Trace elements in olivine characterise the mantle source of subduction related potassic magmas

Trace elements in olivine have recently been shown to provide useful means to investigate the upper mantle, allowing identification of olivine-free (pyroxenitic) reservoirs in the source of OIBs [1]. But few studies are available of magmatic rocks at destructive plate margins [2]. Here we present the first comprehensive set of major and trace element analyses on olivine from subduction-related Plio-Quaternary magmatism of the Italian Peninsula. 16 rock samples covering the full compositional range from calc-alkaline to ultrapotassic occurring within the Italian magmatic region were selected for their primitive character (Mg# = 60-80) and the presence of olivine at the liquidus. 200 olivine crystals were analyzed for major and trace elements (Ca, Mn, Ni) using EPMA, and more than 100 have been analyzed for trace elements using LA ICP-MS. Olivine phenocryst cores show a large compositional variability among the different magmatic suites. Olivine from the Lucanian region has the highest Na and Al, whereas most elements have lower concentrations compared to the other Italian regions and fall within typical mantle values. The olivine crystals from the lamproite-like samples (Tuscan Region) bear evidence of an olivine-depleted, pyroxenitebearing source resulting from Si- and K-rich metasomatism of previously depleted mantle: the olivine crystals are remarkably enriched in Ni (up to 5,000 ppm) with high Cr and low Mn and Ca contents. Olivines from plagio-leucititic rocks of the Roman Region show trace element contents that differ from those of the lamproite-shoshonite suite (extremely low Ni, low Cr and elevated Mn and Ca). This could be explained by a metasomatic reaction in the presence of excess Ca. Li enrichment (up to tens of ppm) in olivine from the Tuscan and the Roman Province indicate the recycling of crustal material into the mantle wedge as a primary cause of the metasomatism