Behaviour of fluid-mobile elements across a high-pressure serpentinization front (Monte Maggiore unit, Alpine Corsica)

Subducted sections of dry oceanic mantle can undergo serpentinization through interaction with metamorphic fluids, recording a sequence of serpentinization events from the subseafloor to serpentine-out reactions. The geochemical patterns of these different stages of fluid-rock interaction trace the context and mechanisms of serpentinization processes. In this study, we used petrographic observations, micro-Raman spectroscopy, as well as bulk and in situ trace element analyses across a serpentinization front adjacent to a subduction-related tectonic contact in the Monte Maggiore massif, Alpine Corsica, France. Using a high-density sampling approach throughout the entire massif, we identified multiple and consistently distributed serpentine generations, reflecting successive hydration events in a chemically open system. Fluid-mobile elements (FME) exhibit a slight increase from lizardite- to antigorite-dominated generations and a more substantial rise with increasing serpentinization degrees towards the tectonic contact. When compared to other serpentinization fronts originated at different geodynamic settings – e.g., Semail Ophiolite (Oman) as a subduction serpentinization front; Lanzo Massif (W. Alps) as a subducted oceanic serpentinization front–, the Monte Maggiore FME patterns indicate a subduction-related serpentinization front, rather than subducted oceanic serpentinites. Our results and the comparison with literature data suggest that decreasing in situ serpentine FME inventory concentrations across serpentinization fronts may be characteristic of subduction-zone serpentinization, whereas opposite trends may be diagnostic of subducted oceanic fronts. Our study also highlights the importance of tracing serpentinization as a spatially evolving process (across fronts), with least serpentinized rocks recording the latest stage(s) of serpentinization instead of relicts of incipient serpentinization.