Constraints on the genesis of the potassium-rich Italian volcanics from U/Th disequilibrium.

We present new U-series isotope, 87Sr/ 86Sr, 143Nd/144Nd and trace element data for a set of mafic, K-rich rocks from volcanoes in Central-Southern Italy. These shoshonitic to ultrapotassic lavas display strongly depleted high field strength element (HSFE) abundances with respect to other incompatible trace elements together with high but variable 87Sr/ 86Sr and low but variable 143Nd/144Nd values. Such characteristics are thought to be due to addition of subducted crust of variable amount and composition to their mantle sources prior to magma genesis. Rocks from the northernmost region (i.e.Tuscan Magmatic Province and Northern Roman Magmatic Province) display (230Th/ 238U) activity ratios close to radioactive equilibrium, suggesting that metasomatism of their sources occurred before 400 ka and recent melting took place at shallow depths, in the absence of garnet. A 238U excess of up to 27% has been measured in rocks from the Neapolitan District. The occurrence of significant U excesses is a feature of arc magmas, but is typically seen in depleted lavas rather than in highly enriched rocks such as these (20 ppm Th).This signature requires a recent addition of a U-rich component to the already strongly enriched mantle wedge beneath this region of Italy.We suggest that a supercritical liquid, from deeply subducted carbonate-rich sediments of the still-active Ionian slab, is responsible for generating a high-U, low-Th component, which produces the observed disequilibria. A 30% 230Th excess measured in a single unaltered sample from the Lucanian Magmatic Province, along with a less marked negativeHFSE anomaly, suggests the contribution of a deeper, garnet-bearing component in the genesis of these magmas, plausibly related to the upwelling of asthenospheric mantle around the corner of the Ionian slab.