Afifé El Korh
email: afife.elkorh@unifr.ch

Etienne Deloule

Béatrice Luais

Marie-Christine Boiron

Luc Bastian

Nathalie Vigier

Lithium behaviour and isotope fractionation during fluid-rock interactions in Variscan oceanic suture zones: Limousin ophiolite and Ile de Groix high-pressure terrane (France)

Keywords: lithium isotopes, hydrothermal alteration, subduction zone metamorphism, ophiolite, metabasite, serpentinite, ion-microprobe, ICP-

Ophiolites and high-pressure/low-temperature (HP-LT) terranes are important sites for the study of geochemical cycling in ancient oceanic lithosphere. We have analysed Li abundances and isotope composition in a series of ultrabasic and basic rocks from the Variscan Limousin ophiolite, as well as in basic and pelitic rocks from the Ile de Groix HP-LT terrane. Both bulk and in-situ analyses are employed to evaluate Li mobility and isotope fractionation in the oceanic lithosphere during fluid-rock interactions related to seafloor and sub-seafloor hydrothermal alteration, subduction and exhumation processes.

In the Limousin ophiolite, early stages of high-temperature (high-T) hydrothermal alteration of oceanic ultrabasic rocks produce serpentine with low Li abundances (0.9-4.6 ppm) and low δ 7 Li (-8.9‰). The δ 7 Li increase from -2.2 to +4.2‰ in the following generations of serpentine during late-stage hydrothermal alteration results from changes in the fluid composition and temperature conditions. Therefore, even if dehydrating subducted serpentinites generate high amounts of fluids during subduction, abyssal serpentinites do not constitute an important source of Li for Li-rich metabasic rocks. In the associated amphibolites, hornblende displays typical Li contents (3.1-8.2 ppm) and isotopic compositions (+3.5 to +12.5‰) similar to hydrothermally altered sheeted dykes and gabbros.

By contrast, the low Li abundances and extremely high δ 7 Li values recorded by omphacite and pargasitic amphibole in the ultra-high-pressure (UHP) zoisite-eclogite from the Limousin probably reflect interaction with a heavy-Li sediment-derived fluid.

The HP-LT metabasites of the Ile de Groix record different Li behaviour, with high Li abundances and low δ 7 Li. They contain Li abundances significantly higher than fresh midocean ridge basalts (MORB) (16-124 ppm), indicating a metasomatic overprint by fluids derived from the neighbouring Li-rich micaschist (15-52 ppm) in addition to seawater during the early stages of subduction. Lithium is mainly hosted by: 1) glaucophane and omphacite in blueschists and eclogites; 2) chlorite and albite in retrograde greenschists; 3) phengite and chlorite in micaschists. The metabasites have δ 7 Li values of -4.8 to +3.2‰ that are generally lower than fresh and altered MORB. The intercalated micaschists display δ 7 Li values ranging

INTRODUCTION

Lithium, the lightest alkali metal with its two isotopes 6 Li and 7 Li, is a fluid mobile element that can be used as a tracer of fluid-mediated mass transfer between oceanic crust and mantle in subduction zones (e.g. [START_REF] Brenan | The role of aqueous fluids in the slab-tomantle transfer of boron, beryllium, and lithium during subduction: Experiments and models[END_REF][START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF][START_REF] Zack | Extremely light Li in orogenic eclogites: the role of isotope fractionation during dehydration in subducted oceanic crust[END_REF]Elliott et al., 2004;[START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF]Tang et al., 2010;[START_REF] Penniston-Dorland | Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA[END_REF], 2012, 2017;[START_REF] Wan | Lithium isotopic geochemistry in subduction zones: retrospects and prospects[END_REF]. Fresh MORB and mantle have generally low Li contents (3-8 ppm; [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF][START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF]Tomascak et al., 2008), while Li can be abundant in the altered oceanic crust (1-119 ppm; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF][START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF] and pelitic sediments (1-80 ppm; Bebout et al., 1993;[START_REF] Bouman | Lithium inputs to subduction zones[END_REF][START_REF] Chan | Lithium isotopic composition of marine sediments[END_REF]. Interaction with heavy Li-rich seawater (+31‰; [START_REF] Millot | Accurate and high-precision measurement of lithium isotopes in two reference materials by MC-ICP-MS[END_REF] during hydrothermal alteration of the oceanic crust triggers Li isotope fractionation, resulting in a δ 7 Li increase in low-T altered rocks (-1.7 to +20.8‰; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF][START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF] compared to fresh MORB (+3.4 ± 1.4‰; Tomascak et al., 2008). Devolatilisation reactions during subduction zone metamorphism (or HP-LT metamorphism) of Li-rich altered oceanic crust may release Li-rich fluids at shallow levels in subduction zones (e.g. [START_REF] You | Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones[END_REF]. Studies on HP-LT rocks have also observed that Li may be mobilised from metabasic [START_REF] Marschall | Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks[END_REF] and metasedimentary rocks (Bebout et al., 1993[START_REF] Bebout | Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high-P/T metasedimentary rocks[END_REF] during devolatilisation reactions at intermediate depths (40-100 km), i.e. at the transition from blueschist to eclogite facies. However, significant amounts of Li are retained in blueschists and eclogites in amphiboles (glaucophane and, to a lesser extent, barroisite), pyroxenes (omphacite, aegyrine/jadeite) and high-pressure phengite (e.g. [START_REF] Woodland | Lithium abundances in eclogite minerals: a clue to a crustal or mantle origin?[END_REF]Spandler and Hermann, 2006;[START_REF] John | Trace-element mobilization in slabs due to non steady-state fluid-rock interaction: Constraints from an eclogite-facies transport vein in blueschist (Tianshan, China)[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2011). [START_REF] Scambelluri | The fate of B, Cl and Li in subducted oceanic mantle and in the antigorite breakdown fluids[END_REF] have shown that the Li content of the serpentinised upper mantle does not vary during early hydrothermal processes and is preserved in high-pressure (HP) antigorite serpentinites. The serpentinised oceanic mantle can carry Li until depths >100 km, below which Li-rich fluids are released during antigorite breakdown, which is considered as a major source of fluids for arc lava magmatism [START_REF] Scambelluri | The fate of B, Cl and Li in subducted oceanic mantle and in the antigorite breakdown fluids[END_REF][START_REF] Bouvier | Slab-derived fluids in magma sources of St. Vincent (Lesser Antilles Arc): Volatile and light element imprints[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF].

Lithium isotopes can fractionate during fluid-rock interactions related to subduction zone metamorphism (e.g. [START_REF] Zack | Extremely light Li in orogenic eclogites: the role of isotope fractionation during dehydration in subducted oceanic crust[END_REF][START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Penniston-Dorland | Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA[END_REF], 2012;[START_REF] Wan | Lithium isotopic geochemistry in subduction zones: retrospects and prospects[END_REF]. Because of the large relative mass difference between the two isotopes (16%; Tomascak, 2004) and the higher diffusivity of 6 Li compared to 7 Li, the two Li stable isotopes can fractionate significantly by kinetic fractionation during chemical diffusion [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF]. With increasing metamorphic degree and temperature, kinetic isotope fractionation becomes predominant over equilibrium fractionation [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF][START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF]. Kinetic isotope fractionation during fluid-rock interactions is enhanced by dissolution-precipitation reactions between mineral assemblages stable under varying P-T conditions [START_REF] John | Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs[END_REF]. Variations in Li concentration and isotope composition in subduction-related rocks have been employed as tracers of metasomatic processes during subduction zone metamorphism [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF], of significant Li fractionation towards low δ 7 Li at shallow depth during subduction. By contrast, the model of [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF] suggests that the δ 7 Li of the subducting crust should only weakly decreases during subduction. They conclude that the light-Li signature recorded by orogenic eclogites is not only due to subduction-related metamorphism, but also results from the mobilisation of Li during eclogitisation or subsequent exhumation. This model predicts that the deeply subducted eclogites should have a higher δ 7 Li than the mantle.

Thus, the model defined by [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF] shows that the heavy-Li fluids released into the fore-arc mantle wedge are consequently responsible for heavy-Li arc magmas or enriched MORB (E-MORB), distinct from lighter-Li normal-MORB (N-MORB) (Tomascak et al., 2000(Tomascak et al., , 2002;;[START_REF] Elliott | The terrestrial Li isotope cycle: light-weight constraints on mantle convection[END_REF][START_REF] Elliott | Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts[END_REF]. This expectation is supported by Li isotopic data from pore fluids and sediments in active subduction zone, which allowed estimation of δ 7 Li values of deep slab-derived fluids to vary between +22 and +38‰ [START_REF] Chan | Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: implications for fluid processes and sediment contribution to the arc volcanoes[END_REF]. In addition, oceanic crust recycling in subduction zones is thought to be the source of abnormal Li isotopic composition of mantle xenoliths (-35 to +8‰;Tang et al., 2007Tang et al., , 2010Tang et al., , 2012Tang et al., , 2014;;[START_REF] Agostini | Drying and dying of a subducted slab: coupled Li and B isotope variations in Western Anatolia Cenozoic Volcanism[END_REF][START_REF] Wan | Lithium isotopic geochemistry in subduction zones: retrospects and prospects[END_REF] compared to the "normal" MORB mantle (+4 ± 2‰; Chan et al., 1992[START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Jeffcoate | Li isotope fractionation in peridotites and mafic melts[END_REF]Tomascak et al., 2008;Gao et al., 2011).

Lithium partitioning and isotope fractionation during fluid-rock interactions through hydrothermal alteration, subduction zone metamorphism and post-collision exhumation processes is still a matter of debate within the geoscience community. This study focuses on Li behaviour and isotope fractionation in a series of rocks from two units from the Moldanubian zone of the Variscan belt: 1) hydrothermally altered basic and ultrabasic rocks from the Limousin low-pressure ophiolite and 2) metabasic and metasedimentary rocks for the Ile de Groix HP-LT terrane. It aims to examine Li mobility and isotopic fractionation in whole rocks and metamorphic minerals with regards to the variations of pressure and temperature and fluid-rock interactions during seafloor hydrothermal alteration and subduction zone metamorphism. We employ a novel approach combining bulk and in-situ elemental and isotopic analyses to investigate Li mobility during fluid-rock interactions related to low-T and high-T hydrothermal alteration, subduction-related devolatilisation and rehydration reactions in ancient oceanic rocks. The results will allow deciphering the modes of Li isotopic fractionation in remants of ancient Variscan oceanic lithosphere under various pressure and temperature conditions related to seafloor and sub-seafloor hydrothermal alteration, subduction and exhumation processes.

GEOLOGICAL CONTEXT AND INVESTIGATED SAMPLES

The Limousin and Ile de Groix areas are part of the European Variscan belt, which results from the Devonian to Carboniferous collision of two major continental domains, Laurussia (formed of Laurentia, Baltica and Avalonia) and Gondwana (Fig. 1a). The continental domains were separated by the Rheic and Palaeotethys Oceans, two major pre-Variscan oceanic domains opened during the Cambrian-Ordovician (e.g. Stampfli et al., 2011;[START_REF] Nance | Evolution of the Rheic Ocean[END_REF]von Raumer et al., 2013 Kroner and[START_REF] Kroner | Two plates-many subduction zones: the Variscan orogeny reconsidered[END_REF]. Three main collision events were involved during the Variscan orogeny: 1) Middle Devonian subduction of the oceanic crust under Gondwana and Laurussia, followed by the dislocation of the northern margin of the Gondwana; 2) Early Carboniferous continental collision between Avalonia-Laurussia and Gondwana-derived continental terranes; and 3) Late Carboniferous final collision [START_REF] Von Raumer | Pre-Mesozoic Alpine basements -their place in the European Paleozoic framework[END_REF][START_REF] Kroner | Two plates-many subduction zones: the Variscan orogeny reconsidered[END_REF]. The allochthonous domain (Moldanubian Zone; Fig. 1a) consists of superposed peri-Gondwanan nappes, and includes a series of Devonian ophiolites, which can be follow from the Sudetes to the Iberian Peninsula (see von Raumer et al., 2013, 2015 andreferences therein). The ophiolites are emplaced along the northern Gondwana margin, and derive from magmatic rocks that are contemporaneous with the Cambro-Ordovician rifting associated to the opening of the Rheic Ocean [START_REF] Von Raumer | Pre-Mesozoic Alpine basements -their place in the European Paleozoic framework[END_REF]. In western France, the Limousin ophiolite and the Ile de Groix blueschist terrane are recognised as remnants of rocks with oceanic affinity within the same Allochthon domain (Figs. 1b andc).

Limousin ophiolite

The Limousin ophiolite belongs to the French Massif Central (FMC), which is part of the western European Variscan belt (e.g. [START_REF] Dubuisson | Evidence for a lost ocean in Variscan terranes of the Western Massif Central, France[END_REF][START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]2010a). Formation of the FMC results from the piling of a series of nappes during the Devonian-Early Carboniferous [START_REF] Girardeau | Cinématique de mise en place des ophiolites et nappes crystallophiliennes du Limousin, Ouest du Massif Central français[END_REF][START_REF] Ledru | Où sont les nappes dans le Massif Central Français?[END_REF][START_REF] Ledru | Lithostratigraphy of Variscan terranes in the French Massif Central: a basis for paleogeographical reconstructions[END_REF][START_REF] Demange | Antevariscan evolution of the Montagne Noire (France): from a passive margin to a foreland basin[END_REF][START_REF] Faure | A review of the pre-Permian geology of the Variscan French Massif Central[END_REF]: 1) the Upper Allochthon, called Gartempe Unit in the North Limousin, corresponds to a group of low-grade Palaeozoic rocks; 2) the Middle Allochthon (Upper Gneiss Unit), is a composite unit that contains rocks from the "leptyno-amphibolite groups'' (LAGs) [START_REF] Santallier | Les complexes leptynoamphiboliques (C.L.A.): revue critique et suggestions pour un meilleur emploi de ce terme[END_REF], formed by paragneisses, leptynites and amphibolites of medium to high grade metamorphism, as well as migmatitic metagreywackes and relicts of eclogites and granulites;

3) the Lower Allochthon (Lower Gneiss Unit) consists of metasediments (paragneisses, micaschists, metashales and metagreywackes) and Late Proterozoic-Early Cambrian and Ordovician leucocratic orthogneisses; 4) the Parautochthon basement, deriving from metasediments and metagranites; and 5) the Southern Palaeozoic Nappes, corresponding to continental margin/platform series.

The Limousin ophiolite occurs as a 25 km long belt of basic and ultrabasic rocks, located in the upper part of the Middle Allochthon unit (Fig. 1b). It consists of a series of 1-5 km wide ophiolite massifs segmented by Variscan and late Carboniferous faults [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF][START_REF] Berger | A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry[END_REF]. Based on petrological and geochemical data, the Limousin ophiolite has been interpreted as the remnants of an ancient oceanic lithosphere [START_REF] Dubuisson | Evidence for a lost ocean in Variscan terranes of the Western Massif Central, France[END_REF][START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF][START_REF] Berger | A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry[END_REF]. It corresponds to a lherzolite-harzburgite ophiolite type, emplaced in a slow-spreading mid-ocean ridge [START_REF] Berger | A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry[END_REF]. Main lithologies include diopsidebearing harzburgites, harzburgites, dunites, wehrlites, troctolites, (meta)gabbros and amphibolites [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]. The ultrabasic rocks are highly serpentinised, with only rare relicts of olivine, spinel or pyroxene. Alteration of abyssal peridotites and amphibolitefacies metamorphism of the gabbros and mafic dykes result from intensive seafloor hydrothermal alteration under low-P conditions (~0.2 GPa) and temperature decreasing from high-T late-magmatic conditions to greenschist-zeolite metamorphic facies, following magma emplacement [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF].

The base of the Middle Allochthon unit also includes lenses of high-pressure (HP) to ultra-high-pressure (UHP) zoisite-eclogites, zoisite-kyanite-eclogites and kyanite-eclogites (Berger et al., 2010a). Eclogites were formed by deep subduction at a depth of 100 km (P ~ 2.9 ± 0.5 GPa, T ~ 660 ± 70°C). Ultra-high-pressure eclogites show variable degrees of fluidrock interactions. Kyanite-eclogites have a composition similar to supra-subduction zone basalts and were relatively preserved from metasomatism (Berger et al., 2010a). Zoisiteeclogites were interpreted as former iron-rich plagioclase cumulates and display geochemical evidences of interaction with fluids derived from subducted terrigenous sediments during the UHP metamorphic stage (Berger et al., 2010a). Geochronological data show that the zoisiteeclogite has recorded a protolith age of 475-489 Ma (U-Pb; zircon) (Berger et al., 2010a).

Crystallisation of the magmatic protolith of the UHP eclogites is contemporaneous to the emplacement of the orthogneisses from the Lower Allochthon unit (446 ± 6 to 521 ± 7 Ma; [START_REF] Melleton | Precambrian protoliths and early Paleozoic magmatism in the French Massif Central: U-Pb data and the North Gondwana connection in the west European Variscan belt[END_REF] and occurred during the Cambro-Ordovician rifting. The UHP event was dated at 412 ± 5 Ma (Berger et al., 2010a).

In this study, we have investigated the Li trace element and isotope composition in six serpentinites, four amphibolites and one zoisite-eclogite from La Flotte, Le Cluzeau and Saint-Laurent ophiolite massifs (Fig. 1c; Table 1). The whole rock compositions are given in the Supplementary Table A1. Serpentinites display the typical mesh textures resulting from hydrothermal alteration of ultrabasic rocks at mid-ocean ridges [START_REF] Bach | Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274)[END_REF], with brown serpentine cores rimmed by a second yellow generation (Figs. 2a andb). The serpentinites from Saint-Laurent (LAU1, LAU2) are rich in MgO, and show low abundances of Fe 2 O 3 , FeO, Al 2 O 3 and CaO. They consist of an assemblage of Mg-rich serpentine, chlorite forming veins, spinel, amphibole (composition from tremolite to Mg-hornblende) and rare relicts of olivine (Fig. 2a). These samples may correspond to the highly serpentinised dunites described by [START_REF] Dubuisson | Evidence for a lost ocean in Variscan terranes of the Western Massif Central, France[END_REF]. Core parts of serpentine are slightly richer in FeO. Serpentine along contact zones with chlorite veins are also richer in FeO and contain Fe-hydroxides. The serpentinites from La Flotte (FLOT1, FLOT2a, FLOT2b) and Le Cluzeau (CLUZ6) are slightly richer in Al 2 O 3 , Fe 2 O 3 and FeO, and have a lower XMg than samples from Saint-Laurent. They are composed of serpentine that have partially to totally replaced olivine and pyroxene, and also contain large lenses of chlorite + amphibole (tremolite to Mghornblende), spinel, iron oxides and sulphides (Fig. 2b). Serpentine is often zoned in FeO and MgO, displaying a rimward increase of the Mg# (Supplementary Table A2). These samples may be related to the serpentinised harzburgites or troctolites described by [START_REF] Dubuisson | Evidence for a lost ocean in Variscan terranes of the Western Massif Central, France[END_REF] and [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]. The studied amphibolites consist of two fine grained slightly foliated amphibolites (CLUZ 1 and CLUZ 1a) and two isotropic amphibolites (CLUZ 4 and CLUZ 5), which derive from isotropic or layered gabbros [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF].

They display an assemblage of green-brown amphibole (mainly Mg-hornblende), plagioclase, chlorite and iron oxides (Figs. 2c andd). Magmatic plagioclase relicts are sometimes present. Amphibole often shows evidences of alteration in iron hydroxydes along grain rims and fractures. The studied zoisite-eclogite L04-143 (from Berger et al. 2010a) is composed of garnet, zoisite, omphacite, pargasitic amphibole, and rutile rimmed by ilmenite.

Ile de Groix

The Ile de Groix (Armorican Massif, France) corresponds to the outcropping part of a HP-LT unit, also located in the Middle Allochthon unit of the Variscan belt. The metamorphic rocks of the Ile de Groix are composed of 20% metabasic rocks of hydrothermally altered E-MORB affinity interlayered with 80% pelitic micaschists [START_REF] Audren | Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique[END_REF][START_REF] Bernard-Griffiths | Geochemical and isotopic characteristics of blueschist facies rocks from the Ile de Groix, Armorican Massif (northwest France)[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013, 2017a) (Fig. 1c). The rocks recorded two metamorphic phases: 1) a prograde blueschist to eclogite facies metamorphism M1 related to subduction, and 2) a greenschist facies overprint M2 related to exhumation (e.g. [START_REF] Carpenter | Petrogenetic study of the glaucophane schists and associated rocks from the Ile de Groix[END_REF][START_REF] Quinquis | Schistes bleus et déformation progressive: l'exemple de l'île de Groix[END_REF][START_REF] Quinquis | Les schistes bleus de l'île de Groix dans la chaîne Variscenne: implications cinématiques[END_REF][START_REF] Barrientos | Infiltration vs. thermal overprinting of epidote blueschists, Ile de Groix, France[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] Ballèvre | Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican massif, France): a potential geobarometer[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013). The eastern part of the Ile de Groix is dominated by eclogites, and epidote-blueschist facies rocks, whereas greenschist facies rocks are dominant on the western part. The difference in mineral assemblages between eclogites and epidote-blueschist facies metabasites may result from small variations in the protolith composition and by small differences in temperature (50-75°C) for the peak metamorphic conditions [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. The peak P-T conditions, estimated at 1.6-2.5 GPa and 500-600°C in eclogites correspond to the blueschist to eclogite facies transition, which is expected to have taken place at c. 60-70 km depth in a subduction zone with an intermediate thermal regime [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. Based on P-T calculations using the NFMASH system for metapelites, [START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF] have defined two metamorphic units divided by a ductile thrust: the Upper Unit (1.6-1.8 GPa, 450-500°C) in the eastern part of the island, and the Lower Unit (1.4-1.6 GPa, 400-450°C) in the West.

The HP-LT event was dated at 358-365 Ma with the 40 Ar/ 39 Ar (phengite) and Rb-Sr (whole rock, phengite and epidote) methods, while a younger age of 345-353 Ma was determined for the greenschist metamorphic event [START_REF] Bosse | Rb-Sr and 40Ar/39Ar ages in blueschists from the Ile de Groix (Armorican Massif, France): Implications for closure mechanisms in isotopic systems[END_REF]. Rare albite gneisses interlayered within micaschists and chloritoschists on the Ile de Groix provided protolith ages of 480.8 ± 4.8 Ma and 492.7 ± 3.2 Ma, respectively (U-Pb dates on zircon; El [START_REF] El Korh | Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith[END_REF][START_REF] Paquette | From opening to subduction of an oceanic domain constrained by LA-ICP-MS U-Pb zircon dating (Variscan belt, Southern Armorican Massif, France)[END_REF], contemporaneous with the Cambro-Ordovician rifting, widely recognised in the internal parts of the Variscan belt (e.g. von [START_REF] Von Raumer | Pre-Mesozoic Alpine basements -their place in the European Paleozoic framework[END_REF]. Fe isotope measurements have shown that the metabasites of the Ile de Groix derive from an unusual heavy-Fe mantle source, and were probably emplaced during the intracontinental back-arc basin rifting that occurred along the northern Gondwana margin after the closure of the Proto-Rheic ocean [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]El Korh et al., 2017a). During the Variscan subduction, the metabasalts and their sedimentary cover were sheared off and imbricated in an accretionary prism [START_REF] El Korh | Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith[END_REF].

Based on trace element and δ 18 O analyses, El [START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF] have shown that large-scale fluid-rock interactions occurred as open system only during seafloor hydrothermal alteration and early subduction prograde metasomatism. The retrograde metamorphism was interpreted as an overprint of fluids likely derived from the basic rocks without involving infiltration of externally derived fluids at P-T conditions lower than 1 GPa and 400°C [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF]. Metabasites that underwent pervasive alteration by pre-HP metasomatism processes, show greater effects of retrogression [START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF]. Consequently, the large-scale zonation of the Ile de Groix was interpreted as the result of superimposed effects of the low-temperature hydrothermal alteration and variable dehydration states due to distinct peak P-T conditions in the two tectonic units [START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF].

Peak metamorphic assemblages in metabasites (blueschists and eclogites) are composed of garnet, glaucophane, epidote, phengite, apatite, quartz, titanite and/or rutile.

Omphacite porphyroblasts are only present in eclogites (Figs. 2e andf). Blueschist facies rocks often contain pseudomorphs after lawsonite, composed of epidote, white mica (phengite, paragonite), actinolite, and albite [START_REF] Ballèvre | Lawsonite growth in the epidote blueschists from the Ile de Groix (Armorican massif, France): a potential geobarometer[END_REF]. Partial retrogression of eclogites and blueschists is evidenced by: 1) barroisite + albite symplectites replacing omphacite, 2) barroisite and actinolite overgrowths along glaucophane rims, and 3) partial replacement of garnet by chlorite and rutile by titanite (Figs. 2e andf). Retrograde assemblages in greenschists correspond to epidote, chlorite, actinolite, barroisite, albite, magnetite, and titanite. High-pressure garnet is partially to totally replaced by chlorite, actinolite and by a retrograde generation of epidote. "Albitic greenschists" contain large albite porphyroblasts ± calcite, and have experienced intensive fluid-rock interactions during retrogression [START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF].

Eighteen samples were studied, typical of each metamorphic facies and lithology (Fig. 1b; Table 1). They include thirteen metabasites (blueschists GR02,04,11b,12b,25a and GROA56;eclogites GR21,24a and 29;greenschists GR06a,23 and 25b,and albitic greenschists GROA43 and 52), two blueschist-facies metapelites (GROA110 and 111a) and two greenschist-facies metapelites (GR26b and GROA104). For the detailed description of the samples, see El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF][START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF][START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF].

ANALYTICAL TECHNIQUES

Whole rock Li elemental and isotopic composition

The whole rock Li abundances in the samples from the Ile de Groix and Limousin were measured using a Varian 220FS Atomic Absorption Spectrometer at the SARM (Service The Li isotope analyses of the whole rock samples from the Ile de Groix and separated glaucophane and omphacite from sample GR29 were analysed at the CRPG-Nancy. Lithium was separated following the procedure detailed in [START_REF] Vigier | Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle[END_REF]modified after James & Palmer, 2000). In summary, 10 mg of powdered samples were dissolved in three steps using: 1) a 2:1 mixture of HF (28 N) and HNO 3 (15 N) on a hot plate at 120°C; 2) HNO 3 (15 N) at 60°C; and 3) HCl (1 N) at 60°C. The total dissolution of the samples took approximately 1 week. After total dissolution of the samples and centrifugation, Li was eluted and purified through cationic AG50X12 resin-exchange chromatography columns. The same chemical procedure has been applied to blank solutions. Chemistry blanks were systematically less than 15 pg Li.

Lithium isotopes were measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) using a NeptunePlus instrument (ThermoFisher Scientific, Germany and USA) at the CRPG-Nancy. Samples were introduced in the MC-ICPMS via a cyclonic spray chamber. The instrument was operating in static mode at a low resolution (M/ M = 400). The cup configuration consisted of: Low 4 ( 6 Li) and High 4 ( 7 Li). The gas flow rates, torch parameters and ion lenses were optimised using a solution of the NIST L-SVEC RM 8545 Li standard. The correction coefficients between the Faraday cups were obtained by a gain calibration before optimising the peak shapes and centring the peaks. Analyses were performed on 25 ppb Li solutions, diluted in HNO 3 (0.05 N). Thirty-five integration cycles were measured during 294 s (+ 300 s take up time), after a washout time of 440 s in HNO 3 (0.05 N) and a background measurement of 67 s.

Typical acquisition series consisted of alternate measurements of the L-SVEC standard and a sample. Each measurement is corrected for the background value (measured in 0.05 N HNO 3 ). Background intensities for the 7 Li isotope were 0.06-0.1 V. Backgroundcorrected intensities for the 7 Li isotope were ≤ 0.01 V for blank solutions, and generally 2-3 V for standards and samples solutions. The Li isotope ratios normalised to the L-SVEC standard, expressed as δ 7 Li L-SVEC , were calculated using the sample standard bracketing method (SSB), as follows: ‰ δ 7 Li L-SVEC (sample) = [( 7 Li/ 6 Li) sample / ( 7 Li/ 6 Li) L-SVEC -1] × 1000 The Li 7 N standard (nominal δ 7 Li value: +30.1‰; +30.1 ± 1.1‰ to +30.2 ± 0.3‰; Carignan et al., 2007) served as external secondary standard to monitor the accuracy and the precision of the isotopic measurements. An average δ 7 Li value of +30.1 ± 0.2‰ was obtained (2 SE as external reproducibility; n = 12), consistent with the reference and published values. The δ 7 Li L-SVEC of the samples is calculated by averaging replicate analyses of the same sample solution. For this reason, uncertainties are given as 2 standard errors (2 SE).

XRD measurements

The serpentine species in sample FLOT2b were characterised on a 50 m polished thin section using a Rigaku Ultima IV X-ray diffractometer (XRD) at the University of Fribourg (Switzerland). Analyses were carried out in continuous scan mode using Bragg-Brentano geometry. Operating conditions included a 2θ step size of 0.02°/step, a counting time/step of 0.5°/min between 5 and 70°, and X-ray tube conditions of 40 kV and 40 mA (Cu K ). The detection limit for mineral determination is c. 5 wt%. The mineral assemblage was identified using the software PDXL2 (Rigaku).

Determination of mineral Li abundances

Limousin ophiolite

Because of the low Li content of serpentinites and amphibolites, the mineral Li abundances in the rocks of the Limousin area were measured by secondary ion mass spectrometry (SIMS) using a Cameca ims1280 instrument at the CRPG. Analyses were performed on gold-coated thin sections, using a 8-14 nA primary projected O -beam with 15-20 m spot size. Secondary ions were measured by peak hopping in monocollection mode using SEM detector for 7 Li, and Faraday cup FC2 for 28 Si, at a resolution of 1500 (M/ M). The primary and secondary beam accelerating voltages were -13 kV and +10kV, respectively. Analyses were performed using a 28 eV energy window, with an energy offset of -50 eV. 20 integration cycles were measured during c. 15 min, after 120s of pre-sputtering. Si was employed as internal standard, using the mineral SiO 2 content (see Electronic Appendix). Reference basaltic glasses KL2-G (5.1 ppm Li; 50.5% SiO 2 ) and StHs60/80-G (20.7 ppm Li; 63.7% SiO 2 ) of the Max-Planck-Institut-Dingwell (MPI-DING) served as external standards to determine the Li ionisation yield relative to Si, as a function of the sample Si content. The daily 2σ external reproducibility of the standards, given as 2σ standard error (2σ SE), was 0.69-1.8%.

Ile de Groix

A part of the Li abundances in minerals from the samples of the Ile de Groix were analysed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) during previous studies (El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF][START_REF] El Korh | Geochemical fingerprints of devolatilisation reactions in the high-pressure rocks of Ile de Groix, France[END_REF]. Supplementary Li abundances in minerals from the samples of the Ile de Groix were performed by LA-ICPMS at the GeoRessources laboratory, University of Lorraine (Vandoeuvre-lès-Nancy, France). The instrument consists of an Agilent 7500c quadrupole ICPMS interfaced to a GeoLas Pro 193 nm ArF excimer laser ablation system (Lambda Physik, Germany). The laser was operating at a 5 Hz repetition rate, a fluence of ~10 J/cm 2 , and a 44 m spot size. Helium was used as the cell gas (0.8 L/min) and was mixed with Ar prior to its introduction in the plasma (1.5 L/min). Peak hopping mode was employed, as well as dual (counting and analogue) secondary electron multiplier (SEM) detector mode. Dwell times were 20 ms for 7 Li and 10 ms for 42 Ca and 29 Si.

The 248 ThO + / 232 Th + and Ca 2+ /Ca + ratios were optimised to about 0.3-0.7 and 0.3-0.5% respectively, by ablation of the National Institute of Standards and Technology reference material SRM 610, a synthetic glass doped with trace elements. Acquisition times for background and ablation intervals amounted to 60 and 40 s, respectively. The NIST SRM 610 reference material was employed as external standard, using the preferred values of Analytical series consisted of 10-16 measurements of samples, bracketed by a run of 2 measurements of the standard before and after the samples. Intensity vs. time data were reduced using an Excel spreadsheet, following the procedure defined in [START_REF] Longerich | Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation[END_REF]. Internal standards were 42 Ca (epidote, garnet, omphacite, titanite, calcite) and 29 Si (glaucophane, albite, chlorite, phengite), based on the mineral major element compositions [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. Intervals of 45 and 25 s were selected for the background and the ablation signal, respectively, with an offset of 3 s from the beginning of the ablation. No correction for down-hole fractionation was applied. The instrumental mass bias was corrected using a linear regression function, considering an equal time interval between each analysis. The minimum detection limit, corresponding to three times the standard deviation of the net background measurement, is based on the 99% confidence level over the Gaussian counting statistics. Limits of detection were 1.5-20 ppm. Uncertainties on individual measurements at 1σ are generally 2.0-12%. The NIST SRM 612 glass was employed as secondary standard and yielded a Li concentration of 39.4 ± 2.2 ppm (95% confidence level (CL); n = 11), similar within uncertainty to the GeoRem preferred value (40.2 ± 1.3 ppm; 95% CL).

In-situ Li isotope analysis in minerals

In-situ Li isotopes in minerals were measured on gold-coated thin sections by SIMS using a Cameca ims1280 instrument at the CRPG. Analyses in the Li-rich minerals (omphacite and glaucophane from the metabasites of the Ile de Groix) were performed using a 5.3-6.3 nA projected primary O -beam with c. 20 m spot size. The Li-poor minerals (serpentine, olivine and hornblende in the samples from the Limousin) were analysed using a 15-20 nA focused primary O -beam with c. 35 m spot size. Secondary 6 Li and 7 Li ions were measured by peak hopping in monocollection mode using SEM detector for intensities < 5.10 5 cps, and FC2 detector for intensities > 5.10 5 cps, at a resolution of 1500 (M/ M). The primary and secondary beam accelerating voltages were of -13 kV and +10 kV, respectively. Analyses were performed using a 50 eV energy window, without energy offset. 20 integration cycles were measured during c. 20 min, after 120 s of pre-sputtering.

Studies of matrix effects during Li isotope analyses by SIMS [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF][START_REF] Kasemann | Lithium isotope composition of basalt glass reference material[END_REF] have shown that there is no significant matrix dependence on the δ 7 Li values between different mineral phases with close Mg#. Besides, matrix effects occur if the Mg# value of standards and samples varies. External calibration to the L-SVEC and correction of the daily instrumental fractionation was performed using a series of clinopyroxenes, olivine and orthopyroxenes of various Mg# and δ 7 Li L-SVEC values, which were measured at the beginning and at the end of each analytical series [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF]Su et al., 2015; see the Supplementary Table A3 for the detailed list of the standards). 1σ internal errors on individual measurements were 0.3-2.7‰. 1-day 2σ external reproducibility of the standards was 0.4-2.7‰ (2σ SE).

During this study, no significant variation of the instrumental mass bias between different phases of similar Mg# (e.g. olivine, clinopyroxene and glass) occurred. Therefore, we observed a variation of the instrumental mass bias with the Mg#. As the Mg# of the studied samples of the Ile de Groix is significantly lower than that of standards (0.52-0.66 and 0.86-0.92, respectively), we have calculated the mass bias, or instrumental mass fractionation (IMF), for the sample from the Ile de Groix as follow: IMF = α * Mg# + β.

Coefficients α and β were obtained by a linear regression using the mean IMF value plotted against the mean Mg# of the various standards. We have also employed MC-ICPMS data of mean glaucophane and omphacite from eclogite GR 29 to constrain the linear regression.

The final δ 7 Li values, relative to the NIST L-SVEC RM 8545, are calculated as follow: δ 7 Li LSVEC (sample) = δ 7 Li LSVEC (SIMS) -IMF. Minerals from the Limousin ophiolite with a lower Mg# (0.71-0.77) than standards were corrected as well.

During the different sessions, no significant variation of the measured δ 7 Li could be observed in association with the orientation of the serpentine, chlorite or amphibole minerals, In this study, the range of mineral δ 7 Li values in each sample is discussed using mean δ 7 Li values. Despite large 2σ SD uncertainties due to mineral heterogeneities, the mean δ 7 Li values are employed to calculate the fractionation factors between mineral pairs and whole rocks. This makes it possible to minimise extreme values, which can be unrepresentative, especially since no outlier rejection is undertaken. Fractionation factors between minerals and whole rocks provide information on Li isotopic equilibrium or disequilibrium, as well as on the extent of Li isotope fractionation, during the various stages of hydrothermal alteration and subduction zone metamorphism.

RESULTS

Li elemental and isotopic composition in the rocks from the Limousin ophiolite

Whole rocks

The studied serpentinites from the Limousin ophiolite have Li abundances of 2.4-4.6 and 0.9-3.0 ppm, respectively. The UHP eclogite has a Li content of 2.3 ppm, while the amphibolites display significant higher Li contents (3.1-8.2 ppm). These values are comparable with the range of values of fresh MORB (3-8 ppm) [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF][START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF] and altered MORB (1-119 ppm; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF][START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]) (Fig. 3; Table 2).

Minerals

Minerals from the Limousin ophiolite generally display low Li abundances (Fig. 3 and Table 3). In serpentinites, serpentine is the main host for Li (0.33-8.2 ppm). Lithium abundances in Fe-richer serpentine (1.2-8.2 ppm) are generally higher than in Mg-rich serpentine (0.33-3.6 ppm). The less abundant olivine and amphibole (tremolite to Mg-hornblende) display similar Li contents as Mg-rich serpentine (1.5-3.0 and 1.1-3.5 ppm, respectively), while chlorite has low Li abundances (0.24-1.0 ppm). In amphibolites, Li abundances are higher in hornblende (1.3-11 ppm) than in plagioclase (0.09-1.0 ppm). In the UHP zoisite-eclogite, Li is mainly hosted by pargasitic amphibole (1.5-31 ppm) and omphacite (1.5-10 ppm), while low Li concentrations are measured in garnet (0.15-0.90 ppm) and zoisite (0.03-0.38 ppm).

The detailed study of serpentinite LAU2 shows that serpentine has a mean δ 7 Li value of -8.9 ± 5.2‰ (2σ SD). Altered serpentine, richer in FeO, has a similar δ 7 Li value of -8.7 ± 3.1‰ (2σ SD), leading to a fractionation factor 7 Li Fe-serp-Mg-serp value of +0.2 ± 6.1‰ (2σ) (Table 3). Even if the two generations of serpentine are identical within uncertainty, the large 2σ SD values result from mineral heterogeneities, which are probably related to local isotopic disequilibria during hydrothermal alteration processes. Serpentine from the sample FLOT-2a has a δ 7 Li value of -2.5 ± 5.5‰ (2σ SD), while relicts of olivine have a δ 7 Li value of +6.3 ± 2.9‰ (2σ SD) (Fig. 4; Table 3). During olivine alteration in serpentine, the 7 Li serp-ol value amounts to -8.8 ± 6.3‰ (2σ) (Table 3; Fig. 4a). Serpentine from sample CLUZ6 is strongly zoned in δ 7 Li: Fe-rich cores, replacing olivine, have a δ 7 Li of -2.2 ± 3.0‰ and Mg-rich rims have a δ 7 Li of +4.2 ± 3.5‰ (2σ SD). Co-existing amphibole (tremolite to Mg-hornblende) has a δ 7 Li of +6.6 ± 4.6‰ (2σ SD) (Fig. 4; Table 3). 7 Li Fe-serp-Mg-serp , 7 Li Fe-serp-amph , and 7 Li Mg- serp-amph values correspond to -6.4 ± 4.6, -8.8 ± 5.5 and -2.5 ± 5.8‰, respectively (Table 3).

Hornblende in amphibolite CLUZ1 has a MORB-like δ 7 Li of value of +3.5 ± 4.4‰ (2σ SD). A higher value of +9.8 ± 0.9‰ (2σ SD) was recorded in altered parts. Hornblende in amphibolite CLUZ4 (metagabbro) has a higher value of +12.5 ± 9.6‰ (2σ SD) (Table 3).

In the UHP zoisite-eclogite L04-143, omphacite is the main host for Li during the UHP metamorphic stage. It has a positive δ 7 Li value of +21.9 ± 5.0‰ (2σ SD) (Fig. 4; Table 3).

Secondary pargasitic amphibole has a δ 7 Li value of +18.7 ± 7.2‰ (2σ SD). The resulting 7 Li amph-omph value consists of -3.3 ± 8.7‰ (2σ) and indicates that isotopic equilibrium between omphacite and amphibole is not globally reached during partial retrogression, even if mineral pairs may be locally in equilibrium (Table 3).

Li elemental and isotopic composition in the rocks from the Ile de Groix

Whole rocks

F o r P e e r R e v i e

w

The metabasites of the Ile de Groix have Li abundances of 16-124 ppm (Table 4). These values are significantly higher than those of fresh MORB and Limousin UHP eclogite. Even if they cover a large range of values, the Li concentrations of the metabasites of the Ile de Groix do not vary according to the different metamorphic facies (Fig. 5) but are controlled by the mineral assemblage (Fig. 5). The studied micaschists have Li contents of 15-52 ppm, in agreement with the values measured in subducted metasediments (2-78 ppm, Bouman et al., 2004;[START_REF] Chan | Lithium isotopic composition of marine sediments[END_REF]. No change of Li concentration is observed between blueschist and greenschist facies micaschists.

The metabasites have δ 7 Li values of -4.8 ± 0.5 to +3.2 ± 0.6‰ (2σ SE) (Fig. 6a; Table 4) that are generally lower than those of fresh MORB (+3.4 ± 1.4‰, 2σ; Tomascak et al., 2008). Most samples have δ 7 Li within the range of data for heavy-Li low-T altered oceanic crust (-1.7 to +20.8‰; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]. Even if Li abundances do not show any correlation with the metamorphic facies, the δ 7 Li values of the metabasites vary between the metamorphic stages (Figs. 6a andb). The δ 7 Li values of blueschists vary from -0.6 ± 0.3 to +0.7‰ (2σ SE), i.e. from MORB-like values to lower negative values. Eclogites have negative δ 7 Li values ranging from -4.8 to -2.5‰, which are lower than those of the low-T altered oceanic crust (Fig. 6; Table 4). Greenschists show a larger range of δ 7 Li values from -4.5 ± 0.5 to +3.2 ± 0.6‰ (2σ SE). Greenschists that underwent restricted fluid rock interactions during retrogression (GR23 and 25b) display negative values (-4.5 ± 0.5 to -0.1 ± 0.5‰). Greenschists displaying a higher degree of retrogression and rehydration (GR06 and albite-bearing greenschist GROA52) have positive values of +1.5 ± 0.4 to +3.2 ± 0.6‰ (Fig. 6; Table 4). Micaschists have δ 7 Li values of -1.7 ± 0.7 to +0. The Li contents in the minerals from the Ile de Groix (from this study and El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF] are reported in Fig. 5 and Table 5. In blueschists and eclogites, main Li-hosting minerals are glaucophane (48-319 ppm), omphacite (27-145 ppm) and phengite (4.4-39 ppm).

Retrograde barroisite and actinolite formed after glaucophane have lower Li contents of 3.1-9.6 ppm. Epidote, garnet and accessory titanite have Li abundances of 0.67-41, 0.47-1.5 and 1.3-17 ppm, respectively. In the retrograde greenschists, Li is mainly hosted by chlorite (13-182 ppm), phengite (5.8-46 ppm), and epidote (1.3-31 ppm) (see also El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. Retrograde albite also host significant amounts of Li (41-212 ppm). Values are consistent with the values reported by [START_REF] Marschall | Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks[END_REF] for HP metamorphic rocks of the Greek island of Syros (Cyclades). In micaschists, main Li-hosting minerals are white micas (phengite + paragonite; 28-61 ppm) and retrograde chlorite (150-180 ppm).

Accessory tourmaline (detrital) also contains high amounts of Li (19-47 ppm), while Li abundances are low in garnet, chloritoid, epidote, and accessory titanite and apatite (<5 ppm) (Fig. 5; Table 5).

These results point out the importance of glaucophane and omphacite for the Li budget and isotopic fractionation during prograde and HP-LT metamorphic stages. The Li isotopic composition of glaucophane and omphacite are scattered within each sample, and vary on average from one sample to the other (Fig. 7; Table 6).

Glaucophane in blueschists has positive mean δ 7 Li values of +1.3 ± 8.5 and +1.3 ± 4.3‰ (2σ SD) (Fig. 7a; Table 6). However, δ 7 Li variations are observed within glaucophane grains. Glaucophane core parts have generally positive MORB-like δ 7 Li values (+2.5 ± 3.4 to +3.8 ± 6.1‰; 2σ SD), while the δ 7 Li values decrease rimwards up to negative values (-2.6 ± 4.9 to -0.1 ± 3.6‰; 2σ SD) (Figs. 7b andc; Table 6). No core-to-rim zonation of Li abundances is observed in blueschist GR02. Blueschist GR25a shows a relatively good correlation between individual Li abundances and δ 7 Li values, suggesting kinetic diffusion of Li during glaucophane crystallisation.

In eclogites, glaucophane has negative mean δ 7 Li values of -4.8 ± 2.3 to -2.3 ± 4.4‰ (2σ SD) (Fig. 7a; Table 6). Contrary to glaucophane from blueschists, glaucophane core have negative δ 7 Li values. δ 7 Li values in glaucophane generally decrease from core (-4.5 ± 2.9 to -1.1 ± 3.4‰; 2σ SD) to rim (-6.6 ± 2.8 to -3.8 ± 3.8‰; 2σ SD). Lithium abundances do not vary from core to rim (Figs. 7d-f; Table 6). The absence of core-to-rim zonation of Li abundances and the absence of correlation between the Li contents and Li isotopic compositions indicates that no late Li diffusion along grain boundaries occurred. Omphacite have negative mean δ 7 Li varying from -15.6 ± 9.4 to -3.5 ± 5.2‰ (2σ SD) (Fig. 7a; Table 6), contrasting with the positive δ 7 Li recorded by omphacite in the UHP eclogite from the Limousin ophiolite. No distinct rimward zonation of Li abundances and δ 7 Li values is observed (Fig. 7d-f), excepting omphacite from eclogite GR24a, where δ 7 Li values lower than -20‰ are measured in rim parts (Fig. 7e).

Despite the large 2σ SD uncertainites of the mean δ 7 Li values obtained on the different phases, due to mineral heterogeneities and core-to-rim zonation, fractionation factors between glaucophane, omphacite and whole rock were calculated using these mean δ 7 Li values to determine if the main Li-hosting minerals show isotopic equilibrium with their respective whole rocks. Glaucophane is a major contributor for the whole rock isotopic budget, with 7 Li WR-gln values ranging from -1.9 ± 4.3 to +0.0 ± 2.3‰ (2σ) (Table 6). In eclogites GR21 and 29, omphacite display δ 7 Li values close to its host rock, with 7 Li WR-omph values varying between -0.6 ± 5.2 and -0.2 ± 6.9‰ (2σ), and with bulk glaucophane [ 7 Li gln- omph values of -0.2 ± 7.3 to -0.1 ± 8.3‰ (2σ)] (Table 6), and thus can be considered at isotopic equilibrium, despite large uncertainties due to mineral heterogeneities. However, considering that omphacite have crystallised at the same time as glaucophane rims, the two minerals do not appear to be at isotopic equilibrium with 7 Li gln rim-omph values from -3.1 ± 5.9‰ to -0.5 ± 5.9‰ (2σ), suggesting Li isotope fractionation under HP-LT conditions. At the opposite, omphacite from eclogite GR24a shows isotopic disequilibrium with whole rock and glaucophane, as evidenced by 7 Li WR-omph, 7 Li gln-omph and 7 Li gln rim-omph values of +13.1 ± 9.4, +13.2 ± 10.4‰ and +11.7 ± 8.3‰ (2σ) (Table 6). Retrograde barroisite formed along omphacite rims have mean δ 7 Li values within the same range as omphacite (-15.4

CALCULATION OF THE FLUID Li ISOTOPIC COMPOSITION

The fluid δ 7 Li fluid composition is sensitive to temperature variations in the subduction zone [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF]. Mineral-fluid fractionation factors obtained from experimental studies on Li fractionation between silicates (clinopyroxene: spodumene, staurolite, Li-amphibole and Li-mica) and H 2 O indicate that fluids generally display a higher δ 7 Li fluid relative to rocks [START_REF] Wunder | Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids[END_REF][START_REF] Wunder | Lithium isotope fractionation between Li-bearing staurolite, Li-mica and aqueous fluids: an experimental study[END_REF][START_REF] Wunder | Li-isotope fractionation between silicates and fluids: pressure dependence and influence of the bonding environment[END_REF]. In our study, we have employed fractionation factors determined for clinopyroxene ( 7 Li spudomene-fluid = -4.61 × (1000/T) + 2.48; [START_REF] Wunder | Temperature-dependent isotopic fractionation of lithium between clinopyroxene and high-pressure hydrous fluids[END_REF], assuming a similar Li partition behaviour in clinopyroxene and amphiboles (inosilicates) relative to fluid (see also [START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF]. We have also employed the same fractionation factors to calculate the δ 7 Li fluid of the fluid in equilibrium with the whole rocks from the Ile de Groix. The δ 7 Li fluid of the fluid in equilibrium with serpentine (phyllosilicate) was calculated using the fractionation factor determined for micas (phyllosilicates) ( 7 Li mica-fluid = -4.52 × (1000/T) + 4.74; [START_REF] Wunder | Lithium isotope fractionation between Li-bearing staurolite, Li-mica and aqueous fluids: an experimental study[END_REF][START_REF] Wunder | The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation[END_REF]. However, the large range of temperature at which the different minerals were formed is a major source of uncertainty on the fluid isotopic composition.

Limousin

The temperature of hydrothermal of oceanic basalts and serpentine formation is rather poorly defined [START_REF] Mével | Serpentinization of abyssal peridotites at mid-ocean ridges[END_REF]. Ocean-floor serpentinites in ophiolites commonly follow a retrograde reaction and often display the crystallisation sequence antigorite > chrysotile > lizardite with decreasing temperature [START_REF] O'hanley | Serpentinites[END_REF]. Hydrothermal alteration of ultrabasic rocks is assumed to have occurred between 350 and 500°C, because of the presence of tremolitic amphibole and absence of talc in the mineral assemblage [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]. In serpentinite LAU2, δ 7 Li fluid calculated for Mg-rich serpentine and altered Fe-richer serpentine yield undistinguishable negative values of -7.8 to -6.4‰ and -7.6 to -6.2‰, respectively, for temperatures decreasing from 500°C to 350°C (Fig. 8a; Table 7).

Formation of the second generation of serpentine in samples FLOT2a and CLUZ6, evidenced by a rimward increase in MgO, may result from changes in hydrothermal alteration temperature conditions. This may correspond to the formation of different serpentine species: transition from antigorite to chrysotile/lizardite is typical of a temperature decrease during hydrothermal alteration (e.g. [START_REF] Evans | Stability of chrysotile and antigorite in the serpentine multi-system[END_REF][START_REF] Mével | Serpentinization of abyssal peridotites at mid-ocean ridges[END_REF][START_REF] Wunder | The effect of chrysotile nanotubes on the serpentine-fluid Li-isotopic fractionation[END_REF]. XRD measurements in sample FLOT 2b have shown that the three serpentine species are present. Variation in the Mg# may also reflect the incorporation of various small-size minerals within serpentine layers [START_REF] Viti | Mesh textures and bastites in the Elba retrograde serpentinites[END_REF][START_REF] Früh-Green | Serpentinization of oceanic peridotites: Implications for geochemical cycles and biological Activity[END_REF][START_REF] Bach | Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274)[END_REF]: Feoxides, hydroxides or clay minerals in equilibrium with the first generation of serpentine and Mg-hydroxides (brucite) in equilibrium with the second generation. Fe-rich serpentine in serpentinite CLUZ6 has totally replaced olivine, and yield a δ 7 Li fluid value of -1.1 to +0.4‰ for temperatures decreasing from 500°C to 350°C. The δ 7 Li fluid of the fluid in equilibrium with the Mg-richer serpentine (rimming the Fe-rich serpentine) and amphibole (tremolite to Mghornblende) have been estimated to be +5.3 to +6.7‰ and +10.1‰, respectively (Fig. 8a; Table 7). A δ 7 Li fluid value varying between -1.4 and 0.0‰ has been calculated for the Mg-rich serpentine rimming olivine in the serpentinite FLOT2a, at a temperature decreasing from 500°C to 350°C (Table 7).

Based on thermometric calculations, [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF] have determined the temperature of equilibration of amphibole and plagioclase in amphibolites to vary between 572 and 749°C. For this range of temperatures, the δ 7 Li fluid of the fluid in equilibrium with hornblende in sample CLUZ1 varies from +5.5 to +6.4‰ (Fig. 8a; Table 7). The altered zones in amphibole appear to have equilibrated with a heavy-Li fluid (δ 7 Li fluid = +11.8 to +12.7‰). Amphibolite CLUZ4 yields a high δ 7 Li fluid value of +14.5 to +15.5‰ (Table 7).

The UHP zoisite-eclogite L04-143 has recorded peak metamorphic P-T conditions of 3.0 GPa and 600°C (Berger et al., 2010a). The fluid in equilibrium with omphacite has a high δ 7 Li value of +24.7‰ (Fig. 8a; Table 7). Secondary pargasitic amphibole crystallisation occurred during partial retrogression of the rock (Berger et al., 2010a). A δ 7 Li fluid was calculated to be +21.5‰ at 600°C and +22.1‰ at 500°C.

Ile de Groix

The δ 7 Li of the fluid in equilibrium with the metabasites and blueschist-facies micaschists can be calculated using the peak metamorphic temperature conditions for blueschist and eclogite facies rocks (500 and 550°C, respectively; El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. In blueschists, the δ 7 Li of the fluid in equilibrium with the whole rocks have been calculated at 500°C to be +2.9 to +4.2‰ (Fig. 8b; Table 8). The detailed calculation of the δ 7 Li values of the fluid in equilibrium with glaucophane cores (T ~ 450°C) and glaucophane rim (T ~ 500°C) shows that the δ 7 Li values of the fluids decreased from +6.4-7.7 to +0.9-3.4‰ with increasing temperature during glaucophane formation, i.e. during the prograde metamorphism. The δ 7 Li fluid of the fluid in equilibrium with the blueschist-facies micaschists have been calculated to vary between +1.8 to +3.7‰, at a temperature assumed to be 500°C (Table 8). These values are typical of subduction fluids with a mixed composition between altered oceanic crust and Global Subducting Sediment (GLOSS) (+7 ± 5‰; [START_REF] Simons | Oxygen isotope heterogeneities and diffusion profile in composite metamorphic-magmatic garnets from the Pyrenees[END_REF].

In eclogites, the δ 7 Li fluid of fluids in equilibrium with whole rocks are -1.6 to +0.7‰, indicating equilibrium with a fluid with a heavier Li isotopic composition than with blueschists (Fig. 8b; Table 8). The fluid in equilibrium with glaucophane cores (T ~ 500°C) has δ 7 Li fluid values of -1.1 to +2.4‰. The δ 7 Li fluid of fluids in equilibrium with glaucophane rims (δ 7 Li fluid -3.5 to -0.7‰) becomes Li-heavier with increasing temperature up to 550°C. The δ 7 Li of fluids in equilibrium with omphacite vary between -1.4 and -0.4‰ for eclogites GR21 and 29 (Fig. 8b; Table 8). Omphacite GR24a in eclogite yields a negative δ 7 Li fluid of -12.4‰, which is lower than that of whole rock (-2.5‰; Table 4), suggesting intensive fluid migration and metasomatism under HP conditions and quartz vein formation [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF]; see also Fig. 2f).

Retrogression in the greenschist facies occurred under P-T conditions lower than 1.0 GPa and 450°C [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF], 2013). In greenschists, the δ 7 Li fluid was calculated to be -0.6 to +7.1‰ at a temperature of 450°C (Fig. 8b; Table 8). In the most retrogressed sample (albitic greenschist GROA52), the δ 7 Li fluid was estimated to be +5.8‰ at 400°C. The results highlight heavy-Li fluid migration during retrogression.

DISCUSSION

Lithium behaviour and fractionation during hydrothermal alteration of the ocean crust: the case of Limousin ophiolite

The studied serpentinites from the Limousin ophiolite have low Li concentrations (0.9-4.6 ppm), as commonly observed for ocean floor serpentinites and serpentinised peridotites [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF][START_REF] Scambelluri | The fate of B, Cl and Li in subducted oceanic mantle and in the antigorite breakdown fluids[END_REF][START_REF] Vils | The Lithium, Boron and Beryllium content of serpentinized peridotites from ODP Leg 209 (Sites 1272A and 1274A): Implications for lithium and boron budgets of oceanic lithosphere[END_REF]. In the studied serpentinites from Saint-Laurent (probably deriving from dunites), the Li content in serpentine increases in the Fe-rich serpentine generation, while the δ 7 Li values remain constant (c. -8.9‰). Even if lithium in abyssal serpentinites is generally heavy with positive whole rock δ 7 Li values, serpentinised oceanic peridotites may display a large range of δ 7 Li values from -19 to +28‰ [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF][START_REF] Benton | Lithium abundance and isotope systematics of forearc serpentinites, Conical Seamount, Mariana forearc: Insights into the mechanics of slab-mantle exchange during subduction[END_REF]. Serpentine formed early during hydrothermal alteration of oceanic peridotites preferentially incorporates 6 Li [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF]. The low Li abundances in serpentinites and negative δ 7 Li values recorded by serpentine indicate that hydrothermal alteration occurred in equilibrium with a Li-poor fluid during the early stages of hydrothermal alteration, such as seawater or hydrothermal vent fluids [START_REF] Chan | Variation of lithium isotope composition in the marine environment: A preliminary report[END_REF].

Hence, Li is leached from the basic oceanic crust by the passing fluid, resulting in a Li recycling during hydrothermal alteration of ultrabasic rocks [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF]. At a constant temperature, Mg-rich serpentine and Fe-richer altered zones should have equilibrated with a fluid with a similar δ 7 Li, varying from -7.6 to -6.4‰ (Fig. 8a).

The studied serpentinites from La Flotte and Le Cluzeau (deriving from harzburgites or troctolites) have Li contents slightly lower than MORB suggesting only a minor Li loss during hydrothermal alteration. Lithium is mainly distributed between olivine, serpentine and amphibole (Fig. 3). During hydrothermal alteration, Li abundances slightly increase from olivine to the replacing Fe-rich serpentine and decrease in the Mg-rich serpentine generation, i.e. with decreasing temperature. The δ 7 Li values also vary during hydrothermal alteration. In serpentinite FLOT2a, the δ 7 Li values decreased from olivine relicts to Mg-rich serpentine from +6.3 to -2.5‰ (Fig. 4a). In sample CLUZ6, the Fe-rich serpentine that has totally replaced olivine has a value similar to Mg-rich serpentine in sample FLOT2a (-2.16‰). In sample CLUZ6, the δ 7 Li values increase in the second generation of serpentine (Mg-rich serpentine: +4.2‰) and amphibole (+6.6‰), while the Li abundances decrease.

Consequently, the chemistry of serpentine does not seem to be the only factor controlling Li fractionation.

The negative δ 7 Li recorded by serpentine formed after olivine indicates that hydrothermal alteration occurred in equilibrium with a fluid with a δ 7 Li fluid value between -1.4 to +0.4‰ at a temperature lower than 500°C (Fig. 8a). The low δ 7 Li of serpentine formed after olivine can be explained by fluid-induced kinetic fractionation during the early stage of hydrothermal alteration [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF]. 6 Li is preferentially diffusing from the fluid phase to the rock during fluid-rock interactions due to its higher diffusivity compared to 7 Li [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF]. The higher δ 7 Li values and lower Li abundances recorded by the Mgrich serpentine and amphibole in sample CLUZ6 may reflect a change in the physicochemical conditions of fluid-rock interactions (e.g. fluid source, temperature, redox conditions). According to [START_REF] Decitre | Behavior of Li and its isotopes during serpentinization of oceanic peridotites[END_REF], the higher δ 7 Li values recorded by serpentine formed during a later stage of hydrothermal alteration reflect derivation from more evolved hydrothermal fluids, the latter becoming heavier during their passage through the oceanic crust.

In amphibolites from the Limousin ophiolite, hornblende is the main Li-bearing phase.

Thus, its mean δ 7 Li value mirrors the whole rock δ 7 Li value. Hornblende display δ 7 Li values of +3.5 to +12.5‰, typical of Li isotopic compositions of hydrothermally altered deep oceanic basic rocks (sheeted dykes and gabbros: -1.7 to +7.9‰; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF]. Hornblende in the metamorphosed dyke CLUZ1 is isotopic lighter than in metagabbro CLUZ4. This result is in agreement with data obtained on the Troodos ophiolite (Cyprus), which show that the uppermost part of the altered oceanic crust is isotopically lighter than the deeper part, and probably reflect variable secondary mineral precipitation with depth during hydrothermal alteration [START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]. The δ 7 Li fluid values of the fluid in equilibrium with amphibolites range from +5.5 to 15.5‰, indicating interaction with heavy-Li fluids -seawater or upwelling hydrothermal fluids -under high-T to low-T hydrothermal conditions. Actually, samples that have incorporated a seawater Li component during low-T hydrothermal alteration have a δ 7 Li value of c. +10 ± 2‰ [START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]. However, the MORB-like Li abundances in amphibolites (3.1-8.2 ppm) reflect limited fluid-rock interactions during hydrothermal alteration [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF].

Even if serpentinites play a major role in fluid-mobile element recycling processes during subduction [START_REF] Scambelluri | The fate of B, Cl and Li in subducted oceanic mantle and in the antigorite breakdown fluids[END_REF][START_REF] Kodolányi | Geochemistry of ocean floor and fore-arc serpentinites: constraints on the ultramafic input to subduction zones[END_REF], our results support that abyssal serpentinites are not significant Li-carrier rocks in the subduction zone. Serpentinites dehydration would release Li-poor fluids with variable δ 7 Li, and does not constitute an important Li source for Li-rich HP-LT metamorphic rocks (see also [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF]. Dehydrating altered amphibolites would release Li-richer fluids with higher δ 7 Li than serpentinised rocks. Therefore, the high Li content in subducted metabasites can only be explained if a Li-rich sedimentary source is involved [START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF].

Lithium fractionation during subduction zone metamorphism

Early subduction hydrothermal processes in the samples from the Ile de Groix

The high whole rock Li contents in the metabasic rocks of the Ile de Groix (16-124 ppm) compared to fresh MORB (3-8 ppm; [START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF] indicates that Li enrichment results from fluid overprint during pre-HP low-T hydrothermal processes. Lithium concentrations in the metabasites of the Ile de Groix are significantly higher than those of the Limousin ophiolite (altered oceanic crust) or altered MORB (ODP Sites 504B and 896A; 1-35 ppm; [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF] (Fig. 5). However, Li contents of metabasites are similar to those of altered oceanic crust in the Troodos ophiolite (Cyprus), which range from 3 to 119 ppm [START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]. The uppermost part of the altered oceanic crust, i.e. the part in contact with the overlying sediments (mostly composed of limestones), shows the highest Li contents and ligher δ 7 Li. The changes in Li compositions were interpreted as the result of changes in the secondary minerals with depth, rather than interaction with sediment-derived pore fluids [START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF].

By contrast, the high Li abundances in the metabasites of the Ile de Groix, as well as the large volume of metapelites in contact with the metabasites, would suggest a sedimentary source in addition to seawater for Li. Subducted sediments (such as the GLOSS) contain high amounts of Li (2-75 ppm; [START_REF] Bouman | Lithium inputs to subduction zones[END_REF] and may release Lirich fluids in the subduction zone due to the prograde breakdown of Li-rich chlorite into garnet + H 2 O [START_REF] Bebout | Rb, Ba, Sr) into subduction zones: SIMS evidence from micas in high-P/T metasedimentary rocks[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF]. The high Li abundances measured in glaucophane indicates Li enrichment during the early stages of subduction, i.e. until the lawsonite-blueschist facies were reached at P-T conditions lower than 1.0-2.0 GPa and 350-450°C [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. Contrary to the Fe isotope data from the same series of samples, which only show slight Fe isotope fractionation during low-temperature hydrothermal alteration (El Korh et al., 2017a), Li is strongly sensitive to hydrothermal processes. However, the Li compositions of the metabasites of the Ile de Groix do not follow the common fractionation trend of low-temperature hydrothermal alteration observed in the altered oceanic crust (increase of Li contents and increase of the δ 7 Li value; e.g. [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Woodland | Lithium abundances in eclogite minerals: a clue to a crustal or mantle origin?[END_REF].

Blueschist to eclogite facies metamorphism in the samples from the Ile de Groix: effects of kinetic isotope fractionation

The δ 7 Li decrease from the blueschist to the eclogite facies metabasites of the Ile de Groix do not correlate with the large range of Li abundances (Figs. 6a, andb), indicating that Li may have fractionated during the subduction zone metamorphism. Enrichment of Li coupled to a δ 7 Li decrease in orogenic eclogites and HP-LT rocks after the onset of subduction is a common feature [START_REF] Zack | Extremely light Li in orogenic eclogites: the role of isotope fractionation during dehydration in subducted oceanic crust[END_REF][START_REF] Marschall | Partitioning and budget of Li, Be and B in high-pressure metamorphic rocks[END_REF][START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Agostini | Drying and dying of a subducted slab: coupled Li and B isotope variations in Western Anatolia Cenozoic Volcanism[END_REF][START_REF] Penniston-Dorland | Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA[END_REF], 2012;[START_REF] Simons | Oxygen isotope heterogeneities and diffusion profile in composite metamorphic-magmatic garnets from the Pyrenees[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF][START_REF] Xiao | Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China[END_REF][START_REF] Romer | Lithium and boron isotopic fractionation in sedimentary rocks during metamorphism -The role of rock composition and protolith mineralogy[END_REF]. The pioneering study of [START_REF] Zack | Extremely light Li in orogenic eclogites: the role of isotope fractionation during dehydration in subducted oceanic crust[END_REF] argued that light Li signatures of eclogites from Trescolmen (Adula nappe, Central Alps, Switzerland) do not originate from the surrounding metasediments, but follow a Rayleigh distillation during dehydration of clays and/or chlorite during the early stages of subduction zone metamorphism. However, based on experimental and natural data, [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF] have shown that the high Li concentrations coupled to negative δ 7 Li values observed in most eclogites do not follow a Rayleigh dehydration process. Thus, Li isotope fractionation is the result of kinetic fractionation during eclogitisation and/or exhumation [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF]. The changes in the Li isotope ratios in the subducted rocks is coupled to a significative mobilisation of Li during dehydration reactions, with a loss of 40 to 60% of the initial whole rock Li content [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF]Beinlich et al., 2010).

During dehydration reactions, the fluid regime and rock porosity evolves continuously. [START_REF] John | Trace-element mobilization in slabs due to non steady-state fluid-rock interaction: Constraints from an eclogite-facies transport vein in blueschist (Tianshan, China)[END_REF]Taetz et al., 2018). The change in the fluid regime allows diffusive transport of Li from the unfractured rocks to the wall rock and vein, and thus kinetic isotope fractionation during fluid-induced eclogitisation [START_REF] John | Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs[END_REF][START_REF] Jonas | The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation[END_REF]Taetz et al., 2018).

Fluids released by dehydration processes of pelitic sediments and basic rocks of the Ile de Groix were channelised during the HP-LT event, as shown by the large vein network developed in the metamorphic rocks [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF]. Fluid-rock interactions mainly involved internally-derived fluids, or fluids derived from the deeper part of the subducted crust and equilibrated with their host rocks. Most of the studied massive blueschists and eclogites do not crop in contact with large veins, and did not interact with the passing fluids owing to their low permeability and porosity [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF], 2013). Consequently, the whole rock δ 7 Li decrease from MORB-like values in blueschists to negative values in eclogites (Fig. 6a) during HP-LT metamorphism may be interpreted as the result of progressive fluid-induced Li kinetic diffusion and isotope fractionation during dehydration reactions already under prograde epidote-blueschist facies P-T conditions, without involving high fluid fluxes (Fig. 6a). The importance of the transition from blueschist facies to eclogite facies is also evidenced by the results of [START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF], who showed that zoisiteeclogites from the Raspas Compex (Ecuador) experienced more intensive metasomatic effects than blueschists. Li diffusion on the intragrain scale and along grain boundaries is triggered by mineral appearance and disappearance related to metamorphic reactions and fluid-related dissolution-precipitation processes [START_REF] Jonas | The role of grain boundaries and transient porosity in rocks as fluid pathways for reaction front propagation[END_REF]Taetz et al., 2018). In blueschists, the δ 7 Li decreases from MORB-like values in glaucophane core to negative values in rims (Figs. 7b andc). Thus, the metabasites were able to conserve a MORB-like isotopic signature after the onset of subduction, i.e. until blueschist facies conditions were reached, even if Li was enriched during the pre-HP hydrothermal processes.

The metamorphic minerals (glaucophane and omphacite) carry high Li contents in the subduction zone, owing to their large stability field during the prograde metamorphism, from -f). As observed for blueschists, the 7 Li WR-gln , 7 Li WR- omph values and 7 Li gln-omph of -0.6 ± 5.2 to 0.0 ± 2.3‰ in eclogites GR21 and 29 indicate that bulk glaucophane and bulk omphacite are relatively at isotopic equilibrium with their host rock. However, the negative 7 Li gln rim-omph values from -3.1 ± 5.9 to -0.5 ± 5.9 reveal Li isotope fractionation under peak metamorphic P-T conditions. For only one sample (eclogite GR24a), omphacite has a low δ 7 Li compared to the whole rock and glaucophane, resulting from metasomatic processes under eclogite facies P-T conditions, as suggested by the presence of HP quartz veins [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF]. Retrograde barroisite formed along omphacite rims have a mean δ 7 Li value similar to that of omphacite, but exhibits a large spread of δ 7 Li values, suggesting localised Li mobilisation and isotope fractionation during early retrogression. The calculated mean δ 7 Li fluid of fluids in equilibrium with dehydrating metabasic and metapelitic rocks decrease from +8 to -2‰ with increasing P-T conditions, i.e. from the lawsonite-blueschist facies (1.0-2.0 GPa, 350-450°C) to the eclogite facies (1.6-2.5 GPa, 500-600°C) (Fig. 8b), even if the δ 7 Li fluid values remain higher than the corresponding δ 7 Li solid (see also [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF].

Consequently, our study argues that the δ 7 Li decrease during subduction zone prograde metamorphism results from cumulative effects of dehydration and fluid-induced Li kinetic diffusion and isotope fractionation.

Our data agree with the δ 7 Li fluid decrease in dehydrating metasediments of high metamorphic grade (lawsonite-blueschist and epidote-amphibolite facies; 275-750°C; 0.5-1.2 GPa; [START_REF] Bebout | Metasomatism during subduction: products and possible paths in the Catalina Schist, California[END_REF] from the Catalina Schist [START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF].

The highest δ 7 Li fluid (+6.4 to +7.7 ‰; Fig. 8b) at equilibrium with glaucophane cores in blueschists (lawsonite-blueschist facies) plot within the field of the δ 7 Li fluid recorded by lawsonite-blueschist and amphibolite facies metabasites from the Catalina Schist (+7 to +13‰; [START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF]. However, the δ 7 Li fluid of fluids in equilibrium with the metabasites and metapelites of the Ile de Groix of higher metamorphic grade (epidoteblueschist to eclogite facies; δ 7 Li fluid of -3.5 to +4.2‰; Fig. 8b) are generally lower than the δ 7 Li fluid of the fluids derived from the metasedimentary and metabasic rocks of the Catalina Schist (c. +5-18‰; [START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF], but plot within the field of kinetically altered amphibolites (0 to +4‰; [START_REF] Penniston-Dorland | Lithium and its isotopes as tracers of subduction zone fluids and metasomatic processes: Evidence from the Catalina Schist, California, USA[END_REF]. The heavy-Li fluids in equilibrium with blueschist-facies metabasites and micaschists are typical of subduction fluids with a mixed composition between altered oceanic crust and GLOSS (+7 ± 5‰; [START_REF] Simons | Oxygen isotope heterogeneities and diffusion profile in composite metamorphic-magmatic garnets from the Pyrenees[END_REF]. The release of these fluids will result in a decrease of the δ 7 Li solid, and could be at the origin of the low δ 7 Li values of eclogites and fluids at equilibrium with them.

Fluids in equilibrium with eclogites have a lower δ 7 Li fluid than subduction zone fluids. It is suggested that the low δ 7 Li fluid under eclogite facies conditions may not reflect their real δ 7 Li fluid value, and would result from kinetic isotope fractionation that becomes predominant over equilibrium fractionation with increasing metamorphic grade and temperature [START_REF] Richter | Isotope fractionation by chemical diffusion between molten basalt and rhyolite[END_REF][START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF].

Li isotope fractionation during greenschist facies retrograde metamorphism related to exhumation of the Ile de Groix

During exhumation of the rocks from the Ile de Groix, the degree of retrogression of metabasites is directly related to the free fluid phase, i.e. to the porosity. During the early stages of exhumation (<1.6 GPa, 450-500°C; El [START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], retrograde reactions occured under restricted fluid-rock interactions and involved internally-derived fluids [START_REF] El Korh | Trace element and isotopic fingerprints in HP-LT metamorphic rocks as a result of fluid-rock interactions (Ile de Groix, France)[END_REF]. The similar Li abundances and δ 7 Li values of blueschists and eclogites (-4.8 to +0.7‰) and greenschists (-4.5 to -0.1‰) that underwent restricted fluid-rock interactions during retrogression (δ 7 Li fluid of -0.6 and +3.8‰ for samples GR23 and 25b, respectively; Fig. 8b), indicate that fluid-induced metasomatism was not able to reset the Li isotope distribution. During the advanced stages of retrogression, the permeability and porosity of the metabasites increase with the decrease of the P-T conditions (<1.0 GPa, <400-450°C), allowing migration of higher amounts of fluids derived from the dehydrated metabasites [START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF], 2013). The increase of the δ 7 Li values toward positive values in the most retrogressed samples (+3.2 and +1.5‰ for samples GR06a and GROA52, respectively) highlights a Li mineral/fluid isotopic exchange and migration of fluids with a heavier-Li 2011) observed heavier Li isotopic compositions in retrograde amphibolites (-0.4 ± 0.3 to +0.9 ± 0.2‰) compared to neighbouring UHP eclogites (-6.9 ± 0.2 to -6.5 ± 0.1‰) from the Chinese Continental Scientific Drilling Program (Sulu, East China).

The authors suggest that Li budget and isotope fractionation is mostly controlled by the mineral assemblage, as significant fractionation occur both in closed and open fluid-rock systems [START_REF] Xiao | Fluid/rock interaction and mass transfer in continental subduction zones: constraints from trace elements and isotopes (Li, B, O, Sr, Nd, Pb) in UHP rocks from the Chinese Continental Scientific Drilling Program, Sulu, East China[END_REF].

Our data confirm that Li isotopes are strongly sensitive to retrograde fluid-rock interactions related to exhumation of HP rocks. Significant Li isotope fractionation during rehydration reactions in open system result in apparent Li compositions that do not reflect the subduction signature anymore. A similar behaviour has also been reported for Ge isotopes [START_REF] El Korh | Investigation of Ge and Ga exchange behaviour and Ge isotopic fractionation during subduction zone metamorphism[END_REF]: rehydration reactions triggered Ge isotope fractionation towards heavier compositions in the most retrogressed greenschists, while the least retrogressed samples (closed system) can retain the Ge isotope signature of high grade facies metabasites.

Li isotope fractionation in the Limousin UHP zoisite-eclogite

The Li abundances and isotopic data in the UHP zoisite-eclogite from the Limousin significantly contrast with the results obtained for the Ile de Groix. The UHP zoisite eclogite, deriving from an iron-rich plagioclase cumulate such as those emplaced in continental intrusions or forming Fe-Ti oceanic gabbros (Berger et al., 2010a), has a Li abundance slightly lower than MORB (Fig. 3). Garnet and zoisite are the most abundant minerals, but do not play an important role in the Li budget due to their low Li contents (Fig. 3). Omphacite and pargasitic amphibole are the main hosts for Li, but display lower Li contents than in eclogites from the Ile de Groix. However, they show a heavy-Li signature typical of lowtemperature altered oceanic crust [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF], with δ 7 Li values considerably higher than those of the Ile de Groix (Figs. 4 and6).

The positive δ 7 Li value recorded by omphacite agrees with the model of Li fractionation determined by [START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF] predicting that deeply subducted eclogites have a heavier Li signature than the mantle. However, the abnormally high δ 7 Li recorded by omphacite probably reflects an interaction with a heavy-Li fluid migrating in the subduction zone (Fig. 8a). Zoisite-eclogites have been intensively metasomatised during the UHP event, by interaction with a fluid derived from dehydrating subducted sediments at the interface slab/mantle (Berger et al., 2010a). Extremely heavy-Li isotopic signatures related to high Li mobility are characteristic of upwelling fluids and sediments in subduction zone decollements [START_REF] Chan | Lithium isotopic compositions of pore fluids and sediments in the Costa Rica subduction zone: implications for fluid processes and sediment contribution to the arc volcanoes[END_REF]. However, the low Li abundances do not argue for any Li transport and enrichment from the fluid phase, but may reflect Li leaching and preferential 6 Li transfer towards the passing fluid. The decrease of the δ 7 Li value in secondary pargasitic amphibole (from +21.9‰ in omphacite to +18.7‰ in amphibole) indicates interaction with a Li-richer and lighter fluid during partial retrogression (Figs. 4b and8a).

Muscovite and quartz in pegmatites from Black Hills (South Dakota, USA) also exhibit heavy-Li isotopic signatures (up to +21.3‰) and result from crystal melt fractionation (Teng et al., 2006). The high δ 7 Li values in omphacite from the UHP zoisite-eclogite may also reflect interaction with crustal-derived fluids during subduction or, more unlikely a crustal contamination during differentiation of the magmatic protolith (see Berger et al., 2010a for discussion of the protolith origin). In any case, the zoisite-eclogite L04-143 derives from a protolith of a different composition than the MORB-derived rocks of the Ile de Groix, and have undegone a different metamorphic history during subduction.

SYNTHESIS AND CONCLUSIONS

This study provides bulk and in-situ analyses to reconstruct Li behaviour and isotope fractionation during the various stages of fluid-rock interactions occurring in the ancient Variscan oceanic lithosphere, including seafloor high-T and low-T hydrothermal alteration, subduction and exhumation processes. 2) Li concentrations in the metabasites of the Ile de Groix are significantly higher than in fresh MORB. The elevated Li content in the subducted metabasites of the Ile de Groix can only be explained by a metasomatic overprint of fluids derived from Li-rich sediments during the early stages of subduction, as observed in other orogenic eclogites. Lithium abundances remain high in the metabasites of the Ile de Groix during subduction zone metamorphism, owing to the large stability field of Li-hosting minerals from the lawsonite-blueschist to the eclogite facies, and in retrograde greenschist facies rocks. Lithium is mainly hosted by 1) glaucophane and omphacite in blueschists and eclogites; 2) chlorite and albite in retrograde greenschists; 3) phengite and chlorite in micaschists.

3) The metabasites of the Ile de Groix have whole rock δ 7 Li values decreasing from blueschists to eclogites. Both bulk and in-situ δ 7 Li data clearly demonstrate that Li isotopes fractionate under blueschist facies conditions in metabasic rocks, i.e. before eclogite facies conditions are reached in the subduction zone. Glaucophane in blueschists have δ 7 Li values decreasing rimwards from positive δ 7 Li values to negative values, indicating that significant Li 6) Lithium isotope fractionation in blueschists and in the most retrogressed greenschists from the Ile de Groix highlights the migration of heavy-Li fluids along the oceanic crust-mantle interface in the subduction zone. As predicted by former models of Li fractionation in subduction zones [START_REF] Elliott | The terrestrial Li isotope cycle: light-weight constraints on mantle convection[END_REF][START_REF] Elliott | Lithium isotope evidence for subduction-enriched mantle in the source of mid-ocean-ridge basalts[END_REF][START_REF] Marschall | The lithium isotopic composition of orogenic eclogites and deep subducted slabs[END_REF][START_REF] Halama | A stable (Li, O) and radiogenic (Sr, Nd) isotope perspective on metasomatic processes in a subducting slab[END_REF], our data suggest that migration of heavy-Li fluids along the crust-mantle wedge may be responsible for an enrichment of the fore-arc mantle wedge along the subduction channel. [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF][START_REF] Ballèvre | Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French-Spanish project[END_REF]. The Limousin ophiolite and the Ile de Groix HP terrane are parts of the Middle Allochthon domain and belong to the series of ophiolites recognised along the Galicia-Brittany-French Massif Central suture zone. The latter was interpreted as the remnant of a narrow ocean between Gondwana and Armorica [START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF] or a late-Cambrian active margin setting along the Gondwana [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]. (b) Geological map of the studied area in the Limousin ophiolite (modified after [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]Berger et al., , 2010a[START_REF] Berger | Origin and tectonic significance of corundum-kyanite-sapphirine amphibolites from the Variscan French Massif Central[END_REF]. c) Geological map of the Ile de Groix (after [START_REF] Audren | Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013). *The beach "Plage des Grands Sables" has moved with the oceanic currents. FMC: French Massif Central; VM: Vosges Massif; BF: Black Quartz veins parallel to the main schistosity are also present. Mineral abbreviations are from [START_REF] Kretz | Symbols for rock-forming minerals[END_REF]. "Sympl" = Albite and barroisite symplectites. The Li composition of fresh MORB is from [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF] and [START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF]. (1987) and [START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF]. The Li isotopic composition is from [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF], [START_REF] Bouman | Lithium inputs to subduction zones[END_REF]Tomascak et al. (2008).

WR: whole rock

Figure 5. Variation in Li abundances in whole rocks and minerals from the Ile de Groix HP terrane. The Li composition of fresh MORB is from [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF] and [START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF]. The Li composition of altered MORB is from [START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF], [START_REF] Bouman | Lithium inputs to subduction zones[END_REF] and [START_REF] Coogan | The role of low-temperature (offaxis) alteration of the oceanic crust in the global Li-cycle: Insights from the Troodos ophiolite[END_REF]. BS: blueschists; Ecl: eclogites; GS: greenschists; MS: micaschists; Micas: phengite + paragonite [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF][START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF][START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF]Tomascak et al., 2008) and belong to the series of ophiolites recognised along the Galicia-Brittany-French Massif Central suture zone. The latter was interpreted as the remnant of a narrow ocean between Gondwana and Armorica [START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF] or a late-Cambrian active margin setting along the Gondwana [START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]. (b) Geological map of the studied area in the Limousin ophiolite (modified after [START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]Berger et al., , 2010a[START_REF] Berger | Origin and tectonic significance of corundum-kyanite-sapphirine amphibolites from the Variscan French Massif Central[END_REF]. c) Geological map of the Ile de Groix (after [START_REF] Audren | Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013). *The beach "Plage des Grands Sables" has moved with the oceanic currents. of fresh MORB [START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF][START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF][START_REF] Chan | Lithium and lithium isotope profiles through the upper oceanic crust: a study of seawater-basalt exchange at ODP sites 504b and 896a[END_REF][START_REF] Bouman | Lithium inputs to subduction zones[END_REF]Tomascak et al., 2008)

  des Roches et Minéraux; CRPG, Nancy). 1 uncertainties are 2%, 5%, 10 % and 20% for abundances of >100 ppm, 50-100 ppm, 10-50 ppm and 0.5-10 ppm, respectively.

  from the GeoRem database (Max Planck Institut für Chemie, Mainz, Germany; http://georem.mpch-mainz.gwdg.de).

  the 1-day 2σ external reproducibility. This agrees with the observation ofSiron et al (2017Siron et al ( , 2018) ) for δ 18 O, H 2 O, F and Cl in-situ measurement by SIMS on biotite.

  2 ± 0.4‰ (2σ SE), consistent with the δ 7 Li values of subducted sediments (mean value: +3.01‰, Chan et al. (2006); -2.3 to +6.8‰, Penniston-Dorland et al., 2012; -6 to +14.5‰, Romer & Meixner, 2014) and upper continental crust (0 ± 2‰; Teng et al., 2004).

First

  , fluids migrate in small scale and form dehydration veins (= closed system; slow and low fluid fluxes). With increasing rock porosity during metamorphic reactions, the fluid regime evolves to channelised fluid flow (or transport veins; fast and high fluid fluxes) carrying externally derived fluids in open system on the outcrop scale

  lawsonite-blueschist facies (1.0-2.0 GPa, 350-450°C; El[START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF] to eclogite facies P-T conditions (1.6-2.5 GPa, 500-600°C; El[START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF]. The high Li content in glaucophane indicate that glaucophane is the main mineral controlling Li fractionation during prograde blueschist facies metamorphism up to HP-LT peak metamorphic conditions.Lithium isotope fractionation occurred during the prograde dehydration with increasing temperature conditions, probably at the transition from the lawsonite-blueschist facies (= glaucophane core crystallisation) to the epidote-garnet-blueschist facies (= glaucophane rim formation). The blueschists from the Ile de Groix contain frequent lawsonite pseudomorphs. Lawsonite breakdown during prograde metamorphism produces large amounts of fluids, which may trigger fluid-induced Li kinetic diffusion and isotope fractionation already under lower grade conditions (transition from lawsonite-blueschist to epidote-blueschist facies). The decrease of the δ 7 Li values may be associated to Li enrichment in glaucophane rims (sample GR25a; Fig. 7c) during metasomatic processes. Omphacite and glaucophane in eclogites have lower δ 7 Li values than MORB. The rimward δ 7 Li decrease in glaucophane indicates that fluid-induced kinetic Li diffusion still

  .8 to +7.6‰; Fig.8b) during rehydration reactions. Our results agree with[START_REF] Penniston-Dorland | Lithium isotopes as a tracer of fluids in a subduction zone mélange: Franciscan Complex, CA[END_REF], who observed retrograde reactions related to an infiltration of an aqueous fluid with high δ 7 Li fluid in HP rocks from the Franciscan Complex (California, USA). Similarly,Xiao et al. (

  the Limousin ophiolite, high-T hydrothermal alteration of ultrabasic rocks produces serpentinites with a Li concentration lower than 5 ppm during interaction with hydrothermal fluids and/or seawater. Serpentine formed during the early stages of alteration has a low δ 7 Li. The δ 7 Li increase in serpentine formed during the late stages of hydrothermal alteration indicates changes in the fluid δ 7 Li fluid composition and, probably, a temperature decrease from high-T to low-T conditions. Amphibolites display Li contents and isotopic compositions typical of hydrothermally altered sheeted dykes and gabbros. The high δ 7 Li values of amphibolites indicate an interaction with heavy-Li fluids -seawater or upwelling hydrothermal fluids -under high-T conditions. These results confirm that Li-poor abyssal serpentinites do not constitute an important source of Li for Li-rich metabasic rocks during subduction zone metamorphism.

  the metabasites of the Ile de Groix occurred at the transition from the lawsonite-blueschist facies to the epidote-garnet-blueschist facies. Lawsonite breakdown releases high amounts of fluids, which can trigger fluid-induced kinetic fractionation of Li. In the eclogites facies metabasites from the Ile de Groix, omphacite and glaucophane have lower δ 7 Li values than glaucophane in blueschists, indicating that fluid-induced kinetic Li diffusion carried on during eclogitisation. However the calculated δ 7 Li fluid at equilibrium may be not representative, because of the possible predominance of kinetic isotope fractionation compared to equilibrium fractionation. Calculation of the fluid δ 7 Li values in equilibrium with the metabasites and metapelites of the Ile de Groix reveals that the dehydrating metabasites release heavy-Li fluids typical of subduction fluids. The fluid δ 7 Li values in equilibrium with metabasites decrease with increasing metamorphic grade up to eclogite facies conditions. Overprint of a fluid under HP conditions with an extremely low δ 7 Li value is evidenced by omphacite in one eclogite sample. However, the δ 7 Li of fluids under epidote-blueschist and eclogite facies may not be representative of the real composition because of the predominance of kinetic isotope fractionation compared to equilibrium fractionation with increasing temperature conditions. 4) The low Li abundances and high δ 7 Li values recorded by omphacite and pargasitic amphibole in the UHP zoisite-eclogite from the Limousin significantly contrast with the results obtained for the Ile de Groix. This probably reflects Li leaching and Li isotopic fractionation during intensive metasomatism by a fluid derived from dehydrating sediments. It may also reflect crustal contamination in the protolith composition. 5) Retrograde greenschists from the Ile de Groix have whole rock Li abundance and δ 7 Li values within the same range as blueschists and eclogites. Therefore, the increase of the δ 7 Li values toward positive values in the most retrogressed samples highlight a Li mineral/fluid isotopic exchange during rehydration reactions and interaction with a Li-heavy fluid.

  into the role of sediment-derived fluids during subduction. Geochimica et Cosmochimica Acta 74, 3621-3641.Siron, G., Baumgartner, L. P.,Bouvier, A.-S., Putlitz, B. & Vennemann, T. (2017). Biotite reference materials for secondary ion mass spectrometry 18 O/ 16 O measurements.Geostandards and GeoanalyticalResearch 41, 243-253. Siron, G., Baumgartner, L. P., Bouvier, A.-S. & Vennemann, T. (2018). Accurate Measurements of H 2 O, F and Cl Content in Biotite using Secondary Ion Mass Spectrometry. Geostandards and Geoanalytical Research, doi: 10.1111/ggr.12235 Spandler, C. & Hermann, J. (2006). High-pressure veins in eclogite from New Caledonia and their significance for fluid migration in subduction zones. Lithos 89, 135-153. Stampfli, G. M., von Raumer, J. F., Wilhem, C. (2011) The distribution of Gondwana derived terranes in the early Paleozoic. In: Gutiérrez Marco JC, Rábano I, García-Bellido I (eds) The ordovician of the world. Instituto Geológico y Minero de España, Madrid, Cuadernos del Museo Geominero 14, 567-574. Su, B.-X., Gu, X.-Y., Deloule, E., Zhang, H.-F., Li, Q.-L., Li, X.-H., Vigier, N., Tang, Y.-J., Tang, G.-Q., Liu, Y., Pang, K.-N., Brewer, A., Mao, Q. & Ma, Y.-G. (2015). Potential orthopyroxene, clinopyroxene and olivine reference materials for in situ lithium isotope determination. Geostandards and Geoanalytical Research 39, 357-369. Taetz, S., John, T., Bröcker, M., Spandler, C. (2016). Fluid-rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes. Contributions to Mineralogy and Petrology 171, 90. Taetz, S., John, T., Bröcker, M., Spandler, C. & Stracke, A. (2018). Fast intraslab fluid-flow events linked to pulses of high pore fluid pressure at the subducted plate interface. Earth and Planetary Science Letters 482, 33-43. Tang, Y. J., Zhang, H. F., Deloule, E., Su, B. X., Ying, J. F., Xiao, Y. & Hu, Y. (2012). Slabderived lithium isotopic signatures in mantle xenoliths from northeastern North China Craton. Lithos 149, 79-90.

Figure 1 .

 1 Figure 1. (a) General sketch of the Variscan Belt in Western Europe (after Berger et al.,

  Forest; LC: Lizard Complex; AM: Armorican Massif; Py: Pyrénées; ECM: External Crystalline Massifs of the Alps; RM: Rhenish Massif; RHZ: Rheno-Hercynian zone; STZ: Saxo-Thuringian zone; MZ: Moldanubian zone.

Figure 2 .

 2 Figure 2. Photomicrographs of studied samples from the Limousin ophiolite (a-d) and Ile de Groix HP-LT terrane (e-f) in plane-polarised light. (a) Typical assemblage of serpentine, spinel, chlorite and tremolite in the serpentinite LAU2 from Saint-Laurent; (b) Serpentinite CLUZ6 from Le Cluzeau composed of serpentine, spinel, chlorite and tremolite, with relicts of olivine and pyroxene; (c) Fine grained and slightly foliated amphibolite CLUZ1 consisting of hornblende and plagioclase; (d) Plagioclase-amphibole symplectite around a hornblende aggregate in isotropic amphibolite CLUZ 4; (e) Garnet-omphacite-glaucophane-clinozoisite assemblage in eclogite GR 29. Partial retrogression is evidenced by titanite overgrowths around rutile, barroisite overgrowths on the rims of glaucophane and by the presence of barroisite-albite symplectites on the rims of omphacite; (f) Retrogression is more pronounced in eclogite GR 24a: rutile is totally replaced by titanite, barroisite-albite symplectites along omphacite are well developed and garnet is partially altered in chlorite + iron hydroxides.

Figure 3 .

 3 Figure 3. Variation in Li abundances in whole rocks and minerals from the Limousin ophiolite.

Figure 4 .

 4 Figure 4. Variation of the mean δ 7 Li values relative to Li abundances in the minerals from the rocks of the Limousin ophiolite. Amphibole in serpentinites and amphibolites is mainly tremolite and hornblende, while amphibole in the UHP eclogite L04-143 is pargasite. Error bars are 2σ SD. The Li elemental composition of fresh MORB is from Ryan & Langmuir

Figure 6 .

 6 Figure 6. (a) Li isotopic composition of the metabasites and micaschists of the Ile de Groix. The δ 7 Li values decrease from the blueschist facies to the eclogite facies. During retrogression, the δ 7 Li values increase with the intensity of rehydration. (b) δ 7 Li values vs. Li abundances. No correlation is observed with the variation of the metamorphic facies. Error bars for δ 7 Li values are 2σ SE, as the whole rock δ 7 Li is calculated by averaging replicate analyses of the same sample solution.

Figure 7 .

 7 Figure 7. Variations in Li content and isotopic composition of glaucophane and omphacite from the metabasites of the Ile de Groix. (a) Mean δ 7 Li vs. Lithium contents. Mean Li abundances were calculated using the LA-ICPMS data. Mean δ 7 Li values correspond to unweighted average of all in-situ δ 7 Li values measured by SIMS in each sample. Error bars are 2σ SD. The dark grey field represents the Li composition of fresh MORB (Ryan &

  (b-f) Core-to-rim variations of the δ 7 Li values and Li contents in (b-c) blueschists and, (d-f) eclogites. The dotted lines and grey field give the corresponding whole rock and 2σ SE values. Individual Li abundances were calculated based on the mean Li contents obtained by LA-ICPMS and 6+7 Li intensities, as follows: 𝐶(𝐿𝑖) 𝑆𝐼𝑀𝑆 𝑖 = [𝐼(𝐿𝑖) 𝑆𝐼𝑀𝑆 𝑖 𝑥 𝐶(𝐿𝑖) 𝐿𝐴 -𝐼𝐶 𝑀𝑆 ]/𝐶(𝐿𝑖) 𝑆𝐼𝑀𝑆

Figure 8 .

 8 Figure 8. Calculated Li isotope composition of fluids in equilibrium with rocks and minerals in the rocks from: (a) the Limousin ophiolite and, (b) the Ile de Groix. δ 7 Li fluid calculations were made using the clinopyroxene-fluid fractionation factors for amphiboles, omphacite and whole rocks, and the mica-fluid fractionation factor for serpentine (Wunder et al., 2006, 2011): δ 7 Li fluid = δ 7 Li sample -7 Li mineral-fluid . The 2σ uncertainties, represented by the error bars, are identical to those of the mean δ 7 Li of minerals and whole rocks employed for calculation.(a) The δ 7 Li values of the fluids in equilibrium with serpentine were calculated for 350°C and 500°C, as the temperature of hydrothermal alteration is not precisely constrained. (b) It is assumed that glaucophane rims have formed at a temperature 50°C higher than glaucophane core.

  -chl-spl-ol-opx-Fe ox-Fe sulf --barr-ep-phe-qtz-ttn-rt-ap--barr-ep-phe-qtz-ttn-rt-ap--barr-ep-phe-chl-ab-ttn-Fe ox (mt)--ep-phe-chl-ab-ttn-ap-qtz-Fe ox (hm)--ep-phe-act-qtz-chl-ttn-Fe ox (mt)--barr-ep-phe-qtz-ttn-chl-ap-Fe ox (hm, mt) -omph-barr-ep-phe-chl-qtz-Fe ox (hm) -barr-ep-omph-phe-chl-qtz-Fe ox (hm, mt) -omph-barr-ep-phe-chl-qtz-ap-rt-ttn-± ◊ lws -ab-phe-barr-qtz-Fe ox (mt)Grt-ep-chl-ab-barr-phe-act-gln-ttn-rt-Fe ox (mt) -◊ lws [-chl-gln-barr-act-ab-qtz-ap-ttn-rt-Fe ox (mt) -cc-phe-ilm-czo-Fe ox (mt) -qtz-ep-act-barr-cc-ttn-Fe ox (mt) -phe-qtz-ap-pg-rt-chl-Fe ox (hm)(± piemt)-phe-pg-qtz-ap-± ab--ep-qtz-chl-ab-ttn-ap-Fe ox (mt)-pg-gln-chl-qtz-ap-act-rt -ilm [4] BS: blueschist; GS: greenschist; ◊ lws: pseudomorphs after lawsonite * see Figure 1 ** The mineral abreviations are from Kretz (1983) *** [1]

F

  o r P e e r R e v i e w Table 8. Calculated Li isotopic composition of fluids in equilibrium with glaucophane, omphacite and whole metabasic rocks from the Ile de Groix (in ‰to the 2σ standard deviations calculated for the mean δ 7 Li values of the different mineral generations for fluids in equilibrium with glaucophane and omphacite, and to the 2σ standard error of the whole rock δ 7Li values for fluids in equilibrium with the whole rock.

Figure 1 .

 1 Figure 1. (a) General sketch of the Variscan Belt in Western Europe (after[START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF][START_REF] Ballèvre | Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French-Spanish project[END_REF]. The Limousin ophiolite and the Ile de Groix HP terrane are parts of the Middle Allochthon domain and belong to the series of ophiolites recognised along the Galicia-Brittany-French Massif Central suture zone. The latter was interpreted as the remnant of a narrow ocean between Gondwana and Armorica[START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF] or a late-Cambrian active margin setting along the Gondwana[START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]. (b) Geological map of the studied area in the Limousin ophiolite (modified after[START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]Berger et al., , 2010a[START_REF] Berger | Origin and tectonic significance of corundum-kyanite-sapphirine amphibolites from the Variscan French Massif Central[END_REF]. c) Geological map of the Ile de Groix (after[START_REF] Audren | Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013). *The beach "Plage des Grands Sables" has moved with the oceanic currents. FMC: French Massif Central; VM: Vosges Massif; BF: Black Forest; LC: Lizard Complex; AM: Armorican Massif; Py: Pyrénées; ECM: External Crystalline Massifs of the Alps; RM: Rhenish Massif; RHZ: Rheno-Hercynian zone; STZ: Saxo-Thuringian zone; MZ: Moldanubian zone.

Figure 2 .

 2 Figure 1. (a) General sketch of the Variscan Belt in Western Europe (after[START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF][START_REF] Ballèvre | Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French-Spanish project[END_REF]. The Limousin ophiolite and the Ile de Groix HP terrane are parts of the Middle Allochthon domain and belong to the series of ophiolites recognised along the Galicia-Brittany-French Massif Central suture zone. The latter was interpreted as the remnant of a narrow ocean between Gondwana and Armorica[START_REF] Matte | The Variscan collage and orogeny (480-290 Ma) and the tectonic definition of the Armorica microplate: a review[END_REF] or a late-Cambrian active margin setting along the Gondwana[START_REF] Von Raumer | Ediacaran to Cambrian oceanic rocks of the Gondwana margin and their tectonic interpretation[END_REF]. (b) Geological map of the studied area in the Limousin ophiolite (modified after[START_REF] Berger | Ocean-floor hydrothermal metamorphism in the Limousin ophiolites (western French Massif Central): evidence of a rare preserved Variscan oceanic marker[END_REF]Berger et al., , 2010a[START_REF] Berger | Origin and tectonic significance of corundum-kyanite-sapphirine amphibolites from the Variscan French Massif Central[END_REF]. c) Geological map of the Ile de Groix (after[START_REF] Audren | Notice explicative de la feuille Ile de Groix à 1/25000, carte géologique[END_REF][START_REF] Bosse | Ductile thrusting recorded by the garnet isograd from blueschist-facies metapelites of the Ile de Groix, Armorican Massif, France[END_REF][START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF], 2013). *The beach "Plage des Grands Sables" has moved with the oceanic currents. FMC: French Massif Central; VM: Vosges Massif; BF: Black Forest; LC: Lizard Complex; AM: Armorican Massif; Py: Pyrénées; ECM: External Crystalline Massifs of the Alps; RM: Rhenish Massif; RHZ: Rheno-Hercynian zone; STZ: Saxo-Thuringian zone; MZ: Moldanubian zone.

Figure 3 .Figure 4 .

 34 Figure 3. Variation in Li abundances in whole rocks and minerals from the Limousin ophiolite. The Li composition of fresh MORB is from[START_REF] Ryan | The systematics of lithium abundances in young volcanic rocks[END_REF] and[START_REF] Niu | Trace element evidence from seamounts for recycled oceanic crust in the eastern Pacific mantle[END_REF]. WR: whole rock

Figure 5 .ManuscriptFigure 6 .

 56 Figure 5. Variation in Li abundances in whole rocks and minerals from the Ile de Groix HP terrane. The Li composition of fresh MORB is from Ryan & Langmuir (1987) and Niu & Batiza (1997). The Li composition of altered MORB is from Chan et al. (2002), Bouman et al., (2004) and Coogan et al. (2017). BS: blueschists; Ecl: eclogites; GS: greenschists; MS: micaschists; Micas: phengite + paragonite Page 123 of 126

Figure 7 .

 7 Figure 7. Variations in Li content and isotopic composition of glaucophane and omphacite from the metabasites of the Ile de Groix. (a) Mean δ 7 Li vs. Lithium contents. Mean Li abundances were calculated using the LA-ICPMS data. Mean δ 7 Li values correspond to unweighted average of all in-situ δ 7 Li values measured by SIMS in each sample. Error bars are 2σ SD. The dark grey field represents the Li composition of fresh MORB (Ryan & Langmuir, 1987; Niu & Batiza, 1997; Chan et al., 2002; Bouman et al., 2004; Tomascak et al., 2008) (b-f) Core-to-rim variations of the δ 7 Li values and Li contents in (b-c) blueschists and, (d-f) eclogites. The dotted lines and grey field give the corresponding whole rock and 2σ SE values. Individual Li abundances were calculated based on the mean Li contents obtained by LA-ICPMS and 6+7 Li

Figure 8 .

 8 Figure 8. Calculated Li isotope composition of fluids in equilibrium with rocks and minerals in the rocks from: (a) the Limousin ophiolite and, (b) the Ile de Groix. δ 7 Li fluid calculations were made using the clinopyroxenefluid fractionation factors for amphiboles, omphacite and whole rocks, and the mica-fluid fractionation factor for serpentine (Wunder et al., 2006, 2011): δ 7 Li fluid = δ 7 Li sample -7 Li mineral-fluid . The 2σ uncertainties, represented by the error bars, are identical to those of the mean δ 7 Li of minerals and whole rocks employed for calculation. (a) The δ 7 Li values of the fluids in equilibrium with serpentine were calculated for 350°C and 500°C, as the temperature of hydrothermal alteration is not precisely constrained. (b) It is assumed that glaucophane rims have formed at a temperature 50°C higher than glaucophane core.

  2σ SD), but exhibits a large spread of δ 7 Li values, resulting in large 2σ uncertainties on the 7 Li omph-barr values [-0.1 ± 20.5 to +1.0 ± 9.5‰; (2σ); Table6].

	5.6 ± 6.5‰;
	F
	o r
	P
	e
	e
	r
	R
	e
	v
	i e
	w
	± 18.2 to -

Table 1 .

 1 Provenance and mineral assemblage of the studied rocks

Table 2 .

 2 Lithium abundances in the rocks from the Limousin ophiolite (in ppm)

Table 3 .

 3 In-situ SIMS measurements of mineral Li contents (in ppm) and isotopic composition (in ‰) for the Limousin ultrabasic and basic rocks. Calculated Li fractionation factors (in ‰) are given for mineral-mineral pairs.

Table 4 .

 4 Lithium contents (in ppm) and MC-ICPMS isotopic data (in ‰) of the rocks from the

	Ile de Groix

Table 5 .

 5 Lithium abundances in the metamorphic minerals from the rocks of the Ile de Groix measured by LA-ICPMS (in ppm)

Table 6 .

 6 Lithium isotopic composition measured by SIMS in the metabasites of the Ile de Groix (in ‰). Calculated Li fractionation factors (in ‰) are given for mineral-mineral and whole rock-mineral pairs. WR: whole rock

Table 7 .

 7 Calculated Li isotopic composition of fluids in equilibrium with the minerals of the

	Limousin ultrabasic and basic rocks (in ‰)

Table 8 .

 8 Calculated Li isotopic composition of fluids in equilibrium with glaucophane, omphacite and whole metabasic rocks from the Ile de Groix (in ‰)

	F o r P e e r R e v i e w

Table 1 .

 1 Provenance and mineral assemblage of the studied rocks Sample

Table 3 .

 3 [START_REF] Berger | New occurrence of UHP eclogites in Limousin (French Massif Central): Age, tectonic setting and fluid-rock interactions[END_REF] [2] El[START_REF] El Korh | Trace element partitioning in HP-LT metamorphic assemblages during subduction-related metamorphism, Ile de Groix, France: a detailed LA-ICP-MS study[END_REF] [3] El Korh et al., 2013; [4] El[START_REF] El Korh | Trace element and Oisotope composition of polyphase metamorphic veins of the Ile de Groix (Armorican Massif, France): implication for fluid flow during HP subduction and exhumation processes[END_REF] In-situ SIMS measurements of mineral Li contents (in ppm) and isotopic composition (in ‰) for the Limousin ultrabasic and basic rocks. Calculated Li fractionation factors ( 7 Li, in ‰) are given for mineral-mineral pairs.

	Page 111 of 126

Table 4 .

 4 Lithium contents (in ppm) and MC-ICPMS isotopic data (in ‰) of the rocks from the Ile de Groix

		Li	δ 7 Li	2σ SE*	n**
	Blueschists
	GR 02	45	0.74	-	1
	GR 04	16.2	n.a.	-
	GR 11b	70	-0.60	0.23	3
	GR 12b	124	n.a.	-
	GR 25a	39	-0.60	0.10	3
	GROA 56	55	n.a.	-
	Eclogites
	GR 21	18	-4.12	0.38	3
	GR 24a	28	-2.46	0.12	3
	GR 29	102	-4.75	0.26	3
	Greenschists
	GR 06a	74	3.16	0.59	3
	GR 23	58	-4.51	0.51	3
	GR 25b	37	-0.14	0.52	3
	GROA 43	62	n.a.	-
	GROA 52	45	1.45	0.39	2
	Micaschists
	GROA 110	15	0.18	0.43	3
	GROA 111a	48	-1.68	0.73	3
	GR 26b	50	n.a.	-
	GROA 104	52	n.a.	-
	* 2σ standard error (i.e. 2σ standard deviations of the mean)
	calculated on the basis of replicate analyses of the same
	sample solution
	** n = number of replicates for Li isotope measurements;

Table 5 .

 5 Lithium abundances in the metamorphic minerals from the rocks of the Ile de Groix measured by LA-ICPMS (in ppm)

	Mineral	Sample	Li abundances		mean	2σ SD
			this study	[1, 2]*
	Prograde minerals in metabasites
	ap	GR29	-	2.6	2.6	-
	ap	GR25b	-	0.49-0.58	0.53	0.12
	ep	GR02	-	0.78-3.4	2.3	19
	ep	GR11b	-	4.8-9.4	6.6	4.9
	ep	GR12b	2.0-41	0.67-26	9.9	22
	ep	GR25a	-	1.6-9.7	5.8	7.4
	ep	GR21	-	1.2-5.1	3.1	2.7
	ep	GR24a	-	2.6-5.7	4.1	2.7
	ep	GR29	1.3-10	5.6	3.9	17
	ep	GR23	17-31	1.3-22	7.9	14
	ep	GR25b	-	2.2-7.2	3.7	3.5
	grt	GR02	-	0.47-1.1	0.72	0.52
	grt	GR21	-	0.49-0.63	0.57	0.15
	grt	GR29	1.5	-	1.5	-
	grt	GR23	-	0.86-1.3	1.1	0.5
	grt	GR25b	-	0.57-1.5	0.95	1.03
	gln	GR02	84-103	-	94	28
	gln	GR11b	-	209-319	270	106
	gln	GR12b	295-401	-	360	253
	gln	GR25a	67-90	-	77	35
	gln	GR21	-	81-215	115	91
	gln	GR24a	88-113	48-74	72	46
	gln	GR29	110-295	-	179	142
	gln	GR25b	77-252	121-129	144	119
	omph	GR21	-	41-61	47	14
	omph	GR24a	32-33	27-31	31	5
	omph	GR29	111-145	90-118	107	28
	phe	GR02	-	16	16	-
	phe	GR11b	-	26-39	32	8.0
	phe	GR12b	-	9-33	23	25
	phe	GR21	-	21-33	26	9.2
	phe	GR24a	-	4.4-9.1	6.8	3.9
	phe	GR23	25-27	41-46	37	20
	phe	GR25b	-	5.8-7.1	6.5	1.8
	ttn	GR02	-	1.3-8.2	4.6	6.1
	ttn	GR11b	-	7.3	7.4	-
	ttn	GR12b	-	17	17	-
	ttn	GR25a	1.6-10	-	5.9	12
	ttn	GR24a	-	4.8-7.0	5.9	1.8
	ttn	GR23	-	14-15	15	1
	ttn	GR25b	-	2.9	2.9	-
	Retrograde minerals in metabasites
	ab	GR29	174-212	-	193	53
	ab	GR25b	-	41	41	-
	act	GR12b	5.7-8.1	-	6.7	2.3
	act	GR25b	-	2.0-19	7.7	11.8
	barr	GR11b	-	5.1-9.4	7.3	6.1
	barr	GR21	-	9	9	-
	barr	GR24a	-	3.1-9.6	6.4	9.2
	chl	GR11b	-	69-182	107	87
	chl	GR12b	106-126	126-127	119	17
	chl	GR25a	27-33	-	30	8
	chl	GR25b	13-50	46-61	43	33
	Minerals in micaschists
	cld	GROA110	-	0.31-1.2	0.75	1.24
	phe	GROA110	-	20-26	22	4.0
	phe	GR26b	33-52	38-61	45	15
	pg	GROA110	-	28	28	-
	grt	GR26b	-	3.1-5.0	3.9	2.0
	chl	GR26b	180	150-185	170	28
	ep	GR26b	-	4.2	4.2	-
	ap	GR26b	-	2.8	2.8	-
	ttn	GR26b	-	3.6	3.6	-
	tur	GR26b	47	19-27	29	25
	* Data from: [1] El Korh et al. (2009); [2] El Korh (2010)

Table 6 .

 6 Lithium isotopic composition measured by SIMS in the metabasites of the Ile de Groix (in ‰). Calculated Li fractionation factors ( 7 Li, in ‰) are given for mineral-mineral and whole rock-mineral pairs. WR: whole rock Mean δ 7 Li values for bulk mineral correspond to unweighted average of all measured data, without considering core/rim proportion.

		bulk δ 7 Li *				core			rim	7 Li
		min	max	mean 2σ SD n	mean 2σ SD n	mean 2σ SD n	mean 2σ
	SIMS data
	Blueschist GR02
	gln	-7.1	+8.8	+1.3	8.5	33	+3.8 6.1	20	-2.6 4.9	13
	WR-gln										-0.5	8.5
	Blueschist GR25a
	gln	-4.8	+5.2	+1.3	4.3	34	+2.5 3.4	18	-0.1 3.6	16
	WR-gln										-1.9	4.3
	Eclogite GR21
	gln	-8.0	+0.8	-3.6	6.5	13	-1.1 3.4	7	-6.6 2.8	6
	omph	-7.2	+3.3	-3.5	5.2	17	-2.9 7.5	7	-3.9 3.0	10
	WR-gln										-0.5	6.5
	WR-omph										-0.6	5.2
	gln-omph										-0.1	8.3
	gln rim-omph									-3.1	5.9
	Eclogite GR24a
	gln	-8.6	+1.4	-2.3	4.4	26	-1.1 3.2	14	-3.8 3.8	12
	omph	-22.4	-9.0	-15.6	9.4	12	-10.9 6.0	4	-17.9 7.0	8
	barr	-24.9	-6.7	-15.4	18.2 3	-	-		-	-
	WR-gln										-0.1	4.4
	WR-omph										+13.1 9.4
	gln-omph										+13.2 10.4
	gln rim-omph									+11.7 8.3
	omph-barr										-0.1	20.5
	Eclogite GR29
	gln	-6.2	-1.9	-4.8	2.2	14	-4.5 2.9	7	-5.0 1.3	7
	omph	-11.2	+2.4	-4.5	6.9	15	-3.0 7.2	8	-6.2 5.5	6
	barr	-8.4	-2.0	-5.6	6.5	3	-	-		-	-
	WR-gln										+0.0	2.3
	WR-omph										-0.2	6.9
	gln-omph										-0.2	7.3
	gln rim-omph									-0.5	5.9
	omph-barr										+1.0	9.5
	MC-ICPMS data
	Eclogite GR29
	gln			-4.8	0.6	4
	omph			-4.5	2.3	3

*

Table 7 .

 7 Calculated Li isotopic composition of fluids in equilibrium with the minerals of the Limousin ultrabasic and basic rocks (in ‰) 2σ uncertainties correspond to the 2σ standard deviations calculated for the mean δ 7 Li values of the different mineral generations

	serp-H 2 O
	amph-H 2 O

http://www.petrology.oupjournals.org/ Manuscript submitted to Journal of Petrology

ACKNOWLEDGMENTS

Thanks to Chantal Peiffert for technical assistance in the LA-ICPMS laboratory, to the team of the SARM-chimie (SARM, CRPG, Nancy) for whole rock Li analyses. Aimeryc Schumacher is acknowledged for his help with the MC-ICPMS analyses. We also thank Nordine Bouden, Andrey Gurenko and Denis Mangin for technical assistance during SIMS analyses, and Olivier Rouer for help in the microprobe laboratory. Bernard Grobéty is thanked for his help with XRD analyses. Jürgen von Raumer is thanked for fruitful discussions on the Variscan palaeogeography. We thank Julien Berger for sending UHP zoisite-eclogite sample L04-143. This study benefited from detailed reviews of four anonymous reviewers. We also thank Simon Turner for the editorial handling of our manuscript. The research was supported by project P300P2_147749 of the Swiss National Science Foundation, and benefited from a funding from the Observatoire de la Terre et de l'Environnement en Lorraine (OTELO-CNRS). This work has also been supported by the French National Research Agency through the national program "Investissements d'avenir" (ANR-10-LABX-21-LABEX RESSOURCES21). This is a CRPG-CNRS contribution n° 2701.