Boron isotopic variations in lavas of the Aeolian volcanic arc, South Italy

The Aeolian arc (South Italy) overlies a subduction zone formed during a complex collision between the African and European tectonic plates. Thinned continental crust beneath the eastern part of the arc and subduction of continental lithosphere distinguish this arc from its counterparts in intraoceanic settings. Boron isotopic systematics were determined for selected Aeolian lavas to evaluate processes of magma genesis, and potential complications arising from involvement of continental materials. The most mafic lavas show significant variation in δ11B (−6.1 to +2.3‰) which is positively correlated with mobile/immobile element ratios (e.g. B/Th (0.7–5.1), B/Nb (0.4–4.9), B/Zr (0.05–0.37), Cs/Nb (0.04–0.42). Plots of δ11B vs. ratios of B to immobile trace elements (e.g. B/Nb, B/Zr, B/La, etc.) give similar δ11B intercepts which are typical of primitive oceanic mantle (ca. −7‰). However, detailed assessment of δ11B–Nb/B–Th/B systematics and other trace element data suggests that the Aeolian mantle wedge is laterally inhomogeneous. The overall cross-arc variation in δ11B in Aeolian calcalkaline lavas can be explained by addition to these mantle sources of a subduction component with relatively high but variable δ11B (−3.5 to +5‰). Progressively higher δ11B and ratios of fluid-mobile elements to Nb with distance eastward from Alicudi is indicative of greater subduction contributions toward the volcanic front. Only the potassic series lavas from Vulcano and Stromboli deviate from this pattern in that they have relatively low δ11B and distinctive Sr–Nd–Pb isotopic compositions that suggest strong involvment of a lithospheric component in addition to subduction effects in their mantle. Comprehensively, δ11B is poorly correlated with Sr–Nd–Pb isotopic compositions allowing clear distinction of subduction related processes from source composition effects.