Unravelling the radiogenic isotope signature of the mantle source from evolved magmas: the case study of Ischia Volcano

The active volcano of Ischia, the well-known island off-shore the city of Naples, has been having a discontinuous volcanic activity characterized by caldera-forming paroxysmal eruptions, lava flows, and lava domes for > 150 ka. The overall geochemical composition of erupted magmas varies from shoshonite, latite, and trachyte/trachyphonolite. Casalini et al.(2017) demonstrated that the radiogenic Sr isotope variation of Ischia magmas with Sr < 100 ppm occurred in a closed system environment, and depicted a scenario contemplating a complex magma chamber reservoir made up of multiple melt lenses isolated by largely crystalline mush portions. Here, we focus on the origin of the radiogenic isotope (Sr, Nd, Hf, Pb) signature of Ischia latites and trachytes with 100en route to the surface. Parental mantle-derived basalts with MgO > 10 wt.% outcrop in the nearby, a few kilometers apart, Procida island suggesting that the evolved latites and trachytes at Ischia might be originated by crustal assimilation processes affecting Procida basalts. However, Energy Constrained Assimilation and Fractional Crystallization modelling (Spera & Bohrson, 2001) set robust constraints against this hypothesis and provide geochemical arguments for a pristine mantle-derived radiogenic isotope signature for trachytes and latites, implying that they also evolved in a closed-system environment akin trachytes with Sr < 100 ppm. This result corroborates the model of Casalini et al.(2017) suggesting a complex magma chamber reservoir formed by discrete melt lenses and mush portions. The Sr, Nd, Pb, Hf isotope composition of latites and trachytes permits to set constraints on the metasomatic process affecting this sector of the Apennine chain even though no mantle-derived magma outcrop in the island. Casalini, M., Avanzinelli, R., Heumann, A., de Vita, S., Sansivero, F., Conticelli, S., Tommasini, S. (2017): Geochemical and radiogenic isotope probes of Ischia volcano, Southern Italy: Constraints on magma chamber dynamics and residence time. Am. Mineral., 102, 262-274. Spera, F.J. & Bohrson, W.A. (2001): Energy-constrained open-system magmatic processes I: general model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation. J. Petrol., 42, 999-1018.