Record of de-serpentinization and re-serpentinization of an exhumed slab sliver: Implications for fluid circulation in subduction zones

Fluid release associated with serpentinite dehydration (de-serpentinization) during subduction plays a key role in fundamental geological processes such as element transport and recycling, seismicity, and arc magmatism. Although the importance of these fluids is well-known, evidence of de-serpentinization remains scarce in the rock record. Here, we investigated the effects of de-serpentinization and fluid circulation in exhumed metaperidotites from the Raspas Complex (Ecuador). This Early Cretaceous complex records warm subduction (similar to 13.5 degrees C/km) and has been hypothesized to represent a coherent slab sliver that preserves the mantle-crust contact (moho) between eclogite-facies metaperidotites and the corresponding crustal section. Petrological observations reveal that titanian-clinohumite-bearing metadunites and banded metaperidotites underwent de-serpentinization after reaching peak pressure-temperatures (P-T) of similar to 1.3-1.6 GPa and 620-650 degrees C. The peak paragenesis is partially obscured by a strong retrograde overprint, driven by crust-derived metamorphic fluids (delta 11B similar to -6 to +8 parts per thousand) that infiltrated at varying fluid/rock ratios, triggering the re-serpentinization of metaperidotites during exhumation (P < 1.3 GPa and 320-400 degrees C). Thermodynamic forward modeling reveals that fluid release in the Raspas paleo-subduction zone is controlled by brucite breakdown and de-serpentinization, which occur at depths of 25-30 km and similar to 50 km, respectively, accounting for a total of up to 10 wt. % H2O of water stored in the rock. Comparatively, dehydration of the crustal section, albeit a minor component, promotes enhanced fluid circulation between 25 and 45 km. During exhumation, circulating crust-derived metamorphic fluids heavily metasomatized the ascending slab sliver and effectively modified its geochemical signature. The depth range of the dehydration reactions overlap the depth of non-volcanic tremors and slow-slip events in warm, active subduction zones worldwide (25-65 km). Thus, the Raspas Complex offers an in-situ window into the fluids responsible for triggering these seismic events.