Lithology of recycled sediments controls metasomatism in the lithospheric mantle: Examples from Italian ultrapotassic volcanics

Mafic ultrapotassic rocks from central Italy exhibit enrichment in mantle incompatible trace elements, continental crust-like Sr-Nd-Pb isotope ratios, and elevated δ18O values - signatures that point to metasomatism of their mantle source by crust-derived materials. Yet, the precise origin of these materials and the processes leading to formation of these rocks remain elusive. We present new TIMS Sr-Nd-Pb isotope and ICP-MS trace element analyses from neighboring Cimini and Sabatini Mts. Volcanic districts. Our goal is to constrain the nature of the involved crustal components and to investigate the extreme enrichment and fractionation of mantle incompatible trace elements that deviate from proposed crustal and mantle endmembers.Trace element and isotopic modelling indicate that the Cimini mantle source formed via interaction between peridotite and melts of subducted siliciclastic metasediment, producing phlogopite- and orthopyroxene rich (± accessory apatite and titanite) metasomatic veins. Stability at depth of accessory phases in the metasediment caused fractionation of trace element ratios such as Th/La and Sm/La in the vein rocks. Subsequent melting of the metasomatic veins produced the Cimini lavas. The Sabatini lavas formed from melts of subducted mixed silica and carbonate rich metasediment that reacted with peridotite and crystallized clinopyroxene and phlogopite. The compositional variation of Sabatini lavas reflects differences in metasediment proportions, melt-rock reaction, and immiscibility between carbonate and silicate melts. This study highlights the importance of recycled crustal lithologies in mantle metasomatism.