GNGTS 2019 - Atti del 38° Convegno Nazionale

208 GNGTS 2019 S essione 1.3 Chiodini, G., Caliro, S., Aiuppa, A., Avino, R., Granieri, D., Moretti, M., Parello, F., 2011: First 13C/12C isotopic characterisation of volcanic plume CO 2 . Bulletin of Volcanology, 73(5) : 531-542, http://dx.doi.org/10.1007/ s00445-010-0423-2. Heap M. J., Mollo S., Vinciguerra S., Lavallée Y., Baud P., Dingwell D.B., Iezzi G., von Aulock F.W.: Thermal weakening of the carbonate basement under Mt. Etna volcano (Italy): Implications for volcano instability . Journal of Volcanology and Geothermal Research , 250 , 42-60. Mollo S., Heap M. J., Dingwell D. B., Hess K.-U., Iezzi G., Masotta M., Scarlato P., Vinciguerra S.: Decarbonation and thermal microcracking under magmatic P-T-fCO2 conditions: the role of skarn substrata in promoting volcanic instability . Geophysical Journal International, 195 , 369–380. Parisio F., Vinciguerra S., Nagel T., Kolditz O.: Lithology controls on the brittle-ductile transition in volcanic areas . Nature Scientific Reports, 9 ,143, DOI:10.1038/s41598-018-36505-x. CONSTRAINING RATES OF SYN-ERUPTIVE MAGMA ASCENT AND DEGASSING AT MT. ETNA BY MODELING THE DIFFUSION OF VOLATILES IN MINERAL-HOSTED MELT EMBAYMENTS F. Zuccarello 1 , M. Giuffrida 1 , M. Viccaro 1,2 1 Dipartimento di Scienze Biologiche Geologiche e Ambientali, Università di Catania, Catania, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Catania, Osservatorio Etneo, Catania, Italy Introduction. Many factors control the explosive intensity of an eruption, like the magma viscosity, gas content, crystallinity, size of the conduit. Recent studies have highlighted how the ascent rate is a crucial parameter for determining the explosivity of an eruption, playing a key control on the bubble nucleation, growth and coalescence, and affecting the modes of degassing during the syn-eruptive stages (Humpreys et al. , 2008; Lloyd et al. , 2014). Diffusion chronometry used routinely in the investigations of the pre-eruptive dynamics (i.e. magma storage, transfer and mixing processes) are not able to give us information about kinetics of ascent rates, since degassing processes induced by decompression are characterized by very short timescales, preventing any compositional variations for what concerns major elements in crystals (e.g. Zellmer, 1999; Costa et al. , 2008). However, new petrological approaches based on diffusion chronometry of elements characterized by high diffusivity in minerals and glasses can provide direct measurements of the kinetics of such short-lived processes. We propose an approach of investigation based on diffusion of volatile species (H 2 O, CO 2 and S) along olivine- hosted embayments (Llyod et al. , 2014). An embayment is a melt inclusion which is not fully enclosed by the host crystal, but it communicates with the external melts and generally shows tubular shapes. Compositional gradients of volatile elements observed along embayment reflect exchange of volatile elements by diffusion with the adjacent melt in response to the degassing- induced decompression. Application of numerical models on compositional gradients can provide information on the timescales over diffusion processes occur, thus allowing estimation of ascent rates. Such approach has been successfully employed in the determination of ascent rates of magma erupted in closed conduit volcanoes with intermediate or acid compositions (Liu et al. , 2007; Humpreys et al. , 2008; Lloyd et al., 2014), whereas estimates of ascent rates in open conduit volcanoes are still missing. The role of ascent rate must be evaluated especially for intense explosive eruptions involving basic magmas in open conduit volcanoes, where the relatively low magma viscosity promotes chiefly effusive or weak explosive eruptions. Constraints on ascent rates can provide useful elements in the evaluation of the volcanic hazard in such contests. Choice of the case studies. This study is aimed at obtaining a spectrum of ascent rates related to different explosive intensity. In this regard, Mt. Etna volcano has been chosen as case of study, being one of the most active and well monitored open conduit volcano in the world. Although the