Benjamin Lefeuvre
email: benjamin.lefeuvre@sorbonne-universite.fr

Philippe Agard

Anne Verlaguet

Benoît Dubacq

Alexis Plunder

Massive formation of lawsonite in subducted sediments from the Schistes Lustrés (W. Alps): Implications for mass transfer and decarbonation in cold subduction zones

Keywords: lawsonite, metasediments, fluid/mass transfer, decarbonation, Schistes Lustrés, seismogenic depths

This study investigates the reactions allowing crystallization of large amounts of lawsonite (CaAl2Si2O7(OH)2•[H2O]) found in calcschists metamorphosed in a subduction zone setting. Previous studies of lawsonite-forming reactions in metasediments have highlighted its importance for the largescale budget of CO2, as the calcium required to form lawsonite is thought to originate from decarbonation reactions. Yet, thermodynamic modelling as well as isotopic measurements have indicated that 80 to 90% of the carbon is retained in sediments, and there is no evidence of major decarbonation in the field. As lawsonite contains abundant H2O and has a large stability field, understanding its crystallization is also important to assess fluid migration and mass transfer in a critical part of the subduction system where slow earthquakes are nucleating (such as LFEs or ETS).

INTRODUCTION

Sediments buried in subduction zones contain large amounts of volatiles (up to ~7 wt.% H2O and 30 vol.% carbonates according to [START_REF] Plank | The chemical composition of subducting sediment and its consequences for the crust and mantle[END_REF][START_REF] Rea | Composition and mass flux of sediment entering the wo rld's subduction zones: implications for global sediment budgets, great earthquakes, and volcanism[END_REF][START_REF] Clift | A revised budget for Cenozoic sedimentary carbon subduction: Cenozoic Carbon Subduction[END_REF].

Understanding their behaviour is key to assess the fate of carbon and fluids in subduction settings [START_REF] Gorman | Modeling open system metamorphic decarbonation of subducting slabs: Metamorphic decarbonation[END_REF][START_REF] Bebout | Metamorphic chemical geodynamics of subduction zones[END_REF]Cook-Collars et al., 2014;[START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up[END_REF][START_REF] Dragovic | Fluid production rate during the regional metamorphism of a pelitic schist[END_REF][START_REF] Plank | Subducting carbon[END_REF]. Metamorphosed sediments also play an important role along the subduction interface [START_REF] Agard | The subduction plate interface: rock record and mechanical coupling (from long to short timescales)[END_REF][START_REF] Raimbourg | Fluid properties and dynamics along the seismogenic plate interface[END_REF]: production and expulsion of metasediment-derived fluids is proposed to control the periodicity of slow earthquakes at the base of the seismogenic zone (and more specifically around depths of 35 km) through changes in permeability and dissolution-precipitation creep (Fig. 1a; [START_REF] Rogers | Episodic Tremor and Slip on the Cascadia subduction zone : the chatter of sile nt slip[END_REF][START_REF] Audet | Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing[END_REF][START_REF] Osei Tutu | Evaluating the Influence of Plate Boundary Friction and Mantle Viscosity on Plate Velocities: Plate boundary effect on plate motion[END_REF][START_REF] Behr | Sediment control on subduction plate speeds[END_REF]. The associated element mobility, metasomatism and changes in rheology must be extensive, for example [START_REF] Audet | Possible control of subduction zone slow-earthquake periodicity by silica enrichment[END_REF] suggest kmscale transfer of silica from metasediments to the overriding plate based on high compressional-to shear-wave velocity ratios (Vp/Vs). The interplay between deformation, mineral reactions and J o u r n a l P r e -p r o o f rheology along and above the plate interface, however, remains largely unknown [START_REF] Breeding | Slab-derived fluids and quartz-vein formation in an accretionary prism, Otago Schist[END_REF][START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF][START_REF] Bebout | Fluid and mass transfer at subduction interfaces -the field metamorphic record[END_REF].

One target outcropping unit to study this interplay is the Schistes Lustrés complex of the Western/Central Alps and Corsica , an accretionary prism including several units and subunits preserving seafloor material, i.e. calcschists, metapelites and remnants of crustal material (e.g., [START_REF] Tricart | Les Schistes lustrés piémontais des Alpes Occidentales : approche stratigraphique, structurales et sédimentologique[END_REF]. Metamorphism of the Schistes Lustrés originates from subduction during closure of the Liguro-Piemontese ocean (see e.g., [START_REF] Lagabrielle | Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model[END_REF]. The selected area belongs to the uppermost units of the Schistes Lustrés complex, which experienced blueschist-facies metamorphism with peak conditions between 1.2 GPa -300°C and 1.9 GPa -450°C (yellow dots; Fig. 1d). Mass transfer via metamorphic fluids appears extensive, as shown by the presence of ubiquitous veins bearing high-pressure minerals [START_REF] Chopin | High-pressure metamorphism in the Western Alps : zoneography of metapelites, chronology and consequences[END_REF]Agard et al., 2001b[START_REF] Agard | Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms[END_REF][START_REF] Bousquet | The tectono -metamorphic history of the Valaisan domain from the Western to the Central Alps: New constraints on the evolution of the Alps[END_REF][START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF]Vitale Brovarone et al., 2014).

A notable feature of the upper unit of the Schistes Lustrés complex is the high abundance of lawsonite (CaAl2Si2O7(OH)2•[H2O]), commonly greater than 20 vol.% and frequently associated with quartz [START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF][START_REF] Sicard-Lochon | La lawsonite et ses pseudomorphes[END_REF][START_REF] Agard | Retrograde mineral and fluid evolution in high-pressure metapelites (Schistes lustrés unit, Western Alps)[END_REF]. With 12 wt.% H2O, lawsonite is a major carrier of H2O. Lawsonite is stable up to ultra-high pressures, implying it may release water at depths greater than 200 km in cold subduction zones (Schmidt andPoli, 1998, 1994;[START_REF] Van Keken | Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide[END_REF][START_REF] Faccenda | Water in the slab: A trilogy[END_REF]. Lawsonite allows for many cation exchanges, especially in its large Ca site prone to J o u r n a l P r e -p r o o f strain [START_REF] Martin | Experimental Measurements of Trace Element Partitioning Between Lawsonite, Zoisite and Fluid and their Implication for the Composition of Arc Magmas[END_REF]Vitale Brovarone and Beyssac, 2014;[START_REF] Dubacq | Controls on Trace Element Distribution in Oxides and Silicates[END_REF]. It is thought to play a key role in trace element cycling into the deep Earth (Schmidt andPoli, 1998, 1994;[START_REF] Zack | An evaluation of reactive fluid flow and trace element mobility in subducting slabs[END_REF][START_REF] Tsujimori | Lawsonite blueschists and lawsonite eclogites as proxies for palaeo-subduction zone processes: a review[END_REF][START_REF] Fornash | Lawsonite composition and zoning as an archive of metamorphic processes in subduction zones[END_REF].

Yet the formation of lawsonite remains poorly understood and models are fraught with contradictions (see [START_REF] Clarke | The lawsonite paradox: a comparison of field evidence and mineral equilibria modelling[END_REF]. Explaining large modal amounts of lawsonite in metasediments is problematic due to the low amounts of Ca initially present in pelitic rocks. Decarbonation and metasomatism are necessarily invoked processes, with a huge impact on modelling the mobility of carbon in its deep cycle [START_REF] Kerrick | Metamorphic devolatilization of subducted marine sediments and the transport of volatiles into the Earth's mantle[END_REF][START_REF] Gorman | Modeling open system metamorphic decarbonation of subducting slabs: Metamorphic decarbonation[END_REF][START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up[END_REF][START_REF] Stewart | Infiltration-driven metamorphism, New England, USA: Regional CO 2 fluxes and implications for Devonian climate and extinctions[END_REF]. Local accumulations of lawsonite in hybrid metasediments from Corsica ('lawsonitites'; >75 vol.%) have been interpreted as the result of large-scale metasomatism through serpentinite-derived metasomatic fluids, with and without decarbonation (Vitale-Brovarone et al., 2014[START_REF] Piccoli | Carbonation by fluid-rock interactions at high-pressure conditions: Implications for carbon cycling in subduction zones[END_REF]respectively).

In contrast to Corsican lawsonitites, the lack of significant isotopic shift in volatiles (C, O, H) from the Schistes Lustrés suggests limited external fluid infiltration, except possibly along tectonic contacts [START_REF] Henry | Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major-and trace-element systematics[END_REF][START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF][START_REF] Collins | Subduction zone metamorphic pathway for deep carbon cycling: II. Evidence from HP/UHP metabasaltic rocks and ophicarbonates[END_REF][START_REF] Jaeckel | Deformation-enhanced fluid and mass transfer along Western and Central Alps paleo-subduction interfaces: Significance for carbon cycling models[END_REF][START_REF] Epstein | Scales of Fluid-Rock Interaction and Carbon Mobility in the Deeply Underplated and HP-Metamorphosed Schistes Lustrés[END_REF]. Fluid-mobile elements (e.g., H, N, B, Li, Ba, K, Rb, Cs) overall show little evolution along comparable rocks in the Cottian Alps [START_REF] Busigny | Massive recycling of nitrogen and other fluid-mobile elements (K, Rb, Cs, H) in a cold slab environment: evidence from HP to UHP oceanic metasediments of the Schistes Lustrés nappe (western Alps, Europe)[END_REF][START_REF] Bebout | Devolatilization history and trace element mobility in deeply subducted sedimentary rocks: Evidence from Western Alps HP/UHP suites[END_REF][START_REF] Barnes | Chlorine and lithium behavior in metasedimentary rocks during prograde metamorphism: A comparative study of exhumed subduction complexes (Catalina Schist and Schistes Lustrés)[END_REF].

Most fluids percolating through these rocks appear internally-derived and/or derived from similar J o u r n a l P r e -p r o o f lithologies, in support of closed-system behaviour at the meter scale and possibly up to 100 meterscale, at least for components other than H2O. This is in stark opposition to large-scale element mobility and extensive metasomatism.

Given the volumetric extent of the Schistes Lustrés metasediments, understanding the formation of lawsonite in terms of reactions, decarbonation, loss or gain of water and silica is crucial to quantifying the element budget along and across the plate interface. The lawsonite-forming reaction is not readily accessible since lawsonite rarely hosts silicate mineral inclusions and is generally poorly preserved in exhumed rocks. Lawsonite is commonly pseudomorphed and replaced by a variety of minerals, including phyllosilicates (mostly chlorite and white mica) and epidote, during burial (as in Syros and Turkey: [START_REF] Hamelin | Pseudomorphs after Lawsonite from Syros, Greece[END_REF][START_REF] Fornash | Lawsonite composition and zoning as an archive of metamorphic processes in subduction zones[END_REF] and exhumation (e.g., in the Cottian Alps: [START_REF] Sicard-Lochon | La lawsonite et ses pseudomorphes[END_REF][START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF]. Reactions suggested so far for lawsonite crystallization in metasediments include: pyrophyllite + calcite + water = lawsonite + quartz + CO2 [START_REF] Nitsch | Das P-T-XCO2 Stabilitätsfeld von Lawsonit[END_REF] (1)

Al2Si4O10(OH)2 + CaCO3 + H2O = CaAl2Si2O7(OH)2•[H2O] + 2 SiO2 + CO2 kaolinite + aragonite = lawsonite + CO2 (Cook-Kollars et al., 2014) (2) Al2Si2O5(OH)4 + CaCO3 = CaAl2Si2O7(OH)2•[H2O] + CO2
J o u r n a l P r e -p r o o f

Both reactions involve decarbonation and production of CO2, since Ca is mobilized from calcium carbonate. Lawsonite, however, was shown experimentally to be unstable in CO2-rich fluids (reaction 1 being valid for XCO2<0.05; [START_REF] Nitsch | Das P-T-XCO2 Stabilitätsfeld von Lawsonit[END_REF]. Based on thermodynamic modelling of reaction (2), [START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF] suggested that only moderate amounts (10-20 vol.%) of lawsonite could be produced given the bulk composition of the Schistes Lustrés and that the Schistes Lustrés retained 80 to 90% of their initial C content. Yet larger amounts of lawsonite pseudomorphs are found in most of these rocks [START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF][START_REF] Sicard-Lochon | La lawsonite et ses pseudomorphes[END_REF], and whether lawsonite formation requires addition of H2O to the sediments or not is unknown. Based on field, mineralogical and micro-structural observations together with geochemical analysis, the present contribution documents lawsonite concentrations along a representative transect of the uppermost units of the Schistes Lustrés. This study aims at explaining how such concentrations have been reached. Mineral reactions forming lawsonite and the scales of associated fluid/mass transfer are critically discussed to shed light on the involvement of large-scale transfer, silica deposition, and decarbonation.

GEOLOGICAL SETTING

The Schistes Lustrés paleo-accretionary complex in the Alpine orogenic belt

The Schistes Lustrés complex belongs to the internal domain of the Alpine orogenic wedge (Figs.

1b,c, inset; for more details on the overall Alpine structure, see: [START_REF] Coward | an overview-Alpine tectonics[END_REF] J o u r n a l P r e -p r o o f

Lagabrielle and [START_REF] Lagabrielle | Alpine, Corsican and Apennine ophiolites: the slow-spreading ridge model[END_REF][START_REF] Rosenbaum | The Western Alps from the Jurassic to Oligocene: spatio -temporal constraints and evolutionary reconstructions[END_REF][START_REF] Handy | Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological-geophysical record of spreading and subduction in the Alps[END_REF]. The Western Alps formed as a result of east-dipping, slow and cold subduction (~8°C/km at a maximum 2 cm/year; Agard et al., 2001a;[START_REF] Le Pichon | Plate kinematics and tectonics leading to the Alpine belt formation; a new analysis[END_REF][START_REF] Lapen | Burial rates during prograde metamorphism of an ultra-high-pressure terrane: an example from Lago di Cignana, J o u r n a l P r e -p r o o f western Alps, Italy[END_REF] of the Valais and Liguro-Piemontese slow-spreading oceans [START_REF] Lagabrielle | Alpine Jurassic ophiolites resemble the modern central Atlantic basement[END_REF]. Oceanic subduction lasted from c. 100 to 40

Ma and was followed by continental subduction and collision of the European plate with Apulia.

The Schistes Lustrés complex is mostly made of metasediments of seafloor origin showing alternations of thinly bedded carbonate-rich and pelitic layers. It also contains slivers and blocks of various scales, more abundant in the lower units, of mafic and ultramafic bodies from the Liguro-Piemontese ocean (Fig. 1b; [START_REF] Lagabrielle | Origine volcano-détritique de certaines prasinites des Schistes lustrés du Queyras (France) : arguments texturaux et géochimiques[END_REF][START_REF] Lagabrielle | Fossil oceanic core complexes recognized in the blueschist metaophiolites of Western Alps and Corsica[END_REF]. The Schistes Lustrés sediments were deposited from the Upper Jurassic (~150 Ma, De Wever and Caby, 1981) to the Late Cretaceous (~70-65 Ma, [START_REF] Lemoine | The continental margin of the Mesozoic Tethys in the Western Alps[END_REF] and formed a 100-400 m thick sediment pile (De Wever and Caby, 1981). During subduction and exhumation, metasediments were intensely folded, leading to an important thickening of the initial sediment pile [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF].

Approximately 30 to 50% of the total sedimentary cover was eventually exhumed, much more than its oceanic basement (<5 %; [START_REF] Agard | Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms[END_REF]. Based on lithological differences in the Schistes Lustrés complex, three to four main units have been distinguished [START_REF] Lagabrielle | Alpine Jurassic ophiolites resemble the modern central Atlantic basement[END_REF][START_REF] Deville | From oceanic closure to continental collision: A synthesis of the" Schistes lustrés" metamorphic complex of the Western Alps[END_REF]Fudral, 1996;Agard et al., 2001a;[START_REF] Tricart | A north-south section across the Queyras Schistes lustrés (Piedmont zone, Western Alps): Syn-collision refolding of a subduction wedge[END_REF][START_REF] Lagabrielle | Fossil oceanic core complexes recognized in the blueschist metaophiolites of Western Alps and Corsica[END_REF]. [START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF] showed that the upper, median and lower units experienced increasing metamorphic conditions.

J o u r n a l P r e -p r o o f

Thermobarometric estimates show an eastward increase from the upper (1.2 GPa -300°C west of Fraiteve) to the lower units (2.1 GPa -500°C) as shown in figure 1d (Agard et al., 2001a;[START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: a new geothermometer[END_REF][START_REF] Plunder | How continuous and precise is the record of P-T paths? Insights from combined thermobarometry and thermodynamic modelling into subduction dynamics (Schistes Lustrés, W. Alps)[END_REF].

Tectono-metamorphic evolution along the Cottian Alps transect

The Schistes Lustrés complex recorded several deformation stages which are well constrained in terms of age and pressure-temperature (P-T) conditions (Agard et al., 2001a[START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF][START_REF] Tricart | A north-south section across the Queyras Schistes lustrés (Piedmont zone, Western Alps): Syn-collision refolding of a subduction wedge[END_REF]. This study focusses on the upper unit of the Schistes Lustrés (LPU) along a ~15 km SW-NE traverse between Mt. Fraiteve and Colle delle Finestre, in the French-Italian Cottian Alps (Figs. 1b, c;[START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF][START_REF] Polino | Les séries océaniques du Haut val de Suse (Alpes Cottiennes): analyse des couvertures sédimentaires[END_REF]. Four deformation stages are recognized in the LPU [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF]. The first episode D1 is linked to peak or near-peak burial, as indicated by the crystallization of [START_REF] Tricart | A north-south section across the Queyras Schistes lustrés (Piedmont zone, Western Alps): Syn-collision refolding of a subduction wedge[END_REF].

FIELD CHARACTERIZATION AND SAMPLING

Lithologies and mineral abundances were investigated along the 15-km traverse through the LPU unit (Fig. 1b). While detailed observations and sampling were carried out along the whole section, this study documents in greater details the ~200 m long Mt. Triplex locality (Figs. 2, 3; metamorphic pick conditions of 1.6GPa -350°C, Fig. 1d).

Lithologies and distribution of hydrous metamorphic minerals

Figure 2 presents the five types of lithologies distinguished between carbonate-rich and pelite-rich layers in the LPU unit. Amounts of lawsonite and Fe-Mg carpholite vary systematically between rock types, although lithologies appear rather similar at first examination. The pelitic endmember (lithology 1) is composed of dark, organic-rich and phyllosilicate-rich material where carbonate layers are absent

or not visible at the cm scale (Figs. 2a,b). By contrast, the carbonate endmember (lithology 5; Fig. 2a) consists of metric (1-5 m.) carbonate horizons, often with subordinate horizons containing phyllosilicates (<5 vol.%; Fig. 2b). Lithologies 2, 3 and 4 (hereafter referred to as calcschists) are composed of a mixture of pelitic layers and carbonate beds in various proportions (Figs. 2a,b).

Lithology 2 contains thinly bedded, typically cm-scale carbonate layers in a shaly matrix (pelite to J o u r n a l P r e -p r o o f

Journal Pre-proof carbonate volume ratio ~3:1). Lithology 3 shows regularly alternating pelitic and carbonate horizons with roughly equivalent thicknesses (commonly 5-10 cm, and up to ~20 cm). Lithology 4 is characterized by more compact calcschists in which phyllosilicate-rich layers do not exceed cm-scale (pelite to carbonate ratio ~1:3).

The metasediments contain significant amounts of hydrous minerals typical of blueschist facies metamorphism such as lawsonite and Fe-Mg-carpholite, or preserve evidence of their former presence.

Fe-Mg-carpholite is much more sensitive to retrogression and alteration than lawsonite. Fe-Mgcarpholite is readily replaced by chlorite and white mica during exhumation and frequently only preserved as micrometric needles in quartz [START_REF] Chopin | High-pressure metamorphism in the Western Alps : zoneography of metapelites, chronology and consequences[END_REF]Agard et al., 2001b). Lawsonite is commonly pseudomorphed, allowing for minimum estimates of its former abundance. Figure 2b displays modal amounts of lawsonite determined in the field. Lawsonite is present in all lithologies but mostly found in calcschists among which it is heterogeneously distributed: it makes up to 40 vol.% of rock type 3 (e.g., at Mt. Triplex and Genevris). Fe-Mg-carpholite is mainly observed in lithology 1, although it may have occurred in other lithologies prior to retrogression. In general, these two minerals are not observed coexisting in hand specimen or thin-section. Along the transect between Mt. Fraiteve and the Assietta pass 2c), most rock types correspond to lithologies 2, 3 and 4 (calcschists). Some locations present much more pelitic material classified as lithology 1 (Mt. Fraiteve and Assietta pass). The carbonate-rich endmember of our classification (lithology 5) is only sparsely J o u r n a l P r e -p r o o f encountered; these rocks are found in greater proportions in (and are typical of) the median unit of the Schistes Lustrés further east. ,4) and in quartz ± ankerite veins (Fig. 3f). Lawsonite is by far the predominant metamorphic hydrous phase (rare Fe-Mg-carpholite pseudomorphs are found in some layers), locally up to 45 vol.% of the host rock.

Outcrop-scale observations in Triplex

LAWSONITE TYPES

At macroscopic scale, three types of lawsonite have been characterized based on their texture, association with other minerals, and presence in the schistose matrix or high-pressure veins (Figs. 3e, f, 5). For simplicity, these different types are termed LwsA, LwsB and LwsC. The lettering does not imply cross-cutting relationship or relative chronology as these remain unclear, as detailed below.

Calcschist lawsonite (LwsA in lithologies 2 and 3)

LwsA, by far the most common type of lawsonite in this area, is mostly found in pelitic calcschists interlayered with carbonate-rich horizons (Figs. 3e,4a) and makes up to 30% of lithology 2 and 40% of lithology 3. In addition to lawsonite, the pelite-rich layers are commonly composed of quartz (20 to 30%), calcite (0 to 30%), ankerite (< 20%), phengite (< 20%), chlorite (< 10%) and organic matter found as inclusions (Figs. 4e,f,g). LwsA forms millimetric to centimetric black prisms (Figs. 4c-g), generally well-preserved, texturally associated with quartz and oxidized reddish ankerite (Fig. 4d).

Ankerite is found in carbonate layers and all across the pelitic layers. Ankerite is particularly concentrated at the edge of folded carbonate-rich layers where it forms diffuse, centimetre-thick J o u r n a l P r e -p r o o f domains along lithological contacts (Figs. 4a,b). Well-defined horizons of ankerite are less frequent (Fig. 4d). The association of LwsA and ankerite is maximum at the interface between carbonate and pelitic layers, and decreases towards pelitic domains and towards thick carbonate horizons. The textural relationships between LwsA and ankerite are best preserved in thin carbonate layers (Fig. 4d).

Figures 4e-f-g show fractured, organic-matter-rich, dark lawsonite crystals overgrown with pale newly-formed lawsonite, indicating incremental growth. When pseudomorphed, LwsA is replaced by a mixture of chlorite, phengite and calcite, as described in previous studies [START_REF] Sicard-Lochon | La lawsonite et ses pseudomorphes[END_REF]Vitale-Brovarone et al., 2014).

Lawsonite-bearing veins (LwsB and LwsC)

The second type of lawsonite (LwsB) is observed in pluri-centimetric to metric quartz veins following the main schistosity (Fig. 5a). LwsB forms elongated, cream-coloured crystals with long axis parallel to vein selvages and aligned with quartz fibres (Figs. 5a-c). Crystal size reaches 20 cm long in places.

LwsB shows incremental growth and also crystallized in pressure shadows around boudinaged quartz veins (Figs. 5c-e). Lawsonite and quartz often evidence ductile deformation in LwsB-bearing veins (Fig. 5d), advocating for syn-deformation incremental growth and mode II vein opening. LwsB is generally only associated with quartz, but ankerite crystals have been observed in places, in particular in the vicinity of pelitic horizons (Fig. 5e). The third type of lawsonite (LwsC) is observed in millimetric to centimetric tensile veins (Fig. 5f,g

PETROGRAPHY AND MINERALOGY

Textural relationships

In the calcschists, LwsA forms euhedral rhombi to oval crystals (Figs. 6a,d,e). Inclusions of organic matter, responsible for the black colour of hand specimens, are concentrated in LwsA cores (Figs. 4eg,6d). These inclusions form trails related to deformation accompanying burial and may record early episodes like D1 [START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF]. Inclusion trails are found in adjacent crystals of lawsonite and ankerite (Fig. 6a). As for outcrop-scale observations, systematic relationships are found between J o u r n a l P r e -p r o o f lawsonite, ankerite and quartz. These three minerals are commonly found in apparent textural equilibrium (Fig. 6) and included in one another: figure 6f shows LwsA and quartz included in rhombic ankerite crystals, and figure 6c shows ankerite and quartz included in lawsonite (see also Sup.

mat.1, 3). The calcite -chlorite assemblage cross-cuts domains with lawsonite, ankerite and quartz. In carbonate-rich layers, the habitus of LwsA is different from that in the adjacent pelite-rich schist (Figs. 6b,c). Crystals are much smaller, better preserved and do not contain organic matter inclusions.

Lawsonite crystals in veins never contain organic matter inclusions. Fine-scale alternations of LwsC, quartz and ankerite are observed along small veins, highlighting their close association and cocrystallization. Lawsonite is only partially preserved in LwsC veins (LwsB is better preserved in veins). Crystals of LwsC are fractured, replaced and pseudomorphed by a fine mixture of chlorite, phengite and calcite along cracks.

The amounts of ankerite and lawsonite were estimated for LwsA, LwsB and LwsC-bearing thin sections from optical microscopy images (Fig. 7). LwsA-bearing rocks show Lws/Ank ratios varying around 1.5. Vein samples with LwsB and LwsC show more scattered ratios, between 1 and 3 for LwsB and 0 and 1 for LwsC. Since LwsC are much more overprinted by phengite and chlorite than LwsA, the LwsC/Ank ratios were corrected using pseudomorphs (see Sup. metamorphism. Phengite appears in the main foliation as well as cross-cutting it. Chlorite is found in lawsonite pseudomorphs together with phengite and calcite, a typical lawsonite retrogression reaction in the SL [START_REF] Sicard-Lochon | La lawsonite et ses pseudomorphes[END_REF]. Chlorite is also found as retrogression product between lawsonite and ankerite.

Rock and mineral composition

Whole-rock major element chemistry was performed with an Agilent 5100 ICP-OES at the ALIPP6 analytical facility (Sorbonne Université, Paris) after acid attack of finely-ground samples. A HF-HNO3 mixture was used for dissolution of samples in closed tubes over 24 hours, and boric acid added immediately after opening to prevent loss of silicium. Dilution and measurement followed shortly (within 24 hours). Analytical uncertainties are estimated to be better than 5% of the measured value.

Loss on ignition was obtained by mass loss after heating approximately 4 grams of samples at 110°C

for one night for adsorbed water and at 1000°C for three hours for remaining volatiles. Mineral analysis was carried out at the CAMPARIS facility (Sorbonne Université, Paris) with a CAMECA SX-FIVE electron microprobe and the data reducing method of [START_REF] Pouchou | Quantitative Analysis of Homogeneous or Stratified Microvolumes Applying the Model 'PAP[END_REF].

Analytical conditions were 15 kV-10 nA in WDS mode, for analysis of major elements. Conditions were changed to 25 kV and 100 nA when measuring Sr and Ti, which are in low concentrations. Fe2O3

(Fe), MnTiO3 (Mn, Ti), diopside (Mg, Si), Cr2O3 (Cr), orthoclase (Al, K), anorthite (Ca) and albite

J o u r n a l P r e -p r o o f

Journal Pre-proof (Na) were used as standards for elements in parentheses. Representative analyses are provided in Table 1 for lawsonite. All Fe is assumed to be in divalent state and is expressed as FeO.

Figure 8a shows the composition of ankerite, chlorite and white mica. Variations in minor to trace element concentrations of lawsonite are shown in figures 8b, c, d. Figure 9 shows selected chemical maps for major and minor to trace elements. Ankerite shows a composition close to the ideal Ca1Fe0.5Mg0.5(CO3)2 with very constant Fe/Mg ratio around 1 (Fig. 8a; Table 1). In places, Mn zoning is observed (Fig. 9d), with rims enriched in Mn. White micas are of muscovite and phengite type (Fig. 8a), a solid solution between muscovite and celadonite with minor (~10%) pyrophyllite content.

Crystals with the highest celadonite content probably preserve the high pressure equilibrium stage (Agard et al., 2001b). Chlorite is fairly homogeneous and compares well with previous data, plotting in a restricted composition interval of fiigure 8a at low sudoite content. The Mg/Fe ratio of chlorite is similarly homogeneous. Together with textural evidence, this points to chlorite being mostly retrograde (Fig. 9a). Lawsonite is close to its ideal composition, and no difference in major elements was found between the various lawsonite types and samples. Variations in trace amounts of Ti and Sr were detected (Figs. 8b,c,d,9b,c; Table 1). Optically-zoned LwsA appears also zoned in Ti and shows higher Ti content in the core, linked to hour-glass zoning (Figs. 8b, 9b; see Sup. mat. 3).

Crystals of LwsA also show enrichment in Sr at their rims, while LwsA cores are 9b,c). Similarly, LwsC crystals are either slightly enriched in Ti or in Sr (Fig. 8d). Only LwsB can J o u r n a l P r e -p r o o f Journal Pre-proof reach higher Sr contents and contain both Sr and Ti (Fig. 8c).The close association of lawsonite, ankerite and quartz is readily apparent from chemical mapping (Fig. 9a).

DISCUSSION

Co-crystallization of lawsonite and ankerite

Co-crystallization of lawsonite, quartz and ankerite is supported by their close association, both as intricate, parallel fibres seeded in vein walls (LwsC; Figs. 5f,g) and in the host-rock (LwsA-rich domains, Figs. 4,6). The abundance of lawsonite in calcschists (lithologies 2 and 3) with mixed carbonate-rich and pelite-rich layers is consistent with the fact that the latter contain most of the cations needed for lawsonite crystallization, i.e. Al, Si, H. Calcium, however, is not present in sufficient amounts in pelite-rich layers to explain the formation of up to 30-40 vol% of lawsonite. This requires adding massive amounts of Ca to pelitic domains via either (i) infiltration of external Ca-rich fluids, (ii) local decarbonation and/or (iii) reaction between carbonate and pelitic layers.

The first hypothesis is not supported by isotope data on C-O-H which should be concomitantly altered [START_REF] Henry | Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major-and trace-element systematics[END_REF][START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF]. Field and petrographic evidences do not support Element transfer through dissolution-precipitation reactions explains the systematic association of lawsonite with ankerite at all scales (e.g., Figs. 4a-d, 5f, g, 6b-f) and the presence of ankerite haloes between pelitic and carbonate horizons (Figs. 4a,b). Based on (i) observations from this study and former descriptions [START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF]Sicard-Lochon, 1986;[START_REF] Caron | Solution-deposition processes and mass transfer in the deformation of a minor fold[END_REF], in particular that reactants J o u r n a l P r e -p r o o f

Thermodynamic modelling of formation of the lawsonite-ankerite assemblage

In order to locate reaction (3) in P-T space, a first pseudosection was computed using a bulk composition corresponding to the stoichiometry of reaction (3) assuming excess water in the CaFMASHC system (Fig. 10a). Computations were made using Perple_X version 6.8.4 with the updated database of Holland and Powell (1998) and corresponding solution models (for chloritoid, dolomite, Fe-Mg-carpholite, phengite) except for chlorite (after Holland et al., 1998). As shown on figure 10a, during burial along a 8°C/km metamorphic gradient, the lawsonite-forming reaction starts at 170°C and 0.44 GPa (i.e., ~15 km depth) and is completed across a restricted P -T range ending at 190°C (Fig. 10b). This reaction forms ~60 vol.% of lawsonite. In this case all lawsonite crystals would be formed during a continuous reaction occurring over a short period of time (a minimum of ~250 000 years at a convergence rate of 2 cm.y -1 ).

In order to quantify the extent to which lawsonite and ankerite may co-crystallize from the interaction between carbonate and pelitic-rich layers, the mineralogical evolution of the calcschists has been modelled in the KCaFMASHC system using the same thermodynamic set up and water saturated conditions. Two pseudosections were built using compositions representative of typical pelitic and carbonate-rich domains (i.e., on either side of the lithological boundary shown in figure 4a). Sample SL16-11c, a type 3 calcschist from Mt. Triplex, was separated in pelitic (R11SCH) and carbonate-rich (R11CARB) domains. For these two bulk rock compositions, CO2 and H2O are declared as J o u r n a l P r e -p r o o f components. CO2 content is inferred from the CaO content, assuming that carbonates host all the CO2 of the rock. H2O content is fixed at 8 wt.% in R11SCH and 5.2 wt.% for R11CARB. These values ensure water saturation across the whole P-T domain, yet represent the minimum H2O content to reach water saturation at low P and T conditions. They are fixed to allow chlorite and kaolinite to form at P -T ≤ 0.2 GPa -150°C while minimizing the amount of stilbite which take over for greater H2O contents (see Sup. mat. 4). The amount of stilbite in itself does not affect the stability field of lawsonite but the presence of excessive stilbite (hosting Ca instead of calcite) would imply an additional lawsoniteforming reaction with addition of CO2, which was deemed unlikely. It is also noted that H2O and CO2 are immiscible at these conditions (so that presence of CO2 will not modify water activity, equal to 1 here) and that the mixing model of Holland and Powell (2003) does not reproduce this accurately (see [START_REF] Dubacq | An activity model for phase equilibria in the H_2O-CO_2-NaCl system[END_REF]. The water content does not affect significantly phase relations at higher grade. It is stressed that the lingering presence of detrital minerals, unobserved in the present rocks but likely dominant in the lowest-grade parts of the pseudosections, will have a greater impact on the reactions experienced by the rocks than details of the modelling before appearance of lawsonite.

Pseudosections predict up to 34 vol.% of lawsonite for the pelitic domains and 23 vol.% for carbonaterich domains at peak burial conditions (1.6 GPa and 350°C;Figs. 10c,d). Calculated Lws/Ank volume ratios are close to 1.5 in both cases but this ratio increases slightly close to peak conditions for the pelitic domain. Most of the lawsonite forms through reaction (3) with ankerite and quartz at the J o u r n a l P r e -p r o o f expense of chlorite, kaolinite and calcite, which releases water. This mechanism predominates in carbonate-rich domains. In pelitic domains, lawsonite formation continues during breakdown of kaolinite and chlorite, which is expected to form Fe-Mg-carpholite above 210°C. Above 330°C, a continuous reaction forms lawsonite and siderite from Fe-Mg-carpholite (which provides Al and Si)

and ankerite (which becomes the source of Ca after exhaustion of calcite, Figs. 10c,e). The corresponding reaction, written for end-members in the CaFASHC system for simplification, is:

Fe-carpholite + ankerite = lawsonite + siderite ( 4)

FeAl2Si2O6(OH)4 + CaFe(CO3)2 = CaAl2Si2O7(OH)2•[H2O] + Fe2(CO3)2
Note that in reaction ( 4), Fe-Mg-carpholite is then consumed: in addition to being much more sensitive to retrogression and alteration than lawsonite, this may explain why both minerals are very rarely found in equilibrium [START_REF] Plunder | Geodynamics of the Tavşanlı zone, western Turkey: Insights into subduction/obduction processes[END_REF]. Further quartz production accompanies to the disappearance of kaolinite above 310 °C (Fig. 10e).

The Tschermak exchange in micas (2 Al 3+ = (Mg 2+ , Fe 2+ ) + Si 4+ ), which releases Al and consumes Mg, However, such reaction is not supported in our modelling, as shown by the parallel modal evolution of lawsonite, ankerite and quartz until 330°C (Fig. 10e). In carbonate domains, the continuous reaction producing small amount of lawsonite and ankerite above 180°C consumes quartz as well as chlorite and aragonite (as in reaction ( 3)). Considering that Al is provided by the Tschermak exchange in micas, the reaction would consume H2O but not release CO2: muscovite + aragonite + quartz + chlorite + water = lawsonite + Fe-celadonite + ankerite + diaspore Presence of white mica is common in lawsonite-bearing rocks, both as high-pressure phengitic mica and as retrogression products (see Sicard-Lochon, 1986;[START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF][START_REF] Hamelin | Pseudomorphs after Lawsonite from Syros, Greece[END_REF] among others). However it is noteworthy that the Tschermak exchange is limited here, even in the high-pressure phengites (Fig. 8a), showing that reactions ( 5) and ( 6) are subordinate to reactions (3) and ( 4).

J o u r n a l P r e -p r o o f

How progressive is lawsonite crystallization?

The large amounts of lawsonite observed in the Triplex area and lawsonite-ankerite relative proportions point to reaction (3) being prominent. Reaction (3) would account for the formation of most of the massive amounts of lawsonite in the Schistes Lustrés, especially LwsA. The relatively small P-T domain over which reaction (3) takes place suggests one large pulse of crystallization.

Overstepping may increase this character by delaying nucleation of lawsonite and favouring faster growth due to a large distance to equilibrium. However this reaction is a medium to high-entropy reaction producing free fluid: ΔS 0 r ≈ -2.3 J/K or -2.5 J/K per mole of oxygen for Mg and for Fe endmembers, respectively. This entropy production is ~5 times larger than the andalusite-sillimanite reaction (using the database of Holland and Powell, 2011). Reaction (3) is therefore unlikely to be largely overstepped (see [START_REF] Pattison | Petrological consequences of variations in metamorphic reaction affinity: Petrological consequences of variations in metamorphic reaction affinity[END_REF]. The carpholite-consuming reaction (4) appears more sensitive to overstepping (ΔS 0 r ≈ 1.5 J/ /K per mole of oxygen; no fluid production) but continuous lawsonite growth may have benefited from the presence of lawsonite crystals nucleated during reaction (3). In addition, deformation during early stages of burial will increase chances of lawsonite crystallization.

Rapid growth of lawsonite is supported by hourglass Ti zoning in LwsA cores (Figs. 4e-g, Sup. mat.

3). Preferential incorporation of Ti on some crystal faces has been related to out-of-equilibrium Ti incorporation [START_REF] Dowty | Crystal structure and crystal growth: I. The influence of internal structure on morphology[END_REF] and compares to sector zoning implying fast growth for lawsonite (Ueno,

J o u r n a l P r e -p r o o f 1999; Vitale-Brovarone et al., 2014). Rapid growth is also supported by the thin, cm-scale reaction fronts (Fig. 4d) showing high reactivity of the fluid (large Damköhler number, the ratio of reaction rate to element advection rate in fluids; see [START_REF] Skelton | Fluid-flux and reaction rate from advective-diffusive carbonation of mafic sill margins in the Dalradian, southwest Scottish Highlands[END_REF][START_REF] Wigley | Controls of sluggish, CO2-promoted, hematite and Kfeldspar dissolution kinetics in sandstones[END_REF]. In this view the production of lawsonite appears transport-limited.

Therefore the restricted P-T window (hence time) over which lawsonite crystallized is likely underestimated by equilibrium modelling and reaction (3) may be more progressive. First, rock compositions considered in the modelling may represent already evolved or mixed lithologies rather than the exact initial protoliths. Instead, calcium may not be readily available in pelite-rich domains, depending on the distance to carbonate layers (i.e., carbonate/pelitic layers alternation), thus transport mechanisms may significantly delay lawsonite crystallization. Around 200°C, cation diffusivity is sluggish yet at least five orders of magnitude faster in free fluid than grain boundary diffusion (e.g., [START_REF] Carlson | Dependence of reaction kinetics on H2O activity as inferred from rates of intergranular diffusion of aluminium: Reaction kinetics and aluminium diffusion[END_REF][START_REF] Verlaguet | Selective transfer of Li-Al-rich phyllosilicate to metamorphic veins (Western Alps): Laser Induced Breakdown Spectroscopy (LIBS) compositional profiles and microstructural characterization[END_REF]. Therefore the presence of free fluids in otherwise isolated carbonate domains will be key to initiating reactions. Lawsonite-bearing veins also attest the presence of a free fluid phase. It is noteworthy that dehydration of smectite and replacement by illite, precursor to phengitic mica, provides water in pelitic domains over this temperature range [START_REF] Vidal | Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H2O activity[END_REF].

This points in turn to additional lawsonite-forming reactions not considered in the modelling. Detrital phases such as feldspar and clay minerals are expected at conditions below appearance of lawsonite.

J o u r n a l P r e -p r o o f

Journal Pre-proof

Their breakdown to mineral assemblages including lawsonite will be progressive. In particular in pelitic domains, the smectite -illite -phengitic mica transition may contribute as a source of Fe and Mg to form ankerite and lawsonite from calcite, as in reaction (3), enlarging the P -T field of lawsonite crystallization. This is estimated as subordinate to reaction (3) because chlorite typically contains 10 to 25 times more Fe and Mg than illitic clays equilibrated around 200°C [START_REF] Dubacq | Dehydration of dioctahedral aluminous phyllosilicates: thermodynamic modelling and implications for thermobarometric estimates[END_REF], their figure 2).

In summary, an array of evidence points to fast initial growth of lawsonite at moderate pressure, at least for most of LwsA (cores), which makes up a large volumetric proportion of lawsonite. More progressive events of crystallization and growth of lawsonite along the prograde path are attested and tentatively set in the frame of deformation events below.

Different lawsonite types and veins: nature and timing of crystallization

-LwsA: The large amounts of LwsA in pelitic layers (Fig. 3) are consistent with relatively rapid lawsonite growth, possibly as early in the burial history as around 0.4 GPa (Fig. 10). LwsA crystals also evidence incremental growth during deformation as shown by the trails of organic matter preserved in their cores and rotation of the cores with respect to the inclusion-free crystals (Figs. 4e-g, 6d; see also [START_REF] Caron | Rapports entre diverses "générations" de lawsonite et les déformations dans les Schistes lustrés des Alpes cottiennes septentrionales (France et Italie)[END_REF]. This event is possibly associated with D1 deformation phase [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF]. Deformation analysis is unfortunately complicated by the succession of deformation stages and J o u r n a l P r e -p r o o f strain localization in the rheologically weak pelitic layers [START_REF] Agard | Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps[END_REF]. Rapid and incremental growth could result from response to rearrangement of porosity, fluid pressure fluctuations and/or seismic cycles [START_REF] Fagereng | Incrementally developed slickenfibers -Geological record of repeating low stress-drop seismic events?[END_REF][START_REF] Viete | Metamorphic records of multiple seismic cycles during subduction[END_REF][START_REF] Ganzhorn | Porosity of metamorphic rocks and fluid migration within subduction interfaces[END_REF].

-LwsC: Small (<5 cm) tensile veins hosting LwsC are best preserved around D2 folds (Figs. 11a, 3d, 5f-g). These veins contain all products of reaction (3), i.e. lawsonite-ankerite-quartz, in proportions similar to those modelled in pseudosections and with textures evidencing co-crystallization before transposition into the S2 schistosity (Figs. 5,11c,d). The mineralogical homogeneity of LwsC veins is interpreted as reflecting growth during progressive deformation, along the prograde path and from reaction (3), in deformation-controlled sites. These tensile veins evidence mode I opening and point to short-lived events, possibly repeated over time (through a delayed reaction (3)? Figs. 11d, 12a). Such fibrous veins are commonly interpreted as the result of mineral growth in volumetrically limited fluidfilled dilating space [START_REF] Bons | The formation of veins and their microstructures[END_REF][START_REF] Oliver | Mechanisms of fluid flow and fluid-rock interaction in fossil metamorphic hydrothermal systems inferred from vein-wallrock patterns, geometry and microstructure[END_REF][START_REF] Verlaguet | Metamorphic veining and mass transfer in a chemically closed system: a case study Alpine metabauxites (western Vanoise)[END_REF][START_REF] Fisher | The role of silica redistribution in the evolution of slip instabilities along subduction interfaces: Constraints from the Kodiak accretionary complex, Alaska[END_REF]. The constant width of fibres across the vein indicates that crystals grew at the same rate, independent on the mineral or nuclei orientation, suggesting that their growth was limited by the vein opening rate. The earliest LwsC veins have probably been largely transposed into the schistosity.

-LwsB: Veins hosting LwsB generally lack ankerite or only in subordinate and variable amounts. The fact that LwsB veins do not show products of reaction (3) (unlike LwsC veins), suggests that LwsBbearing veins formed either during replacement of ankerite and carpholite (e.g., through reaction (4),

J o u r n a l P r e -p r o o f >330°C on figures 10c, 12a) and/or by pressure solution of lawsonite, in the presence of either local or external fluids. The latter is supported by the growth of some LwsB in pressure shadows around former quartz vein boudins. These veins are by far larger and longer (>5-10 cm) than LwsC veins.

They evidence boudinage stretching and internal rotation (Fig. 5a) indicative of formation throughout progressive deformation.

As a final note, the initial boundaries between the carbonate and pelitic layers are attenuated through deformation and migration of reaction fronts (Fig. 11b). This contributes to the progressive chemical mixing of the subducted sediment at the local scale, enhancing the stability of carbonates.

Implications for mass transfer and the carbon budget in subduction zone

The major lawsonite-in reaction reported here (reaction (3)) for the first time allows explaining the high lawsonite content observed in the calcschists. Importantly, the mineralogical and petrological evolution indicates that no significant decarbonation took place during lawsonite formation in the Schistes Lustrés calcschists up to 1.6 GPa (maximum pressure in Triplex area). Calcite dissolution led instead to local co-crystallization of lawsonite and ankerite during relatively fast reactions. In the debate on how much carbon is recycled to subarc depths or even beyond into the deep mantle (i.e., significant: Gorman et al., 2008;[START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF][START_REF] Epstein | Scales of Fluid-Rock Interaction and Carbon Mobility in the Deeply Underplated and HP-Metamorphosed Schistes Lustrés[END_REF]minor: Kelemen and J o u r n a l P r e -p r o o f [START_REF] Kelemen | Reevaluating carbon fluxes in subduction zones, what goes down, mostly comes up[END_REF], the results of this study show that there is no major decarbonation of slab sediments at least to depths of ̴ 40km.

Further, in our samples, early-crystallized lawsonite stored much organic matter as inclusions, leading to minimum C export from the rock, at least until 35-40 km depth (Fig. 12). Assemblages bearing

LwsA and LwsC imply that C remained within centimetres of its dissolution site, consistent with the isotopic signatures of C, O and H [START_REF] Henry | Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major-and trace-element systematics[END_REF][START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF][START_REF] Collins | Subduction zone metamorphic pathway for deep carbon cycling: II. Evidence from HP/UHP metabasaltic rocks and ophicarbonates[END_REF][START_REF] Jaeckel | Deformation-enhanced fluid and mass transfer along Western and Central Alps paleo-subduction interfaces: Significance for carbon cycling models[END_REF]. There is also no evidence of carbonation by external C-enriched fluid as reported in ultramafic horizons in Alpine Corsica [START_REF] Piccoli | Carbonation by fluid-rock interactions at high-pressure conditions: Implications for carbon cycling in subduction zones[END_REF].

Intense veining together with the high abundance of water-rich minerals are direct witnesses of high fluid activity in the Schistes Lustrés. Internally-derived fluids contribute to pervasive, cm-scale element transfer between pelite and carbonate-rich domains (Figs. 4a,b), attenuating lithological contrasts (Figs. 11b,c) and restricting the scale of carbon mobility (black and blue boxes on figure 12b). Excess fluids may nevertheless be expelled into larger-scale vein networks (i.e., LwsB-bearing veins; blue arrows on figure 12b), possibly contributing to migration of fluids, silica and volatiles beyond the cm-m-scale.

Large-scale silica production at seismogenic depths was hypothesized to account for high Vp/Vs ratios (Figs. 11d, 12;[START_REF] Audet | Possible control of subduction zone slow-earthquake periodicity by silica enrichment[END_REF]. Lawsonite-quartz-bearing veins and reaction (3) do show mobilization and production of quartz. However, observations of the bulk mineralogy and J o u r n a l P r e -p r o o f thermodynamic modelling imply small amounts of quartz produced, as silica is consumed in other reactions such as the Tschermak exchange of phengitic micas. There is also no field evidence for massive addition of silica via quartz veins, and none was detected by whole-rock chemistry [START_REF] Henry | Evolution of synmetamorphic veins and their wallrocks through a Western Alps transect: no evidence for large-scale fluid flow. Stable isotope, major-and trace-element systematics[END_REF]. We conclude that large-scale silica migration is not likely in paleo-subduction complexes hosting Tethyan-type metasediments such as Schistes Lustrés.

CONCLUSIONS

This study shows that the mineralogical evolution and voluminous formation of lawsonite during prograde metamorphism of the Schistes Lustrés complex indicate element redistribution mediated by fluids from low-grade metamorphic conditions (180°C, 0.4 GPa) to seismogenic depths (Fig. 12b).

Lawsonite mostly grows in stoichiometric proportions with ankerite and quartz from calcite and phyllosilicates. Stabilization of a carbonate phase during lawsonite formation precludes any significant decarbonation of the calcschists. While large amounts of fluids are involved, these results are consistent with the Schistes Lustrés complex behaving mostly as a closed system at the m-scale, in agreement with previous studies. Generalized mobilization of elements and dissolution/crystallization of water-rich phases set up rapid reactions leaving a wealth of small reaction fronts and veins consistent with local redistribution. In this view most of the lawsonite crystallized in a relatively short period of time from a continuous reaction in a narrow P-T field. One type of lawsonite-bearing vein J o u r n a l P r e -p r o o f (LwsB) is consistent with progressive and incremental crystallization, and potentially larger transport distances, but is found in lesser amounts. MV: Mont Viso, [START_REF] Locatelli | Intermediate -depth brecciation along the subduction plate interface (Monviso eclogite, W. Alps)[END_REF]DM: Dora Maira, Chopin et al., 1991). Sample locations are reported on the geological map (B) and on the cross-section (C). Fraiteve and the Assietta pass (along the path of Fig. 1b), presented on a simplified cross-section (topography not to scale). Colours as on A) and B). Calcschists are dominant overall and lawsonite is found all across the section apart from the vicinity of Testa dell'Assietta where it is in minor proportions. Hourglass zonations are highlighted by colour variations and by inclusion trails of organic matter.

FIGURES CAPTIONS

Pale, inclusion-free lawsonite overgrows zoned, boudinaged and fractured lawsonite cores, sometimes leading to amalgamation of crystals. Qz: quartz; OM: organic matter. For LwsC veins, the ratios estimated including pseudomorphs (stars) are significantly higher than without pseudomorphs and are those considered, as LwsC crystals are highly retrogressed compared to LwsA and LwsB.

  figure 3 is a composite D1-D2 structural pattern. The D2 shear bands are cross-cut by later, ductile to

  ) cutting across the host rock. They show the same J o u r n a l P r e -p r o o f cream-coloured, elongated fibres as LwsB-bearing veins but are systematically associated with ankerite and quartz. The LwsC-bearing veins are particularly abundant in calcschists hosting LwsA and mostly located around fold hinges (in extrado domains) and near larger LwsB-bearing quartz veins. LwsC-ankerite-quartz fibres are perpendicular to the vein walls and these veins are dominated by mode I opening (Figs.5f, g). Sometimes, these veins cross-cut LwsB-bearing veins and apparently opened in a single stage, possibly indicating shorter-lived tensile events than for LwsB formation.Early veins transposed along the schistosity have been identified in places as former, strongly deformed LwsC-bearing veins, suggesting that only the latest generation of LwsC-bearing veins is observed. Local accumulations of LwsB-and LwsC-bearing veins are found in m-scale domains scattered around Mt. Triplex, in lithologies where LwsA is also abundant (Figs.3d, f).

  mat. 2). The corrected LwsC/Ank estimates plot around 1.5, as for LwsA/Ank. Large variations in LwsB/Ank ratios reflect the largely varying amounts of ankerite(Figs. 7, 5a, e). Most phyllosilicates appear linked to peak or retrograde J o u r n a l P r e -p r o o f

  significant decarbonation: there is crystallization of ankerite and overall no major dissolution of carbonate. The third hypothesis is favoured: replacement of primary calcite from the carbonate-rich lithologies by ankerite allows substituting half of the initial Ca amount with Fe and Mg, with zero net J o u r n a l P r e -p r o o f export of carbon. The net reaction releases sufficient Ca to form lawsonite in the more pelitic layers from mixed lithologies. An indication on the spatial range of element transfer is provided by the very low content of lawsonite in pelitic lithologies with no carbonate horizon within a few meters (almost zero in lithology 1).

  may have been somewhat similar to minerals overgrowing lawsonite and ankerite on exhumation (i.e., chlorite, calcite Fig.9a), and (ii) the large amounts of hydrous aluminosilicates present in common sediments(Planck and Langmuir, 1998), lawsonite and ankerite are expected to form at the expense of chlorite, calcite and kaolinite, themselves formed at the expense of former detrital and early diagenetic phases. The observed overall 1.5 volume ratio of lawsonite to ankerite (controlled by LwsA-bearing rocks) corresponds to a 1:1 Lws/Ank molar ratio. This points to a reaction such as: chlorite + kaolinite + aragonite = lawsonite + ankerite + quartz + water (3) 0.2 Mg2.5Fe2.5Al(Al,Si3)O10(OH)8 + 0.8 Al2Si2O5(OH)4 + 2 CaCO3 = CaAl2Si2O7(OH)2•[H2O] + Ca(Mg0.5,Fe0.5)(CO3)2 + 0.2 SiO2 + 0.4 H2O. (assuming XMg=0.5 for chlorite)

Fe

  and Si at constant amount of mica, could also contribute to lawsonite formation, as proposed by[START_REF] Cook-Kollars | Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps[END_REF]. In pelite-rich domains, the reaction, balanced here for phengite endmembers in the KCaFASHC system, would produce CO2 and consume H2O, as suggested by Cook-Kollars et al. (2014): J o u r n a l P r e -p r o o f muscovite + ankerite + quartz + water = lawsonite + Fe-celadonite + CO2 (5) KAl2(Al,Si3)O10(OH)2 + CaFe(CO3)2 + SiO2 + 2 H2O = CaAl2Si2O7(OH)2•[H2O] + KAlFeSi4O10(OH)2 + 2 CO2.

  ,Si3)O10(OH)2 + 2 CaCO3 + 1.8 SiO2 + 0.4 Fe5Al(Al,Si3)O10(OH)8 + 0.8 H2O = CaAl2Si2O7(OH)2•[H2O] + KAlFeSi4O10(OH)2 + CaFe(CO3)2 + 0.8 AlO(OH).

Figure 1 :

 1 Figure 1: Geological setting

Figure 2 :

 2 Figure 2: Distribution of metasediments in the upper unit of the Schistes Lustrés complex

Figure 3 :

 3 Figure 3: Structural framework and field observations on Monte Triplex

Figure 4 :

 4 Figure 4: Lawsonite in calcschists: LwsA (lithology type 3)

Figure 5 :

 5 Figure 5: Lawsonite in metamorphic veins: LwsB and LwsC

Figure 6 :Figure 7 :

 67 Figure 6: Textural observations of the LwsA-ankerite-quartz assemblage

Figure 8 :

 8 Figure 8: Mineral composition.

Figure 9 :

 9 Figure 9: Element maps for lawsonite-ankerite-quartz assemblages.

Figure 10 :Figure 1 Figure 2 Figure 3

 10123 Figure 10: Thermodynamic modelling of reactions and phase equilibria

Figure 4 Figure 5

 45 Figure 4

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Figure 8 Figure 9 Figure 10 Figure 11

 891011 Figure 8

Figure 12

 12 Figure12

Acknowledgements

This work was financially supported by the CNRS-INSU SYSTER programs, by ISTeP and by the ZIP project (REA grant agreement no. 604713 (ZIP "Zooming In between Plates") from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013). We thank O. Boudouma and D. Deldique for their expertise with SEM characterisation; M. Fialin and N. Rividi for help with EPMA; B. Caron and B. Villemant for ICP -OES measurements; E. Delairis for making the thin sections and H. Raimbourg and L. Jolivet for discussions. We also thank P. Goncalves and B. Dragovic for their constructive reviews, which helped clarify the manuscript. J o u r n a l P r e -p r o o f

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

J o u r n a l P r e -p r o o f

A) Summary of the inferred metamorphic evolution of the upper unit of the Schistes Lustrés complex.

LwsA crystallizes rapidly compared to the complex vein system formed progressively over most of the prograde history in the stability field of lawsonite. The small sketches and the text on the right sum up the main information regarding lawsonite formation at three different step time reported on the P -T grid. B) Schematic model of the varying properties, in particular water retention versus production of the Schistes Lustrés units at seismogenic depths. Hypothetical fluid pathways are likely found in calcschists, which produce the most water (reaction (3)). Insert on the top right as been modified after Audet and Bürgmann, 2009 and locate the studied area at depth (LFSEs = Low-frequency earthquakes).

TABLE CAPTION

Table1: Representative analysis of each type of lawsonite encountered in the studied samples.

SUPPLEMENTARY MATERIALS CAPTIONS

Sup


S Salvador
email: salvador@enstimac.fr.

J.-M Commandre

B R Stanmore

R Gadiou
email: r.gadiou@uha.fr.

L ; Granatstein

D L Fouling

The Catalytic Effect of Vanadium on the Reactivity of Petroleum Cokes with NO

Petroleum cokes are widely used in cement plants for energy generation. Because of their very low volatile content, the physicochemical processes occurring during their combustion should be mainly controlled by heterogeneous reactions. The reactivities of some petroleum cokes toward NO between 800 and 1000 °C have been characterized through experiments in an entrained flow reactor. These carbonaceous materials exhibit very high reactivities toward NO. Their capacity to reduce NO concentration in the surrounding gas is significant, so that this reaction can have a significant impact in terms of NO emissions resulting from their combustion. The influence of surface area and of the heavy metal content on surface reactivity was tested. As it was the case for carbon-oxygen reaction, the surface measured by mercury porosimetry seems to be a good parameter to calculate surface reaction rate. No correlation was found between calcium or iron content and NO-carbon reaction rate. As for the reaction of petroleum coke with oxygen, vanadium appears to have a catalytic effect on the reduction reaction of NO with carbon.

Introduction

Petroleum coke, a residue of the petroleum refining industry, is used today in large quantities as a source of energy. This solid fuel has the advantage of being easy to grind, of containing a high calorific value (between 32.5 and 34.5 MJ kg -1 ), and of being cheap in the market place. For cement plant precalciner applications, it is ground to approximately 50% < 90 µm, and used as a coal replacement fuel. It has also been used in fluidized bed combustors, generally co-fired with coal. 1,2 The previous years of utilization have identified a number of specificities in their utilization, particularly in terms of process control and of pollutant formation. Very little literature has been devoted to these fuels, making it difficult to interpret the differences observed in their behavior as compared with coal.

Because of the low volatile matter content of this type of fuel, the importance of heterogeneous reactions should be enhanced both for the oxidation of the carbonaceous material or for the formation of pollutants. The wide variation of reactivities of petroleum cokes toward oxygen has already been demonstrated in a previous paper; [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF] it has been shown that these variations can be correlated with the catalytic effects of vanadium. The emissions of nitrogen oxides at the exhaust of a combustor are the result of a balance between the formation of NO and its reduction by homogeneous and heterogeneous reactions. Therefore, the heterogeneous reduction of NO on petroleum coke is expected to influence the nitrogen oxide level in the exhaust, and the determination of its reactivity toward NO is of interest.

The reduction of nitrogen monoxide by different sorts of carbonaceous solids has been the aim of many studies since the 1980's, because this phenomenon is important in fluidized bed combustion. [START_REF] Johnsson | Formation and reduction of nitrogen oxides in fluidized bed combustion[END_REF] Despite this large amount of published material, numerous uncertainties still remain. The mechanism of the reaction itself is not fully understood although many attempts have been presented. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF][START_REF] De Soete | Heterogeneous N2O and NO formation from bound nitrogen atoms during coal char combustion[END_REF] Because of this lack of established mechanism, most authors have tried to fit their experimental data with global kinetic laws. But, as it has been shown by Aarna and Suuberg in their review, [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] large variations are observed between published data, whatever the temperature.

To explain these differences, the chemical structure and the textural properties of the solid are the most often investigated parameters. Rodriguez-Mirasol et al. [START_REF] Rodriguez-Mirasol | NO and N2O decomposition over coal char at fluidizedbed combustion conditions[END_REF] studied the influence of the rank of the parent coal on the reactivity of the char toward NO in a small scale fixed bed. He observed that lignite chars are more reactive than chars obtained from high rank coals; the graphitic structure of the char is therefore an important parameter in the determination of the reactivity. A study by Zevenhoven and Hupa 9 on a larger number of chars led to the same conclusion. Nevertheless, these results may be mainly a consequence of the differences in texture and chemical composition of the samples. A more precise study of this point has been made by Gulyurtlu et al. who used char prepared at differents temperatures from the same raw coal. [START_REF] Gulyurtlu | N2O formation during fludized bed combustion of chars[END_REF] The authors observed that the higher the preparation temperature, the more reactive the sample is. This result could be explained by an increase of surface area and also by the graphitization of the carbon structure.

To take into account the influence of the texture of the samples, most authors calculated the specific reaction rate by dividing the global rate of reaction by a surface area. This last parameter is either the CO 2 adsorption surface or the N 2 surface computed by the BET equation. Shimizu used the former surface and found a constant value of the specific reaction rate for nine samples which had surfaces ranging between 250 and 900 m 2 /g. [START_REF] Shimizu | Conversion of char-bound nitrogen to nitric oxide during combustion[END_REF] Illian-Gomez et al. [START_REF] Illan-Gomez | NO reduction by activated carbons. 1. The role of carbon porosity and surface area[END_REF] observed also a linear correlation between the reaction rate of carbon samples at 873 K and the BET surface area. Nevertheless, other studies showed that the specific reaction rate presents large variations as a function of the chemical composition of the carbonaceous samples.

With an experimental setup similar to Rodriguez-Mirasol, Chan et al. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF] examined the influence of mineral matter, and observed that the demineralization of a lignite char leads to a decrease in its reactivity with NO by an order of magnitude. As for the carbon-oxygen reaction, many elements can have a catalytic effect on the NO-char and the N 2 O-char reactions. Chan et al. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF] and Zevenhoven and Hupa 9 observed that the calcium content of the most reactive char, i.e., the lignite char, was high. Illan-Gomez et al. studied the influence of mineral matter on the rate of NO reduction over chars. They observed that whatever the temperature, the addition of a metal such as Fe, Cu, or Ni decreases the activation energy of the reaction. [START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 5. Catalytic Effect of Iron[END_REF][START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 6. Catalysis by transition metals[END_REF] The catalytic activity of potassium and calcium was also observed. [START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 2. Catalytic Effect of Potassium[END_REF][START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 3. Catalysis by Calcium[END_REF] Some authors tried to remove the mineral matter of chars in order to quantify the decrease in the reaction rate. [START_REF] Guo | Effects of CaO and burnout on the kinetics of reduction by Beulah zap char[END_REF][START_REF] Aarna | The role of carbon monoxide in the NO-carbon reaction[END_REF][START_REF] Peter | Reactivite des oxydes d'azote formes lors de la combustion de charbon en lit fluidise circulant[END_REF] The main problem is that the demineralization is never complete, so that a catalytic activity remains; another problem is that the preparation of the sample changes the textural properties of the carbonaceous solid. Despite these difficulties, a decrease of the rate of the reduction of NO on the char surface was observed in all cases.

In this work, we have characterized the kinetics of the reaction of NO reduction on the char surface of a number of petroleum cokes between 800 and 1000 °C. This temperature range is representative either for FBC combustion of petroleum coke or for the precalciner of cement plants. As we will demonstrate, this reaction has a significant impact on the global emissions of NO resulting from the combustion of the petcokes.

Experimental Section

2.1. Experimental Setup. To characterize the reaction kinetics in conditions close to those of a fluidized bed or of an entrained flow such as a cement plant precalciner, a set of experiments was carried out in a laboratory scale entrained flow reactor (EFR). Although many different experimental devices may be used to obtain heterogeneous reactions kinetics such as a thermogravimetric apparatus or fixed beds, [START_REF] De Soete | Heterogeneous N2O and NO formation from bound nitrogen atoms during coal char combustion[END_REF][START_REF] Rodriguez-Mirasol | NO and N2O decomposition over coal char at fluidizedbed combustion conditions[END_REF][START_REF] Suuberg | Studies on the kinetics and mechanism of the reaction of NO with carbon[END_REF] the EFR has been widely used because it allows the dispersion of particles in a gas flow. Therefore the diffusion limitations observed in fixed beds can be avoided. Another advantage is that high temperatures may be reached. [START_REF] Levy | NO-char reactions at pulverized coal flame temperatures[END_REF][START_REF] Song | Reduction of Nitric Oxide by Coal Char at Temperatures of 1250-1750 K[END_REF] The reactor design has already been detailed elsewhere. [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF][START_REF] Van De Steene | Controlling powdered fuel combustion at low temperature in a new entrained flow reactor[END_REF] The apparatus shown in Figure 1 consists of an 80 mm i.d. and 1 m long isothermal fused silica reactor fed with an electrically preheated gas flow. A stream of powdered fuel particles was injected through a water-cooled feeding probe. Care was taken to obtain a good dispersion of the particles on a cross section of the reactor by placing a dispersion dome 1 mm downstream from the tip of the feeding probe.

The gas was sampled after a controlled residence time through a water-cooled nitrogen-quenched probe. It was then analyzed; NO, CO, and CO2 were quantified by on-line NDIR analyzers, and gaseous hydrocarbons were measured with a flame ionization analyzer.

Samples Characterization.

Eight samples were used in this work, six petroleum cokes, a bituminous coal and an activated carbon. The petcokes used in this study were sampled at different industrial sites; no data concerning their origin was available. Table 1 gives the results of the proximate and ultimate analyses. They show very similar compositions in terms of the percentages of C and H. As expected for heavy petroleum fuels, the natural sulfur contents are high, and range between 4% and 7.5%. The fuel nitrogen concentrations are similar for all of the petcokes. The proximate analysis indicates low volatile matter and low ash content. The aim of this work was to extract kinetic data for a number of petcokes and to study the influence of the nature of the sample. Then, all the chars were prepared at the same temperature and their reactivity was studied as a function of temperature. The chars were obtained by pyrolyzing the fuel samples in the EFR at 900 °C. The bituminous coal (Ivan Coal) was used as a reference to a classical fuel, and the activated carbon (AC) was retained as a high BET area carbon for considerations of reactive surface, as discussed further. The analyses of these two fuels are also given in Table 1.

Prior to all analysis and experiments, all the fuels were ground to less than 200 µm, dried, and the fraction 30-100 µm was selected by pneumatic sieving. Particles of less than 30 µm were removed to enable regular feeding of the fuel during EFR experiments.

To obtain a good characterization of the texture of the char samples, the surface areas were measured using two methods:

• nitrogen adsorption at 77 K with the BET equation,

• mercury porosimetry to quantify the surface area of the mesopores and macropores of the particles. This method was used to measure the surface area of pores larger than 20 nm (Hg20).

The surface Hg20 was here derived from mercury porosimetry tests, using the standard pore model proposed by Satterfield, 27 in which the pore diameter is calculated from the porosity , the particle density Fp, and the pore surface area S as follows: 26 The adsorption of CO2 at 293 K also allows the determination of a surface area with the help of models such as the Dubinin-Polanyi equation. The problem is that in many experiments, the access of the gaseous reactant to micropores is difficult. As a result, this method was not used in this work. The porosity was obtained by helium pycnometry. Table 1 presents the results of this characterization. The values of the surface areas of the petcokes are very low compared, for instance, to that of the coal, which typically exhibits BET area of several tens or hundreds of m 2 g -1 . It should be noted that the petcoke samples exhibit lower surface area than many samples used in previously published studies. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF][START_REF] Rodriguez-Mirasol | NO and N2O decomposition over coal char at fluidizedbed combustion conditions[END_REF][START_REF] Shimizu | Conversion of char-bound nitrogen to nitric oxide during combustion[END_REF] The concentrations of 7 elements known for their catalytic activity in carbon reactions were analyzed: vanadium, copper, molybdenum, iron, potassium, calcium, and manganese; the results are reported in Table 2. As in heavy fuel oils, large amounts of vanadium are found. This element is present in quantities as large as 0.26% of the dry mass in Asland petcoke. The other metals are much less concentrated, apart from iron in St Constant (0.17%) and in Lily (0.061%). The BET, the Hg20 surface area, and the porosity are those of the chars.

Reaction of

d p ) 4 F p S
experiments. The chars were previously prepared by passing the fuels in 4.16% O2 in nitrogen through the EFR at 900 °C.

The experiments to characterize the chars reactivities toward NO were carried out at three temperaturess800, 900, and 1000 °C, so that an activation energy can be determined. The particles feed flowrate was 1.5 g/min. The gas feed flow was a mixture of 955 ppm of NO in nitrogen, and the flow rate was 10 L/min (NTP). Under these experimental conditions, the residence time ranged between 3.25 and 3.6 s, depending on temperature. If we assume a homogeneous dispersion of the particles in the volume of the reactor, the solid concentrations in the tubular reactor are equal to 38, 35, and 32 g/m [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF] for experiments at 800, 900, and 1000 °C, respectively.

We report in Table 3 the NO reduction yield for the different samples as a function of temperature. The decrease of NO concentration between injection and the bottom of the reactor vary between 11 ppmv for Lily petcoke char at 800 °C and 920 ppmv for the coal char at 1000 °C. It is obvious from these results that the quantity of NO that can be reduced on petroleum coke, in the presence of an NO concentration similar to that in an industrial furnace, is very large. In agreement with previous studies in this temperature range, the influence of temperature is important. For example, Chan et al. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF] reported activation energies larger than 200 kJ/mol for graphite and char samples.

For all experiments, the concentration of hydrocarbons at the exhaust of the reactor was below 20 ppmv. Small amounts of CO were detected which could be related mainly to the reaction 2NO + 2C f 2CO + N2. In most cases, the CO measured was higher than the NO consumed; the excess of CO ranged between 0 and 500 ppmv for all experiments. This excess can be attributed to the desorption of CO from the oxygen adsorbed on the char.

There have been numerous studies in which the enhancement of the NO reduction rate by CO was reported. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF][START_REF] Aarna | The role of carbon monoxide in the NO-carbon reaction[END_REF] This leads to difficulties in the kinetic interpretation of the experimental results since CO is a product and a reactant. [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] Two explanations have been presented to explain this behavior:

• the stripping of surface oxides by CO through the reaction CO + C(O) f CO2 + C*. This process can be significant at low temperature when the desorption of surface oxides is slow,

• the catalysis of the reaction NO + CO f 0.5N2 + CO2 by various substances such as the char itself.

It must be noticed that, in other studies, it has been observed that the addition of low concentrations of CO (i.e., below 700 ppmv) does not lead to an increase of the char-NO reaction rate. [START_REF] Johnsson | Formation and reduction of nitrogen oxides in fluidized bed combustion[END_REF] The order of the reaction with respect to CO is also not known. The study of this complex phenomenon was outside the scope of this work, and it was decided to analyze the experimental results in a classical way with a single firstorder kinetic law.

The concentration of CO2 was not significant. This is consistent with previously published studies in which it has been shown that the CO/CO 2 ratio is very high in this temperature range. [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] The low amount of carbon oxides and hydrocarbons shows also that there is no significant further devolatilization of the chars during the experiments in the EFR.

As expected, the coal char and the activated carbon lead to the highest reaction yield; this can be related to the high surface areas of these two samples. The elemental and proximate analyses of the six petcokes are similar, so that one could expect to observe the influence of the texture of the samples. Among the petroleum coke samples, Havre CDP, which exhibits the highest surface area, does not lead to the highest reduction yield. Therefore, other parameters such as catalytic elements must influence the reactivity.

Discussion

The rate constants were determined by fitting the experimental results with those from a model. As pointed out by Aarna and Suuberg in their review, [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] many published results use the mass of NO divided per mass of char per unit time per unit pressure of NO to define their rate constant. In this work we use a similar definition for the chemical reaction rate constants, but we take into account the surface area of the samples. Therefore, the reaction rate was expressed as follows: with where [NO] is the molar concentration of NO in the gas and C c is the concentration of carbonaceous material in the gas. η NO is the effectiveness factor, as calculated from the Thiele modulus for a reaction order equal to 1. Some authors have observed reaction orders toward NO which range between 0.2 and 0.8 for this reduction reaction. [START_REF] Rodriguez-Mirasol | NO and N2O decomposition over coal char at fluidizedbed combustion conditions[END_REF]28 Nevertheless, most studies used a reaction order of 1, whatever the experimental setup used to obtain the results. [START_REF] Suuberg | Studies on the kinetics and mechanism of the reaction of NO with carbon[END_REF][START_REF] Song | Reduction of Nitric Oxide by Coal Char at Temperatures of 1250-1750 K[END_REF] To fit experimental data, a mass balance was done between the entry and the exhaust of the entrained flow reactor. The main assumptions of this model are the following:

• the zone close to the axis is a plug flow reactor,

• the dispersion of the particles in the gas is homogeneous,

• the particles velocity is equal to the gas velocity. The details of the calculations are given elsewhere. 26 An iterative computation was used to obtain the effectiveness factor η NO and the Arrhenius parameters for each sample.

The plots in Figure 2 and Figure 3 allow the comparison between the reactivities of the different petcoke chars and that of the coal char. Shimizu

dm NO dt ) -k m c S sp η NO P NO k ) A e (-E/RT)
char reactivity, 11 but this was not possible in some others studies. [START_REF] Rodriguez-Mirasol | NO and N2O decomposition over coal char at fluidizedbed combustion conditions[END_REF] Aarna and Suuberg [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] have proposed a synthesis of the reaction rates, specified on BET areas, derived by a number of authors. Their correlation was converted to the same units as the ones used in this work, and plotted in Figures 2 and3. The spread in the reaction rates is much more important in Figure 2, on BET area base, than in Figure 3, which was plotted on Hg 20 base. Radovic et al. 24 expressed as early as 1983 that BET area is an inadequate basis for the expression of reactivity. Aarna and Suuberg 25 showed that micropores hardly participate in the reaction between oxygen and carbon. In a recent work by Salvador et al., [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF] it was shown that the scatter in the reactivities of carbons of various forms was much smaller when the surface of the pores larger than 20 nm (Hg 20 ) was used to express the reactive surface, instead of the total pore surface. This tends to confirm that micropores smaller than 20 nm do not participate in the reaction. A study on the reactivity of carbons issuing from different origins with NO lead Commandre et al. 26 to the same conclusion.

It appears to be the case that excluding the surface area of the small pores (i.e., with diameters lower than 20 nm) from the reactive surface reconciles the data from various forms of carbon; and thus Hg 20 surface area is likely to be a better indicator of the real reactive surface than BET. We have already reported this result for carbon oxidation in a previous work. [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF] Petcokes again exhibit very high reactivities as compared with results from the literature. [START_REF] Aarna | A review of the kinetics of the nitric oxide-carbon reaction[END_REF] Most authors have observed that the activation energy increases in value at temperatures between 800 and 1000 K. Although there are large variations in the measured activation energies, the values generally measured change from 75 kJ/mol below 950 K to 180 kJ/mol above this temperature. [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF][START_REF] Suuberg | Studies on the kinetics and mechanism of the reaction of NO with carbon[END_REF]29 The low activation energy observed at low temperature has been related to the limitation by the desorption of reaction products; a limitation which disappears as the temperature increases. The rate of the high temperature reaction is therefore controlled by the dissociative adsorption of NO on the carbon surface. As shown in Table 4, we observed large variations of the activation energies of the different samples tested in this study, as the values obtained with the six petcokes range from 115 to 240 kJ/mol. These values are in good agreement with previously published data. It must be noticed that the three samples which lead to activations energies close to 235 kJ/mol are the ones which have a low vanadium content. This value is in the highest range of activation energies found in the literature, [START_REF] Chan | Kinetics of the NOcarbon reaction at fluidized bed combustion conditions[END_REF][START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 2. Catalytic Effect of Potassium[END_REF]29 and may be related to the non-catalyzed reaction of NO with carbon.

The catalytic effect of metals on the reaction of C with NO can be estimated from Figures 4 and5 where the metal concentration was plotted versus the reaction rate at 1200 K, using Hg 20 as the area base. Figure 5 shows that a clear correlation appears between the vanadium content of the petcokes and reactivity toward NO. The correlation factor for the linear least-squares estimation is r 2 ) 0.72. This shows that vanadium can have a catalytic effect even in the case of a rapid reaction. The ratio between the reaction rate of the more reactive petcoke and that of the less reactive one is 3.5. The two other elements which are presented in this figure are iron and calcium; it can be observed that there is no clear correlation between reaction rate and the concentrations of these two elements. As pointed out in the Introduction, Ca has often been presented as an active catalyst for reduction reactions. [START_REF] Zevenhoven | The reactivity of chars from coal, peat, and wood toward NO, with and without CO[END_REF][START_REF] Guo | Effects of CaO and burnout on the kinetics of reduction by Beulah zap char[END_REF][START_REF] Peter | Reactivite des oxydes d'azote formes lors de la combustion de charbon en lit fluidise circulant[END_REF] It must be noticed that the concentration of Ca in petcoke is lower than that of the char samples used in most studies. The concentration of iron in petcoke is high and an influence of this metal on the NO-har reaction rate could be expected. 30 Nevertheless, this catalytic activity of iron was not observed in some other studies, [START_REF] Zevenhoven | The reactivity of chars from coal, peat, and wood toward NO, with and without CO[END_REF] and Illian-Gomez et al. [START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 5. Catalytic Effect of Iron[END_REF] observed a high activity of Fe only at low temperature (i.e., below 700 K). Figure 5 presents the four metals which are present in low concentration in petroleum cokes. Although potassium has often been found as an active element whatever the temperature, [START_REF] Illan-Gomez | NO Reduction by Activated Carbons. 2. Catalytic Effect of Potassium[END_REF] it is obvious that its low concentration leads to a negligible influence on char reactivity compared to that of vanadium. The same explanation can be applied for copper, molybdenum, and manganese.

The calculation of the particle-related effectiveness factor (following the model of Commandre et al. 26 ) leads to values between 0.7 and 1 if BET surface area is used. As it is presented in Table 4, if Hg 20 surface area is used, η NO is above 0.88 for all the samples whatever the experimental temperature. This shows that NO mass transfer in the porous structure of the chars does not control the reaction yield. This explains why the catalytic effect of vanadium on the reaction is clearly evident here. This was not the case for the reaction of C with O 2 from air during our TG experiments, where oxygen transfer was apparently limiting the combustion velocity. [START_REF] Salvador | Reaction rate for the oxidation of highly sulphurised petroleum cokes: the influence of thermogravimetric conditions and some coke properties[END_REF]

Conclusion

A study of the reaction of NO on petroleum cokes has been carried out in an entrained flow reactor. It was observed that petcokes are very reactive forms of carbon in the heterogeneous reaction of carbon with NO, resulting in NO concentration depletions of hundreds of ppmv after a residence time of less than 1.5 s at temperatures in the range 800-1000 °C.

For the heterogeneous reaction considered, we have shown that excluding the surface of the pores smaller than several tens of nanometers from the total pore surface lead to more uniform values for the reaction rates of chars from different origins. This tends to confirm the assumption that small pores do not participate in these heterogeneous reactions. We think that the surface area of the pores larger than 20 nm which is calculated from mercury porosimetry measurements is a better estimate of the reactive surface than the total pore area determined by classical BET tests or by CO 2 adsorption experiments.

A ratio of 3.5 is indicated between the reaction rates at 1200 K of the more reactive and the less reactive petcoke chars. Seven elements (vanadium, copper, molybdenum, iron, potassium, calcium, and manganese) which are known as catalysts for this sort of reaction were analyzed to explain the different reactivities of petcokes. Although petroleum cokes are low-ash-content fuels, the analysis of these seven elements indicated vanadium contents as high as 0.26%. Among these elements, vanadium appears to be responsible for the high variation of reactivities which are observed for the petcoke chars, and a clear correlation could be found between the vanadium content of the sample and its reactivity toward NO.

Figure 1 .

 1 Figure 1. Entrained flow reactor: (1) Conveyor belt/vibrating feeder/ejector; (2) pneumatic transport; (3) electrical preheater; (4) flow meters; (5) water-cooled feeding probe; (6) dispersion dome; (7) three-zone electrical furnace; (8) 80 mm i.d. quartz tube reactor; (9) cyclone collector; (10) exhaust fan; (11) water-cooled nitrogen-quenched sampling probe; (12) particle collector (filter); (13) sampling pump; (14) gas conditioner; (15) gas analyzers (infrared/paramagnetic); M, mass flow meters and controllers; N2, nitrogen; C.A., compressed air; W, water (probes cooling).

(

  29) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Nitric oxide reduction by char and carbon monoxide. Fuel 1985, 64 (9), 1306-1309. (30) Allen, D.; Hayhurst, A. N. FBC Technology and the Environmental Challenge; Adam Hilger: Bristol, U.K., 1991.

Figure 2 .

 2 Figure 2. Arrhenius representation for the reaction rate of carbon toward NO, specified on BET area.

Figure 3 .

 3 Figure 3. Arrhenius representation for the reaction rate of carbon toward NO, specified on Hg20 area.

Figure 4 .

 4 Figure 4. V, Fe, and Ca content of the petcokes versus their reaction rate toward NO at 1200 K. The line is a linear correlation for vanadium.

Figure 5 .

 5 Figure 5. Trace metals content of the petcokes versus their reaction rate toward NO at 1200 K.

Chars with NO in the EFR.

  The reactivity toward NO of chars has been determined from EFR

(24) Radovic, L. R.; Walker, P. L.; Jenkins, R. G. Fuel 1983, 62, 849-857. (25) Aarna, I.; Suuberg, E. M. 27th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1998; pp 2933-2939. (26) Commandre ´, J. M.; Stanmore, B.; Salvador, S. The High Temperature Reaction of Carbon with Nitric Oxide. Combust. Flame 2002, 128, 211-216. (27) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; Krieger Publishing Company: 1981.

Table 1 . Ultimate and Proximate Analysis and Lower Calorific Value of the Samples a

 1

			ultimate analysis (% raw)		proximate analysis (% raw)		surface analysis
	sample	C	H	O	S	N	VM	ash	LCV (kJ/g) BET (m 2 /g) Hg20 (m 2 /g)
	petcoke Lily	87.8 3.51	1.1 7.47 1.61	10.5	0.33	34.45	4.5	1.77	0.57
	petcoke Havre CDP	87.9 3.51	1.1 7.08 1.51	10.1	0.38	34.35	10.0	2.20	0.57
	petcoke Asland	89.4 3.60		4.02 1.88	10.1	0.47	34.50	5.3	1.82	0.59
	petcoke Villauengas 88.4 3.70		6.16 1.56	11.0	0.12	34.35	6.2	2.00	0.65
	petcoke St Constant 88.5 1.94		6.00 1.63	5.2	0.50	32.50	6.3	1.33	0.54
	petcoke Niebla	85.8 3.50		5.85 1.70	11.1	3.35	33.40	4.8	1.80	0.66
	coal Ivan	67.0 4.54 11.3 0.24 1.85	30.7	13.23	25.95	64.0	8.30	0.73
	activated carbon	85.9 0.60		0.50 0.11		2.70		890.0	12.89	0.78
	a

Table 2 . Analysis of Five Metals, K, and Ca in the Petcokes (ppm raw)

 2 et al. were able to find a correlation between CO 2 surface area and NO-

	sample	V	Cu	Mo	Fe	K	Mn	Ca
	Lily	507	9.1	7.0	615	45	5.1	220
	Havre CDP	819			6			180
	Asland	2570	1.9	1.4	93	10	0.5	20
	Villauengas	1390	3.8	17.3	20	17	2.6	100
	St Constant	899	6.1	26.3	1740	15	17.1	150
	Niebla	1830	3.1	16.6	381	57	6.1	2190

(28) Schuler, J.; Baumann H.; Klein, J. Kinetics of NO reduction by coal char. Conf. (Int.) Coal Sci. 1987, 857-860.

Table 3 . Reduction Yields of NO by the Different Chars during EFR Experiments (in mol %) a

 3

	sample	800 °C	900 °C	1000	°C
	petcoke Lily	1.2	15.7	59.9
	petcoke Havre CDP	6.3	15.2	58.8
	petcoke Asland	11.1	35.0	74.7
	petcoke Villauengas	23.0	42.4	77.4
	petcoke St Constant	3.8	18.4	63.4
	petcoke Niebla	9.4	42.4	74.6
	coal Ivan	57.6	84.7	96.3
	activated carbon	27.2	44.8	69.1

a The initial NO concentration was 955 ppmv.

Table 4 . Activation Energies and Effectiveness Factor ηNO for NO Reduction (computed with Hg20 surface area)

 4

		activation energy	effectiveness
	sample	(kJ/mol)	factor (-)
	petcoke Lily	240	0.928-0.999
	petcoke Havre CDP	230	0.936-0.999
	petcoke Asland	185	0.883-0.994
	petcoke Villauengas	115	0.940-0.990
	petcoke St Constant	235	0.902-0.998
	petcoke Niebla	145	0.944-0.995
	coal Ivan	85	0.879-0.968
	activated carbon	115	0.992-0.998