Constraining the recent plumbing system of Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: The 1739 A.D. Pietre Cotte lava flow

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Constraining the recent plumbing system of Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: The 1739 A.D. Pietre Cotte lava flow
  Constraining the recent plumbing system of Vulcano(Aeolian Arc, Italy) by textural, petrological, and fractalanalysis: The 1739 A.D. Pietre Cotte lava flow  Monica Piochi Sezione Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, I-80124 Napoli, Italy ( Gianfilippo De Astis Sezione Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia, I-80124 Napoli, Italy  Now at Sezione di Sismologia e Tettonofisica, Istituto Nazionale di Geofisica e Vulcanologia, I-00143 Roma, Italy ( Maurizio Petrelli Dipartimento di Scienze della Terra, Universita` di Perugia, I-06123 Perugia, Italy ( Guido Ventura  Sezione di Sismologia e Tettonofisica, Istituto Nazionale di Geofisica e Vulcanologia, I-00143 Roma, Italy ( Roberto Sulpizio Dipartimento Geomineralogico, Universita` di Bari, I-70125 Bari, Italy (  Alberto Zanetti Consiglio Nazionale delle Ricerche, Istituto di Geoscienze e Georisorse, I-27100 Pavia, Italy ( [ 1 ]  The 1739 A.D. Pietre Cotte lava flow is part of a sequence of low-explosive to weak effusion eventsoccurred at La Fossa Cone, the active vent on Vulcano Island (Aeolian Arc, Italy). This lava is rhyolitic,texturally heterogeneous, and contains lati-trachytic enclaves. These compositions are recurrent in the LaFossa volcanic products and are representative of the recent Vulcano plumbing system. The host lava isvesicular, relatively phenocryst-free, and locally contains microlites and millimeter-sized spherulites. Theenclaves are up to 10 cm in size, display angular to spherical shapes, and can form the core of spherulites.Enclaves mostly consist of plagioclase and augitic phenocrysts set in a weakly vesicular groundmasscharacterized by variable abundance of glass and feldspar microlites. Field, textural, and fractal data allowus to constrain the rheological features of the rhyolitic and lati-trachytic magmas. In situ major, trace, andvolatile element analyses provide evidence for heterogeneities in the glassy matrix and zoning of  phenocrysts. Processes of magma evolution have been quantitatively constrained by using the apparent distribution ratios of trace elements measured between mineral phases and glassy matrices. The collecteddata in combination with petrological and fluid inclusion data from the literature provides evidence for (1) agenetic relationship between the two magmas through assimilation fractional crystallization process; (2) amingling mechanism between an uprising rhyolitic magma and a shallower partly crystallized lati-trachytic G 3 G 3 GeochemistryGeophysicsGeosystems Published by AGU and the Geochemical SocietyAN ELECTRONIC JOURNAL OF THE EARTH SCIENCES GeochemistryGeophysicsGeosystems  Article  Volume 10 , Number 1 30 January 2009Q01009, doi:10.1029/2008GC002176ISSN: 1525-2027 Click Here for Full Article Copyright 2009 by the American Geophysical Union 1 of 28  magma plug; (3) the desegregation (enclaves) at variable scales of the lati-trachyte within the rhyolite; and(4) the possible eruptive scenarios consequent to a future magmatic unrest. Components:  11,415 words, 11 figures, 4 tables . Keywords:  crustal assimilation; enclaves; lava flow; magma mingling; rheology. Index Terms:  3625 Mineralogy and Petrology: Petrography, microstructures, and textures; 8488 Volcanology: Volcanichazards and risks; 8414 Volcanology: Eruption mechanisms and flow emplacement. Received  17 July 2008;  Revised  1 December 2008;  Accepted  9 December 2008;  Published  30 January 2009.Piochi, M., G. De Astis, M. Petrelli, G. Ventura, R. Sulpizio, and A. Zanetti (2009), Constraining the recent plumbing systemof Vulcano (Aeolian Arc, Italy) by textural, petrological, and fractal analysis: The 1739 A.D. Pietre Cotte lava flow,  Geochem.Geophys. Geosyst. ,  10 , Q01009, doi:10.1029/2008GC002176. 1. Introduction [ 2 ] Silicic volcanism characterizes all island arcs incontinentalmarginsettings,althougheruptivestylesmay vary in different volcanoes (see  Eichelberger  [1995] and  Petford   [2003] for reviews). Intermedi-ate to high-silica magmas are typically involved inthe formation of caldera structures and dome com- plexes, and they commonly retain petrologicalevidence for interaction with less evolved magmas.[ 3 ] Several studies have dealt with the genesis andevolution of these magmas from their residence incrustal magma chambers until their more or lessdramatic arrival at the Earth’s surface [e.g.,  Wilson ,1989, and references therein]. In particular, inter-mediate- to high-silica magmas bearing enclaveswith contrasting compositions, have received atten-tion because they provide insights into (1) themechanisms of magma interaction, (2) the pro-cesses of magma chamber refilling, and (3) the possible influence exerted by these processes onthe dynamics of magma storage, ascent, anderuption [e.g.,  Eichelberger  , 1980;  Bacon , 1986; Turner and Campbell  , 1986;  Koyaguchi and  Blake , 1989;  Fernandez and Gasquet  , 1994;  Murphy et al. , 2000;  Holtz et al. , 2004;  Ban et al. ,2005]. Likewise, small volume eruptions withlow-explosive to nonexplosive degassing of silica-rich magmas offers a chance to study the interrela-tion between eruption dynamics and viscosity of chemically comparable magmas [see  Eichelberger etal. ,1986;  Eichelberger  ,1995;  Martel etal. ,1998,2000;  Melnik and Sparks , 2005, and referencestherein]. Despite a wealth of published researches,these topics are still not exhaustively investigated.[ 4 ] High-silica lava flows and domes containingmafic enclaves have frequently formed at theAeolian volcanoes, in Italy [e.g.,  Clocchiatti et al. , 1994;  Gioncada et al. , 1998, 2005;  De Rosaet al. , 2003;  Davı`  , 2007]. These domes and lavas provide a unique opportunity to clarify the relation-ships among magma chambers located at different levels in the crust and the interaction mechanismsamong magmas with different physical-chemicalfeatures.[ 5 ] This study deals with the most recent lava (i.e.,the 1739 A.D. Pietre Cotte lava flow) erupted fromVulcano Island, in the Aeolian Arc (Figure 1). Thissmall-volume lava flow (Figure 2) was emplacedtoward the end of a sequence of low-explosive tomild-explosive events erupting magmas with vari-able compositions from La Fossa Cone (Figures 1band 1c). It is rhyolitic in composition and containsabundant dark gray to reddish lati-trachytic lavaenclaves (Figure 1c and Figures 2c, 2d, and 2e) of variable aspect and debated srcin (see  Perugini et al.  [2007] and references therein for a review).Since the latite-to-rhyolite variation, the 1739 A.D. products span almost the whole compositionalrange of the youngest Vulcano magmas and can provide information about the plumbing systemactive on the island.[ 6 ] This paper presents new field, morphological,textural, fractal, and in situ major and trace element data on the Pietre Cotte lava and enclaves, usingthem in conjunction with petrological and fluidinclusion information from the literature on themost recent rocks from La Fossa Cone [  De Astiset al. , 1997;  Gioncada et al. , 1998;  Frezzotti et al. ,2004]. The aim is to define (1) the srcin of theenclaves in the 1739 A.D. lava; (2) the mechanismof interaction between compositionally and rheo-logically different magmas represented in the flow;(3) the thermobarometric conditions of the differ-entiation processes and their effects on the eruptive GeochemistryGeophysicsGeosystems G 3 G 3  piochi et al.: vulcano system by pietre cotte lava flow  10.1029/2008GC002176 2 of 28  dynamics; (4) the possible rheological behavior of the magmas along the conduit and during emplace-ment. Furthermore, our data set has been used toconstrain the shallow plumbing system of La FossaCone. We believe that in such a highly populated(in particular during the tourist season) and volca-nically active area this information is useful for thecorrect evaluation of the volcanic hazard and can be used to define the triggering mechanism of eruptions, the magma ascent dynamics, and thekinematics of lava flows. In general, the present study indicates that field, morphological, textural, Figure 1.  (a) Vulcano Island, Southern Tyrrhenian Sea (Italy). Inset (modified from  Peccerillo  [2003]): different grayscale indicates Italian volcanic areas. Legend (from  De Astis et al.  [1997]): 1, alluvial deposits; 2, 183 B.C. to1550 A.D. lava and pyroclastic deposits from Vulcanello cones; 3, lava preceding the Vulcanello composite cone;4, La Fossa Cone deposits dated at 6 ka to 1890 A.D.: pyroclastic rocks (shaded area) and lava flow (black); 5, LaFossa Caldera volcanic products dated at 15–8 ka; 6, Mastro Minico-Lentia Complex (28–13 ka); 7, undifferentiated products dated at 120–21 ka; 8, faults; 9, crater rim. (b) Stratigraphy of La Fossa Cone with the time position of thePietre Cotte Formation [from  Dellino and La Volpe , 1997;  De Astis et al. , 2006]. Pietre Cotte Formation: pc1 is thinlyand plane bedded deposits and pc2 is lava flow. (c) Chemostratigraphy of rocks from La Fossa Cone (whole rock data:gray diamonds from  De Astis et al.  [1997] and dashed gray square from G. De Astis (unpublished data, 2008)). Notewith different symbols the two compositions of the Pietre Cotte Lava flow products (light squares are host rock lava;dark squares are enclaves) and the chemical evolution trend of the Pietre Cotte Formation (arrow). Also note thechemical range of products preceding and following the eruption of the Pietre Cotte Formation. VL indicates the latiticscoria used as the magmatic end-member in the AFC model. Refer to the text for further details. GeochemistryGeophysicsGeosystems G 3 G 3  piochi et al.: vulcano system by pietre cotte lava flow  10.1029/2008GC002176 piochi et al.: vulcano system by pietre cotte lava flow  10.1029/2008GC002176 3 of 28  fractal, and petrological data should be integratedin order to clarify petrological and volcanologicalissues about the behavior of magmatic and volca-nic systems. 2. Volcanological Data  2.1. Volcanic Activity at La Fossa Cone [ 7 ] During the volcanism that developed within LaFossa Caldera (15–6 ka; Figure 1a), the Vulcanofeeding system erupted rhyolites, with formation of domes and lava flows, and latitic to trachyticmagmas (Figure 1c), as well as K-alkaline basalts(Vulcanello, Figure 1a) [  De Astis et al. , 1997].[ 8 ] In the following 6 ka, La Fossa Cone grewwithin the caldera, generating eruptions from dif-ferent vents (Figures 1a and 1b) [  Dellino and LaVolpe , 1997;  De Astis et al. , 2006]. Volcanic products consist of compositionally different pyro-clastic successions that, in some cases, may includelava flows [  Dellino and La Volpe , 1997;  De Astis et al. , 2006]. As previously observed, rock composi-tions range from latite to rhyolite, alternating alongone or two adjoining successions (Figures 1b and1c). Overall, these data indicate that the shallow plumbing system of La Fossa Cone was character-ized by the coexistence of magmas with different composition, forming multiple pockets and feedingfractures that favored mutual interaction [  De Astiset al. , 1997].[ 9 ] The very recent Pietre Cotte Formation [  De Astis et al. , 2006] (Figure 1b) represents one of themost intriguing examples of this type of heteroge-neous magmatism. Latitic surges erupted at the beginning are closely followed by trachy-rhyolitic pumices and by a rhyolitic lava containing lati-trachytic enclaves (Figure 1c and Figures 2c, 2d,and 2e). In particular, a thinly and plane parallel bedded pyroclastic succession with thickness up to3 m and limited dispersion around the crater represents the early deposit of the Pietre CotteFormation. It consists of variously colored, vesic-ular, fine ashes alternated with thin-to-thick graylaminas of poorly coherent medium to coarseashes, which increase in frequency toward the topof the unit. The composition of fresh glass shardssampled from the basal layers is latitic [  Dellino and  La Volpe , 1997]. A massive layer (up to 2.5 mthick near the vent) of pumiceous lapilli andsubordinate trachy-rhyolitic bread crust bombsrelated to fall activity overlays the basal unit of the Pietre Cotte Formation. These products arewidespread up to 2 km away from the La FossaCone.[ 10 ] According to historical reports, the PietreCotte lava flow closed an eruptive period, whichlasted not more than tens of years. The renewal of the volcanic activity about 80 years later and until1888 A.D. [  Mercalli , 1891] produced trachy-rhyo-litic pumices followed by latitic scoriaceous bombs[  De Astis et al. , 2006] (Figure 1c). Noteworthy,these later (younger) eruptions produced a mag-matic sequence that replicates the compositionalrange observed in the Pietre Cotte products andindicates an unchanged magma feeding system. 2.2. Stratigraphy and Field Observations of the Pietre Cotte Lava Flow  [ 11 ] The Pietre Cotte lava flow outcrops on thenorthern slope of La Fossa Cone (Figures 1a Figure 2.  (a) General view of the Pietre Cotte lavaflow along the La Fossa Cone. The red line delimits theflow; note its variable thickness and low aspect ratio, aswell as the basal slope. (b) The front of the Pietre Cottelava flow; note the basal brecciated zone and the flowlayering. (c) Photograph of a large enclave (enclosed bythe yellow line). (d) Photograph showing varioussmaller enclaves (indicated by the red arrows) withinthe host lava. (e) Photograph showing various smallenclaves (indicated by the red arrows) deformed withinthe host lava; note the flow layering, as indicate by theyellow dot lines. Blue arrow directly drawn on thesample indicates the base-to-top direction. GeochemistryGeophysicsGeosystems G 3 G 3  piochi et al.: vulcano system by pietre cotte lava flow  10.1029/2008GC002176 4 of 28  and 2a). It is  350 m in length and  200 m wide.The thickness progressively increases from the vent to the front, where it reaches the maximum value of   10–12 m, owing to the decrease of the groundslope. The lava flow exhibits a low aspect ratio(length/height) of    35 (Figure 2a) and the basalslope on which it emplaced was  25–30  .[ 12 ] Different lithological facies can be distin-guished along the flow and at its margins. A basalcarapace breccia is the most frequent facies ob-served along the contact between lava and paleo-topographic surfaces (Figure 2b). Massive lavawith facies transitions to layering (foliation), fold-ing, and extended obsidian portions occurs wherethe flow is thicker and mainly at the front. Foliationis defined by the alternation of black to grayobsidian and white gray, millimeter-tick, irregular vesicular laminas (Figures 2b and 2e).[ 13 ] Near the vent, the lava contains abundant quartzite xenoliths (made up of 95 vol.% quartz;hereinafter referred to as quartz xenoliths; see  Frezzotti et al.  [2004] for details), as well asfew-to-ten centimeters in size darker enclaves(Figures 2c, 2d, and 2e) and whitish clasts. Bothclasts and enclaves have variable shapes and con-tacts with the host lava. The whitish clasts couldrepresent extremely modified enclaves, whose li-thology has been almost completely transformed by intense hydrothermal activity (i.e., argillic alter-ation). At the front (Figure 2b), the lava is massiveor thinly layered for about 50–65% of its thick-ness, whereas the basal part is fragmented with brecciated and/or small-sized (millimeter to tens of centimeters scale) folding structures. The better exposures of the enclaves are in the distal outcrops.Here, the enclaves range from millimeter-sizeddrop-like, oxidized inclusions to few to tens of centimeters in size irregularly shaped, porphyriticclasts (Figures 2c, 2d, and 2e). These enclaves show both smooth and sharp contacts with the host lava.[ 14 ] Some loosen clinopyroxene phenocrysts can be found in the lava. They derive from the partialdisjointing of the enclaves. 3. Analytical Methods [ 15 ] The enclaves have been studied by means of fractal geometry. The chemical compositions of glasses and minerals have been analyzed usingelectron and ion microprobes and laser ablationinductively coupled plasma-mass spectrometry(LA-ICP-MS). Analytical methods are reported below. 3.1. Geometric and Fractal Geometry  Analyses [ 16 ] The size, axial ratio, and fractal dimension(D  box ) of enclaves within the Pietre Cotte lava flowhave been measured adopting the method detailed by  De Rosa et al.  [2002]. Photographs (  30   30 cm large) have been acquired by means of ahigh-resolution (5 Mpixel) camera at a distance of 1 m from the outcrop. The color to gray scaleconversion has been performed using the AdobePhotoshop 6.0 software. For each image, a stan-dard density slicing technique has been applied inorder to select the interface between enclaves andsurrounding host using the Scion Image software([ 17 ] The size of the enclaves is given by theequivalent radius, which represents the radius of a circle having the same area of the enclave. Theaxial ratio of the enclaves is the ratio betweenthe major and minor axis of the enclaves. D  box of the interface has been determined followingthe box counting method of   Mandelbrot   [1982]and  Sreenivasan and Meneveau  [1986]. The meth-od involves the subdivision of the image intosquare boxes of size  r   and in counting the number of boxes  N  ( r  ) containing the interface (Figure 3).This is repeated for different sizes of the boxes.  D  box  is determined using the equation:  D  box  ¼ log  N r  ð Þ = log r  The analysis of the interfaces has been performedusing the HarFA software developed by  Zmeskal et al.  [2001]. The error in the determination of   D  box  is<5% [  Buchnicek et al. , 2000]. 3.2. Electron and Ion Microprobe Analyses [ 18 ] Major element and Cl analyses of glasses andcrystals (Tables 1 and 2) were obtained from 30and 80  m m-thick polished thin sections using aCameca SX-50 Electron Microprobe at the ‘‘Isti-tuto di Geologia Ambientale e Geoingegneria’’(Consiglio Nazionale delle Ricerche) in Roma,Italy. The following operating conditions for Wave-length Dispersive Spectroscopy (WDS) analysishave been adopted: 10 nA beam current, 15 keVaccelerating voltage, and 100 s counting time. For each spot analysis on glass, alkalis were alwaysconcurrently counted during the first 15 s using adefocused beam size of 10  m m in order to limit thedegree of volatilization during probing. Otherwise,the beam size was 5  m m. Data reduction was madeusing the ZAF4/FLS software by Link Analytical. GeochemistryGeophysicsGeosystems G 3 G 3  piochi et al.: vulcano system by pietre cotte lava flow  10.1029/2008GC002176 5 of 28
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