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Zandomeneghi, D.
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Zandomeneghi, D.
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- PublicationRestrictedQuantitative analysis of X-ray microtomography images of geomaterials: Application to volcanic rocks(2010-12)
; ; ; ; ; ; ;Zandomeneghi, D.; International Centre for Theoretical Physics, Trieste, Italy ;Voltolini, M.; Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy ;Mancini, L.; Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy ;Brun, F.; Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy; Dipartimento di Elettronica, Elettrotecnica ed Informatica, Universitá di Trieste, Trieste, Italy ;Dreossi, D.; Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy ;Polacci, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; ; ; ; ; X-ray computed microtomography is an excellent tool for the three-dimensional analysis of rock microstructure. Digital images are acquired, visualized, and processed to identify and measure several discrete features and constituents of rock samples, by means of mathematical algorithms and computational methods. In this paper, we present digital images of volcanic rocks collected with X-ray computed microtomography techniques and studied by means of a software library, called Pore3D, custom-implemented at the Elettra Synchrotron Light Laboratory of Trieste (Italy). Using the Pore3D software, we analyzed the fabrics and we quantified the characteristics of the main constituents (vesicles, crystals, and glassy matrix) of four different types of pyroclasts: frothy pumice, tube pumice, scoria, and “crystalline” scoria. We identified the distinctive features of these different types of volcanic rocks. The frothy pumices show vesicles that coalesce in isotropic aggregates, especially toward the sample interior, while the scoriae have a low porosity and an abundance of isolated vesicles. In the “crystalline” scoria sample most of the vesicle separation is due to the presence of crystals of different types, while the tube pumice shows an anisotropic distribution of vesicles and crystals at the microscale, as also observed at the scale of the hand sample. Quantitative analysis and textural information may supply an additional tool to investigate the eruptive processes and the origin of volcanic rocks.153 24 - PublicationRestrictedRole of conduit shear on ascent of the crystal-rich magma feeding the 800-year-b.p. Plinian eruption of Quilotoa Volcano (Ecuador)(2004-05)
; ; ; ; ; ;Rosi, M.; Dipartimento di Scienze della Terra, Pisa, Italy ;Landi, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia ;Polacci, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Di Muro, A.; Dipartimento di Scienze della Terra, Pisa, Italy ;Zandomeneghi, D.; Dipartimento di Scienze della Terra, Pisa, Italy; ; ; ; We have characterized pumice products belonging to the climactic phase of the 800-year-b.p. Quilotoa eruption. Bulk rock compositions, petrography, mineral, and glass chemistry and textural investigations were performed on the three end-member pumice types, namely white, gray, and mingled pumices. All the investigated pumice clasts are dacites characterized by the same bulk rock composition and mineralogical assemblage, but glass compositions and bulk textures change according to different pumice types. White pumice has higher crystallinity (~48 wt%), abundant euhedral pheno/microphenocrysts, no groundmass microlites, the most evolved glass compositions (7478 wt% SiO2), and heterogeneous vesicle populations marked by deformed and highly coalesced vesicles with thin walls. Gray pumice exhibits lower crystallinity (2936 wt%), abundant broken and/or resorbed crystals, ubiquitous groundmass phenocryst fragments and microlites, the widest range of glass compositions (6978 wt% SiO2), and quite homogeneous poorly deformed and coalesced vesicles with thicker walls. Mingled pumices are characterized by the alternation of bands or patches with white and gray pumice compositional and textural characteristics. We attribute heterogeneities in glass compositions and crystal and vesicle textures to processes occurring within volcanic conduits as magma is ascending to the surface. In particular, the above observations and results are consistent with an origin of a gray magma by heating of the original white magma in a strongly sheared region of the conduit because of a mechanism of viscous dissipation and crystal grinding and resorption at the conduit walls. The less viscous gray magma, therefore, would enable the onset and preservation of a high mass flux of the eruption otherwise difficult to explain for highly viscous crystal-rich dacitic magmas.188 85 - PublicationRestrictedSeismic tomography of Central Sao Miguel, Azores(2008-03)
; ; ; ; ;Zandomeneghi, D.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain ;Almendros, J.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain; Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain ;Ibanez, J. M.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain; Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain ;Saccorotti, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; ; ; We determine the three-dimensional distribution of P- and S-wave velocities for Central São Miguel Island (Azores, Portugal) by tomographic inversion of local earthquake arrival times. We use P- and S-phases from 289 earthquakes recorded by a network of 20 seismometers. The model shows good resolution in the shallowest 5–6 km, as illustrated by different resolution tests. There are several velocity anomalies, interpreted as pyroclastic deposits, intrusive bodies, geothermal fields, and the effects of tectonics. A low Vp zone marks Furnas caldera, probably evidencing volcaniclastic sediments with development of intense geothermal activity. Another low Vp zone extends in correspondence of the highly fractured area between Fogo and the north coast. Conversely, strong positive anomalies are found south of Fogo and northwest of Furnas. They are interpreted in terms of high-density deposits and remnants of a plutonic intrusion. These interpretations are supported by the distribution of Vp/Vs, and are consistent with previous geological, geochemical, and geophysical data.179 25 - PublicationRestrictedTexture analysis of volcanic rock samples: Quantitative study of crystals and vesicles shape preferred orientation from X-ray microtomography data(2011-04-30)
; ; ; ; ;Voltolini, M.; Sincrotrone Trieste, Basovizza (TS), Italy ;Zandomeneghi, D.; International Centre for Theoretical Physics, Trieste, Italy ;Mancini, L.; Sincrotrone Trieste, Basovizza (TS), Italy ;Polacci, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Pisa, Pisa, Italia; ; ; In the texture analysis of volcanic rocks, the preferred orientation of the constituents can provide useful information for the interpretation of the processes involved in the rock formation. We present here a new data analysis technique, based on X-ray microtomography measurements and on shape preferred orientation analysis, to obtain the orientation distribution functions of the constituents of volcanic rocks. This procedure proved to be very suitable for volcanic samples, where diffraction-based techniques, developed for crystallographic preferred orientation studies, are of limited utilization, in addition to the fact that they cannot provide any information about vesicles or bubbles. Moreover the analysis performed directly in three dimensions (3D) overcomes the problems that usually occur when employing stereological methods for the analysis of the images obtained via microscopy-based techniques. In this study, two scoriae (from Stromboli and Etna) and a tube pumice (from Campi Flegrei) were measured via X-ray microtomography and then the resulting volumes were analyzed following the proposed procedure. Results highlight little preferred orientation for the vesicles in the two scoria samples, whereas the pumice shows a marked preferred orientation. Crystals (also divided by mineral species) were taken into account as well and in the two scoria samples there is no crystal preferred orientation, in contrast with the pumice, where crystal preferred orientation features are very similar to the ones found for the vesicles. Overall we found strong differences in preferred orientation: weak for vesicles in scoriae, showing an axial symmetry with the axis parallel to the elongation axis of the sample, and a stronger and more complex orientation texture in the pumice sample for both crystals and vesicles. The promising results obtained suggest that this procedure is potentially very useful for the analysis of preferred orientation in volcanic rocks and geomaterials in general.138 22