GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.3 209 ordinary activity of Mt. Etna is generally Strombolian with effusion of lava flows, some violent manifestations have been recorded both in historical and recent times, involving basic and ultrabasic compositions in the most explosive records (Coltelli et al. , 1998; Kamenetsky et al, 2007; Cannata et al. , 2018). Moreover, the choice of a single volcano offers the advantage to consider a unique feeding system characterized by similar physical and chemical conditions, including temperature, pressure, magma compositions and viscosity. In this way, it is possible to evaluate if differences in ascent rates can produce different eruptive styles starting from similar magmatic conditions. Some eruptions of the post-2011 activity, characterized by variable intensities, have been selected as case studies (Fig.1). The eruptions selected, with increasing explosiveness, are: i) the flank eruption of the December 24, 2018, which affected the eastern side of the volcanic edifice coupled with an intense seismic activity (Cannavò et al. , 2019); ii) the lava fountaining of the February 19, 2013, which is part of the 2011-2013 paroxysmal activity occurred at the New South East Crater (NSEC; Behncke et al. , 2014; Giuffrida and Viccaro, 2017); iii) the paroxysmal episode occurred on December 3, 2015 at Voragine Crater (VOR), which represents the most violent eruptive event at Mt. Etna over the last 20 years (Cannata et al. , 2018). Results and discussions. Olivine-hosted melt inclusions and embayments have been analyzed through Raman spectroscopy for quantification of the water content; then, major elements, S and Cl content inmelt inclusions have been determined through electronmicroprobe. Measurements of volatile elements in olivine-hosted melt inclusions allow to recognize the original volatile contents in the magma and to track the degassing path. Water contents in melt inclusions range between 0.3 and 2.2 wt.%, with the highest average values observed for products erupted during the December 3, 2015 eruption. PEC correction has been applied to the major element compositions highlighting two types of melt inclusions. The first type is characterized by K 2 O ~2.4 - 2.9 wt.%, CaO ~6.0 - 9.2 wt.%, Mg# ~48 - 54% and S ~275 - 1245 ppm, whereas the second type shows K 2 O content of ~1.7 - 2.1 wt.%, CaO ~9.8 - 12.3, Mg# ~55 - 61% and S ~1500 - 2900 ppm. The second group of melt inclusions generally contain higher water content and are entrained in olivine crystals with high forsterite contents of 80 - 85 mol%, indicating the more primitive nature of these melt inclusions with respect to the first type, which are entrapped in olivine crystals with forsterite contents of 76 - 79 mol%. Preliminary results of H 2 O measurements along olivine-hosted melt tubes show an average value of 1.3 - 1.4 wt.% in the inner part of the embayments for the February 19, 2013 products and are characterized by a gradual decrease towards the mouth of the melt tubes. On the contrary, December 3, 2015 products are characterized by higher average values at 1.6 - 1.7 wt.% in the inner parts of the melt tubes, showing a sharper decrease to the end. The December 24, 2018 products show a more water-depleted conditions, with average values at 0.3 - 0.5 Fig. 1 - a) Paroxysmal episode of the December 3, 2015 at VOR (photo credit: Marco Restivo https://www.flickr.com/photos/marcorestivo/ ); b) One of the eruptive episodes of the 2011-2013 paroxysmal sequence at NSEC; c) Flank eruption of the December 24, 2018.