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 delta Mo-98/95 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 delta Mo-98/95 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 delta Mo-98/95 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. Plain Language Summary Subduction is one of the most important processes of plate tectonics and controls the global cycle of elements from surface to Earth's interior. Recycling via subduction is an efficient mechanism for returning light and incompatible trace elements to the mantle. Trace elements and isotopic compositions represent a unique tool for investigating the different components in defining the geochemical signature of orogenic magmas. We apply the study of Mo elemental and isotopic systematics to some Italian potassic volcanic rocks (i.e., leucite-free rocks from Tuscany and leucite-bearing rocks from Vesuvius), which are from mantle sources clearly dominated by different recycled sediments (carbonate poor vs. carbonate rich). Mo isotope compositions of these two groups are similar, at odds with the significantly different trace element and radiogenic isotope signature, and are also among the heaviest recorded in subduction-related magmas arguing for originally heavy recycled sediment components. The distribution of Mo, Ce, and Nb between the two groups requires specific fractionation that can be explained by the role of different accessory phases (sulfides vs. epidote) occurring as residual phases after sediment melting. We present a model to constrain the process liable to reproduce the melt components responsible for mantle metasomatism of the studied rocks.