GNGTS 2018 - 37° Convegno Nazionale

254 GNGTS 2018 S essione 1.3 fundamental framework for the interpretation of geodetic data associated to complex volcanic phenomena. Based on an extensive dataset of concentration and diffusion profiles of olivine crystals combined with geodetic data, we are able to interpret the geodetic evidence of volcanic unrest throughout the 2017 eruptive period in terms of deep and subsurface pre- eruptive processes such as the storage, transport and interactions of magmas. Such approach of investigation allows the spatial localization of active magmatic sources, and also defines their temporal activation before and during each eruptive episode. This enables to address some important changes in the modes of magma supply into the Etnean plumbing system during 2017, which seem to be a direct consequence of the last violent paroxysmal episodes occurred at Voragine Crater on May 2016. Geochemical and geodetic data elaboration and integration. We elaborated and integrated information preserved in olivine crystals with GPS observations collected from the permanent network and spanning the May 19, 2016 - May 25, 2017 period to explore dynamics of magma ascent and interaction leading to the volcanic activity of 2017. Olivine cores spanning from more basic (Fo 80-81 ) to slightly evolved compositions (Fo 72-74 ) reflect crystal growth in separated magma volumes residing at different depths beneath Mt. Etna, each defined by precise physical and chemical conditions. We have identified four olivine populations which fall in the same range of compositions of olivine groups recognized for past effusive and explosive eruptions of Mt. Etna (Giuffrida and Viccaro, 2017; Cannata et al., 2018; Fig.1). Such observations lead us to infer that the residence and growth of olivine crystals during 2017 occurred within the same magmatic environments (indicated as M i ) that were selectively reactivated during the 2011-2013 eruptive sequence at the NSEC, and later between 2015 and 2016 during the volcanic activity at VOR (Giuffrida and Viccaro, 2017; Cannata et al., 2018). Thus, olivines with Fo 80-81 cores are representative of the M 0 environment located at pressure of 420-380 MPa; Fo 78 cores refer to the M 1a environment at 290-230 MPa; olivines with Fo 76 core compositions belong to the M 1b environment at 160-120 MPa, whereas the Fo 72-74 core compositions encompass a compositional range which is intermediate between M 1b and M 2 (30-40 MPa). In spite of the complex diversity of zoning patterns, some evolutionary paths are dominant in the history recorded by each olivine population. For instance, the prevalence of normal zoning pattern characterizing more basic crystals (Fo 80-81 and Fo 78 ) for the eruptions of February 28 and March 19 indicates ascent of basic magmas from deep reservoirs (M 0 and M 1a ) to shallow crustal levels. Starting from March 25, compositions of olivine cores drop to lower forsteritic concentrations (Fo 75 to Fo 72 ). Such a compositional change accounts for the reactivation of the shallow magmatic environments M 1b and M 2 , and therefore the emission of more evolved magmas during the episodes of March 25, Fig. 1. Representative core-to-rim compositional profiles for the identified olivine populations. Yellow lines in the BSE images indicate the direction of the SEM-EDS/WDS transect.