Metasomatism and melting in subduction–related volcanics: U-Th-Pa constraints from Vesuvius

We argue that the highly undersaturated, incompatible element enriched lavas of southern Italy reflect the influence of a metasomatic component from subducted carbonate-rich sediments. Vesuvius in particular, provides a natural laboratory to study this process, located on an active margin where abundant clay-carbonate rich lithologies are currently subducting. Here we present new trace element and U-Th-Pa isotope data on historic potassic to ultrapotassic Vesuvius samples in order to constrain melting and metasomatic processes responsible for such petrologically distinctive rocks. The samples display ubiquitous 238U excess (up to 27%) at relatively constant (230Th/232Th). This combination of significant 238U excesses in enirched arc lavas is extremely unusual and we attribute this to the influence of carbonate component. In striking contrast to the 238U excess, our new (231Pa/235U) are among the highest reported for arc rocks. This further emphasises the decoupled U-Th and U-Pa systems seen elsewhere in arc volcanics. We reproduced the observed U-Th-Pa data with a dynamic melting model modified with the assumption that the amount of added U-rich hydrous melts is reflected by an increase in degree of melting (and in melting rate). In this model, 231Pa-excesses are generated by ‘ingrowth’ but the 238U-230Th excesses still dominantly reflect addition of a subduction related component. More generally we try to relate the case study of Vesuvius to the array of highly alkalic Italian magmatism. We suggest two distinct episodes of melting and metasomatism occurring at different depths and timescales in the down-going slab beneath Vesuvuis. Firstly, an episode of sediment melting below the solidus of carbonate phases at shallow depths which can generally explain the K-rich magmatism of the Italian volcanoes. Secondly, a recent episode of U-rich hydrous melts from carbonate- and clay-rich subducted metasediments, generated at the current depth of the slab beneath Vesuvius (>350km), can explain the observed 238U excesses.