Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting
lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel
the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were
used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external
fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation
processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine
high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction
metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from
core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while
no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an
extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence
to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in
garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal
extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the
serpentinised lithospheric mantle or hydrated portions of the basement.