Trace elements in olivine from Italy convergent margin and inferences on the mineralogy of the mantle wedge

Major and trace elements in forsteritic olivine from mafic igneous rocks has proven a valuable tool to infer on metasomatic processes that affect the mantle-source of magmas prior to melting. The deviation of some compatible trace elements (e.g. Ni, Co, Cr) and their ratios to major elements (Fe, Mg), from the values of olivine from MORB, has been used to quantify the contribution of an olivine-free mantle source in the origin of some OIB. Despite the potential of this approach, very few studies exist on olivine from convergent settings, leaving a gap in our knowledge of magma genesis in the mantle wedge. Here we present the first comprehensive set of major ant trace elements analyses of high-Mg olivine crystals from the Plio-Qyaternary magmatic rocks of the Italian Peninsula. The samples are among the most primitive members of the relevant petrographic series: from Ca-alkaline to strongly alkaline rocks ('ultrapotassic') with lamproitic and leucititic affinity. Olivine crystals were analyzed for major (EPMA) and a wide palette of trace elements (EPMA and LA ICP- MS). The olivine crystals display a wide variety of Ni, Cr, Mn and Ca contents, which encompass the whole spectrum of magmatic olivines worldwide. Olivines from the lamproite family have remarkably high Ni (up to 5.000 ppm) and Cr, low Ca and Mn/Fe. These values are consistent with a contribution from a mantle source that had undergone depletion and then silica- and K-rich metasomatism prior to melting, leading to olivine-consumption in favour of orthopyroxene. Conversely, the olivine crystals from the rocks of the leucititic family have diametrically opposed trace elements contents. These olivines have variably high Ca (up to 4.000 ppm), Mn/Fe, and low Ni (less than 2,000 ppm) and Cr. This is compatible with a metasomatic reaction in the presence of excess Ca, that would stabilize olivine and clinopyroxene at the expenses of orthopyroxene. Olivine crystals from both groups are characterized by remarkably high-Li contents (consistently above 5 ppm), pointing toward the involvement of crustal material that have been subducted and then recycled into the mantle wedge. Our results provide further evidence that two different types of sediments are responsible for the two metasomatic processes depicted above: silico-clastic sediments in the source of the rocks of the Lamproite group, carbonate-rich sediments in that of the rocks of the Leucitite group. This duality is strictly linked to the geodynamic of the subduction front beneath Italy and to its changes with time and space.