A two-component mantle below Mt Etna volcano: evidences from noble gas and trace element geochemistry of primitive products

A general geochemical study comprehensive of major elements, trace elements and Sr-Nd isotopes in the bulk rock, coupled to noble gases analyses from fluid inclusions retained in minerals, was performed. The studied samples ( basalts, trachybasalts and basanites) represent some among the most primitive products of Etnean history. The variable composition measured in trace elements (i.e. Zr/Nb=2.81–4.98, Ce/Yb=35.02–66.90, La/Yb=15.36–35.52, Th/Y=0.17–0.43) was modeled as due to varying degrees of melting of a common mantle source. We numerically simulated the process by MELTS code to calculate a melting percentage for each product, and we accordingly estimated the pristine trace-elements content of their mantle source. This latter resulted to be common to all of the investigated samples and evidenced a close affinity between Etnean and Hyblean mantle. The observed coupling between trace elements and noble gases allow us to better define the Etnean mantle, which is compatible with a peridotitic matrix veined by 10% of clinopyroxenites. The geochemistry of the Sr-Nd-He isotopes (87Sr/86Sr= 0.703321–0.703910, 143Nd/144Nd= 0.512836 – 0.512913 and 3He/4He= 6.7–7.6 Ra), evidence also that crustal-derived fluids contaminated at various degrees the inferred source (?) The 87Sr/86Sr enrichment appears correlated to estimated degree of melting for each product, therefore the metasomatic fluids probably refertilized some portions of the mantle prior to partial melting. In contrast, the observed decoupling between 3He/4He and 87Sr/86Sr ratios could be related to shallower processes, due to magma aging or to a contribution of shallow fluids, responsible of the more radiogenic values of 3He/4He