Mantle melting in within-plate continental settings: Sr–Nd–Pb and U-series isotope constraints in alkali basalts from the Sicily Channel (Pantelleria and Linosa Islands, Southern Italy)

Abstract

The origin of the mantle sources of the Na-alkaline magmas erupted within the continental rift of the Sicily Channel and their melting behaviour are here investigated through the determination of Sr, Nd and Pb isotope ratios and U-series disequilibria on basaltic volcanic rocks from Linosa and Pantelleria. The isotope data, along with trace element ratios are used to assess the possible role of the interaction with the continental crust and/or the Sub-Continental Lithospheric Mantle (SCLM). The data show little variation in Sr and Nd isotopes and a continuous trend toward more radiogenic Pb isotope composition from Linosa to the oldest mafic activity of Pantelleria (i.e. Paleo-Pantelleria), with intermediate values measured in the youngest Pantelleria lavas (Neo-Pantelleria). Pantelleria basalts have ubiquitous 230Th-excess ranging from 7% to 20%. These data suggest the magmas are originated within the asthenospheric mantle, with little or no interaction with either the continental crust or the SCLM. The increasing FOZO-like character of the studied magmas and the variation of some key trace element ratios (e.g. Rb/La) argue for an increasing role of recycled oceanic material in the form of eclogite/pyroxenite dispersed within the mantle sources of these magmas. A completely distinct isotope composition is recorded in Neo-Pantelleria hawaiites from Khartibucale, which show significantly higher 87Sr/86Sr and lower 143Nd/144Nd, 206Pb/204Pb, (238U/232Th) and (230Th/232Th), but comparable (230Th/238U) with respect to all the other rocks studied. These rocks cannot be considered co-genetic with other Pantelleria basalts and are interpreted either as related to interaction with partial melts of the SCLM or to be originated from a mantle source enriched by recycled crustal material (EM-like). 235U–231Pa disequilibria were also measured in one Neo-Pantelleria hawaiite and one Neo-Pantelleria basalt. The coupled (230Th/238U)=1.20 and (231Pa/235U)=1.39 of the latter were used to perform quantitative dynamic melting models in order to constrain physical parameters of mantle melting in the Sicily Channel. The combined modelling yielded positive solutions only for high DU/DTh (≥2.5) and low melting rates (Γ<1×10−4kg/m3/a). These data argue against any important role for amphibole in the genesis of these magmas and are consistent with a peridotite source possibly well mixed with recycled components. The modelled values of Γ can also be converted into estimates of the upwelling rate of the mantle that are compatible with slow passive upwelling along the Sicily Channel rift.