Plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: The 2001–2006 eruptive period at Mt. Etna

Earth-Science Reviews 138 (2014) 371–393

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Plagioclase as archive of magma ascent dynamics on “open conduit” volcanoes: The 2001–2006 eruptive period at Mt. Etna P.P. Giacomoni a,⁎, C. Ferlito b, M. Coltorti a, C. Bonadiman a, G. Lanzafame b a b

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

a r t i c l e

i n f o

Article history: Received 16 July 2013 Accepted 29 June 2014 Available online 14 July 2014 Keywords: Plagioclase Etna Feeding system Intensive variables Crystal growth Crystal textures

a b s t r a c t Plagioclase is the most common phenocryst in all Etnean magmatic suites (~50% in volume), as well as in most lavas erupted worldwide. Its stability field is strongly dependent on the physico-chemical conditions of the melt 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 emitted during the 2001, 2002–2003, 2004–2005 and 2006 eruptions. Four distinct textures were recognized at the crystal cores: (1) clear and rounded (An73–85), (2) dusty and rounded (An73–85), (3) sieved (An82–88) and (4) patchy (An60–81), while two distinct textures are commonly observed at the crystal rim: (1) dusty (An73–90) and (2) with melt inclusion alignments (An70–76). Moreover all plagioclases present a thin (10–20 μm) outermost less calcic (An53–76) rim. For each crystal a complex evolutionary path was reconstructed, 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 melt and, in turn, to reconstruct the primitive magma composition by adding a wehrlitic assemblage to the least evolved 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, plagioclase and spinel, decreasing T from the liquidus down to 1000 °C at steps of 20 °C. P–T and water contents were also determined using geothermobarometers and plagioclase–melt hygrometers respectively, aiming at verifying the parameters used in the MELTS modeling. At this point plagioclase textural features and compositions were related to specific P–T–fO2–H2O conditions. Plagioclase stability models indicate that: (1) H2O strongly influences the plagioclase–melt equilibrium allowing the crystallizations of more calcic compositions only at shallow levels; (2) patchy cores form at high pressure (up to 350 MPa) and low water content (b 1.7 wt.%); (3) clear dissolved cores form at lower pressure (150 MPa) and higher water content (1.5–2.8 wt.%); (4) dusty rims form at even lower pressure straddling the H2O-saturation curve and, (5) melt alignments form during degassing. According to experimental works each of 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 and degassing (melt inclusion alignment at rims). These inferences were successfully compared with the eruptive evolution of each event as deduced from direct observations, and geophysical and petrological data. The overall picture shows that plagioclase crystallizes under polybaric conditions in a vertically extended and continuous feeding system in which at least two magma crystallization levels were identified. Plagioclase stability also indicates that a large variability in water content characterizes the magma within the feeding system. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Volcanological evolution of 2001–2006 eruptive events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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

http://dx.doi.org/10.1016/j.earscirev.2014.06.009 0012-8252/© 2014 Elsevier B.V. All rights reserved.

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3.1. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 3.3. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 3.4. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Intensive variables and parental magma composition . . . . . . . . . . . . . . . 4.1. Oxygen fugacity and primary magma . . . . . . . . . . . . . . . . . . . 4.2. Temperature and pressure . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Water content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. MELTS phase stability modeling . . . . . . . . . . . . . . . . . . . . . . 5. Plagioclase textures, compositional profiles and water estimates . . . . . . . . . . 5.1. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 5.3. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 5.4. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Relationships between magmatic processes and plagioclase petrological features 6.2. 2001 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. 2002–2003 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 6.4. 2004–2005 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . 6.5. 2006 eruptive event . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Volcano feeding system and magma storage . . . . . . . . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Plagioclase is the most common mineral phase crystallizing in basaltic magmas. Several authors have shown that its abundance, chemistry and texture reflect the composition of the magma and the crystallization conditions (T, P, XH2O, fO2) (Lofgren, 1980; Tsuchiyama, 1985; Cashman and Marsh, 1988; Nelson and Montana, 1992; Landi et al., 2004; Viccaro et al., 2010; Iezzi et al., 2011). Such variables may change during magma permanence in intratelluric conditions or ascent to the surface thus causing remarkable variations in the growth of plagioclases and therefore in their final morphology and composition. In particular, temperature decrease and loss of H2O during degassing will promote crystal nucleation and growth, while temperature and/or H2O increase, due to magma mixing and/or volatiles flushing (Ferlito et al., 2014), will induce dissolution of crystals. Thus phenocrysts in the lava are the result of a complex and 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 the scientific community have been devoted in disentangling this story to reconstruct magma dynamics as well as the geometry of the feeding system. Usually, such reconstruction is biased by too many assumptions; especially when attempts are done for past eruptions or for eruptive events of remote volcanoes. In fact, only a few volcanoes are heavily monitored to have an array of seismological and ground deformation data that allow constraining the timing of the eruptive events and magma ascent. In the last 15 yrs the Istituto Nazionale di Geofisica e Vulcanologia (INGV) has developed an instrumental network on and around Mt. Etna that allows an efficient monitoring during the eruptive events as well as the quiescent periods. Together with these favorable conditions Mt. Etna has been recently characterized by important eruptive events (2001, 2002– 2003, 2004–2005 and 2006), which have provided a variable spectrum of activity, from purely effusive to strongly explosive, and different duration times, from days to several months. Given these premises we choose to study the plagioclase phenocrysts produced during these eruptive events and to relate their compositions and morphological features to magma conditions within the feeding system. Plagioclases have been analyzed in 150 samples from the above-mentioned eruptions. Morphological textures have been studied by high contrasting imagery with a

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scanning electron microscope (SEM) and mineral compositions have been determined through core to rim profiles with an electron microprobe (EMPA). This very detailed study allowed us to recognize several growth modalities within each individual crystal that in turn were related to different magmatic processes. Even though Mt. Etna is one of the most studied volcanoes in the world, a large consensus is not yet reached on physical–chemical crystallization conditions. Most of the uncertainties come from the lack of primitive lava composition and in the determination of oxygen fugacity (fO2) conditions. The latter was determined by using Plg–Cpx/liquid equilibrium (France et al., 2010) allowing the reconstruction of the original Fe3+/Fe2+ ratio in the melt and in turn the primitive magma composition by backward fractionation until equilibration with peridotitic mantle paragenesis. Then MELTS simulations (Ghiorso and Sack, 1995) were performed to constrain the mineral phase relationships and composition, particularly regarding plagioclase, during magma differentiation at different pressures and dissolved water contents. Results were compared with estimates obtained by means of the clinopyroxene–liquid geothermometer of Putirka et al. (2003) on clinopyroxenes. T estimated on clinopyroxene rims or microphenocrysts coexisting with plagioclase were then used to predict the melt–water content by using the hygrometers of Putirka (2005) and Lange et al. (2009), assuming that the most basic magma erupted during each event was in equilibrium with the plagioclase core.

2. Geological setting Mt. Etna is a 3340-m-high stratovolcano located on the eastern coast of Sicily, covering an area of over about 1418 km2 (Tanguy et al., 1997), and with its continuous activity represents the most important active volcano in Europe. The complex volcanic edifice is grown at the intersection of (1) two major fault belts trending NNW–SSE (Tindari–Letojanni– Malta) and NNE–SSW (Messina–Giardini) and (2) three structural domains: (1) to the north, the Peloritani mountain range corresponding to the Apennine–Maghrebian belt which extends westward; (2) to the south, the undeformed northern margin of the African plate constituted by the Hyblean Plateau and represents the foreland which 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 products shift toward a marked Na-alkalic character (cf. Tanguy et al., 1997). Several distinct stratovolcanoes formed ~130 ka BP (Catalano et al., 2004) (Ancient Alkaline Centers, AAC, cf. Romano, 1982), which evolved into edifices of considerable dimension, whose remains are still visible in the Valle del Bove area. Finally, all ancient edifices were buried by the

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lava flows and tephra originated by the activity of the Ellittico, the biggest volcanic center that appeared on the Etnean region. The activity continued with the “Recent Mongibello” (Romano, 1982), which started ~15 ka BP, displaying a wide range of eruptive styles from effusive and mildly strombolian to sub-plinian. The volcanic pile, resting on the top of the sedimentary substratum with an elevation of about 1300 m a.s.l., reaches the thickness of about 2000 m in the central portion of the edifice. A reconstruction of

Fig. 1. (a) Regional geological map of Sicily showing position of Mt. Etna and main structural features; (b) map of 2001 eruptive fissures, vents and lava flows. SE-Pl: South East Crater-Piano del Lago, LAG: Laghetto vent, C-L: Calcarazzi vent; (c) map of 2002–2003 Northeast Rift eruptive fissures, vents and lava flows; (d) map of 2002–2003 Southeast Rift eruptive fissures, vents and lava flows; (e) 2004 eruptive fissures and lava flows; (f) 2006 eruptive fissures and lava flows.

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the hidden morphology allowed the estimation that the total bulk of the Etnean volcanics sums up to 373 km3 (Neri and Rossi, 2002). Most of this volume (at least two/third) can be referred to the activity occurred in the last 100 ky (Catalano et al., 2004). The periodic eruptions of poorly evolved magma, the persistent degassing from summit craters together with geophysical data (i.e. deformation of the edifice, persistent shallow seismic tremor) suggest that the feeding system can be considered as an “open conduit” persistently filled with magma. Geological, geophysical and geochemical evidences support the hypothesis that the structure of the plumbing system consists of a plexus of dikes and sills, that can constitute a series of magma batches at a depth of 3–5 km b.s.l, not connected to the surface (Cristofolini et al., 1985; Allard, 1997; Patanè et al., 2003; Allard et 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 persistent activity of Mt. Etna, four eruptive events encompassing different eruptive behaviors, from purely effusive to strongly explosive, were chosen. Moreover, these events have been studied in detail by numerous researchers and are the subject of several multidisciplinary works. This provides an indirect opportunity to test the hypothesis put forward by studying plagioclase textural and compositional features. In order to elucidate eruptive dynamics, the following section will present a summary of the volcanological and geophysical characteristics of each eruptive episode. More detailed descriptions of these events can be found in Viccaro et al. (2006), Ferlito et al. (2009a,b), Corsaro et al. (2009), Ferlito et al. (2010) and Ferlito et al. (2012). 3.1. 2001 eruptive event The 2001 eruption occurred in the southern sector of Mt. Etna between July 13th and August 9th 2001, and produced an estimated volume of about 25 × 106 m3 of lava and 7 × 106 m3 of tephra (Behncke and Neri, 2003; Clocchiatti et al., 2004). It was characterized by two eruptive fissures that were active simultaneously. In the high slopes 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 Laghetto area (PL), the fissure system was trending NNW–SSE (SE–PL system,

Fig. 1b). Lava fountaining was present on the upper part, while lava flows developed in the lower part. A N–S oriented fissure formed at 2100 m near Calcarazzi and migrated upwards reaching the Laghetto area at 2550 m (C–L system) with strombolian activity and lava flows (Fig. 1b). The two fissure systems crossed at Piano del Lago forming a distinct and highly explosive vent (LAG) (Viccaro et al., 2006) (Fig. 1b). All phases of the eruptive event were preceded and accompanied by seismic swarms; below the SE Crater, foci depths were located between 4.5 and 7.0 km below the vent (Alparone et al., 2004), while below C–L and LAG vents earthquakes had deeper foci (about 8 km below the vent, Monaco et al., 2005). Magma emitted throughout the eruption was trachybasaltic in composition (Fig. 2). At C–L fracture it was slightly more evolved, low porphyritic with amphibole phenocrysts (Clocchiatti et al., 2004; Viccaro et al., 2006) that are uncommon in the recent Etnean activity. Magma from SE–PL fracture was instead highly porphyritic and characterized by the occurrence of olivine, clinopyroxene and abundant plagioclase. Products emitted by the LAG cone are low porphyritic and similar to the C–L in terms of phenocryst compositions and modal proportions. Amphibole was also present but smaller in size and with significant resorbed glassy rims (cf. Viccaro et al., 2006; Ferlito et al., 2008, 2009b). 3.2. 2002–2003 eruptive event This eruptive event was bilateral, involving both southern (South Rift System, SRS) and northern (NE Rift System, NERS) flanks of the volcano simultaneously. It started on October 26th and ended on January 28th. During the eruption an estimated volume of 32–44 106 m3 of lavas and 50–60 106 m3 of pyroclastic material was emitted (Andronico et al., 2005). The activity was preceded and accompanied by a seismic swarm with hypocenters distributed below the summit area which moved northeastwards after the first days and remained stationary for one entire week underneath the Pernicana fault (Monaco et al., 2005 and references therein). At NERS the entire eruptive fracture opened in less than 14 h and was composed of three main eruptive segments at an elevation of 2500–2300 m, 2300–2100 m and 2100–1950 m, each of them erupted distinct magma types (Ferlito et al., 2009a) (Figs. 1c and 2). A hawaiite (Low Potassium Oligophyric, LKO magma, Ferlito et al., 2009a) was emitted from the uppermost segment (2500–2300 m a.s.l.), with a

Fig. 2. Total alkali vs Silica classification diagram (Le Maitre, 2002) of erupted lavas from 2001, 2002/2003, 2004/2005 and 2006 eruptive events. Melt inclusion compositions from Spilliaert et al. (2006) and Métrich et al. (2004) are plotted with Maletto and Montagnola primitive lava compositions and starting compositions of MELTS simulation (2006 mantle equilibrated basalt and 2002/2003 trachybasalt) composition and the 2002/2003 trachybasalts.

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Porphyritic Index (P.I.) of about 12% and phenocrysts made up of Ol (10%), Cpx (35%) and Plg (55%). On the other hand, trachybasaltic magma 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; Ferlito et al., 2009a) and phenocrysts composed of Ol (10%), Cpx (30%) and Plg (60%). Activity ceased on this side of the volcano on November 3rd. At SRS lava fountains and strombolian activity started on October 26th, while lava flow emission occurred on the 27th contemporary with the opening of the lowermost segment of the NERS (Ferlito et al., 2009a). The activity on the southern flank continued until January 28th. During these three months activity underwent sudden increments with numerous episodes of gas explosions and lava fountaining, in particular those of November 12th, November 30th and December 18th (Andronico et al., 2005) (Figs. 1d and 2). This magma had textural features perfectly comparable to those erupted at the lower segment of NERS but with slightly less evolved composition (Ferlito et al., 2009a, b; Giacomoni et al., 2012). 3.3. 2004–2005 eruptive event Eruptive activity resumed after 20 months of quiescence on September 7th 2004 and lasted for about 6 months with a total estimated volume of emitted lava ranging from 40 to 60 × 106 m3 (Neri and Acocella, 2006). The eruption onset was neither heralded nor accompanied by recorded precursory signals, such as seismicity and ground deformation, nor by explosions at the summit craters (Burton et al., 2005). It started from an articulated fracture zone, which extended ESE from the Southeast Crater (SEC) toward the rim of Valle del Bove over a length of about 200 m (Fig. 1e). A small SE-directed lava flow poured out at 2920 m a.s.l. and stopped few hours later. A new fracture and effusive vent opened at 2620 m a.s.l. on September 10th and fed a lava flow expanding toward the Serra Giannicola Piccola ridge (SGP) (Fig. 1e). Then, the fracture zone propagated downslope and a new effusive vent opened at about 2320 m a.s.l., close to Serra Giannicola Grande. 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 reached about 1600 m a.s.l. On March 8th, the eruption ended. Plg, Cpx, Ol and Ti-magnetite form the mineralogical assemblage of products emitted during this eruptive event, with limited variability in the mineral proportions throughout the entire eruption. All products are trachybasalts with an average SiO2 of 47.92 wt.%, with the most evolved magmas being erupted before September 24th (Corsaro et al., 2009).

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edifice began to be affected by rock sliding, while the volcanic tremor amplitude increased (cf. Behncke et al., 2008). At 6:15 UTC, eruptive activity began near the summit of the SEC with strong strombolian activity, accompanied by gray ash and steam explosions, and lava flows directed toward the Valle del Bove. At 14:00 a series of collapse episodes started, culminating at 14:28 when a few-meters-high billow of brown ash-laden steam was observed at the base of the niche (~3050 m a.s.l.), immediately joined by another ca. 20 m upslope. The billows evolved into ~150-m-high brownish plumes. These were immediately followed by an ESE–WNW oriented fracture opening along the gully (Fig. 2 in Ferlito et al., 2010) and a series of explosions, which gave rise to a 300-m-high eruptive curtain, bearing juvenile and lithic clasts. The curtain was sustained for a short time (a little more than 1 min) and then collapsed, giving rise to a gravity driven pyroclastic flow, which moved downslope along the flank of the SEC down to ~2800 m. After the November 16th peak, the eruption continued with alternating phases of effusive and strombolian activity. Four vents were fed at variable rates until the end of the eruption on December 15th, 2006. Lavas are porphyritic trachybasalts (Fig. 2) with phenocrysts composed of a variable amount of Plg (10%), Cpx (8%), Ol (5%) and Ti-magnetite (3%). Products are quite homogeneous, with slightly more basic features (SiO2 b 47 wt.%) in those emitted during the November 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). Almost invariably, Etnean rocks display the same mineral assemblage made of plagioclase, clinopyroxene, olivine, Ti-magnetite ± apatite and ±amphibole (Tanguy et al., 1997). Contrary to the hawaiites, Etnean lavas contain lower TiO2, and higher Al2O3 and CaO contents. In recent times (after the 1971 eruption), the lavas show a tendency toward higher K2O/Na2O ratio leading to classify them as trachybasalts. However, K-rich lavas are also present in 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 during magma uprise makes the reconstruction of the primary composition rather 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 recent alkaline erupted products are less evolved with Mg# varying between 63.4 and 49.0. None of these compositions, however, result in equilibrium with mantle peridotite.

3.4. 2006 eruptive event 4.1. Oxygen fugacity and primary magma After the 2004–2005 event in Valle del Bove, the activity resumed on July 14th 2006 on the eastern flank of South East summit Crater (SEC). Activity continued by lava effusion and lava fountaining until December 15th with a total estimated volume of emitted products of 13 × 106 m3. Two vents opened on July 14th emitting lava flows (b5 m3/s) that expanded into the Valle del Bove. On July 15th a third vent opened, characterized by strombolian activity with emission of lithics and juvenile ejecta. Strombolian activity produced an ~30 m high cinder cone, which collapsed in the evening of July 19th when intensity of explosion increased and a new lava flow was emitted (~10 m3/s). In the following days, the activity at the SEC showed intermittent strombolian explosions, lava fountains and lava flows (Fig. 1f). Intensity varied widely, from a single lava flow, to weak strombolian explosions and to strong lava fountains up to 250 m above the crater rim. The activity continued alternating strombolian explosions, lava effusions and intra-crateric emissions 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 on November 16th. In the early morning, the eastern sector of the SEC

A problem in calculating the Mg# and by consequence assessing the evolution degree of magmas is the amount of FeO, which is controlled by the oxygen fugacity (fO2) of the magmatic system. When Fe3+/Fe2+ ratio is directly measured on lavas it may be affected by oxidation processes occurring during eruption and/or alteration and it is unlikely that such ratio would reflect the magma oxidation state at depth (Mollo et al., 2013 and references therein). More appropriate ways for measuring the original fO2 of magmas are based on mineral/ mineral and mineral/melt equilibria. The most commonly used method is the ilmenite/magnetite paragenesis, which is however prevented at Etna due to the absence of ilmenite. A different tool was proposed by France et al. (2010), which uses the partition of ferrous and ferric iron between plagioclase, clinopyroxene and the coexisting melt. Plagioclase can incorporate more Fe3+ than Fe2+ while clinopyroxene can incorporate more Fe2+ than Fe3+. The effect of oxidizing a partly molten basaltic system is to stabilize Fe3+, thus FeOTot increases in plagioclase and decreases in the associated clinopyroxene. The model is calibrated for pressure b 0.5 GPa and (ΔQFM N 0). The application of this model to

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the 2002–2003 products (at T = 1150 °C) has produced values of logfO2 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 + 1.5 ΔQFM (Fig. 3) (see Supplementary Material for the compositions used for calculations). These results are consistent with those calculated by Mollo 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) and from −6.8 to −6.5 logfO2 (+2.4 b ΔQFM b +2.1) using the equation of Kress and Carmichael (1991) based on Fe3 +/Fe2 + distribution on clinopyroxene. Similar values were obtained by D'Orazio et al. (1998), who calculated a fO2 ranging from +1 b ΔQFM b +3 using the Eu partition between plagioclase and melt. According to the equation of Kress and Carmichael (1988) at this fO2 condition corresponds a Fe3+/Fe2+ ratio of about 0.15, that is the most common ratio used for basaltic magmas. Several authors have identified the lavas erupted during the Recent Mongibello 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 primary magma only if a considerable amount of Fe is oxidized, with a Fe3+/ Fe2+ ratio ranging from 0.49 to 0.89 (Viccaro and Cristofolini, 2008). According to the equilibrium stated by Kress and Carmichael (1988) this ratio would correspond to ΔQFM N + 4, rather unusual for intraplate or even for suprasubduction environments (Rilling and Barton, 2005; Rowe et al., 2009). As indicated above for fO2 = + 1.5 ΔQFM at T of 1170 °C, a Fe3+/Fe2+ ratio of 0.15 is considered more appropriate for these magmas. In this case the Mg# of the Mt. Maletto, Montagnola or 2006 eruption varies from 53.5 to 63.4, quite far from Mg# = 68 estimated for melt in equilibrium with a fertile (OlFo = 88) peridotitic mantle assemblage, according to the Fe/Mg ol/liq partitioning of 0.3 determined by Roedder and Emslie (1970). A variable amount of wehrlitic assemblage (10–15% of fractionation of an assemblage constituted by 70.6–61.2% Ol + 38.8–29.4% Cpx; Table 1 and Fig. 2; see also Alesci et al., 2013) was determined by mass balance backward fractionation and added to these magmas to equilibrate them to mantle conditions. The obtained compositions are well comparable with those measured in melt inclusions. In particular, the reconstructed composition of 2006 primary magma falls in the field of melt inclusions (Mis) found

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

Montagnola

2006

2002/2003

SiO2 TiO2 Al O 2 3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 Total Mg#

48.31 1.62 16.56 8.10 1.22 0.20 7.89 11.92 2.56 1.25 0.37 100 63.36

48.31 1.68 17.90 8.28 1.22 0.18 5.96 10.97 3.75 1.31 0.42 100 56.11

47.94 1.82 18.07 8.47 1.27 0.19 5.49 10.88 3.53 1.90 0.45 100 53.53

49.49 1.64 17.96 7.85 1.77 0.17 4.03 9.27 4.19 2.16 0.54 100 47.68

Equilibrated SiO2 TiO2 Al O 2 3 FeO Fe O 2 3 MnO MgO CaO Na2O K2O P2O5 Total Mg# Ol Cpx F%

47.96 1.51 15.33 8.36 1.12 0.19 10.21 11.45 2.37 1.15 0.34 100 68.43 70.59 29.41 10

47.85 1.52 16.09 7.39 2.54 0.16 8.96 10.57 3.37 1.17 0.38 100 68.27 61.18 38.82 12

47.22 1.61 15.54 7.36 2.96 0.17 9.10 10.40 3.39 1.85 0.39 100 68.71 64.71 35.29 15

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 exchange reactions of Diopside/Hedenbergite–Jadeite and Hedenbergite/ Ca-Tschermak between clinopyroxene and the equilibrium melt (Putirka et al., 1996; Putirka, 2008). Crystal/melt equilibrium conditions however must be checked in order to apply geo-thermobarometers, which rise the question of magma composition. Clinopyroxene equilibrium was tested on the basis of Cpx–liqKdMg–Fe (Putirka, 2008) for crystals embedded in lavas of the 2002/2003 eruptive event choosing the most primitive erupted composition as equilibrium melt. Cpx–liqKdMg–Fe ranges between 0.26 and 0.28 (Fig. 4a), well within the experimentally determined for T N 1050 °C (0.24–0.30, Putirka, 2008). Temperature and pressure were determined using the P-independent equation of Putirka et al. (1996) and the H2O-independent equation of Putirka (2008), respectively. Results indicate that clinopyroxene crystallized between 1189 ± 26 °C and 1132 ± 26 °C at a pressure varying between 740 ± 30 and 330 ± 30 MPa. 4.3. Water content

Fig. 3. Oxygen fugacity determinations plotted on the (Fe2+/Fe3+)/logfO2 curve calculated using the equation of Kress and Carmichael (1991). Black circles represent the values determined in this study using the method of France et al. (2010). Black and gray squares are the 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 by D'Orazio et al. (1998) on the basis of Eu partitioning in plagioclase.

A similar approach for checking equilibrium conditions was followed for plagioclase (Fig. 4b). Crystal cores and dissolution zones were considered in equilibrium with the most primitive melt erupted during each single event, while rims were assumed in equilibrium with their whole rock. According to Putirka (2008) equilibrium can be assessed when Plg–liqKdAb–An ranges 0.27 ± 0.11 for T N 1050 °C. Fig. 4b shows that most of the plagioclases are in equilibrium, although disequilibrium occurs in some An-rich portion of the crystals.

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(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 the melt using a plagioclase hygrometer has been done using the equation of Lange et al. (2009) only on equilibrated crystals. This method is based on the exchange reactions between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8) components calibrated on a dataset spanning a wide range of liquid compositions (SiO2, 46–74 wt.%), plagioclase compositions (An93–37), temperatures (825–1230 °C), pressures (0–300 MPa), and dissolved water concentrations (0–7 wt.%) in both saturated and undersaturated conditions. Temperature was fixed at 1132 °C, that is the lower T estimated in Cpx. Pressure was arbitrarily fixed at 250 MPa, having checked that the effect of pressure on hygrometers is small (±100 MPa corresponds to ~ 0.1 wt.% H2O calculated in the melt). With these assumptions water estimates range between 0.2 ± 0.23 and 3.6 ± 0.23 wt.%, in accordance with Mi determinations in olivine from the 2001 and 2002–2003 eruptions, which suggest that up to 3.5 ± 0.23 wt.% of H2O is in the melt at 490 MPa, whereas erupted lavas have H2O contents down to 0.5–1.0 wt.% (Métrich et al., 2004; Spilliaert et al., 2006). The obtained geothermobarometric data and the calculated water contents were used to constrain the range of intensive variables introduced in the MELTS model.

4.4. MELTS phase stability modeling Stability of mineral phases and equilibrium compositions have been modeled with MELTS code (Ghiorso and Sack, 1995). Calculations were performed using two different compositions: the 2006 mantleequilibrated basaltic composition and the 2002–2003 trachybasalt (Fig. 2 and Table 1). For each composition, calculations were repeated using different water contents (0–3 wt.%, step 0.5) and pressures (400–50 MPa, step 5), corresponding to 11–1 km below the Central Craters following the density model of Corsaro and Pompilio (2004). T decreases from 1300 °C to 1000 °C at steps of 20 °C at constant fO2, corresponding to ΔQFM = + 1. In Fig. 5a–d liquidus temperatures of olivine, clinopyroxene, spinel and plagioclase are shown as function of pressure at different H2O contents. Olivine is the first phase to crystallize in both basaltic and trachybasaltic melts. As the amount of dissolved H2O increases by about 1 wt.%, the liquidus temperature decreases by about 50 °C for all phases. In anhydrous melt plagioclase becomes stable at higher temperature than spinel for both magma compositions (Fig. 5a). Such result does not fit with petrographic observations that indicate an early appearance of spinel in the magma, often enclosed in the

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1300

Ol

Ol

1300

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Cpx

Cpx

Plg

Plg

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Sp

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0wt% H2O

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250

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2wt% H2O

1000 0

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P (MPa)

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Cpx Plg

Sp Plg

1200

Cpx

1150

Sp

Sp 1100

1100

Plg

1050

(b) 350

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1wt% H2O 300

250

200

150

100

50

0

P (MPa)

(d) 350

300

1050

3wt% H2O 250

200

150

100

50

1000 0

P (MPa)

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), 1 H2O wt.% (b), 2 H2O wt.% (c) and 4 H2O wt.% (d).

outer portions of clinopyroxenes, thus suggesting that anhydrous primitive magmas are unlikely on Mt. Etna (Giacomoni et al., 2012). The main effect of increasing H2O is to strongly depress the plagioclase liquidus with respect to olivine and clinopyroxene (Fig. 5a–d). MELTS simulations of plagioclase composition and stability were performed using two different melt compositions: the reconstructed mantle-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 against pressure and dissolved H2O content; to the left plagioclase is stable while to the right it can crystallizes only at unrealistic temperatures (b1000 °C) for Etnean lavas. Results indicate that the plagioclase stability field is strongly affected by pressure and H2O content of the melt, it expands by lowering pressure and with low H2O content. On the other hand, crystal composition becomes more anorthitic in H2O-rich melts and at low-pressure conditions (Fig. 6 a–b). Moreover, it is evident that: (1) with increasing pressure, plagioclase becomes stable at higher T, (2) an increase of H2O depresses the plagioclase stability field, crystallizing a more anorthitic plagioclase at

lower temperature, and (3) the slightly evolved trachybasaltic magma stabilizes plagioclase earlier with respect to undifferentiated basaltic compositions. 5. Plagioclase textures, compositional profiles and water estimates Etnean plagioclase phenocrysts in the lavas erupted during the 2001–2006 period are generally euhedral, with major axis of 0.5–2 mm, and characterized by very diverse textural and growth features. An attempt to classify these textures and to relate them to magmatic processes has been recently proposed by Viccaro et al. (2010), who recognized seven crystal types. However, this classification describes the entire crystal and is unable to take into account the various parts that may constitute the whole plagioclase. We believe that an improvement of this classification is necessary since each growth feature can be related to specific physical and/or chemical conditions of the system; many crystals are, in fact, constituted by a core, one or two overgrowth portion/s, a rim and an outermost rim. Textural observations, with optical and electron microscopes, has been performed on a total

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(a) 300

1185 (69)

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1187 (72)

200 150

1100 (71)

50

11.3

1106 (74)

9.4

1188 (73)

1103 (75)

7.5

1184 (74)

1109 (78)

ve

1050 (80)

1105 (79)

1180 (75)

at Os

cur

5.6

1060 (82)

1101 (80)

1166 (76)

tion

ura

H2

100

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2006 mantle-equilibrated basaltic composition

Depth (km)

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3.7 1080 (86)

1080 (83)

1.9

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1185 (69)

1100 (71)

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1106 (74)

1080 (80)

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1188 (73)

1103 (75)

1080 (80)

11.3

1184 (74)

1109 (78)

1050 (80)

100

1180 (75)

1105 (79)

1060 (82)

50

1166 (76)

1101 (80)

150

13.2

2002/2003 trachy-basalt

9.4 7.5 u

nc

ratio

rve

5.6

atu

s HO

Depth (km)

P (MPa)

(b)

1140 (72)

350

2

1080 (83)

1040 (88)

3.7

1080 (86)

1.9

0 0.5

1

1.5

2

2.5

3

3.5

4

4.5

H2O wt% Fig. 6. MELTS pressure vs. H2O content for 2006 mantle-equilibrated basaltic composition (a) and 2002/2003 trachybasalt (b). Plagioclase liquidus (black line) is reported with the expected 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 150 samples of the 2001–2006 eruptive period. The main morphological features, together with a sketch that underlines the peculiar characteristics are shown in Fig. 7. A first subdivision can be done between simple zoned crystals, which preserve their euhedral habitus and complex zoned crystals that can be rounded or anhedral. Simple zoning includes oscillatory zonation and can be recognized in every crystal zone, often characterizing the overgrowth between portions of the crystal with complex zoning. Using BSE images and compositional profiles two types of oscillatory zoning have been recognized: Low Amplitude High Frequency (LAHF) and High Amplitude Low Frequency (HALF) patterns (cf. Viccaro et al., 2010). The LAHF pattern shows little An variations (~ ΔAn5) with zone widths of 5–10 μm. The HALF pattern displays higher An variations (N ΔAn10) with wider zones (20–30 μm) and is often associated with crenulated edges due to dissolution and angular unconformities (Fig. 7a). Complex zoning can be recorded in anhedral crystal cores and/or rims. Four different core types can be recognized, three of them usually having a rounded edge core and an overgrowth characterized by an abrupt 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 melt pockets (Fig. 7c); and (3) patchy rounded cores are characterized by complex juxtapositions of An-rich (~ An80) and An-poor (~ An55) domains, arranged in irregular patches and covering an approximate area of 400 μm2 (Fig. 7d). Clear rounded cores are characterized by both LAHF and HALF patterns. Oscillatory zonation is abruptly interrupted by the rounded edge and compositional profiles evidence a decrease in An content (ΔAn ≥ 20%) in the overgrowth just beyond the ovoidal edge. In dusty cores, glassy μm-sized melt pockets are distributed randomly within the crystal core (Fig. 7c). Sometimes, glassy channels interconnect the pockets, modifying the preexisting oscillatory zonation inside the crystal. A rounded edge marks the dusty zone and an overgrowth envelopes the core, often with a decrease in An content (ΔAn ≥ 20%). In patchy cores, the rounded edge is followed by less anorthitic overgrowths (Fig. 7d). Within the patchy cores, sieve textures may occur (see sieve textured cores in Fig. 7e). Patchy rounded cores are frequent in historic products but are rare in products from recent eruptive events (Viccaro et al., 2010). The fourth type consists of sieve-textured cores that are characterized by pockets of partially recrystallized melt randomly distributed within the crystal core (Fig. 6e). Compared with dusty cores, they do not present edge (neither rounded nor rough) that marks the

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Fig. 7. Light microscope photomicrographs and sketches of the recognized textures of plagioclase in the 2001–2006 eruptions. Simple zoning: Oscillatory zoning — Oz (a); complex zoning at 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); melt inclusion alignment — Mi (g).

boundary of core and no significant compositional variations have been detected; compositional profiles show oscillatory LAHF or HALF patterns 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 μm thick) rounded zone composed by isolated or interconnected partially re-crystallized melt pockets (Fig. 7f). Remarkable An (ΔAn15–30) and FeO enrichments mark the edge of the dusty zone. Most crystals show one rounded dusty rim; phenocrysts with two or more dusty zones at rim are very rare, spaced by oscillatory zoning. (2) Rims with melt inclusion alignments are characterized by iso-orientation of μm-sized (~5–10 μm wide) melt inclusions polygonal in shape (Fig. 7g). Differently from dusty rims, they do not present a rounded inner edge. Compositional profiles show a decrease in An (ΔAn 10–20%) and FeO contents. With this in mind, each phenocryst is the result of several events of growth and/or dissolution and can be summarized as an evolutionary path (Fig. 8). The most complex phenocrysts record a maximum of five events, starting from the first nucleation and initial growth (1st event). A major perturbation in the physical–chemical conditions of the system can be recorded by the formation of complex zoning with (Clear, C; Dusty, D1; Patchy, P) or without (Sieved, S) rounded edges (2nd event). Thereafter crystal growth is resumed and an oscillatory Overgrowth (3rd event, O1) surrounds the edge of the complex zoned cores. If a new perturbation in the physical–chemical conditions occurs, crystal can acquire again Dusty (D2) rounded zones or alignments

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

of melt inclusions (Mis) (4th event). An oscillatory zonation (O2, 5th event) usually envelopes the alignment, suggesting that crystallization occurred just before or even during the eruption. As evidenced in the diagram, not all phenocrysts record the entire sequence. In many cases a simple oscillatory zoning may characterize the entire crystal (Oscillatory Zoning, Oz, Fig. 8). In other cases simple oscillatory zoning may develop

Fig. 9. Back scattered SEM images and core–rim compositional profiles of An% and FeO wt.% of plagioclases emitted during the 2001 eruptive event. The growth path of each crystal is presented together with H2O determinations performed with the hygrometer of Lange et al. (2009). (a) Plagioclase from lavas emitted at the SE–PL fracture; (b) plagioclase from lavas emitted at the C–L fracture; (c) plagioclase from lavas emitted at the Lag cinder cone.

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after the first stage of overgrowth (O1 to eruption) or characterize the initial growing stage of the crystal, which later can be affected by complex textures at the rim (Nucleation and Growth to Mi/D2). Frequently, different types of plagioclases are found within the same sample or in samples that can be attributed to the same eruptive period. In this respect temporal relationships between the various samples are crucial. In the following paragraphs each plagioclase type is attributed to a specific period on the base of the most frequent textures at similar size, taking in mind that the presence of different crystals may provide useful information on the process/es occurring to the magma during uprising. It is evident that if we assume that different crystal zonings are related to different processes occurring within the feeding system the contemporaneous presence of two (or more) crystal textures cannot be 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 each eruptive event. In Figs. 9–11 these are reported together with the H2O content of the melt (Lange et al., 2009) calculated on plagioclase with Kd within the equilibrium range (Putirka, 2008) and the evolutionary path of the most abundant type of plagioclase found in the erupted product. 5.1. 2001 eruptive event Plagioclase phenocrysts in lavas of the 2001 eruption are mainly euhedral from 0.5 to 2 mm in size. In SE–PL lavas, most of the plagioclases (90%, Table 2) have simple textures with oscillatory zoning (Fig. 7a). The composition varies from An84 at the cores to An63 at the rims. 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 HALF pattern characterized by cross cutting edges and neat dissolution surfaces. An variation during oscillation is ~5%, and no significant changes in An content are associated to sieved cores. The path diagram represents only nucleation and growth (1st event) followed by an oscillatory zoned continuous growth (5th event). Oscillatory zoned crystals in SE–PL lavas indicate equilibrium with an average H2O content of 2.3 wt.% (Fig. 9a). In C–L lavas, phenocrysts with clear oscillatory zoning are rare (10%, Table 2); the majority of the crystals (90%, Table 2) have complex zoning textures, with HALF oscillatory zoning core or patchy zoning. With respect to SE–PL, these cores are less calcic with remarkable compositional variations within oscillatory zones (An47–63, ΔAn15–20%), which are not in equilibrium with whole rock. The core is surrounded by a dusty rim which is associated with An increment. Oscillatory overgrowth envelopes the dusty rim characterized by a less calcic composition (Fig. 9b). The following evolution can be depicted for these crystals: after nucleation, probably from a colder magma (1st event), the crystals reacted with a hotter magma producing a dusty rounded rim (D2, 4th event). A clear and oscillatory outermost overgrowth (O2, 5th event) followed the dusty rim just before the eruption (Fig. 9b). The hygrometer 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), varying in composition from An55 to An65 (Fig. 9c) with Plg–liqKdCa–Na = 0.42, clearly out of equilibrium. They are surrounded by an oscillatory zoned overgrowth (O1, 3rd event), a strong ovoidal dusty rim with Plg–liq KdCa–Na = 0.18 (D2, 4th event) characterized by an An and FeO increment (N15–20% An; ±0.7 wt.% FeO), and, finally an outermost less calcic overgrowth (O2, 5th event). The hygrometer estimates a high

383

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

5.2. 2002–2003 eruptive event The plagioclase in the products erupted by the highest part of the fracture (2550 to 2300 m a.s.l., T3, Ferlito et al., 2009a) on the NERS presents two distinct types of cores: (1) dissolved rounded cores with an oscillatory LAHF zoning (60%, Table 2), bytownitic in composition (An88–82) (Fig. 10a) and (3) patchy cores (40%, Table 2) with convoluted edges and labradoritic composition (An81–An71) (Fig. 10b). A less calcic overgrowth (O1, 3rd event), characterized by an oscillatory zoning ranging from An57 to An73, surrounds both cores. The evolutionary path shows that after nucleation (1st event) plagioclase cores become clear rounded 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 content on clear rounded (in equilibrium) cores indicates H2O contents of 2.8 wt.% (C), (Fig. 10a). Patchy cores are not in equilibrium with the whole rock however; the hygrometer in crystal overgrowth estimates 1.2 wt.% of H2O in the melt (Fig. 10b). Plagioclase in lavas and tephra erupted by the intermediate segment of 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 oscillatory zoning (An69–76), surrounded by an oscillatory and less calcic overgrowth (An63–67). After nucleation and growth (1st event), a dissolution stage occurred (2nd event). Growth is resumed with an oscillatory overgrowth until crystallization ends (from 3rd to 5th event). H2O hygrometer estimation suggests a value of 1.4 wt.% in the clear rounded cores (C), slightly decreasing to 0.8 wt.% at the overgrowth (O2) (Fig. 10c). Another type (40%, Table 2) of plagioclase within these products consists of crystals with oscillatory bytownitic cores (An80–87), rimmed by melt inclusions (Mis, 4th event), associated with a drastic drop in An content (ΔAn ~ 15%). An oscillatory overgrowth (O2, 5th event) mantles the outermost portion of the crystal characterized by less An (An54–62). H2O estimation inside oscillatory-zoned cores indicates a H2O content of 2.7 wt.% (Fig. 10d). Phenocrysts embedded in lavas emitted at the lower segment of NERS (2100–1950 m a.s.l.) during the initial phase of fracture opening (early T5), have oscillatory cores (80%. Table 2) (rarely slightly sievetextured, 20% in Table 2). Cores are bytownitic (An82–90) and rimmed by melt inclusion alignment (Mi, 4th event) followed by an outermost overgrowth (O2, 5th event) (Fig. 10e). Chemical profiles reveal a strong decrease in An content corresponding to the melt inclusion alignment (ΔAn ≥ 10%). A second drop in An (ΔAn ≥ 10%) characterizes the outermost oscillatory rim reaching labradoritic compositions (An ~ 60) (Fig. 10e). Inner rounded cores are not in equilibrium with the host rock, 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 rounded clear cores (C) but with a slightly less calcic composition (An75–83), an oscillatory-zoned overgrowth (O1) surrounds the cores showing an abrupt An drop (An50–58) (Fig. 10f). Path diagram (Fig. 10f) differs to those of the initial T5 crystal by the absence of melt inclusion alignments. The plagioclase records the initial nucleation and growth of crystal core (1st event), dissolution of the core (2nd event) and an oscillatory overgrowth until crystallization ends (5th event) (Fig. 10b and Table 2). Crystals in late T5 (Fig. 10f) indicate remarkably lower water content at the clear rounded cores (C — 2.3 wt.%) and a decrease (0.8 wt.%) associated with the overgrowth (O1).

Fig. 10. Back scattered SEM images and core–rim compositional profiles of An% and FeO wt% of plagioclases emitted during the 2002–2003 eruptive event. The growth path of each crystal is presented together with H2O determinations performed with the hygrometer of Lange et al. (2009). (a) and (b) Plagioclases from Low Potassium Oligophiric (LKO) lavas from North East Rift System (NERS); (c) and (d) plagioclase from High Potassium Oligophiric (HKO) lavas from NERS; (e) plagioclase from High Potassium Porphyritic (HKP) lavas emitted at early T5; (f) plagioclase from HKP lavas emitted at T5 from the NERS contemporaneously with (g) plagioclase emitted from HKP lavas emitted from the South Rift System (SRS). (h) Plagioclase emitted from HKP lavas at T6 on the SRS.

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Fig. 10 (continued).

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385

Plagioclases erupted at T5 on SRS, have a clear rounded core (C) similar to those erupted at T5 on NERS; chemical profiles reveal a comparable bytownitic composition (An79–86) (Fig. 10g). The core is surrounded by an oscillatory overgrowth (O1) with a relevant drop in An (ΔAn ~ 10%). The lack of melt inclusion alignment in these crystals is also to be noted. Path diagram shows a first event of nucleation and growth (1st event), an event of dissolution of the core (2nd event) and a last overgrowth (5th event) (Fig. 10g and Table 2). H2O estimations are similar to those obtained in products emitted on NERS at the same time. Water content in the clear rounded cores (C) is 2.3 wt.% decreasing at the overgrowth (O2) to 1.1 wt.% (Fig. 10g). Plagioclase phenocrysts in products erupted in the following phases of the eruption on the SRS (T6, T7 and T8 in Giacomoni et al., 2012), are characterized by patchy or clear rounded cores (70% and 30% respectively, Table 2) with andesinic composition (An ~ 60%). A very thin oscillatory overgrowth (O1, 3rd event) precedes a dusty rim (D2, 4th event) associated with an increase in An content (ΔAn ~ 20%); an outermost oscillatory rim with labradoritic composition (An56–60) mantles the crystal (O2, 5th event) (Fig. 10h and Table 2). The dusty rim is the only portion of the crystal in equilibrium with the host rock and the dissolved H2O in the melt is 2.2 wt.%.

Plagioclases in lavas emitted during November 16th show more calcic 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 sieved cores becomes progressively less anorthitic down to An77. This zone is surrounded by a resorbed dusty rim (D2, 4th event) always associated with strong An and FeO (~ΔAn 15%) enrichments. An outermost rim envelopes the phenocrysts and presents a strong An drop down to An60 (Fig. 11d). The path diagram in Fig. 11d shows four main events: nucleation and growth (1st event), sieve textures at the core (2nd event, S), dusty rounded rim formation (4th event, D) and an overgrowth at the outermost portion of the crystal before eruption (5th event, O2). Hygrometer estimation of H2O dissolved in the melt indicates a value of 3.0 wt.% in sieved cores (S), gradually increasing to 3.2 wt.% at the dusty rim (D2). The amount of estimated H2O abruptly decreases in the outermost overgrowth (1.5 wt.%) (Fig. 11d).

5.3. 2004–2005 eruptive event

Plagioclase is a good candidate to monitor the magma ascent dynamics in the last 12 km, through the various growth and dissolution/ resorption episodes occurring when changes in P–T–X conditions occur. This approach takes into account several experimental and theoretical studies that investigated the mechanisms and the kinetic of processes responsible for crystal growth and stability. Growth kinetic is the main 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 oscillatory pattern (LAHF and HALF, Ginibre et al., 2002; Viccaro et al., 2010) that may be ascribed to kinetic effects and/or minor changes in bulk chemistry or physical parameter of the melt. Clear rounded edges and dusty zoning plagioclases in equilibrium with an andesitic melt have been experimentally reproduced by Tsuchiyama (1985): crystals become smaller and rounded when T rises above the plagioclase liquidus temperature, while they maintain the original shape below the liquidus. At constant T, crystal/melt interface remains smooth if the crystal is more calcic than the equilibrium plagioclase, while it becomes rough and dusty if the crystal is less calcic. Tsuchiyama (1985) recognized two types of dissolution: (1) a crystal dissolves in the melt which is undersaturated with respect to the phase resulting in clear rounded cores (simple dissolution); and (2) a crystal is partially dissolved to form a dusty plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). A few hypotheses have been proposed for the development of patchy zoning. Anderson (1984) suggested that patchy-zoned regions could be formed during oversaturation and crystal growth episodes. Similarly, Kuritani (1999) invoked a skeletal growth rather than a dissolution process to explain this texture. On the other hand Humphreys et al. (2006) and Ginibre and Wörner (2007) proposed that patchy zoning might be acquired during major events of plagioclase destabilization as repeated episodes of dissolution and re-growth occurring in a convective system. Such conclusions are also supported by the experimental 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 sieve textures is related to the decompression rate and that plagioclase composition becomes more albitic with increasing pressure. Although not all the textural features have been so far experimentally 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

Plagioclases in lavas emitted in the first phase of the eruption from the fracture located at the SE flank of the South East Crater (SEC) are mainly euhedral (90%, Table 2), with oscillatory zoning (Fig. 11a). Crystals vary in composition from An88 to An78 at the cores and exhibit a LAHF pattern. Phenocrysts with sieved cores are subordinately present (10%, Table 2); they do not show significant core to rim compositional variations. A less calcic overgrowth (An b 70%) characterizes both clear and sieved crystals at the rim. The path diagram shows a single event of nucleation (1st event) followed by an oscillatory-zoned growth (Fig. 7a and Table 2). Hygrometer estimation indicates a constant value of 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 sieved cores (S, 2nd event, Fig. 11b) become more abundant (90%, Table 2). In these crystals, the anorthite content varies from An85–An70 at the cores and is surrounded by a less calcic (An74–68) oscillatory-zoned overgrowth (O1, 3rd event). Most of phenocryst rims are characterized by dusty zoning (D2, 4th event) with an increment in An content (ΔAn ≥ 10%). An outermost overgrowth (O2, 5th event) surrounds the dusty rims and shows a strong depletion in An content (An70–55) (Fig. 11b). Sieve-textured cores (S) are in disequilibrium with whole rock however, the hygrometer estimates an amount of 3.1 wt.% of H2O in the dusty zone D2, decreasing to 1 wt.% H2O in the outermost rim (Fig. 11b). 5.4. 2006 eruptive event Due to the large number of eruptive episodes and to the complexity of the compound lava field that characterized this multiple eruptive event, attention was exclusively focused on the paroxystic episode of November 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 a LAHF pattern with An varying from An88 to An70 with several episodes of calcic enrichments. However, most of the oscillations are within An80–An85. The oscillatory cores are surrounded by a dusty rim (D2, 4th event) associated with an increment in An content (ΔAn ~ 10%). A subsequent oscillatory-zoned overgrowth (O2, 5th event) mantles the crystal. H2O content estimation in the dusty (D2) portion of the plagioclase 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).

6. Discussion 6.1. Relationships between magmatic processes and plagioclase petrological features

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2 2001

SE-PL (3040–2940 m a.s.l.) C-L (2620–2320 m a.s.l.)

2002–2003

2006

a.s.l.) a.s.l.) a.s.l.) a.s.l.)

4

5

– – – – – O1 O1 O1 O1 O1 O1 – – O1 O1

– – – – D2 D2 D2 D2 – – – Mi Mi Mi –

Oz Oz Oz Oz O2 O2 O2 O2 Oz Oz Oz O2 O2 O2 Oz

90% 5% 5% 5% 90% 5% 60% 40% 60% 40% 60% 40% 80% 20% 100%

C

O1

–

Oz

100%

P C – S S – Oz S Oz S

O1 O1 – – O1 – – O1 – O1

D2 D2 – – D2 – D2 D2 D2 D2

O2 O2 Oz Oz O2 Oz O2 O2 O2 O2

70% 30% 90% 10% 90% 10% 95% 5% 10% 90%

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

7.5 ve

150

ur nc

5.6

tio

ura

sat

plg. stable

100

3.7 (88)

50

1.9

(b)

2001 C-L

350

13.2 11.3

300 250

9.4

plg. unstable

200

(76)

7.5

Dusty (D2) e

urv

150

1

H2O

5.6

t

ura

(80)

c ion

sat

plg. stable

100

a.s.l.)

a.s.l.)

9.4

plg. unstable

Core (Oz)

1

H 2O

a.s.l.)

a.s.l.)

250

3.7

50

1.9

2 Overgrowth (O2)

350

(c)

2001 Lag

(56)

13.2

Patchy (P) (60)

300

11.3

1

9.4

250 plg. unstable

200

7.5

(74)

Dusty (D2)

150

H2O

(81)

sat

5.6

t

ura

2

ion

Depth (km)

– S D1 – Oz P P C C P C Oz Oz S C

11.3

(75)

200

13.2

Depth (km)

2004–2005

Lag (2620–2320 m NERS (2500–2300 m NERS (2300–2100 m NERS Early T5 (2100–1980 m NERS Late T5 (2100–1980 m SRS T5 (2850–2600 m SRS T6–T8 (2850–2600 m SEC (2920 m a.s.l.) SGP (2620–2320 m Pre Nov 16th (3040 m a.s.l.) Nov 16th (3040 m a.s.l.)

3

Frequency

P (MPa)

Recorded events

P (MPa)

Vent or fracture location

300

P (MPa)

Eruptive event

(a)

2001 SE-PL

350

Depth (km)

Table 2 Summary of the recognized plagioclase evolutionary path and recorded events in lavas emitted by the distinct vents or fractures during the 2001, 2002/2003, 2004/2005 and 2006 eruptions. The frequency of appearance of each growth path is also reported and expressed as relative percentage.

387

3.7

100 plg. stable

50

1.9

3 Overgrowth (O2)

6.2. 2001 eruptive event Several studies focused on this event have highlighted the involvement of various magmas with distinct petrographical and geochemical features that were erupted by different fracture segments (Viccaro et al., 2006; Ferlito et al., 2008; Coulson et al., 2011). Plagioclases in SE–PL lavas are oscillatory zoned (1) without complex dissolution/re-growth textures at core or at rim. Crystals begin to crystallize between 270 and 200 MPa, that correspond to a depth ranging from 10 to 7.5 km in a magma containing 1.6–2.4 wt.% of H2O (Figs. 7 and 12a). Plagioclases in lavas emitted at the C–L fracture display a HALF oscillatory zoning ranging from An81 to An45. Such low anorthite content, in disequilibrium with the melt (as indicated by Kd Plg–liqKdCa–Na), suggests that these cores crystallized in a cooled and degassed magma and could be interpreted as antecrysts recycled from a previously intruded magma batch. A dusty rounded zone (1) envelopes the core thus recording a reaction of the antecrysts with an incoming more basic (H2O-rich) melts. Such mixing processes probably occurred between depth 6–4 km (Fig. 12b). The outermost overgrowth (2) formed after decompression and volatile loss at lower pressure (crystallization at P b 40 MPa) (Fig. 12b). Plagioclases in lavas emitted at the LAG have patchy cores (1) that are in disequilibrium with the host magma. According to MELTS modeling they could be formed at pressure, water content and temperature of 350–300 MPa, 1.0–1.5 wt.% and 1050–1070 °C respectively. Both disequilibrium and low crystallization temperature suggest than an

0

0.5

1

1.5

2

2.5

3

3.5

4

Fig. 12. P/H2O plagioclase stability field in lavas emitted during the 2001 eruptive event calculated with MELTS. Plagioclase liquidus (black line) is reported with the expected composition (An%) and temperature. H2O saturation curve has been calculated with Solex 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 from C–L fracture.

antecrystic origin cannot be excluded for these cores. The cores are enveloped by a dusty zone (2), associated with a strong increment of An 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 pressure (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 the evidence of a magma mixing between a shallow, cold and amphibolebearing magma with an incoming more basic melt. Mixing probably occurred between 6 and 4 km of depth and increased the explosive style at the Laghetto cinder cone (Ferlito et al., 2009b). 6.3. 2002–2003 eruptive event Several studies focused on the role of the NE Rift feeding system and two distinct models were elaborated: (1) the eruption was driven by a

Fig. 11. Back scattered SEM images and core–rim compositional profiles of An% and FeO wt.% of plagioclases emitted during the 2004/2005 and 2006 eruptive events. The growth path of each crystal is presented together with H2O determinations performed with the hygrometer of Lange et al. (2009). (a) Plagioclase from lavas emitted at the SEC fracture; (b) plagioclase from lavas emitted at the SGP fracture; (c) plagioclase in lavas emitted before November 16th; (d) plagioclase in lavas emitted during November 16th paroxystic event.

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2002 LKO T3 (a) 13.2

350

11.3

300

plg. unstable

(77) (80)

250

7.5

(82) e

urv

150

Clear (C) plg. stable

1

100

(86)

5.6

t

ura

H 2O

c ion

sat

11.3

250

Clear (C)

3

200

9.4

plg. unstable

(83)

7.5 e

urv

150

(85)

plg. stable

c ion

5.6

t

ura

H2O

1

100

3.7

13.2

sat

Depth (km)

200

9.4

Patchy (P)

2

(c)

2002 HKP T5+T5S

(74)

Depth (km)

P (MPa)

300

P (Mpa)

350

3.7

(87)

4

3 2002 HKO T3

350

(b)

(73)

Clear (C)

250

7.5

2 (86)

n atio

cur

5.6

ur

H 2O

sat

100 50 Overgrowth (O1) 3 1

4 1.5

300

ve

Core (Oz)

0.5

11.3 9.4

(82)

plg. stable

0

350

(80)

200 150

Overgrowth (O1)

2

Melt Alignment (MI)

2.5

3

3.5

11.3 (73)

50

9.4

Dusty (D2)

plg. unstable

1

150

1.9

1.9

Melt Alignment (MI)

2002 T6+SRS (d) 13.2

200

100

4

2

250

3.7

H2O wt

4

ve

plg. stable

0

0.5

1

sat

ur nc

5.6

tio

ura

H 2O

Overgrowth (O1)

7.5

(79)

3.7 1.9

2 1.5

Depth (km)

1

plg. unstable

13.2

Depth (km)

P (Mpa)

300

50

1.9 Overgrowth (O2)

P (Mpa)

50

2

2.5

3

3.5

4

H2O wt

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 expected composition (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 upper portion of the central conduits (Andronico et al., 2005); and (2) the NE Rift was fed by an independent system, which allowed the ascent and 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.%, and patchy cores (2) crystallized at 250 to 200 MPa and water content varying between 1.7 and 2.3 wt.%. Both core types are in equilibrium with the whole rock and are followed by an overgrowth occurred at shallower pressure (40 and 20 MPa; 3 and 4 respectively) after decompression and volatile loss (Fig. 13a). Plagioclases in HKO (Fig. 13b) lavas present different cores: i) clear cores (1) crystallized at 280–230 MPa with H2O content varying between 1.5 and 2.0 wt.%; ii) oscillatory cores (2) nucleated between 170 and 120 MPa and H2O content ranging between 2.3 to 2.7 wt.%. Clear cores are followed by an oscillatory overgrowth (3) crystallized at very shallow pressure with water content ranging between 0.8 and 1.3 wt.%. Oscillatory-zoned cores are enveloped by an alignment of 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 erupted during the early T5 phase at NERS nucleated and grew at pressures straddling the volatile saturation curve (150–70 MPa, Fig. 13c) with water content from 2.6 to 3 wt.% (1). Further overgrowth with alignment of melt inclusions (2) was promoted by decompression and volatile loss at ca. 100 MPa and 2–2.3 wt.% of H2O. Plagioclases in lava emitted during late T5 at NERS and contemporaneously at SRS grew at deeper pressure (260–180 MPa) and water content ranging from 1.7 to 2.3 wt.% (3) (Fig. 13c). An increase in water content induced plagioclase liquidus depression and crystal rounding. A new growth stage occurred only after a fast decompression (4) that induced volatile loss and undercooling at very low pressure b20 MPa (Fig. 13c).

Patchy cores in subsequent eruptive phase (T6 to T8) are not in equilibrium with the whole rock. They are surrounded by a dusty zone (1) crystallized between 250 and 200 MPa in a more basic and volatile-rich (1.8–2.3 H2O wt%) magma (Fig. 13d). Subsequent overgrowth (2) was promoted by decompression and volatile loss at shallow depth b 20 MPa (Fig. 13d). The presence of clear rounded cores (C) in the 2002–2003 products complicates the interpretation of plagioclase evolutionary path, introducing a dissolution event, which can be associated with an increase of H2O in the system. As shown above for the 2001 eruption input of a more basic and/or hotter magma in equilibrium with a more anorthic plagioclase causes the reaction and resorption of the pre-existing more albitic crystal core, as testified by a dusty texture. A clear rounded core cannot be explained by the same process, but suggests that magma becomes suddenly undersaturated in plagioclase causing the dissolution of the pre-existing crystal (Tsuchiyama, 1985). However, neither textural nor chemical evidences for a mixing with such a primitive melt were observed. Thus, it is most likely that the clear dissolved cores are associated with an increase of volatiles in the system, which is not necessarily related to the input of new magma (see Section “Volcano feeding 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 at depth (Fig. 13a, b). Plagioclase in lavas erupted in the early phase of T5 indicates a shallow crystallization in H2O-rich magma. Such high H2O content delayed the appearance of plagioclase, which becomes stable only after a massive volatile loss and decompression due to fracture opening (Fig. 13c). In lavas erupted contemporaneously on the NE and S Rifts at late T5 similar clear rounded cores suggest a common deep feeding system at an approximate depth of 9–6 km. The presence of dusty rims in plagioclase emitted on the S Rift from T6 to the end of the eruptive event indicates that several inputs of basic magma occurred and fed the activity (Giacomoni et al., 2012).

P.P. Giacomoni et al. / Earth-Science Reviews 138 (2014) 371–393

plg. unstable

250

13.2

350

11.3

300

9.4

250

150

plg. stable

tio

ura

(86)

1

H2O

sat

5.6

Core (Oz)

100

200 plg. stable

(83)

2

plg. unstable

13.2

350

11.3

300

(86)

ve

n atio

cur

5.6

ur

H2O

100

sat

3.7

(87)

50

Overgrowth (O1)

3

Dusty (D2)

0.5

1

1.5

2

2.5

3

3.7

(86) (87)

1.9 Overgrowth (O2)

2006 16th Nov

1.9

2

250

13.2

9.4

200

4

7.5

(80)

150 100

H2O

e

urv

Sieved (S)

1

plg. stable

5.6

t

ura

sat

c ion

3.7

(83)

50

(88) (85)

3

Overgrowth (O1)

3.5

(d)

11.3

(74)

Dusty (D2)

(60)

0

5.6

0

0.5

1

H2O wt

1.5

2

2.5

Depth (km)

150

7.5

Sieved (S)

P (Mpa)

9.4 (83)

1

tio

ura

sat

plg. unstable

250

plg. stable

H2O

ur nc

50

Depth (km)

P (Mpa)

300

200

(86)

3

(b)

ve

Core (Oz)

Dusty (D2)

2004-2005 SGP

11.3

7.5

(83)

1

100

1.9

13.2

9.4

2 350

(c)

plg. unstable

150

3.7 (88)

Overgrowth (O1)

50

2006 Pre 16th Nov

Depth (km)

ve

ur nc

Depth (km)

7.5

200

P (Mpa)

300

P (Mpa)

(a)

2004-2005 SEC

350

389

1.9

2 3

3.5

4

H2O wt

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 expected 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 2004/2005 eruption; (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 emitted during the November 16th paroxysm of 2006 eruptive event.

6.4. 2004–2005 eruptive event

6.5. 2006 eruptive event

This effusive eruptive event occurred after 20 months of inactivity and was characterized by the absence of significant geophysical signals preceding and accompanying the eruption onset (Bonaccorso et al., 2006; Di Grazia et al., 2006). Corsaro et al. (2009) presented a petrologic study on sequential samples collected throughout the eruption. The authors supported the model in which a fractionated magma intruded in the shallow portions of the lateral feeding system during the 2002– 2003 eruption and was progressively mixed with magma rising along the central conduits. In SEC lavas (Fig. 14a), plagioclases began to crystallize at 130–60 MPa from a magma containing 2.6–3 wt.% of H2O (1). Such observation suggests that nucleation occurred in an undegassed magma intruded in a lateral feeding system linked with the central conduits just before the eruption. The effect of an increasing H2O-content in the magma is to prevent plagioclase nucleation at depth. The outermost oscillatory-zoned overgrowth (2) is likely associated with depressurization and volatile loss that stabilize a more albitic crystal. Plagioclases in lavas 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 resorbed rim (D2) often envelopes sieved cores (2) suggesting a reaction with a more basic magma during the SGP phase. Subsequent overgrowth (O2-3) occurred as a consequence of a second episode of volatile loss and decompression (Fig. 14b). Plagioclase textures, geochemical features and stability MELTS model suggest a scenario that fairly agrees with the model presented by Corsaro et al. (2009). Eruption was initially fed by a shallow and volatile-rich magma becoming progressively mixed by an incoming more primitive melt. Pre-existing long lived phenocrysts recorded at least two episodes of degassing that occurred in the magma stored in the central open conduits (Ferlito et al., 2012).

The flank collapse of the sub-terminal SEC that occurred on November 16th has been the subject of numerous studies that proposed two contrasting triggering mechanisms. The first model presented in Behncke et al. (2008) and Behncke (2009) explains the observed violent explosive activity as due to a superficial interaction between lava flowing from the top of the SEC and the moisture-rich tephra constituting the cone. An alternative explanation was presented by Ferlito et al. (2010), where the authors associate the explosive emission of the magma with the rapid opening of the ESE–WNW oriented fracture at the base of the crater. In lavas emitted before November 16th plagioclase is mainly oscillatory zoned (1) and MELTS modeling suggests that it crystallizes between 170 and 110 MPa. Some phenocrysts have dusty resorbed rims (2), formed between 130 and 60 MPa, suggesting an input of basic magma between 5 and 3 km. Outermost rims (3) formed after decompression and volatile loss, during final magma uprise (b 50 MPa) (Fig. 14c). Plagioclases in lavas emitted during November 16th have sieved cores (1) formed at pressures of 170 to 80 MPa. A dusty zone surrounds the cores (2) suggesting a reaction with a more basic and undegassed magma at very shallow depth (70– 30 MPa). Subsequent overgrowth (3) occurred, after decompression and H2O loss, at P b 50 MPa (Fig. 14d). According to MELTS modeling, plagioclase records the arrival of a slightly more basic magma, while markedly basic magmas were erupted only during the paroxystic episode. 6.6. Volcano feeding system and magma storage Following Rittman (1973) no major long-lived crustal reservoirs exist beneath Etna, even though some magma ponding appears to be necessary to explain magma differentiation (Allard et al., 2006 and

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Fig. 15. Sketched reconstruction of Mt. Etna volcano feeding system, superimposed on the stratigraphy as deduced from the seismic study of Finetti (2005). In the right column the depth of crystallization of the main 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). Vertical bars 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 beneath the rift before the 2002–2003 eruption (Ferlito et al., 2009a). These magma batches can be envisaged as a network of dykes and sills (e.g. Guest and Duncan, 1981; Murray, 1990), as it is clearly observed in ancient eroded volcanic edifices outcropping in Valle del Bove (Branca and Del Carlo, 2004; Ferlito and Nicotra, 2010). This is also 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 below the volcano there is a large zone with about 14% of molten rocks, accounting for an approximate magma volume of 1600 km3 (Sharp et al., 1980). However, more recent tomographic studies did not confirm the presence of such deep magma reservoir (Patanè et al., 2006). At a depth of 12 ± 3 km, Murru et al. (1999) identified a smaller lowvelocity zone, which can be interpreted as another level of magma storage, most probably corresponding to the boundary between sedimentary cover and crystalline basement. The same authors recognized a second shallower low velocity zone at about 5 ± 2 km, located at the discontinuity between the Hyblean carbonate platform and the Flysch units (Lentini, 1982). The latter geological unit is probably the source of the quartz-arenitic xenoliths frequently found in Etnean lavas (Clocchiatti et al., 2004; Ferlito et al., 2009a,b; Coulson et al., 2011). These three recognized levels represent only the most prominent portions of the feeding system below the volcano, which can be considered almost continuous from the mantle to the surface.

Further seismic tomograms revealed the presence of a vast high velocity zone, probably composed by plutonic rocks that extend down to a depth 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 beneath Mt. 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 up to a shallow depth of about 10 km. At similar crustal pressure also plagioclase begins its crystallization history; the deepest crystals are characterized by patchy textures (14–11.5 km); clear rounded cores crystallize between 9.3 and 5 km, followed by melt alignment and dusty textures at the rim (4.5–2.7 km). Crystal overgrowths and melt inclusion alignments form over the volatile saturation depth, above 2.8 km and inside the volcanic edifice. Crystallization and development of patchy cores, rare in recent products (Viccaro et al., 2010), are in disequilibrium with the host rock and can be considered as antecrysts whose texture can be associated with repeated magma inputs. They formed at the same depth of the low velocity zone recognized by Murru et al. (1999) (12 km). The deepest patchy cores cannot be formed in equilibrium with water content higher than 1.7 wt.% (Figs. 12c and 15).

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Clear rounded cores characterize most of the plagioclase in the 2002–2003 eruptive event (Ferlito et al., 2009a; Giacomoni et al., 2012) and crystallize between 9.5 and 5 km, corresponding to the depth of precursor earthquakes (Andronico et al., 2005) with a water content ranging between 1.5 and 2.8 wt.%. Dusty rims are common in all the studied plagioclases, testifying the relevance of magma mixing in the Etnean feeding system and the ability of plagioclase to record such processes although involving magmas with similar compositions. Depth estimations suggest that these rims can form between 4.5 and 2.7 km, across the H2O-saturation depth. The alignments of melt inclusions formed above 2.5 km of depth and well above the H2O saturation level. Melt inclusion alignments are frequent in products emitted on the NERS during the 2002–2003 event. The outermost albitic overgrowth is associated with the last degassing phase and can occur in syn-eruptive conditions. The overall picture that can be drawn putting together plagioclase textures and seismic anomalies supports the non-existence of major crustal magma chambers, but depicts a vertically extended and continuous feeding system which leads magma from the mantle to the surface. Short-lived ponding reservoirs may exist where the tectonics is less intense, 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 stable and/or can grow in a H2O-rich magma only at relatively low pressure according with the experimental data of Métrich and Rutherford (1998). Ab-rich plagioclase cores that nucleate at depth (12 km) can be in equilibrium only with a basaltic magma containing H2O b 1.5 wt.%, as also suggested by Lanzafame et al. (2013). Alternatively, these cores could crystallize from a more evolved melt (mugearitic–benmoreitic composition). However, such evolved melts have neither been erupted during the period considered nor in historical time, thus making unlikely this second scenario. Our findings must be discussed against the MI data (Métrich et al., 2004; Spilliaert et al., 2006), which indicate the presence of H2O-rich magmas at high depth (e.g. H2O 3.4 wt.%). These contrasting evidences can be reconciled by only admitting that magmas within the Etnean feeding system are enriched in H2O. Since Etnean basalts with similar composition are produced by comparable degrees of mantle partial melting (Corsaro and Cristofolini, 1996; Peccerillo, 2005; Alesci et al., 2013), it is highly implausible that they can be originated with such drastically different H2O contents. Furthermore, magma differentiation is unable to significantly increase water in Etnean magmas, since fractionation percentage is relative low and its effect on water increase is negligible (0.2 wt.% of water every 10% of fractionation, Nichols et al., 2002). This evidence implies that magmas are enriched in H2O after their formation and very likely within the feeding system. It is therefore necessary to envisage an independent contribution of volatiles (e.g. H2O) that migrate upward and enrich the originally water-undersaturated magma ponding at various levels within the feeding system (Ferlito et al., 2014). 7. Conclusions Plagioclase is a ubiquitous mineral in magmatic products, with a large spectrum of compositions and textural features. Here we present a systematic study on textures and compositions of plagioclases from lavas of four significant, well studied and monitored eruptive events recently occurred on Mt. Etna volcano. Different compositions and textures have been related to physico-chemical conditions of the system, which have been further constrained with thermodynamic modeling and compared with results from experimental studies. The estimation of the intensive variables of the system (P–T–fO2) and the H2O content, allowed the reconstruction of the parental magma composition and plagioclase stability fields. We have therefore attempted to associate magmatic processes (e.g. decompression, magma mixing, degassing or volatile influx) with specific textures and compositions; finally a

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comparison with volcanological evolution of each eruption has been carried out. A good correspondence between plagioclase-inferred magmatic processes and the volcanological evolutionary models put forward by several authors was found. For the studied period, plagioclase textures and compositions indicate that crystallization occurs in a polybaric system, suggesting a vertically developed and rather continuous plumbing system beneath the volcano edifice, which would rule out the presence of significant magma chambers. A relatively small number of plagioclases nucleate at greater depth (12 km), with a composition in equilibrium with water-poor magmas. Most of the plagioclases nucleate at pressure of 200–250 MPa, corresponding to the depth (5–6 km) at which basaltic magmas reach saturation level for H2O that is consequently exsolved. Interestingly the deformation pattern reconstructed for the 2008 eruption was strongly controlled by magma overpressure generated by gas exsolution at this level, underlining the significant contribution of volatile in the eruptive style of the volcano (Aloisi et al., 2011). These two levels recognized through plagioclase textures and composition match the low-velocity zones found by the seismic tomography at about 12 ± 3 and 5 ± 2 km, probably corresponding to the base of the sedimentary cover and the Numidian Flysch respectively. The plagioclase stability fields also indicate a large variability in water content within the magmatic system. This confirms that primitive Etnean melts are water-poor and that the amount of water increases during magma ascent to the surface. Fractionation is not a feasible mechanism to account for this enrichment. Volatile fluxing can be a viable mechanism but further studies are necessary to precisely constrain the physico-chemical parameters that control this process (Ferlito et al., 2014). In summary it is demonstrated the relevance of the systematic study of plagioclase texture and composition to constrain volcanological processes on basaltic magmas in open conduit volcanoes. This study is a first attempt that can be extended to future eruptive events taking advantage that Mt. Etna volcano is extremely well monitored as one of the most active volcanoes in the word (more than twenty paroxysmic events from August 2010 to February 2013). Acknowledgments Authors are thankful to the national PRIN 2012 (Piano Ricerca di Interesse Nazionale — “Volatile transfer at convergent plate margins: linking COH fluids/melts heterogeneities to tectonic anomalies in subduction zones”) funding that supported the research. A special thanks to Raul Carampin for his priceless expertise and competence during in situ mineral analyses at the Microprobe Analytical Laboratory of IGGCNR (Padua, Italy). Two anonymous referees and the editor in chief are also acknowledged for their constructive criticism, which substantially 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. References Alesci, G., Giacomoni, P.P., Coltorti, M., Ferlito, C., 2013. Primary magmas modelling and mantle source of Etnean lavas. Mineral. Mag. 77 (5), 571. Allard, P., 1997. Endogenous magma degassing and storage at Mount Etna. Geophys. Res. Lett. 24, 2219–2222. Allard, P., Behncke, B., D'Amico, S., Neri, M., Gambino, S., 2006. Mount Etna 1993–2005: anatomy of an evolving eruptive cycle. Earth Sci. Rev. 78, 85–114. Allegre, C.J., Provost, A., Jaupart, C., 1981. Oscillatory zoning: a pathological case of crystal growth. Nature 294, 223–229. Aloisi, M., Mattia, M., Ferlito, C., Palano, M., Bruno, V., Cannavò, F., 2011. Imaging the multi-level magma reservoirs at Mt. Etna (Italy). Geophys. Res. Lett. 38, 16. Alparone, S., Andronico, D., Giammanco, S., Lodato, L., 2004. A multidisciplinary approach to detect active pathways for magma migration and eruption at Mt. Etna (Sicily, Italy) before the 2001 and 2002–2003 eruptions. J. Volcanol. Geotherm. Res. 136, 121–140.

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