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Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt

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Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt minerals Article Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt. Etna Volcano Manuela Nazzari 1,2,*, Flavio Di Stefano 1, Silvio Mollo 1,2, Piergiorgio Scarlato 2 , Vanni Tecchiato 1, Ben Ellis 3, Olivier Bachmann 3 and Carmelo Ferlito 4 1 Department of Earth Sciences, Sapienza-Università di Roma, 00185 Rome, Italy; fl[email protected] (F.D.S.); [email protected] (S.M.); [email protected] (V.T.) 2 Istituto Nazionale di Geofisica e Vulcanologia, 00143 Rome, Italy; [email protected] 3 Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH, 8092 Zurich, Switzerland; [email protected] (B.E.); [email protected] (O.B.) 4 Department of Biological, Geological and Environmental Sciences, University of Catania, 95129 Catania, Italy; [email protected] * Correspondence: [email protected] Received: 9 January 2019; Accepted: 19 February 2019; Published: 21 February 2019 Abstract: This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 ◦C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 ◦C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to DNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface. Keywords: sill magma; magma decompression and degassing; REE fractionation; Mt. Etna volcano 1. Introduction Mt. Etna volcano (Sicily, Italy; Figure1a) is the largest volcano in Europe, and one of the most active and most intensely monitored on Earth. The volcanic edifice is 1200 km2 wide and 3.3 km high. The greater part of the stratovolcano was constructed by overlapping products with a Na-alkaline affinity and principally erupted in the last 220 ka (e.g., Reference [1]). Magma dynamics are frequently referred to the pulsatory upward migration of H2O-rich magmas and/or fluxes of abundant volatiles from depth [2–6]. The explosive activity of the volcano mostly involves gas-charged Minerals 2019, 9, 126; doi:10.3390/min9020126 www.mdpi.com/journal/minerals Minerals 2019, 9, x FOR PEER REVIEW 2 of 24 frequently referred to the pulsatory upward migration of H2O-rich magmas and/or fluxes of Mineralsabundant2019 ,volatiles9, 126 from depth [2–6]. The explosive activity of the volcano mostly involves2 gas- of 23 charged magmas formed at mantle depths with primitive compositions [7,8]. However, distinct magma storage regions at deep, intermediate and shallow levels have been recognized along the magmasvertically-developed formed at mantle plumbing depths withsystem primitive [9–15]. compositions The migration [7,8 ].of However, magma distincttowards magma the surface storage regionschanges at its deep, physicochemical intermediate and state, shallow thus levelscontrolling have been mineral recognized and melt along compositions, the vertically-developed degree of plumbingcrystallization, system magma [9–15 ].ascent The migration velocity and of magma type of towards eruption the [2,9,10,16–20]. surface changes A distinctive its physicochemical feature of state,the recent thus activity controlling (post-1971) mineral is andthe persistent melt compositions, eruption of degree magmas of with crystallization, K-affinity and magma buffered ascent to velocitytrachybasaltic and type compositions of eruption that [2,9 ,10sporadically,16–20]. A distinctiveevolve towards feature basaltic of the recenttrachyandesites activity (post-1971) [21]. In iscontrast, the persistent historic eruption(pre-1971) of and magmas prehistoric with K-affinityproducts show and buffered a distinctive to trachybasaltic Na-affinity, compositionsranging from thathawaiites sporadically to mugearites. evolve towardsThe Valle basaltic del Bove trachyandesites (Figure 1b) volcanic [21]. In succession contrast, historic is characterized (pre-1971) by and a prehistoricrarity of primitive products products show a distinctiveand its prehistoric Na-affinity, eruptions ranging evolve from hawaiitesup to trachyte to mugearites. encompassing The Vallehigh deldegrees Bove of (Figure crystallization1b) volcanic (>40%). succession This remarkable is characterized compositional by a rarity variability of primitive has been products attributed and its to prehistoric eruptions evolve up to trachyte encompassing high degrees of crystallization (>40%). crystal fractionation processes at shallow crustal levels mostly controlled by degassing and H2O Thisexsolution remarkable phenomena compositional [16,22–25]. variability has been attributed to crystal fractionation processes at shallowIn this crustal study, levels we mostlypresent controlled new geochemical by degassing data from and Ha2 sillO exsolution (~80 m long phenomena and ~30 m [ 16thick;,22– 25Figure]. 1c) atIn ~1780 this study, m altitude we present in the new southern geochemical wall of datathe Valle from adel sill Bove (~80 (Figure m long and1b). ~30The m intrusive thick; Figure body1 c)is at ~1780 m altitude in the southern wall of the Valle del Bove (Figure1b). The intrusive body is in in continuity with the prehistoric lava flows of Trifoglietto (80–60 ka) and Ellittico (60–15 ka) continuityeruptive centers with the [26]. prehistoric The sill lavais in flows sharp of Trifogliettocontact with (80–60 the host ka) andlavas Ellittico and its (60–15 age ka)is therefore eruptive centersconstrained [26]. as The post sill flow is inemplacement. sharp contact The with exposu there host is 80 lavas m long, and but its agethe sill is therefore continues constrained buried out asof sight. post flow Nine emplacement. rock samples have The exposurebeen collected is 80 mat long,the central but the portion sill continues of the sill buried using outa sampling of sight. Ninedistance rock of samples 1.5 m. Major have beenand trace collected elements at the of central bulk rocks portion and of single the sill minerals using a samplinghave been distance analyzed of 1.5 m. Major and trace elements of bulk rocks and single minerals have been analyzed for each rock for each rock sample and then discussed in the framework of magma modeling based on P–T–fO2– sample and then discussed in the framework of magma modeling based on P–T–f O –H O-lattice H2O-lattice strain equations. This approach constrains the crystallization and 2emplacement2 strainconditions equations. of the This sill, approach illustrating constrains that thethe crystallizationdifferentiation and of emplacement prehistoric conditionsmagmas started of the sill,at illustratingintermediate that (~6 the km) differentiation depths [27–30] of prehistoric and proceede magmasd through started at fractional intermediate crystallization, (~6 km) depths degassing [27–30] and proceededcooling phenomena through fractional at shallow crystallization, crustal levels degassing or during and cooling ascent phenomena towards the at shallow surface crustal (e.g., levelsReferences or during [16,24,25]). ascent towards the surface (e.g., References [16,24,25]). Figure 1. SchematicSchematic map map showing showing the the location location of of Mt. Mt. Etna Etna volcano volcano (a), ( aValle), Valle del del Bove Bove depression depression (b) (sillb) sillformation formation (c). (c). 2. Methods Photomicrographs ofof thethe sill sill samples samples were were collected collected with with the backscatteredthe backscattered electron electron (BSE) mode(BSE) ofmode a field of emissiona field emission gun-scanning gun-scanning electron microscopy electron microscopy (FE-SEM) Jeol (FE-SEM) 6500F (JEOL, Jeol 6500F Ltd., Tokyo,(JEOL, Japan) Ltd., equippedTokyo, Japan) with equipped an energy-dispersive with an energy-dispersive spectrometer (EDS) spectrometer detector and (EDS) installed detector at the and HPHT installed Laboratory at the ofHPHT Experimental Laboratory Volcanology of Experimental and Geophysics Volcanology of the and Istituto Geophysics Nazionale of dithe Geofisica Istituto eNazionale Vulcanologia di (INGV)Geofisica in e Rome, Vulcanologia Italy. (INGV) in Rome, Italy. Microchemical analysesanalyses of of minerals minerals were were performed performed at the at INGV the INGV
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