MOLYBDENUM ISOTOPE SYSTEMATICS DURING REACTIVE FLUID FLOW IN SUBDUCTION ZONES: AS CASE STUDY FROM THE VOLTRI MASSIF, ITALY

Primitive arc lavas display heavier Mo isotope compositions relative to mid-ocean ridge basalts. This isotopic difference was attributed to distinct contributions from serpentinite-derived fluids into the arc magma source. While an increasing number of studies highlight the utility of Mo isotopes in tracing fluid-mediated processes in subduction zones, the mobility and distribution of Mo along the slab-mantle interface remain poorly understood. For instance, a limited number of studies have alluded to the importance of both mineralogical and redox controls on the partitioning behavior of Mo during fluid-rock interactions at relevant P-T conditions. This study investigates the distribution and fluid-mediated transfer of Mo and its isotopes between juxtaposed mafic (metagabbro) and ultramafic (serpentinite) rocks at the slab-mantle interface, using the exhumed high-pressure Voltri Massif (Ligurian Alps, Italy) as a natural laboratory. The transect, composed of five mineralogically distinct zones, is characterized by systematic bulk-rock δ^98/95Mo, [Mo], fluid-mobile elements (FMEs; e.g., Ba, Sr) and Fe³⁺/ΣFe_total trends. Zone 1 serpentinites display low [Mo] and [FMEs], and heavy δ^98/95Mo which we interpret to have resulted from interaction with externally derived crustal fluids during early stages of burial. Metagabbros (Zones 2 and 3) reacted by fluids previously in equilibrium with serpentinites (Zone 1) display low [Mo], [FMEs], Fe³⁺/ΣFe_total, and light δ^98/95Mo relative to more distal metagabbros (Zone 4). Zone 4 metagabbros with high [Mo], [FMEs] and Fe³⁺/ΣFe_total display heavy δ^98/95Mo isotope composition. Zone 5 metagabbros display low [FMEs] and Fe³⁺/ΣFe_total and a MORB-like [Mo] and δ^98/95Mo isotope compositions. We discuss the geochemical trends in terms of the evolving mineralogy and oxidation state during reactive infiltration of fluid previously in equilibrium with serpentinite and subsequent reaction with the adjacent metagabbro. This study highlights the complex partitioning behavior of Mo at the slab-mantle interface and proposes pathways of delivering isotopically heavy Mo from the slab to the source region of arc magmas.