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Subducted sections of dry oceanic mantle can undergo serpentinization through interaction with metamorphic fluids, recording a sequence of serpentinization events from the subseafloor to serpentine-out reactions. The geochemical patterns of these different stages of fluid-rock interaction trace the context and mechanisms of serpentinization processes. In this study, we used petrographic observations, micro-Raman spectroscopy, as well as bulk and in situ trace element analyses across a serpentinization front adjacent to a subduction-related tectonic contact in the Monte Maggiore massif, Alpine Corsica, France. Using a high-density sampling approach throughout the entire massif, we identified multiple and consistently distributed serpentine generations, reflecting successive hydration events in a chemically open system. Fluid-mobile elements (FME) exhibit a slight increase from lizardite- to antigorite-dominated generations and a more substantial rise with increasing serpentinization degrees towards the tectonic contact. When compared to other serpentinization fronts originated at different geodynamic settings – e.g., Semail Ophiolite (Oman) as a subduction serpentinization front; Lanzo Massif (W. Alps) as a subducted oceanic serpentinization front –, the Monte Maggiore FME patterns indicate a subduction-related serpentinization front, rather than subducted oceanic serpentinites. Our results and the comparison with literature data suggest that decreasing in situ serpentine FME concentrations across serpentinization fronts may be characteristic of subduction-zone serpentinization, whereas opposite trends may be diagnostic of subducted oceanic fronts. Our study also highlights the importance of tracing serpentinization as a spatially evolving process (across fronts), with least serpentinized rocks recording the latest stage(s) of serpentinization instead of relicts of incipient serpentinization.

Geodynamic models implying subduction of continental crust either consider this process happening during collision, when the continental margin of the lower plate attempts subduction, or in pre-collisional stages, when tectonic erosion of the upper plate or subduction of continental extensional allochthons drag continental crust in the subduction channel. In the Zagros orogen (W Iran), high-pressure rocks are known only from the Sanandaj-Sirjan Zone, NE of the Main Zagros Thrust. Here, eclogites of the North Shahrekord Metamorphic Complex suggest subduction of continental crust slices derived from the upper plate (Central Iran) during the onset of the Neo-Tethys subduction along the southern margin of Iran. Eclogites record a clockwise pressure-temperature-time path, with pre-eclogitic epidote-amphibolites-facies phase assemblages preserved in garnet cores, a high-pressure stage, and a subsequent retrogression at amphibolite-facies conditions. By means of forward thermodynamic modelling and 40Ar/39Ar geochronology, the peak metamorphism has been constrained at 1.9-2.1 GPa and 550-600 °C, in the 191-194 Ma time span. The following retrogression during exhumation lasted at least until 144 Ma. Our data suggest that the onset of the Neo-Tethys subduction traces back prior to 190 Ma, involving together with the Neo-Tethys oceanic lithosphere also slices of the upper plate continental crust scraped off by means of tectonic erosion processes.

The water-mediated mineral carbonation represents a promising solution for the capture and the storage of atmospheric CO2. Even though this reaction might be spontaneous for a number of Mg- and Ca-rich mineral phases, it is characterized by considerable activation barriers. In order to make it effective, associated energy costs related to the achievement of adequate reaction conditions must be minimized. Microwave chemistry is known to provide for substantial increments of the reaction rate for several systems. We applied here microwave chemistry to the process of carbonation of aqueous slurries of brucite, a model system of Mg-rich mineral, subjected to partial pressures of CO2 as low as 6 bar and to no other additive. The temperature of the reactor was finely varied while the radiation power and the reactor pressure were monitored in real-time. The radiation power was used to estimate the radiation energy budget needed to complete the carbonation process, whereas the reactor pressure was used as a proxy of reaction progression. We show a detailed evolution of the carbonate products obtained in terms of mineral phases, morphological properties, and degree of crystallinity, both as precipitate and as solid residue in the exsiccated supernatant reaction liquid.

Understanding the interactions between surface and deep Earth processes is important for research in many diverse scientific areas including climate, environment, energy, georesources and biosphere. The TOPO-EUROPE initiative of the International Lithosphere Program serves as a pan-European platform for integrated surface and deep Earth sciences, synergizing observational studies of the Earth structure and fluxes on all spatial and temporal scales with modelling of Earth processes. This review provides a survey of scientific developments in our quantitative understanding of coupled surface-deep Earth processes achieved through TOPO-EUROPE. The most notable innovations include (1) a process-based understanding of the connection of upper mantle dynamics and absolute plate motion frames; (2) integrated models for sediment source-to-sink dynamics, demonstrating the importance of mass transfer from mountains to basins and from basin to basin; (3) demonstration of the key role of polyphase evolution of sedimentary basins, the impact of pre-rift and pre-orogenic structures, and the evolution of subsequent lithosphere and landscape dynamics; (4) improved conceptual understanding of the temporal evolution from back-arc extension to tectonic inversion and onset of subduction; (5) models to explain the integrated strength of Europe's lithosphere; (6) concepts governing the interplay between thermal upper mantle processes and stress-induced intraplate deformation; (7) constraints on the record of vertical motions from high-resolution data sets obtained from geo-thermochronology for Europe's topographic evolution; (8) recognition and quantifications of the forcing by erosional and/or glacial-interglacial surface mass transfer on the regional magmatism, with major implications for our understanding of the carbon cycle on geological timescales and the emerging field of biogeodynamics; and (9) the transfer of insights obtained on the coupling of deep Earth and surface processes to

We disclose the crystallisation evolution of magnetite-bearing multiphase inclusions hosted in metamorphic olivine of harzburgites from the Cerro de Almirez (Betic Cordillera, Spain), which have been interpreted as final products of the trapping of the aqueous fluid produced by the subduction-zone dehydration of former serpentinites. The chemical exchange between inclusion fluid and olivine started soon after entrapment, at peak P-T conditions of 1.6–1.9 GPa and 650–700 °C, and continued during cooling along the retrograde path, with the coexistence of olivine and magnetite with orthopyroxene, chlorite, talc, antigorite and the destabilisation of olivine and antigorite into brucite and low-temperature chrysotile serpentine, as recognised by Raman analyses. Thermodynamic modelling and mass balance calculations demonstrate that the water component of fluid trapped in the inclusions of metamorphic olivine is expected to trigger the oxidation of the fayalite component in olivine, producing a mineral assemblage made of magnetite + orthopyroxene and molecular hydrogen, where the elemental redox processes are Fe2+ of olivine that oxidizes to Fe3+ and H+ of water that reduces to H2. Probable H2 trapped in the olivine host close to the inclusion wall has been detected by Raman spectroscopy. To corroborate its presence, we performed quantitative mass spectrometry analyses of the fluid phase trapped in the multiphase inclusions and of the olivine crystals hosting the inclusions, revealing that 1 kg of olivine matrix contains 6.2 ± 0.1 mmol of H2. We identify two synergistic driving forces of the whole process, which has the peculiarity to produce molecular hydrogen at apparently oxidising conditions: i) the building up of an epitaxial interface between olivine and magnetite, and ii) the olivine ability to trap H2 at high pressures. The olivine + H2O system of these natural microreactors simulates a process of oxidation of the mantle olivine by water, with production of H2 at p

Silicate nanoparticles occur in various astrophysical environments where they experience substantial processing due to events such as grain-grain collisions and irradiation. However, the structure and chemical evolution together with the origin of these grains are still poorly understood and intensively debated. For this purpose, we performed liquid-phase nanosecond pulsed laser ablation on olivine single crystals to (i) simulate space weathering in a water environment (e.g., hydrous or volatile-rich bodies) and (ii) study the chemical and structural evolution of both the target surface and the ablated material. In particular, optical spectroscopy analyses have been performed on the ablated material and correlated with high-resolution transmission electron microscopy and diffraction, whereas compositional variations of the ablated target surface were determined by X-ray photoelectron spectroscopy. Our results show that the target material is enriched in Fe and depleted in Mg after the ablation process, with the water environment triggering the oxidation of Fe2+ into Fe3+ in a region confined at the solid-liquid interface and thus promoting the formation of magnetite on the sample surface. On the other hand, in the ablated material we find olivine crystalline fragments with shock features together with Mg-rich crystalline nanoparticles. Notably, no metallic iron nanoparticles have been detected in the ablated material. Our simulation of space weathering in the water environment revealed structural and chemical changes which are expected to give rise to distinctive features in the reflectance spectra when compared to those from airless bodies of the inner solar system.

The formation of stable and insoluble Mg- and Ca-carbonates and/or hydroxy-carbonate hydrates, through water-mediated reaction of Mg(OH)2 (brucite) and Ca(OH)2 (portlandite) with carbon dioxide, is referred to as mineral carbonation (MC). MC is among the most promising potential solutions for long-term carbon capture and storage (CCS), since it is spontaneous under a wide range of conditions. However, kinetic barriers pose severe limitations to the practical exploitation of MC, implying energy requirements to reach sufficiently high reaction rates. Trying to overcome these hindrances, we show here the application of microwave (MW)-assisted processes for the carbonation of brucite, used as a model system for the carbonation occurring in a number of widely diffused minerals, such as serpentine. The mechanism, kinetics, and energy costs of the reaction, together with the chemical characteristics of the products obtained, are inferred by a combined study, carried out by both a MW synthesizer on bulk brucite water slurries and a purposedly developed microscopy platform, working on nano-scale regions of the surface of brucite single crystals.

Mineral Carbonation (MC) is included among the most promising potential solutions for long-term carbon capture and storage (CCS). This mineral treatment consists in the water-mediated reaction of the CaO and MgO components of minerals with carbon dioxide, with formation of stable and insoluble Ca- and Mg-carbonates and/or hydroxy-carbonate hydrates. Noteworthy, these latter compounds constitute also valuable secondary raw materials employed, e.g., in the construction sector. Despite the carbonation reaction is spontaneous under a wide range of conditions, kinetic barriers pose sever limitations for its practical exploitation. High fractions of MgO are available in serpentine minerals, characterized by alternated layers of silica [SiO2] and brucite [Mg(OH)2], quarried in a number of sites in Europe, with a relevant role played by Italy. This industrial activity is characterized by the production of a fraction as large as 50% of waste in the form of serpentine pebble and powder, representing potential reservoirs for fixing a quantity as large as 20 ktons of CO2 per site/per year in the form of re-utilizable Mg-carbonates. With the scope to contribute in making this possibility a reality, we show here the application of microwave (MW)-assisted processes for the in-depth study of the carbonation of brucite, representing a model systems for the reactive component of serpentine. MW reactors allow for a fine tuning of the temperature, pressure and irradiation energy, enabling us to infer the mechanism, kinetics, and energy costs of the reaction and to identify the chemical characteristics of the products.

The Borgo outcrop of the Monte Duria Area (Adula-Cima Lunga unit, Central Alps, Italy) is an excellent example of melt-peridotite interaction which occurred under a deformation regime at high pressure, that enabled the combination of porous and focused flow of eclogite-derived melts into garnet peridotites. Migmatised eclogites are in direct contact with retrogressed garnet peridotites and the contact is marked by a tremolitite layer, also occurring as boudins parallel to the garnet layering in the peridotites, derived from a garnet websterite precursor produced by the interaction between eclogite-derived melts with the peridotite at high pressure. LREE concentrations of tremolitite along a 120 m length profile, starting from the eclogite-peridotite contact to the inner part of the peridotite, show a progressive enrichment coupled with a peculiar fractionation. Numerical modelling assuming the eclogitic leucosome as the starting percolating melt reproduces the REE enrichment and LREE/HREE fractionation observed in tremolitite bulk rocks within the first 30 m. The comparison between the REE composition of the retrogressed garnet websterites along the profile and the result of our model suggests that reactive melt infiltration at high pressure is a plausible mechanism to modify the REE budget of mantle peridotites that lie on top of the subducting crustal slab.

Pyroxenites are common compositional heterogeneities in the upper mantle and represent key lithologies in mantle deformation processes, as the local presence of pyroxene-rich compositions can weaken the mantle strength. Pyroxenites occur ubiquitously as variably deformed layers in most of oceanic and orogenic peridotite massifs, and thus can be used as a proxy to investigate the rheological behavior of the mantle in different geodynamic settings, including subduction zones. In the Ulten Zone (Tonale nappe, Eastern Alps, N Italy), numerous peridotite bodies occur within high-grade crustal rocks. Peridotites show a transition from coarse protogranular spinel lherzolites to finer-grained amphibole + garnet peridotites (Obata and Morten, 1987). Pyroxenites veins and dikes, transposed along the peridotite foliation, show a similar evolution from coarse garnet-free websterites to finer-grained garnet clinopyroxenites (Morten and Obata, 1983). This evolution has been interpreted to reflect cooling and pressure increase of pyroxenites and host peridotites from spinel- (1200 °C, 1.3-1.6 GPa) to garnet-facies conditions (850 °C and 2.7 GPa) within the mantle corner flow (Nimis and Morten, 2000). This results in the consequent formation of garnet at the expense of spinel. In particular, garnet initially formed as coronas around spinel and as exsolution lamellae in high-T pyroxenes, and later as neoblasts along the foliation of pyroxenites and host peridotites. Microstructures and crystallographic orientation data indicate that the transition from spinel- to garnet-facies conditions occurred in a deformation regime. Pyroxene porphyroclasts in garnet clinopyroxenites show well-developed crystallographic preferred orientation, high frequency of low-angle misorientations, and non-random distribution of the low-angle misorientation axes. These features indicate that pyroxene porphyroclasts primarily deformed by grain size insensitive (GSI) creep. Core-and-mantle microstructures in

In the last decades many studies have been focusing on Carbon Capture and Storage(CCS) to find a possible remedy to reduce the large increase of anthropogenic carbon dioxide. Mineral Carbonation (MC) is a potential solution for almost irreversible chemical long-term CCS. It concerns the combination of CaO and MgO with carbon dioxide forming spontaneously and exothermically dolomite and magnesite. However, kinetic barriers pose sever limitations for the practical exploitation of this reaction. High fractions of MgO are available in silicates such as olivine, orthopyroxene, clinopyroxene and serpentine. To date, data reported that serpentine polymorphs, above all antigorite, is an excellent candidate for fixing the carbon dioxide as the reaction efficiency is approximately 92% compared to lizardite (40%) and olivine (66%). This is due to the surface reactivity of approximately 18.7 m^2/g for the dehydrated antigorite comparedto10.8 m^2/g for dehydrated lizardite and 4.6 m^2/g for olivine. The microwave assisted process for CCS is an innovative technology that can be employed to catalyze the reaction through thermal and non-thermal mechanisms. Some pioneering tests of direct carbonation by microwave hydrothermal equipment have been performed on olivine, lizardite and chrysotile powders [1] but not on antigorite. The structure of serpentine is characterized by corrugated stacked layers of silica and brucite. For this reason, MC involves dissolution of silica layers, dissolution/dehydration of brucite layers, and precipitation of magnesium carbonate. To address the chemical response of the single phases, experiments have been performed by both a local microwave-source acting locally on a specific crystal surface and a volume source interacting with an ensemble of grains on synthetic powders and single crystals of pure brucite and talc. In a second step, treatments have been extended to chrysotile, lizardite and antigorite. A characterization of the mechanism and kinetics

Magnetite microinclusions in metamorphic harzburgites, derived from the deserpentinization of the subducted hydrated oceanic lithospheric mantle, were examined by synchrotron Mössbauer spectroscopy to investigate the chemical and magnetic environments of the Fe nuclei. The data reveal a critical susceptibility of the octahedral sites of the cubic structure of magnetite to chemical variations, which, in turn, influences their magnetic properties in terms of hyperfine magnetic field intensity and direction. Micromagnetites display substantial remanent magnetization; however, the magnetic moment direction can be significantly different among inclusions, even for those in close spatial proximity. This evidence points to a kinetic control of the composition of microcavities at mantle depths, implying that the use of the remanent magnetic field of included magnetic phases to infer large-scale implications on the Earth's magnetic field requires the development of complex geochemical and geodynamical models.

Peridotites of Ulten Zone (Eastern Alps, N Italy) show a transition from coarse protogranular spinel lherzolites to fine-grained amphibole + garnet peridotites, recorded by the crystallization of garnet coronas around spinel. Pyroxenite veins, transposed along the peridotite foliation, show a similar metamorphic evolution from coarse-grained (garnet-free) websterites to fine-grained garnet websterites. In both peridotites and websterites, garnet previously exsolved from porphyroclastic high-temperature pyroxenes and later crystallized along the foliation. This evolution has been interpreted to reflect cooling and pressure increase of websterites and host peridotites from spinel- to garnet-facies conditions. Microstructures and crystallographic orientation data indicate that the re-equilibration of garnet websterites in the garnet stability field occurred during deformation. Porphyroclastic pyroxenes have been interpreted to deform by dislocation glide and creep. In particular, TEM observations indicate the activation of the (100)[010] slip system in orthopyroxene. Core-and-mantle microstructures also suggest that dislocation creep was aided by subgrain rotation recrystallization, leading to the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion-accommodated grain boundary sliding, as indicated by the occurrence of quadruple junctions and straight, aligned grain boundaries. The transition from dislocation creep to diffusion creep in websterites was accompanied by the crystallization of garnet along foliation, which triggered the pinning of the recrystallized matrix and stabilized the fine-grained microtexture for diffusion creep, promoting rheological weakening. Garnet websterites of Ulten Zone thus offer a unique opportunity to investigate the effects of reaction softening during the corner flow in the supra-subduction lithospheric mantle induced by the descending slab.

In the Monte Duria area (Adula-Cima Lunga unit, Central Alps, N Italy) garnet peridotites occur in direct contact with migmatised orthogneiss (Mt. Duria) and eclogites (Borgo). Both eclogites and ultramafic rocks share a common high pressure (HP) peak at 2.8 GPa and 750 °C and post-peak static equilibration at 0.8–1.0 GPa and 850 °C. Garnet peridotites show abundant amphibole, dolomite, phlogopite and orthopyroxene after olivine, suggesting that they experienced metasomatism by crust-derived agents enriched in SiO2, K2O, CO2 and H2O. Peridotites also display LREE fractionation (LaN/NdN = 2.4) related to LREE-rich amphibole and clinopyroxene grown in equilibrium with garnet, indicating that metasomatism occurred at HP conditions. At Borgo, retrogressed garnet peridotites show low strain domains characterised by garnet compositional layering, cut by a subsequent low-pressure (LP) chlorite foliation, in direct contact with migmatised eclogites. Kfs + Pl + Qz + Cpx interstitial pocket aggregates and Cpx + Kfs thin films around symplectites after omphacite parallel to the Zo + Omp + Grt foliation in the eclogites suggest that they underwent partial melting at HP. The contact between garnet peridotites and eclogites is marked by a tremolitite layer. The same rock also occurs as layers within the peridotite lens, showing a boudinage parallel to the garnet layering of peridotites, flowing in the boudin necks. This clearly indicates that the tremolitite boudins formed when peridotites were in the garnet stability field. Tremolitites also show Phl + Tc + Chl + Tr pseudomorphs after garnet, both crystallised in a static regime postdating the boudins formation, suggesting that they derive from a garnet-bearing precursor. Tremolitites have Mg# > 0.90 and Al2O3 = 2.75 wt% pointing to ultramafic compositions but also show enrichments in SiO2, CaO, and LREE suggesting that they formed after the reaction between the eclogite-derived melt and the garnet peridotite at HP. To test t

On page 76, line 13 of Section 4.1.2. and caption of Figure 6, the sentence “chemographic representation for the system Fe3O4-CaMgO2-KAlSiO4 at fixed SiO2 chemical potential, projected through the FeMg−1 exchange vector” must be completed adding “and projected from CaMgO2”. The correction does not change any fundamental conclusions of the paper. It only clarifies the relative positions of the plotted mineral phases.

The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite, ± garnet ± K-feldspar. Amphibole-plagioclase geothermobarometry and thermodynamic modeling indicate that pristine and mylonitized pseudotachylytes formed at 650–750°C and 0.7–0.8 GPa. Thermodynamic modeling indicates that a limited amount of H2O infiltration (0.20–0.40 wt. %) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites.

Multiphase inclusions represent microenvironments where the interaction between fluid and host mineral is preserved during the rock geological path. Under its peculiar chemical-physical constraints, the entrapped solute-rich fluid might follow a crystallisation mechanism which is not predictable through simple equilibrium arguments. In this letter, by the modelling of solid-solution equilibrium and the application of principles of mass conservation, we demonstrate that cavities in mantle garnet filled with slab-derived fluids can re-equilibrate to a pyrope + spinel + chlorite assemblage at the same high P-T of their formation. The basis of this occurrence is a dissolution-reprecipitation mechanism, triggered by a dilute, non-equilibrated slab fluid.

The valence of carbon is governed by the oxidation state of the host system. The subducted oceanic lithosphere contains considerable amounts of iron so that Fe3+/Fe2+ equilibria in mineral assemblages are able to buffer the (intensive) fO2 and the valence of carbon. Alternatively, carbon itself can be a carrier of (extensive) ‘excess oxygen’ when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle. Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle oxidation by C-bearing fluids is of primary importance when considering different fluid/rock ratios. Fluid-mediated processes at the slab–mantle interface can also be investigated experimentally. The presence of CO2 (or CH4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation or decarbonation reactions and of the solidus. Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes. The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models and the message of nature.

The Duria Area is located in the southern part of the Adula Nappe complex on the north-eastern side of the Como Lake (N Italy). Garnet/chlorite peridotites outcrop within migmatitic gneiss or in direct contact with amphibole-bearing migmatites containing boudins of variably granulitised eclogites. The contact between mafic and ultramafic rocks is marked by a metasomatic rim composed by tremolite and dispersed round-shaped chlorite aggregates, which also occurs within the peridotite body. The occurrence of lobes and cusps structures and Qtz+Pl+Kf+Bt leucosomes in eclogites provide evidence for partial melting of these rocks. Petrography and mineral chemistry indicate that peridotites and associated eclogites experienced a HP metamorphic peak at ≈ 2.8 GPa and ≈ 730 ± 20 °C, followed by a granulitic overprint at 0.8-1.2 GPa and 850 °C during their exhumation path (Tumiati et al. 2018). The micostructural evidence of Qtz+Pl+Kf+Bt pockets in eclogites support the field evidence of a partial melting event. Eclogites are characterised by a HP assemblage formed by Grt+Ky+Omp+Kf, suggesting that a former phengite was completely consumed by the reaction Phe+Cpx+SiO2=Grt+Ky+Kfs+Melt. At this stage a clinopyroxenite reaction front probably results from a Ca-rich mafic silicate melt-peridotite interaction. Therefore, we suggest that tremolite-rich metasomatic rim probably represents a former grt- clinopyroxenite reaction front subsequently retrogressed at fluid-present conditions through the reaction Cpx+Opx+Grt+H2O=Tr+Chl. Bulk rock trace element analyses of peridotites show a strong fractionation in REE (La/NdN=2.4) likely indicating that these rocks record an interaction with a silicate melt. The metasomatic rim shows higher REE concentrations with a LREE enrichment and a slight Eu negative anomaly. This pattern broadly resembles the trace elements composition of pyroxenites from other localities of eastern Europe (i.e. Gföhl Moldanubian Nappe), interpreted as the product of

The action of slab-derived fluids on mantle rocks frequently brings about the formation of microcavities in minerals, which represent microenvironments where the interaction between fluid and host mineral is preserved during the whole geological path of the rock. The entrapped fluid and the host matrix constitute a system evolving towards equilibrium under peculiar chemical-physical constraints. By expliciting these constraints through mass conservation laws and stoichiometry relations accounting for the composition of the host mineral and of the initial fluid, and for the presence of a finite fluid-solid interface, we show that cavities in minerals filled with slab-derived fluids can re-equilibrate following a crystallisation mechanism which is not predictable through simple equilibrium arguments holding for open systems (Campione et al., 2017). The occurrence of phases stable at ultrahigh pressure (UHP), such as coesite or microdiamond, has long been considered evidence of precipitation from such fluids at P > 3 GPa. Here, we demonstrate that even a mineral association characterised by phases usually considered stable at relatively low pressure in mantle systems (e.g., spinel + chlorite), would potentially crystallise at UHP by chemical fluid/host interaction. We will consider as a case study a well-known example of multiphase solid inclusions occurring in the cores of garnets forming orthopyroxenites from the Maowu Ultramafic Complex (Eastern China), interpreted as hybrid rocks resulting from the interaction of previous harzburgites and slab-derived silica-rich liquids (P = 4 GPa, T = 800 °C) at the slab-mantle interface. Malaspina et al. (2015) demonstrated the epitaxial relationship between spinel and garnet, which suggested nucleation of spinel under near-to-equilibrium conditions. On the contrary, hydrous phases (amphiboles, chlorite and ±talc ±phlogopite) nucleate in a non- registered manner and likely under far-from-equilibrium conditions. The epitaxial gro

Processes taking place in subduction zones are the main controller of the chemical cycle of volatile and incompatible elements in the Earth system. Metamorphic devolatilization reactions occurring during slab burial play a key role in the transfer of elements to the supra-subduction mantle, from forearc to sub-arc depth. Here, we discuss the elements released in fluids and melts from oceanic (i.e., sediments, altered oceanic crust, and hydrated lithospheric mantle) and continental slab materials during prograde subduction and the consequent implications in the chemical evolution of the supra-subduction mantle. We use bulk data and fluid and melt inclusions analyses to show and to constrain the mobility of elements from top to bottom in the subduction zone setting. The development of mélange domains at the slab-mantle interface and its influence in the element cycle are also taken into account. Coupled with trace-element mobility, we review the redox evolution of slab materials during subduction and its implication in the redox conditions of the supra-subduction mantle due to fluid and melt infiltration down to sub-arc depth.

Peridotites and different types of eclogites occurring in the Monte Duria area (Adula-Cima Lunga unit, Central Alps, Italy) share a common eclogite-facies peak at P=2·6-3·0GPa and T=710-750°C, constrained by conventional thermobarometry and thermodynamic modelling. High-pressure minerals are replaced both in peridotites and in eclogites by lower-P and high-T assemblages. In peridotites, the zirconium titanate srilankite occurs asmicrometre-sized crystals in textural equilibriumwith spinel, clinopyroxene and orthopyroxene in kelyphites developed between garnet and olivine. By using a new ZrO2-TiO2 solid-solution model, we provide evidence that srilankite is stable in peridotites relative to zircon+rutile for T > 810°C at an assumed P=0·9GPa, consistent with estimates of T≈850°C (at assumed P=0·9GPa) determined for symplectites made of sapphirine+spinel+Al-rich orthopyroxene+amphibole found in fractures within garnet. In eclogites, kyanite is replaced by symplectites made of anorthite-rich plagioclase+spinel6sapphirine6corundum, formed at T≈850°C and P=0·8-1·0GPa, conditions that are coincident with the high-T overprint observed in the peridotites. Thermodynamic modelling coupled with a material-transfer study provides constraints for these sapphirine-bearing symplectites. In thesemicro-domains, the 'inert' components could not fully equilibrate with the surrounding rock, and the locally high Al content promoted the stability of the Al-rich phases (i.e. mosaic equilibrium). This is the first report from the Alps of eclogite-facies rocks of supposed Alpine age showing a granulite-facies metamorphic overprint, which is, in contrast, well documented in the Variscan belt. On these grounds, although the age of the high-pressure and hightemperature stages in the Monte Duria rocks is still not constrained, the possibility that they reached eclogitic and granulitic conditions in pre-Alpine times should be taken into account.