EVALUATING REDOX EVOLUTION AT THE SUBDUCTION INTERFACE FROM THE MO PERSPECTIVE: A CASE STUDY FROM THE LIGURIAN ALPS

The observation of heavy Mo isotopes in arc lavas is inferred to result from the reactive flow of serpentinite-derived fluids through subducted crustal rocks under oxidizing conditions. This interpretation is supported by the retention of light Mo (in phases such as rutile) in subducted oceanic rocks. However, the redox state of the metasomatic fluid and its effect on Mo partitioning and isotopic fractionation during high-pressure (HP) metamorphism remains enigmatic. We attempt to ascertain the redox gradient formed by HP fluid-mediated mass transfer in the subducted slab by analyzing a metasomatic reaction zone between juxtaposed serpentinite and metagabbro from the Voltri Massif using Mo isotopes, [Mo], [fluid-mobile elements; FMEs], Fe³⁺/ΣFe_total, and [S].

The HP metasomatic reaction zone shows systematic trends in [Mo], Fe³⁺/ΣFe_total, and δ^98/95Mo across the contact. Serpentinites display low [Mo], [FMEs], and a heavy Mo signature, indicative of reducing (H₂S) fluids from externally-derived crustal sources during burial. Metagabbros proximal to the contact exhibit lower [Mo], Fe³⁺/ΣFe_total, and δ^98/95Mo values compared to distal metagabbros. This trend is consistent with the redistribution of Mo from the metagabbros, the crystallization of Fe²⁺ -bearing minerals (e.g., ilmenite, chlorite), and the partitioning of heavy Mo into a progressively oxidizing (HSO₄^--bearing) fluid during Mg-metasomatism. At ~35 cm from the contact, [FMEs] and Fe³⁺/ΣFe_total increase with a shift to heavy δ^98/95Mo, suggesting interaction with a reduced (H₂S-bearing) fluid and the crystallization of Fe³⁺-bearing (i.e., epidote) minerals, accompanying Ca metasomatism/redistribution. Distal metagabbros maintain MORB-like δ^98/95Mo values and low [FMEs], remaining largely unaltered by metasomatism.

The results and interpretations in this study have two implications: 1) fluid-mediated mass transfer between serpentinite and crustal rocks can generate isotopically heavy Mo that can be delivered to the source of arc magmas via reactive fluid flow, and 2) this process alters the redox state of the slab and associated fluid. This study helps bridge the gap in understanding the fate of Mo in oceanic rocks, and the evolving redox gradient during metasomatism at HP metamorphic conditions in subduction zones.