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International audience; The Lago Superiore Unit (LSU, Monviso Massif, Italian Western Alps) is a section of fossil oceanic lithosphere equilibrated to eclogite facies conditions (550 °C – 2.8 GPa) during Alpine subduction (45–40 Ma). It is cut by two major shear zones, namely the Intermediate (ISZ) and Lower Shear Zone (LSZ), mostly consisting of serpentinite. The lowermost, serpentine-rich, section of the Lago Superiore Unit, the Basal Serpentinite, separates the HP ophiolite domain from the underlying continental Dora-Maira Unit.Here we show that the LSZ and the Basal Serpentinite were active at different stages of the subduction and exhumation history of the complex. Most of retrograde deformation and mineral re-equilibration were localized in the LSZ. Channelized fluids percolating during this phase chemically homogenized the LSZ serpentinites, that preserved their HP mineralogy only locally; the best-preserved relicts of the eclogite-facies high pressure stage within the LSZ serpentinite are nodules of magnesite (representing former veins) and eclogite blocks. Differently, the underlying Basal Serpentinite largely escaped the exhumation-related processes and still records the prograde chemical and petrological history of the LSU serpentinite, from ocean-floor hydration to HP metamorphic conditions.The Lago Superiore Unit thus represents a snapshot of major Alpine metamorphic and shearing events, from prograde subduction to exhumation. Its km-scale thickness, and the oriented antigorite fabric in the Lower Shear Zone and Basal Serpentinite makes it a good seismic reflector. This HP ophiolite complex can thus be used as proxy of a deep (70–80 km) Alpine-type subduction zone, and to better constrain and interpret seismic images of present-day convergent margins.

The key role of serpentinites in the global cycles of volatiles, halogens and fluid-mobile elements in oceans and in subduction zones is now ascertained by many studies quantifying their element budgets and the composition of fluids they release during subduction. Geochemical tracers (e.g. B, As, Sb; stable B and radiogenic Sr and Pb isotopes) have also been employed to trace the provenance of serpentinites (slab or forearc mantle?) accreted to the plate interface of fossil subduction zones. In turn, this helps defining the tectonic processes, seismicity and mass transfer attending rock burial and exhumation within subduction zones. The results suggest that the sole use of geochemical data is insufficient to track the origin of subduction-zone serpentinites and the timing of serpentinization, whether oceanic or subduction-related. Integrated multidisciplinary studies of ophiolitic serpentinites show that pristine, oceanic, geochemical imprints (e.g. high 11B, marine Sr isotopes, low As + Sb) become reset towards more radiogenic Sr, lower 11B, and higher As + Sb via metasomatic exchange with crust-derived fluids during subduction accretion to the plate interface.The dehydration fluids released by serpentinite dehydration at various subduction stages and still preserved in these rocks as inclusions, carry significant amounts of halogens and fluid-mobile elements. The key compositional similarities of antigorite-breakdown fluids from different localities (Betic Cordillera, Spain; Central Alps, Switzerland) indicate that rocks record comparable subduction processes. We individuate the fluid-mediated exchange with sedimentary and/or crustal reservoirs during subduction as the key mechanism for geochemical hybridization of serpentinite. The antigorite dehydration fluids produced by hybrid serpentinites have high Cs, Rb, Ba, B, Pb, As, Sb and Li overlapping those of the arc lavas and representing the mixed serpentinite-sediment (crustal) component released to arcs. This helps discriminating the mass transfer processes responsible for supra-subduction mantle metasomatism and arc magmatism. The studied plate-interface hybrid serpentinites are also proxies of forearc mantle metasomatized by slab fluids. Based on the above observations, we propose that the mass transfer from slabs to plate interface and/or forearc mantle and the subsequent down-drag of this altered mantle to subarc depths potentially is a major process operating in subduction zones.The nominally anhydrous olivine, orhopyroxene, clinopyroxene and garnet produced by serpentinite dehydration host appreciable amounts of halogens and fluid-mobile elements that can be recycled in the deep mantle beyond arcs. Involvement of de-serpentinized residues in lower mantle metasomatism begins to be increasingly recognized by studies of ocean island basalts (OIB) and of B-bearing blue diamonds and by the isotopic serpentinite compositions presented here

Much of the long-term carbon cycle in solid earth occurs in subduction zones, where processes of devolatilization, partial melting of carbonated rocks, and dissolution of carbonate minerals lead to the return of CO2 to the atmosphere via volcanic degassing. Release of COH fluids from hydrous and carbonate minerals influences C recycling and magmatism at subduction zones. Contradictory interpretations exist regarding the retention/storage of C in subducting plates and in the forearc to subarc mantle. Several lines of evidence indicate mobility of C, of uncertain magnitude, in forearcs. A poorly constrained fraction of the 40–115 Mt/yr of C initially subducted is released into fluids (by decarbonation and/or carbonate dissolution) and 18–43 Mt/yr is returned at arc volcanoes. Current estimates suggest the amount of C released into subduction fluids is greater than that degassed at arc volcanoes: the imbalance could reflect C subduction into the deeper mantle, beyond subarc regions, or storage of C in forearc/subarc reservoirs. We examine the fate of C in plate-interface ultramafic rocks, and by analogy serpentinized mantle wedge, via study of fluid–rock evolution of marble and variably carbonated serpentinite in the Ligurian Alps. Based on petrography, major and trace element concentrations, and carbonate C and O isotope compositions, we demonstrate that serpentinite dehydration at 2–2.5 GPa, 550 °C released aqueous fluids triggering breakdown of dolomite in nearby marbles, thus releasing C into fluids. Carbonate + olivine veins document flow of COH fluids and that the interaction of these COH fluids with serpentinite led to the formation of high-P carbonated ultramafic-rock domains (high-P ophicarbonates). We estimate that this could result in the retention of ∼0.5–2.0 Mt C/yr in such rocks along subduction interfaces. As another means of C storage, 1 to 3 km-thick layers of serpentinized forearc mantle wedge containing 50 modal % dolomite could sequester 1.62 to 4.85 Mt C/yr. We stress that lithologically complex interfaces could contain sites of both C release and C addition, further confounding estimates of net C loss at forearc and subarc depths. Sites of C retention, also including carbonate veins and graphite as reduced carbonate, could influence the transfer of slab C to at least the depths beneath volcanic fronts.

At Cima di Gagnone garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750 850 degrees C and pressures < 3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite suggesting that these rocks recrystallized cofacially at similar to 800 degrees C. Alternatively they might have decoupled during subduction as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid mobile element enrichments (B Pb As Sb Cs Li U Be) which confirm derivation from variably serpentinized protoliths. Similarity in the U Pb B Li and Sr contents of the Gagnone peridotites to present day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater derived fluids. Positive Be As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid mobile element enrichment characterizes all peak eclogitic minerals implying that multiple hydration events and element influx pre dated the eclogite facies dehydration. Peak anhydrous minerals retain B Li As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite despite showing evidence for chlorite dehydration retains significant amounts of fluid mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi stage peridotite hydration and subsequent dehydration in a plate interface setting.

We discuss geochemical tracers in serpentinite for assessing the fluidrock interactions and the element exchange processes experienced by these rocks during subduction. This enables to define the timing of serpentinite accretion to plate interface domains and the evolution of such tectonically active settings. Uptake of As, Sb, Be and reset of B, Sr and Pb isotopes are due to interaction with sediment-derived fluids during prograde subduction metamorphism. This implies the early slicing and accretion of serpentinite (of slab and/or mantle wedge origin) to plateinterface domains, where they uptake crust- and sediment-derived fluids. Serpentinite metasomatism by sediment-derived fluids enable transfer of these elements to subarc depths and their release to de-serpentinization fluids affecting the mantle sources of arc magmas.

This work investigates the interplay between fluids and metamorphic recrystallisation in the Austroalpine Dent Blanche Nappe (Western Alps, Italy). This nappe consists of the Arolla and Valpelline Units, respectively dominated by metagranitoids and paraschists with Pre-Alpine to Alpine polymetamorphic history. In the Arolla Unit the Alpine tectonitic to mylonitic foliation anastomoses around weakly deformed bodies preserving the pre-Alpine igneous textures. In the Valpelline Unit, the Alpine structures are concentrated within a narrow shear horizon at the contact with the Arolla Unit, which parallels the pre-Alpine foliation. This contrast in the Alpine deformation between the Arolla and the Valpelline Units suggests heterogeneous fluid distribution in the Dent Blanche system during the Alpine orogeny. The relict (meta)igneous Arolla rocks record a post-magmatic, pre-Alpine stage of amphibolitefacies hydration at relatively low pressure. A deeper crustal origin of the Valpelline Unit than the Arolla Unit can be envisaged due to the presence of high temperature and pressure assemblages (clinopyroxene, garnet, plagioclase, amphibole) in the mafic rocks. In Arolla granitoids igneous plagioclase records two hydration events recorded by aggregates of: (i) pre-Alpine amphibolite-facies andesine + epidote, and (ii) Alpine greenschist-facies albite + epidote. The quartz crystals associated with the altered magmatic plagioclase contain two-phase (liquid + vapour) and three-phase (liquid + vapour + chloride daughter crystals) inclusions. Isochores of the salt-saturated inclusions plot in the andesine + epidote stabi - lity field, pointing to pre-Alpine plagioclase alteration in response to infiltration of saline aqueous solutions at amphibolite-facies conditions. Most isochores of the liquid + vapour inclusions plot in the albite + epidote stability field, suggesting entrapment during the lower temperature greenschist-facies hydration of plagioclase. Pre - ferential fluids influx in the Arolla Unit during pre-Alpine amphibolite- facies metamorphism enhanced the reaction softening of plagioclase. Alteration of plagioclase played a major role in promoting the deformation of granitoids during Alpine tectonics

We review three case studies emphasizing the role of ultramafic rocks in the recycling of volatiles and trace elements at convergent plate margins. Serpentinites are major water carriers in subduction zones and their breakdown liberates large quantities of water at sub-arc depths. The incompatible elements incorporated during oceanic serpentinization are released into the fluid phase produced once antigorite dehydrates to olivine + orthopyroxene. Importantly, the antigorite breakdown can trigger either wet melting or production of supercritical fluids in altered basalts and sediments. The produced fluid phases incorporate substantial amounts of incompatible element, initially residing in the crustal reservoirs. The fluid phase which exits the slab is highly reactive with respect to the overlying, silica undersaturated, mantle rocks. This leads to formation of reactive (ortho)pyroxenite layers, which filter the uprising hydrous melt/supercritical fluid to produce aqueous, solute-rich solutions. This fluid has equilibrated with peridotites and is mobile in the mantle. A consequence of these subduction fluid/mantle reactions is that the mantle wedge domains overlying the slabs can be heterogeneous in composition and layered, due to the presence of reactive pyroxenite bodies. Another aspect regards the debate whether supercritical fluids or hydrous melts are effective media for trace element transport. Since both agents are saturated in silica, they will react with the silica undersaturated mantle wedge peridotites to produce aqueous, incompatible trace element-rich residual fluids. Hence, while hydrous melt and/or supercritical fluids are important for scavenging incompatible elements from the slab, they may not be the agents that transfer the metasomatic subduction signature to the inner parts of the mantle wedges.