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1. Exhumed Serpentinites and Their Tectonic Significance in Non-Collisional Orogens

   Donoso-Tapia, Damián; Flores, Kennet E; Martin, Celine; Gazel, Esteban; Marsh, Jeffrey (February 2024, Geochemistry geophysics geosystems)

   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. 

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2. Exhumed Serpentinites and Their Tectonic Significance in Non‐Collisional Orogens

   https://doi.org/10.1029/2023GC011072  

   Donoso‐Tapia, Damián; Flores, Kennet E; Martin, Celine; Gazel, Esteban; Marsh, Jeffrey (February 2024, Geochemistry, Geophysics, Geosystems)

   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. 

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

   https://doi.org/10.1016/j.epsl.2025.119213  

   Donoso-Tapia, Damián; Flores, Kennet E; Martin, Celine; Hull, Sarah; Hernández-Uribe, David; Gazel, Esteban (January 2025, Earth and Planetary Science Letters)

   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. 

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4. Emergent Feedbacks Between Progressive Serpentinization, Interface Weakening, and Subduction Rates

   https://doi.org/10.1029/2025GC012488  

   Stoner, R K; Holt, A F; Epstein, G S; Guevara, V E; Condit, C B (November 2025, Geochemistry, Geophysics, Geosystems)

   Abstract During subduction, the downgoing oceanic crust is exposed to high temperatures in the mantle wedge, causing volatile‐bearing minerals to break down and release hydrous fluids into the forearc. These fluids percolate upwards, reacting with the mantle wedge to form hydrated ultramafic lithologies, including serpentinite. To accurately track the fate and impact of water on the forearc, we develop time‐dependent models that self‐consistently capture both serpentinite ingrowth and the associated rheological weakening of the plate interface. Unlike many subduction models that investigate forearc serpentinization and prescribe plate velocities, geometries, or steady‐state conditions, our approach allows plates to evolve dynamically without predefined velocities or geometries. During subduction infancy, serpentinite accumulates rapidly. As subduction matures, serpentinite ingrowth decreases, and more serpentinite is also dragged downward by the slab. To elucidate the links between subduction dynamics and serpentinization, we consider variations in serpentinite strength and hydration state of the incoming plate. Subducting fully water‐saturated sediments yield ∼3× greater forearc serpentinite than within the moderately hydrated reference case. The water‐saturated case produces a weaker interface and, in turn, subduction zone convergence rates ∼40% higher than in an endmember case with anhydrous sediment. A lower serpentinite strength also produces higher convergence rates despite more downdragging of serpentinite from the forearc. We find that hydrous sediments not only lubricate the interface directly by weakening it, as previously suggested, but also by dehydrating and releasing water that produces weak serpentinite in the mantle wedge, with such feedback only able to be captured within fully coupled dynamic models. 

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5. A c. 1900 Ma Tethyan-type ophiolite in the Penokean Orogen, Pembine, Wisconsin (USA): Insights from the volcanic stratigraphy

   https://doi.org/10.1016/j.precamres.2023.107223  

   Guice, George L.; Viete, Daniel R.; Holder, Robert M.; Roy, Supratik (December 2023, Precambrian Research)

   Orogenic ophiolites are a hallmark of Phanerozoic plate tectonics, containing igneous lithologies that provide constraints on fundamental tectono-magmatic processes. The c. 1900Ma Pembine Ophiolite (Wisconsin, USA) is associated with the Penokean Orogen and represents a rare example of a proposed Paleoproterozoic ophiolite. The Penokean Orogen shares broad characteristics with Phanerozoic (<541 Ma) orogens, but the origin of the Pembine Ophiolite remains unclear, with the mafic volcanic rocks interpreted as representing either an intra-oceanic arc or continental back arc setting. To test these hypotheses, we present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of 34 Pembine rocks, as well as U-Pb zircon geochronology for two samples. Based on trace elements established as immobile in the studied rocks, we demonstrate that mafic volcanism progressed (up-stratigraphic-section) from mid-ocean ridge-like to boninitic. The chemical evolution is identical to that observed in < 250 Ma ophiolites in the Himalayan–Alpine Orogen, which record forearc spreading during the nascent stages of subduction in the Tethys Ocean. We interpret the Pembine Ophiolite as forearc lithosphere formed during subduction initiation and obducted to the margin of the Superior Craton during the Penokean Orogeny. The processes responsible for forming (and preserving) this example of a Paleoproterozoic ophiolite may not have been dissimilar to those operating on the Phanerozoic Earth. 

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