Kinematic significance of morphological structures generated by mixing of magmas: a case study from Salina Island (southern Italy

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Kinematic significance of morphological structures generated by mixing of magmas: a case study from Salina Island (southern Italy
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  Kinematic significance of morphological structures generated bymixing of magmas: a case study from Salina Island (southern Italy) D. Perugini a, *, G. Ventura  b , M. Petrelli a  , G. Poli a  a   Department of Earth Sciences, University of Perugia, Piazza Universita`  , I-06100 Perugia, Italy  b Osservatorio Vesuviano, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Via Diocleziano, 328, I-80124 Napoli, Italy Received 7 November 2003; received in revised form 26 February 2004; accepted 24 March 2004 Abstract Morphological features of mixing/mingling structures of heterogeneous juveniles from the 13-ka-old Upper Pollara eruption(Salina Island, southern Italy) are studied. These heterogeneous rocks result from the mixing between an andesitic and arhyolitic magma. Concentration patterns generated by magma mixing are analyzed on digital images and results have beencompared with those obtained from numerical simulations of mixing processes coupling chaotic advection and chemicaldiffusion. Results show that the Upper Pollara rocks were produced by mixing 24–49% of andesite and 76–51% of rhyolite.Multifractal analysis of the mixing structures is performed and the mixing intensity, i.e. the degree of interaction between thetwo end-member magmas, has been deduced from the analysis of the singularity spectrum. Results show that narrower is themultifractal spectrum, the higher is the degree of homogenization of magmas. Reynolds number (  Re ) during mixing has beenestimated from the geometrical analysis of mixing structures by using experimentally defined relationships between shape parameters and  Re . Results show that   Re  is between ca. 500 and 7000. There is a positive correlation between the estimatedinitial percentage of mafic magma in the different analyzed samples and  Re , in agreement with the observation that the higher isthe percentage of the mafic, lower viscosity magma, the higher is the turbulence of the mixing system. In addition, our analysisreveals an unexpected inverse relationship between the calculated  Re  and the degree of magma homogeneity suggesting that energy dissipation may have had a major role in the controlling mixing process because dissipation is inversely proportional tothe mixing efficiency. Results suggest that mixing processes between the andesitic and rhyolitic magmas mainly developed inthe conduit. It is also suggested that mixing occurred in a shear layer-type or pipe-type flow. D  2004 Elsevier B.V. All rights reserved.  Keywords:  magma mixing; morphological structures; concentration patterns; image analysis; multifractals; fluid dynamic regime 1. Introduction Textural heterogeneity due to magma mixing/ mingling is a common feature of volcanic rocks(e.g. Refs. [1–5]), and the processes responsible for them have been discussed extensively [5–10]. This heterogeneity includes enclaves, banding, and‘streaky’ structures [4,11–13]. Although studies on the mineralogical and geochemical features of mixedrocks are abundant (e.g. Refs. [12,14–17]), only few studies focus on quantitative analyses of morpholo-gies related to textural heterogeneity [18–24]. These 0012-821X/$ - see front matter   D  2004 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2004.03.038* Corresponding author. Tel.: +39-75-585-2652; fax: +39-75-585-2603.  E-mail address:  diegop@unipg.it (D. Perugini).www.elsevier.com/locate/epslEarth and Planetary Science Letters 222 (2004) 1051–1066  studies mainly focus on (1) the deformation of theinterface between the two interacting magmas inlaminar flows (e.g. Refs. [20,21]) and turbulent flows [18]; (2) the effects of  the vesiculation on the deformation of enclaves [23]; and (3) the relation-ships between the morphology of the mingling/mix-ing structures and the degree of magma interaction[22,24].Studies on the fluid-dynamics of interactingliquids (e.g. Refs. [25–30]), as well as analogue models on magma mixing (e.g. Refs. [31–33]), haveshown that different morphological structures (e.g.folds, blobs, filaments, vortexes) give useful infor-mation about the flow regime and the dynamics of mixing.In this study, we analyze mixing morphologicalstructures of heterogeneous juvenile fragments from a13-ka-old flow deposit of the Upper Pollara eruption(Salina Island, southern Italy). In a first step, weshow that different colours of the glass correspond todifferent magma compositions. In a second step, we perform image and multifractal analysis of the mix-ing structures in order to determine (a) the initial percentages of the end-member magmas, (b) themixing intensity and (c) the flow regime actingduring the mixing process. In a third step, we discussthe relationships between (a), (b) and (c), and pro- pose a kinematic model for magma interaction. Weshow that the analytical approach proposed here can be used to extract petrological and physical informa-tion from the analysis of the concentration patterns of mixed rocks. 2. Geological setting and the Upper Pollaraeruption The Pollara depression is located in the north-western sector of Salina Island (Aeolian Archipela-go, Southern Tyrrhenian Sea), which consists of fivemain volcanoes (Corvo, Rivi-Capo, Fossa delleFelci, Porri and Pollara) formed between 170 and13 ka (Fig. 1A; Ref. [34]). Volcanism within the Pollara depression started with the emission of thePunta Perciato lava flow (30 ka [35]) and endedwith two explosive eruptions known as Lower Pollara (24 ka [12]) and Upper Pollara (13 ka [36]). The Lower Pollara deposits consist of threemain fall units. Magma compositions are andesitic(lowermost fall unit), andesitic and dacitic (interme-diate unit) and rhyolitic (uppermost unit). Suchvariability has been interpreted as the result of thesyn-eruptive turbulent mixing between an aphyricrhyolitic magma and an andesitic, partly crystal-lized, magma [12].The Upper Pollara deposits (Fig. 1C) consist of a70-m-thick sequence of fall beds (lithofacies A1 inFig. 1C) followed by a rhythmic alternation of massive (lithofacies B) to crudely stratified (lithofa-cies A) coarse ash and lapilli layers. In the upper part of the sequence, thick, inversely graded beds of medium-coarse ash and lapilli (lithofacies C) occur [37,38]. The sequence ends with a massive bed of  fine to medium ash (lithofacies D). According to Ref.[39], the Upper Pollara sequence mainly resultedfrom the emplacement of highly concentrated pyro-clastic flows. 3. The Upper Pollara juvenile component:evidence of magma mixing processes On the basis of petrographic investigations on thecoarse (2 mm) to medium (1/16 mm) ash fraction of the deposits, the juvenile glass po pulation consists of  four main classes [38] (Fig. 1C): (i) dark glass, (ii) colorless glass, (iii) heterogeneous glass and (iv)gray glass with microlites. Both (i) and (iv) glassesare less than 20 vol.% in the sequence. Glass (ii)decreases with increasing stratigraphic height, where-as glass (iii) increases. Detailed observations (Fig.2A–C) show that glass (iii) consists of micrometer-to centimeter-sized filament-like structures, plane- parallel and poorly folded bands to vortex- andstrongly folded-like structures. In detail, inside thesame mixing system, it is possible to observe thecontemporaneous occurrence of (i) filament-likeregions in which magmas are in contact along wideinterfaces and (ii) coherent regions, showing a glob-ular shape, and occurring between filament-likeregions (Fig. 2A–C). It is noteworthy that the occurrence of such structures has been widely docu-mented in mixed volcanic rocks and it is interpretedas the result of chaotic dynamics of the magmamixing process [22,24]. As shown by Ref. [34], glass compositions range from high-potassium  D. Perugini et al. / Earth and Planetary Science Letters 222 (2004) 1051–1066  1052  Fig. 1. (A) Structural scheme of the Aeolian Islands. (B) Simplified geological map of  Salina Island (from Ref. [37]) with the distribution of the Pollara pyroclastics. (C) Stratigraphic column of the Upper Pollara deposits (from Ref. [38]). The vol.% variation of the different types of   juvenile fragments with stratigraphy is also reported (data from Ref. [38]).  D. Perugini et al. / Earth and Planetary Science Letters 222 (2004) 1051–1066   1053  (HK) andesites (SiO 2 f 63%) to high-silica rhyolites(SiO 2 f 76 wt.%). According to Refs. [22,39], the geochemical data indicate, together with the macro-scopic and petrographic features of the juvenilecomponent, that the Upper Pollara magmas are the product of the mixing of a HK andesitic magma witha high-silica rhyolite before the magma fragmenta-tion in the conduit. The whole eruptive sequence has been interpreted as the result of the emptying of anormally zoned magmatic reservoir. 4. Analysis of the concentration patterns of heterogeneous juveniles Digital images of heterogeneous juveniles showthat the dispersion of the mafic magma into the felsicone generated complex morphological structureswhose gray value is highly variable (Fig. 2A–C).[22] selected poorly vesiculated (vesicles<5 vol.%)and subaphyric (crystals V 5 vol.%) heterogeneous juvenile clasts from the Upper Pollara sequence in Fig. 2. (A–C) Pictures of representative magma mixing structures occurring in the Upper Pollara sequence; (D) SiO 2  vs. grey level plot for theUpper Pollara juveniles (data from Ref. [22]). The best fit line among the data and the 0.95 confidence is also reported.  D. Perugini et al. / Earth and Planetary Science Letters 222 (2004) 1051–1066  1054  order to calibrate the gray value of digital images withthe glass composition. Glass composition has beendetermined by electron microprobe on homogeneousglass portions with variable gray intensity. Detailsabout the analytical methods can be found in Ref.[22]. Results of the coupled image/chemical analysisof the selected Upper Pollara clasts are reported inFig. 2D, where a positive correlation between grayvalues and chemical composition (SiO 2  wt.%) of glasses is observed. The best fitting cur ve is the polynomial curve reported in the graph [22]. This result indicates that differences in the gray value of images reflect different chemical compositions. It should be noted that the gray value equal to 20(SiO 2 f 63 wt.%) corresponds to the most maficmagma and that gray values below 20 are associatedto the few vesicles occurring in the samples. Since thefocus here is on the analysis of concentration patternsof samples, gray values below 20 will be not consid-ered. Gray values above 250 (SiO 2 f 76 wt.%) arenot detected in the analyzed samples meaning that thisgray value corresponds to the most felsic magma.Hereafter, results based on analysis of digital imagesof samples will be presented considering the SiO 2 composition of glasses calculated by using the equa-tion reported in Fig. 2D. 5. Determination of initial percentages of magmas Weselected 10heterogeneousclasts from theUpper Pollara sequence. These samples have been collectedwithin twoinversely graded beds(lithofacies B) at25– 30 m from the base of the stratigraphic column, wherethe heterogeneous clasts represent over 70 vol.% of thetotal juvenile component. All samples have vesicles<5 vol.% and a crystal content   V 5 vol.%. Polishedthick slices from these samples have been acquired ingray scale by means of a high-resolution opticalscanner with resolution 1 pixel=0.075 mm (1 mm=13.35 pixels) and gray values have been extracted fromdigital images using the National Institute of Health(NIH) image analysis software.Fig. 3 reports frequency histograms for SiO 2  calcu-lated from digital images of some representative sam- ples (Fig. 2A–C). Histograms are bell-shaped and encompass all compositions between SiO 2 = 63%(mafic magma) and SiO 2 =76% (felsic magma) with Fig. 3. Frequency histograms of concentration for the three representative magma mixing structures shown in Fig. 2. Along the  x -axis both graylevel (GL) and corresponding SiO 2  values are reported. Light gray areas on the left and right side of the graph represent gray levels related tovesicles (GL<20) and not detected (GL>250) in the mixing structures, respectively.  D. Perugini et al. / Earth and Planetary Science Letters 222 (2004) 1051–1066   1055
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