The fate of high-angle dipping slabs in the subduction factory: An integrated trace element and radiogenic isotope (U, Th, Sr, Nd, Pb) study of Stromboli volcano, Aeolian Arc, Italy

Abstract

The subaerial part of the Stromboli stratovolcano was built up in the last 100 kyr through six periods of activity; the erupted magmas record the largest compositional variation of all the Aeolian arc volcanoes (calc-alkaline, shoshonitic, and potassic alkaline magma series). The trace element characteristics of the less evolved magmas of each period of activity are coherently correlated with their radiogenic isotope (Sr, Nd, Pb) composition, and are typical of volcanic arc rocks. In terms of U-series isotopes, samples from the different magma series have both U-238 and Th-230 excesses, and this distinctive feature provides additional constraints on source enrichment processes within the mantle wedge and on the mechanism of partial melting. Overall the complete set of data demonstrates that the genesis of the different magma series at Stromboli can be accommodated in a mantle source that experienced two distinct enrichment processes by different parts of the subducting oceanic crust of the Ionian slab. The first was caused by supercritical liquids originating from the basaltic and sedimentary parts of the subducting slab at >5 GPa and similar to 900 degrees C. The second was induced by aqueous fluids, again originating from the basaltic and sedimentary parts of the slab, released from a shallower part of the subducted Ionian slab (<5GPa and similar to 800 degrees C). U-Th disequilibria constrain the timing of the first metasomatic event (Stage I: supercritical liquids) at >435 ka, whereas the second event (Stage II: aqueous fluids) occurred at similar to 100 ka. The high-angle dip of the Ionian slab (similar to 70 degrees) caused the superimposition of the metasomatizing agents of the two enrichment processes in the same volume of the mantle wedge, explaining the occurrence of such different magma series in a single volcanic edifice. The U-Th disequilibria provide evidence for dynamic melting of the metasomatized mantle wedge combined with an ageing effect resulting from the restoration of secular equilibrium after the perturbation caused by the U-rich aqueous fluids of Stage II. The trace element and radiogenic isotope (U, Th, Sr, Nd, Pb) signature of the mantle source of the magmas at Stromboli is thus dependent upon the amount of supercritical liquids and aqueous fluids released by the two components of the subducted slab, whereas the distinctive U-238 and Th-230 excesses of the magmas result from a combination of mantle ageing and time-dependent dynamic melting. The geochemical and radiogenic isotope signature of the mantle source beneath Stromboli places important constraints on the isotopic polarity from Southern Latium to the Aeolian arc attributed to the effect of a HIMU mantle component following either lateral inflow of foreland mantle material or upwelling of a mantle plume in the centre of the Tyrrhenian basin. Our geochemical model demonstrates that the high Pb-206/Pb-204 of the putative 'HIMU' mantle component could be equally formed during metasomatism of the pre-existing mantle wedge by either the supercritical liquid (Stage I) or aqueous fluid (Stage II) released by the subducted altered basalt of the Ionian plate.