PREHISTORIC LAVAS ON MT ETNA SHOW ISOTOPIC EVIDENCE FOR SUBDUCTION COMPONENT

Mt. Etna is a composite stratovolcano located near the eastern coast of Sicily where three structural domains converge: the Appennine–Maghrebian overthrust belt (formed by collision of the European and African plates); the Iblean Plateau to the south; and Ionian oceanic lithosphere to the east. Magmatic evolution broadly involves a transition from an early tholeiitic phase (ca. 500 ka) to the current Na-alkaline phase. While evidence for active subduction beneath Sicily is currently lacking, the compositions of Etna’s OIB-like lavas show increasing orogenic-type geochemical signatures over time. However, >95% of the volcano’s history is only poorly known, because at least 85% of the surface is covered by volcanic products erupted in the last 15 ka. The purpose of this study is to bridge the gap between Etna’s tholeiitic beginnings and its current alkaline character through geochemical analysis of ~85-15 ka lavas exposed in the southern wall of the Valle del Bove (VdB) and to evaluate whether they provide evidence for a subduction component during this phase of evolution. The lavas are mildly alkaline trachybasalts to trachyandesites that are more evolved than recent and historic Etna, but they overlap the compositions of previously studied prehistoric and ancient lavas. Sr-Nd-Pb-Hf isotopes support the existence of at least three different mantle components. The oldest VdB lavas (>85ka Salifizio) are similar to recent (<200 yrs) Etna lavas (⁸⁷Sr/⁸⁶Sr 0.70357; ¹⁴³Nd/¹⁴⁴Nd 0.51285; ¹⁷⁶Hf/¹⁷⁷Hf 0.28295; ²⁰⁶Pb/²⁰⁴Pb 19.92), whereas most VdB lavas <85 ka tapped a source similar to that of historic Etna (⁸⁷Sr/⁸⁶Sr 0.7033-0.7034; ¹⁴³Nd/¹⁴⁴Nd 0.51288-0.51290; ¹⁷⁶Hf/¹⁷⁷Hf 0.28294-0.28297; ²⁰⁶Pb/²⁰⁴Pb 19.86-20.02). The third component, recorded in lavas from Piano Provenzana (~42-30 ka), has unusually low ¹⁷⁶Hf/¹⁷⁷Hf (0.28288) and ²⁰⁶Pb/²⁰⁴Pb (19.79) and is currently unique among analyzed lavas from Mt. Etna. Hf-Pb isotope mixing models suggest the Provenzana lavas can be explained by introduction of zircon-rich sediments into the mantle wedge prior to mixing with an upwelling melt generated from a more depleted mantle end member (HIMU-DMM). The new Sr-Nd-Pb-Hf isotopic data indicate that several isotopically distinct sources have been available in the magma generation process since prehistoric times.