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Mantle peridotites exhumed in mountain belts provide important insights on the composition and evolution of the upper mantle, and additionally inform on metamorphic, geochemical, and tectonic processes -including seismic activity-at convergent margins. In this contribution, we present field, microstructural, and mineralogical data of fresh, pseudotachylyte-bearing mantle peridotites from the lawsonite eclogite-facies San Petrone unit, Alpine Corsica, France. The present case study represents the first example of subducted fresh peridotite associated with fresh lawsonite eclogite-facies assemblages. Two bodies of fresh peridotite are embedded in fully serpentinized ultramafic rocks forming the substratum of a subducted ocean-continent transition of the Piemonte-Liguria Basin. Clinopyroxene and spinel mineral chemistry indicates that the investigated peridotite samples were part of a refertilized mantle and, therefore, the San Petrone unit likely belonged to the more distal part of the ocean-continent transition. Because small bodies of fresh peridotite embedded in fully serpentinized rocks can hardly be identified by means of geophysical investigations, this finding suggests that small, yet disseminated bodies of fresh mantle peridotite can potentially be more abundant than previously supposed at ocean-continent transition and, potentially, at mid-ocean ridges and in subduction zones. The preservation of fresh mantle peridotite bodies in subducting slabs is also discussed with respect to its potential implications on intermediate depth seismicity and geochemical cycling -including production of natural energy sources-from rifting to subduction. [ABSTRACT FROM AUTHOR]

Fluid release from subducting oceanic lithosphere is a key process for subduction zone geodynamics, from controlling arc volcanism to seismicity and tectonic exhumation. However, many fundamental details of fluid composition, flow pathways, and reactivity with slab‐forming rocks remain to be thoroughly understood. In this study we investigate a multi‐kilometer‐long, high‐pressure metasomatic system preserved in the lawsonite‐eclogite metamorphic unit of Alpine Corsica, France. The fluid‐mediated process was localized along a major intra‐slab interface, which is the contact between basement and cover unit. Two distinct metasomatic stages are identified and discussed. We show that these two stages resulted from the infiltration of deep fluids that were derived from the same source and had the same slab‐parallel, updip flow direction. By mass balance analysis, we quantify metasomatic mass changes along this fluid pathway and the time‐integrated fluid fluxes responsible for them. In addition, we also assess carbon fluxes associated with these metasomatic events. The magnitude of the estimated fluid fluxes (104–105) indicates that major intra‐slab interfaces such as lithological boundaries acted as fluid channels facilitating episodic pulses of fluid flow. We also show that when fluids are channelized, high time‐integrated fluid fluxes lead to carbon fluxes several orders of magnitude higher than carbon fluxes generated by local dehydration reactions. Given the size and geologic features of the investigated metasomatic system, we propose that it represents the first reported natural analogue of the so‐called high permeability channels predicted by numerical simulations. Key Points: Analysis of metasomatic rocks formed along a slab‐scale lithological interface indicates that extremely high fluid fluxes were realizedReactive fluid flow lead to CO2‐rock interactions, which can sequester as much carbon as the amount liberated from decarbonation/dissolutionThe investigated metasomatic system is the first report of a natural analogue of intra‐slab high permeability channel [ABSTRACT FROM AUTHOR]

Carbonate-bearing lithologies are the main carbon carrier into subduction zones. Their evolution during metamorphism largely controls the fate of carbon, regulating its fluxes between shallow and deep reservoirs. Recent estimates predict that almost all subducted carbon is transferred into the crust and lithospheric mantle during subduction metamorphism via decarbonation and dissolution reactions at high-pressure conditions. Here we report the occurrence of eclogite-facies marbles associated with metasomatic systems in Alpine Corsica (France). The occurrence of these marbles along major fluid-conduits as well as textural, geochemical and isotopic data indicating fluid–mineral reactions are compelling evidence for the precipitation of these carbonate-rich assemblages from carbonic fluids during metamorphism. The discovery of metasomatic marbles brings new insights into the fate of carbonic fluids formed in subducting slabs. We infer that rock carbonation can occur at high-pressure conditions by either vein-injection or chemical replacement mechanisms. This indicates that carbonic fluids produced by decarbonation reactions and carbonate dissolution may not be directly transferred to the mantle wedge, but can interact with slab and mantle-forming rocks. Rock-carbonation by fluid–rock interactions may have an important impact on the residence time of carbon and oxygen in subduction zones and lithospheric mantle reservoirs as well as carbonate isotopic signatures in subduction zones. Furthermore, carbonation may modulate the emission of CO 2 at volcanic arcs over geological time scales. [ABSTRACT FROM AUTHOR]

The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays a crucial role in carbon distribution and fluxes regulation. Here we present field, microstructural, petrological and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites from the Chinese southwestern Tianshan ultrahigh-pressure (UHP) metamorphic belt. The mafic eclogites recorded two successive, superimposed, metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies (Stage#2). This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites by carbonate + paragonite (± zoisite) + quartz. Carbon and oxygen isotopic data and Deep Earth Water (DEW) thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions equal or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Modeling results also show that the saturation of carbonic fluid with respect to graphite can boost retrograde carbonation processes. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts. • Metasediment-derived carbonic fluids were responsible for mafic eclogite carbonation upon exhumation. • DEW thermodynamic modeling reveals that graphite saturation boosts carbonation reaction. • Fluid/rock ratio controls the metasomatic product. [ABSTRACT FROM AUTHOR]