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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 (∼13.5 °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 ∼1.3–1.6 GPa and 620–650 °C. The peak paragenesis is partially obscured by a strong retrograde overprint, driven by crust-derived metamorphic fluids (δ11B ∼ -6 to +8 ‰) that infiltrated at varying fluid/rock ratios, triggering the re-serpentinization of metaperidotites during exhumation (P < 1.3 GPa and 320–400 °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 ∼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. 

Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid-rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non-collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non-collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in-situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene high-pressure (HP)–low-temperature metamorphic sequences. Whole-rock trace element data and in-situ B isotopes favor serpentinization by a crust-derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid-Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back-arc basins. Subsequent compressional phases trigger short-lived subduction in the back-arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non-collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back-arc basins. 

Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid-rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non-collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non-collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in-situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ11B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ11B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic-Early Cretaceous, and potentially Late Cretaceous-Paleocene high-pressure (HP)–low-temperature metamorphic sequences. Whole-rock trace element data and in-situ B isotopes favor serpentinization by a crust-derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid-Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back-arc basins. Subsequent compressional phases trigger short-lived subduction in the back-arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non-collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back-arc basins. 

Abstract Exhumed serpentinites are fragments of ancient oceanic lithosphere or mantle wedge that record deep fluid‐rock interactions and metasomatic processes. While common in suture zones after closure of ocean basins, in non‐collisional orogens their origin and tectonic significance are not fully understood. We study serpentinite samples from five river basins in a segment of the non‐collisional Andean orogen in Ecuador (Cordillera Real). All samples are fully serpentinized with antigorite as the main polymorph, while spinel is the only relic phase. Watershed delineation analysis and in‐situ B isotope data suggest four serpentinite sources, linked to mantle wedge (δ¹¹B = ∼−10.6 to −0.03‰) and obducted ophiolite (δ¹¹B = −2.51 to +5.73‰) bodies, likely associated with Triassic, Jurassic‐Early Cretaceous, and potentially Late Cretaceous‐Paleocene high‐pressure (HP)–low‐temperature metamorphic sequences. Whole‐rock trace element data and in‐situ B isotopes favor serpentinization by a crust‐derived metamorphic fluid. Thermodynamic modeling in two samples suggests serpentinization at ∼550–500°C and pressures from 2.5 to 2.2 GPa and 1.0–0.6 GPa for two localities. Both samples record a subsequent overprint at ∼1.5–0.5 GPa and 680–660°C. In the Andes, regional phases of slab rollback have been reported since the mid‐Paleozoic to Late Cretaceous. This tectonic scenario favors the extrusion of HP rocks into the forearc and the opening of back‐arc basins. Subsequent compressional phases trigger short‐lived subduction in the back‐arc that culminates with ophiolite obduction and associated metamorphic rock exhumation. Thus, we propose that serpentinites in non‐collisional orogens are sourced from extruded slivers of mantle wedge in the forearc or obducted ophiolite sequences associated with regional back‐arc basins.