Numerous active and potentially high-risk volcanoes do occur in the Italian peninsula 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. Ischia has erupted in a subduction-related setting and it is characterised by a continuous transition from trachy-basalt to phonolite. The geochemical and radiogenic isotope data of its volcanic products demonstrate a two-step evolutive process: the first step, controlled by fractional crystallization plus crustal assimilation (AFC), drives magma composition from trachy-basalt to moderately differentiated trachyte; the second step, controlled only by fractional crystallisation (FC), drives the magma composition to the more differentiated products (phonolite) determining very low Sr (a few ppm) and high Rb (>500 ppm) contents due to extreme plagioclase and K-feldspar fractionation. Pantelleria is located in a witin-plate setting and it is characterised by a bimodal magmatism of alkali-balsalt and differentiated products passing from trachyte to peralkaline ryolite (i.e. Pantellerite), through FC processes. Pantelleritic rocks also show extremely low Sr and high Rb contents. 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 anomalously high Sr isotope compositions that cannot be justified by the assimilation of crustal material. This characteristic could be explained by 87Sr in-growth in long-lived 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 isotope composition. The extremely low diffusion coefficients of Sr in feldspar and clinopyroxene makes them perfect candidates to estimate the timing of crystallisation and, by inference, the magma residence time. The calculated crystallization times are here discussed in terms of the chemical and physical characteristic of the magmas.
Residence time analysis of active volcanic systems:Rb-Sr isotope study of Ischia and Pantelleria
CASALINI, MARTINA;
2013-01-01
Abstract
Numerous active and potentially high-risk volcanoes do occur in the Italian peninsula 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. Ischia has erupted in a subduction-related setting and it is characterised by a continuous transition from trachy-basalt to phonolite. The geochemical and radiogenic isotope data of its volcanic products demonstrate a two-step evolutive process: the first step, controlled by fractional crystallization plus crustal assimilation (AFC), drives magma composition from trachy-basalt to moderately differentiated trachyte; the second step, controlled only by fractional crystallisation (FC), drives the magma composition to the more differentiated products (phonolite) determining very low Sr (a few ppm) and high Rb (>500 ppm) contents due to extreme plagioclase and K-feldspar fractionation. Pantelleria is located in a witin-plate setting and it is characterised by a bimodal magmatism of alkali-balsalt and differentiated products passing from trachyte to peralkaline ryolite (i.e. Pantellerite), through FC processes. Pantelleritic rocks also show extremely low Sr and high Rb contents. 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 anomalously high Sr isotope compositions that cannot be justified by the assimilation of crustal material. This characteristic could be explained by 87Sr in-growth in long-lived 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 isotope composition. The extremely low diffusion coefficients of Sr in feldspar and clinopyroxene makes them perfect candidates to estimate the timing of crystallisation and, by inference, the magma residence time. The calculated crystallization times are here discussed in terms of the chemical and physical characteristic of the magmas.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
