Long-term vs. short-term geochemical changes of Mt. Etna lavas: the puzzling effect of a variegated mantle

Mt. Etna volcano fascinated the scientific community during the last decades due to its unusual geodynamic location and geochemical signature of the erupted lavas. Although the edifice is placed above the front of a compressive regional tectonic setting, volcanic products have a marked intraplate signature. The volcano has shown a complex geochemical variability during the half-million years of life, with sub-alkaline products erupted during the early stage of volcanism up to ~220 ka ago, then followed by a Na-alkaline stage. The alkaline stage displays long-term geochemical variations, as a result of evolutionary processes acting at various levels of the feeding system. The Etnean lavas also exhibit short-term changes in some periods. For example, an increase in some LILEs and volatiles together with marked changes of the Sr-Nd-Pb-Hf isotope ratios are observed since the 1971 (Viccaro and Cristofolini, 2008; Viccaro et al., 2011). Mt. Etna can surely be considered as one among the most studied volcanoes on Earth, although the features of the source are still matter of discussion. The absence of mantle material (xenoliths), which is efficiently fractionated in the deep levels of the feeding system, makes more intricate the picture. Thus, indirect information can be only attained from magma compositions. The behavior of incompatible trace elements for mantle-equilibrated magma compositions of the Ellittico (60-15 ka) and Recent Mongibello (15 ka-present), and of their enrichment ratio show that the Etnean magmas are produced by a variable, low degrees of partial melting. The enrichment ratio also reveals that magmas can be generated from a classic garnet peridotite variably enriched by metasomatic mineral phases (amphibole-phlogopite). The Zr/Nb vs. Ce/Y and Ba/Th vs. Lu/Hf diagrams highlight the enriched signature of the Etnean mantle. The Zr/Nb vs. Ce/Y diagram evidences the progressive contribution of metasomatic phases with time. The Ba/Th vs. Lu/Hf diagram shows the short-term coupled increase of the contribution by metasomatic agents and partial melting degree during the last 400 years of volcanic activity. In terms of mantle components, the integration of Sr-Nd-Pb-Hf isotopic ratios suggested that FOZO is the dominant component in the Etnean source (Viccaro et al., 2011). However, the FOZO component is not able to fully account for the isotopic changes from the Ellittico to Recent Mongibello magmas. Indeed, the isotopic compositions of the Ellittico and Recent Mongibello volcanic products can be explained by addition of an EM1-type component (up to 10%) to a dominant FOZO. The integrated analysis of Sm/Hf and Th/Hf ratios, together with Hf isotope ratios, gives evidence that the enriched component may be metasomatizing silicate melts. Long-term vs. short-term compositional changes at Mt. Etna are here related to partial melting of a recycled, altered oceanic lithosphere, infiltrated by metasomatizing silicate melts. Variable amounts of the enriched component participating to partial melting are able to affect its degree and the geochemical signature of the produced magmas.