Numerous active and potentially high-risk volcanoes do occur in the Italian peninsula (e.g. Aeolian islands, Vesuvio etc.) and therefore understanding their dynamics is crucial for volcanic hazard assessment. Here we present a study on the active volcanic systems of Ischia and Pantelleria, representing two high-silica volcanoes emplaced in subduction related and within-plate geodynamic settings, respectively. The volcanic history of Ischia, started more than 150 ka and proceded until 1302 AD, is characterized by monogenic lava domes, pyroclastic and lava flows with composition from trachy-basaltic, trachytic,alkalitrachytic to phonolitic and latitic. Green-Tuff of Mount Epomeo represents an important level which evidences the occurrence of a catastrophic eruption. Pantelleria, the largest Italian volcanic island, is located in the middle of the spreading Sicily Channel within-plate rift. This magmatic system, active at least for the last 150 ka, shows a clear volcanic cyclicity with periods of quiescence followed by emplacement of peralkaline rhyolites and trachytes in the form of lava flows and ignimbrites. Pantelleria is characterized by bimodal volcanism with the lack of intermediate products between mafic (alkali basalts and hawaiites) and differentiated products (trachytes and pantellerites). The geochemical and radiogenic isotope data of the volcanic products of Ischia demonstrate a two-step evolutive process: the first step was controlled by fractional crystallization plus crustal assimilation (AFC) and drove magma composition from “basalt” to least differentiated trachyte; the second step was controlled only by fractional crystallization (FC), driving the magma composition to the more differentiated products (trachyte-phonolite), and determined very low Sr (a few ppm) and high of Rb (>500 ppm) content due to extreme plagioclase and K-feldspar fractionation. A number of evolved volcanic rocks from Ischia (i.e. low Sr and high Rb content) have anomalously high Sr isotope composition (87Sr/86Sr>0.7075), which cannot be justified by the assimilation of crustal material during the first step evolutive process and demand for other explanations. The relationship between mafic and felsic products at Pantelleria has been explained alternatively with extensive FC processes from basalt to pantellerites or remelting of gabbroic bodies producing the trachytic magmas, which in turn differentiated to pantellerites by fractional crystallization. The active volcanic systems of Ischia and Pantelleria, although belonging to different geodynamic settings, are characterized by the occurrence of strongly differentiated products with high Rb/Sr and anomalous high Sr isotope compositions. This characteristic could be explained by 87Sr in-growth in longlived magma chambers. To explore this hypothesis we carefully screened a number of evolved samples from Ischia and Pantelleria, on which we separated their rock-forming minerals (sanidine and clinopyroxene) and glass to determine Rb and Sr content by isotope dilution, along with Sr isotopic composition. The extremely low diffusion coefficients of Sr in feldspar and clinopyroxene makes them perfect candidates to estimate the timing of crystallization and, by inference, the magma residence time. The obtained Rb-Sr mineral-glass ages are older than K-Ar eruption ages, allowing the determination of the magma residence time following sanidine and clinopyroxene crystallization. These estimates, representing minimum ages, yield magma residence times of some 70 ka and 30-40 ka for Pantelleria and Ischia, respectively, with the exception of one sample from Ischia yielding much higher magma residence time of some 800 ka before eruption. The results imply evolved magma storage at high temperature in closed and thermally insulated portions of the magma chamber and demand for a continuous heat supply from the feeding system of both Ischia and Pantelleria.

Residence time analysis of active volcanic systems: the case study of Ischia and Pantelleria / M. CASALINI; R. AVANZINELLI; S. TOMMASINI. - ELETTRONICO. - (2012), pp. 52-52. (Intervento presentato al convegno Conferenza A. Rittmann 2012 tenutosi a Nicolosi (CT) nel 12-14 Dicembre 2012).

Residence time analysis of active volcanic systems: the case study of Ischia and Pantelleria

M. CASALINI;AVANZINELLI, RICCARDO;TOMMASINI, SIMONE
2012

Abstract

Numerous active and potentially high-risk volcanoes do occur in the Italian peninsula (e.g. Aeolian islands, Vesuvio etc.) and therefore understanding their dynamics is crucial for volcanic hazard assessment. Here we present a study on the active volcanic systems of Ischia and Pantelleria, representing two high-silica volcanoes emplaced in subduction related and within-plate geodynamic settings, respectively. The volcanic history of Ischia, started more than 150 ka and proceded until 1302 AD, is characterized by monogenic lava domes, pyroclastic and lava flows with composition from trachy-basaltic, trachytic,alkalitrachytic to phonolitic and latitic. Green-Tuff of Mount Epomeo represents an important level which evidences the occurrence of a catastrophic eruption. Pantelleria, the largest Italian volcanic island, is located in the middle of the spreading Sicily Channel within-plate rift. This magmatic system, active at least for the last 150 ka, shows a clear volcanic cyclicity with periods of quiescence followed by emplacement of peralkaline rhyolites and trachytes in the form of lava flows and ignimbrites. Pantelleria is characterized by bimodal volcanism with the lack of intermediate products between mafic (alkali basalts and hawaiites) and differentiated products (trachytes and pantellerites). The geochemical and radiogenic isotope data of the volcanic products of Ischia demonstrate a two-step evolutive process: the first step was controlled by fractional crystallization plus crustal assimilation (AFC) and drove magma composition from “basalt” to least differentiated trachyte; the second step was controlled only by fractional crystallization (FC), driving the magma composition to the more differentiated products (trachyte-phonolite), and determined very low Sr (a few ppm) and high of Rb (>500 ppm) content due to extreme plagioclase and K-feldspar fractionation. A number of evolved volcanic rocks from Ischia (i.e. low Sr and high Rb content) have anomalously high Sr isotope composition (87Sr/86Sr>0.7075), which cannot be justified by the assimilation of crustal material during the first step evolutive process and demand for other explanations. The relationship between mafic and felsic products at Pantelleria has been explained alternatively with extensive FC processes from basalt to pantellerites or remelting of gabbroic bodies producing the trachytic magmas, which in turn differentiated to pantellerites by fractional crystallization. The active volcanic systems of Ischia and Pantelleria, although belonging to different geodynamic settings, are characterized by the occurrence of strongly differentiated products with high Rb/Sr and anomalous high Sr isotope compositions. This characteristic could be explained by 87Sr in-growth in longlived magma chambers. To explore this hypothesis we carefully screened a number of evolved samples from Ischia and Pantelleria, on which we separated their rock-forming minerals (sanidine and clinopyroxene) and glass to determine Rb and Sr content by isotope dilution, along with Sr isotopic composition. The extremely low diffusion coefficients of Sr in feldspar and clinopyroxene makes them perfect candidates to estimate the timing of crystallization and, by inference, the magma residence time. The obtained Rb-Sr mineral-glass ages are older than K-Ar eruption ages, allowing the determination of the magma residence time following sanidine and clinopyroxene crystallization. These estimates, representing minimum ages, yield magma residence times of some 70 ka and 30-40 ka for Pantelleria and Ischia, respectively, with the exception of one sample from Ischia yielding much higher magma residence time of some 800 ka before eruption. The results imply evolved magma storage at high temperature in closed and thermally insulated portions of the magma chamber and demand for a continuous heat supply from the feeding system of both Ischia and Pantelleria.
2012
Riassunti Estesi. Conferenza A. Rittmann 2012
Conferenza A. Rittmann 2012
Nicolosi (CT)
M. CASALINI; R. AVANZINELLI; S. TOMMASINI
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Utilizza questo identificatore per citare o creare un link a questa risorsa: https://hdl.handle.net/2158/957155
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