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The Lyngen Magmatic Complex (LMC) is the lowest unit of the Lyngsfjellet Nappe (Upper Allochthon, North Norwegian Caledonides). The fabrics of the LMC rocks range from undeformed to mylonitic. The undeformed rock is a gabbro-norite formed primarily by anorthite-rich (93%) plagioclase, enstatite, and augite. Two deformation events are distinguished in the LMC: (D1) an earlier shearing that has produced a N—S trending vertical foliation with sub-horizontal stretching lineation and dextral sense of shear, and (D2) a top-to-SE-directed thrust contact with the lower nappe series at the base of the meta-gabbro-norite. In the thrust contact region, the early vertical foliation is rotated into a flat-lying orientation and shows an ESE-trending stretching lineation. Deformed fabrics of D1 have developed successively from lower amphibolite, to epidote-amphibolite, and to greenschist metamorphic grades, i.e., on a retrograde temperature-path. The fabrics of the thrust contact have also developed from amphibolite to greenschist conditions.Rock fabrics associated to D1 are dominantly located in the northern portion of the LMC (from Lyngstuva to the north side of the Kjosen fjord). The amphibole compositions of these rocks vary from core to rim, showing a trend from pargasitic to actinolitic composition, consistent with the transition from high- to low-temperature (amphibolite to greenschist facies). U-Pb dating of titanite associated with the greenschist grade in meta-gabbro-norite assemblages indicates a date of 485±9 Ma. This date is interpreted as a deformation/metamorphic age, because the analysed titanite forms from pargasite breakdown and is aligned parallel to the deformed fabric. As this deformation event is synchronous with the crystallization age of the LMC (481±6 Ma, Augland et al., 2014), the deformation associated to the N—S oriented stretching lineation and vertical foliation is linked to sea floor strike slip movements during back-arc spreading of the LMC. D2-rock-fabrics are dominantly located in the southern portion of the LMC and represent typical structures of nappe stacking during the Scandian collisional stage of the Caledonian orogeny. Close to the lower boundary of the LMC, garnet-bearing amphibolites, allow refining the P and T conditions for this unit. Thermobarometric estimates result in conditions of 650°C and 10kbar. This temperature is in contrast with the Raman spectroscopy values averaging around 530°C for the graphite bearing sediments below the lower contact of the LMC, i.e. sediments between the meta-gabbro-norite and the underlying Reisa nappe. The temperature difference between the two deformation events indicates re-heating of the meta-gabbro-norite during the Scandian thrusting.The D1 structural relationships described in the LMC appears common for supra-subduction zone settings, and could potentially be observed at deeper mantle sections as reported in younger analogue tectonic settings as the Wadi al Wasit area of the Oman ophiolite. D2 appears linked to out-of-sequence thrusting at the base of the LMC with respect to the surrounding nappes, contributing to the north Norwegian Caledonides nappe transport sequence.

Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na2O + K2O = 4.15–14.97 wt%) and silica-undersaturated (SiO2 = 36.73–45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025–1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations.

International audience; The metamorphic sole of the Mt. Albert ophiolitic complex (Gaspé peninsula, Quebec, Canada) is a sliver of Ordovician oceanic crust accreted to the base of an incomplete ophiolitic sequence, along a suture zone throughout the north-eastern Appalachians linked to the Taconian orogeny. A detailed mineralogical study of the rocks in the metamorphic section of the sole is provided in this publication: these rocks record valuable information in terms of petrological processes and conditions of accretion, with limited retrogression. The petrology of the metamorphic sole shows that it originates from ocean floor and that it is the equivalent of the Shick-Shock volcanics group, metamorphosed to granulite/higher amphibolite facies. Presence of aluminosilicate bearing metapelites allows constraining pressure conditions in a more robust way than in the case of the famous Semail ophiolite (Oman). Thermobarometric estimates for peak metamorphic conditions for the metamorphic sole of the Mont Albert ophiolitic complex indicate temperatures above 800 °C close to the contact with the overlying peridotite, decreasing to ~650 °C within

International audience; The Queyras-Monviso traverse of the Schistes Lustrés complex, is a stack of underplated nappes of oceanic rocks subducted from blueschist- to eclogite-facies conditions.This study reports on salinities and gas contents of primary fluid inclusions trapped in high pressure veins from metasediments and metamafic rocks (445 fluid inclusions in 22 samples). These data provide snapshots of the compositions of fluids present at peak burial conditions, varying from ~30 to 80 km depth and illustrate the evolution of fluid composition with burial along a cold subduction zone.Fluid inclusions trapped in lawsonite- and carpholite-bearing veins in metasediments contain moderately saline aqueous fluids (average salinity of 4.6 wt% NaCl eq.) with subordinate amounts of CO2 and CH4 in the vapor phase. The observed salinity decrease with increasing grade is interpreted as reflecting progressive dilution of initial seawater-like pore fluid by low-salinity fluids released locally by dehydration reactions. Less-frequently measured higher salinities in the uppermost metasedimentary-dominated tectonic unit suggest brine infiltration from embedded blocks of margin units containing evaporites. CO2 and CH4 (and scarce potential hydrocarbons) appear to be locally released from fluid interaction with carbonates and carbonaceous matter-rich pelitic horizons, respectively.Fluid inclusions in high pressure omphacite veins in metagabbros record higher salinities (mean salinity about 17 wt% NaCl eq.) with small amounts of N2 in eclogitic veins only, and a variety of minerals (calcite, white mica, salts) indicative of complex chemical systems. These high salinities are interpreted as partly inherited from seafloor high-temperature hydrothermal alteration of gabbros, resulting in phase separation and brine formation. Progressive breakdown of hydrothermal Cl-rich amphibole to glaucophane (blueschist-facies) and then omphacite (eclogite-facies) and release of brines trapped in fluid inclusions could account for the high salinity fluids. Therefore, in metagabbros, fluid inclusions record progressive release of Cl in the fluid phase with increasing grade.Fluid signatures of metasediments and metamafics appear characteristic of each rock type. Local fluid signatures and redox conditions were preserved within units, possibly due to restricted and transient fluid circulation (at the hm-scale at most).Fluid inclusions in Alpine metasediments show salinities and gas contents comparable with other subducted fragments of oceanic lithosphere worldwide, whereas fluid salinities of Alpine metagabbros are higher than salinities recorded elsewhere, either due to (1) higher-temperature hydrothermal alteration and brine formation in Alpine metagabbros (compared to metavolcanics) or (2) more restricted infiltration by sediment-derived fluids compared to block-in-mélange subduction complexes.

Subduction zones are crucial features of Earth’s plate tectonics, yet subduction initiation remains enigmatic and controversial. Herein, we reappraise the timing of formation of the first fragments detached from the leading edge of the downgoing slab during subduction initiation (i.e., the Semail metamorphic sole; Oman–United Arab Emirates). Based on geochronology and phase equilibrium modeling, we demonstrate that subduction initiated prior to 105 Ma and at a slow pace (< cm/yr). Subduction stagnated at relatively warm conditions (15–20°C/km) for at least 10 Myr before evolving into a faster (≥ 2–5 cm/yr) and colder (~7°C/km) self-sustained regime. Subduction unlocking at 95-96 Ma, through the progressive change of the interplate thermo-mechanical structure, triggered the onset of slab retreat, large-scale corner flow and fast ocean spreading in the overriding plate. These results reconcile conflicting analogue and numerical subduction initiation models and reveal the thermal, mechanical and kinematic complexity of early subduction steps.

Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is commonly used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous. This study investigates the reactions producing a phyllosilicate, chlorite, in and below greenschist-facies conditions and the variations in chlorite composition, along a strain gradient in the Bielsa granitoid (Axial Zone, Pyrenees). Compositional maps of chlorite (including iron speciation) are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation and scales of compositional homogenisation. In the Bielsa granitoid, altered at the late Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement and high compositional heterogeneities in chlorite. This is attributed to: (i) variable element supply and reaction mechanisms observed at nanoscale and (ii) little interconnected intra- and inter-grain nanoporosity causing isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro-, nano-cracks and defects allows the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive replacement of late-Variscan chlorite by Alpine chlorite. Local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according (i) matrix-fracture porosity contrasts at nanoscale and (ii) the location and interconnection of nanoporosity between crystallites of phyllosilicates that control reaction mechanisms and element mobility. In low grade mylonites, mineral and compositional replacement remains incomplete despite the high strain.

International audience; Understanding how halogens are distributed among usual hydrous silicates in the lithosphere is important to constrain their deep geochemical cycle and fluid-rock interactions in subduction zones. This article presents firstprinciples modelling of halogen (F-, Cl-, Br-) incorporation in hydrous silicates including mica, chlorite, serpentine, amphibole, epidote and carpholite. The approach allows studying the impact of crystal chemistry on halogen partitioning by quantification of the energetic cost of halogen incorporation in minerals. Calculations are carried out in large systems where halogens are in minor to trace concentrations. Estimations show that F-bearing defects must be separated at least 9 Å from one another to reproduce trace element behaviour, this value increasing to at least 10 Å for Cl and Br. Results highlight the competition between the effects of electrostatic interactions and steric hindrance for incorporation of halogens, where steric hindrance has greater importance for heavy halogens, in particular for Br. Interaction with alkalis is a major control for F incorporation, especially in mica. Other parameters such as octahedral site occupancy, Si/Al ratio of tetrahedral sites and the nature of alkalis in amphibole and mica (K or Na) appear to play subordinate roles. Partition coefficients have been estimated in mineral assemblages in an effort to be representative of subduction zone metamorphism. Results show that pargasite, biotite and lizardite are favoured hosts for all three halogens, followed by clinochlore, tremolite and carpholite. The energetic cost of incorporating halogens into dioctahedral phyllosilicates and epidote is comparatively higher, and partitioning is predicted as unfavourable to these minerals. Fractionation between halogens in subduction zones is predicted by the evolution of mineral assemblages and partition coefficients, a consequence of the influence of crystal chemistry over halogen incorporation in hydrous silicates.

International audience; Strain accommodation in upper crustal rocks is often accompanied by fluid-mediated crystallization of phyllosilicates, which influence rock strength and shear zone formation. The composition of these phyllosilicates is frequently used for pressure-temperature-time constraints of deformation events, although it is often highly heterogeneous, even in mylonites. This study investigates the reactions producing a phyllosilicate, chlorite, in and below grenschist-facies conditions and the variations in chlorite composition along a strain gradient in the Variscan Bielsa granitoid (Axial Zone, Pyrenees). Compositional maps of chlorite including iron speciation are compared to nanostructures observed by transmission electron microscopy in increasingly-strained samples and related to mechanisms of fluid percolation. In the Bielsa granitoid, altered at the late Variscan, Alpine-age shear zones are found with high strain gradients. The undeformed granitoid exhibits local equilibria, pseudomorphic replacement and high compositional heterogeneities in chlorite. This is attributed to variable reaction mechanisms at nanoscale and element supply, little interconnected intra-and inter-grain nanoporosity and isolation of fluid evolving in local reservoirs. In samples with discrete and mm-sized fractures, channelized fluid triggered the precipitation of homogeneous Alpine chlorite in fractures, preserving late-Variscan chlorite within the matrix. In low-grade mylonites, where brittle-ductile deformation is observed, micro and nanocracks and defects allows the fluid percolating into the matrix at the scale of hundreds of µm. This results in a more pervasive but incomplete replacement of late-Variscan chlorite by Alpine chlorite, despite the high strain. In studied granitoids deformed under greenschist-facies conditions, local equilibria and high compositional heterogeneities in phyllosilicates as chlorite are therefore preserved according reaction mechanisms and element mobility controlled by (i) matrix-fracture porosity contrasts at nanoscale and (ii) the location and interconnection of nanoporosity between crystallites of phyllosilicates. This preservation influences our ability to reconstruct the pre-and syn-kinematic metamorphic history of granitic rocks in low-grade units of orogens.

International audience; This study investigates the reactions allowing crystallization of large amounts of lawsonite (CaAl 2 Si 2 O 7 (OH) 2 • [H 2 O]) found in calcschists metamorphosed in a subduction zone setting. Previous studies of lawsonite-forming reactions in metasediments have highlighted its importance for the large-scale budget of CO 2 , as the calcium required to form lawsonite is thought to originate from decarbonation reactions. Yet, thermodynamic modelling as well as isotopic measurements have indicated that 80 to 90% of the carbon is retained in sediments, and there is no evidence of major decarbonation in the field. As lawsonite contains abundant H 2 O and has a large stability field, understanding its crystallization is also important to assess fluid migration and mass transfer in a critical part of the subduction system where slow earthquakes are nucleating (such as low frequency earthquakes or episodic tremor and slip). The upper units of the blueschist facies metasediments of the Schistes Lustrés complex (Western Alps), buried to depths of~30-40 km, have been selected as an ideal case study as they host up to 40 vol% of lawsonite. Lawsonite is found crystallized over several generations, in veins and in reactions fronts as well as in the rock matrix. Three types of lawsonite were identified. The most abundant type of lawsonite is associated with quartz and ankerite. This assemblage formed from phyllosilicates and calcite in a continuous reaction: chlorite + calcite + kaolinite = lawsonite + ankerite + quartz + H 2 O. According to thermodynamic modelling, this reaction is restricted to a narrow pressure-temperature domain and initiates around 180°C and 0.4 GPa. Lawsonite is also observed and predicted to grow from Fe-Mg-carpholite at higher metamorphic conditions. None of these reactions allow efficient net export of carbon, as one carbonate replaces another, and most observations are consistent with closed-system behaviour at outcrop-scale, in agreement with geochemical studies. At sample-scale and below, crystallization of lawsonite is linked to homogenization of carbonate and pelitic domains in geologically fast reactions. Dissolution of calcite produces reactive fluids prone to react with pelitic domains and crystallize lawsonite and another carbonate. Although metamorphic veins are ubiquitous to the upper units of the Schistes Lustrés Complex, most of them result from local reactions and do no indicate large-scale mass transfer.

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large- scale ophiolites (usually ~20 km thick). They typically show a steep inverted metamorphic structure where the pressure and temperature (T) conditions of crystallization increase upward, from the base of the sole (500 ± 100°C at 0.5 ± 0.2 GPa) to the contact with the overlying peridotite (800 ± 100°C at 1.0 ± 0.2 GPa). The inverted T gradient was historically interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab synchronously with the overlying ophiolite formation (within only 1-2 Myrs). Their mineralogical assemblage and deformation pattern provide major constraints on the nature and the timing of the processes controlling the dynamics of the plate interface during early subduction.Soret et al. (2017, 2019) recently reappraised the tectonic–petrological model for the formation of metamorphic soles below ophiolites, showing that the present-day structure of the sole results from the successive stacking of several homogeneous oceanic crustal slivers (without internal T gradient). This stacking marks the evolution of rheological properties of slab material and peridotites of the upper plate as the plate interface progressively cools (Agard et al., 2016). These findings outline the thermal and mechanical complexity of early subduction dynamics, and highlight the need for refined numerical modelling studies.Lu-Hf geochronology on garnet from the Oman metamorphic sole has recently shown that the earliest accreted subunit, found directly against the upper plate mantle, was initially buried ≥ 8 Ma earlier than previously estimated (Guilmette et al., 2017). These results imply initiation ≥ 8 Ma before the formation of the ophiolite, which underscores the common belief that ophiolite-sole couples record spontaneous subduction initiation and rather indicates far-field forcing long before upper plate extension and mantle upwelling.We herein present new U-Pb titanite and monazite petrochronology across the different sub-units of the Oman metamorphic sole. Our results confirm the time lag of several million years between subduction initiation and the ophiolite formation, therefore supporting the recently proposed model of far-field forced subduction initiation. They also reveal a significant time lag between the underplating and exhumation of each sub-unit of the sole.

Halogens are volatile elements of great interest for the study of fluid-rock interactions between minerals of metamorphic and mantle rocks. Constraining the partitioning of these elements between minerals is also key to understanding their deep geochemical cycle. Hydrous silicates such as micas, amphiboles, chlorites, epidotes or serpentines often contain minor to trace amounts of halogens incorporated by the OH- = X- (X- = F-, Cl-, Br- or I-) mechanism. Their abundance in metamorphic and mantle rocks grants them a major role in storing and transporting halogens through the subduction zone. However, low halogen concentrations hamper in situ analyses, and quantifying the partitioning of low-concentrated halogens remains then very challenging.The present study focusses on incorporation of halogens (F-, Cl-, Br-, I-) into hydroxyl sites in phyllosilicates and amphiboles, on both analytical and theoretical grounds.In situ measurements in minerals using electron probe microanalysis and LA-ICP-MS/MS have been carried out, allowing investigation of minor to ultra-trace halogen concentrations. Average detection limits with the electron probe are of 200 ppm for F and 35-40 ppm for Cl, Br and I. LA-ICP-MS/MS allowed simultaneous measurement of Cl, Br and I, reaching detection limits of about 50-100 ppm of Cl, 1-10 ppm of Br and well below 1 ppm for I. Calibrations have been carried out using international and house standards. Halogen ratios and partition coefficients between minerals have been measured.Ab-initio modelling of the OH- = X- exchange in phyllosilicate end-members of interest (e.g. phlogopite, muscovite, clinochlore) is underway (CRYSTAL17, Dovesi et al., 2014). Halogen-bearing defects are modelled as diluted as much as possible in crystals (≤ 1 wt. %) to mimic trace concentrations. Crystal strain and energetic cost of the substitution as well as theoretical partition coefficients have been computed and compared between optimised structures. Comparison of F and Cl partitioning between Mg-biotite and muscovite shows that the effect of dioctahedral vacancies over the position of hydroxyl groups strongly influences halogen partitioning, where F and Cl distribute in favour of biotite. Forthcoming modelling will quantify the strain and energetic impact of halogen incorporation in chlorite and amphibole end-members. ReferenceDovesi, R.; Orlando, R.; Erba, A.; Zicovich-Wilson, C. M.; Civalleri, B., … Kirtman, B. (2014). International Journal of Quantum Chemistry, 114(19), 1287–1317.

Using partition coefficients is extremely useful to model melting processes and fluid-rock interactions. However, partition coefficients values remain scarce in regard of their sensitivity to mineral composition and to the variability of mineral composition. In addition, the inferred equilibrium between phases is not necessarily reached, even in high-grade metamorphic conditions associated to melting. Disequilibrium may dramatically hamper the effective mobility of species and lead to element distribution far from the predicted values.This contribution aims at estimating partition coefficients for chromium (Cr) between garnet and clinopyroxene, and testing them in natural rocks of various metamorphic grades. As a poorly mobile trivalent element, Cr is chosen as a proxy to rare earth elements.Theoretical partition coefficients for Cr between garnet and clinopyroxene are calculated ab initio from structures where Cr3+ is modelled as a defect in Al3+ sites using CRYSTAL17 (Dovesi et al., 2014) and the thermodynamic description of Dubacq and Plunder (2018). Results are compared to electron microprobe measurements in mineral assemblages containing tens to thousands of ppm of Cr, where element mapping brings much information. Results of ab initio computations highlight the role of crystal-chemistry over the strain field around point defects, controlling the dynamics of the Cr3+ = Al3+ exchange between clinopyroxene and garnet. As expected, the partitioning of Cr between garnet and clinopyroxene depends strongly on the grossular and pyrope content: Cr incorporates grossular preferentially to jadeite, but jadeite incorporates Cr preferentially to pyrope.Comparison between predicted and measured partition coefficients allowed to estimate the deviation from equilibrium. Disequilibrium is evidenced even for samples metamorphosed around 850°C, as shown by the distribution of Cr-rich and Cr-depleted domains. Disequilibrium is attributed to slow diffusivity of Cr in fluid and at grain boundaries during crystal growth, leading to interface-coupled dissolution-precipitation.Dovesi, R., Orlando, R., Erba, A., Zicovich‐Wilson, C. M., Civalleri, B., Casassa, S., ... & Noël, Y. (2014). CRYSTAL14: A program for the ab initio investigation of crystalline solids. International Journal of Quantum Chemistry, 114(19), 1287-1317.Dubacq, B., & Plunder, A. (2018). Controls on trace element distribution in oxides and silicates. Journal of Petrology, 59(2), 233-256.

International audience; This contribution investigates mechanisms controlling subduction development and stabilization over time (coined as 'slabitization'), from a nascent slab to a mature slab viscously coupled to mantle convection, from grain scale to plate tectonics scale. Frozen-in, deep and warm portions of the subduction plate interface with both sides still preserved are found at the base of ophiolites in almost pristine state. Both sides record changes in the mineralogy, structure, fluid content and rheology due to devolatilization of subducting metamorphic rocks. They allow characterizing the evolution, shortly after subduction initiation (~1–10 Ma), of interplate coupling, mantle resistance to slab penetration or incipient mantle wedge metasomatism, as well as transformations occurring at depth in warm or cold subduction zones today.This study combines structural field work, mineralogical and crystallographic data, detailed petrology, thermodynamic modelling and geochemistry from/on both sides of the plate interface, i.e. the base of the mantle wedge (basal ophiolitic peridotites) and crustal fragments from the slab (metamorphic soles). Data collected across the entire Semail ophiolite (Oman, UAE territory) and other similar settings worldwide (e.g., Canada, Turkey, New Caledonia) show a continuous evolution of the subduction plate interface from 1.2–0.9 GPa 900–750 °C to 0.7–0.5 GPa 750–600 °C, with progressive localization of strain and fluid transfer. Crystallization of neo-formed minerals, enrichment in fluid-mobile elements and their isotopic signature (e.g., for boron) indicate that metasomatism of the mantle base results from interaction with subduction fluids derived from the dehydrating metamorphic sole and slab tip, migrating at velocities ~1–10 m/a. Coeval deformation and metamorphic reactions in metabasalts of the downgoing slab reveal the importance of mineral changes (e.g., amphibole content) and deformation modes in controlling fluid delivery, stepwise detachment and accretion of successive slices from the downgoing slab (HTa, HTb and then LT soles) to the mylonitized mantle.This study demonstrates how the interplay between metamorphic reactions, fluid/melt transfer and deformation mechanisms, in particular dissolution-precipitation creep (DPC), controls the mechanical coupling state of the plate interface: (i) suppression of fluid transfer and DPC at depth triggers the onset of viscous coupling. This occurs near ~ 30 km depth during subduction infancy and HTa sole formation; (ii) with increased cooling and fluid availability, strain localization progressively develops downwards and unzips the subduction interface. The downward migration of viscous coupling triggers localized mantle wedge upwelling, potentially leading to short-lived suprasubduction ophiolite or forearc lithosphere formation; (iii) the locus of viscous coupling stabilizes near ~80–100 km in mature (and cold) subduction zones, and sets mantle counterflow. This is where and when plates get reattached and slabs become part of the mantle convection system.Recent geochronological data suggest that the duration from subduction nucleation to ophiolite formation is probably slower than suspected (~5–10 Ma), and that another 5–10 Ma may be needed to reach mature subduction and profuse arc magmatism. These results refine our view of the subduction factory and have important implications for how, how much, and which sort of fluid is being fluxed into the mantle wedge at depths where serpentine is no longer stable.

International audience; Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions.

The strength of the lithosphere may be constrained qualitatively by field observations on localized vs distributed modes of deformation and by the mineral assemblages formed during deformation. The internal deformation of the Bielsa basement unit of the Pyrenean Axial zone is investigated through structural, microstructural and thermometric data. In this area, shortening is widely distributed as attested by the folded attitude of the interface between the basement and its sedimentary Triassic cover. Shortening is estimated around 1.7 km (13%) from a regional balanced cross-section and should be considered in pre-Pyrenean reconstructions. Shortening probably occurred before strain localization on crustal ramps as suggested by zircon fission-track analysis. Distributed shortening is characterized at small-scale by low-temperature mylonites and cataclasites. In thin-section, feldspar originally present in the magmatic protolith is partially to totally sericitized. This transformation led to significant weakening of the rock and took place in the 250–350 °C temperature range. Sericitization is ubiquitous, even in un-deformed granodiorites. This shows that the weakening effect of sericitization not only occurs in ultra-mylonites, ultra-cataclasites and phyllonites but also more generally in the upper crust early during the shortening history, with implications for the shortening style. Estimates of the geothermal gradient suggest that inherited thermicity may also have influenced the style of shortening. We propose that the upper crust was very weak before or at the onset of its shortening due to high-thermal gradients and fluid circulation that induced large-scale sericitization in greenschist facies conditions. This has strong implications on the rheological evolution of the upper crust submitted to metamorphic alteration in the greenschist facies and below.

International audience; Chlorite is a ubiquitous product of metamorphism, alteration of magmatic rocks and hydrothermal processes owing to its large stability field and wide compositional range. Its composition is governed by several substitutions and has been used as a geothermometer, on the basis of empirical, semi-empirical, and thermodynamic models. As in some other phyllosilicates of petrological interest, the oxidation state of iron in chlorite may differ from the usually assumed divalent state. However, the crystal chemistry of trivalent iron in chlorite remains poorly known, and the thermodynamic properties of ferric chlorite are missing from databases used for petrological modeling. As part of an attempt to fill this gap, we present results from in situ, micrometer-scale measurements of the oxidation state of iron in various chlorite-bearing samples. X-ray absorption near-edge spectroscopy (XANES) was combined with electron probe microanalysis (EPMA) on the same crystals. Results show iron oxidation states varying from ferrous to ferric; iron is in octahedral coordination in all ferromagnesian chlorites but to ~25% tetrahedral in the lithian chlorite cookeite (1.0 wt% Fe2O3(total)). Absolute amounts of ferric iron cover an unprecedented range (0 to ~30 wt% Fe2O3). For highly magnesian, ferric chlorite, Fe concentrations are low and can be accounted for by Al = Fe3+ substitution. In Fe-rich samples, Fe3+ may exceed 2 atoms per formula unit (pfu, 18 oxygen basis). When structural formulas are normalized to 28 charges corresponding to the standard O10(OH)8 anionic basis, these measurements define the exchange vector of a di-trioctahedral-type substitution: 3 VI(Mg, Fe2+) = VI☐ + 2 VIFe3+, as described in earlier studies. However, structural formulas calculated on the basis of the oxygen contents actually measured by EPMA show that this trend is an artifact, due to the neglect of variations in the number of protons in the structure. Our measurements indicate increasing hydrogen deficiency with increasing Fe3+ content, up to ~ 2 H+ pfu in the Fe3+-rich chlorite samples, corresponding to a net exchange vector of the type R2+ + H+ = Fe3+. These results do not support substitutions toward di-trioctahedral ferric end-members, and highlight the need for considering substitution toward an “oxychlorite” (i.e., H-deficient) ferric component, close to tri-trioctahedral, with an O12(OH)6 anionic basis, even in green, pristine-looking chlorite. The effects of iron oxidation and H deficiency on chlorite geothermometers were explored. They are deterring if H deficiency is ignored but, given the sensitivity of most thermometers to octahedral vacancy, the assumption FeTotal = Fe2+ is still safer than using high measured Fe3+ contents and the standard 28 charge basis, which artificially increases vacancies. In such ferric chlorites, EPMA measurement of oxygen allows a fair estimate of H content if Fe3+/Fe2+ is known; it should be more systematically implemented. For the same reasons, literature data reporting Fe3+-rich chlorite with vacancy content along the possibly artificial di-trioctahedral-type substitution should be verified. With the help of constraints from thermodynamic models, charge balance, crystal symmetry, and proton loss, a new cation site distribution is proposed for di-tri- to tri-trioctahedral chlorites in the Fe2+-Fe3+-Mg-Al-Si-O-H system, allowing a more realistic thermodynamic handling of their solid solutions.

This study sheds light on the deformation mechanisms of subducted mafic rocks metamorphosed at amphibolite and granulite facies conditions and on their importance for strain accommodation and localization at the top of the slab during subduction infancy. These rocks, namely metamorphic soles, are oceanic slivers stripped from the downgoing slab and accreted below the upper plate mantle wedge during the first million years of intraoceanic subduction, when the subduction interface is still warm. Their formation and intense deformation (i.e., shear strain ≥5) attest to a systematic and transient coupling between the plates over a restricted time span of ∼1 Myr and specific rheological conditions. Combining microstructural analyses with mineral chemistry constrains grain-scale deformation mechanisms and the rheology of amphibole and amphibolites along the plate interface during early subduction dynamics, as well as the interplay between brittle and ductile deformation, water activity, mineral change, grain size reduction and phase mixing. Results indicate that increasing pressure and temperature conditions and slab dehydration (from amphibolite to granulite facies) lead to the nucleation of mechanically strong phases (garnet, clinopyroxene and amphibole) and rock hardening. Peak conditions (850 ∘C and 1 GPa) coincide with a pervasive stage of brittle deformation which enables strain localization in the top of the mafic slab, and therefore possibly the unit detachment from the slab. In contrast, during early exhumation and cooling (from ∼850 down to ∼700 ∘C and 0.7 GPa), the garnet–clinopyroxene-bearing amphibolite experiences extensive retrogression (and fluid ingression) and significant strain weakening essentially accommodated in the dissolution–precipitation creep regime including heterogeneous nucleation of fine-grained materials and the activation of grain boundary sliding processes. This deformation mechanism is closely assisted with continuous fluid-driven fracturing throughout the exhumed amphibolite, which contributes to fluid channelization within the amphibolites. These mechanical transitions, coeval with detachment and early exhumation of the high-temperature (HT) metamorphic soles, therefore controlled the viscosity contrast and mechanical coupling across the plate interface during subduction infancy, between the top of the slab and the overlying peridotites. Our findings may thus apply to other geodynamic environments where similar temperatures, lithologies, fluid circulation and mechanical coupling between mafic rocks and peridotites prevail, such as in mature warm subduction zones (e.g., Nankai, Cascadia), in lower continental crust shear zones and oceanic detachments.

International audience; The basal mantle of the Semail ophiolite directly overlying the metamorphic sole has been affected by a late and relatively low temperature ductile deformation event, ascribed to the deformation of the mantle just above the plate interface during subduction infancy (which ultimately led to ophiolite obduction). We show that this deformation results in the formation of proto-mylonitic (∼850–750°C) to ultra-mylonitic (∼750–650°C) shear zones wrapping lenses of porphyroclastic tectonites deformed at higher temperature (∼1200°C). In proto- to ultra-mylonites, syn-deformation hydrous fluid/peridotite interaction triggered the dissolution of coarser grains (olivine1, orthopyroxene1 and clinopyroxene1) and the crystallization of polymineralic mixtures of smaller grains (olivine2, orthopyroxene2, clinopyroxene2, spinel2 ± sulfide, together with magnesio-hornblende to pargasitic amphibole). This modal metasomatism is associated with cryptic metasomatism marked by striking mineral enrichment in some fluid-mobile-elements (especially B and Li). From the major and trace elements composition of the newly precipitated grains and estimated conditions of fluid/peridotite interaction (∼800°C), we interpret the metasomatic agent as a fluid derived from the dehydration of the amphibolitic to granulitic metamorphic sole (i.e. the metamorphosed crust from the downgoing plate) underlying the mantle during incipient subduction. We propose that this high temperature sole was accreted to the proto-mylonitic banded unit while dehydrating and that both units were exhumed together during peridotite ultra-mylonitic deformation, thereby preserving a fossilized warm subduction interface and evidence of an incipiently (de)forming mantle wedge.

International audience; Despite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle–crust transition of the New Caledonia obducted ophiolite and ~ 50 to 100 m above the basal thrust contact of the ophiolite. These m- to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40Ar/39Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction.Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700–800 °C and 3–5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40Ar/39Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650–750 °C); (3) the last, low temperature (< 600 °C) metasomatic stage results in the formation of deformed veinlets containing talc, chlorite and serpentine.

Metamorphic soles are tectonic slices welded beneath most large-scale ophiolites. These slivers of oceanic crust metamorphosed up to granulite facies conditions are interpreted as forming during the first million years of intra-oceanic subduction following heat transfer from the incipient mantle wedge towards the top of the subducting plate. This study reappraises the formation of metamorphic soles through detailed field and petrological work on three key sections from the Semail ophiolite (Oman and United Arab Emirates). Based on thermobarometry and thermodynamic modelling, it is shown that metamorphic soles do not record a continuous temperature gradient, as expected from simple heating by the upper plate or by shear heating as proposed in previous studies. The upper, high-temperature metamorphic sole is subdivided in at least two units, testifying to the stepwise formation, detachment and accretion of successive slices from the down-going slab to the mylonitic base of the ophiolite. Estimated peak pressure-temperature conditions through the metamorphic sole, from top to bottom, are 850°C and 1 GPa, 725°C and 0.8 GPa and 530°C and 0.5 GPa. These estimates appear constant within each unit but differing between units by 100 to 200°C and ~0.2 GPa. Despite being separated by hundreds of kilometres below the Semail ophiolite and having contrasting locations with respect to the ridge axis position, metamorphic soles show no evidence for significant petrological variations along strike. These constraints allow us to refine the tectonic–petrological model for the genesis of metamorphic soles, formed via the stepwise stacking of several homogeneous slivers of oceanic crust and its sedimentary cover. Metamorphic soles result not so much from downward heat transfer (ironing effect) as from progressive metamorphism during strain localization and cooling of the plate interface. The successive thrusts originate from rheological contrasts between the sole, initially the top of the subducting slab, and the peridotite above as the plate interface progressively cools. These findings have implications for the thickness, the scale and the coupling state at the plate interface during the early history of subduction/obduction systems. This article is protected by copyright. All rights reserved.

International audience; The dissolution of CO 2 into formation brines and the subsequent reactions of the CO 2-charged brines with reservoir minerals are two key processes likely to increase the security of geological carbon-dioxide storage. These processes will be dependent on the permeability structure and mineral compositions of the reservoirs, but there is limited observational data on their rates. In this paper we report the cation and anion concentrations and Sr, oxygen and carbon isotopic compositions of formation waters from four extraction wells sampled at surface, over~6 months after commencement of CO 2 injection in a five spot pattern for enhanced oil recovery at the Salt Creek field, Wyoming. Sampled fluids, separated from the minor oil component, exhibit near-monotonic increases in alkalinity and concentrations of cations but little change in Cl and Br concentrations and oxygen and deuterium isotope ratios. The increases in alkalinity are modelled in terms of reaction with reservoir calcite and silicate minerals as the changes in fluid chemistry and Sr-isotopic compositions are inconsistent with simple addition of injected fluids sampled over the course of the experiment. The reservoir mineral chemical and isotopic compositions are characterised by sampling core as well as surface exposures of the Frontier Formation elsewhere in Wyoming. The evolution of the fluid chemistries reflects extensive dissolution of both carbonate and silicate minerals over the course of the six months sampling implying rapid dissolution of CO 2 in the formation waters and reaction of CO 2-bearing brines with formation minerals. Rates of CO 2 diffusion into the brines and advection of CO 2 charged brines in the reservoir are sufficiently slow that, if present, calcite should react to be close to equilibrium with the fluids. This allows estimation of the CO 2 partial pressures in the sampled fluids and comparison with the thermodynamic driving force for the relatively rapid average plagioclase dissolution rates of~10 − 12 mol•m − 2 •s − 1 .

International audience; Subduction infancy corresponds to the first few million years following subduction initiation, when slabs start their descent into the mantle. It coincides with the transient (yet systematic) transfer of material from the top of the slab to the upper plate, as witnessed by metamorphic soles welded beneath obducted ophiolites. Combining structure–lithology–pressure–temperature–time data from metamorphic soles with flow laws derived from experimental rock mechanics, this study highlights two main successive rheological switches across the subduction interface (mantle wedge vs. basalts, then mantle wedge vs. sediments; at ∼800 °C and ∼600 °C, respectively), during which interplate mechanical coupling is maximized by the existence of transiently similar rheologies across the plate contact. We propose that these rheological switches hinder slab penetration and are responsible for slicing the top of the slab and welding crustal pieces (high- then low-temperature metamorphic soles) to the base of the mantle wedge during subduction infancy. This mechanism has implications for the rheological properties of the crust and mantle (and for transient episodes of accretion/exhumation of HP-LT rocks in mature subduction systems) and highlights the role of fluids in enabling subduction to overcome the early resistance to slab penetration.

Chorite is a phyllosilicate crystallizing in a wide range of pressure and temperature conditions, among a diversity of rocks : from diagenesis to metamorphic conditions, typically in greenschist facies but also in blueschist and amphibolite facies. Chlorite is useful for metamorphic petrology as a geothermometer, as its composition is sensitive to the temperature of crystallization and chlorite is commonly found within mineral assemblages. Chlorite shows many solid solutions, among which Fe may be incorporated in large proportions in divalent (Fe2+) and/or trivalent (Fe3+) state. Unfortunately current models for geothermometry on chlorite either neglect Fe3+ or require evaluating the thermodynamic properties of Fe3+-rich chlorite for increased accuracy. This study aims at providing new crystal-chemistry data on ferric chlorite from a wide range of composition and origin, and to answer the following questions : (1) how much Fe3+ may be incorporated in chlorite ? (2) what are the cationic substitutions and end-members for Fe3+ incorporation ? (3) how is Fe3+ distributed within the chlorite structure ? and (4) what are the dependencies between Fe3+ in chlorite and the pressure-temperature conditions, and oxygen fugacity, at crystal and mineral scales ? This work investigates the speciation of iron in chlorite with different techniques; X-ray absorption near-edge spectroscopy at the Fe-K edge (K XANES), and electron energy-loss spectroscopy (EELS) combined with electron microprobe major element analyzes. X-ray diffraction investigation brings structural information on the structure of ferric chlorite. This analytical strategy is focused on chlorite from natural rock samples and experimental syntheses made at fixed pressure and temperature, and under buffered oxygen fugacity.A new database on ferric chlorite crystal-chemistry is the major output of this work, which highlights substitutions involving Fe3+. A Fe3+ -rich, vacant endmember is required to account for the di-trioctahedral substitution, and a magnesian end-member with 1 Fe3+ replacing Al for the homovalent Al - Fe3+ substitution. These results are observed in natural specimens and confirmed by experimental synthsesis of ferric chlorite. In addition, chlorite with Fe3+ > 1.5 p.f.u. shows systematic deviation from the ideal O10(OH)8 anionic basis, where proton deficit has been inferred from indirect measurements of H+ content. These results are consistent with the existence of an "oxychlorite" group within the classification of phyllosilicates. At mineral scale, the variations of the oxidation state take place at nanoscale and are unrelated to variations in the amount of Fe. Chlorite crystallized via experimental synthesis shows similar features. These results explain the poor success of geothermometry on some iron-rich chlorite, and allow us to propose improved cation distribution algorithms for geo thermometry. Our study paves the way for future experimental synthesis focused on oxychlorite.; Les chlorites sont des phyllosilicates ayant la particularité de cristalliser sur une large gamme de conditions de pression et température, et dans une variété de roches et d’environnements géologiques pouvant aller du contexte diagénétique au contexte métamorphique de type schiste bleu à amphibolitique. La composition des chlorites est sensible à leur température de cristallisation, et avec leur présence récurrente au sein des paragénèses, celles-ci sont couramment utilisées pour l’estimation des conditions thermobarométriques par les pétrologues. Cependant la plupart des modèles thermobarométriques actuels ne permettent pas d’estimer de manière fiable des températures au sein de l’ensemble des compositions des chlorites. Les modèles thermodynamiques en particulier ne prennent pas en compte ou sous-estiment l’état d’oxydation du fer dans les chlorites qui peut être sous forme Fe2+ ou Fe3+. Ce travail de thèse a ainsi pour objectif d’apporter de nouvelles données cristallochimiques sur un large panel de chlorites ayant incorporé du fer trivalent de compositions et contextes variés, tout en répondant aux questions sur : (1) la quantité de Fe3+ qui peut être incorporée dans les chlorites, (2) les substitutions cationiques permettant l’incorporation du Fe3+ dans les chlorites et leurs pôles purs associés, (3) la localisation du Fe3+ et des autres cations dans les sites cristallographiques des chlorites et enfin, (4) sur le lien éventuel entre le Fe3+ dans les chlorites et les conditions thermodynamiques du milieu que sont la pression, température, et la fugacité en oxygène à l’échelle du cristal et du minéral.Ce travail de thèse utilise différentes méthodes de mesure du Fe3+/FeTOT ; la spectroscopie d’absorption du fer au seuil K (K XANES), et la spectroscopie par perte d’énergie des électrons (EELS) couplées à l’utilisation de la microsonde électronique. Ce volet analytique est complété par des analyses de diffraction des rayons X. L’approche utilisée repose sur l’étude de chlorites naturelles échantillonnées pour leur représentativité du domaine de composition, ainsi que sur des chlorites synthétisées expérimentalement sous pression, température, et fugacité en oxygène contrôlées.Une nouvelle base de données cristallochimiques sur les chlorites ferriques constitue l’apport principal de ce travail, et a permis en particulier de mettre en évidence les substitutions contrôlant l’incorporation du Fe3+ dans la structure. Ainsi la substitution di-trioctaédrique 3R2+ = 2Fe3+ + _ et l’échange Al = Fe3+ semblent expliquer le domaine de composition des chlorites ferriques jusqu’à Fe3+ ∼ 1.5 p.f.u., permettant de définir un ou plusieurs pôles purs de type ferri-sudoite en plus d’un pôle ferrique non lacunaire et magnésien, dans les chlorites naturelles et synthétiques. Mais l’analyse de l’oxygène dans les chlorites les plus ferriques montrent un déficit de protons interprété comme lié à l’oxydation du fer divalent, tel que : Fe2+ + H+ = Fe3+, mettant en évidence un composant "oxychlorite" à base anionique O12(OH)6. Une implication importante est que la présence de lacunes et la substitution di-trioctaédrique précédemment révélées sont largement un artefact de normalisation sur la base anionique O10(OH)8.A l’échelle du minéral la distribution du rapport Fe3+/FeTOT dans les chlorites montre des variations pouvant atteindre jusqu’à ∼ 30 %, sur une échelle de la centaine à dizaine de nanomètres. Cette hétérogénéité se retrouve également à l’échelle du micromètre mais n’est pas corrélée à la distribution du FeTOT et des autres éléments majeurs dans la lame. Enfin en plus de confirmer les substitutions des chlorites naturelles, les synthèses de chlorites ont permis d’identifier un probable contrôle de la fugacité en oxygène sur le rapport Fe3+/FeTOT mais pas sur la quantité de Fe3+ incorporée, indépendamment de la composition de départ.Ces résultats améliorent les connaissances cristallochimiques sur les chlorites ferriques et permettent de proposer un nouvel algorithme d’allocation des cations dans les sites, en vue de l’élaboration d’un modèle thermodynamique prenant en compte l’incorporation du Fe3+.

Chorite is a phyllosilicate crystallizing in a wide range of pressure and temperature conditions, among a diversity of rocks : from diagenesis to metamorphic conditions, typically in greenschist facies but also in blueschist and amphibolite facies. Chlorite is useful for metamorphic petrology as a geothermometer, as its composition is sensitive to the temperature of crystallization and chlorite is commonly found within mineral assemblages. Chlorite shows many solid solutions, among which Fe may be incorporated in large proportions in divalent (Fe2+) and/or trivalent (Fe3+) state. Unfortunately current models for geothermometry on chlorite either neglect Fe3+ or require evaluating the thermodynamic properties of Fe3+-rich chlorite for increased accuracy. This study aims at providing new crystal-chemistry data on ferric chlorite from a wide range of composition and origin, and to answer the following questions : (1) how much Fe3+ may be incorporated in chlorite ? (2) what are the cationic substitutions and end-members for Fe3+ incorporation ? (3) how is Fe3+ distributed within the chlorite structure ? and (4) what are the dependencies between Fe3+ in chlorite and the pressure-temperature conditions, and oxygen fugacity, at crystal and mineral scales ? This work investigates the speciation of iron in chlorite with different techniques; X-ray absorption near-edge spectroscopy at the Fe-K edge (K XANES), and electron energy-loss spectroscopy (EELS) combined with electron microprobe major element analyzes. X-ray diffraction investigation brings structural information on the structure of ferric chlorite. This analytical strategy is focused on chlorite from natural rock samples and experimental syntheses made at fixed pressure and temperature, and under buffered oxygen fugacity.A new database on ferric chlorite crystal-chemistry is the major output of this work, which highlights substitutions involving Fe3+. A Fe3+ -rich, vacant endmember is required to account for the di-trioctahedral substitution, and a magnesian end-member with 1 Fe3+ replacing Al for the homovalent Al - Fe3+ substitution. These results are observed in natural specimens and confirmed by experimental synthsesis of ferric chlorite. In addition, chlorite with Fe3+ > 1.5 p.f.u. shows systematic deviation from the ideal O10(OH)8 anionic basis, where proton deficit has been inferred from indirect measurements of H+ content. These results are consistent with the existence of an "oxychlorite" group within the classification of phyllosilicates. At mineral scale, the variations of the oxidation state take place at nanoscale and are unrelated to variations in the amount of Fe. Chlorite crystallized via experimental synthesis shows similar features. These results explain the poor success of geothermometry on some iron-rich chlorite, and allow us to propose improved cation distribution algorithms for geo thermometry. Our study paves the way for future experimental synthesis focused on oxychlorite.; Les chlorites sont des phyllosilicates ayant la particularité de cristalliser sur une large gamme de conditions de pression et température, et dans une variété de roches et d’environnements géologiques pouvant aller du contexte diagénétique au contexte métamorphique de type schiste bleu à amphibolitique. La composition des chlorites est sensible à leur température de cristallisation, et avec leur présence récurrente au sein des paragénèses, celles-ci sont couramment utilisées pour l’estimation des conditions thermobarométriques par les pétrologues. Cependant la plupart des modèles thermobarométriques actuels ne permettent pas d’estimer de manière fiable des températures au sein de l’ensemble des compositions des chlorites. Les modèles thermodynamiques en particulier ne prennent pas en compte ou sous-estiment l’état d’oxydation du fer dans les chlorites qui peut être sous forme Fe2+ ou Fe3+. Ce travail de thèse a ainsi pour objectif d’apporter de nouvelles données cristallochimiques sur un large panel de chlorites ayant incorporé du fer trivalent de compositions et contextes variés, tout en répondant aux questions sur : (1) la quantité de Fe3+ qui peut être incorporée dans les chlorites, (2) les substitutions cationiques permettant l’incorporation du Fe3+ dans les chlorites et leurs pôles purs associés, (3) la localisation du Fe3+ et des autres cations dans les sites cristallographiques des chlorites et enfin, (4) sur le lien éventuel entre le Fe3+ dans les chlorites et les conditions thermodynamiques du milieu que sont la pression, température, et la fugacité en oxygène à l’échelle du cristal et du minéral.Ce travail de thèse utilise différentes méthodes de mesure du Fe3+/FeTOT ; la spectroscopie d’absorption du fer au seuil K (K XANES), et la spectroscopie par perte d’énergie des électrons (EELS) couplées à l’utilisation de la microsonde électronique. Ce volet analytique est complété par des analyses de diffraction des rayons X. L’approche utilisée repose sur l’étude de chlorites naturelles échantillonnées pour leur représentativité du domaine de composition, ainsi que sur des chlorites synthétisées expérimentalement sous pression, température, et fugacité en oxygène contrôlées.Une nouvelle base de données cristallochimiques sur les chlorites ferriques constitue l’apport principal de ce travail, et a permis en particulier de mettre en évidence les substitutions contrôlant l’incorporation du Fe3+ dans la structure. Ainsi la substitution di-trioctaédrique 3R2+ = 2Fe3+ + _ et l’échange Al = Fe3+ semblent expliquer le domaine de composition des chlorites ferriques jusqu’à Fe3+ ∼ 1.5 p.f.u., permettant de définir un ou plusieurs pôles purs de type ferri-sudoite en plus d’un pôle ferrique non lacunaire et magnésien, dans les chlorites naturelles et synthétiques. Mais l’analyse de l’oxygène dans les chlorites les plus ferriques montrent un déficit de protons interprété comme lié à l’oxydation du fer divalent, tel que : Fe2+ + H+ = Fe3+, mettant en évidence un composant "oxychlorite" à base anionique O12(OH)6. Une implication importante est que la présence de lacunes et la substitution di-trioctaédrique précédemment révélées sont largement un artefact de normalisation sur la base anionique O10(OH)8.A l’échelle du minéral la distribution du rapport Fe3+/FeTOT dans les chlorites montre des variations pouvant atteindre jusqu’à ∼ 30 %, sur une échelle de la centaine à dizaine de nanomètres. Cette hétérogénéité se retrouve également à l’échelle du micromètre mais n’est pas corrélée à la distribution du FeTOT et des autres éléments majeurs dans la lame. Enfin en plus de confirmer les substitutions des chlorites naturelles, les synthèses de chlorites ont permis d’identifier un probable contrôle de la fugacité en oxygène sur le rapport Fe3+/FeTOT mais pas sur la quantité de Fe3+ incorporée, indépendamment de la composition de départ.Ces résultats améliorent les connaissances cristallochimiques sur les chlorites ferriques et permettent de proposer un nouvel algorithme d’allocation des cations dans les sites, en vue de l’élaboration d’un modèle thermodynamique prenant en compte l’incorporation du Fe3+.

In subduction zones, the migration of fluids derived from the dehydration of metasediments has important consequences on the mechanic properties of rocks, hence on seismicity. Geophysical imaging of the velocity of pressure and shear waves shows that fluids are present in large quantities and migrating in the 30-40 km depth range. This range corresponds to the depth at which slow slip events are supposed to take place, and it is currently assumed that fluids play a large part in the onset of these slow slip events. Deformation mechanisms accompanying slow slip events are still debated, as well as the exact role of fluids which remain enigmatic in their source, origin, volumetric importance together with the scale and the mode of their migration.To better understand how such fluids migrate in subducted metasediments and refine the budget of fluids in this context (from ~150°C-0.3 GPa to 350°C-1.2 GPa), we have studied subunits of the Schistes Lustrés Complex of the Western Alps.Lawsonite (ideally CaAl2Si2O7(OH)4.H2O) is ubiquitous in these rocks, in the schistose matrix as well as in metamorphic veins, attesting to fluid-rock interactions at depth. Lawsonite, a highly hydrated mineral (~12wt.% H2O) is of particular interest as it is known to play a key role in recycling fluids to mantle depths for it is stable up tu ultra-high pressures.Yet the formation of large amounts of lawsonite in subducted pelagic sediments – where pelitic and carbonated horizons alternate – is unexplained and lawsonite-forming reactions are poorly documented. In particular, the need for large amounts of calcium sourced from carbonated horizons led some authors to suggest ample decarbonation, allowing for export of carbon, a major constituent of these rocks.This study aims first at defining which reactions allowed lawsonite crystallization to shed light on element transfer at depth. The petrologic study allowed distinguishing several types of lawsonite based on textural, micro-structural and morphologic criteria. The vast majority of the lawsonite crystals show textural equilibrium with ankerite, and the following reaction is proposed: chlorite + kaolinite + aragonite = lawsonite + ankerite + quartz + H2O. This reaction shows zero net export of carbon during lawsonite crystallization. Thermodynamic modelling suggests lawsonite crystallization initiates early during the metamorphic history of the Schistes Lustrés (~13 km depth) and is on-going the metamorphic peak, due to sluggish kinetics and the apparition of additional yet subordinate reactions.A detailed geochemical study of the lawsonite types has been carried out in this study. Results highlight the differences in fluid-rock interactions and their time evolution. The earliest, most abundant lawsonite type shows closed-system crystallization with small equilibrium length-scales (micro- to millimetres) whereas later lawsonite types indicate system opening with increased fluid mobility and element transfer via advection.Observations concur to showing progressive system opening in the Schistes Lustrés via increased fluid mobility. This took place increment by increment as shown by textural and compositional analysis, such as strontium-rich lawsonite overgrowths. Scales of fluid mobilization remain uncertain and analysis of strontium isotopic ratios will help further constraining the source of fluids and their interactions with the host rock.; La circulation des fluides issus des réactions de déshydratation, intenses dans les métasédiments, a des conséquences majeures sur les propriétés rhéologiques des roches, tout particulièrement dans les zones de subduction, à l'interface entre les plaques. Les méthodes géophysiques y suggèrent la présence, au travers des vitesses des ondes sismiques (Vp/Vs) notamment, de fortes concentrations de fluides entre 30 et 40 km sous la surface. C’est également dans cette gamme de profondeur que de nouveaux types de séismes ont été décrits au début des années 2000 : les séismes lents, pour lesquels il est admis que les fluides jouent un rôle important mais dont le déclenchement, les mécanismes de déformations associés, de même que la source des fluides, leur origine, leur volume et l’échelle et le mode de leur circulation demeurent débattus. Pour améliorer les bilans de production de fluides dans ces gammes de pression – température (de 150 à 350 °C, de 0.3 à 1.2 GPa) et affiner la compréhension de la migration des fluides dans ces roches, nous avons recours à un analogue de terrain maintenant à l’affleurement : le complexe des Schistes Lustrés alpins. La présence ubiquiste de lawsonite dans ces métasédiments, dans la matrice et dans différentes générations de veines, localement en grande abondance, y témoigne d’intenses interactions fluides-roches en profondeur. La lawsonite est d'autant plus importante que ce minéral hydraté (~12 % poids de H2O; de formule théorique CaAl2Si2O7(OH)4.H2O) est aussi connu pour jouer un rôle important dans le recyclage des fluides en contexte de subduction, jusqu’aux conditions de ultra-haute pression (>300 km). La formation d'une telle quantité de lawsonite dans les sédiments pélagiques (alternances de bancs pélitiques et de bancs carbonatés) est toutefois difficile à expliquer et les réactions permettant sa formation sont encore peu documentées. En particulier, la nécessité d’un apport en Ca pour sa cristallisation amène certains auteurs à suggérer la décarbonatation des horizons calcaires, ce qui semble en désaccord avec les observations de terrain et a des implications fortes pour la dynamique du carbone présent en quantités importantes dans ces roches. Dans un premier temps, cette thèse vise à mieux contraindre les réactions de formation de la lawsonite afin de clarifier les échanges de matières effectifs en profondeur. L’étude pétrologique révèle la présence de plusieurs types de lawsonite sur des critères texturaux, micro-structuraux et morphologiques. La majorité des lawsonites observées sur le terrain montre des textures de co-cristallisation avec l’ankérite, et la réaction de formation suivante a été identifiée : chlorite + kaolinite + aragonite = lawsonite + ankérite + quartz + H2O. Cette réaction indique que la formation de lawsonite ne constitue pas une source de carbone, son bilan net étant nul. La modélisation thermodynamique de cette réaction l’identifie comme précoce dans l’enfouissement des Schistes Lustrés (s’initiant vers environ 13 km de profondeur). La cristallisation de lawsonite dans ces roches apparaît toutefois étendue dans le temps et se prolongeant jusqu’au pic métamorphique, du fait du délai cinétique de cette réaction et de l’apparition de réactions additionnelles. Dans un deuxième temps, une étude géochimique fine des éléments traces réalisée sur les différents types de lawsonite rencontrés sur le terrain permet de mettre en évidence deux comportements très différents en terme d’interactions fluides-roches. Si la première génération (LwsA) témoigne d’une cristallisation en système fermé (échelle micro- à millimétrique), la deuxième génération (LwsB) atteste au contraire de la remobilisation de fluides dans un système plus ouvert (échelle métrique à hectométrique). Les observations faites dans cette étude mettent en évidence une ouverture progressive du système que représente les Schistes Lustrés, en terme de circulation de fluides. Cette ouverture se fait de manière incrémentale comme en témoignent les textures et les surcroissances des lawsonites, enrichies en Sr. Les échelles sur lesquelles sont remobilisés les fluides sont encore incertaines et l'étude en cours sur les rapports isotopiques du strontium apportera des éléments de réponses pour mieux contraindre la migration de ces fluides.

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large-scale ophiolites (usually ≤ 20 km thick). They typically show a steep inverted metamorphic structure where the pressure (P) and temperature (T) conditions of crystallization increase upward, from the base of the sole (500±100ºC at 0.5±0.2 GPa) to the contact with the overlying peridotite (800±100ºC at 1.0±0.2 GPa). Soles are interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab during the first My of intra-oceanic subduction. Metamorphic soles are therefore direct witnesses of petrological processes during early subduction. Their mineralogical assemblage and deformation pattern provide major constraints on the evolution of the thermal structure, on the migration of fluids and on the effective rheology along the nascent slab interface. We present a detailed petrological, (micro-)structural and experimental study, with refined P–T estimates obtained with pseudosection modelling and EBSD measurements, on the garnet-bearing and garnet-free (natural and synthetized) amphibolite. We suggest a new tectonic–petrological model for the formation of metamorphic soles below ophiolites, which involves the stacking of several homogeneous slivers (without any T gradient) of oceanic crust to form the present-day structure of the sole. These successive thrusts are the result of rheological contrasts between the slab material and the peridotites of the upper plate as the plate interface progressively cools. This model outlines the thermal and mechanical complexity of the early subduction dynamics, and highlights the need for more refined numerical modelling studies.; Les semelles métamorphiques sont des unités d’origine océanique (≤ 500 m d’épaisseur) situées à la base des grandes ophiolites obductées (≤ 20 km d’épaisseur). Ces unités sont caractérisées par un gradient métamorphique inverse, où les conditions de pression (P) et de température (T) de cristallisation augmentent de la base vers le contact avec l’ophiolite sus-jacente : depuis 500±100˚C et 0.5±0.2 GPa jusqu'à 800±100˚C et 1.0±0.2 GPa. Formées et exhumées au cours des 2 Ma suivant l’initiation des subductions océaniques, les semelles sont des témoins directs de leur dynamique précoce. Les assemblages minéralogiques qu’elles portent et leur déformation fournissent des contraintes majeures, et rares, sur l’évolution de la structure thermique et sur le comportement mécanique de l’interface de subduction naissante. Au terme d'une étude pétrologique, (micro-) structurale et expérimentale sur les amphibolites naturelles de la semelle de Semail (Oman, UAE) et synthétisées en laboratoire, nous proposons un modèle où la semelle métamorphique résulte d’épisodes multiples d’accrétion d’unités homogènes en P–T (donc sans gradient métamorphique) au cours des premières étapes de subduction océanique. L’écaillage subséquent résulte de changements majeurs dans la distribution de la déformation, du fait des variations des propriétés mécaniques des roches à l’interface de subduction lors de son équilibration thermique et de l’augmentation au cours du temps de la proportion de sédiments entrant en subduction. Ce modèle rend compte d’une grande complexité thermique et mécanique à l’interface de subduction, encore insuffisamment examinée dans les études numériques actuelles.