Abstract
Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the African and Eurasian plates, near the Quaternary subduction-related Aeolian arc and above the down-going Ionian oceanic slab. Magmatic evolution broadly involves a transition from an early tholeiitic phase (~ 500 ka) to the current alkaline phase. Most geochemical investigations have focussed on either historic (> 130-years old) or recent (< 130-years old) eruptions of Mt. Etna or on the ancient basal lavas (ca. 500 ka). In this study, we have analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove, which represent a time span of Mt. Etna’s prehistoric magmatic activity from ~ 85 to ~ 4 ka. They exhibit geochemical variations that distinguish them as six separate lithostratigraphic and volcanic units. 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 the magma source during the ~ 80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed Valle del Bove unit (Salifizio-1) erupted basaltic trachyandesites with variations in 143Nd/144Nd and 87Sr/86Sr ratios indicating a magma source remarkably similar to that of recent Etna eruptions, while four of the five subsequent units have isotopic compositions resembling those of historic Etna magmas. All five magma batches are considered to be derived from melting of a mixture of spinel lherzolite and pyroxenite (± garnet). In contrast, the sixth unit, the main Piano Provenzana formation (~ 42–30 ka), includes the most evolved trachyandesitic lavas (58–62 wt% SiO2) and exhibits notably lower 176Hf/177Hf, 143Nd/144Nd, and 206Pb/204Pb ratios than the other prehistoric Valle del Bove units. This isotopic signature has not yet been observed in any other samples from Mt. Etna and we suggest that the parental melts of the trachyandesites were derived predominantly from ancient pyroxenite in the mantle source of Etna.
Highlights
Mount Etna, Europe’s largest active volcano, is located on the Mediterranean island of Sicily (Fig. 1) and lies in a complex tectonic setting near the convergence of the African and Eurasian plates
We provide new radiogenic isotope data (Sr, Nd, Pb, and Hf) for these prehistoric Valle del Bove units to investigate whether the broad trends in magma source evolution from ancient Etna eruptions to the modern day continued systematically throughout the ~ 80 kyr period that they cover
Salifizio-1 and Piano Provenzana lavas exhibit lower concentrations of total alkalis for a given S iO2 concentration and are transitional to silica oversaturated in composition, whereas the Mongibello lavas are all silica undersaturated, with up to 6 wt% normative nepheline
Summary
Mount Etna, Europe’s largest active volcano, is located on the Mediterranean island of Sicily (Fig. 1) and lies in a complex tectonic setting near the convergence of the African and Eurasian plates. Mount Etna is located near the convergence of the African and Eurasian tectonic plates (Fig. 1a). The stratigraphy, field relations, and petrography of the lavas can be inferred from the recent geological map of Etna by Branca et al (2011b). Note that we have tried to follow the terminology of Branca et al (2011b), choosing terms from the geological map of Etna that was published after field sampling had been completed. We have inadvertently employed a mixture of terms relating to different volcanic centres (e.g., Cuvigghiuni, Mongibello) and lithostratigraphic units (e.g., Piano Provenzana, Tripodo).
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