Inverse modelling to unravel the radiogenic isotope signature of 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 havinghad a discontinuous volcanic activity characterised by caldera-forming paroxysmal eruptions, lava flows, and lava domes for >150 kyr. The overall geochemical composition of erupted magmas varies includes from shoshonite, latite, and trachyte/trachyphonolite. In a complementary study, we demonstrated that the evolution of Ischia trachytes 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 100< Sr <800 ppm, in the context of the recent Neapolitan District volcanic activity, and bearing in mind that no parental mantle-derived magma outcrops at Ischia. Parental mantle-derived basalts with MgO > 10 wt.% outcrop in the nearby, a few kilometers apart at Procida island, and suggest that the evolved latites and trachytes at Ischia might be have originated by crustal assimilation processes affecting Procida basalts. However, Energy Constrained Assimilation and Fractional Crystallization modelling sets 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 from parental basaltic magmas in a closed-system environment akin trachytes with Sr <100 ppm. This result corroborates the model presented in the complementary study, suggesting a complex magma chamber reservoir formed by isolated magma batches with distinct radiogenic isotope compositions that evolved along similar liquid lines of descent and are separated by crystal mush zones. The Sr, Nd, Pb, and Hf isotope composition of Ischia latites and trachytes suggests metasomatic enrichment of a MORB-type asthenospheric mantle source by composite supercritical liquids originating from the altered oceanic basalt and pelagic sediment of the subducting Adriatic-Ionian slab. The results obtained on Ischia latites and trachytes, can be tentatively extended to the entire Neapolitan District volcanic system with important consequences for the isotopic signature of the mantle source and volcanic hazard assessment.