Noble Gases Investigation on Etnean Volcanic Gases and on the Erupted Products During the 2001-2006 Period

Data acquired during the last 20 years of geochemical monitoring of volcanic gases lead us to better understand how volcanoes work. According to theoretical and experimental investigations, both the chemical and isotopic changes in sampled volcanic gases have been interpreted in terms of magma ascent using the models proposed by Nuccio & Paonita, (2001) and Caracausi et al. (2003). On the basis of numerical simulations of volatile degassing, we have been able to recognize episodes of magma migration from deeper reservoirs of Mount Etna to the shallower storage volume, until magma is erupted. The 3He/4He isotope ratios of gas emitted at the periphery of Mount Etna volcanic edifice exhibit synchronous variations, typically ranging between 7.5 Ra and 5.9 Ra, but displaying different average values characterizing each single sampling site. These isotope ratios normally show decreasing trends following outgassing processes, in contrast, increasing values indicate injection of new volatile-rich magma. The highest values are surprisingly similar to those measured in peridotite xenoliths from the neighbour Hyblean Plateau (Sapienza et al., 2005), suggesting it probably is the value which characterizes the upper mantle of the region. Since 2001, Mount Etna has been characterized by an intense eruptive activity with the emission of petrologically different products from various vents at the same time. Fluid inclusions trapped in olivines and pyroxenes of the erupted products have been investigated by analysing He and Ar isotope composition and abundance, and compared with those recorded by volcano monitoring of the same eruptive period. Our results confirm that olivine has the most efficient crystalline structure for preserving the pristine composition of entrapped gases, while pyroxene can suffer diffusive He loss. Significant differences were also observed among olivines of the same parental magma erupted during the 2001-2004 period, with 3He/4He isotope ratios moving from about 7.0 Ra in 2001 volcanites to 6.6 Ra in 2004-2005 products. Both abundances and isotope ratios of helium were attributed to protracted degassing of the same magma bodies from the 2001 to the 2004-2005 eruptive events. The general decrease in 3He/4He ratios measured in fluid inclusions is similar to that recorded in etnean peripheral gases by volcano monitoring during the same period, providing strong evidence of the real-time feeding of peripheral emissions by magmatic degassing