GEOCHEMISTRY AND GEODYNAMICS OF THE PREHISTORIC MANTLE SOURCE BENEATH MT. ETNA, SICILY: EVIDENCE FROM THE VALLE DEL BOVE

Although located near the convergence of the African, European, and Ionian plates, Mt Etna, Europe’s largest active volcano, occurs in an unusual tectonic position, i.e., near but not within a subduction zone, and in an accretionary foreland rather than above a mantle wedge. Passive mantle upwelling from beneath the adjacent African plate, due to Ionian slab rollback, may play a role, but Etna’s magma compositions are more ocean island basalt-like than subduction-related, so the nature of the mantle source is still debated. Most geochemical investigations have focussed on either historic (>130yr) or recent (<130yr) eruptions or on the ancient basal lavas (~500 ka). These studies note a temporal isotopic trend of more DMM (Depleted MORB Mantle)-like for Plio-Pleistocene-age Iblean Plateau lavas to more enriched EAR (European asthenospheric reservoir)-like for younger Etnean magmatism.

In this study, we analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove (VdB), which represent a less well studied time span (~85 to ~4 ka) in Mt. Etna’s prehistoric magmatic activity. Isotopic data (¹⁴³Nd/¹⁴⁴Nd = 0.51283-0.51291; ⁸⁷Sr/⁸⁶Sr = 0.70332-0.70363; ¹⁷⁶Hf/¹⁷⁷Hf = 0.28288-0.28298; ²⁰⁶Pb/²⁰⁴Pb = 19.76-20.03) indicate changes in magma source during the ~80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed VdB unit has Nd- and Sr-isotope ratios similar to recent Etna eruptions, while four of the five subsequent units resemble historic Etna magmas. A sixth unit, the main Piano Provenzana formation (~42-30 ka), exhibits notably lower ¹⁷⁶Hf/¹⁷⁷Hf, ¹⁴³Nd/¹⁴⁴Nd, and ²⁰⁶Pb/²⁰⁴Pb ratios than the other prehistoric VdB units. This isotopic signature has not yet been observed in any other samples from Mt. Etna.

While relatively recent influx of a subduction component cannot be ruled out, the available geochemical data are more consistent with derivation of VdB lavas from a heterogeneous, marble-cake-style mantle source variably metasomatised by hydrous phases (amphibole and/or phlogopite) and pyroxenite veins. We suggest the parental melts for the Piano Provenzana unit were derived predominantly from ancient pyroxenite in the mantle source. The proportion of pyroxenite appears to be greater in historic-to-recent Etna than in the VdB source.