristofolini et al., 1985; Allard, 1997; Patanè et al., 2003; Allardet al., 2006; Viccaro and Cristofolini, 2008; Ferlito and Nicotra, 2010).

3. Volcanological evolution of 2001–2006 eruptive events

Among the many volcanic episodes that characterize the persistentactivity of Mt. Etna, four eruptive events encompassing different erup-tive behaviors, from purely effusive to strongly explosive, were chosen.Moreover, these events have been studied in detail by numerous re-searchers and are the subject of several multidisciplinary works. Thisprovides an indirect opportunity to test the hypothesis put forward bystudying plagioclase textural and compositional features. In order toelucidate eruptive dynamics, the following section will present a sum-mary of the volcanological and geophysical characteristics of each erup-tive episode. More detailed descriptions of these events can be found inViccaro et al. (2006), Ferlito et al. (2009a,b), Corsaro et al. (2009), Ferlitoet al. (2010) and Ferlito et al. (2012).

3.1. 2001 eruptive event

The 2001 eruption occurred in the southern sector of Mt. Etnabetween July 13th and August 9th 2001, and produced an estimatedvolume of about 25 × 106 m3 of lava and 7 × 106 m3 of tephra(Behncke and Neri, 2003; Clocchiatti et al., 2004). It was characterizedby two eruptive fissures that were active simultaneously. In the highslopes of the volcano, from an elevation of 3100 m at the South East(SE) crater down to about 2650 m at the Piano del Lago or Laghettoarea (PL), the fissure system was trending NNW–SSE (SE–PL system,

Fig. 2. Total alkali vs Silica classification diagram (Le Maitre, 2002) of erupted lavas fromfrom Spilliaert et al. (2006) andMétrich et al. (2004) are plotted with Maletto and Montagnolaequilibrated basalt and 2002/2003 trachybasalt) composition and the 2002/2003 trachybasalts


Fig. 1b). Lava fountaining was present on the upper part, while lavaflows developed in the lower part. A N–S oriented fissure formed at2100 m near Calcarazzi and migrated upwards reaching the Laghettoarea at 2550 m (C–L system) with strombolian activity and lava flows(Fig. 1b). The two fissure systems crossed at Piano del Lago forming adistinct and highly explosive vent (LAG) (Viccaro et al., 2006)(Fig. 1b). All phases of the eruptive event were preceded and accompa-nied by seismic swarms; below the SE Crater, foci depths were locatedbetween 4.5 and 7.0 km below the vent (Alparone et al., 2004), whilebelow C–L and LAG vents earthquakes had deeper foci (about 8 kmbelow the vent, Monaco et al., 2005). Magma emitted throughout theeruption was trachybasaltic in composition (Fig. 2). At C–L fracture itwas slightly more evolved, low porphyritic with amphibole phenocrysts(Clocchiatti et al., 2004; Viccaro et al., 2006) that are uncommon in the re-cent Etnean activity. Magma from SE–PL fracture was instead highly por-phyritic and characterized by the occurrence of olivine, clinopyroxeneand abundant plagioclase. Products emitted by the LAG cone are lowporphyritic and similar to the C–L in terms of phenocryst compositionsand modal proportions. Amphibole was also present but smaller in sizeand with significant resorbed glassy rims (cf. Viccaro et al., 2006; Ferlitoet al., 2008, 2009b).

3.2. 2002–2003 eruptive event

This eruptive event was bilateral, involving both southern (South RiftSystem, SRS) and northern (NE Rift System, NERS) flanks of the volcanosimultaneously. It started on October 26th and ended on January 28th.During the eruption an estimated volume of 32–44 106 m3 of lavas and50–60 106 m3 of pyroclastic material was emitted (Andronico et al.,2005). The activity was preceded and accompanied by a seismic swarmwith hypocenters distributed below the summit area which movednortheastwards after the first days and remained stationary for oneentire week underneath the Pernicana fault (Monaco et al., 2005 andreferences therein).

At NERS the entire eruptive fracture opened in less than 14 h andwas composed of three main eruptive segments at an elevation of2500–2300 m, 2300–2100 m and 2100–1950 m, each of them erupteddistinct magma types (Ferlito et al., 2009a) (Figs. 1c and 2). A hawaiite(Low Potassium Oligophyric, LKO magma, Ferlito et al., 2009a) wasemitted from the uppermost segment (2500–2300 m a.s.l.), with a

2001, 2002/2003, 2004/2005 and 2006 eruptive events. Melt inclusion compositionsprimitive lava compositions and starting compositions of MELTS simulation (2006 mantle.




Porphyritic Index (P.I.) of about 12% and phenocrysts made up of Ol(10%), Cpx (35%) and Plg (55%). On the other hand, trachybasalticmagma was erupted from the central (2300–2100 m a.s.l.) and lower(2100–1950 m a.s.l.) segments with a P.I. varying from 15 to 24%(High Potassium Oligophyric and Porphyritic, HKO and HKP; Ferlitoet al., 2009a) and phenocrysts composed of Ol (10%), Cpx (30%) andPlg (60%). Activity ceased on this side of the volcano on November3rd. At SRS lava fountains and strombolian activity started on October26th, while lava flow emission occurred on the 27th contemporarywith the opening of the lowermost segment of the NERS (Ferlito et al.,2009a). The activity on the southern flank continued until January28th. During these threemonths activity underwent sudden incrementswith numerous episodes of gas explosions and lava fountaining, in par-ticular those of November 12th, November 30th and December 18th(Andronico et al., 2005) (Figs. 1d and 2). This magma had textural fea-tures perfectly comparable to those erupted at the lower segment ofNERS but with slightly less evolved composition (Ferlito et al., 2009a,b; Giacomoni et al., 2012).


Eruptive activity resumed after 20 months of quiescence onSeptember 7th 2004 and lasted for about 6 monthswith a total estimat-ed volume of emitted lava ranging from 40 to 60 × 106 m3 (Neri andAcocella, 2006). The eruption onset was neither heralded nor accompa-nied by recorded precursory signals, such as seismicity and ground de-formation, nor by explosions at the summit craters (Burton et al., 2005).It started from an articulated fracture zone, which extended ESE fromthe Southeast Crater (SEC) toward the rim of Valle del Bove over alength of about 200 m (Fig. 1e). A small SE-directed lava flow pouredout at 2920 m a.s.l. and stopped few hours later. A new fracture andeffusive vent opened at 2620 m a.s.l. on September 10th and fed alava flow expanding toward the Serra Giannicola Piccola ridge (SGP)(Fig. 1e). Then, the fracture zone propagated downslope and a new ef-fusive vent opened at about 2320 m a.s.l., close to Serra GiannicolaGrande. After September 13th, the fracture zone did not evolve further,and the effusive activity was stabilized at both 2620 and 2320 m vents.During the next months, a compound lava field developed and reachedabout 1600 m a.s.l. On March 8th, the eruption ended. Plg, Cpx, Ol andTi-magnetite form the mineralogical assemblage of products emittedduring this eruptive event, with limited variability in the mineral pro-portions throughout the entire eruption. All products are trachybasaltswith an average SiO2 of 47.92 wt.%, with the most evolved magmasbeing erupted before September 24th (Corsaro et al., 2009).


After the 2004–2005 event in Valle del Bove, the activity resumed onJuly 14th 2006 on the eastern flank of South East summit Crater (SEC).Activity continued by lava effusion and lava fountaining until December15th with a total estimated volume of emitted products of 13 × 106 m3.

Two vents opened on July 14th emitting lava flows (b5 m3/s) thatexpanded into the Valle del Bove. On July 15th a third vent opened,characterized by strombolian activity with emission of lithics and juve-nile ejecta. Strombolian activity produced an ~30 m high cinder cone,which collapsed in the evening of July 19th when intensity of explosionincreased and a new lava flowwas emitted (~10m3/s). In the followingdays, the activity at the SEC showed intermittent strombolian explo-sions, lava fountains and lava flows (Fig. 1f). Intensity varied widely,from a single lava flow, to weak strombolian explosions and to stronglava fountains up to 250 m above the crater rim. The activity continuedalternating strombolian explosions, lava effusions and intra-cratericemissions of ashes and lithics until November 16th (Behncke et al.,2008; Ferlito et al., 2010).

The most relevant episode of the entire eruptive event occurred onNovember 16th. In the early morning, the eastern sector of the SEC


edifice began to be affected by rock sliding, while the volcanic tremoramplitude increased (cf. Behncke et al., 2008). At 6:15 UTC, eruptive ac-tivity began near the summit of the SEC with strong strombolian activ-ity, accompanied by gray ash and steam explosions, and lava flowsdirected toward theValle del Bove. At 14:00 a series of collapse episodesstarted, culminating at 14:28 when a few-meters-high billow of brownash-laden steamwas observed at the base of the niche (~3050 m a.s.l.),immediately joined by another ca. 20 m upslope. The billows evolvedinto ~150-m-high brownish plumes. These were immediately followedby an ESE–WNW oriented fracture opening along the gully (Fig. 2 inFerlito et al., 2010) and a series of explosions, which gave rise to a300-m-high eruptive curtain, bearing juvenile and lithic clasts. The cur-tain was sustained for a short time (a little more than 1 min) and thencollapsed, giving rise to a gravity driven pyroclastic flow, whichmoved downslope along the flank of the SEC down to ~2800 m. Afterthe November 16th peak, the eruption continued with alternatingphases of effusive and strombolian activity. Four vents were fed at var-iable rates until the end of the eruption on December 15th, 2006.

Lavas are porphyritic trachybasalts (Fig. 2) with phenocrystscomposed of a variable amount of Plg (10%), Cpx (8%), Ol (5%) andTi-magnetite (3%). Products are quite homogeneous, with slightlymore basic features (SiO2 b 47 wt.%) in those emitted during theNovember 16th paroxystic episode (Ferlito et al., 2010).

4. Intensive variables and parental magma composition

Etnean magmas, except those emitted during the tholeiitic period,are alkaline falling in the field of hawaiites and mugearites (Fig. 2). Al-most invariably, Etnean rocks display the same mineral assemblagemade of plagioclase, clinopyroxene, olivine, Ti-magnetite ± apatiteand ±amphibole (Tanguy et al., 1997).

Contrary to the hawaiites, Etnean lavas contain lower TiO2, andhigher Al2O3 and CaO contents. In recent times (after the 1971 erup-tion), the lavas show a tendency toward higher K2O/Na2O ratio leadingto classify them as trachybasalts. However, K-rich lavas are also presentin old products (Ferlito and Lanzafame, 2010) and, on the other hand,hawaiites were emitted by the NERS in 2002 (Ferlito et al., 2009a).The evidence of efficient differentiation processes occurring duringmagma uprise makes the reconstruction of the primary compositionrather difficult. The most evolved trachytic magmas (Mg# = 38.4,where Mg# = (MgO ∕ (MgO + FeO)) mol % with Fe2O3/FeO = 0.15)were erupted during the Ellittico period (60–15 Ka ago), while recentalkaline erupted products are less evolved with Mg# varying between63.4 and 49.0. None of these compositions, however, result in equilibri-um with mantle peridotite.

4.1. Oxygen fugacity and primary magma

A problem in calculating the Mg# and by consequence assessingthe evolution degree of magmas is the amount of FeO, which is con-trolled by the oxygen fugacity (fO2) of the magmatic system. WhenFe3+/Fe2+ ratio is directly measured on lavas it may be affected by oxi-dation processes occurring during eruption and/or alteration and it isunlikely that such ratio would reflect the magma oxidation state atdepth (Mollo et al., 2013 and references therein). More appropriateways for measuring the original fO2 of magmas are based on mineral/mineral and mineral/melt equilibria. The most commonly used methodis the ilmenite/magnetite paragenesis, which is however prevented atEtna due to the absence of ilmenite. A different tool was proposed byFrance et al. (2010), which uses the partition of ferrous and ferric ironbetween plagioclase, clinopyroxene and the coexistingmelt. Plagioclasecan incorporatemore Fe3+ than Fe2+while clinopyroxene can incorpo-rate more Fe2+ than Fe3+. The effect of oxidizing a partly molten basal-tic system is to stabilize Fe3+, thus FeOTot increases in plagioclase anddecreases in the associated clinopyroxene. The model is calibrated forpressure b0.5 GPa and (ΔQFM N 0). The application of this model to



Table 1Major element analysis of Mt. Maletto, Montagnola, 2006 basic magmas and re-equilibration to their primitive composition. The 2002–2003 MELTS modeling startingcomposition is reported. Mg# = MgO ∕ (MgO + FeO) mol%, F (%) is the amount of frac-tionated solid that has been added to obtain the equilibrium composition.

MELTS magmas

Mt Maletto Montagnola 2006 2002/2003

SiO2 48.31 48.31 47.94 49.49TiO2 1.62 1.68 1.82 1.64Al2O3 16.56 17.90 18.07 17.96FeO 8.10 8.28 8.47 7.85Fe2O3 1.22 1.22 1.27 1.77MnO 0.20 0.18 0.19 0.17MgO 7.89 5.96 5.49 4.03CaO 11.92 10.97 10.88 9.27Na2O 2.56 3.75 3.53 4.19K2O 1.25 1.31 1.90 2.16P2O5 0.37 0.42 0.45 0.54Total 100 100 100 100Mg# 63.36 56.11 53.53 47.68

EquilibratedSiO2 47.96 47.85 47.22TiO2 1.51 1.52 1.61Al2O3 15.33 16.09 15.54FeO 8.36 7.39 7.36Fe2O3 1.12 2.54 2.96MnO 0.19 0.16 0.17MgO 10.21 8.96 9.10CaO 11.45 10.57 10.40Na2O 2.37 3.37 3.39K2O 1.15 1.17 1.85P2O5 0.34 0.38 0.39Total 100 100 100Mg# 68.43 68.27 68.71Ol 70.59 61.18 64.71Cpx 29.41 38.82 35.29F% 10 12 15


the 2002–2003 products (at T = 1150 °C) has produced values oflogfO2 in the range from −8.3 to −6.7, corresponding to +0.8 b

ΔQFM b +2.3, with a medium value of −7.7 logfO2, that is +1.5ΔQFM (Fig. 3) (see Supplementary Material for the compositions usedfor calculations). These results are consistent with those calculated byMollo et al. (2011) who obtained fO2 values from −8.0 to −7.3 (+1 b

ΔQFM b +2.2) using the same method of France et al. (2010) andfrom −6.8 to −6.5 logfO2 (+2.4 b ΔQFM b +2.1) using the equationof Kress and Carmichael (1991) based on Fe3+/Fe2+ distribution onclinopyroxene. Similar values were obtained by D'Orazio et al. (1998),who calculated a fO2 ranging from+1 b ΔQFM b+3 using the Eu par-tition between plagioclase and melt. According to the equation of Kressand Carmichael (1988) at this fO2 condition corresponds a Fe3+/Fe2+

ratio of about 0.15, that is the most common ratio used for basalticmagmas.

Several authors have identified the lavas erupted during the RecentMongibello activity such as Mt. Maletto (pre-historic flank eruption),Montagnola (1763 flank eruption), and 2006 (subterminal eruption),as the most primitive compositions that can be found on Mt. Etna(Table 1 and Fig. 2). Their Mg# however is comparable to that of prima-ry magma only if a considerable amount of Fe is oxidized, with a Fe3+/Fe2+ ratio ranging from0.49 to 0.89 (Viccaro and Cristofolini, 2008). Ac-cording to the equilibrium stated by Kress and Carmichael (1988) thisratio would correspond to ΔQFM N +4, rather unusual for intraplateor even for suprasubduction environments (Rilling and Barton, 2005;Rowe et al., 2009). As indicated above for fO2 = +1.5 ΔQFM at T of1170 °C, a Fe3+/Fe2+ ratio of 0.15 is considered more appropriate forthese magmas. In this case the Mg# of the Mt. Maletto, Montagnola or2006 eruption varies from 53.5 to 63.4, quite far from Mg# = 68 esti-mated for melt in equilibrium with a fertile (OlFo = 88) peridotiticmantle assemblage, according to the Fe/Mg ol/liq partitioning of 0.3 de-termined by Roedder and Emslie (1970). A variable amount of wehrliticassemblage (10–15% of fractionation of an assemblage constituted by70.6–61.2% Ol + 38.8–29.4% Cpx; Table 1 and Fig. 2; see also Alesciet al., 2013) was determined by mass balance backward fractionationand added to these magmas to equilibrate them to mantle conditions.The obtained compositions are well comparable with those measuredin melt inclusions. In particular, the reconstructed composition of2006 primary magma falls in the field of melt inclusions (Mis) found

Fig. 3.Oxygen fugacity determinations plotted on the (Fe2+/Fe3+)/logfO2 curve calculatedusing the equation of Kress and Carmichael (1991). Black circles represent the values de-termined in this study using themethod of France et al. (2010). Black and gray squares arethe values determined by Mollo et al. (2011) using the equations of France et al. (2010)and Kress and Carmichael (1991) respectively. Gray triangle is the value estimated byD'Orazio et al. (1998) on the basis of Eu partitioning in plagioclase.


in 2001 and 2002–2003 products (Fig. 2) (Métrich et al., 2004;Spilliaert et al., 2006).

4.2. Temperature and pressure

Temperature and pressure have been estimated by means of the ex-change reactions of Diopside/Hedenbergite–Jadeite and Hedenbergite/Ca-Tschermak between clinopyroxene and the equilibrium melt(Putirka et al., 1996; Putirka, 2008). Crystal/melt equilibrium conditionshowever must be checked in order to apply geo-thermobarometers,which rise the question of magma composition. Clinopyroxene equilibri-um was tested on the basis of Cpx–liqKdMg–Fe (Putirka, 2008) for crystalsembedded in lavas of the 2002/2003 eruptive event choosing the mostprimitive erupted composition as equilibriummelt. Cpx–liqKdMg–Fe rangesbetween 0.26 and 0.28 (Fig. 4a), well within the experimentally deter-mined for T N 1050 °C (0.24–0.30, Putirka, 2008). Temperature and pres-sure were determined using the P-independent equation of Putirka et al.(1996) and theH2O-independent equation of Putirka (2008), respective-ly. Results indicate that clinopyroxene crystallized between 1189 ±26 °C and 1132 ± 26 °C at a pressure varying between 740 ± 30 and330 ± 30 MPa.

4.3. Water content

A similar approach for checking equilibrium conditions was follow-ed for plagioclase (Fig. 4b). Crystal cores and dissolution zones wereconsidered in equilibrium with the most primitive melt erupted duringeach single event, while rims were assumed in equilibrium with theirwhole rock. According to Putirka (2008) equilibrium can be assessedwhen Plg–liqKdAb–An ranges 0.27 ± 0.11 for T N 1050 °C. Fig. 4b showsthatmost of theplagioclases are in equilibrium, althoughdisequilibriumoccurs in some An-rich portion of the crystals.



(a)

(b)

Fig. 4. (a) Cpx–meltKdFe–Mg equilibrium test for clinopyroxene from 2002–2003 eruption. Temperature has been determined using the Cpx–melt geothermometer by Putirka (2008).(b) Plg–liqKdAb–An equilibrium test for plagioclase 2001, 2002/2003, 2004 and 2006 eruptive events.


An effort to estimate the amount of H2O dissolved in themelt using aplagioclase hygrometer has been done using the equation of Lange et al.(2009) only on equilibrated crystals. This method is based on the ex-change reactions between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8)components calibrated on a dataset spanning awide range of liquid com-positions (SiO2, 46–74 wt.%), plagioclase compositions (An93–37), tem-peratures (825–1230 °C), pressures (0–300 MPa), and dissolved waterconcentrations (0–7 wt.%) in both saturated and undersaturated condi-tions. Temperature was fixed at 1132 °C, that is the lower T estimatedin Cpx. Pressure was arbitrarily fixed at 250 MPa, having checked thatthe effect of pressure on hygrometers is small (±100 MPa correspondsto ~0.1 wt.% H2O calculated in the melt). With these assumptionswater estimates range between 0.2±0.23 and 3.6± 0.23 wt.%, in accor-dance with Mi determinations in olivine from the 2001 and 2002–2003eruptions, which suggest that up to 3.5 ± 0.23 wt.% of H2O is in themelt at 490 MPa, whereas erupted lavas have H2O contents down to0.5–1.0 wt.% (Métrich et al., 2004; Spilliaert et al., 2006).

The obtained geothermobarometric data and the calculated watercontents were used to constrain the range of intensive variables intro-duced in the MELTS model.


4.4. MELTS phase stability modeling

Stability of mineral phases and equilibrium compositions havebeen modeled with MELTS code (Ghiorso and Sack, 1995). Calculationswere performed using two different compositions: the 2006 mantle-equilibrated basaltic composition and the 2002–2003 trachybasalt(Fig. 2 and Table 1). For each composition, calculations were repeatedusing different water contents (0–3 wt.%, step 0.5) and pressures(400–50 MPa, step 5), corresponding to 11–1 km below the CentralCraters following the density model of Corsaro and Pompilio (2004). Tdecreases from 1300 °C to 1000 °C at steps of 20 °C at constant fO2,corresponding to ΔQFM = +1. In Fig. 5a–d liquidus temperatures ofolivine, clinopyroxene, spinel and plagioclase are shown as function ofpressure at differentH2O contents. Olivine is thefirst phase to crystallizein both basaltic and trachybasaltic melts. As the amount of dissolvedH2O increases by about 1 wt.%, the liquidus temperature decreases byabout 50 °C for all phases. In anhydrousmelt plagioclase becomes stableat higher temperature than spinel for both magma compositions(Fig. 5a). Such result does not fit with petrographic observations that in-dicate an early appearance of spinel in themagma, often enclosed in the



Ol

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(a)

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(c)

(d)

Ol

Fig. 5.MELTS pressure vs. temperature for 2006 mantle-equilibrated basaltic (solid line) and trachybasaltic composition (dashed line). Calculations were performed at 0 H2O wt.% (a), 1H2O wt.% (b), 2 H2O wt.% (c) and 4 H2O wt.% (d).


outer portions of clinopyroxenes, thus suggesting that anhydrous prim-itive magmas are unlikely on Mt. Etna (Giacomoni et al., 2012). Themain effect of increasing H2O is to strongly depress the plagioclaseliquidus with respect to olivine and clinopyroxene (Fig. 5a–d).

MELTS simulations of plagioclase composition and stability wereperformed using two different melt compositions: the reconstructedmantle-equilibrated basalt from the 2006 (Fig. 6a) most basic lava,and a common trachybasalt emitted during the 2002–2003 event(Fig. 6b). Plagioclase liquidus (black solid line) has been plotted againstpressure and dissolved H2O content; to the left plagioclase is stablewhile to the right it can crystallizes only at unrealistic temperatures(b1000 °C) for Etnean lavas. Results indicate that the plagioclase stabil-ity field is strongly affected by pressure and H2O content of the melt, itexpands by lowering pressure and with low H2O content. On theother hand, crystal composition becomes more anorthitic in H2O-richmelts and at low-pressure conditions (Fig. 6 a–b).

Moreover, it is evident that: (1) with increasing pressure, plagio-clase becomes stable at higher T, (2) an increase of H2O depresses theplagioclase stability field, crystallizing a more anorthitic plagioclase at


lower temperature, and (3) the slightly evolved trachybasaltic magmastabilizes plagioclase earlier with respect to undifferentiated basalticcompositions.

5. Plagioclase textures, compositional profiles and water estimates

Etnean plagioclase phenocrysts in the lavas erupted duringthe 2001–2006 period are generally euhedral, with major axis of0.5–2 mm, and characterized by very diverse textural and growth fea-tures. An attempt to classify these textures and to relate them to mag-matic processes has been recently proposed by Viccaro et al. (2010),who recognized seven crystal types. However, this classification de-scribes the entire crystal and is unable to take into account the variousparts that may constitute the whole plagioclase. We believe that an im-provement of this classification is necessary since each growth featurecan be related to specific physical and/or chemical conditions of the sys-tem; many crystals are, in fact, constituted by a core, one or two over-growth portion/s, a rim and an outermost rim. Textural observations,with optical and electron microscopes, has been performed on a total



1185 (69)

1100 (71)

1187 (72)

1188 (73)

1184 (74)

1180 (75)

1166 (76)

1106 (74)

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1109 (78)

1105 (79)

1101 (80)

1050 (80)

1060 (82)

1080 (83)

1080 (86)

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MP

a)

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1185 (69)

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1166 (76)

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1140 (72)

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00.5 1 1.5 2 2.5 3 3.5 4

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2006 mantle-equilibrated basaltic

composition

2002/2003 trachy-basalt

Dep

th (

km)

Dep

th (

km)

(a)

(b)

Fig. 6.MELTS pressure vs. H2O content for 2006mantle-equilibrated basaltic composition (a) and 2002/2003 trachybasalt (b). Plagioclase liquidus (black line) is reportedwith the expect-ed An composition and temperature (°C) (An% — circles). H2O saturation curve has been calculated with Solex software (Witham et al., 2011).


of 250 thin sections of lavas from historic to recent eruptions although,for the scope of the present work attention is mainly focused on 150samples of the 2001–2006 eruptive period.

Themainmorphological features, together with a sketch that under-lines the peculiar characteristics are shown in Fig. 7. A first subdivisioncan be done between simple zoned crystals, which preserve theireuhedral habitus and complex zoned crystals that can be rounded oranhedral. Simple zoning includes oscillatory zonation and can be recog-nized in every crystal zone, often characterizing the overgrowth be-tween portions of the crystal with complex zoning. Using BSE imagesand compositional profiles two types of oscillatory zoning have beenrecognized: Low Amplitude High Frequency (LAHF) and High Ampli-tude Low Frequency (HALF) patterns (cf. Viccaro et al., 2010). TheLAHF pattern shows little An variations (~ΔAn5) with zone widths of5–10 μm. The HALF pattern displays higher An variations (NΔAn10)with wider zones (20–30 μm) and is often associated with crenulatededges due to dissolution and angular unconformities (Fig. 7a).

Complex zoning can be recorded in anhedral crystal cores and/orrims. Four different core types can be recognized, three of them usuallyhaving a rounded edge core and an overgrowth characterized by anabrupt change in the chemical composition: (1) clear rounded cores,are frequent and in some cases preserve oscillatory zoning (Fig. 7b);


(2) dusty cores, characterized by glassy or partially crystallized meltpockets (Fig. 7c); and (3) patchy rounded cores are characterized bycomplex juxtapositions of An-rich (~An80) and An-poor (~An55) do-mains, arranged in irregular patches and covering an approximatearea of 400 μm2 (Fig. 7d).

Clear rounded cores are characterized by both LAHF and HALFpatterns. Oscillatory zonation is abruptly interrupted by the roundededge and compositional profiles evidence a decrease in An content(ΔAn ≥ 20%) in the overgrowth just beyond the ovoidal edge. In dustycores, glassy μm-sized melt pockets are distributed randomly withinthe crystal core (Fig. 7c). Sometimes, glassy channels interconnect thepockets, modifying the preexisting oscillatory zonation inside the crys-tal. A rounded edgemarks the dusty zone and an overgrowth envelopesthe core, often with a decrease in An content (ΔAn ≥ 20%). In patchycores, the rounded edge is followed by less anorthitic overgrowths(Fig. 7d). Within the patchy cores, sieve textures may occur (see sievetextured cores in Fig. 7e). Patchy rounded cores are frequent in historicproducts but are rare in products from recent eruptive events (Viccaroet al., 2010). The fourth type consists of sieve-textured cores that arecharacterized by pockets of partially recrystallized melt randomly dis-tributed within the crystal core (Fig. 6e). Compared with dusty cores,they do not present edge (neither rounded nor rough) that marks the



Fig. 7. Lightmicroscope photomicrographs and sketches of the recognized textures of plagioclase in the 2001–2006 eruptions. Simple zoning: Oscillatory zoning—Oz (a); complex zoningat the cores: Clear rounded core — C (b); dusty rounded core — D (c); patchy rounded core — P (d); sieved core — S (e); complex zoning at the rim: Dusty rounded rim — D (f); meltinclusion alignment — Mi (g).

Fig. 8. Evolutionary path diagram of core and rim textures. Starting from the initialnucleation and growth conditions, each texture records an event related to changes inthe P–T–X conditions of the magmatic system.


boundary of core and no significant compositional variations have beendetected; compositional profiles show oscillatory LAHF or HALF pat-terns inside the sieve textured zone continuing at the overgrowth.

Two types of complex zoning can be recognized at the rim:(1) rounded dusty rims which are characterized by a dusty (~50 μmthick) rounded zone composed by isolated or interconnected partiallyre-crystallized melt pockets (Fig. 7f). Remarkable An (ΔAn15–30) andFeO enrichments mark the edge of the dusty zone. Most crystals showone rounded dusty rim; phenocrysts with two or more dusty zones atrim are very rare, spaced by oscillatory zoning. (2) Rims with meltinclusion alignments are characterized by iso-orientation of μm-sized(~5–10 μm wide) melt inclusions polygonal in shape (Fig. 7g). Differ-ently from dusty rims, they do not present a rounded inner edge.Compositional profiles show a decrease in An (ΔAn 10–20%) and FeOcontents.

With this in mind, each phenocryst is the result of several events ofgrowth and/or dissolution and can be summarized as an evolutionarypath (Fig. 8). The most complex phenocrysts record a maximumof five events, starting from the first nucleation and initial growth(1st event). A major perturbation in the physical–chemical conditionsof the system can be recorded by the formation of complex zoningwith (Clear, C; Dusty, D1; Patchy, P) or without (Sieved, S) roundededges (2nd event). Thereafter crystal growth is resumed and an oscilla-tory Overgrowth (3rd event, O1) surrounds the edge of the complexzoned cores. If a new perturbation in the physical–chemical conditionsoccurs, crystal can acquire againDusty (D2) rounded zones or alignments

Fig. 9. Back scattered SEM images and core–rim compositional profiles of An% and FeO wt.% opresented together with H2O determinations performed with the hygrometer of Lange et al. (2emitted at the C–L fracture; (c) plagioclase from lavas emitted at the Lag cinder cone.


of melt inclusions (Mis) (4th event). An oscillatory zonation (O2, 5thevent) usually envelopes the alignment, suggesting that crystallizationoccurred just before or even during the eruption. As evidenced in the di-agram, not all phenocrysts record the entire sequence. In many cases asimple oscillatory zoning may characterize the entire crystal (OscillatoryZoning, Oz, Fig. 8). In other cases simple oscillatory zoning may develop

f plagioclases emitted during the 2001 eruptive event. The growth path of each crystal is009). (a) Plagioclase from lavas emitted at the SE–PL fracture; (b) plagioclase from lavas




Please cite this article as: Giacomoni, P.P., et al., Plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: The 2001–2006eruptive period at Mt. Etna, Earth-Sci. Rev. (2014), http://dx.doi.org/10.1016/j.earscirev.2014.06.009



after the first stage of overgrowth (O1 to eruption) or characterize the ini-tial growing stage of the crystal, which later can be affected by complextextures at the rim (Nucleation and Growth to Mi/D2).

Frequently, different types of plagioclases are foundwithin the samesample or in samples that can be attributed to the same eruptive period.In this respect temporal relationships between the various samples arecrucial. In the following paragraphs each plagioclase type is attributed toa specific period on the base of the most frequent textures at similarsize, taking in mind that the presence of different crystals may provideuseful information on the process/es occurring to the magma duringuprising. It is evident that if we assume that different crystal zoningsare related to different processes occurring within the feeding systemthe contemporaneous presence of two (ormore) crystal textures cannotbe neglected (see the Discussion section below).

In the following sections textural features and chemical variations(An and FeO core–rim profiles) of plagioclases are described for eacheruptive event. In Figs. 9–11 these are reported together with the H2Ocontent of the melt (Lange et al., 2009) calculated on plagioclase withKd within the equilibrium range (Putirka, 2008) and the evolutionarypath of the most abundant type of plagioclase found in the eruptedproduct.


Plagioclase phenocrysts in lavas of the 2001 eruption are mainlyeuhedral from 0.5 to 2 mm in size. In SE–PL lavas, most of the plagio-clases (90%, Table 2) have simple textures with oscillatory zoning(Fig. 7a). The composition varies from An84 at the cores to An63 at therims. FeOtot content is notable (0.4–1.03 wt.%). Some crystals (5%,Table 2) have sieved bytownitic cores, or dusty rounded rims (5%,Table 2). Compositional profiles show that oscillations have a HALFpattern characterized by cross cutting edges and neat dissolution sur-faces. An variation during oscillation is ~5%, and no significant changesin An content are associated to sieved cores. The path diagram repre-sents only nucleation and growth (1st event) followed by an oscillatoryzoned continuous growth (5th event). Oscillatory zoned crystals inSE–PL lavas indicate equilibrium with an average H2O content of 2.3wt.% (Fig. 9a).

In C–L lavas, phenocrysts with clear oscillatory zoning are rare(10%, Table 2); themajority of the crystals (90%, Table 2) have complexzoning textures, with HALF oscillatory zoning core or patchy zoning.With respect to SE–PL, these cores are less calcic with remarkable com-positional variations within oscillatory zones (An47–63, ΔAn15–20%),which are not in equilibrium with whole rock. The core is surroundedby a dusty rim which is associated with An increment. Oscillatory over-growth envelopes the dusty rim characterized by a less calcic composi-tion (Fig. 9b). The following evolution can be depicted for these crystals:after nucleation, probably from a coldermagma (1st event), the crystalsreacted with a hotter magma producing a dusty rounded rim (D2, 4thevent). A clear and oscillatory outermost overgrowth (O2, 5th event)followed the dusty rim just before the eruption (Fig. 9b). The hygrome-ter indicates an H2O content of 2.3 wt.% in dusty textures (D2).

Plagioclases in lava from the LAG cinder cone present rounded clear(40%, Table 2) or patchy (60%, Table 2) cores (C or P, 2nd event), varyingin composition from An55 to An65 (Fig. 9c) with Plg–liqKdCa–Na = 0.42,clearly out of equilibrium. They are surrounded by an oscillatoryzoned overgrowth (O1, 3rd event), a strong ovoidal dusty rim withPlg–liqKdCa–Na= 0.18 (D2, 4th event) characterized by an An and FeO in-crement (N15–20% An; ±0.7 wt.% FeO), and, finally an outermost lesscalcic overgrowth (O2, 5th event). The hygrometer estimates a high

Fig. 10. Back scattered SEM images and core–rim compositional profiles of An% and FeOwt% of pis presented togetherwithH2Odeterminations performedwith the hygrometer of Lange et al. (2Rift System (NERS); (c) and (d) plagioclase from High Potassium Oligophiric (HKO) lavas from(f) plagioclase from HKP lavas emitted at T5 from the NERS contemporaneously with (g) plagemitted from HKP lavas at T6 on the SRS.


H2O content at the dusty rim (D2 — 2.3 wt.%) decreasing in the secondovergrowth (O2 — 1.3 wt.%).


The plagioclase in the products erupted by the highest part of thefracture (2550 to 2300m a.s.l., T3, Ferlito et al., 2009a) on the NERS pre-sents two distinct types of cores: (1) dissolved rounded cores with anoscillatory LAHF zoning (60%, Table 2), bytownitic in composition(An88–82) (Fig. 10a) and (3) patchy cores (40%, Table 2) with convolutededges and labradoritic composition (An81–An71) (Fig. 10b). A less calcicovergrowth (O1, 3rd event), characterized by an oscillatory zoning rang-ing from An57 to An73, surrounds both cores. The evolutionary pathshows that after nucleation (1st event) plagioclase cores become clearrounded with Plg–liqKdCa–Na = 0.27 or patchy with Plg–liqKdCa–Na = 0.10(2nd event; Fig. 10a, b and Table 2). Hygrometer estimation of H2O con-tent on clear rounded (in equilibrium) cores indicates H2O contents of2.8 wt.% (C), (Fig. 10a). Patchy cores are not in equilibrium with thewhole rock however; the hygrometer in crystal overgrowth estimates1.2 wt.% of H2O in the melt (Fig. 10b).

Plagioclase in lavas and tephra erupted by the intermediate segmentof the fracture (2300 to 2100 m a.s.l., T4 in Ferlito et al., 2009a)(Fig. 10c) shows clear rounded cores (60%, Table 2) with oscillatoryzoning (An69–76), surrounded by an oscillatory and less calcic over-growth (An63–67). After nucleation and growth (1st event), a dissolutionstage occurred (2nd event). Growth is resumedwith an oscillatory over-growth until crystallization ends (from 3rd to 5th event). H2O hygrom-eter estimation suggests a value of 1.4 wt.% in the clear rounded cores(C), slightly decreasing to 0.8wt.% at the overgrowth (O2) (Fig. 10c). An-other type (40%, Table 2) of plagioclasewithin these products consists ofcrystals with oscillatory bytownitic cores (An80–87), rimmed bymelt in-clusions (Mis, 4th event), associated with a drastic drop in An content(ΔAn ~ 15%). An oscillatory overgrowth (O2, 5th event) mantles theoutermost portion of the crystal characterized by less An (An54–62).H2O estimation inside oscillatory-zoned cores indicates a H2O contentof 2.7 wt.% (Fig. 10d).

Phenocrysts embedded in lavas emitted at the lower segment ofNERS (2100–1950 m a.s.l.) during the initial phase of fracture opening(early T5), have oscillatory cores (80%. Table 2) (rarely slightly sieve-textured, 20% in Table 2). Cores are bytownitic (An82–90) and rimmedby melt inclusion alignment (Mi, 4th event) followed by an outermostovergrowth (O2, 5th event) (Fig. 10e). Chemical profiles reveal a strongdecrease in An content corresponding to the melt inclusion alignment(ΔAn ≥ 10%). A second drop in An (ΔAn ≥ 10%) characterizes the out-ermost oscillatory rim reaching labradoritic compositions (An ~ 60)(Fig. 10e). Inner rounded cores are not in equilibrium with the hostrock, and hygrometer estimation indicates an H2O content of 2.1 wt.%in the overgrowth with melt inclusion alignments (Mi — Fig. 10e).

The latest products of this phase have plagioclases with roundedclear cores (C) but with a slightly less calcic composition (An75–83), anoscillatory-zoned overgrowth (O1) surrounds the cores showing anabrupt An drop (An50–58) (Fig. 10f). Path diagram (Fig. 10f) differs tothose of the initial T5 crystal by the absence of melt inclusionalignments. The plagioclase records the initial nucleation and growthof crystal core (1st event), dissolution of the core (2nd event) and anoscillatory overgrowth until crystallization ends (5th event) (Fig. 10band Table 2). Crystals in late T5 (Fig. 10f) indicate remarkably lowerwater content at the clear rounded cores (C— 2.3 wt.%) and a decrease(0.8 wt.%) associated with the overgrowth (O1).

lagioclases emitted during the 2002–2003 eruptive event. The growth path of each crystal009). (a) and (b) Plagioclases fromLowPotassiumOligophiric (LKO) lavas fromNorth EastNERS; (e) plagioclase from High Potassium Porphyritic (HKP) lavas emitted at early T5;

ioclase emitted from HKP lavas emitted from the South Rift System (SRS). (h) Plagioclase



Fig. 10 (continued).





Plagioclases erupted at T5 on SRS, have a clear rounded core (C) similarto those erupted at T5 on NERS; chemical profiles reveal a comparablebytownitic composition (An79–86) (Fig. 10g). The core is surroundedby an oscillatory overgrowth (O1) with a relevant drop in An (ΔAn~ 10%). The lack of melt inclusion alignment in these crystals is also tobe noted. Path diagram shows a first event of nucleation and growth(1st event), an event of dissolution of the core (2nd event) and a lastovergrowth (5th event) (Fig. 10g and Table 2). H2O estimations aresimilar to those obtained in products emitted on NERS at the sametime.Water content in the clear rounded cores (C) is 2.3wt.% decreasingat the overgrowth (O2) to 1.1 wt.% (Fig. 10g).

Plagioclase phenocrysts in products erupted in the following phasesof the eruption on the SRS (T6, T7 and T8 in Giacomoni et al., 2012), arecharacterized by patchy or clear rounded cores (70% and 30% respec-tively, Table 2) with andesinic composition (An ~ 60%). A very thinoscillatory overgrowth (O1, 3rd event) precedes a dusty rim (D2, 4thevent) associatedwith an increase in An content (ΔAn ~ 20%); an outer-most oscillatory rim with labradoritic composition (An56–60) mantlesthe crystal (O2, 5th event) (Fig. 10h and Table 2). The dusty rim is theonly portion of the crystal in equilibrium with the host rock and thedissolved H2O in the melt is 2.2 wt.%.


Plagioclases in lavas emitted in the first phase of the eruption fromthe fracture located at the SE flank of the South East Crater (SEC) aremainly euhedral (90%, Table 2), with oscillatory zoning (Fig. 11a). Crys-tals vary in composition from An88 to An78 at the cores and exhibit aLAHF pattern. Phenocrysts with sieved cores are subordinately present(10%, Table 2); they do not show significant core to rim compositionalvariations. A less calcic overgrowth (An b 70%) characterizes bothclear and sieved crystals at the rim. The path diagram shows a singleevent of nucleation (1st event) followed by an oscillatory-zoned growth(Fig. 7a and Table 2). Hygrometer estimation indicates a constant valueof dissolved water of 3.3 wt.% in oscillatory-zoned crystals (Fig. 11a).

In the lavas emitted from the fracture at Serra Giannicola Piccola(SGP), during the second half of the eruption, plagioclases with sievedcores (S, 2nd event, Fig. 11b) become more abundant (90%, Table 2).In these crystals, the anorthite content varies from An85–An70 at thecores and is surrounded by a less calcic (An74–68) oscillatory-zonedovergrowth (O1, 3rd event). Most of phenocryst rims are characterizedby dusty zoning (D2, 4th event) with an increment in An content(ΔAn ≥ 10%). An outermost overgrowth (O2, 5th event) surroundsthe dusty rims and shows a strong depletion in An content (An70–55)(Fig. 11b). Sieve-textured cores (S) are in disequilibrium with wholerock however, the hygrometer estimates an amount of 3.1 wt.% of H2Oin the dusty zone D2, decreasing to 1 wt.% H2O in the outermost rim(Fig. 11b).


Due to the large number of eruptive episodes and to the complexityof the compound lava field that characterized this multiple eruptiveevent, attention was exclusively focused on the paroxystic episode ofNovember 16th.

Phenocrysts in lavas emitted before November 16th are euhedral(95%, Table 2) and present resorbed dusty rims (D2, 4th event)(Fig. 11d). Oscillatory zoning (Oz) in the core is characterized by aLAHF pattern with An varying from An88 to An70 with several episodesof calcic enrichments. However, most of the oscillations are withinAn80–An85. The oscillatory cores are surrounded by a dusty rim (D2,4th event) associated with an increment in An content (ΔAn ~ 10%). Asubsequent oscillatory-zoned overgrowth (O2, 5th event) mantles thecrystal. H2O content estimation in the dusty (D2) portion of the plagio-clase indicates equilibrium with 2.8 wt.% of dissolved H2O in the melt.Such H2O content decreases to 2.1 wt.% in the outermost rim (Fig. 11d).


Plagioclases in lavas emitted during November 16th showmore cal-cic cores, ranging from An90–80 and characterized by oscillatory zoning(Oz) and sieve textures (S) (Fig. 11d), in proportions of 10% and 90%respectively (Table 2). The HALF oscillatory zoning inside the sievedcores becomes progressively less anorthitic down to An77. This zone issurrounded by a resorbed dusty rim (D2, 4th event) always associatedwith strongAn and FeO (~ΔAn 15%) enrichments. An outermost rim en-velopes the phenocrysts and presents a strong An drop down to An60

(Fig. 11d). The path diagram in Fig. 11d shows four main events: nucle-ation and growth (1st event), sieve textures at the core (2nd event, S),dusty rounded rim formation (4th event, D) and an overgrowth at theoutermost portion of the crystal before eruption (5th event, O2).Hygrometer estimation of H2O dissolved in the melt indicates a valueof 3.0 wt.% in sieved cores (S), gradually increasing to 3.2 wt.% at thedusty rim (D2). The amount of estimated H2O abruptly decreases inthe outermost overgrowth (1.5 wt.%) (Fig. 11d).

6. Discussion

6.1. Relationships betweenmagmatic processes and plagioclase petrologicalfeatures

Plagioclase is a good candidate to monitor the magma ascent dy-namics in the last 12 km, through the various growth and dissolution/resorption episodes occurring when changes in P–T–X conditionsoccur. This approach takes into account several experimental and theo-retical studies that investigated the mechanisms and the kinetic of pro-cesses responsible for crystal growth and stability. Growth kinetic is themain factor influencing the oscillatory zoning in plagioclase (Lofgren,1974a,b; Kirkpatrick et al., 1979; Haase et al., 1980; Lofgren, 1980;Allegre et al., 1981; Lasaga, 1982; Loomis, 1982; Cashman, 1990;Ortoleva, 1990; Wang and Wu, 1995; L'Heureux and Fowler, 1996a,b).Pearce and Kolisnik (1990) first recognized a small and a large-scale os-cillatory pattern (LAHF and HALF, Ginibre et al., 2002; Viccaro et al.,2010) that may be ascribed to kinetic effects and/or minor changes inbulk chemistry or physical parameter of the melt.

Clear rounded edges and dusty zoning plagioclases in equilibriumwith an andesitic melt have been experimentally reproduced byTsuchiyama (1985): crystals become smaller and rounded when Trises above the plagioclase liquidus temperature, while they maintainthe original shape below the liquidus. At constant T, crystal/melt inter-face remains smooth if the crystal is more calcic than the equilibriumplagioclase, while it becomes rough and dusty if the crystal is less calcic.Tsuchiyama (1985) recognized two types of dissolution: (1) a crystaldissolves in the melt which is undersaturated with respect to thephase resulting in clear rounded cores (simple dissolution); and (2) acrystal is partially dissolved to form a dusty plagioclase by reactionbetween sodic plagioclase and calcic melt (partial dissolution).

A few hypotheses have been proposed for the development ofpatchy zoning. Anderson (1984) suggested that patchy-zoned regionscould be formed during oversaturation and crystal growth episodes.Similarly, Kuritani (1999) invoked a skeletal growth rather than a disso-lution process to explain this texture. On the other hand Humphreyset al. (2006) and Ginibre andWörner (2007) proposed that patchy zon-ingmight be acquired duringmajor events of plagioclase destabilizationas repeated episodes of dissolution and re-growth occurring in a con-vective system. Such conclusions are also supported by the experimen-tal work of Hammouda and Pichavant (2000).

Sieve textured plagioclase has been studied by Nelson and Montana(1992), who carried out a series of high-pressure (1200 to 600 MPa)experiments in a high-K andesite suggesting that the density of sievetextures is related to the decompression rate and that plagioclase com-position becomes more albitic with increasing pressure.

Although not all the textural features have been so far experimental-ly reproduced, it is possible to link most of them to a particular event,which in turn is related to a physical–chemical change occurring in



(88)


plg. stable

plg. unstableCore (Oz)

2001 SE-PL (a)

(75)


plg. unstable

2001 C-L

Dusty (D2)

(80)

(76)

0 0.5 1 1.5 2 2.5 3

300

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150

100

50

3.5

350

4

H2O saturation

1.9

3.7

5.6

7.5

11.3

9.4

13.2

plg. stable

2001 Lag

Depth (km

)Dusty (D2)

Overgrowth (O2)

2

(74)

(81)

(56)

(60)

Patchy (P)

1

3

plg. unstable

P (

MP

a)

Overgrowth (O2)

2

1

plg. stable

300

250

200

150

100

50

350

P (

MP

a)

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)

1

300

250

200

150

100

50

350

P (

MP

a)

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)

(b)

(c)

Fig. 12. P/H2O plagioclase stability field in lavas emitted during the 2001 eruptive eventcalculated with MELTS. Plagioclase liquidus (black line) is reported with the expectedcomposition (An%) and temperature. H2O saturation curve has been calculated withSolex software (Witham et al., 2011). (a) Plagioclase in lavas emitted from SE–PL fracture;(b) plagioclase in lavas emitted from Lag cinder cone (c) plagioclase in lavas emitted fromC–L fracture.

Table 2Summary of the recognized plagioclase evolutionary path and recorded events in lavasemitted by the distinct vents or fractures during the 2001, 2002/2003, 2004/2005 and2006 eruptions. The frequency of appearance of each growth path is also reported andexpressed as relative percentage.

Eruptive event Vent or fracture location Recorded events Frequency

2 3 4 5

2001 SE-PL(3040–2940 m a.s.l.)

– – – Oz 90%S – – Oz 5%D1 – – Oz 5%

C-L(2620–2320 m a.s.l.)

– – – Oz 5%Oz – D2 O2 90%P O1 D2 O2 5%

Lag(2620–2320 m a.s.l.)

P O1 D2 O2 60%C O1 D2 O2 40%

2002–2003 NERS(2500–2300 m a.s.l.)

C O1 – Oz 60%P O1 – Oz 40%

NERS(2300–2100 m a.s.l.)

C O1 – Oz 60%Oz – Mi O2 40%

NERS Early T5(2100–1980 m a.s.l.)

Oz – Mi O2 80%S O1 Mi O2 20%

NERS Late T5(2100–1980 m a.s.l.)

C O1 – Oz 100%

SRS T5(2850–2600 m a.s.l.)

C O1 – Oz 100%

SRS T6–T8(2850–2600 m a.s.l.)

P O1 D2 O2 70%C O1 D2 O2 30%

2004–2005 SEC(2920 m a.s.l.)

– – – Oz 90%S – – Oz 10%

SGP(2620–2320 m a.s.l.)

S O1 D2 O2 90%– – – Oz 10%

2006 Pre Nov 16th(3040 m a.s.l.)

Oz – D2 O2 95%S O1 D2 O2 5%

Nov 16th(3040 m a.s.l.)

Oz – D2 O2 10%S O1 D2 O2 90%


the feeding system. In the following sections the current eruptivemodelsof the 2001, 2002–2003, 2004–2005 and 2006 events will be revised andcompared with the plagioclase textural and geochemical features.


Several studies focused on this event have highlighted the involve-ment of various magmas with distinct petrographical and geochemicalfeatures that were erupted by different fracture segments (Viccaroet al., 2006; Ferlito et al., 2008; Coulson et al., 2011).

Plagioclases in SE–PL lavas are oscillatory zoned (1) without complexdissolution/re-growth textures at core or at rim. Crystals begin to crystal-lize between 270 and 200 MPa, that correspond to a depth ranging from10 to 7.5 km in amagma containing 1.6–2.4wt.% of H2O (Figs. 7 and 12a).

Plagioclases in lavas emitted at the C–L fracture display a HALF oscil-latory zoning ranging from An81 to An45. Such low anorthite content, indisequilibrium with the melt (as indicated by Kd Plg–liqKdCa–Na), sug-gests that these cores crystallized in a cooled and degassed magmaand could be interpreted as antecrysts recycled from a previously in-truded magma batch. A dusty rounded zone (1) envelopes the corethus recording a reaction of the antecrysts with an incoming morebasic (H2O-rich) melts. Such mixing processes probably occurred be-tween depth 6–4 km (Fig. 12b). The outermost overgrowth (2) formedafter decompression and volatile loss at lower pressure (crystallizationat P b 40 MPa) (Fig. 12b).

Plagioclases in lavas emitted at the LAG have patchy cores (1) thatare in disequilibriumwith the hostmagma. According toMELTSmodel-ing they could be formed at pressure, water content and temperature of350–300 MPa, 1.0–1.5 wt.% and 1050–1070 °C respectively. Bothdisequilibrium and low crystallization temperature suggest than an

Fig. 11. Back scattered SEM images and core–rim compositional profiles of An% and FeO wt.% ofeach crystal is presented together with H2O determinations performed with the hygrometer offrom lavas emitted at the SGP fracture; (c) plagioclase in lavas emitted before November 16th


antecrystic origin cannot be excluded for these cores. The cores areenveloped by a dusty zone (2), associated with a strong increment ofAn and FeO, suggesting a reaction with a more mafic (H2O-rich)magma at a pressure ranging from 190 to 130 MPa, i.e. 6.3–4.0 km(Fig. 12c). An outermost overgrowth (3) formed at very shallow pres-sure (b50 MPa) in equilibrium with volatile-poor, i.e. a degassed melt(1.0–1.3 H2O wt%).

The study of the plagioclase of the 2001 eruption supports theevidence of a magma mixing between a shallow, cold and amphibole-bearing magma with an incoming more basic melt. Mixing probablyoccurred between 6 and 4 km of depth and increased the explosivestyle at the Laghetto cinder cone (Ferlito et al., 2009b).


Several studies focused on the role of the NE Rift feeding system andtwo distinct models were elaborated: (1) the eruption was driven by a

plagioclases emitted during the 2004/2005 and 2006 eruptive events. The growth path ofLange et al. (2009). (a) Plagioclase from lavas emitted at the SEC fracture; (b) plagioclase; (d) plagioclase in lavas emitted during November 16th paroxystic event.



(82)


2002 LKO T3

Patchy (P)

Overgrowth (O2)

(80)

Clear (C)

0 0.5 1 1.5 2 2.5 3

300

250

200

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100

50

3.5

(82)

350

P (

Mpa

)

H2O wt H2O wt4


1.9

3.7

5.6

7.5

11.3

9.4

13.2

plg. unstable

2002 HKO T3

Clear (C)

Melt Alignment (MI)

Depth (km

)

plg. stable

(86)

(80)

Core (Oz)

34

(73)

Overgrowth (O1)

1

2

(83)


plg. unstable

2002 HKP T5+T5S

Clear (C)

plg. stable(85)

(74)

(87)

(79)


plg. unstable

2002 T6+SRS

Dusty (D2)

plg. stable

(73)

300

250

200

150

100

50

350

P (

MP

a)

(a)

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)1

2

34

(77)

(86)plg. stable

plg. unstable

0 0.5 1 1.5 2 2.5 3 3.5 4

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)

300

250

200

150

100

50

350

P (

Mpa

)

2Overgrowth (O1)

(b) (d)

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)

300

250

200

150

100

50

350

P (

Mpa

)

1

1

2

3

4 Melt Alignment (MI)

Overgrowth (O1)

(c)

Fig. 13. P/H2O plagioclase stability field in lavas emitted during the 2002/2003 eruptive event calculated with MELTS. Plagioclase liquidus (black line) is reported with the expectedcomposition (An%) and temperature. H2O saturation curve has been calculated with Solex software (Witham et al., 2011). (a) Plagioclase in LKO lavas emitted during T3 at NERS;(b) plagioclase in HKO lavas emitted during T4 at NERS; (c) plagioclase in HKP lavas emitted on early T5 (black areas) at NERS, at T5 at NERS and contemporaneously at SRS(gray areas); (d) plagioclase in HKP lavas emitted at T6 on SRS.


hydraulic fracturing and draining of the magma occupying the upperportion of the central conduits (Andronico et al., 2005); and (2) theNE Rift was fed by an independent system, which allowed the ascentand storage of small magma batches (Ferlito et al., 2009a).

Plagioclase in LKO lavas (Ferlito et al., 2009a) have two core types(Fig. 10a,b): clear rounded cores (1) nucleated at lower pressure(180–120 MPa) and water content between 2.3 and 2.7 wt.%, andpatchy cores (2) crystallized at 250 to 200 MPa and water contentvarying between 1.7 and 2.3 wt.%. Both core types are in equilibriumwith the whole rock and are followed by an overgrowth occurred atshallower pressure (40 and 20 MPa; 3 and 4 respectively) after decom-pression and volatile loss (Fig. 13a).

Plagioclases in HKO (Fig. 13b) lavas present different cores: i) clearcores (1) crystallized at 280–230 MPa with H2O content varyingbetween 1.5 and 2.0 wt.%; ii) oscillatory cores (2) nucleated between170 and 120 MPa and H2O content ranging between 2.3 to 2.7 wt.%.Clear cores are followed by an oscillatory overgrowth (3) crystallizedat very shallow pressure with water content ranging between 0.8and 1.3 wt.%. Oscillatory-zoned cores are enveloped by an alignmentof melt inclusions (4) formed at low pressure and H2O content(b50 MPa, 1.4–1.8 wt.% H2O) due to rapid volatile loss.

The stability conditions indicate that plagioclases in lavas eruptedduring the early T5 phase at NERS nucleated and grew at pressuresstraddling the volatile saturation curve (150–70 MPa, Fig. 13c) withwater content from 2.6 to 3 wt.% (1). Further overgrowth with align-ment of melt inclusions (2) was promoted by decompression and vola-tile loss at ca. 100 MPa and 2–2.3 wt.% of H2O. Plagioclases in lavaemitted during late T5 at NERS and contemporaneously at SRS grew atdeeper pressure (260–180 MPa) and water content ranging from 1.7to 2.3 wt.% (3) (Fig. 13c). An increase in water content induced plagio-clase liquidus depression and crystal rounding. A new growth stageoccurred only after a fast decompression (4) that induced volatile lossand undercooling at very low pressure b20 MPa (Fig. 13c).


Patchy cores in subsequent eruptive phase (T6 to T8) are not inequilibrium with the whole rock. They are surrounded by a dustyzone (1) crystallized between 250 and 200 MPa in a more basic andvolatile-rich (1.8–2.3 H2O wt%) magma (Fig. 13d). Subsequent over-growth (2)waspromoted bydecompression and volatile loss at shallowdepth b20 MPa (Fig. 13d).

The presence of clear rounded cores (C) in the 2002–2003 productscomplicates the interpretation of plagioclase evolutionary path, intro-ducing a dissolution event, which can be associated with an increaseof H2O in the system. As shown above for the 2001 eruption input of amore basic and/or hotter magma in equilibrium with a more anorthicplagioclase causes the reaction and resorption of the pre-existingmore albitic crystal core, as testified by a dusty texture. A clear roundedcore cannot be explained by the same process, but suggests thatmagmabecomes suddenly undersaturated in plagioclase causing the dissolu-tion of the pre-existing crystal (Tsuchiyama, 1985). However, neithertextural nor chemical evidences for a mixing with such a primitivemelt were observed. Thus, it is most likely that the clear dissolved coresare associated with an increase of volatiles in the system, which is notnecessarily related to the input of new magma (see Section “Volcanofeeding system and magma storage”).

Plagioclase differences indicate that distinct magma batches(LKO and HKO) intruded below the NE Rift between 6 and 2.3 km atdepth (Fig. 13a, b). Plagioclase in lavas erupted in the early phase ofT5 indicates a shallow crystallization in H2O-rich magma. Such highH2O content delayed the appearance of plagioclase, which becomes sta-ble only after a massive volatile loss and decompression due to fractureopening (Fig. 13c). In lavas erupted contemporaneously on theNE and SRifts at late T5 similar clear rounded cores suggest a commondeep feed-ing system at an approximate depth of 9–6 km. The presence of dustyrims in plagioclase emitted on the S Rift from T6 to the end of the erup-tive event indicates that several inputs of basicmagma occurred and fedthe activity (Giacomoni et al., 2012).



0 0.5 1 1.5 2 2.5 3

300

250

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)

H2O wt4

1.9

3.7

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9.4

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1

2

0 0.5 1 1.5 2 2.5 3 3.5 4

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3.7

5.6

7.5

11.3

9.4

13.2

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)

300

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Mpa

)

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5.6

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11.3

9.4

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250

200

150

100

50

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Mpa

)


plg. unstable

2004-2005 SEC

plg. stable (86)

Core (Oz)

Overgrowth (O1)(88)

(83)

Sieved (S)


2004-2005 SGP

(86)

(87)

(a)

(b)

1.9

3.7

5.6

7.5

11.3

9.4

13.2

Depth (km

)

300

250

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150

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50

350

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Mpa

)

Overgrowth (O1) 2

1

3

plg. stable

plg. unstable

(83)


2006 Pre 16th Nov

Core (Oz)(86)

(87)

(83)

(86)

plg. unstable

plg. stable1

2

3

Overgrowth (O2)

Dusty (D2)


2006 16th Nov

Sieved (S)(80)

(88)

Dusty (D2)(85)

(83)

(74)(60)

(d)

plg. stable

plg. unstable

1

2

3

Overgrowth (O1)Dusty (D2)

H2O wt

(c)

Fig. 14. P/H2O plagioclase stability field in lavas emitted during the 2004/2005 and 2006 eruptive events calculated with MELTS. Plagioclase liquidus (black line) is reported with the ex-pected composition (An%— circles). H2O saturation curve has been calculated with Solex software (Witham et al., 2011). (a) Plagioclase in lavas from SEC fracture in lavas of 2004/2005eruption; (b) plagioclase from SGP in lavas of 2004/2005 eruption; (c) plagioclase in lavas emitted before November 16th during the 2006 eruption and (d) plagioclase in lavas emittedduring the November 16th paroxysm of 2006 eruptive event.



This effusive eruptive event occurred after 20 months of inactivityand was characterized by the absence of significant geophysical signalspreceding and accompanying the eruption onset (Bonaccorso et al.,2006; Di Grazia et al., 2006). Corsaro et al. (2009) presented a petrologicstudy on sequential samples collected throughout the eruption. Theauthors supported the model in which a fractionated magma intrudedin the shallow portions of the lateral feeding system during the 2002–2003 eruption and was progressively mixed with magma rising alongthe central conduits. In SEC lavas (Fig. 14a), plagioclases began to crys-tallize at 130–60 MPa from a magma containing 2.6–3 wt.% of H2O (1).Such observation suggests that nucleation occurred in an undegassedmagma intruded in a lateral feeding system linkedwith the central con-duits just before the eruption. The effect of an increasingH2O-content inthemagma is to prevent plagioclase nucleation at depth. The outermostoscillatory-zoned overgrowth (2) is likely associatedwith depressuriza-tion and volatile loss that stabilize a more albitic crystal. Plagioclases inlavas outpoured in the subsequent phase of the eruption (SGP fracture,Fig. 14b) formed at a pressure varying between 200 and 150 MPa (1),with H2O-content ranging from 2.4 to 2.7 wt.% (Fig. 11b). A dusty re-sorbed rim (D2) often envelopes sieved cores (2) suggesting a reactionwith a more basic magma during the SGP phase. Subsequent over-growth (O2-3) occurred as a consequence of a second episode of volatileloss and decompression (Fig. 14b).

Plagioclase textures, geochemical features and stability MELTSmodel suggest a scenario that fairly agrees with the model presentedby Corsaro et al. (2009). Eruption was initially fed by a shallow andvolatile-rich magma becoming progressively mixed by an incomingmore primitive melt. Pre-existing long lived phenocrysts recorded atleast two episodes of degassing that occurred in the magma stored inthe central open conduits (Ferlito et al., 2012).



The flank collapse of the sub-terminal SEC that occurred onNovember 16th has been the subject of numerous studies that proposedtwo contrasting triggering mechanisms. The first model presented inBehncke et al. (2008) and Behncke (2009) explains the observed violentexplosive activity as due to a superficial interaction between lavaflowing from the top of the SEC and themoisture-rich tephra constitut-ing the cone. An alternative explanation was presented by Ferlito et al.(2010), where the authors associate the explosive emission of themagma with the rapid opening of the ESE–WNW oriented fracture atthe base of the crater. In lavas emitted before November 16th plagio-clase is mainly oscillatory zoned (1) and MELTS modeling suggeststhat it crystallizes between 170 and 110 MPa. Some phenocrysts havedusty resorbed rims (2), formed between 130 and 60 MPa, suggestingan input of basic magma between 5 and 3 km. Outermost rims(3) formed after decompression and volatile loss, during final magmauprise (b50 MPa) (Fig. 14c). Plagioclases in lavas emitted during No-vember 16th have sieved cores (1) formed at pressures of 170 to80 MPa. A dusty zone surrounds the cores (2) suggesting a reactionwith a more basic and undegassed magma at very shallow depth (70–30 MPa). Subsequent overgrowth (3) occurred, after decompressionand H2O loss, at P b 50 MPa (Fig. 14d). According to MELTS modeling,plagioclase records the arrival of a slightly more basic magma, whilemarkedly basic magmas were erupted only during the paroxysticepisode.

6.6. Volcano feeding system and magma storage

Following Rittman (1973) no major long-lived crustal reservoirsexist beneath Etna, even though some magma ponding appears to benecessary to explain magma differentiation (Allard et al., 2006 and



Fig. 15. Sketched reconstruction ofMt. Etna volcano feeding system, superimposedon the stratigraphy as deduced from the seismic studyof Finetti (2005). In the right column thedepth ofcrystallization of themain plagioclase textures at the cores and at the rims is plotted, together with clinopyroxenes from 2002/2003 event (this study) and Armienti et al. (2007). Verticalbars represent the range of depth of the tomographic low velocity zones recognized by (1) Bonaccorso et al. (2011); (2) Lundgren et al. (2003); (3) Murru et al. (1999); (4) Sharp et al.(1980) and (5) Di Stefano and Branca (2002). The corresponding depth is calculated on an average rock density of 2.7 g/cm3 and expressed as below the summit (3000 m a.s.l.).


references therein). Small and shallow reservoirs probably intruded be-neath the rift before the 2002–2003 eruption (Ferlito et al., 2009a).These magma batches can be envisaged as a network of dykes andsills (e.g. Guest and Duncan, 1981; Murray, 1990), as it is clearly ob-served in ancient eroded volcanic edifices outcropping in Valle delBove (Branca and Del Carlo, 2004; Ferlito and Nicotra, 2010). This isalso supported by the chemistry of the lavas that are poorly evolved,implying that magmas must rise quite rapidly from the mantle.

Early seismic refraction studies suggested that at 20–25 km belowthe volcano there is a large zone with about 14% of molten rocks,accounting for an approximate magma volume of 1600 km3 (Sharpet al., 1980). However,more recent tomographic studies did not confirmthe presence of such deep magma reservoir (Patanè et al., 2006). At adepth of 12 ± 3 km, Murru et al. (1999) identified a smaller low-velocity zone, which can be interpreted as another level of magmastorage, most probably corresponding to the boundary between sedi-mentary cover and crystalline basement. The same authors recognizeda second shallower low velocity zone at about 5 ± 2 km, located atthe discontinuity between the Hyblean carbonate platform and theFlysch units (Lentini, 1982). The latter geological unit is probably thesource of the quartz-arenitic xenoliths frequently found in Etneanlavas (Clocchiatti et al., 2004; Ferlito et al., 2009a,b; Coulson et al.,2011). These three recognized levels represent only themost prominentportions of the feeding system below the volcano, which can be consid-ered almost continuous from the mantle to the surface.


Further seismic tomograms revealed thepresence of a vast high veloc-ity zone, probably composed by plutonic rocks that extend down to adepth of about 15 km b.s.l. and that increase its width with depth(Hirn et al., 1997; Chiarabba et al., 1999; Laigle et al., 2000; Rollin et al.,2000; Patanè et al., 2003; Chiarabba et al., 2004; De Gori et al., 2005;Patanè et al., 2006). This domain, located south-east of the Central Craters,represents the fossil feeding system of the past of Mt. Etna activity.

In Fig. 15a tentative reconstruction of the geological section beneathMt. Etna is reported, based on the CROP Project (CROsta Profonda —

Deep Crust) results (Finetti, 2005). According to Armienti et al. (2013)clinopyroxene starts crystallizing at mantle depth and continues upto a shallow depth of about 10 km. At similar crustal pressure alsoplagioclase begins its crystallization history; the deepest crystals arecharacterized by patchy textures (14–11.5 km); clear rounded corescrystallize between 9.3 and 5 km, followed by melt alignment anddusty textures at the rim (4.5–2.7 km). Crystal overgrowths and meltinclusion alignments form over the volatile saturation depth, above2.8 km and inside the volcanic edifice.

Crystallization and development of patchy cores, rare in recentproducts (Viccaro et al., 2010), are in disequilibrium with the hostrock and can be considered as antecrysts whose texture can be associat-ed with repeated magma inputs. They formed at the same depth of thelow velocity zone recognized by Murru et al. (1999) (12 km). Thedeepest patchy cores cannot be formed in equilibrium with water con-tent higher than 1.7 wt.% (Figs. 12c and 15).




Clear rounded cores characterize most of the plagioclase in the2002–2003 eruptive event (Ferlito et al., 2009a; Giacomoni et al.,2012) and crystallize between 9.5 and 5 km, corresponding to thedepth of precursor earthquakes (Andronico et al., 2005) with a watercontent ranging between 1.5 and 2.8 wt.%.

Dusty rims are common in all the studied plagioclases, testifying therelevance ofmagmamixing in the Etnean feeding systemand the abilityof plagioclase to record such processes although involvingmagmaswithsimilar compositions. Depth estimations suggest that these rims canform between 4.5 and 2.7 km, across the H2O-saturation depth.

The alignments ofmelt inclusions formed above 2.5 kmof depth andwell above the H2O saturation level. Melt inclusion alignments are fre-quent in products emitted on the NERS during the 2002–2003 event.The outermost albitic overgrowth is associated with the last degassingphase and can occur in syn-eruptive conditions.

The overall picture that can be drawn putting together plagioclasetextures and seismic anomalies supports the non-existence of majorcrustal magma chambers, but depicts a vertically extended and contin-uous feeding systemwhich leadsmagma from themantle to the surface.Short-lived ponding reservoirs may exist where the tectonics is less in-tense, slowing the magma ascent or even preventing the eruption(Ferlito et al., 2009a; Giacomoni et al., 2012).

Another important outcome from this study is that plagioclase is sta-ble and/or can grow in a H2O-richmagma only at relatively low pressureaccordingwith the experimental data of Métrich and Rutherford (1998).Ab-rich plagioclase cores that nucleate at depth (12 km) can be inequilibrium only with a basaltic magma containing H2O b 1.5 wt.%, asalso suggested by Lanzafame et al. (2013). Alternatively, these corescould crystallize from a more evolved melt (mugearitic–benmoreiticcomposition). However, such evolved melts have neither been eruptedduring the period considered nor in historical time, thusmaking unlikelythis second scenario. Our findings must be discussed against the MI data(Métrich et al., 2004; Spilliaert et al., 2006), which indicate the presenceof H2O-richmagmas at high depth (e.g. H2O 3.4 wt.%). These contrastingevidences can be reconciled by only admitting that magmas within theEtnean feeding system are enriched in H2O. Since Etnean basalts withsimilar composition are produced by comparable degrees of mantlepartial melting (Corsaro and Cristofolini, 1996; Peccerillo, 2005; Alesciet al., 2013), it is highly implausible that they can be originated withsuch drastically different H2O contents.

Furthermore, magma differentiation is unable to significantlyincrease water in Etneanmagmas, since fractionation percentage is rel-ative low and its effect onwater increase is negligible (0.2 wt.% of waterevery 10% of fractionation, Nichols et al., 2002).

This evidence implies that magmas are enriched in H2O after theirformation and very likely within the feeding system. It is therefore nec-essary to envisage an independent contribution of volatiles (e.g. H2O)that migrate upward and enrich the originally water-undersaturatedmagma ponding at various levels within the feeding system (Ferlitoet al., 2014).

7. Conclusions

Plagioclase is a ubiquitous mineral in magmatic products, with alarge spectrum of compositions and textural features. Here we presenta systematic study on textures and compositions of plagioclases fromlavas of four significant, well studied and monitored eruptive eventsrecently occurred on Mt. Etna volcano. Different compositions and tex-tures have been related to physico-chemical conditions of the system,which have been further constrained with thermodynamic modelingand compared with results from experimental studies. The estimationof the intensive variables of the system (P–T–fO2) and the H2O content,allowed the reconstruction of the parental magma composition andplagioclase stability fields. We have therefore attempted to associatemagmatic processes (e.g. decompression, magma mixing, degassing orvolatile influx) with specific textures and compositions; finally a


comparison with volcanological evolution of each eruption has beencarried out. A good correspondence between plagioclase-inferred mag-matic processes and the volcanological evolutionary models put for-ward by several authors was found.

For the studied period, plagioclase textures and compositionsindicate that crystallization occurs in a polybaric system, suggesting avertically developed and rather continuous plumbing system beneaththe volcano edifice, which would rule out the presence of significantmagma chambers. A relatively small number of plagioclases nucleateat greater depth (12 km), with a composition in equilibrium withwater-poor magmas. Most of the plagioclases nucleate at pressure of200–250 MPa, corresponding to the depth (5–6 km) at which basalticmagmas reach saturation level for H2O that is consequently exsolved.Interestingly the deformation pattern reconstructed for the 2008 erup-tion was strongly controlled by magma overpressure generated by gasexsolution at this level, underlining the significant contribution ofvolatile in the eruptive style of the volcano (Aloisi et al., 2011).

These two levels recognized through plagioclase textures and com-position match the low-velocity zones found by the seismic tomogra-phy at about 12 ± 3 and 5 ± 2 km, probably corresponding to thebase of the sedimentary cover and the Numidian Flysch respectively.

The plagioclase stability fields also indicate a large variability inwater contentwithin themagmatic system. This confirms that primitiveEtnean melts are water-poor and that the amount of water increasesduring magma ascent to the surface. Fractionation is not a feasiblemechanism to account for this enrichment. Volatile fluxing can be aviable mechanism but further studies are necessary to preciselyconstrain the physico-chemical parameters that control this process(Ferlito et al., 2014).

In summary it is demonstrated the relevance of the systematic studyof plagioclase texture and composition to constrain volcanologicalprocesses on basaltic magmas in open conduit volcanoes. This study isa first attempt that can be extended to future eruptive events taking ad-vantage thatMt. Etna volcano is extremely well monitored as one of themost active volcanoes in the word (more than twenty paroxysmicevents from August 2010 to February 2013).

Acknowledgments

Authors are thankful to the national PRIN 2012 (Piano Ricerca diInteresse Nazionale — “Volatile transfer at convergent plate margins:linking COH fluids/melts heterogeneities to tectonic anomalies in sub-duction zones”) funding that supported the research. A special thanksto Raul Carampin for his priceless expertise and competence during insitu mineral analyses at the Microprobe Analytical Laboratory of IGG-CNR (Padua, Italy). Two anonymous referees and the editor in chiefare also acknowledged for their constructive criticism, which substan-tially improved a previous version of this manuscript.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.earscirev.2014.06.009.

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plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: the 2001–2006...

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