Camille Clerc
email: clerc@geologie.ens.fr

Philippe Boulvais

Yves Lagabrielle

Michel De Saint Blanquat

Ophicalcites fr om the Nor ther n Pyr enean Belt: a field, petr ographic and stable isotope study

Keywords: ophicalcite, ophicarbonate, stable isotopes, oxygen, carbon, veins, matrices, Pyrenees, Lherz, Urdach, mantle exhumation, tectonic fracturation, hydrothermalism, sedimentary deposits

Brecciated and fractured peridotites with a carbonate matrix, referred to as ophicalcites, are common features of mantle rocks exhumed in passive margins and midoceanic ridges. Ophicalcites have been found in close association with massive peridotites, which form the numerous ultramafic bodies scattered along the North Pyrenean Zone (NPZ), on the northern flank of the Pyrenean belt. We present the first field, textural and stable isotope characterization of these rocks. Our observations show that Pyrenean ophicalcites belong to three main types: (1) a wide variety of breccias composed of sorted or unsorted millimeter-to meter-sized clasts of fresh or oxidized ultramafic material, in a fine-grained calcitic matrix; (2) calcitic veins penetrating into fractured serpentine and fresh peridotite; and

(3) pervasive substitution of serpentine minerals by calcite. Stable isotope analyses (O, C) have been conducted on the carbonate matrix, veins and clasts of samples from 12 Pyrenean ultramafic bodies. We show that the Pyrenean ophicalcites are the product of three distinct genetic processes: i) pervasive ophicalcite resulting from relatively deep and hot hydrothermal activity; ii) ophicalcites in veins resulting from tectonic fracturing and cooler hydrothermal activity; and iii) polymictic breccias resulting from sedimentary processes occurring after the exposure of subcontinental mantle as portions of the floor of basins which opened during the mid-Cretaceous. We highlight a major difference between the Eastern and Western Pyrenean ophicalcites belonging respectively to the sedimentary and to the hydrothermal types. Our data set points to a possible origin of the sedimentary ophicalcites in continental endorheic basins, but a post-depositional evolution by circulation of metamorphic fluids or an origin from relatively warm marine waters cannot be ruled out. Finally, we discuss the significance of such discrepancy in the characterics of the NPZ ophicalcites in the frame of the variable exhumation history of the peridotites all along the Pyrenean realm.

I. Introduction

During uplift and exhumation of the sub-continental mantle, the peridotites are commonly serpentinized through interactions with fluids, with direct consequences on their bulk density and their rheological, seismic, gravimetric and magnetic properties [START_REF] Brun | Mantle exhumation at passive margins[END_REF][START_REF] Boschi | Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif (MAR 30°N)[END_REF]. In oceanic and passive margin environments, besides serpentinization, the peridotites show evidence of carbonation expressed through the occurrence of a bimodal, ultramafic and carbonate association known as ophicalcites or ophicarbonates [START_REF] Spooner | Sub-sea-floor metamorphism, heat and mass transfer[END_REF][START_REF] Bonatti | Ultramafic carbonate breccias from the equatorial Mid-Atlantic Ridge[END_REF][START_REF] Dietrich | Alpine metamorphism of mafic rocks[END_REF][START_REF] Gianelli | Northern Apennine ophiolite: an ancient transcurrent fault zone[END_REF][START_REF] Ohnenstetter | La série ophiolitifère de Rospigliano (Corse) est-elle un témoin des phénomènes tectoniques, sédimentaires et magmatiques liés au fonctionnement des zones transformantes?[END_REF][START_REF] Lemoine | Serpentinites, gabbros and ophicalcites in the Piemont-Ligurian domain of the Western Alps: Possible indicators of océanic fracture zone and of associated serpentinite protrusions in the Jurassic-Cretaceous Thetys[END_REF][START_REF] Cortesogno | Descrizione dettagliata di alcuni caratteristici affioramenti di brecce serpentiniche della Liguria orientale ed interpretazione chiave geodinamica[END_REF][START_REF] Lagabrielle | Alpine Jurassic ophiolites resemble the modern central Atlantic basement[END_REF]. More recently, the carbonation of the exhumed mantle rocks has been clearly described as the last event affecting faulted rocks uprising along oceanic detachment faults at the axis of slow-spreading ridges [START_REF] Picazo | Deformation associated with the denudation of mantle-derived rocks at the Mid-Atlantic Ridge 13°-15°N: the role of magmatic injections and hydrothermal alteration[END_REF].

Ophicalcites were first discovered in the Ligurian Alps [START_REF] Bonney | Notes on some Ligurian and Tuscan serpentinites[END_REF], and then described in many ophiolite sequences (see [START_REF] Artemyev | The types and genesis of ophicalcites in Lower Devonian olistostromes at cobalt-bearing massive sulfide deposits in the West Magnitogorsk paleoisland arc (South Urals)[END_REF][START_REF] Artemyev | The types and genesis of ophicalcites in Lower Devonian olistostromes at cobalt-bearing massive sulfide deposits in the West Magnitogorsk paleoisland arc (South Urals)[END_REF][START_REF] Bogoch | Classification and genetic models of ophicarbonate rocks[END_REF] for a comprehensive historical literature on ophicalcites). Ophicalcites commonly display large variations in the proportions of ultramafic and carbonate material. They range from in-situ fractured peridotites with carbonate infill, through clast-supported breccias with multiple generations of carbonate infilling and internal sediments, to matrix-supported breccias. [START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic imodel[END_REF] distinguish two main types of ophicalcite. Ophicalcite type 1 (OC1) is represented by massive serpentinites exhibiting a dense mesh of calcite-infilled fractures.

Ophicalcite type 2 (OC2) refers to sedimentary breccias having a calcitic matrix, often deposited above ophicalcite of type 1. The clasts comprise sorted or unsorted millimeter-to meter-sized fresh or oxidized ultramafic fragments. In some cases, exotic clasts of gabbroic or basaltic composition can be observed. The matrix of OC2 is either a fine-grained calcitic sediment or a cement consisting of sparry calcite. It varies in color from red to pink and gray to green, depending on the chlorite or hematite content; sparry calcite is generally white.

Ophicalcites often record a polyphase history, revealed by different generations of cements and sediment infill, which can be highlighted by color changes, and bimodal grain distribution [START_REF] Abbate | Olistostromes and olistoliths[END_REF][START_REF] Bonatti | Ultramafic carbonate breccias from the equatorial Mid-Atlantic Ridge[END_REF][START_REF] Bernoulli | Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland[END_REF][START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic imodel[END_REF][START_REF] Früh-Green | A multiple fluid history recorded in Alpine ophiolites[END_REF]).

Because of the wide variety of ophicalcites and their occurrence in different oceanic or continental margin settings, it is important to recall that the term ophicalcite does not refer to a genetic process, but to a generic rock-type. Among the processes invoked for their formation one group refers to endogenic evolution, involving various deep seated phenomena such as: (1) mantle originated gas seeps [START_REF] Bonatti | Ultramafic carbonate breccias from the equatorial Mid-Atlantic Ridge[END_REF][START_REF] Haggerty | Late mantle evolution of the Pyrenean sub-continental lithospheric mantle in the light of new 40Ar-39Ar and Sm-Nd ages on pyroxenites and peridotites (Pyrenees, France)[END_REF]Kelemen et al. 2004); (2) magmatic intrusions [START_REF] Cornelius | Petrographische untershungen in den Bergen Zwischen Septinerund Julierpass[END_REF][START_REF] Bailey | Some Aspects of the Steinmann Trinity, Mainly Chemical[END_REF]; (3) contact and regional metamorphism [START_REF] Peters | A water-bearing andradite from the Totalp serpentine (Davos, Switzerland)[END_REF][START_REF] Trommsdorff | Ophicarbonate rocks: metamorphic reactions and possible origin[END_REF]) and (4) hydrothermal fluid interactions [START_REF] Cornelius | Petrographische untershungen in den Bergen Zwischen Septinerund Julierpass[END_REF][START_REF] Lavoie | Ordovician ophicalcites of southern Quebec Appalachians; a proposed early seafloor tectonosedimentary and hydrothermal origin[END_REF][START_REF] Artemyev | The types and genesis of ophicalcites in Lower Devonian olistostromes at cobalt-bearing massive sulfide deposits in the West Magnitogorsk paleoisland arc (South Urals)[END_REF]. Another group refers to surficial processes involving mechanical mixing of carbonates and ultramafic rocks through tectonic crushing, sedimentary reworking and gravity-driven infilling of veins and fractures (Bortolotti & Passerini 1970;[START_REF] Knipper | Ophicalcites and some other types of breccias accompanying the preorogenic formation of ophiolite complex[END_REF][START_REF] Bernoulli | Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland[END_REF][START_REF] Früh-Green | A multiple fluid history recorded in Alpine ophiolites[END_REF][START_REF] Treves | Exposure of serpentinites on ocean floor. Sequence of faulting and hydrofracturing in the Northern Apennine ophicalcites[END_REF][START_REF] Treves | Texture and microchemical data of oceanic hydrothermal calcite veins, Northern Apennine ophicalcites[END_REF][START_REF] Knipper | Exhumation of the upper-mantle and lower-crust rocks during rifting[END_REF].

In the French Pyrenees, ophicalcites have been reported in some ultramafic bodies associated with mid-Cretaceous basins and recently re-interpreted as resulting from mantle exhumation during Mid-Cretaceous rifting [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF][START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF]Clerc et al. 2012). In order to document and to characterize the variable typology of ophicalcites and to decipher their origin with respect to exhumation processes, we have performed a comprehensive field, petrological and geochemical study of samples taken throughout the Pyrenees (fig. 1). We identify three types of ophicalcites related to three distinct genetic processes: i) pervasive ophicalcite resulting from replacement of ultramafic minerals due to relatively deep and hot hydrothermal activity, ii) ophicalcites as veins resulting from tectonic fracturing and cooler hydrothermal activity and iii) polymictic breccias resulting from syn-sedimentary processes. The first two types record the activity associated with the final emplacement of the peridotites, whereas the last one is associated with the final exhumation of mantle rocks as portions of the basement of newly formed basins.

The Pyrenees are a narrow, 400 km long continental fold and thrust belt resulting from the collision of the northern edge of the Iberian Plate and the southern edge of the European Plate during the late Cretaceous to Tertiary [START_REF] Choukroune | The Ecors Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt[END_REF][START_REF] Muñoz | Evolution of a continental collision belt: ECORS-Pyrenees crustal balanced cross-section[END_REF][START_REF] Deramond | Relationships between thrust tectonics and sequence stratigraphy surfaces in foredeeps: model and examples from the Pyrenees (Cretaceous-Eocene, France, Spain)[END_REF][START_REF] Roure | Contribution of the ECORS seismic data to the Pyrenean geology : Crustal architecture and geodynamic evolution of the Pyrenees[END_REF][START_REF] Teixell | Crustal structure and orogenic material budget in the west central Pyrenees[END_REF]. Triassic and Jurassic aborted rifting events predated the development of a major Cretaceous crustal thinning event, which culminated in displacement between the Iberian and European plates [START_REF] Puigdefàbregas | Tecto-sedimentary cycles and depositional sequences of the Mesozoic and Tertiary from the Pyrenees[END_REF][START_REF] Vergés | Mesozoic evolution and Cainozoic inversion of the Pyrenean rift. Peri-Tethys Memoir 6: Peri-Tethyan Rift/Wrench Basins and Passive Margins[END_REF]. Continental rifting in the Pyrenean domain occurred in response to the counterclockwise rotation of Iberia relative to Europe, coeval with the onset of oceanic spreading in the Bay of Biscay between Chron M0 and A33o (approximately 125-83 Ma) [START_REF] Le Pichon | La faille nord-Pyrénéenne: Faille transformante liée à l'ouverture du Golfe de Gascogne[END_REF][START_REF] Choukroune | Tectonique des plaques et Pyrénées: Sur le fonctionnement de la faille transformante nord-Pyrénéenne; comparaisons avec les modèles actuels[END_REF][START_REF] Olivet | La cinématique de la plaque ibérique[END_REF][START_REF] Gong | The rotation of Iberia during the Aptian and the opening of the Bay of Biscay[END_REF][START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF]. About forty metric-to kilometric-sized fragments of subcontinental mantle rocks are found along the northern flank of the Pyrenees, in the North Pyrenean Zone (NPZ). They reside within or next to numerous lozenge-shaped basins flanking the North Pyrenean Fault (NPF). These basins are interpreted as the remnants of isolated, pull-apart or transtensive half graben basins formed in response to the eastward drift of Iberia along the NPF and later inverted during the Late Cretaceous-Early Cenozoic II. Geological setting of the ultr amafic Pyr enean bodies:

Pyrenean orogeny [START_REF] Le Pichon | La faille nord-Pyrénéenne: Faille transformante liée à l'ouverture du Golfe de Gascogne[END_REF][START_REF] Choukroune | Tectonique des plaques et Pyrénées: Sur le fonctionnement de la faille transformante nord-Pyrénéenne; comparaisons avec les modèles actuels[END_REF]. A typical flysch sedimentation started during the mid-Albian within these basins (black flysch), which later enlarged during the Late Albian and connected into one single, wider basin trough during the Cenomanian [START_REF] Debroas | Sédimentogenèse et position structurale des flyschs crétacés du versant nord des Pyrénées centrales[END_REF][START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Debroas | Le flysch noir albo-cenomanien témoin de la structuration albienne à sénonienne de la Zone nord-pyrénéenne en Bigorre (Hautes-Pyrénées, France)[END_REF]).

Various scenarios have been proposed for the emplacement of the ultramafic bodies, ranging from purely tectonic mechanisms, such as solid intrusion of hot or cold mantle rocks into sediments during strike-slip events [START_REF] Avé-Lallemand | Structural and petrofabric analysis of an « Alpine type » peridotite: The lherzolite of the French Pyrénées[END_REF][START_REF] Minnigh | Quenching: An additional model for emplacement of the Iherzolite at Lers (French Pyrenees)[END_REF][START_REF] Vielzeuf | Crustal splitting and the emplacement of Pyrenean lherzolites and granulites[END_REF], to tectono-sedimentary processes in which mantle rocks were exhumed during Variscan time [START_REF] Mattauer | Les lherzolites des Pyrénées sont des extrusions de matériel ancien dans le Mésozoique nord Pyrénées[END_REF][START_REF] Fortane | Lherzolites and the western « Chainons bearnais » (French Pyrenees): Structural and paleogeographical pattern[END_REF]) and reworked in a mid-Cretaceous wild flysch [START_REF] Fortane | Lherzolites and the western « Chainons bearnais » (French Pyrenees): Structural and paleogeographical pattern[END_REF]). In recent re-examinations, various authors propose that some of these bodies are fragments of sub-continental mantle basement partially exhumed during Albian-Cenomanian times [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF][START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF][START_REF] Debroas | Les brèches d'Urdach, témoins de l'exhumation du manteau pyrénéen dans un escarpement de faille vraconnien-cénomanien inférieur (zone nord-pyrénéenne, Pyrénées-Atlantiques, France)[END_REF]Clerc et al. 2012). Within the NPZ, the metasediments are locally strongly deformed and underwent a High Temperature -Low Pressure (HT-LP) mid-Cretaceous metamorphic event, which lasted nearly 30 Ma from 110 Ma to 80 Ma [START_REF] Azambre | Le magmatisme alcalin d'âge crétacé dans les Pyrénées occidentales; ses relations avec le métamorphisme et la tectonique[END_REF]Albarède & Michard-Vitrac 1978b;[START_REF] Montigny | K-Ar Study of cretaceous magmatism and metamorphism in the pyrenees: Age and length of rotation of the Iiberian Peninsula[END_REF][START_REF] Golberg | Petrological and age relationship between emplacement of magmatic breccia, alkaline magmatism, and static metamorphism in the North Pyrenean Zone[END_REF]Goldberg & Maluski 1988;[START_REF] Thiébaut | Métamorphisme et halocinèse crétacés dans les évaporites de Betchat le long du chevauchement frontal Nord-Pyrénéen (Haute-Garonne et Ariège, France)[END_REF][START_REF] Thiébaut | Métamorphisme des évaporites triasiques du versant nord des Pyrénées centrales et Occidentales[END_REF]). This metamorphism is considered as a consequence of crustal thinning [START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)[END_REF]) and developed in relation with hydrothermal circulations [START_REF] Dauteuil | Une circulation de fluides de haute température à l'origine du métamorphisme crétacé nord-Pyrénéen[END_REF]. Hydrothermal circulations are also responsible for extensive albitization of some of the North Pyrenean Massifs [START_REF] Demange | Les syénites du castillet (massif de l'agly, pyrénées orientales, France): une roche exceptionnelle dans les pyrénées[END_REF][START_REF] Boulvais | Cretaceous albitization and dequartzification of Hercynian peraluminous granite in the Salvezines Massif (French Pyrénées)[END_REF][START_REF] Poujol | Regional-scale Cretaceous albitization in the Pyrenees: evidence from in situ U-Th-Pb dating of monazite, titanite and zircon[END_REF], and formation of massive talc deposits [START_REF] Moine | Comparative mineralogy, geochemistry, and conditions of formation of two metasomatic talc and chlorite deposits; Trimouns (Pyrenees, France) and Rabenwald (Eastern Alps, Austria)[END_REF][START_REF] Schärer | Formation of the Trimouns talc-chlorite deposit (Pyrenees) from persistent hydrothermal activity between 112 and 97 Ma[END_REF][START_REF] Boulvais | Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif[END_REF] probably in relation to the activity of major ductile extensive shear-zones [START_REF] Passchier | Mylonite-Dominated Footwall Geometry in a Shear Zone, Central Pyrenees[END_REF][START_REF] St Blanquat | Les zones de cisaillements du massif nord Pyrénéen du Saint-Bartelemy, témoins probables del'extension crustale d'âge crétacé[END_REF][START_REF] Costa | Use of the 40Ar-39Ar stepwise heating method for dating mylonite zones: An example from the St. Barthélémy massif (Northern Pyrenees, France)[END_REF][START_REF] St Blanquat | Petrological arguments for hightemperature extensional deformation in the Pyrenean Variscan crust (Saint Barthélémy Massif, Ariège, France)[END_REF][START_REF] St Blanquat | La faille normale ductile du massif du Saint Barthélémy: Evolution hercynienne des massifs nord-pyrénéens catazonaux considérée du point de vue de leur histoire thermique[END_REF].

III. Geology of the sampling sites

The ophicalcites and related ultramafic breccias selected for the oxygen and carbon isotope study were sampled from peridotite bodies exposed all along the NPZ (fig. 1). We selected nine localities which are representative of the variety of the Pyrenean peridotites, and presumably of the different geological processes involved in their exhumation history. Two sampling sites are located in the western Pyrenees (Urdach and Tos de la Coustette), one in the central Pyrenees (Moncaup) and nine in the Eastern Pyrenees (Ercé-Angladure, Lherz, Fontête Rouge, Freychinède, Berqué, Vicdessos, Urs, Bestiac, Caussou).

III.1. Wester n per idotites

Several peridotite bodies outcrop in the Chaînons Béarnais, within the fold and thrust belt Mesozoic sequence of the NPZ, at a longitude corresponding to the western termination of the Paleozoic Pyrenean axial zone. The base of the stratigraphic sequence, exposed along the Mail Arrouy, Sarrance and Layens post-Cenomanian thrusts [START_REF] Casteras | Carte géol[END_REF], is composed of Late Triassic evaporites, breccias and ophites overlain by Mesozoic platform carbonates forming the original cover of the northern Iberian margin [START_REF] Canérot | Présence d'une marge méridionale à l'emplacement des Chaînons Béarnais (Pyrénées basco-béarnaises)[END_REF][START_REF] Canérot | Tectonic and sedimentation on the north Iberian margin, Chainons Béarnais south Pyrenean zone (Pyrenees basco-béarnaises)-New data about the signification of the lherzolites in the Saraillé area[END_REF]. In contrast to the Eastern NPZ, evidence of HT-LP metamorphism is restricted here to some scarce and narrow regions bordering fault contacts with Triassic and mantle rocks [START_REF] Fortane | Lherzolites and the western « Chainons bearnais » (French Pyrenees): Structural and paleogeographical pattern[END_REF][START_REF] Thiébaut | Métamorphisme des évaporites triasiques du versant nord des Pyrénées centrales et Occidentales[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF]) and the temperatures during peak metamorphism barely exceeded 400°C (Clerc 2012). Petrological and geothermobarometric studies of the western ultramafic bodies show that they underwent a two step exhumation with a first rise from 60 km to 25 km depth (1050-950°C), probably during late Hercynian times, followed by a further step from 25 km to a shallower and cooler (600°C) level [START_REF] Fabriès | Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees[END_REF]. This second step is marked by the development of a mylonitic fabric, from 117 Ma to 109 Ma [START_REF] Vissers | Mylonitic deformation in upper mantle peridotites of the North Pyrenean Zone (France): implications for strength and strain localization in the lithosphere[END_REF][START_REF] Fabriès | Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees[END_REF].

The Urdach body is a 1.5 km wide peridotite slice exposed at the western termination of the Mail Arrouy thrust (fig. 2A). It is overlain by Paleozoic basement slices and surrounded on its western and southern sides by a large volume of unsorted sedimentary breccias and olistoliths composed of peridotite fragments associated with Paleozoic basement clasts from upper, middle and lower crustal levels. These debris are intermingled in the Cenomanian black flysch [START_REF] Casteras | Carte géol[END_REF][START_REF] Vielzeuf | Relations de phases dans le faciès granulite et implications géodynamiques[END_REF][START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF][START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF][START_REF] Debroas | Les brèches d'Urdach, témoins de l'exhumation du manteau pyrénéen dans un escarpement de faille vraconnien-cénomanien inférieur (zone nord-pyrénéenne, Pyrénées-Atlantiques, France)[END_REF]. Some authors considered that the Urdach body itself might be an olistolith settled in Cenomanian sediments [START_REF] Duée | Les lherzolites associées aux Chaînons Béarnais (Pyrénées Occidentales): Mise à l'affleurement anté-dogger et resédimentation albo-cénomanienne[END_REF][START_REF] Fortane | Lherzolites and the western « Chainons bearnais » (French Pyrenees): Structural and paleogeographical pattern[END_REF]. Peridotite hydrothermal alteration led to pervasive serpentinization reaching 80%. This is a dominant character of the Urdach body [START_REF] Fabriès | Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees[END_REF].

The 400 m long Tos de la Coustette ultramafic body is located 3 km west of the Saraillé summit, at the western termination of the Sarrance anticline (fig 2a). Apart from the peridotites, the faulted heart of the Sarrance anticline includes, Paleozoic basement rocks and ophite lenses embedded within cataclastic Triassic sediments. It is thrusted over the verticalized Urgonian limestones and Albian flysch of the Lourdios Syncline [START_REF] Casteras | Carte géol[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF]. The Tos de la Coustette body itself is in tectonic contact with small lenses of Paleozoic crustal rocks and Triassic metaevaporites outcropping both above and beneath the peridotites. Like the Saraillé peridotites, the environment of the Tos de la Coustette body is devoid of sedimentary breccias; instead these bodies are entirely surrounded by cataclastic breccias limited by tectonic contacts and are thought never to have been exposed to the seafloor [START_REF] Canérot | Tectonic and sedimentation on the north Iberian margin, Chainons Béarnais south Pyrenean zone (Pyrenees basco-béarnaises)-New data about the signification of the lherzolites in the Saraillé area[END_REF][START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF].

III.2. Centr al per idotites

The Moncaup ultramafic body is part of a group of peridotites exposures lying around the Milhas massif, in the central Pyrenees (fig. 1). They are associated with basement rocks, variably brecciated Triassic sediments, ophites and Albian mafic intrusions. They are overlain in tectonic contact by highly metamorphosed Mesozoic marbles [START_REF] Debeaux | Les affleurements du socle paléozoique entre les massifs de la Barousse et de Milhas[END_REF][START_REF] Hervouët | Resédimentation chaotique de méta-ophites et de marbres mésozoïques de la vallée du Job (Pyrénées commingeoises): Relations détritisme/métamorphisme en zone nord-Pyrénéenne[END_REF][START_REF] Barrère | Carte géol[END_REF]. Although the peridotites have risen to near surface levels, there is no evidence for sedimentary reworking indicating their exhumation on the basin floor. Indeed, based on their geological setting, it can be deduced that the mantle rocks have remained capped by the Mesozoic marbles together with small slices of continental crust, during their uplift along the detachment fault [START_REF] Lagabrielle | Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies[END_REF].

III.3. Easter n per idotites:

The eastern Pyrenean peridotites are found within narrow belts of Mesozoic sediments of the NPZ, mainly limestones, pinched between the Axial Zone to the south and blocks of Paleozoic crust to the North representing the continental basement of the NPZ (North Pyrenean massifs) (fig. 1). Although the eastern Pyrenean mantle outcrops are often small and disconnected from an original substratum, one can still observe, on a decametric scale, a progressive transition from carbonate-free massive peridotites to high carbonate content breccias with all intermediates. On the rim of the ultramafic bodies, the massive peridotites are often crosscut by millimetric to centimetric calcite veins over the first few meters. In addition, in many localities, the peridotites are reworked within sedimentary polymictic breccias together with highly variable proportions of carbonate clasts. The amount of matrix of these breccias increases toward the carbonated end-member.

Most of the ultramafic bodies sampled for this study, from West to East, namely Ercé-Angladure, Lherz, Fontête Rouge, Freychinède, Berqué, Vicdessos, Urs, Bestiac and Caussou, display geological settings consistent with an origin as olistoliths surrounded by polymictic detritic formation (fig. 2B,3). They are interpreted as sedimentary records of the exhumation of the peridotites on the floor of the Cretaceous basins [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF]Clerc et al. 2012). The ultramafic-bearing breccias show sedimentary features such as grain-sorting and crossbeddings and can be found away from the main bodies, indicating that they have been transported by sedimentary processes. [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF] showed that the polymictic ultramafic-carbonate clastic sediments have been emplaced into fissures opened within the exhumed massive peridotites, in a way similar to OC2 sedimentary ophicalcites of [START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic imodel[END_REF]. Similar features are reported in more detail, together with the presence of ultramafic-rich debris flows and evidence of ultramafic rock-fall in the vicinity of the Lherz body by Clerc et al. (2012). The peridotites show little serpentinization, developed mainly along discrete, localized joints and fissures. The carbonates reworked in the detritic formations surrounding the peridotites are strongly deformed and underwent HT-LP metamorphism with peak temperatures commonly as high as 600°C [START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)[END_REF]Clerc 2012). By contrast to the western peridotites, the eastern ones underwent a single and rapid uplift event, which probably limited hydrothermal alteration and serpentinization (Albarède & Michard-Vitrac 1978a;[START_REF] Fabriès | Evolution of the Upper Mantle beneath the Pyrenees: Evidence from Orogenic Spinel Lherzolite Massifs[END_REF][START_REF] Haggerty | Late mantle evolution of the Pyrenean sub-continental lithospheric mantle in the light of new 40Ar-39Ar and Sm-Nd ages on pyroxenites and peridotites (Pyrenees, France)[END_REF].

IV. Descr iption of the analyzed samples

IV.1. Sampling str ategy and collected samples

The sampling strategy was to collect samples from the four main geological environments identified and distinguished as follows: (1) poorly serpentinized peridotites surrounded by hot metasediments (Eastern and central peridotites) either exhumed to the basin floor (Lherz, Bestiac, Caussou, Vicdessos, Urs, Ercé-Angladure) or only unroofed but never exhumed (Moncaup), and ( 2) highly serpentinized peridotites surrounded by cooler sediments (Western peridotites) either exhumed to the basin floor (Urdach) or only unroofed but never exhumed (Tos de la Coustette). The list of the 48 studied samples is given in Table 1. We focused this study on the Lherz and Urdach bodies since they are among the largest mantle outcrops in the Pyrenees and because they represent two well-studied end-members in terms of their geological environment. Furthermore, these two localities offer better outcrop conditions compared with the smaller bodies poorly exposed in areas presenting important vegetal covering and rock alteration.

IV.2. Field and macr oscopic aspects:

The western ophicalcites appear essentially as veins of calcite infilling fissures and fractures opened within the ultramafic rocks. The fractures present relatively constant and repeated orientations (fig. 4A). Particularly well observable in a quarry opened on the western side of the Urdach lherzolite body, they were first described by [START_REF] Monchoux | Les lherzolites pyrénéennes: contribution à l'étude de leur minéralogie, de leur génèse et de leurs transformations[END_REF] and later interpreted as typical ophicalcite textures by [START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF]. These authors highlighted their similarities with structures observed within exhumed mantle in the Alps and drilled off Iberia [START_REF] Manatschal | New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps[END_REF]). They consist mainly in millimetric to decimetric veins of clear white calcite (fig. 4B). The thickest veins are actually constituted of an accumulation of numerous veins and veinlets separated by thin fragments of peridotite strapped from the rims.

IV.2.a. Western Pyrenean Ophicalcites:

At Tos de la Coustette, the ophicalcites also appear as a dense mesh of very fine veinlets and as a pervasive substitution of serpentine minerals by patches of carbonate, barely visible on a macroscopic scale, invading highly serpentinized peridotites (fig. 4C andD). Similar textures have also been described further east in the Avezac-Moncaut peridotites [START_REF] Fabriès | Petrogenetic evolution of orogenic lherzolite massifs in the central and western Pyrenees[END_REF].

At least two types of ophicalcites were identified in the Moncaup peridotites. The first one, observed close to the damage zone of the tectonic contact between the peridotites and the overlying marbles, is represented by millimetric veins of coarse translucent sparite (fig. 4E).

The veins crosscut and hence post-date a mylonitic fabric affecting the peridotites. The second one, observed in the damage zone of the detachment fault, consists of light brown micro-conglomerates and micrite infilling veins and cavities opened in the altered and dislocated peridotites (fig. 4F). The cavities present contorted and rounded rims. The microsediments show complex multi-generation evolutions with successive stages of deposition indicated by several color shades and crosscutting sparitic veins. The micro-conglomerates are laminated and present clear grain-sorting.

IV.2.b. Central Pyrenean Ophicalcites:

In the Eastern Pyrenees, ophicalcites and polymictic ultramafic-marble bearing breccias are observed within, close to, and even far away from the main peridotite bodies. Most of the clasts are composed either of ultra-fresh subcontinental peridotites or of marbles bearing mineral assemblages typical of the HT-LP mid-Cretaceous metamorphism. They are associated with a minor proportion of fragments deriving from gabbros, Triassic ophites Mesozoic meta-pelites and meta-evaporites and Paleozoic basement rocks [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF]Clerc et al. 2012).

IV.2.c. Eastern Pyrenean Ophicalcites:

In the Etang de Lherz area, Lagabrielle and Bodinier (1998) identified four main types of breccias and ophicalcites that can be extended to the other peridotite outcrops of the Eastern Pyrenees. (i) Type 1 is found in direct contact with or within the ultramafic body, it consists of a carapace of monomictic breccias resulting from the cataclastic deformation of the peridotites during exhumation. These breccias typically lack carbonate clasts and contain little to no carbonate veins and cement. Therefore, they will not be considered further in this study.

(ii) Type 2 breccias, generally found in close contact with type 1 breccias, are ultramaficdominated polymictic breccias resulting from the sedimentary reworking of type 1. (iii) Type 3 breccias consist of thin layers of graded ultramafic litharenites bearing isolated cm-sized clasts of peridotites and marbles and presenting slumps and syn-sedimentary normal faults (fig. 5A andB). Within the type 3 breccias, the peridotite clasts display many different lithologies (lherzolite, harzburgite, websterite, pyroxenite etc.), variable mantle textures (equant coarse-granular to mylonitic) and variable degrees of serpentinization (totally fresh to fully serpentinized, fig. 5C andD). These observations point to mixing and transport from a relatively distant source by sedimentary processes. Furthermore, clasts of former monomictic carbonate breccias and polymictic UM-marble breccia are also reworked in these formations, pointing to their late deposition with regard to the exhumation history (Clerc et al., 2012). (iv) Type 4 breccias correspond to clastic rocks closely resembling the OC2 sedimentary ophicalcites of [START_REF] Lemoine | Ultramafic and gabbroic ocean floor of the Ligurian Tethys (Alps, Corsica, Apennines): In search of a genetic imodel[END_REF]. Clear white calcite veins penetrate fractured ultramafic blocks in which they separate angular fragments. There is a striking association of these veins with matrix-supported microbreccias similar to those forming the matrix of the type 2 breccias (fig. 5E). The veins are smoothly rooted in the matrix and seem to be its extension in narrow domains where the clasts were too big to fit. However, some veins crosscut pre-existing matrices, pointing to a contemporaneous formation of matrices and veins during sedimentation accompanied by multi-stage circulations of cementing fluids. In some outcrops, the ultramafic clasts exhibit a centimeter thick orange-brown oxidation ring on their contact with carbonate matrices and veins (fig. 5 E andF). This feature is also commonly observed in oceanic ophicalcites [START_REF] Boschi | Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif (MAR 30°N)[END_REF][START_REF] Dick | Plutonic foundation of a slowspreading ridge segment: Oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W[END_REF]. This oxidation pattern does not appear on the rims of the thinnest veins.

Ophicalcites at the Bestiac locality show similar features and offer exceptional conditions to investigate the successive generations of serpentinization and carbonation. The peridotite consists of tens of lens-shaped bodies less than a few hundred meters in size and embedded in a metamorphic bimodal ultramafic-marble breccia. Angular to slightly rounded centimeter-sized clasts of lherzolite are found more than 500 m away from the main blocks, a pattern clearly not consistent with any fault-assisted mode of emplacement. A few outstanding outcrops are visible in several abandoned caves and galleries dug for prospection of asbestos.

Fresh to dark green serpentinites are crosscut by networks of cm-thick light green fibrous serpentine (likely chrysotile) delimitating metric lumps which mimict at a meter-scale the classical microscopic mesh-texture of serpentinized peridotites [START_REF] Wicks | Serpentine textures and serpentinization[END_REF] (fig. 5G). The serpentine veins commonly show oblique sigmoidal fibers indicating shearing contemporaneous to brittle deformation responsible for vein development. Pluri-millimetric veins of calcite crosscut through this latest generation of serpentine veins that acted as a weak gateway for sediments and fluids during the ultimate stage of deformation and sedimentary reworking (fig. 5H). Some of the calcitic veins have a reddish coloration and increased concentration of oxides, suggesting possible syn-or post-diagenetic hydrothermal circulation.

As in the Lherz area, these remarkable ultramafic-bearing formations have a relatively restricted extension and appear within large volumes of clastic formations devoid of any ultramafic component. The matrix of the eastern Pyrenean ophicalcites and associated polymictic breccias consists of a pale-orange litharenite composed of infra-millimeter-sized angular clasts of marbles mixed with varyingly serpentinized ultramafic clasts and isolated minerals (pyroxenes, olivine, green and brown spinels). This litharenite sometimes appears laminated and shows graded-bedding due to sedimentary transport [START_REF] Washington | Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees[END_REF]Clerc et al. 2012) (fig. 6E). In contrast to the veins, the matrices of the breccias, show much less evidence for recrystallization: there are, for example, a few newly formed metamorphic phyllosilicates and amphiboles.

Most of the veins observed in the Eastern Pyrenean ophicalcites consist of clear and equant sparry calcites (fig. 6F). The veins cross cut alternatively the ultramafic clasts and the matrix of the breccias in which they often seem to be rooted. In the simplest cases, a single generation of calcite crystal nucleates from the rims of the veins and grows toward the center where it joins in a central suture. In some cases, the growth resumed or the fracture was reopened, leading to the formation of vugs. Most of the veins show multistep histories with successive infillings of sparite particularly well highlighted by varying luminescence in CL (fig. 6G). When pure enough, the veins are clearly recrystallized as evidenced by the conservation of former zoned calcite dogteeth ghosts within bigger equant neoformed crystals. The borders of the neoformed crystals are independent from those of the ghosts that they overprint (fig. 6H). When thick enough, the veins are generally filled with detrital material including micro-fragments of serpentine, oxides and calcite clasts mixed within a micrite. Although they lack microfossils, such veins resemble the neptunian dykes and veins formed on the subaquatic floor and consequently opened to sedimentary influx [START_REF] Smart | Neptunian dikes and fissures fills: an overview and account of some modern exemples[END_REF][START_REF] Laznicka | Breccias and Coarse Fragmentites: Petrology, Environments, Associations, Ores[END_REF]Winterer et al. 1991).

Rock samples were sawed to select well-oriented and relevant planes. The sawed faces were cleaned using water and pulsed with dry air before micro-drilling. A minimum of about 20 mg of powder was collected for each sampling site. We note that the pluri-millimetric size of our sampling drilling spots provide bulk estimates of the isotopic compositions of the veins and matrix but do not allow the complex multistage history recorded in some of the veins to be deciphered (i.e. fig. 6G). The isotope composition of the matrices does not correlate with the variable lithology of the clasts (either ultramafic and/or carbonate), nor with the relative amount of clasts. In contrast the isotopic compositions of the marble clasts are highly variable and plot within a larger field than the matrices and veins (Fig. 7). Also, there is no correlation between the isotopic compositions of clasts and the veins or matrices that host them. Actually, these clasts underwent a strong HT/LP metamorphism and may contain abundant silicates (phyllosilicates, amphiboles, scapolite).

V. Methods for deter mination of O and C isotope compositions

Closed-system isotopic equilibration between the carbonate and the silicate phases likely introduced variable isotopic alteration of the carbonate phase, depending on the initial amount of detrital silicates in the sedimentary precursor (see for example [START_REF] Valley | Stable isotope geochemistry of metamorphic rocks[END_REF]Boulvais et al., 2000). Also, open-system alteration during syn-metamorphic infiltration possibly caused isotopic shifts, which remain difficult to estimate here because we have no more information on the initial geometry of the clast (for example the distance to a lithological discontinuity).

The ophicalcites from the Moncaup body display two distinct generations of carbonates with distinct isotopic compositions (fig. 4C andD). Such differences in isotopic compositions are consistent with the occurrence of two types of textures as described in section IV.

VII.1. Or igin of the var ious types of ophicalcites

VII. Discussion

Based on the petrographic descriptions on the one hand and on the stable isotope compositions on the other, we are able to distinguish three main categories of ophicalcites associated with the subcontinental mantle bodies of the northern Pyrenees (table 2).

The first type of ophicalcites or hydrothermal type, as defined in the Tos de la Coustette body Different generations of veins cross-cutting each other with slight variations in isotopic composition may reflect some temperature variations during the successive steps of fracturing / precipitation. In sample URD 1, a first generation of vein with a lower δ 18 O values (δ 18 O=18.6‰) is cut by a later generation with a higher δ 18 O value (δ 18 O=21.1‰). This is consistent with cooling during vein formations, in consequence of progressive exhumation, provided that the isotopic composition of the invading fluid, and then its source, remained constant throughout the history of this sample.

The third type of ophicalcites, dominant in the eastern Pyrenees, is sedimentary ophicalcite. It consists of a cogenetic association of calcite vein and polymictic breccias. The matrixsupported to clast-supported polymictic breccias are composed of variable proportions of marbles and UM clastic material. Polymictic compositions and typical sedimentary features such as grain-sorting and cross bedding indicate that these ophicalcites have a sedimentary origin (Clerc et al., 2012). Their wide lithological variety, both in UM and metasedimentary material, likely results from sedimentary transport and mixing. By analogy with neptunian veins observed in other extensional settings (Winterer et al. 1991), the micrite-filled brittle fractures have been interpreted as very late, near surface fracturing of the exposed ultramafic basement [START_REF] Lagabrielle | Active in situ disaggregation of oceanic crust and mantle on Gorringe Bank: analogy with ophiolitic massives[END_REF]Morgan & Milliken 1996), possibly leading to gravitational instabilities: slumping, slope failure, and landslides as described at ODP site 899 by [START_REF] Gibson | Serpentinite-Breccia Landslide Deposits Generated during Crustal Extension at the Iberia Margin[END_REF]. The sparite-filled veins and veinlets, either reworked in the breccias, smoothly rooted in the matrix or crosscutting clasts and matrix reveal a multistage deposition history, already implied by the presence of breccias clasts reworked in the breccias (Clerc et al. 2012). [START_REF] Bernoulli | Sedimentary fabrics in Alpine ophicalcites, South Pennine Arosa zone, Switzerland[END_REF] describe similar cogenetic and simultaneous sediment infillings and calcitic cement precipitation in Alpine ophicalcites. Such fractures must have allowed the circulation of sedimentary fluids or early diagenetic fluids in domains where restricted dimension hindered the penetration of sedimentary material. The isotopic compositions of matrices and veins (Fig. 8) show a good correlation in both the oxygen and carbon systems, which confirm the idea that both features developed from the same reservoir.

Since the relationship is valuable on a rather large range of δ 18 O and δ 13 C, one can infer that the system of vein + matrix development was not connected to an external reservoir which would have produced veins with distinct compositions from the ones of the matrices.

Precipitation of acicular aragonite has commonly been correlated with warm water temperatures, high Mg/Ca ratios, high salinity, and high carbonate concentrations, conditions reached in the uppermost levels of serpentine seamounts [START_REF] Haggerty | Petrology and geochemistry of Neogene sedimentary rocks from Mariana forearc seamounts[END_REF][START_REF] Lagabrielle | Mineralogical and Geochemical Analyses of Sedimentary Serpentinites from Conical Seamount (Hole 788A): Implication for the Evolution of Serpentine Seamounts[END_REF]. Similar thermal and chemical conditions are associated with fibrous, botryoidal calcite occurrences [START_REF] Folk | The natural history of crystalline calcium carbonate: effects of magnesium content and salinity[END_REF][START_REF] St Blanquat | Petrological arguments for hightemperature extensional deformation in the Pyrenean Variscan crust (Saint Barthélémy Massif, Ariège, France)[END_REF], and botryoidal calcite could also represent a replacement texture of acicular radiating aragonite [START_REF] Ross | Botryoidal high magnesium calcite cements from the upperCretaceous of the Mediterranean region[END_REF]. More equant and bladed sparry calcite has been correlated with cooler, deeper marine or meteoric settings, typically with low Mg, and carbonate concentrations [START_REF] Folk | The natural history of crystalline calcium carbonate: effects of magnesium content and salinity[END_REF][START_REF] Burton | Relative precipitation rates of aragonite and Mg calcite from seawater: Temperature or carbonate ion control?[END_REF]. Instead, geochemical data (trace element and isotopic signatures) from Iberian margin ophicalcites indicate a seawater imprint at temperatures of 10-20°C, consistent with an early Cretaceous seawater only slightly modified by interaction with serpentinized peridotite basement (Milliken & Morgan 1996). For that reason, Morgan & Milliken (1996) suggested that the temporal evolution in the carbonate phase and morphology, from precipitation of aragonite, followed by fibrous, botryoidal calcite and finally to coarse, bladed sparry calcite may be controlled primarily by fluid flow rates through the vein rather than by variations of the chemical and thermal parameters (fig. 9).

The low temperature ophicalcites from the Moncaup body (MP84a, b, c) are peculiar in that they have extremely low δ 13 C (< -5.00 ‰), indicative of interaction with organic material.

Such carbon isotope compositions are commonly observed in karstic or calcrete precipitations from continental/meteoric waters transiting through soils and vegetation. These peculiar ophicalcites are located immediately below the cataclastic formations staking out the detachment fault between the peridotites and the overlying marbles. Due to the solubility and permeability contrast between these two formations, this interface is prone to concentrating groundwater. However, we cannot specify if the formations observed are contemporaneous of any very early exhumation of the mantle rocks to subaerial (onland) environments in the Cretaceous or if they result from a later karstification and/or replacement of pre-existing ophicarbonates. Regardless the exact explanation, these values strongly differ from the rest of our dataset by a clear shift toward lower δ 13 C values, allowing the distinction to be made between a surface-derived cement precipitated in karstic environments and sedimentary ophicalcites deposited in subaqueous conditions.

The isotopic compositions of the Pyrenean ophicalcites fall into the same field as other ophicalcites from the literature (fig. 7). Only the three samples from Moncaup MP84a, b, and c are out of this range because of their low carbon composition indicative of interactions with carbon from soils, as discussed above. Ophicalcites from the Alps and Apennine were interpreted as having been formed by interaction with seawater moderately heated to 80 to less than 200°C [START_REF] Barbieri | Stable isotope evidence for a marine origin of ophicalcites from the north-central Apennines (Italy)[END_REF][START_REF] Barrett | Stable isotopic composition of atypical ophiolitic rocks from east Liguria, Italy[END_REF][START_REF] Früh-Green | A multiple fluid history recorded in Alpine ophiolites[END_REF][START_REF] Schwarzenbach | Serpentinization, fluids and life: comparing carbon and sulfur cycles in modern and ancient environments[END_REF] or by sedimentary fluids later re-equilibrated during Alpine metamorphism [START_REF] Weissert | Oxygen isotope composition of calcite in Alpine ophicarbonates: a hydrothermal or Alpine metamorphic signal ? Eclogae geol[END_REF][START_REF] Barbieri | Stable isotope evidence for a marine origin of ophicalcites from the north-central Apennines (Italy)[END_REF].

VII.2. Envir onmental conditions for the for mation of Centr al and Easter n Pyr enean ophicalcites

Samples from the Eastern Pyrenees are clearly recognized as sedimentary ophicalcites by their textures. Consistently, the Eastern Pyrenean ophicalcites have the highest δ 18 O values measured in our sample set, a feature which is indicative of low temperatures of precipitation at near surface conditions. However, we notice a major difference in the oxygen isotope composition with ophicalcites from the Iberian margin [START_REF] Agrinier | Hydrothermal Alteration of the Peridotites Cored at the Ocean/Continent Boundary of the Iberian Margin: Petrologic and Stable Isotope Evidence[END_REF][START_REF] Evans | Low-Temperature Alteration of Peridotite, Hole 637A[END_REF][START_REF] Agrinier | Mineralogical and oxygen isotopic features of serpentinites recovered from the ocean/continent transition in the Iberia abyssal plain[END_REF]Milliken & Morgan 1996;[START_REF] Plas | Petrologic and stable isotope constraints on fluid-rock interaction, serpentinization and alteration of oceanic ultramafic rocks[END_REF][START_REF] Skelton | The relative timing of serpentinisation and mantle exhumation at the ocean-continent transition, Iberia: constraints from oxygen isotopes[END_REF]. Indeed, in the Iberian ophicalcites, which precipitated from low temperature seawater, the δ 18 O values of calcite are around 31‰ with δ 13 C values varying between -1.7 and 2.2‰.

Instead, the Eastern Pyrenean sedimentary ophicalcites analyzed in our study have significantly lower δ 18 O values (around 24‰) with carbon isotope compositions ranging from -2.5 to 1.4‰, so that even if the envelope of Pyrenean sedimentary ophicalcites mimic the Iberian margin ones, it is displaced in the δ 13 C vs. δ 18 O space (grey arrow, fig. 7). At least, three hypotheses can be proposed in order to explain these differences (fig. 10).

1. Lowering of the O and C isotope compositions could result from a metamorphic imprint with introduction of externally-derived fluids. Isotopic exchanges between neoformed calcites and the mineral silicates, mainly serpentinite, which form a significant portion of the detrital material associated with the sedimentary ophicalcites, may lower the O isotope composition of the calcite. The breccias and ophicalcites rework clasts of pre-rift material that already bear signs of high-grade recrystallization during the regional HT/LP metamorphism, with the development of scapolite and amphibole. The deposition of the breccias and ophicalcites hence occurs after the peak of metamorphism. But the long-lasting mid-cretaceous thermal anomaly is followed by a lower grade metamorphism that affects the Turonian-Senonian postrift sediments, with a maximum temperature near 350°C [START_REF] Ternet | Notice explicative, Carte géol[END_REF]Clerc 2012). This later and lower grade metamorphism may hence have affected the ophicalcites and breccias presented in this study. However, the petrographical effect of metamorphism on these rocks seems rather limited since the matrices show only little recrystallization. Furthermore, we would expect that the oxygen isotope composition would be much more variable depending on the fluid/rock ratio. This is the case for a metamorphic-driven alteration of the isotopic signal as shown from the study of the Alpine ophicalcites (Fig. 7; [START_REF] Weissert | Oxygen isotope composition of calcite in Alpine ophicarbonates: a hydrothermal or Alpine metamorphic signal ? Eclogae geol[END_REF][START_REF] Früh-Green | A multiple fluid history recorded in Alpine ophiolites[END_REF]). Also, one would have expected that the veins show more constant composition instead of displaying delta values that correlate with the values of matrices (Fig. 8).

2. The low O and C compositions may be the result of a hot diagenesis from marine porewater during carbonation. This hypothesis, which implies active circulation of relatively hot fluids in the boundary layer between the ultramafic basement and seawater, is consistent with the high geothermal gradients known to characterize the basins of the North Pyrenean Zone during the Albian-Cenomanian period [START_REF] Dauteuil | Une circulation de fluides de haute température à l'origine du métamorphisme crétacé nord-Pyrénéen[END_REF][START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)[END_REF].

The thermal gradients for the Albo-Cenomanian metamorphism can be higher than 100°C/km.

In such conditions, we may also consider that unconsolidated sediments still soaked with seawater can be rapidly buried and heated to temperatures as high as 50-80°C. At such temperatures, the calcite precipitated from seawater (δ 18 O = 0‰) would have a δ 18 O value of around 23‰ (considering the isotopic fractionation coefficient of Zheng, 2011), a value that compares well with the data of the Eastern ophicalcites. Thermal gradients as high as 160-180°C/km are known in present days, for instance in the Salton Sea geothermal field [START_REF] Elders | Crustal Spreading in Southern California The Imperial Valley and the Gulf of California Formed by the Rifting Apart of a Continental Plate[END_REF][START_REF] Muffler | Active Metamorphism of Upper Cenozoic Sediments in the Salton Sea Geothermal Field and the Salton Trough, Southeastern California[END_REF]. Comparable environments can also be found on the top of mantle exhumed in oceanic domains, where hydrothermal fields develop over areas several square kilometerswide (around 2.5km 2 at the Rainbow hydrothermal site, German et al., 1996; around 2km 2 at the Lost City hydrothermal site, [START_REF] Kelley | An Off-axis Hydrothermal Vent Field Near the Mid-Atlantic Ridge at 30° N[END_REF], along the Mid-Atlantic Ridge). In similar settings, in the ophiolites of East Liguria, [START_REF] Spooner | Sub-sea-floor metamorphism, heat and mass transfer[END_REF] describe temperatures as high as 400°C for shallow depth of circa 300 m below the water/rock interface.

3. As a last hypothesis, it may be that the Eastern Pyrenean sedimentary ophicalcites formed in a low-temperature but endorheic environment, dominated by continental waters and possibly disconnected from the ocean. Indeed, the oxygen composition measured here is about 7‰ lower than the present-day marine Iberian ophicalcites, a difference consistent with the difference between marine and unspecific waters with a continental affinity. Note first that a continental environment is not precluded by the existence of marine fauna, which would have been observed in sediments associated with ophicalcites. The hypothesis of an endorheic environment dominated by continental waters has to be questioned with respect to the paleogeographic reconstructions of the Pyrenean realm during mid-Cretaceous times. These reconstructions point to the existence of a V-shape opening oceanic domain, narrowing from the Bay of Biscay toward the East where it propagates into the continental crust [START_REF] Jammes | Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees[END_REF] and references therein). The opening of numerous transtensive basins of limited extension in the central and eastern part of the pre-Pyrenean domain may have been such that these basins were endorheic [START_REF] Le Pichon | La faille nord-Pyrénéenne: Faille transformante liée à l'ouverture du Golfe de Gascogne[END_REF][START_REF] Choukroune | Tectonique des plaques et Pyrénées: Sur le fonctionnement de la faille transformante nord-Pyrénéenne; comparaisons avec les modèles actuels[END_REF], partially disconnected from a marine influence at the time of ophicalcite development. This hypothesis is consistent with the stratigraphy of the Albian sediments deposited in disconnected basins separated by positive reliefs [START_REF] Debroas | Sédimentogenèse et position structurale des flyschs crétacés du versant nord des Pyrénées centrales[END_REF][START_REF] Debroas | Le flysch noir albo-cenomanien témoin de la structuration albienne à sénonienne de la Zone nord-pyrénéenne en Bigorre (Hautes-Pyrénées, France)[END_REF][START_REF] Souquet | Le groupe du Flysch noir (albo-cénomanien) dans les Pyrénées[END_REF]. Some of these reliefs such as the future North Pyrenean massifs and the future Axial Zone were emerged, as shown by the outline of the Cenomanian transgression and by evidence of cooling and sedimentary reworking of crustal material [START_REF] Filleaudeau | Thermo-tectonic evolution of the southcentral Pyrenees from rifting to orogeny: insights from detrital zircon U/Pb and (U-Th)He thermochronometry[END_REF]. Such short wave-length and high amplitude morphology likely resulted from the flexural response of the lithosphere to the extreme crustal stretching due to the extensional Albian-Cenomanian tectonics along the Pyrenean realm. In such conditions, we may envision that the area where mantle has been exhumed was surrounded by subaerial catchments and, at that time possibly disconnected from the sea. A possible present-day analog is represented by the Salton Sea basin, which is an endorheic continental basin located ahead of the propagating oceanic spreading axis of the Gulf of California. Circulations of continental waters within sediments are also described in more opened environments, for example at the foot of the Aden Gulf margins [START_REF] Lucazeau | Heat-flow and hydrothermal circulation at the ocean-continent transition of the eastern gulf of Aden[END_REF].

At this time, it is difficult to select between the three hypotheses even if the last one is the simplest in term of the isotopic composition record. Additional informations like fluid inclusion data is needed to strengthen this hypothesis. It remains clear that, regardless of the exact explanation, sedimentary ophicalcites in the Eastern Pyrenees are distinguishable from those in the Central and in the Western Pyrenees.

The three types of ophicalcites identified in this study have to be considered within the frame of the exhumation history of the Pyrenean peridotites presented in section III a and c and as summarized in figure 11. We highlight a clear distinction between the Eastern and Western Pyrenean isotope composition of ophicalcites also evidenced by the different degrees of serpentinization of the mantle that host them, by the temperatures of the metamorphic peak in the surrounding metasediments (Choukroune & Seguret, 1973;[START_REF] Golberg | High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)[END_REF][START_REF] Ravier | Le métamorphisme des terrains secondaires des Pyrénées[END_REF]Clerc 2012) and by the typologies of ophicalcites (fig. 11). Following our observations, and in accordance with phase stability of serpentine mineral [START_REF] Andreani | Dynamic control on serpentine crystallization in veins: Constraints on hydration processes in oceanic peridotites[END_REF]), it appears that the variable serpentinization degree of the Pyrenean peridotites can be linked, primarily, to the thermal anomaly accompanying their exhumation. Since carbonation postdated serpentinization, the degree of serpentinization appears as a key factor influencing the development of ophicalcites. Volume increase and rheological softening induced by serpentinization tend to favor the development of numerous fractures, allowing an endogenic precipitation of carbonates as observed in the Western ophicalcites. In contrast, the less serpentinized peridotites exposed in Moncaup and in the Eastern Pyrenees must have had a different behavior during uprising to crustal levels. Their contrasting rheology with the surrounding rocks implies that they were probably still massive and competent until exhumation. This could explain the predominance of superficial ophicalcites found in these localities. The fact that the Eastern Pyrenean peridotites remained preserved from hydrothermal circulation may explain their scarce serpentinization. In addition, such a lack of fluid activity may also be responsible for the preservation of high temperature mineral VII.3. Wester n and Easter n Pyr enean ophicalcites: why ar e they so differ ent? assemblage since heat was evacuated by convection. The reason of the limited access of fluids to the exhuming peridotites is not yet understood. We could suggest either i) a blanketing effect of the Mesozoic sedimentary cover that would inhibit water infiltration, or ii) fast exhumation in a continental environment with limited amounts of water available for hydrothermal circulations.

On the basis of close fieldwork, petrographic and geochemical considerations, we present the first comprehensive review of the Pyrenean ophicalcites. Our results, in accordance with published studies on worldwide occurrences of ophicalcites allowed us to distinguish and characterize three main types of ophicalcite (table 2)

Conclusion

We further investigated the possible origins of the fluid and temperatures at which the calcite may have precipitated in both hydrothermal and sedimentary domains. We present three possible explanations for the relatively low values of the sedimentary ophicalcites: i) a postsedimentary metamorphic imprint; ii) a hot diagenesis in relation to the high regional thermal gradient; iii) sedimentation in an endorheic basin. This last hypothesis is consistent with the paleogeographic reconstructions of isolated Albo-Cenomanian basins at the tip of a propagating rift. Finally, we highlight a major difference between Eastern and Western ophicalcites, linked primarily to the variable degree of serpentinization. Considering the strong control of serpentinization on the rheology of mantle rocks we propose that the formation of different ophicalcites types is controled by the degree of serpentinization, depending itself on the rate and modalities of exhumation of the subcontinental mantle during extreme crustal stretching.

Winterer, E. L., Metzler, C. V., & Sart, M. (1991). Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems. Sedimentology, 38(3), 381-404. doi:10.1111/j.1365-3091.1991.tb00358.x Zheng, Y. F. (2011). On the theoretical calculations of oxygen isotope fractionation factors for carbonate-water systems. Geochemical Journal, 45, 341-354. [START_REF] Evans | Low-Temperature Alteration of Peridotite, Hole 637A[END_REF]Milliken & Morgan 1996;[START_REF] Plas | Petrologic and stable isotope constraints on fluid-rock interaction, serpentinization and alteration of oceanic ultramafic rocks[END_REF][START_REF] Skelton | The relative timing of serpentinisation and mantle exhumation at the ocean-continent transition, Iberia: constraints from oxygen isotopes[END_REF]; the Alps and Apennines [START_REF] Brotzu | Contributo alla conoscenza delle « Rocce Verdi » dell'Appennino centrale. Nota III. La composizione isotopica della calcite presente in alcuni affioramenti di oficalciti del F 129 (S. Fiora) e sue implicazioni petrologiche[END_REF][START_REF] Barbieri | Stable isotope evidence for a marine origin of ophicalcites from the north-central Apennines (Italy)[END_REF][START_REF] Weissert | Oxygen isotope composition of calcite in Alpine ophicarbonates: a hydrothermal or Alpine metamorphic signal ? Eclogae geol[END_REF][START_REF] Barrett | Stable isotopic composition of atypical ophiolitic rocks from east Liguria, Italy[END_REF][START_REF] Demeny | Stable isotope compositions of the Penninic ophiolites of the Köszeg-Rechnitz series[END_REF]) and from other hydrothermal ophicalcites [START_REF] Lavoie | Ordovician ophicalcites of southern Quebec Appalachians; a proposed early seafloor tectonosedimentary and hydrothermal origin[END_REF][START_REF] Artemyev | The types and genesis of ophicalcites in Lower Devonian olistostromes at cobalt-bearing massive sulfide deposits in the West Magnitogorsk paleoisland arc (South Urals)[END_REF]. (Morgan & Milliken, 1996). This study Morgan & Milliken (1996) 1 mm 0.1 mm My lon itic de fo rm a tio n (Fa briè s, 19 98 ; V is se rs 1 9 9 7 )

IV. 3 .

 3 Micr oscopic descr iption: Ophicalcite from the Urdach ultramafic body have millimetric veins of sparite. Several vein generations crosscut each other with varying angles. The calcite veins commonly show crack-seal aspects. Symmetric layers of calcite with varying intensity in transmitted light and cathodoluminescence (CL), induced by minor variations of composition or inclusion concentration, are separated by a central suture (fig. 6A). Some of the largest veins actually consist of an accumulation of numerous sub-parallel veinlets, less than a micrometer wide, invading the serpentinite (fig. 6B). Finally, some other veins show zonated botryoids of fibrous radiating calcite (fig. 6C) In the Tos de la Coustette ophicalcites, the carbonates mainly appear as micrometric to millimetric patches of calcite extensively dispersed within the serpentinite (fig. 6D). Intimate repartition of calcite and serpentine indicate a pervasive calcification by replacement of some serpentinous phases. The poorly elongated calcite aggregates develop following a general foliation marked by magnetite alignments and thin yellow serpentine veinlets. Surface estimate by digital image treatment indicates that the rock includes up to 55 % calcite, 40 % serpentine and 5 % magnetite with minor phases.

  The O and C isotope compositions were measured using a VG SIRA 10 triple collector mass spectrometer at the University of Rennes 1, on the CO 2 released during reaction of calcite with anhydrous H 3 PO 4 in sealed vessels at 50°C[START_REF] Mccrea | On the isotope chemistry of carbonates and a paleotemperature scale[END_REF]. NBS 19 and internal-lab standard references materials (Prolabo Rennes) were continuously measured during the course of this work. NBS 19 measured values were δ 18 O = 28.26 ± 0.09 (1σ, n=12) ‰ and δ 13 C = 1.86 ± 0.02 (1σ, n=12) ‰. Results were corrected in accordance with the NBS 19 recommended values of 28.65‰ and 1.95‰, for O and C respectively. The analytical uncertainty is estimated at 0.15‰ and 0.1‰ for O and C. The isotope compositions for the 48 analyzed samples are presented in table 1 and figure 7. The calcite phase found in clasts, veins and matrices from the Pyrenean ophicalcites and ultramafic-bearing breccias displays a wide range of oxygen isotope compositions with minimum values of 12.6 and 13.8‰ (vs. SMOW) measured in Moncaup and Tos de la Coustette veins (Western ophicalcites), and maximum values of 25.1‰ in the matrices of samples from Etang de Lherz area (fig. 4 & 6; Eastern ophicalcites). The carbon isotope compositions range from -5.8‰ (vs. PDB) in Moncaup samples to 1.5‰ in Lherz samples (matrices). As a whole, the field of isotopic compositions of Pyrenean ophicalcites is displaced from the one of the Iberian margin ophicalcites by lower δ 18 O values and slightly lower δ 13 C values (arrow in figure 7). Whereas no clear distinction can be made when comparing Pyrenean ophicalcites with Alpine and Apenninic ones, it seems that hydrothermal ophicalcite worldwide compare well with ophicalcites from the Western Pyrenees. First order analysis of the distribution of the oxygen and carbon isotope compositions implies to distinguish two separate domains: i) Ophicalcites from the Urdach and Tos de la Coustette ultramafic bodies have rather low and variable values of δ 18 O and δ 13 C, ranging from δ 18 O = 13.8 to 22.1‰ and δ 13 C = -5.22 to 1.12‰ and scattered around a mean value of δ 18 O = 19.1‰ and δ 13 C = -1,0‰. Ophicalcites from Tos de la Coustette body plot into a distinct field having an extremely low value of δ 18 O. ii) Ophicalcites from the Eastern Pyrenean bodies display low dispersion (21.3 to 25.1‰) in δ 18 O but variable δ 13 C values (1.53 to -2.47‰). Among the Eastern Pyrenean ophicalcites, very poor discrimination can be made based on the composition of veins and matrices since both display almost similar values with mean δ 18 O of 23.4‰ in the veins and 24.1‰ in the matrices. The O and C isotope compositions of the nine samples containing both veins and matrices are reported in figure 8. One can observe good correlations between the compositions of matrices and veins with the exception of samples VI. Results LHZ8 and LHZ64 whose veins are significantly depleted in 18 O and slightly enriched in 13 C.

  results from peridotite carbonation by veins and pervasive substitution of the serpentinite minerals by low δ 18 O calcite. The low δ 18 O values of calcite indicate that carbonation occurred from rather hot fluids. Comparable δ 18 O values have been measured in ophicalcites formed in oceanic and ophiolitic hydrothermal systems (Lavoie & Cousineau 1995; Artemyev & Zaykov 2010). Due to its low δ 18 O, the first generation of coarse crystalline calcite veins described in the Moncaup peridotite likely corresponds to this type of hydrothermal ophicalcite. The difference in texture types between the Moncaup and the Tos de la Coustette ophicalcites may be explained by the very different rheology and chemical response to fluid circulation of the unserpentinized peridotite at Moncaup compared with the totally serpentinized peridotite of Tos de la Coustette. The second type of ophicalcites or tectonically-controlled type is well characterized in the Urdach body. It consists of massive serpentinized peridotites, indifferently lherzolite or harzburgite, crosscut by successive generations of millimetric to centimetric calcite veins with intermediate isotopic compositions. The tectonic control of calcite crystallization is documented by the distribution of veins along preferential planes and their crack-seal geometry. Such calcite crystallizations likely record the arrival of the peridotite close to seafloor environments, directly under the influence of waters with intermediate temperature.

  : (i) hydrothermal ophicalcites resulting in low δ 18 O calcite (13.8‰) pervasively replacing serpentinite; (ii) intermediate or syntectonic ophicalcites developed along with brittle discontinuities in the serpentinized mantle rocks, with intermediate calcite isotope compositions (δ 18 O around 20.0‰; δ 13 C around -1.06‰); (iii) sedimentary ophicalcites occurring as breccias and neptunian dykes, associated with the circulation of syn-sedimentary fluids. The isotopic compositions for this sedimentary type show the highest δ 18 O and δ 13 C values of the set, consistent with the cold temperatures of precipitation expected in a sedimentary environment. We note a non-linear distribution of the different ophicalcite type along the Pyrenean range, with dominant endogenic ones in the West and dominant exogenic ones in the East. Such a distribution is clearly linked to a difference in serpentinization degrees likely related to the different exhumation histories and subsequent variable thermal anomalies.

Figur e Caption Figure 1 .

 Caption1 Figur e Caption

Figure 2 .

 2 Figure 2. A: Simplified geological map of the Urdach and Tos de la Coustette in the Mail

Figure 3 :

 3 Figure 3: Photograph of an ultramafic olistolith in the Aulus basin illustrating the progressive

Figure 5 :

 5 Figure 5: Macroscopic aspects of some of the eastern Pyrenean ophicalcites. A: Bimodal

Figure 6 :

 6 Figure 6: Microscopic aspects of the Pyrenean ophicalcites. A: Seal crack calcite vein from

Figure 7 :

 7 Figure 7: δ 13 C vs. d18O diagram showing the isotopic compositions of the Pyrenean

Figure 8 :

 8 Figure 8: Comparison of the C and O isotope compositions of calcitic veins and matrices in

Figure 9 :

 9 Figure 9: Comparison of calcite microtextures in veins and matrices from this study and from

Figure 10 :

 10 Figure 10: Cartoons illustrating the three possible mechanisms responsible the low O

Figure 11 :

 11 Figure 11: Sketches presenting the exhumation history of the Eastern and Western Pyrenean

  Fig.2a

Table Caption Table 1 :

 Caption1 C (vs. PDB) an O (vs. SMOW) isotopes compositions determined for the carbonate fraction of the veins, matrix and clasts from the Pyrenean ophicalcite.

Table 2 :

 2 Schematic representation of the different types of ophicalcites analyzed in this study.

Acknowledgments: This work was made possible thanks to CNRS, Total, and the Action Marges research group (INSU, Total, IFP, BRGM, IFREMER) through a Ph.D. grant to C. Clerc. We thank G. Frühgreen and G. Manatschal for their valuable comments that helped improve the quality of the manuscript. We are grateful to B. Smith for improving the English, to J. -C. Ringenbach and Benoit Ildefonse for fruitful discussions and improvement on the quality of the figures, and to C. Nevado for the high quality thin-sections realized at the Géosciences Montpellier laboratory.


published or not. The documents may come


T Chevalier

S Rodts

X Chateau

C Chevalier

P Coussot

Breaking of non-Newtonian character in flows through a porous medium

Keywords: number(s): 47.56.+r, 47.50.-d, 83.60.La

From NMR measurements we show that the velocity field of a yield stress fluid flowing through a disordered well-connected porous medium is very close to that for a Newtonian fluid. In particular, it is shown that no arrested regions exist even at very low velocities, for which the solid regime is expected to be dominant. This suggests that these results obtained for strongly nonlinear fluid can be extrapolated to any nonlinear fluid. We deduce a generalized form of Darcy's law for such materials and provide insight into the physical origin of the coefficients involved in this expression, which are shown to be moments of the second invariant of the strain rate tensor.

I. INTRODUCTION

Flows of non-Newtonian fluids through porous media are involved in a wide range of applications [1], such as the injection of cement in soils, penetration of glue in porous substrates, propagation of blood through the kidneys, hydraulic fracture [2], etc. One of the most important applications concerns oil reservoir engineering, in which a wide range of complex fluids (foams, polymers, emulsions) are used to improve oil recovery [3]. In these processes the flow characteristics of such fluids in porous media remain poorly known due to the complexity of the fluid behavior and the opacity of the solid structure.

For Newtonian fluids (of constant viscosity μ) we know that any element of the liquid network flows, and in the absence of inertia effects, the velocity field scaled by the average velocity ( V ) is constant (Stokes flow). This is at the origin of Darcy's law, which tells us that the pressure gradient (∇p) is proportional to V by a factor μ/K, in which K is a characteristic of the porous medium, namely, its permeability. For nonlinear fluids we expect a significant evolution of the distribution of flowing regions with the velocity and thus a more complex relationship between ∇p and V .

For yield stress fluids (mud, cement, concentrated emulsions, or foams), which flow like liquids only beyond a critical stress and behave as solids otherwise, various analytical or numerical approaches [4] have been developed but the validity of their physical assumptions could not be checked. Some in-depth physical studies suggested that two critical effects could occur: (i) at the pore scale the flowing volume increases with the pressure gradient [5] and (ii) at a macroscopic scale the flow starts as a percolation effect, i.e., at a critical pressure drop liquid regions exist only along a specific path throughout the porous medium [5,6], and as ∇p is increased more flowing paths progressively form within the porous medium. Such effects still need experimental validation.

Here we observe a yield stress fluid flow inside a model bead packing with a NMR technique which provides straightforward information on the average local flow characteristics without being affected by any spatial resolution problem. We show that there is no region of fluid at rest, even at low V values, and the velocity density distribution is similar to that of a Newtonian fluid. This makes it possible to deduce the effective form of Darcy's law for yield stress fluids and the physical origin of its coefficients.

II. MATERIALS AND PROCEDURES

Our yield stress fluid is a water-in-oil emulsion (81% water, 19% oil) prepared by dispersing a water + CaCl 2 (150 g/L) salt solution in a surfactant solution (dodecane + 7.5 wt% Span 80 surfactant) with a mixer. The droplet size ranges from 1 to 5 μm. Rheological tests in simple shear show that this is a simple yield stress fluid (with negligible thixotropic character) with a flow curve that can be well represented by a Herschel-Bulkley (HB) model (see Fig. 1): τ < τ c ⇒ γ = 0 (solid regime); τ > τ c ⇒ τ = τ c + k γ n (liquid regime), in which τ is the shear stress, γ the shear rate, τ c the yield stress, and k and n two material parameters [START_REF]τ c = 63 Pa, k = 13 Pa s n[END_REF]. The fluid is pushed by a piston through a duct (8 cm diameter) and then penetrates a bead packing. Two types of beads were used with a similar (when represented as a function of the size rescaled by the mean diameter) triangular grain size distribution in the range 0.8-1.2 times the average diameter D = 0.5 or 2 mm. They were covered with a thin layer (≈0.015D) of resin which stuck them together, leading to a porosity ε ≈ 0.33.

The generalized Reynolds number, i.e., Re = ρ V 2 /τ c , in which ρ is the fluid density, is much smaller than 1, which means inertia effects are negligible. Under these conditions the flow is governed by the Bingham number, Bi = τ c /k γ n , which estimates the ratio of the constant (elastoplastic) to the rate-dependent (viscous) parts of the constitutive equation, and finally gives an idea of the relative importance of the solid and liquid regions in the sample. Thus flows through similar porous media with different pore sizes or yield stress values have similar flow characteristics (after appropriate rescaling) as soon as they have the same Bingham number.

For the flow through a complex geometry we use a characteristic shear rate ( γ = V /d), which is the ratio of the average velocity through the void volume (V ) to a characteristic length scale of the cross section of the geometry (d). For our bead packing we have V = V /ε and we take d = D/6. Our range of velocities ( V from 0.013 to 1.3 mm s -1 ) and bead sizes lead to Bi ranging from 1 to 11.4, which includes the main classes of yield stress fluid flows, i.e., (i) almost purely plastic flows (Bi 1) and (ii) flows with both elastoplastic and viscous effects playing a significant role (Bi ≈ 1).

Measurements were carried out after a time of flow (after saturation of the bead packing) such that the distance (typically 10 cm) covered at the average velocity was several orders of magnitude larger than the bead size. This ascertains that a steady-state flow was reached and residual creep motions in the solid regions were negligible.

The whole setup is inserted vertically in the channel of a vertical proton magnetic resonance imaging (MRI) system (Avance 24/80 DBX by Bruker, 0.5-T superconducting magnet by Oxford, 20-cm inner diameter). Velocity distributions in the pore space are measured by means of pulsed-gradient spin echo (PGSE) proton NMR [START_REF] Stejskal | [END_REF] as follows: After an initial excitation of the hydrogen spin system of liquid molecules, a pair of self-compensating field gradient pulses of value G produces in the spin precession a motion-sensitive

phase shift δϕ = γ G • ( δ 0 r(t)dt - +δ r(t)dt)
, where γ is the gyromagnetic ratio, r(t) the molecular trajectory, and time constants δ and define the timing of the pulse sequence. The NMR protocol uses various gradient intensities to recover statistical information on the bracketed term [START_REF] Callaghan | Principles of Nuclear Magnetic Resonance Microscopy[END_REF]. The latter corresponds to local velocity field components by some known multiplicative factor, provided molecular self-diffusive motions can be neglected and sequence timing is short. The sequence was designed here to be sensitive only to water phase. Water confinement in micrometer-size droplets strongly restricts diffusive motions. Residual diffusion inside droplets was quantified [START_REF] Neuman | [END_REF] so as to find sequence tunings which kept the spurious effects of water diffusion on velocity distributions under some undetectable level [11]. Results are presented in terms of a probability density function of one velocity component (also often called "1D velocity propagator"), which is such that the fraction of fluid having a velocity between v and v + dv is f (v)dv. We checked the ability of the technique to measure the effective velocity distribution for flows in simple geometry, including arrested regions (see, for example, Fig. 2)

In order to appreciate the data in a straightforward way, we think it more appropriate to look at the distribution function, defined as

F (v) = v -∞ f (u)du.
Indeed, the shape of F is less dependent on the measurement uncertainties, in particular, FIG. 2. (Color online) Distribution function vs velocity scaled by the mean velocity (in the small duct) measured for the flow of a yield stress fluid through a 7-cm-diameter and 7-cm-long duct between 3.5cm-diameter ducts for Bi = 14. The colored regions approximately correspond to those which are visible on the velocity field (see inset) obtained from independent measurements (see [13]) and represented by color scale (in mm/s): outer arrested region (red and gray dotted lines), transitional region (light green, continuous line), moving plug region (dark blue, dashed line).

the impact of the noise is damped as the integral filters out some noise components [11]. In the following, except when mentioned, we will focus on the longitudinal velocity, i.e., along the cylindrical sample axis. For example, for the flow through a capillary, F takes very different shapes depending on the rheological behavior of the fluid (see Fig. 1), which means that the distribution function can be a fingerprint of this behavior. For a yield stress fluid there is an unsheared region (plug) around the center of the duct which is associated with a vertical jump at the end of the distribution (see Fig. 1). The size of this region increases with Bi. On the contrary, if the fluid is arrested in a region of volume fraction φ A , such as in an expansion-contraction, the distribution globally starts by a jump to a finite level [F (0) = φ A ] (see Fig. 2).

III. EXPERIMENTAL DATA AND ANALYSIS

The first critical result from our data for the flow through a bead packing is that over our whole range of Bi the distribution function starts around zero, i.e., F (0) ≈ 0.05 (see Fig. 3). More precisely, in contrast with the distribution for the model pore of Fig. 2, the overall shape (i.e., leaving apart possible uncertainties around zero) of the curves for porous media does not exhibit any apparent step around zero which might be associated with a more significant arrested region. Similar conclusions can be drawn from transversal distributions (see inset of Fig. 3). For a yield stress fluid in steady flow this means that the whole fluid is in the liquid regime and flows, i.e., the arrested (solid) regions occupy a negligible volume. This observation is in contradiction with previous conceptual expectations for such flows [5,6]. In particular, our result implies that even at low velocity there is no preferential pathway involving a fraction of the pores through which the fluid would flow while it would be arrested outside. Our data also prove that at the scale of the pore there is no significant This is a surprising result, since in a model pore made of a short duct between two smaller ducts and for a similar range of Bi there remains, around the central flowing region, a wide region of arrested fluid where the stress is necessarily smaller than the yield stress [12,13] (see Fig. 2). Moreover, in order to induce the flow of a yield stress fluid between two long plates situated at a distance h, the minimum pressure gradient to apply is 2τ c /h, which tends to infinity when h tends to zero. Thus in a bead packing we could expect a stoppage in the regions situated between two very close beads. We suggest that this does not occur, because such regions extend over short distance and are connected in various directions with other regions containing larger fluid volumes. Finally, the picture we get from these data is that instead of keeping some restricted regions at rest, a liquid regime sets up everywhere in the fluid network.

Let us now compare the distribution functions obtained for different porous media and at different V by representing them as a function of the velocity scaled by V . Owing to small and δ imposed by NMR sequence tuning, this is equivalent to looking at the distribution of local displacements scaled by the mean displacement. With such a representation, for flows of Newtonian fluids in the absence of inertia effects (i.e., low Re), we expect a single distribution function for different V or similar porous media of various length scales. For example, in a capillary, F is a straight line in any case. On the contrary, for a yield stress fluid F varies widely with the Bingham number, which depends on the velocity and capillary diameter (see Fig. 1).

Surprisingly, for a yield stress fluid through a bead packing, in our range of experimental conditions, the distribution function in the transversal and longitudinal directions are independent of V and D (see Fig. 3). Thus, once again in contrast with yield stress fluid flows through simple geometry, V has no significant impact on the velocity distribution. Actually, such a phenomenon has already been observed for a complex flow (extrusion) of yield stress fluids [14] in a similar range of Bi: over a length of the entrance conduit approximately equal to its diameter, the velocity fields (in a longitudinal cross section) scaled by the piston velocity appear quite similar for different Bi.

Since the independence of F with regard to V and D is typically obtained for a Newtonian fluid, our results suggest that the local flow characteristics of a yield stress fluid through a bead packing are similar to those of a Newtonian fluid. This conclusion is further supported by the observation that the distribution function for the yield stress fluid is approximately similar to that for a Newtonian fluid (see Fig. 3) and to that for a simple shear-thinning fluid (without yield stress).

It could be argued that there is an effect of statistical averaging over the various local geometrical shapes leading to the same average distribution whatever the differences at a local scale. However, even after this transformation the final distribution should depend on the specific local characteristics of the fluid flow. Indeed, if we assume that in a given pore (i.e., a void region), due to a different rheological behavior we have a velocity distribution different from that for a Newtonian fluid, e.g., a larger fraction with high velocities, we will have a similar effect in another pore of different shape and finally a similar effect on the average over all pores, with some impact on F . Thus we can conclude that the local velocity fields for the different fluid types are very close; otherwise we would not get similar distribution functions.

IV. DISCUSSION

These observations lead us to conclude that the strong differences of flow characteristics usually induced by different rheological behavior types, such as can be observed in a capillary (see Fig. 1), are "broken" by the porous medium. This conclusion is further supported by the observation that the yielding character of a fluid is broken around the entrance in a small die, which well represents the situation continuously encountered inside a bead packing: far from the entrance there is a plug flow [14], but just after the entrance the velocity profile is close to that for a Newtonian fluid and only after some distance of the order of the conduit diameter the standard profile including a plug is recovered (see Fig. 4). Finally, the corresponding distribution function in cross section evolves as described in Fig. 1, i.e., from that of a simple fluid to that of a yield stress fluid (see Fig. 4).

We consider that the process at the origin of the breakage of the non-Newtonian character in a bead packing is as follows:

The fluid is continuously and significantly deformed in various directions as it moves through a complex porous network. As soon as it has been significantly deformed so as to reach a velocity field similar to that of a Newtonian fluid, it will never encounter a region allowing it to recover its flow characteristics typical of a yield stress fluid in uniform flow, and thus will keep a Newtonian velocity field.

We can now use these results to establish a general form of Darcy's law for some complex fluids. In particular, for a yield stress fluid the usual three-dimensional form of the constitutive equation [14] Since the viscous dissipation may also be expressed through a macroscopic approach as ∇p • V = ε ∇p • V , we deduce Darcy's law for yield stress fluids (which includes power-law and Newtonian fluids):

∇p = α τ c D + β n k 1 D V D n ( 1
)
The form of this law is consistent with the analysis of a series of measurements of ∇p as a function of V for different yield stress fluids and pore sizes [18]. Our approach now demonstrates that this is the general form of Darcy's law for yield stress fluids and provides explicit expressions and the physical meaning for the two coefficients (α and β n ).

V. CONCLUSION

Finally, our experimental results suggest a change of paradigm for dealing with transport in porous mediums, i.e., the fact that the flow of a nonlinear fluid in a complex geometry tends to somehow cancel its rheological complexity. The complete origin of this effect still needs to be understood by theory or simulations, for example, via the promising Lattice-Boltzmann approaches [19,20].

FIG. 1 .

 1 FIG. 1. Distribution function vs velocity scaled by the maximum velocity for flows through a capillary: Newtonian fluid (dashed line) and yield stress fluid (continuous line) following a HB model with an unsheared region (plug) extending over 2/3 of the capillary radius (i.e., Bi = 5.4). Inset: experimental flow curve of our material (squares) fitted by a HB model [7] (continuous line), and Newtonian flow curve of arbitrary viscosity (dashed line).

FIG. 3 .

 3 FIG. 3. (Color online) Distribution functions as a function of the velocity scaled by the average velocity through the voids. (i) Computed from literature NMR data for f (at sufficiently small ) with Newtonian fluids (water or oil) through bead packings: (squares) Fig. 1 of [15] V = 3.55 mm s -1 ; (circles) data from Fig. 4 of [16] V = 1.1 mm s -1 , and (stars) for a power-law fluid (Xanthan), Fig. 9 of [17], V = 30 mm s -1 . (ii) From our tests with a yield stress fluid: (D = 2 mm) short dash light blue (V = 0.12 mm s -1 , Bi = 7.3) and dotted red line (V = 4 mm s -1 , Bi = 1.8); (D = 0.5 mm) black line (V = 0.12 mm s -1 , Bi = 4.2) and dashed dark blue (V = 4 mm s -1 , Bi = 1.04). Our data for intermediate velocities fall exactly along the other data. The data for Bi = 11.4 are similar although slightly more affected by diffusion. The inset shows the distribution function for the transversal velocity for D = 0.5 mm, and V = 0.4 mm s -1 , Bi = 3 (black), V = 1.2 mm s -1 , Bi = 1.7 (dotted red), and V = 4 mm s -1 , Bi = 1.04 (dashed dark blue).

FIG. 4 .

 4 FIG. 4. (Color online) Yield stress fluid flow (arriving from a 8-cm-diameter duct) through a die of 1 cm diameter. Distribution function in cross sections at 1 mm (circles) and 16 mm (squares) from the die entrance, calculated from the velocity profiles (see inset) (data from [14]). The velocity field in color scale [highest velocities in red (central region), smallest ones in blue (outside region)] is also shown in the inset.

  , which leads to a HB model in simple shear, is τ I I < τ c ⇒ d = 0 (solid regime) and τ I I > τ c ⇒ τ = τ c d/d I I + 2 n kd n-1 I I d (liquid regime), in which τ is the deviatoric stress tensor, d the strain rate tensor, and d I I = trd 2 /2. The viscous dissipation inside the bead packing (of volume ) is P = tr(d • τ )dω. Introducing the constitutive equation we deduce P = τ c 2d I I dω + k (2d I I ) n+1 dω, which may be rewritten as P= ατ c ε (V /D) + β n ε (V /D) n+1 , with α = (1/ε ) 2 D V d I I dω, β n = (1/ε ) (2 D V d I I ) n+1 dω.The above experimental results concerning the approximately unique scaled velocity field obtained for different velocities and bead diameters lead to the conclusion that (D/V )d I I is a unique dimensionless function weakly dependent on the effective behavior of the material. That means that α and β n are two dimensionless coefficients depending only on this distribution of shear rate intensity and on the coefficient n.


Grzegorz Mieczarek

Czeslaw Wozniak

On the dynamic modelling of fibrous composites


Czeslaw Wozniak

Margaret Wozniak

On the micromodelling of dynamic response for thermoelastic periodic composites