Elemental and isotope covariation of noble gases in mineral phases from Etnean volcanics erupted during 2001-2005, and genetic relation with peripheral gas discharges

During 2001–2005, Mount Etna was characterized by intense eruptive activity involving the emission of petrologically different products from several vents, which involved at least two types of magma with different degrees of evolution. We investigated the ratios and abundances for noble-gas isotopes in fluid inclusions trapped in olivines and pyroxenes in the erupted products. We confirm that olivine has the most efficient crystalline structure for preserving the pristine composition of entrapped gases, while pyroxene can suffer diffusive He loss. Both the minerals also experience noble gas air contamination after eruption. Helium isotopes of the products genetically linked to the two different magmas fall in the isotopic range typical of the Etnean volcanism. This result is compatible with the metasomatic process that the Etnean mantle is undergoing by fluids from the Ionian slab during the last ten kyr, as previously inferred by isotope and trace element geochemistry. Significant differences were also observed among olivines of the same parental magma that erupted throughout 2001–2005, with 3He/4He ratios moving from about 7.0 Ra in 2001 volcanites, to 6.6 Ra in 2004–2005 products. Changes in He abundances and isotope ratios were attributed to variations in protracted degassing of the same magma bodies from the 2001 to the 2004–2005 events, with the latter lacking any contribution of undegassed magma. The decrease in 3He/4He is similar to that found from measurements carried out every fifteen days during the same period in gases discharged at the periphery of the volcano. To our knowledge this is the first time that such a comparison has been performed so in detail, and provides strong evidence of the real-time feeding of peripheral emissions by magmatic degassing.