Lithos 184–187 (2014) 447–455

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The volatile flushing triggers eruptions at open conduit volcanoes:Evidence from Mount Etna volcano (Italy)

Carmelo Ferlito a, Massimo Coltorti b,⁎, Gabriele Lanzafame a, Pier Paolo Giacomoni b

a Università di Catania, Dipartimento di Scienze Geologiche, Corso Italia 57, I-95129 Catania, Italyb Università di Ferrara, Dipartimento di Fisica e Scienze della Terra, Via Saragat 1, I-40400 Ferrara, Italy

⁎ Corresponding author.E-mail address: clt@unife.it (M. Coltorti).

0024-4937/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.lithos.2013.10.030

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 April 2013Accepted 25 October 2013Available online 5 November 2013

Keywords:FlushingMg-rich magmaPlumbing systemVolatilesGas plume

At the Mount Etna volcano (Italy) the massive release of magmatic gasses (especially H2O, CO2 and SO2) duringexplosive eruptions and through the persistent gas plume, raises important issues: i) the volume of magmaerupted at given periods is insufficient to feed the gas plume of the related periods; ii) gas-dominated, explosiveeruptions do not emit differentiated products, but relatively Mg-rich magmas; iii) H2O measured in melt inclu-sions (~3.5 wt.% at about 0.4 GPa) exceeds what is expected from intraplate mantle-derived melts (b1.4 wt.%).Literature data onmelt inclusions (MIs) along with textural and compositional analyses of plagioclase and phasestability constrained by MELT calculations, enabled us to build a model that reconciles the abovementioneddiscrepancies with the high variability of the water content along the Etnean feeding system.Wepropose that along anopenmagma conduit, continuous gas loss from the free surface ofmagma at depth pro-motes an almost steady stream of H2O-rich fluid extending well below the gas saturation depth. The velocity ofvolatile migration might be enhanced by the huge amount of CO2 present at Mount Etna, because the CO2

exsolved at high-pressure conditions may act as carrier for other volatile species.In this way the H2O-undersaturated primitive magma present in the plumbing system, may undergo a volatile“flushing” proportional to the residing time. This process is fundamental to increase the originally low H2O con-tent of primitive magma (≤1.4 wt.%) allowing it to overcome the saturation threshold, to exsolve as gas and topromote the eruption of Mg-rich lavas. Such mechanism would also account for the unexpectedly high amountof magmatic water released during non-eruptive periods.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Mount Etna is themost active and largest (40 kmwide, 3340 mhigh)volcano in Europe. It is located on the eastern coast of Sicily at the bound-aries of three structural domains: the Appennine–Maghrebian overthrustbelt to the North, made up of marly clays, marly limestones and quartz-arenitic rocks; the northern margin of the Hyblean block to the South,made up of a thick limestone and dolomite sequence (Grasso et al.,1982; Lentini, 1982; Pedley and Grasso, 1992) and the Ionian lithosphereto the East, overlaid by a thick sedimentary cover (Fig. 1a) (Cristofoliniet al., 1985; Monaco et al., 1997; Scribano et al., 2009).

After an initial period (from 500 to 200 ka ago) characterized by theemission of a small amount of tholeiitic to transitional products, themagma composition shifted to the Na-Alkaline field with a prevalenceof hawaiites and mugearites (Fig. 1b). The compositions of themost re-cent products (after 1971) display a general shift towards mild potassicterms classified as Trachybasalts (Armienti et al., 1989; Clocchiatti et al.,1988; Condomines et al., 1995; Corsaro and Cristofolini, 1996; Peccerilloand Lustrino, 2005; Tanguy et al., 1997; Tonarini et al., 2001). However,

ghts reserved.

K-poor products have been emitted recently (2002 event; Ferlito et al.,2009a; Giacomoni et al., 2012), whereas K-rich lavas have characterizedthe products of the ancient Ellittico center and more recent pre-historiclavas (Ferlito, 1994; Ferlito and Lanzafame, 2010).

The volcanic activity ofMt. Etna includes a variety of eruptive behav-iors, from purely effusive to violent Strombolian. The difference in erup-tive styles cannot be simply ascribed to magma composition, which isroughly homogeneous, but more likely to different amounts of dis-solved volatiles in magmas and to degassing behavior in the shallowplumbing system (see Ferlito et al. 2008; 2009b for the 2001 event).Concerning volatiles, continuous gas release from the summit cratersand from peripheral fumaroles is observed, making Mt. Etna one ofthe largest SO2 emission sources even during non-eruptive periods(Allard, 1997; Allard et al., 1991; Pennisi and Le Cloarec, 1998). Mea-surements of the SO2 volcanic plume and of CO2 soil emissions indicatea SO2 outflux between 840 and 21,000 t d-1 (average 5600 t d-1 duringnon-eruptive periods) and a CO2 average output of 13 ± 3 Tg yr-1

(Allard et al., 1991).The large amount of volatiles emitted in the atmosphere by many

volcanoes on Earth (see Shinohara, 2008; Wallace, 2003 and referencestherein) raises questions on the gas/magmabudget, and in particular onthe discrepancy between the large amount of volatiles released during

Fig. 1. a)Mapof the regional and local structural setting ofMt. Etna (fromGiacomoni et al., 2012). Lavafields of S andNE rifts of the 2002–2003 eruptive event are also represented. b) Totalalkali vs silica diagram (Na2O + K2O and SiO2 in wt%): (1) picrite, (2) basalt, (3) basaltic-andesite, (4) dacite, (5) tephrite, (6) Hawaiite or trachy basalt, (7) mugearite or basaltic trachy-andesite, (8) benmoreite or trachy-andesite, (9) trachyte, (10) phono-tephrite, (11) tephri-phonolite, (12) phonolite, (13) nephelinite.

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degassing and the comparatively small quantity of magma effectivelyemitted. Several models have been proposed to explain such a discrep-ancy, including degassing of non-eruptedmagma (Di Muro et al., 2008;Edmonds et al., 2001; Roberge et al., 2009; Rose et al., 1982), breakdownof sulfur-bearing phases on eruption (Devine et al., 1984), mixing ofmagmas with different redox conditions (Kress, 1997), and thepresence of an excess of vapor prior to eruption (Wallace and Gerlach,1994).

At Mount Etna the high volume of volatiles emitted at the summitcraters during eruptive and non-eruptive periods is not justified bythe relatively small amount of erupted magma, as also observed inother volcanoes. Moreover, several studies point out that explosive vol-canism is not associated with the emission of differentiated productsbut, on the contrary, involves primitive Mg-rich magmas (Ferlito et al.,2009b; Kamenetsky et al., 2007). Finally, the measured contents ofH2O inmelt inclusions (Métrich et al., 2004) largely exceed the amountsexpected from intraplate mantle-derived melts even for a relatively hy-dratedmantle (see Section 2.1). These key issues need careful investiga-tions aimed to shed new light on the role of magmatic volatiles withinthe volcanic feeding system, with implications for eruption predictionsand hazard assessment.

In this work we present i) a critical reappraisal of the existing litera-ture, with emphasis on melt inclusion data and SO2 flux; ii) a study ofplagioclase textures and stability in relation to physical-chemicalcondition of the feeding system; and iii) a petrological modeling of themaximum H2O produced by partial melting of a fertile intraplatemantle, comparable to the sub-Etnean mantle.

The various inconsistencies underlined above can be reconciledwitha large-scale migration of volatiles, which can ultimately be seen as aviable mechanism for eruption triggering.

This new way of considering the feeding system at Mount Etna hassignificant implications: i) deciphering the geophysical signals relatedto themagmatic activity, ii) interpreting the present-day volcanic activ-ity, characterized by short explosive and violent paroxysmal as well asseveral month-lasting lava effusions and iii) conceiving a model ableto predict future eruptions.

2. Volatile-related issues in open-conduit volcanoes

2.1. The origin of magmatic water

The origin of magmatic gasses (particularly H2O) is a major topicwhen studying volcanoes. At Mount Etna, the relative amount of gasspecies dissolved in magmas has been estimated by means of studiesof melt inclusions (MIs) entrapped in olivines, in which S constitutes~6%, whilst H2O is ~80% of the total fluid (Métrich et al., 2004 and

Fig. 2. H2O content in melt inclusions in olivines from 2001 and 2002 eruptions vs. pressure of2004; Spilliaert et al., 2006); the black line is the liquidus line nucleating plagioclase, accordinanorthite contents, which are stable at given pressures and H2O contents and at T N 1010 °C.

Spilliaert et al., 2006). Olivine found in recently erupted products con-tains MIs with extremely high H2O and CO2 contents (3.5 wt.% and4000 ppm, respectively; see data by Métrich et al., 2004 and Spilliaertet al., 2006). They cluster in two groups (Fig. 2), which have beentrapped at pressure up to 400 MPa and lower than 100 MPa. Such ahigh amount of water in magmas is usually associated to slab dehydra-tion in subduction settings, where water is conveyed in the mantlewedge lowering its solidus and facilitating melting. As a matter of factTonarini et al. (2001) based on the study of B isotopes, suggested the in-volvement of slab components in the Etnean lavas and Schiano et al.(2001) identified an increase in 87Sr/86Sr and K2O in lavas emittedafter 1971 as a shift from alkaline towards calc-alkaline, subduction-related products. However, subduction occurs underneath Calabria(Faccenna et al., 2011) and slab dehydrates beneath the Eolian islands(ca. 100 km north-west of Etna), whereas the slab subduction beneathMount Etna is still debated since there is neither physiographic nor geo-physical evidence of this (e.g. lack of Wadati-Benioff zone and trench,Finetti, 2005). On the other hand, the K2O variation in Etnean magmascan be explained by i) melting of a mantle source variably enriched involatile-bearing minerals such as amphibole and/or phlogopite (Alesciet al., 2013; Beccaluva et al., 1998; Viccaro and Cristofolini, 2008) or ii)supercritical fluids carrying alkali Cl-complexes migrating from thedeeper to the shallower portion of the plumbing system (Ferlito andLanzafame, 2010).

In themantle only negligible amounts ofwater are stored in pyroxeneand olivine (Xia et al., 2010). Additional volatile reservoirs in the mantleare represented by pargasite/kaersutite and phlogopite, which are usual-ly present as accessory phases, even when concentrated in veins. Atmantle depth amphiboles are mainly dehydrogenated and can store upto 2 wt.% ofH2O (cf. Tiepolo et al., 2000),whereasH2O in phlogopite rare-ly exceeds 3 wt.% (Righter et al., 2002). According to these constraints,water contents in excess of 3 wt.% in primary mantle melts could notbe obtained even by totally melting of amphibole and/or phlogopite.Moreover in this latter case primitive basalts with K2O N 5 wt.% wouldbe produced, which is implausible since the maximum K2O content inEtnean basic lavas is ~2%. On the other hand, Etnean primitive composi-tions (Mt. Maletto lava, Tanguy et al., 1997) can be modeled by melting~50% amphibole and ~10% phlogopite (Alesci et al., 2013). Assumingthe most favorable conditions, i.e. water behaves as totally incompatibleand volatile-bearing phases are fully released into the magma, a maxi-mum water content of 1.4 wt.% can be obtained.

Magmadifferentiation is likewise unable to significantly increase thewater content of Etnean basalts, which are characterized by Mg# ~55(Mg# = MgO/(MgO + FeO) mol%). Most lavas are basic and haveMg# ranging between 53 and 63; in order to be considered in equilibri-um with a fertile mantle assemblage (Ol Fo88–89), they need to be re-

entrapment and corresponding depth in km beneath summit craters (from Métrich et al.,g to simulations performed with MELTS (Ghiorso and Sack, 1995); gray circles represent

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equilibrated by adding an assemblage of Ol 61–70% and Cpx 29–38% fora total fractionated material of 10–15% (Alesci et al., 2013; Giacomoniet al., 2013), as suggested by the Fe/Mg partitioning between olivineand liquid (Roeder and Emslie, 1970). Considering an increase in0.2 wt.% of water every 10% of fractionation (Nichols et al., 2002), awater content of 3.5 wt.%would be produced bymore than 50% of crys-tallization, yielding a residual melt withMg# ~45, which is significantlylower than the values found in Etnean basalts. Therefore the high watercontent in Etnean olivineMIs cannot be interpreted as a primary featureof mantle melting or due to crystal fractionation. Finally external sourceof H2O and CO2 due to the interaction with the surrounding rocks can-not be excluded, but low values of δ18O (5.4 ± 0.3) in Etnean lavas,comparablewithMORB (Marty et al., 1994), suggest that decarbonationor water influx from external source are not effective mechanisms atMount Etna.

2.2. The persistent gas plume issue

The proportion of water among the volcanic gas is difficult to beassessed. However, the relative proportion of the SO2 flux (~ten timesless abundant than H2O in wt%), measured for monitoring purposeswith the COrrelation SPECtrometer (COSPEC) during the period 1987-2001 indicates a discharge of 5000 t d−1 of SO2 in the plume (Aiuppaet al., 2007, 2010, 2011; Allard et al., 2006). The volatiles releasedfrom the summit craters (Fig. 3) are largely volcanic and, based on thecommon held view, below the exsolution depths volatiles aretransported upwards as components dissolved within the uprisingmagma. This means that huge amounts of magma should be broughtabove these critical depths (7-8 km for H2O; 3-4 km for SO2, or b1 kmfor Cl and F) to determine the massive persistent degassing occurringat the summit craters. A SO2 flux of about 5000 t d-1 at the summit cra-ters (D'Alessandro et al., 1997), considering an average S content dis-solved in magma of ~0.2 wt.% (Métrich et al., 2004; Spilliaert et al.,2006), requires that 3 ∗ 105 m3 of magma should rise above gas exsolu-tion depth every day.

The existence of this large daily input of magma is questionable. Thedifference between the volume of degassing magma necessary to feed

Fig. 3. Photo of the persistent gas plume emitted by the summit craters of Mount Etna (particmagma (solid line, PDM),whose degassing is necessary to sustain themeasured SO2flux in thepline, EEM) from September 1987 to March 2000 is represented. PDMwas calculated accordingerage 5000 tons/day), S = initial sulfur content in the magma 0.29 wt.% (from Spilliaert et al.,vative conditions).

the gas plume and the volume of magma effectively erupted in a givenperiod is impressive: from 1987 to 2001 ~2 km3 of magma shouldhave undergone degassing (presumed degassed magma — PDM;Fig. 3), while only b0.5 km3 of magma was effectively erupted (effec-tively eruptedmagma— EEM; Fig. 3). Similarly, on the basis of radionu-clide activity of 210Pb, 210Bi and 210Po, Le Cloarec and Pennisi (2001)proposed that between 1983 and 1995 only 15–20% of the degassingmagma was erupted, which means that the volume of degassedmagma exceeds 4–5 times the volume of the erupted magma. This evi-dent paradox is often overlooked or explained with a mechanism ofmagma convection in the conduit (Stevenson and Blake, 1998) as away to bring volatiles to the surface without erupting their carriermagmas. The convective recycling model proposed by Dixon et al.(1991) for Kilauea suggests that degassed magma is recycled down inthe deep plumbing system due to differential density and mixed againwith CO2-rich magma prior to eruption. Allard (1997) suggested a sim-ilar mechanism for Mt. Etna, proposing that convectively degassedbatches of dense basalt contribute to the growth of a subvolcanic plu-tonic complex. However, the solution of magma convection has somesignificant weakness: 1) it does not explain why new magma batchesemplaced near the surface lose their gas content but do not erupt;2) the strong undercooling induced by the loss of volatiles promotesa massive crystal nucleation, which would solidify in the uppermostportion of the conduit, hindering its recycling in dike-like conduits(Giacomoni et al., 2013; Mollo et al., 2012); 3) analogously, if theH2O content in the near-surfacemagmawould reach the saturation con-centration, its loss during non-eruptive degassing should promote crys-tallization and solidification within the conduit, causing the closure ofthe open-conduit; 4) if all the magma risen near the surface would un-dergo complete degassing (implausible for the inertia and viscosity ofthe system) an enormous amount of cold and dense basalt (3 ∗ 105 m3

of magma every day) will have to be accommodated within the volcanicedifice or just beneath it, which would imply a continuous and impres-sive inflation of the volcano, far larger than what is effectivelymeasured. In fact, the plutonic complex invoked by Allard (1997) assubvolcanic reservoir for the left-over cold basalts, recently recognizedin seismic tomography (Patanè et al., 2006), is far too small to justify a

ular of the NE crater). In the diagram the cumulative curve i) of the presumed volume ofersistent gas plume and ii) the estimated volume of the effectively eruptedmagma (dottedto the formula of Allard (1997): V = (0.5 ∗ F)/S ∗ P ∗ (1 − Xc), where F = SO2 flux (av-2006), p = density of magma 2.7 g/cm3 and Xc = crystal content 0% (to assume conser-

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sustained summit degassing. Finally, the convective recycling mecha-nism still fails to explain the gas flux observed by Aiuppa et al. (2007)and cannot account for the high CO2/SO2 gas ratio observed prior tothe 2004–2007 activity (Collins et al., 2009).

2.3. Plagioclase as monitor of water saturation of the magma

Plagioclase is a common mineral phase in the alkaline lavas of Mt.Etna and it forms during polybaric crystallization along the cotecticOl–Cpx–Plg (Armienti et al., 2004; Tonarini et al., 2001). Due to itshigh sensitivity to temperature (T), pressure (P), melt composition(Xmelt) and water content (XH2O), chemical and textural zoning of pla-gioclase has been used to track and quantify the processes occurringin the shallow feeding system of active volcanoes (Crabtree andLange, 2010; Davidson and Tepley, 1997; Ginibre and Wörner, 2007;Lanzafame et al., 2013).

Plagioclase in Mt. Etna lavas is usually euhedral in shape with disso-lution–regrowth textures more rarely dendritic or H-shaped (Viccaroet al., 2010). Plagioclase from historic and recent lavas (2001–2006eruptive period, Giacomoni et al., 2012; 2013) present two dissolutiontextures that can be used to track processes occurring in the shallowfeeding system: (i) dissolved rounded bytownitic cores (An85–75),with labradoritic (An60–55) overgrowth (Fig. 4) and (ii) partially dis-solved dusty rims with high An content (An80–90) overgrowth.

The kinetics of plagioclase dissolution has been experimentally in-vestigated by Tsuchiyama (1985) who distinguished simple and partialdissolution. Simple dissolution occurs when plagioclase is surroundedby a melt that is plagioclase-undersaturated; hence the crystal startsdissolving in the melt. Partial dissolution occurs when a plagioclase issurrounded by a melt in which a more An-rich plagioclase is stable. Inthis case dissolution occurs by a reaction of the crystal with the melt,

Fig. 4. SEM image of a plagioclase erupted during the 2002–2003 event with dissolvedAn-rich core and albitic overgrowth (from Fig. 4 of Viccaro et al., 2010); the white solidline indicates the trace of the anorthite content's profile reported below. Compositionsare analyzed by EMP.

which penetrates the crystal edge forming channels and melt pocketsresulting in a “dusty” texture. Contemporaneously a new plagioclasegrows within the reaction zone with a composition in equilibriumwith the new melt (more An-rich).

Partial dissolution with dusty rims can be easily interpreted as aconsequence of mixing between slightly evolved magma (hawaiites)residing in the shallower portions of the feeding system (4–6 km ofdepth) with an incoming more basic and hotter melt.

On the other hand simple dissolution is found exclusively incrystal cores suggesting that plagioclase reacted with a plagioclase-undersaturatedmelt at depthwhere Etneanmagmasmaybe reasonablyconsidered thermally and compositionally more homogeneous.

To constrain the conditions of plagioclase dissolution in the MountEtna feeding system we performed a MELT model using a commonEtnean hawaiite at pressure between 4000 and 500 bars and decreasingtemperature from liquidus (1270 °C) to 1050 °C. Oxygen fugacity hasbeen fixed at QFM + 1 accordingly with mineral-melt equilibriacalculation methods applied to Mt. Etna lavas (D'Orazio et al., 1998;Giacomoni et al., 2013; Mollo et al., 2011). Results can be summarizedin the P/H2O plagioclase liquidus line in Fig. 2. It is evident that plagio-clase stability and composition are highly influenced bywater dissolvedin magma that depolymerizes the melt structure stabilizing a moreanortitic plagioclase at lower pressure. The whole effect is that at con-stant temperature, in a H2O-rich magma, plagioclase that crystallizes atshallower depth has amore calcic composition (Fig. 2). MELT calculationsuggest that dissolved rounded bytownitic cores could have crystallizedin H2O-undersaturated magma (1.5–2 wt.%) between 2500 and 2000bars (~9–7.5 km). Calculations performed using the hygrometer ofLange et al. (2009), after plagioclase/melt equilibrium check followingPutirka (2005), suggest a comparable content of H2O (1.2–1.8 wt.%) inhawaiitic magma from 2002 to 2003 eruptive event at P of 2,500 barsand T of 1130 °C. These results argue against plagioclase crystallizing atthis depth with a water content N2.0 wt.%, as indicated by the MI mea-surements (Métrich et al., 2004; Spilliaert et al., 2006).

We propose that at such conditions themost efficient mechanism topromote dissolution of plagioclase is the addition of H2O which can ef-ficiently undersaturate the melt in plagioclase thus promoting the rec-ognized simple dissolution at cores as also suggested by Blundy andCashman (2001), who experimentally observed that dissolution tex-tures could originate when plg-undersaturated magma come close toH2O saturation.

3. The idea of volatile flushing

3.1. The effect of volatile flushing in the plumbing system

Since the last segment of magma ascent to the surface is controlledby extensional tectonics, it is possible to envisage the uppermost por-tion of the Etnean plumbing system as made up of dikes, either singleor grouped in large swarms (cf. Ferlito and Nicotra, 2010 and referencestherein). Hints for depth and geometry of the intermediate plumbingsystem are provided by Murru et al. (1999), who identified two low-velocity zones at depths of 12 ± 3 km and 5 ± 2 km b.s.l. These shal-low systems are fed by a deeper main magma reservoir, identified at adepth of 20–25 km b.s.l. (Sharp et al., 1980). Information about thesize of this reservoir is provided by the distribution of peripheral gasemission sites, located up to 60 km away from the summit craters,where high fluxes of magmatic CO2 are recorded (Collins et al., 2009;Nuccio et al., 2008), associated to high heat fluxes (up to 100 mW/m2;Cataldi et al., 1995).

The permanent degassing activity in open conduit volcanoes impliesthat the deep, intermediate and superficial portions of the plumbingsystem are necessarily interconnected forming one main feedingsystem, constantly filled with magma and through which volatiles canmigrate upwards (Fig. 5). The effect of volatile migration in magmahas been described in Ferlito et al. (2008) and Ferlito and Lanzafame

Fig. 5. Scheme of the present-day plumbing system beneath Mount Etna volcano (modified after Ferlito and Lanzafame, 2010). Intracrustal reservoirs and the levels of exsolution for thevarious gas species are also reported.

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(2010) who pointed out the role of Cl-rich fluids in transporting K andother metals from the lower to the upper portion of the plumbing sys-tem. Here we speculate that the continuous loss of gasses from thefree surface of the system can enhance volatile migration from below.This provides an almost steady stream of volatile extending wellbelow the gas saturation depth, where volatiles do not form a free gas-eous phase. The way in which volatiles at the ionic state move in a mol-tenmagma is still notwell understood; some available data suggest thatthe velocity of ionic diffusion is apparently too low (Behrens, 2010;Schmidt et al., 2013). However, the large quantity of gas emitted dailyby the summit craters (e.g. SO2 rate measured before 2001 was5000 t/day; H2O amount can be estimated as 10 times SO2), can be jus-tified only by an equivalent amount of volatiles involved in the migra-tion, even if below the saturation depth the velocity of volatiles ationic state is obviously lower than the velocity of the gas phase, risingas bubbles. The large quantity of CO2 (Mount Etna alone is responsible~10% of global Earth CO2 emissions, Allard et al., 1991; Aiuppa et al.,2006) could also play a significant role in volatile migration(Yoshimura and Nakamura, 2013). In fact CO2 becomes exsolved andstarts bubbling at a considerable depth (cf. Barsanati et al., 2009 and ref-erences therein). This rising CO2-rich gas can change the H2O–CO2 equi-librium in the magma affected by the CO2 flux, thereby enhancing theH2O exsolution at depth and causing a local loss of H2O from the system(Armienti et al. 2013; Collins et al., 2009; Dixon et al., 1995; Liu et al.,2005; Papale, 1999) or an enrichment depending on the quantity ofCO2 passing through the system.Microscopic CO2 gas bubbles might ac-celerate the ionic diffusion of H2O below the saturation depth (Dallaiet al., 2011) and could provide preferential ways for water migration,probablyworking as carrier for theOH−, due to the high electronegativ-ity of the hydroxiles (Mysen and Virgo, 1986).

Notwithstanding the striking evidence that volatiles migrate mas-sively through the plumbing system, the precise mechanism of theirmigration is still far from being fully understood (see also Yoshimuraand Nakamura, 2011; 2013). The resolution of this problem will

necessitate a long series of experimental work and goes far beyondthe scopes of this work.

3.2. Fluid flushing and eruption triggering

Along its way up water can be retained in undersaturatedmelts; thepassage of H2O through the plumbing system and its absorption bywater-undersaturatedmagma is here defined as “flushing” and the effi-ciency of the absorption depends on the residing time of the magma(Fig. 6). Under the effect of H2O flushing the undersaturated magmasbehave like sponges absorbing volatiles, which will convert an H2O-undersaturatedmagma, unable to rise and erupt, into a magma saturat-ed in H2O and ready, as a perturbation occurs (e.g. earthquakes), toabundantly exsolve gasses, reach boiling conditions, overpressurize,and eventually erupt. Deeper magmas retain a higher quantity of vola-tiles before achieving saturation, which means that deeply flushedmagmas can originate strong explosive eruptions still maintainingprimitive compositional features (e.g. Mg-rich) (Fig. 6). In other wordsif we imagine the magma as a gun, the fluid flushing will load its car-tridge. The capacity of the cartridge is strictly correlated to the depthat which the magma resides.

This mechanism would also explain the MIs distribution into twoclusters (Fig. 2): the deepest MIs are hosted in olivine growing duringvolatile flushing at high pressure (12 ± 3 km b.s.l.), while the otherMIs are trapped in olivine during degassing. Analogously thismechanismcould account for the sharp decrease of SO2 flux in the persistent gasplume observed after the 2001 and 2002–2003 eruptions (from 5000to 1000 t d−1; Allard et al., 2006): before these two powerful eruptiveevents the magma filling the plumbing system underwent long flushingand therefore it was unable to retain large amounts of fluids, which es-caped through the uppermost plumbing system and fed the persistentgas plume at the summit craters (SO2 flux 5000 tons/day; Allard et al.,2006). As an example just before the 2001 eruption the magma withinthe plumbing system was deeply flushed, i.e. filled with volatiles and

Fig. 6.H2O content vs pressure of magma batches and corresponding depth in km beneath summit craters; dotted arrows represent different paths of H2O enrichment. We can observe avariety of behaviors between two endmembers: the flushing-absent path (a), which would lead to a scarce gas accumulation, insufficient to trigger eruptions and therefore will produceintratelluric crystallized bodies and the H2O enrichment path (c), which would enrich, in proportion to the residing time, magma batches residing at intermediate pressures (350–250 Mpa). In fact, shallower batches will undergo gas exsolution and H2O loss, whereas deeper batches will not have resided for sufficient time to increase the H2O content. It is also im-portant to consider that this range of depth corresponds with the ipocenters of the earthquakes preceding themain explosive eruptions (see text for references). Finally, the intermediatesteady flushing path (b), can be considered as themodality inwhich the volatilesmigrate throughout the plumbing system to feed the steady state degassing at the summit craters, whichis by far the most significant eruptive activity for the Etnean volcano.

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unable to retain more volatiles, in other words the “sponge” wasdrenched and the volcano was ready for two vigorous lateral eruptions.After the 2001 and 2002–2003 events the fresh undersaturated magmathat substituted the erupted one in the plumbing system was like a drysponge able to copiously absorb volatiles, which therefore were notemitted in the plume (SO2 flux decreased to 1000 t d−1; Allard et al.,2006). The progressive increase in SO2 flux in recent years (3000 t d−1

Caltabiano pers. comm.) indicates that themagma in the feeding systemis gradually becoming more saturated and therefore less able to retainvolatiles. The increasing saturation of the system is also revealed by themany short and violent episodes associated with lava fountains, whichduring 2011 and first half of 2013 (see National Institute of Geophysicsand Volcanology — INGV reports, www.ct.ingv.it) have been character-ized by a nearly monthly frequency.

4. Conclusions

Themigration of volatiles from depth provides the volatile budget tofeed the persistent gas plume emitted at the summit craterswithout theneed of an equivalent volume of shallow degassing magma. H2O flush-ing also justifies the dissolution textures in plagioclase: crystals nucleateand grow in primitive H2O-undersaturatedmagmas; theH2O influxwilllower the plagioclase liquidus, crystals will no longer be in equilibriumand will dissolve. When H2O exsolves (superficial and/or eruptive con-ditions) crystal growth restarts, but a less anorthitic plagioclase is stabi-lized. Significant evidence of the effect of volatile flushing in the Etneanplumbing system has lately been presented by Aloisi et al. (2011), whocollected deformation data of the Etnean edifice in the period 2008–2010 by a continuously operating GPS network of 36 stations. Thesemeasurements (carried out for the first time on Etna) enabled the au-thors to track the vertical migration of the inflation source. Surprisingly,the source of inflation did notmove from the lower to the upper portionof the plumbing system, as would be expected assuming an uprisingmagma. Instead, inflation moved from the upper to the lower portionof the plumbing system (from ~4 to ~9 km below the central craters

in two years). Such a behavior can be explained by the upward move-ment of volatiles (especially H2O), which during their migration havebeen retained by volatile-poor magmas, until exsolution thresholdwas overcome. Depending on pressure, gas exsolution will be firstreached at lower H2O content, by the shallow residing magma (4 kmbelow the summit craters; Aloisi et al., 2011) and later, at increasingwater content, in the lower portions of the feeding system (down to9 km below summit craters; Aloisi et al., 2011).

The persistencyof theplume suggests that volatilemigration throughthe plumbing system occurs as a continuous flux. However, earthquakesof moderate to high magnitude can liberate considerable frictional heatduring seismic slip, which could raise the temperature at the focalplane to 1450 °C (Bizzarri, 2009; Di Toro and Pennacchioni, 2004). Thisheat, localized on a vertical volume of the plumbing system, could en-hance volatiles upward migration by promoting deeper exsolution.Deep earthquake swarms (25–15 km b.s.l.) might therefore be relatedto eruptions occurring weeks–months later (Patanè et al., 2003). As anexample, the July 2001 eruption was preceded in April by a seismicswarm with hypocenters between 10 and 6 km b.s.l. (Bonaccorsoet al., 2004). Fluid migration associated with the earthquakesenriched the magma located at 6 km b.s.l. that erupted in July(Ferlito et al., 2008, 2009a,b). Deep seismic swarms could thereforespeed up the volatile influx of magma batches, bringing them closeto H2O-saturation and triggering an eruption within a short timespan.

In conclusion, fluid flushing is the simple answer to the “excess” ofwater in magmas measured at Mount Etna as well as in other volcanoes(cf. Shinohara, 2008) and offers a model for understanding magma dy-namics prior to eruption.We trust that future andwell-aimed interdisci-plinary studieswillmonitor volatilemigration and flushing oversteppingthe difficult problem of predicting eruptions. Mount Etna is amongthe best-studied volcanoes in the world but the flushing modelcould be applied to many other volcanoes (cf. Shinohara, 2008), inparticular those characterized by open-conduit system and excessof volatile budget.

454 C. Ferlito et al. / Lithos 184–187 (2014) 447–455

Acknowledgments

Authors are indebted to Alberto Zanetti and Paola Comodi for shar-ing unpublished data and their expertise on volatile-bearing minerals.Three anonymous referees and the editor in chief are also gratefullyacknowledged for their constructive criticisms, which substantially im-proved a previous version of this manuscript.

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