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