GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.3 187 3D textural and geochemical investigation to explore the trigger mechanisms of plinian eruptions: a Somma-Vesuvius volcano case study G. Buono 1 , L. Pappalardo 2 , P. Petrosino 1 1 Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi Federico II, Napoli, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy Introduction. The knowledge of trigger mechanisms and syn-eruptive dynamic of high- magnitude eruptions is a primary goal in volcanology, due to the influence of these sub-volcanic processes on the behavior of precursory phenomena that are detected by monitoring systems during volcanic crises. In fact a severe difficulty in volcanic forecast is to correlate the evolution of the geochemical and geophysical signals recorded at the surface with the magma transfer at depth. These processes are strongly controlled by pre-eruptive conditions (e.g. depth of magma chambers, magma composition, amount of volatiles) as well as by structural factors (e.g. edifice load and related stress field). In the last decades, quantitative textural studies on volcanic rocks combinedwith conventional geochemical analyses, have proved to be a fundamental approach in exploring the conditions related to magma storage and ascent in volcanic conduit and allowed to improve our ability to interpret volcano-monitoring signals and perform hazard assessments. Particularly 3D textural data have played a key role to assess conditions related to nucleation, growth and coalescence of gas bubbles and magma fragmentation that in turn influence the style and intensity of eruptions. In fact 3D data allow the direct observation and quantification of the orientation and shape of the vesicles, as well as of their degree of interconnectedness and permeability, impossible to determine using the more conventional 2D techniques. Inmore densely populated regions, as the Neapolitan high-risk volcanic area, this information would be essential to avoid a major volcanic disaster. In this case study we have conducted a 3D textural (via X-ray microtomography) and geochemical (major-minor oxides and Sr-Nd isotopic ratios) investigation of volcanic products emitted during the Pomici di Base eruption (about 22 ka). This event represents the first and most intense plinian eruption of Somma-Vesuvius volcano (Bertagnini et al. , 1998; Landi et al. , 1999) and delineates the passage, after a period of prevalent effusive activity, to the explosive character of the volcano as well as the beginning of the caldera collapse. A crucial issue that characterizes this eruption is the high mass discharge rate (MDR, 2-2.5 x 10 7 kg/s) that remains stable during the whole sustained-column phase despite the significant compositional variation towards gas-poor mafic composition. The obtained results allowed us to reconstruct the evolution of the Somma-Vesuvius magmatic system before and during the eruption as well as to obtain information on the trigger mechanisms and eruptive dynamic. Pre-eruptive processes. Our geochemical data show the existence of a chemically-zoned magma chamber, from trachyte to latite. We have estimated pre-eruptive temperature and pressure conditions by using the clinopyroxene-liquid thermo-barometer developed by Masotta et al. (2013). Particularly, thermometric calculations indicate that crystallization temperature increases from 900°C in the upper trachytic layer to 1050°C towards the basal latite with an estimated average pressure of 1.37 ± 0.85 kbar. The last value is in agreement with the results obtained by using Cl-content on matrix-glasses (Balcone-Boissard et al. , 2016), while it differs from the range of pressure values (3-4 kbar) reported by Landi et al. (1999) on the basis of two feldspars and feldspar-liquid barometers. Thermodynamic simulations (trough MELTS software) corroborate the hypothesis of a magma chamber localized at a depth of 4-6 km (about 150 MPa) including cogenetic latitic to trachytic magmas, as a result of fractional crystallization of sanidine, clinopyroxene, plagioclase as dominant phases. Moreover the simulations suggest the occurrence along the liquid line of descend of a pseudo-invariant temperature point (930°C), at which crystal content, exsolved