Mo isotopes as tracers for deep recycling of subducted C-rich, anoxic sediments in the mantle source of the Roman magmatic province

Understanding the role of recycled sedimentary material into the convecting mantle is a key aspect to provide new constraints on the chemical budget of subduction zones. Mo isotopes have been shown to fractionate in the oceans during the incorporation into sediments, being perceptive to redox conditions. The variable composition of Mo isotopes recorded in different geochemical reservoirs offers the opportunity to use these isotopes as tracers of recycled material into the mantle. This is particularly true for sediment formed under anoxic conditions, which inherited from the seawater specifically heavy isotopic compositions. The potassic and ultrapotassic igneous rocks of the Roman magmatic province (Italy) show extremely variable and well distinct geochemical and radiogenic isotopic signatures, which are referred to a strong but variable involvement of a subduction-recycled component in their genesis, hence they represent an interesting case study for tackling the role of different subduction-related metasomatic agents using Mo stable isotopes. We performed Mo isotopes on magmatic rocks and sedimentary end-members as proxy of the recycled component. Roman igneous rocks display variable Mo isotope compositions, which are significantly heavier compared to any sediment-dominated subduction-related magmatic rocks measured so far (e.g., Lesser Antilles). Such heavy Mo isotopic signatures, reveal a sort of “Mo anomaly”, which is not observed in the nearby subduction-related magmatic regions. The heavy isotope composition, along with the pronounced sediment-dominated character, suggests the presence of an isotopically heavy component in the subducted material that is likely to derive from subducted, anoxic organic-rich sediments. In this context, the application of Mo isotopes to complex subduction settings show the potential of Mo isotopes as tracers of recycled anoxic sediment, and thus the fate of organic carbon, in subduction zones.