Ce/Mo and Molybdenum Isotope Systematics in Subduction-Related Orogenic Potassic Magmas of Central-Southern Italy

Several recent studies have employed variations in the concentration and isotopic composition of molybdenum as tracers of igneous processes. In this study we present new Mo concentration and δ^98/95Mo data on the peculiar subduction-related potassic magmas of the Central-Southern Italian peninsula; the leucite-free (lamproite-like) rocks of the Tuscan Magmatic Province and the leucite-bearing rocks of Mt. Vesuvius. These rocks display exotic and distinctive geochemical and isotopic features due to differences in the lithology of the subducted material in their respective mantle sources. We examine the elemental and isotopic systematics of Mo in the context of these geochemical variations. The two different associations of magmas display significantly different Ce/Mo values but surprisingly similar δ^98/95Mo values (0.10–0.26‰ for Vesuvius and 0.07–0.24‰ for Tuscan Magmatic Province), which are significantly heavier than typical mid-ocean ridge basalts. While the δ^98/95Mo implicate an isotopically heavy sedimentary component recycled into their respective mantle sources, the different Ce/Mo ratios reflect contrasting elemental fractionation during sediment melting related to the lithology and consequent residual mineralogy (sulfides vs. epidote) of the subducted sedimentary material undergoing melting (Ca poor vs. Ca rich). This indicates that the heavy Mo isotopic signature of these magmas is independent of the lithology of the recycled material, which instead controls the elemental fractionation of Mo.