GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.2 118 TRACKING VOLATILE LOSS FROM OLIVINE-HOSTED MELT INCLUSIONS IN TEPHRA ERUPTED DURING THE RECENT ACTIVITY OF MT. ETNA VOLCANO F. Zuccarello 1 , F. Schiavi 2 , M. Viccaro 1,3 1 Università degli Studi di Catania, Dipartimento di Scienze Biologiche Geologiche e Ambientali, Catania, Italy 2 Université Clermont-Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, Clermont-Ferrand, France 3 Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania, Osservatorio Etneo, Catania, Italy Introduction The post-2011 activity at Mt. Etna has been characterized by violent lava fountaining occurred during the 2011-2013 paroxysmal series at the New South East Crater and during the 2015-2016 paroxysms at the Voragine Crater (Giuffrida and Viccaro, 2017; Cannata et al., 2018), alternating with more effusive or weak explosive activity, as observed during the eruptions of 2017 and 2018 (Viccaro et al., 2019; Borzì et al., 2020). Few data on olivine-hosted melt inclusions (MIs) are available for this time interval, which provided rather low volatile contents (Gennaro et al., 2019), contrasting the eruptive style observed during the last decades. Here, we provide a new dataset of major, trace and volatile elements (H 2 O, CO 2 , S, Cl and F) measured in MIs from tephra ejected during three variably energetic eruptions from the 2013-2018 activity at Mt. Etna, in order to assess the compositional evolution and pre-eruptive volatile budget of the erupted mag- mas (Zuccarello et al., 2021). Results Major element compositions have been corrected for post entrapment crystallization and allowed us to recognize two types of MIs (Fig. 1). The type 1 MIs are entrapped in Fo 79-85 olivi- nes and have more basic compositions (SiO 2 ~42.7-45.7 wt.%, CaO ~9.9-12.5 wt.%, Na 2 O+K 2 O ~5.1-6.2 wt.%, Mg# ~55-60). The type 2 MIs are found in Fo 69-78 olivines and show more evolved compositions (SiO 2 ~46.5-51.2 wt.%, CaO ~6.1-9.7 wt.%, Na 2 O+K 2 O ~6.2-8.3 wt.%, Mg# ~40-54). Some ratios of incompatible trace elements (i.e., Ba/Sr, Zr/Nb, La/Sm) support similar geoche- mical signature for MIs coming from the three eruptions, with minor differences among the two types of MIs. Dissolved volatile concentrations in glasses are rather low (H 2 O ~0.4-2.4 wt.%, CO 2 ~46-849 ppm, S ~172-3072 ppm, Cl ~870-2884 ppm and F ~670-1546 ppm) compared to those of magmas erupted in 2001 and 2002-03 (Métrich et al, 2004; Spilliaert et al., 2006). Lower H 2 O contents are sometimes correlated with depleted SiO 2 contents. This is particularly evident in some type 1 inclusions. Discussion and conclusions No thermodynamic solution with rhyolite-MELTS allows to reproduce our data using the most primitive composition at Mt. Etna as starting melt (Fig.1; Kamenetsky et al., 2007). Furthermore, low entrapment pressures (<130 MPa) have been inferred for MIs trapped in high-Fo olivine (80- 85 mol%) due to the low volatile concentrations (Fig.2). Our data show evidence of diffusive H 2 O loss affecting both types of MIs, which is coupled with SiO 2 depletion during post-entrapment crystallization at the MI walls (Portnyagin et al., 2019). As well as 20-60% of the CO 2 initially dissolved in the melt was lost to the shrinkage bubble. Restoration of the original SiO 2 -H 2 O-CO 2 concentrations allowed us to get insights on the magma evolution and degassing. Restored initial H 2 O concentrations are on the order of 3.5-4.0 wt.%, comparable with those measured for the