Franck Bourdelle
email: franck.bourdelle@univ-lille.fr

Olivier Beyssac

Teddy Parra

Christian Chopin

Nanoscale chemical zoning of chlorite and implications for low-temperature thermometry: application to the Glarus Alps (Switzerland)

Keywords: chlorite, thermometry, Glarus, low-grade metamorphism

A combination of adapted chlorite thermometry and high-spatial-resolution analytical techniques (TEM-EDX/FIB) shows that the low-grade metamorphic chlorites of the Glarus Alps, Central Alps (Switzerland), do not record the peak metamorphic conditions as commonly assumed in previous studies of this reference area for low-grade metamorphism.

Chlorites have rather recorded several stages of the retrograde path, through an intracrystalline nanometric-scale compositional zoning. The consistency of the nanoscale zoning patterns observed both within sample and along the transect suggests that local equilibrium was achieved at this scale and maintained during growth or re-equilibration.

Applying recent thermobarometers, we highlight that chlorites recorded a distinct behaviour between the northern and southern part of the studied transect: the south of the Glarus area displays a regular P-T exhumation path, from 3.0 ± 0.2 kbar and 310 ± 20 °C for maximum P-T (estimated with conventional thermometers), to ~0.8 kbar and ~220 °C according to chlorite crystal-core analysis, and to ~0.3 kbar and ~190 °C according to chlorite crystal-rim analysis.

On the contrary, the north of the Glarus area shows an apparent break in the exhumation, with P estimates from chlorite crystal rims (~1.3 kbar) higher than from chlorite crystal cores (~0.8 kbar). Even if the absolute pressure values are fraught with large uncertainty, their contrasting core/rim pattern is not model dependent. We try to correlate these new thermobarometric results with independent data to refine the exhumation scenario of the North-Helvetic flyschs, confirming differential uplift along the transect. This reappraisal of Glarus chlorite thermometry demonstrates that an adapted thermometry/analysis protocol opens new prospects for investigating the evolution of low-grade metamorphic terranes.

Introduction

Quantifying the thermal history of low-grade metamorphic rocks, i.e. in the temperature (T) range below 350 °C, is challenging because kinetics are much more sluggish than at higher metamorphic grade. For instance, the short diffusion lengths require the use of advanced in situ analytical approaches to obtain accurate crystallochemical information in chemically heterogeneous crystals down to the nanoscale. A first necessity is therefore to choose the appropriate thermometer for the targeted rocks among the multiple tools available, e.g. composition of phyllosilicates, oxygen isotope partitioning, structural and/or chemical evolution of carbonaceous material, or thermochronologic methods.

In this respect, since the 1980s, the compositional change of chlorite in low-T settings has been considered as a good proxy for the temperature of formation (e.g., [START_REF] Cathelineau | A chlorite solid solution geothermometer. The Los Azufres (Mexico) geothermal system[END_REF]. As a consequence, many chlorite thermometers have been developed, first via empirical equations linking chlorite Al IV content and temperature, with a correction based on the Fe/Mg ratio [START_REF] Kranidiotis | Systematics of chlorite alternation at the Phelps Dodge massive sulfide deposit, Matagami, Quebec[END_REF][START_REF] Jowett | Fitting iron and magnesium into the hydrothermal chlorite geothermometer[END_REF][START_REF] Zang | Chloritization of the hydrothermally altered bedrock at the Igarape-Bahia gold deposit, Carajas, Brazil[END_REF][START_REF] Xie | IIb trioctahedral chlorite from the Barberton greenstone belt: Crystal structure and rock composition constraints with implications to geothermometry[END_REF] or without such [START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF][START_REF] Hillier | Octahedral occupancy and the chemical-composition of diagenetic (low-temperature) chlorites[END_REF]. In the absence of constrained thermodynamic modelsable to take into account all the substitutions occurring in chloritesthese empirical thermometers were widely used during the 1990s to estimate T conditions in diagenetic and low-grade metamorphic environments, like the Glarus Alps complex in Switzerland (e.g., [START_REF] Rahn | Very low-grade metamorphism of the Taveyanne Greywacke, Glarus Alps, Switzerland[END_REF][START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF]. In reality, each of these empirical thermometers was calibrated for a particular context with specific applicability conditions (as reviewed by [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF], and their systematic application without due consideration of these conditions failed or led to erroneous geological interpretations in many instances, as shown by [START_REF] De Caritat | Chlorite geothermometry: a review[END_REF] for instance.

Since the 2000s, chlorite thermometry is reappraised and several thermodynamic [START_REF] Vidal | A thermodynamic model for Fe-Mg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100 ° to 600 °C, 1 to 25 kb range[END_REF][START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe 3+ /Fe 2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF][START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF] or semi-empirical models [START_REF] Inoue | Application of chemical geothermometry to low-temperature trioctahedral chlorites[END_REF]Bourdelle et al., 2013a;[START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF] have been published, with the goal to improve the theoretical background beyond these methods and their reliability. These models consider the chlorite + quartz equilibrium and are based on a combination of chlorite endmembers. They take into account the Tschermak (Al IV Al VI = Si[Mg,Fe 2+ ]), the ferromagnesian (Fe 2+ = Mg) and the di/trioctahedral (3[Mg, Fe 2+ ] =  + 2Al VI ; where  represents an octahedral vacancy) substitutions occurring in chlorites. Each of these thermometers has its own formulation, assumptions (like Fe 3+ consideration, non-ideality, compositional field, etc) and applicability field (low T and P, high T and P). When correctly applied, they have shown their efficiency in various geological contexts (see [START_REF] Inoue | Application of chlorite geothermometry to hydrothermal alteration in Toyoha geothermal system, Southwestern Hokkaido, Japan[END_REF][START_REF] Kameda | Smectite to chlorite conversion by frictional heating along a subduction thrust[END_REF][START_REF] Kameda | Silica diagenesis and its effect on interplate seismicity in cold subduction zones[END_REF][START_REF] Ganne | Modern-style plate subduction preserved in the Palaeoproterozoic West African craton[END_REF][START_REF] Trincal | Temperature micro-mapping in oscillatory-zoned chlorite: Application to study of a green-schist facies fault zone in the Pyrenean Axial Zone (Spain)[END_REF][START_REF] Scheffer | Syn-to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)[END_REF][START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF][START_REF] Chinchilla | Chlorite geothermometry applied to massive and oscillatory-zoned radiated Mn-rich chlorites in the Patricia Zn-Pb-Ag epithermal deposit (NE, Chile)[END_REF] as well as in experimental cases (e.g. [START_REF] Mosser-Ruck | Contribution of longterm hydrothermal experiments for understanding the smectite-to-chlorite conversion in geological environments[END_REF].

Another possible issue at low T is that diffusion is sluggish and therefore the composition of chlorites may be modified only over very short distances, i.e. at the nanoscale, during advanced diagenesis and/or metamorphic evolution. Fortunately, several high-spatialresolution analytical techniques have become more accessible for petrology, such as Focused Ion Beam (FIB), Transmission Electron Microscopy (TEM) coupled with EDX probe (Energy Dispersive X-ray spectroscopy), or synchrotron (EXAFS or STXM-XANES, see for phyllosilicates, Bourdelle et al., 2013b;[START_REF] De Andrade | Retrieving past geodynamic events by unlocking rock archives with μ-XRF and μ-spectroscopy[END_REF]. Recent thermometer formulations coupled with these nanometric-scale analytical techniques (Bourdelle et al., 2013c) allow one to obtain accurate temperature estimations, and open new prospects for the study of low-grade rocks [START_REF] Vidal | Deciphering temperature, pressure and oxygen-activity conditions of chlorite formation[END_REF].

In this study, we use recent analytical developments coupling FIB and TEM-EDX to characterize the chemistry of chlorite down to the nanoscale. We have targeted the Glarus Alps (Switzerland) which are one of the reference zones for low-grade metamorphism and where chlorite occurs all along the metamorphic profile. Eighteen FIB foils (9 samples, 2 FIB foils per sample) were cut in chlorite-illite assemblages to preserve the textural relationships and were analysed with a calibrated TEM-EDX. On this analytical basis, we apply recent chlorite thermometry (Bourdelle et al., 2013a) and compare our results with previous chlorite thermometric studies. We then discuss the results in terms of methodology for the assessment of thermal history in low-grade metamorphism and we highlight some implications for the exhumation history of the Glarus Alps.

Materials and methods

Geological setting and sampling

Geological setting

The mountainous region of Glarus is located in the Central Alps south of the Walensee lake (eastern Switzerland) and is cut by two main valleys along the Linth (west) and Sernft rivers (east). This part of the Central Alps exhibits a remarkable yet rather complex geological structure resulting from at least two orogenic cycles (Hercynian and Alpine) (e.g. [START_REF] Pfiffner | The structure of the Helvetic nappes and its relation to the mechanical stratigraphy[END_REF][START_REF] Schmid | Geophysicalgeological transect and tectonic evolution of the Swiss-Italian Alps[END_REF]. During the Alpine phase, the whole region has been deformed and structured resulting in a spectacular thrust (Glarus thrust) separating the Helvetic nappes in the hanging wall from the Infrahelvetic complex in the footwall (Fig. 1). The Helvetic nappes are composed by Permian conglomerates (Verrucano), Jurassic and Triassic carbonates and Tertiary flyschs (e.g. [START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF][START_REF] Merriman | Patterns of very low-grade metamorphism in metapelitic rocks[END_REF], while the Infrahelvetic complex is made by the crystalline basement of the Aar massif and its parautochthonous cover including Cretaceous and Cenozoic sediments (flyschs). The activity of the Glarus thrust from Eocene to mid-Miocene resulted in burial of the Infrahelvetic units and metamorphism up to greenschist-facies conditions (e.g. [START_REF] Frey | Alpine metamorphism of the Central Alps[END_REF].

As shown below, considering the extensive work that has been done there, the Glarus area is like a showcase of low-grade metamorphism in sediments [START_REF] Merriman | Patterns of very low-grade metamorphism in metapelitic rocks[END_REF].

Further details on the local geology are given by [START_REF] Milnes | Structural development of the Infrahelvetic complex, eastern Switzerland[END_REF], [START_REF] Pfiffner | Evolution of the North Alpine Foreland Basin in the Central Alps[END_REF][START_REF] Pfiffner | Crustal shortening in the Alpine Orogen: results from deep seismic reflection profiling in the eastern Swiss Alps, line NFP 20-east[END_REF].

2.1.2. Metamorphic conditions in the Glarus Alps: a historical case study [START_REF] Hunziker | The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps, Switzerland[END_REF] and [START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF] performed a thorough mineralogical study of the Glarus area and observed the following southward succession of index minerals: kaolinite close to Glarus, pyrophyllite and glauconite in the surroundings of Schwanden, stilpnomelane and pumpellyite between Linthal and Panix Pass, biotite and chloritoid to the south (Panix Pass). These authors also documented a transition from illite to muscovite using chemical and structural analyses, and an illite crystallinity index (IC) decreasing from the north (IC > 0.42 °2θ) to the south (IC < 0.25 °2θ) of the area (see also [START_REF] Frey | Discontinuous inverse metamorphic zonation, Glarus Alps, Switzerland: evidence from illite "crystallinity" data[END_REF][START_REF] Wang | Diagenesis and metamorphism of clay minerals in the Helvetic Alps of eastern Switzerland[END_REF]. In addition, calcite-dolomite thermometry data [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF] combined with phase-relation data show a gradient in metamorphism from NW to SE, i.e. from advanced diagenesis in the surroundings of Walensee lake and Glarus up to greenschist-facies conditions around Panix Pass, reflecting southward increasing thermal overprint due to increasing burial. This gradient was also observed by [START_REF] Mullis | The system methane-water as a geologic thermometer and barometer from the external part of the Central Alps[END_REF], [START_REF] Frey | Very low-grade metamorphism of the Alps -an introduction[END_REF] and [START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF] by investigating fluid inclusions. To the west of Walensee lake, a hydrocarbon-rich zone was identified (HHC: higher hydrocarbons), whereas methane (CH4 zone, anchizone indicator) was found by going south to Linthal (Linth valley) and Engi (Sernft valley). These two locations mark the limit between the CH4 zone and a H2O zone, to the south of which the fluid inclusions consist mostly of H2O. Vitrinite reflectance [START_REF] Rahn | Very low-grade metamorphism of the Taveyanne Greywacke, Glarus Alps, Switzerland[END_REF][START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF][START_REF] Merriman | Patterns of very low-grade metamorphism in metapelitic rocks[END_REF][START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF][START_REF] Mullis | Fluid inclusion microthermometry to calibrate vitrinite reflectance (between 50 and 270 °C), illite Kübler-Index data and the diagenesis/anchizone boundary in the external part of the Central Alps[END_REF]) also records a thermal variation from Walensee to Panix Pass. Whereas vitrinite reflectance data show a very low-rank coal to the north, with Rmax = 0.77 % (indicator of 100-120 °C), they refer to anthracite around Panix Pass, with Rmax > 8 % (indicator of T > 270 °C). This is confirmed by the evolution of the Raman spectrum of carbonaceous material which is characteristic of very disordered graphitic carbon, with a marked southward increase of structural ordering [START_REF] Lahfid | Evolution of the Raman spectrum of carbonaceous material in low-grade metasediments of the Glarus Alps (Switzerland)[END_REF]. All these independent proxies (noted "independent conventional thermometers") give information on peak metamorphism (noted "maximum temperature" below); they concur to the picture of a NW-SE thermal gradient (Fig. 1) which can be approximated by a linear regression linking maximum temperature and structural distance through the Glarus Alps [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF].

Constraints on pressure are poor, and only based on the b0 crystallographic parameter [START_REF] Sassi | The b0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF] of micas in the North-Helvetic flyschs (2.0-2.5 kbar, [START_REF] Frey | Discontinuous inverse metamorphic zonation, Glarus Alps, Switzerland: evidence from illite "crystallinity" data[END_REF], and on fluid inclusion data around Elm (2.0-2.4 kbar; [START_REF] Bowers | Calculation of the thermodynamic and geochemical consequences of non-ideal mixing in the system H2O-CO2-NaCl on phase-relations in geologic systems-metamorphic equilibria at high-pressures and temperatures[END_REF][START_REF] Potter | The volumetric properties of aqueous sodium chloride solutions from 0 ° to 500 °C and pressures up to 2000 bars based on a regression of available data in the literature[END_REF][START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF][START_REF] Caumon | Determination of methane content in NaCl-H2O fluid inclusions by Raman spectroscopy. Calibration and application to the external part of the Central Alps (Switzerland)[END_REF]. By extrapolation, considering these works and the barometric gradient (lithostatic pressure assumed ;[START_REF] Frey | Discontinuous inverse metamorphic zonation, Glarus Alps, Switzerland: evidence from illite "crystallinity" data[END_REF], we can approximately estimate pressures of 1.3-1.5 kbar close to Walensee Lake, 1.5-2.0 kbar in the surroundings of Glarus, 2.0-2.4 kbar around Elm, and 2.4-3.0 kbar at Panix Pass.

Sampling

Samples come from the Cenozoic North-Helvetic flyschs (Infrahelvetic complex; Table 1; Fig. 1) in the footwall of the Glarus thrust, which have undergone only one metamorphic cycle, i.e. the Alpine orogeny. Eleven samples were collected along a N-S profile around Elm (along the Sernft valley, following the regional thermal gradient), and nine were selected (two samples were analysed but finally discarded because FIB foils were not correctly cut through chlorite-illite contacts, see below). As the chlorite thermometer used here is based on chlorite + quartz equilibrium (see section 2.2), all the samples were selected based on the presence of quartz (even samples with calcschist lithology; Table 1). From the data compilation of [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF], a metamorphic peak temperature can be assigned by interpolation to each of our samples (Table 1), from 245 °C north of Elm to 330 °C south of Panix pass.

Chlorite thermometers and barometer

Investigating the relationship between chlorite composition and crystallization temperature in geothermal fields of Salton Sea and Los Azufres, [START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF] showed that chlorite Al IV content increases with temperature, and established the following linear regression: T°C = -61.92 + 321.98 (Al IV apfu)

(1)

where Al IV is expressed in atoms per 18 oxygen formula unit. This pioneer equation became the most used chlorite thermometer (including studies of Glarus metamorphism) and is the base of several other empirical thermometers [START_REF] Kranidiotis | Systematics of chlorite alternation at the Phelps Dodge massive sulfide deposit, Matagami, Quebec[END_REF][START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF][START_REF] Hillier | Octahedral occupancy and the chemical-composition of diagenetic (low-temperature) chlorites[END_REF][START_REF] Jowett | Fitting iron and magnesium into the hydrothermal chlorite geothermometer[END_REF]. However, empirical equations have shown their limits: [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF] demonstrate that these equations are all specific cases of the Si-1-1R2 2+ exchange vector (combining Tschermak and di/trioctahedral substitutions), linked to the geological setting in which they were calibrated, and are consequently limited to restricted chlorite compositional ranges and geological contexts. It is therefore irrelevant to apply one empirical equation rather than another in a different geological setting; Eq. 1 is applied here only to test and compare empirical thermometry to literature and semi-empirical approaches.

To circumvent limitations of empirical calibrations, semi-empirical and thermodynamic chlorite thermometers were later proposed. Semi-empirical thermometers are ideal models based on a ratio of end-member activities and directly link the temperature to a K equilibrium constant (for a chlorite + quartz equilibrium, calculated from chlorite endmembers, H2O and SiO2 activities) as:

(2)

where A and B are constants, the Gibbs free energy of reaction, and R the gas constant.

With this approach, effects of pressure are neglected; this assumption is especially justified in low-T/low-P contexts such as the Glarus Alps.

 

B T A T R G K r      . 303 . 2 log 0 0 r G 
Here, the model of Bourdelle et al. (2013a) was used because (i) it is specially calibrated for low-P-T chlorites, i.e. T < 350 °C and P < 3-4 kbar, and (ii) it does not require a prior quantification of the Fe 3+ content, considering that all iron is divalent. The latter assumption does not compromise reliable temperature estimation, as shown by [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF] and [START_REF] Chinchilla | Chlorite geothermometry applied to massive and oscillatory-zoned radiated Mn-rich chlorites in the Patricia Zn-Pb-Ag epithermal deposit (NE, Chile)[END_REF]. The formulation of Bourdelle et al. (2013a) considers magnesian end-members and an ordered cationic site repartition model following the recommendations of [START_REF] Bailey | Chlorites: structures and crystal chemistry[END_REF] and [START_REF] Vidal | A thermodynamic model for Fe-Mg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100 ° to 600 °C, 1 to 25 kb range[END_REF] for di-trioctahedral chlorites, an approach reinforced by several crystal-chemical studies (e.g. [START_REF] Welch | A multi-nuclear NMR study of clinochlore[END_REF][START_REF] Inoue | High-angle annular dark field scanning transmission electron microscopic (HAADFSTEM) study of Fe-rich 7 Å-14 Å interstratified minerals from a hydrothermal deposit[END_REF]. The calibrated thermometric equation is:

(3) where K is the equilibrium constant of the chlorite + quartz reaction as: Al-free Mg-chlorite + 3 Mg-sudoite = 3 Mg-amesite + 7 Quartz + 4 H2O (4).

This formalism considers only the magnesian system, but that does not affect temperature estimates because the ferromagnesian substitution is not sensitive to temperature in low-T contexts (for more details, see [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF].

Based on the same assumptions as Bourdelle et al. (2013a), but considering the pressure effect and a different set of end-members, [START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF] proposed a semi-empirical thermometer as:

(5) based on the reaction:

15 . 273 log 40 . 23 9400 ) (     K C T 15 . 273 782 . 455 ln ) ( 99 . 4996 203093 ) (       K R kbar P C T 2 Clinochlore + 3 Sudoite = 4 Amesite + 7 Quartz + 4 H2O (6).
It is important to note that this modelas the thermodynamic model of [START_REF] Vidal | Thermodynamic properties of the Tschermak solid solution in Fe-chlorite: Application to natural examples and possible role of oxidation[END_REF][START_REF] Vidal | P-Tdeformation-Fe 3+ /Fe 2+ mapping at the thin section scale and comparison with XANES mapping: application to a garnet-bearing metapelite from the Sambagawa metamorphic belt (Japan)[END_REF] excludes Si-rich chlorites (Si > 3 apfu); it was used here to try to estimate pressure, using as input the temperature obtained with equation (3).

Semi-empirical thermometers were calibrated assuming H2O and SiO2 activities as

aH2O = aSiO2 = 1.
In the Glarus context, chlorites were sampled in the H2O-dominated zone, except for sample GL07 13 from the CH4 zone (Figure 1). [START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF][START_REF] Tarantola | Oxidation of CH4 to CO2 and H2O by chloritization of detrital biotite at 270 ± 5 °C in the external part of the Central Alps, Switzerland[END_REF] showed that fluid inclusions in the H2O zone along the Sernft valley are CO2-free or contain less than 1 mol% of CO2 and are composed by > 99 mol% of H2O. Consequently, the H2O activity could be considered as close to 1. Although it can be assumed that H2O-dominated fluid inclusions are formed at peak metamorphism [START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF], it is not possible to determine whether the H2O activity in the system varied during exhumation. However, the assumption that aH2O = 1 is usually reasonable under diagenetic to low-grade metamorphic conditions [START_REF] Inoue | Application of chemical geothermometry to low-temperature trioctahedral chlorites[END_REF][START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF]. Besides, aSiO2 = 1 is justified in the case of quartz-bearing rocks.

Analytical techniques

Detailed petrologic investigations of petrographic thin sections from the Glarus samples were carried out using optical microscopy and Scanning Electron Microscopy (SEM)

to describe the texture of chlorites and precisely distinguish authigenic from detrital chlorites before FIB milling. The SEM used was a Hitachi S-2500 instrument operating at 15 kV with a 1.5 nA current intensity (ENS Paris).

The FIB foils were cut using the FIB 200 FEI of CP2M (University of Aix-Marseille) with a voltage of 30 kV and a gallium beam. The cutting intensity varied from 5000 to 50 pA, following the protocol for FIB thin foil preparation given by [START_REF] Heaney | Focused ion beam milling: a method of site-specific sample extraction for microanalysis of Earth and planetary materials[END_REF] and [START_REF] Wirth | Focused Ion Beam (FIB): a novel technology for advanced application of micro-and nanoanalysis in geosciences and applied mineralogy[END_REF]. Eighteen FIB foils were cut from nine samples (two FIB foils by sample) across illitechlorite contacts near one end of the interface, where re-equilibration is the most likely. The thin foils were cut out with a size of approximately 15 µm by 5 µm and a thickness between 50 and 300 nm. To check the preservation of the mineral crystalline structure after FIBmilling, lattice-fringe imaging was systematically carried out with the TEM.

Samples were analysed with a TEM-EDX/FIB coupled method and quantitative analyses were deduced and corrected thanks to the t-O-protocol [START_REF] Van Cappellen | Quantitative transmission-X-ray microanalysis of ionic compounds[END_REF][START_REF] Bourdelle | Ultrathin section preparation of phyllosilicates by Focused Ion Beam milling for quantitative analysis by TEM-EDX[END_REF]. The TEM-EDX used was a JEOL 2100-F at the Physics and Microanalysis Department of IFPEN Lyon-Solaize, using a 200 kV voltage, a counting time of 60 s and a dead time lower than 15%. The current density was maintained at 1.3 pA/cm 2 . The sample tilt angle was 7°. In these conditions, the spot size was around 1 nm and defocused to 50 nm. The TEM-EDX K factors [START_REF] Cliff | The quantitative analysis of thin specimens[END_REF] were calibrated on paragonite (Na, Al), pyrophyllite (Al), talc (Mg), muscovite (Fe, Al, K), chlorite (Fe, Al), clintonite (Mg, Ca, Al) and phengite (Mg, Fe, K, Al) and the standardizations were checked against electron microprobe analysis of reference clays (SMB-18; [START_REF] Kohler | Clayey Cap-Rock behavior in H2O-CO2 media at low pressure and temperature conditions: an experimental approach[END_REF].

Given the high spatial resolution, the analysis points on FIB foils were targeted to reveal within-grain compositional variations. The structural formulae of chlorite were calculated on the anhydrous basis of 14 oxygens. Analyses containing K2O + Na2O + CaO > 1 wt% (of the 100 wt% total of TEM-EDX analyses) were discarded.

Results

Petrology of Infrahelvetic nappe samples

The selected samples are schists ranging from calcschists to quartz-schists. SEM and optical microscopy observations reveal that the main minerals are quartz, carbonates, phyllosilicates and feldspars (albite and K-feldspars). Quartz grains are commonly angular, associated with prismatic altered feldspars. Phyllosilicates refer to micas (commonly muscovite/illite, occasionally detrital biotite in the CH4 zone) and chlorite, either detrital or authigenic. Several clusters of pyrite can be identified, with a framboidal or cubic habit.

Generally, a calcareous matrix surrounds other minerals.

Detrital micas and chlorites are mainly observed in GL07 13 sample, i.e. in the CH4 zone (Table 1; Fig. 1), while phyllosilicates are mostly neo-formed in other samplesin the H2O zone. Under the polarizing microscope, detrital chlorites have a dark green colour with a strong pleochroism. These chlorites are well-preserved and oriented sub-parallel to the nascent schistosity of the rock. Authigenic chlorites are light green and form intergrowths with authigenic muscovite/illite sheets, along the entire profile (Fig. 2). These chlorites generally show slight deformation features. According to these microtextural observations, the intergrowths of muscovite/illite and chlorite are quite similar to those observed and considered as authigenic by [START_REF] Tarantola | Oxidation of CH4 to CO2 and H2O by chloritization of detrital biotite at 270 ± 5 °C in the external part of the Central Alps, Switzerland[END_REF]. The FIB foils were cut across authigenic illite-chlorite intergrowths; detrital chlorites were discarded on the basis of the microtextural criteria described above.

Nanoscale chemical zoning in chlorites

All analyses considered, the targeted Glarus chlorites display a Si content between 2.65 and 3.08 apfu and a R 2+ content (i.e. Fe 2+ + Mg 2+ ) between 4.46 and 3.71 apfu, referring to di-tri-octahedral compositions with  < 0.4 apfu.

However, given the high spatial resolution of TEM-EDX and considering that FIB milling allows to preserve the crystal integrity, we can separate analyses into two categories:

(i) crystal rim analyses, performed ~50 nm away from illite-chlorite contacts (this distance allows one to avoid contamination through the FIB foil thickness due to adjacent minerals, see [START_REF] Bourdelle | Ultrathin section preparation of phyllosilicates by Focused Ion Beam milling for quantitative analysis by TEM-EDX[END_REF], and (ii) crystal core analyses (see Bourdelle et al., 2013c). The comparison of core and rim analyses reveals a systematic trend: for each sample, crystal rims are enriched in Si and depleted in divalent cations compared to crystal cores (Table 2). As an example, compositional data obtained on one of GL07-19 FIB foils reveal a chemical variation of chlorite over a few tens of nanometers, with divalent cation content lower than 4.0 apfu in the crystal rim, but always higher than 4.1 apfu in the crystal core (Fig. 3). All the analysed grains show such a compositional variation and, for any given sample, the two analysed grains show the same trend (see section 4.2). This systematic difference between crystal core and rim compositions clearly suggests a chemical zoning at the nanoscale [START_REF] Bourdelle | Chemical variations of the micas and chlorites compositions along natural geothermal gradient: Example of Helvetic Alps (Glarus)[END_REF].

Selection of chlorite analyses and chlorite thermometry

As a reference and like most previous studies, we applied the empirical thermometer of [START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF] to all chlorite analyses. To simulate EMP data, crystal rim and core analyses were interpreted together. The obtained temperatures vary between 255 and 365 °C (Table 2) for all the samples (from north to south), in fair agreement with [START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF] and [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF] from independent thermometers.

However, owing to the spatially resolved techniques employed here, which revealed the chemical zoning of Glarus chlorites, one is now able to consider separately crystal rim and core analyses. In addition, to clearly identify the re-equilibration or growth steps, only the "extreme" compositions should be considered (see Bourdelle et al., 2013c). Indeed, because of the difficulty to locate precisely the rim areas that recorded the last re-equilibration or the core areas that recorded the earliest re-equilibration or growth, we have considered only the three or four rim analyses and the three or four core analyses (noted "extreme" rim and "extreme" core compositions respectively) per sample (i.e. combining the two FIB foils of each sample) that represent the compositions closest to equilibrium. Equilibrium is here assessed by the extremum log K (Eq. 3), calculated from the analyses and considering the direction of the relevant reaction. The retained "extreme" rim compositions actually ignore some analyses having Si > 3 apfu (because they cannot be used further for pressure evaluation with Lanari et al. (2014) model). Therefore, the "extreme" rim results will represent a conservative estimate of (i.e. an upper bound to) the lowest (re)equilibration/growth temperatures.

The chlorite thermometer of Bourdelle et al. (2013a) applied on the "extreme" crystal rim and core compositions yields the following results (Table 2, Fig. 4):

(1) Temperatures (from rim and core analyses) are systematically lower than maximum temperatures obtained with other independent thermometers along the sampling profile (difference in the range of ca. 50-100 °C, depending on rim and core analyses).

(2) Temperatures from rim compositions are always lower than temperatures from core compositions, for any given sample. The difference ranges from 10 to 60 °C.

(3) Temperatures from rim compositions are constant along the N-S profile (always around 190 ± 20 °C, except GL07 13 sample) whereas the maximum temperatures from other independent thermometers display a regular increase (from 245 to 330 °C).

Tentative reconstruction of the exhumation P-T path

To estimate the formation pressure from chlorite core and rim compositions, we used the thermobarometer of [START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF], i.e. the semi-empirical equation ( 5) linking T, P and log K of the chlorite + quartz equilibrium, in which the T estimates obtained with the Bourdelle et al. (2013a) model are used as input data (along with relevant K values). Figure 5 presents the results of P calculation, linking for each sample maximum P-T (estimated from literature, see section 2.1.2), chlorite-core and chlorite-rim thermobarometry. All chlorite core analyses give a pressure around 0.8 kbar (± 0.2 kbar, except for GL07 21), while pressures estimated from chlorite rim analyses are around 0.3 kbar (± 0.2 kbar) for samples from the south of the Glarus area (from GL07 19 sample to Elm) and around 1.3 kbar (± 0.3 kbar) for samples from the north (from Elm to GL01 13 sample). Therefore, the south of the area displays a regular P-T exhumation path (except GL07 20 sample), from 3.0 ± 0.2 kbar and 310 ± 20 °C for maximum P-T, to ~0.8 kbar and ~220 °C for chlorite cores, and to ~0.3 kbar and 190 °C for chlorite rims. On the contrary, the north of the area shows an apparent break in the exhumation, with P estimates higher for chlorite rims (~1.3 kbar) than chlorite cores (~0.8 kbar). All these pressure data may be too low, if one considers the geothermal gradients implied; they should indeed not be taken at face value, considering that the reported uncertainties are just the standard deviation; they do not include propagation of analytical error nor the unknown but likely poor accuracy of the barometer (due to the low pressure dependence of chlorite composition in low-P-T contexts, Bourdelle et al., 2013a). However, regardless of absolute values, the differential of core and rim data and its regional pattern are solid, model-independent results. As will be shown below, the consistency of the results, both in rim/core temperature evolutions and in the areal distribution of the rim/core pressure difference, supports the equilibrium-thermodynamics approach and suggests two different exhumation histories for the northern and the southern part of the Glarus Alps.

Discussion

Micro-vs. nano-scale analysis

Our results show the necessity of using high-resolution spatially resolved techniques to analyse properly chlorites (as well as micas) in a low-grade metamorphic setting like the Glarus Alps. Indeed, the results using a recent chlorite thermometer and considering the nanoscale chlorite zoning are significantly different from those obtained on the same chlorite via conventional thermometers and empirical chlorite thermometers coupled with microscale analysis like EMPA.

Previous studies on Glarus chlorites used EMPA, which has a sampling volume of several µm 3 that does not allow identifying any compositional variation in submicrometric crystals. Glarus chlorites were therefore considered to have a homogeneous composition [START_REF] Rahn | Very low-grade metamorphism of the Taveyanne Greywacke, Glarus Alps, Switzerland[END_REF]. However, several chlorite compositions may coexist in the same rock sample or even in the same crystal, such as detrital, authigenic and/or metamorphic compositions (e.g. Bourdelle et al., 2013c). Only specific low-T cases, such as homogeneous chlorites directly precipitated from fluids make it possible to avoid a nanometric analytical

protocol and yet to obtain compositions suitable for thermometry [START_REF] Lanari | A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO-FeO-Al2O3-SiO2-H2O: applications to P-T sections and geothermometry[END_REF]. Moreover, contaminations of chlorite analyses by quartzwhich can strongly impact thermometric calculation, especially with empirical thermometersare undetectable in EMPA (see [START_REF] Schmidt | Reaction progress in chloritic material: an electron microbeam study of the Taveyanne greywacke, Switzerland[END_REF] on chlorites from Taveyanne greywacke, Switzerland).

Thanks to the excellent microtextural control offered by imaging modes down to the nanoscale, such contaminations are avoidable using TEM; contamination of chlorite analysis by a few smectite or illite layers is still possible, but (i) is limited by exclusion of analyses containing K2O + Na2O + CaO > 1 wt% and (ii) does not influence thermometric calculation [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF].

Significance of chlorite compositional variations, local equilibrium vs. kinetics

The EMP analyses of detrital chlorites (not presented here) show Si contents significantly lower (< 2.6 apfu) than the crystal-core analyses obtained by TEM on authigenic chlorites. As a consequence, crystal core compositions must be considered as the result of growth or re-equilibration of (prograde?) authigenic chlorite and not as a detrital relic.

A key issue is then the significance of compositional variations revealed by this study.

The formation of chlorite takes place during metamorphic reactions which are transient disequilibrium states governed by local chemical potential gradients and kinetic processes. Its re-equilibration may proceed through diffusion or, more likely, dissolution-reprecipitation (cf. [START_REF] Putnis | The mechanism of reequilibration of solids in the presence of a fluid phase[END_REF][START_REF] Putnis | Mineral replacement reactions[END_REF]. Is then the observed compositional variability of chlorite the compositional record of reaction kinetics, or the record that local, grain-boundary equilibrium was achieved and maintained during growth or re-equilibration of a given metamorphic stage? Only in the latter case can the equilibrium-thermodynamic approach inherent to the applied thermometers be justified. The question must be addressed before any thermobarometric or geological consequence can be envisaged.

One way to address the equilibrium issue is through the consistency or heterogeneity of the compositional variations. In case kinetics and local-scale factors prevailed, one would expect different zoning in each grain depending on the local environment. If grain-boundary equilibrium prevailed, a local and larger-scale consistency is to be expected in the zoning patterns. In other words, can the assumption of local equilibrium, which has been shown to offer a valid conceptual framework at the micro-to millimetre scale in higher-grade metamorphic terranes, also be valid in low-grade environments, then of course at nanoscale?

This issue, which is of general bearing, is tested below on the basis of two case studies for which nanoscale data are available, the Glarus Alps (this study) and the advanced diagenesis of Gulf Coast sandstones, Texas (Bourdelle et al., 2013c).

The Glarus samples provide a first test of within-sample consistency. As explained in section 2.3, for each sample, compositional data were obtained from two FIB sections extracted from two different chlorite grains, and merged. They were merged, indeed, because the zoning patterns recorded were consistently the same in the two crystals of each pair of FIB foils. This is illustrated in Figure 6 for the representative sample GL07 17, about halfway in the Glarus transect. This figure depicts the Si and R 2+ contents for rim and core of two different grains named B1 and A2. For clarity, we have considered only the four rim analyses and the four core analyses per grains (i.e. for B1 and A2) that give the extremum log K (Eq. 3; see section 3.3). It appears that crystal rims display a similar composition (Si content between 2.72 and 2.83 apfu, R 2+ content between 4.16 and 4.24 apfu). Likewise the crystal cores of B1 and A2 grains show a similar "extreme" composition. Furthermore, the compositional trend from core to rim is not only the same within sample, but it is consistently observed along the whole transect, in all samples, with calculated rim temperatures consistently lower than core temperatures, whatever they mean. Last, a second type of inter-sample regional-scale consistency exists, with the northern samples showing higher or marginally higher calculated rim pressures than core pressures, whatever they mean, and the southern samples showing the reverse (Fig. 5). It seems unlikely that such within-sample and regional consistencies could be achieved if kinetics and local factors were controlling chlorite composition.

A similar conclusion was reached from the study of Gulf Coast chlorites (Bourdelle et al., 2013c). In chlorites sampled at different depths and various locations of the Gulf Coast area, TEM and FIB techniques highlight consistent within-grain patterns of major-elements variations (see also Fig. 11 in [START_REF] Vidal | Deciphering temperature, pressure and oxygen-activity conditions of chlorite formation[END_REF]. There, the zoning of chlorite at a nanoscale is due to various crystallization stages during burial, with a core composition corresponding to detrital chlorite and a rim composition of neo-formed chlorite. Moreover, the chlorite rim compositions display a T dependence with depth and location that is consistent with the diagenetic history described at a regional scale (see Bourdelle et al., 2013c). This T dependence of chlorite rim compositions, independently of chlorite core, clearly supports the idea that the zoning of low-grade chlorites is not controlled by a kinetic effect but rather by local equilibrium, in this case-study too. These observations lend credence to the thermometric approach used here, the results of which are further discussed below.

Thermometric implications of the compositional zoning of Glarus chlorite

The identification of chemical zoning in the Glarus chlorites has major thermometric implications. The fact that T estimates from EMP analysis of Glarus chlorites using an empirical thermometer are similar to maximum temperatures (Table 2) from other independent thermometers is a coincidence, resulting from the use of a low-spatial-resolution analytical protocol and of an empirical equation not calibrated for this geological context (see [START_REF] Bourdelle | Low-temperature chlorite geothermometry: a graphical representation based on a T-R 2+ -Si diagram[END_REF] for more explanations on the applicability of empirical thermometers). This erroneously led earlier authors to conclude that authigenic chlorites formed during the maximum burial step of the Glarus rocks and were not re-equilibrated during exhumation. In fact, along the sampling profile, temperatures from independent conventional thermometers are clearly higher than results of chlorite semi-empirical thermometry (Bourdelle et al., 2013a) applied to crystal core and rim compositions (Table 2), indicating different temperatures for different re-equilibration steps.

The chemical zoning identified in the Glarus chlorites is approximated through two sets of compositional data (from rims and cores) generating two sets of estimated temperatures, to be compared with the temperatures from independent conventional thermometers (maximum T). Considering that independent thermometers give the peak metamorphic temperature [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF], that crystal-core compositions are not detrital, and that T(core) > T(rim), the three arrays of T estimates (in black, red and blue on Fig. 4) are assumed to correspond to two snapshots along the retrograde path recorded by chlorites. The first one is the re-equilibration of prograde chlorites that may happen due to fluid-rock interactions during the early exhumation and/or by chloritization of residual detrital biotites.

The latter case may be valid when T estimate from core is close to maximum T, considering that biotite chloritization may be assigned to peak metamorphism with a fluid-controlled mineralogical isograd [START_REF] Tarantola | Oxidation of CH4 to CO2 and H2O by chloritization of detrital biotite at 270 ± 5 °C in the external part of the Central Alps, Switzerland[END_REF]. The second one is the re-equilibration of the chlorite rims during the late stages of the exhumation.

Geological interpretations

Compositional variations within a chlorite crystal are not easy to detect. Even using the latest developments of the TEM-EDX/FIB coupling, it is difficult to understand whether the zoning observed represents a progressive zoning from core to rim (continuous evolution from core to rim) or is due to the juxtaposition of different compositions (i.e., several juxtaposed and independent zones in a same chlorite, each with a specific composition).

Therefore, we distinguish only two populations of chlorite compositions: core versus rim.

Assuming that tens of analyses on two FIB foils for each of nine samples are representative (see Bourdelle et al., 2013c) and that targeted crystal rims refer to the late stages of the retrograde path, we can study the exhumation via the difference between the maximum temperature undergone by rocks (from independent conventional thermometers) and the temperature of the last recorded step of equilibration (our data retrieved from crystal rims). Ma ago. To reconcile the rotation scenario (between 10 Ma and present), the maximum temperatures (which increase from North to South), and the variation of ΔT, we can envisage (i) an exhumation that would have begun much earlier than in the scenario proposed by Rahn and Graseman (1999), with northern units initially less buried than the southern ones (crystallisation of chlorite cores), followed by (ii) a step at 10-12 Ma where northern and southern units are approximately at the same depth and during which the chlorite rims (re)crystallized, and finally by (iii) an exhumation to the surface, with the southern part exhumed structurally above the northern one. This scenario is summarized in Figure 8.

The chemical zoning of Glarus chlorite also gives valuable pressure information.

Therefore, chlorite core and chlorite rim thermobarometry allowed us to outline the P-T retrograde path, and to identify a difference in exhumation history between the northern and southern parts of the studied area. These two retrograde paths could be linked to the exhumation scenario proposed above, completed by either (i) a slight compression step for the north of the area around 10-12 Ma, explaining the estimated P increase, or (ii) a differential exhumation rate between the north and the south, or a combination of both scenarios.

Scenario (i) would imply a thrust fault somewhere to accommodate the compression but there is no direct trace or evidence for a blind fault within the studied unit. Scenario (ii) implies that the southern rocks would be exhumed fast enough so that temperature would not reequilibrate during decompression, whereas the northern rocks would be exhumed at a slower rate. This latter scenario is in agreement with the discussion above and observations from [START_REF] Rahn | Fission track and numerical thermal modeling of differential exhumation of the Glarus thrust plane (Switzerland)[END_REF], although we note that their conclusions deal with the shallowest part of exhumation, as the apatite closure temperature is estimated at ca. 110-130 °C.

Conclusion

Over the last years, chlorite thermometry has made important progress and, combined with high-spatial-resolution analytical techniques, it allows one to obtain accurate temperature estimates, even in advanced diagenesis or very low-grade metamorphic contexts. Therein, selecting a thermometer should carefully consider all assumptions and the applicability field for this thermometer with regard to the case study. In addition, the analytical protocol should be chosen according to the targeted chlorites: EMP for homogeneouslarge grain sizeschlorites, TEM-EDX/FIB for heterogeneoussmall grain sizeschlorites. Studying the Glarus Alps metasediments, we show that low-T chlorites can record the retrograde rock history, and that earlier interpretations made on the basis of chlorite empirical thermometry must take this into account. In Glarus, past interpretations imply a crystallization of chlorites during the maximum burial step without re-equilibration during exhumation. These chlorites are actually heterogeneous and record a succession of re-equilibrations, giving new keys to understand the P-T history of this part of the Central Alps.

grateful for the challenging reviews of A. Tarantola and an anonymous referee, and for the editorial insight of M. Scambelluri, which helped us to strengthen and clarify our line of argument. This study was financially supported by IFP Energies Nouvelles, CNRS and ENS Paris. Funding by Partenariat Hubert Curien (Germaine de Stael) to OB is acknowledged.

Tables Table 1: Sample description with name, geographical position (WGS 84 coordinates), area of sampling, lithology and temperature of peak metamorphism estimated by [START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF] and [START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF]. [START_REF] Rahn | Alpine metamorphism in the North Helvetic Flysch of the Glarus Alps, Switzerland[END_REF][START_REF] Merriman | Patterns of very low-grade metamorphism in metapelitic rocks[END_REF][START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF][START_REF] Tarantola | Oxidation of methane at the CH4 / H2O-(CO2) transition zone in the external part of the Central Alps, Switzerland: evidences from stable isotope investigations[END_REF]. For each sample, ΔT was calculated as the difference between maximum T from the literature and the average T from chlorite rim thermometry (based on "extreme" compositions, see text;

Figure 7

 7 Figure7shows this difference (noted ΔT) along the sampling profile. We observe that ΔT

FiguresFig. 1 :

 1 Figures

Fig. 2 :

 2 Fig. 2: Back-Scattered Electron SEM images of several petrographic thin sections showing

Fig. 3 :

 3 Fig. 3: Analysis of an illite-chlorite assemblage (GL07 19 sample). (a) SEM image of the

Fig. 4 :

 4 Fig. 4: Comparison of thermal profiles obtained between East-Schwanden and Panix Pass

Fig. 5 :

 5 Fig. 5: Exhumation path drawn in a P-T diagram by linking maximum P-T, chlorite-core and

Fig. 6 :

 6 Fig. 6: Comparison of analyzed rim and core compositions of two chlorite grains (noted B1 and A2) from one sample (GL07 17). Compositions are represented by Si and R 2+ contents (in

Fig. 7 :

 7 Fig. 7: Evolution of ΔT along the sampling profile (between East-Schwanden and Panix Pass).

  Bourdelle et al. (2013a) thermometer). Error bars refer to the maximum of standard deviations. Dark line: linear regression. Grey area: linear regression ± 15 °C.

Fig. 8 :

 8 Fig. 8: Schematic exhumation scenario for the North-Helvetic flyschs along the studied

Table 2 :

 2 Selection of TEM-EDX analyses of Glarus chlorite. For each sample, a crystal core

	Struct. form. (apfu)
	Si	3.01	2.77	2.83	2.67	2.83	2.86	2.81	2.72	2.76	2.76	2.76	2.70	2.89	2.76	2.77	2.69
	Ti	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.01	0.00	0.00	0.00
	Al(IV)	0.99	1.23	1.17	1.33	1.17	1.14	1.19	1.28	1.24	1.24	1.24	1.30	1.11	1.24	1.23	1.31
	Al(VI)	1.80	1.74	1.61	1.80	1.63	1.51	1.63	1.65	1.74	1.61	1.82	1.74	1.60	1.50	1.63	1.56
	Fe 2+	1.99	2.27	1.77	2.17	1.69	2.03	1.87	1.92	2.27	2.21	2.19	2.35	2.29	2.00	2.35	2.43
	Mn	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
	Mg	1.78	1.73	2.40	1.72	2.38	2.19	2.25	2.24	1.73	1.99	1.70	1.68	1.82	2.36	1.80	1.87
	Ca	0.01	0.02	0.00	0.03	0.05	0.07	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.01
	Na	0.00	0.00	0.00	0.03	0.00	0.04	0.01	0.00	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.00
	K	0.01	0.00	0.01	0.04	0.00	0.00	0.04	0.00	0.01	0.00	0.00	0.00	0.05	0.01	0.03	0.00
	T°C estimates
	max T°C	245	245	280	280	285	285	295	295	310	310	320	320	325	325	330	330
	Cathelineau T°C	255	336	315	363	311	304	321	350	336	337	336	355	294	337	335	359
	Bourdelle T°C	120	183	186	202	176	197	189	217	183	209	172	201	168	245	203	261

and crystal rim analysis are given. Max T°C refers to maximum temperature (peak metamorphism) obtained from independent conventional thermometers

[START_REF] Ebert | Cooling induced strain localization in carbonate mylonites within a large-scale shear zone (Glarus thrust, Switzerland)[END_REF]

.

Cathelineau T°C: temperatures estimated with the

[START_REF] Cathelineau | Cation site occupancy in chlorites and illites as a function of temperature[END_REF]

thermometer (Eq. 1).

Bourdelle T°C: temperatures estimated with the Bourdelle et al. (

2013a

) thermometer (Eq. 3).

Acknowledgements

We are most grateful to the materials characterization department of IFP Energies Nouvelles-Lyon, in particular to F. Moreau, for technical advice. Thanks are also extended to W. Saikaly, C. Dominici and M. Cabié, CP2M-Université Aix-Marseille. We are most


Mateugue Diack

Tegaye Diop

Ramatoulaye Ndiaye

Restoration of Degraded Lands Affected by Salinization Process under Climate Change Conditions: Impacts on Food Security in the Senegal River Valley

Keywords: Climate change, Agriculture, Degraded land, Salinization, Food security

With the progressive degradation of agricultural lands due to climate change, there is a need to document how land responds to management practices and their resilience to salinization processes. Salinization is a problem associated with agriculture and it constitutes a constraint which results in inappropriate practices. In addition, agriculture intensification and changes in temperature and precipitation patterns expected from climate change are likely to affect the salt-water balance of fragile ecosystems. Information on the relationship between climate change and salt-affected land salinization processes is scattered. Little has been done to highlight the most affected or vulnerable areas or to promote practices that can be used to adapt agricultural production in fragile areas to climate change. This study will contribute to food security and reducing the stress on ecosystems. This study relates land management practices to lands degraded and salinized due to climate change. Several land management practices have been used to evaluate the level of restoration of degraded lands. Changes in temperature, relative humidity, evapora-tion, solar radiation, and soil temperature combined with agricultural management practices driven by different cropping systems were used to evaluate the best and most adapted management practices for degraded lands that are dedicated to agriculture. Results suggest that regular seasonal cropping under irrigation might be recommended for degraded lands to assure sustainable food security.

Introduction

Land degradation due to salinization has revealed negative interactions that have been developing for decades from an increase in population and a decrease of crop production in developing countries. This has aggravated inequalities in food production across regions [START_REF] Parry | Climate change, global food supply and risk of hunger[END_REF]. Moreover, food insecurity is likely to increase with climate change unless early warning systems and development programs are used more effectively [START_REF] Brown | Food security under climate change[END_REF]. Developing countries in sub-Saharan Africa are more vulnerable because of the dominance of agriculture in their economies, the scarcity of capital for adaptation measures, warmer baseline climates, and heightened exposure to extreme events (Tubiello andFischer 2007, 2013;[START_REF] Brown | Food security under climate change[END_REF]. Soils affected by salinization processes have become a major environmental issue. Nearly 800 million hectares of land worldwide, more than 6 % of the total surface area of lands, are affected by salinization processes. In addition, climate changes aggravate losses of arable lands in the arid and semi-arid zones [START_REF] Boivin | De ´sertification et salinisation des terres au Se ´ne ´gal: proble `mes et reme `des[END_REF]. In Senegal, estimated total lands affected by salinization processes are 1,000,000 ha to 1,700,000 ha [START_REF] Gobin | Indicators for pan-European assessment and monitoring of soil erosion by water[END_REF][START_REF] Sadio | Pe ´doge `nese et potentialite ´s forestie `res des sols sulfate ´s acides sale ´s des tannes du Sine Saloum, Se ´ne ´gal[END_REF]. More recent studies conducted by the National Institute of Pedology in Senegal ( 2008) have estimated the total land affected by salinization processes at 996,950 ha (Fig. 1). The same studies underlined that the level of salinization requires setting up strategy control the phenomenon and determining the origin or the source of the salt within the considered soil. Those studies did not identify the arable surface area affected by The salinization process or indicate the sources of specific salinization processes for any surface area listed. Salinization processes are near to irreversible in the case of heavy-textured soils with high levels of swelling clay [START_REF] Marius | Effets de la se ´cheresse sur l'e ´volution des mangroves du Se ´ne ´gal et de la Gambie[END_REF]Sadio 1986). The nature of the salt, the plant species, and even the individuality of the plant (e.g., structure and depth of the root system) determine the concentration of soil-salt levels at which a crop or plants will succumb (Photo.1). Consequently, there has been a progres-sive reduction of crop production regardless of whether it is for industrial or feeding purposes [START_REF] Diop | Les techniques de de ´salinisation des terres de la Compagnie Sucrie `re Se ´ne ´galaise: impact sur l'environnement[END_REF]. Therefore, improving and maintaining croplands to improve crop productivity are critical when techniques are adapted to restore lands affected by salinization processes [START_REF] Marius | Contribution a ` l'e ´tude des mangroves du Se ´ne ´gal et de Gambie. Ecologiepe ´dologie-ge ´ochimie. Mise en valeur et ame ´nagement (Re ´sume ´ de the `se)[END_REF]. Although a combination of efficient drainage and flushing of the soil by water is often used, the leaching of salts from the profile is rarely effective [START_REF] Boivin | Les sols argileux de la re ´gion de Nianga-Podor: re ´partition, caracte ´ristiques, aptitude et risques de de ´gradation sous irrigation[END_REF][START_REF] Boivin | Les sols de la moyenne valle ´e du fleuve Se ´ne ´gal: caracte ´ristiques et e ´volution sous irrigation[END_REF]. Because the reclamation, improvement, and management of salt affected soils necessitate complex and expensive technologies, all efforts must be taken for the efficient prevention of these harmful processes [START_REF] Lal | Soil management in the developing countries[END_REF]. Adequate soil and water conservation practices based on a comprehensive soil or land degradation assessment can become an early warning system that provides possibilities for efficient salinity (or alkalinity) control, the prevention of these environmental stresses, and their undesirable ecological, economic, and social consequences (Le [START_REF] Brusq | Evolution de la salinite ´ des sols et des eaux en relation avec la riziculture submerge ´e dans le delta du fleuve Se ´ne ´gal (campagne 1981-1982)[END_REF]. As adaptation practices refer to actual adjustments or changes in decision environments, which might ultimately enhance resilience or reduce vulnerability to observed or expected changes in climate, a changing climate will result in considerable changes in natural vegetation and in land use practices [START_REF] Adger | Assessment of adaptation practices, options, constraints and capacity[END_REF]). These changes in turn result in a feedback effect on the climate, which considerably affects the field water cycle and soil formation/degradation processes [START_REF] Lal | Soil processes and water quality[END_REF][START_REF] Lal | Soil degradation: I. Basic processes[END_REF][START_REF] Va ´rallyay | Potential impacts of climate change on agro-ecosystems[END_REF][START_REF] Harnos | Climate change: environment-risk-society[END_REF][START_REF] Va ´rallyay | Impact of climate change on soils in Hungary[END_REF]. The integral influence of climate, hydrology, and vegetation land use changes are reflected by the fiel water balance and soil moisture regimen [START_REF] Va ´rallyay | Fertilizer -liquid manure -drinking water relationships[END_REF][START_REF] Va ´rallyay | Climate change, soil salinity and alkalinity[END_REF][START_REF] Va ´rallyay | Symposium and 3rd training course 'Climate change: challenge for training of applied plant scientist. Impact of climate change on crop production[END_REF][START_REF] Farkas | A chernozem soil water regime response to predicted climate change scenarios[END_REF]. Climate change and its hydrological consequences may result in the significant modification of soil conditions (Va ´rallyay 2010). Changes in temperature, precipitation, natural vegetation, and land use practices will result in significant changes in crop production. In addition, climatic changes exacerbate the loss of woody lands in the arid and semi-arid regions [START_REF] Zeng | Atmospheric science. Drought in the Sahel[END_REF]. Soil salinity is also a serious problem in areas where the groundwater of high salt content is used for irrigation. The most serious salinity problems are faced by the irrigated arid and semi-arid regions of the world, and it is in these very dry regions that irrigation is essential to increase agricultural production to satisfy food requirements. The problem of soil degradation is a serious threat to the welfare of mankind. Although degradation of the land has always characterized man's systematic use of it, the process has accelerated in recent decades, precisely at a time when population growth and rising expectations have begun to demand enormous increases in food production. The issue is of overwhelming urgency. As the soil is subject to degradation, the cost of reclaiming it becomes higher, rising sharply until the threshold beyond which reclamation is no longer economically feasible is passed. Nearly 50 % of the irrigated land in the arid and semi-arid regions has some soil salinization problems. It is generally agreed that the future food needs of the increasing population will be met by directing the efforts of all concerned stakeholders (Le Gal 2001). D'aquino-Passouant (1995) studied the salinization process of the soils and noticed essentially two issues: the drainage of water and the rising of salt by capillarity. He revealed that the first process depended mostly on the nature and type of soil, as percolation of the vertisols and alfisols varied from 1 to 3 mm/day. Rising water by capillarity was due to the presence of shallow and high in salt groundwater that probably occurred during different periods of sea transgressions and regressions 2,000 years (before JC). This left a high quantity of soluble salt that remained at the fossil state in the soil of the delta, which explains the origin of the saline groundwater [START_REF] Ceuppens | Impact of non-drained irrigated rice cropping on soil salinization in the Senegal River Delta[END_REF]. Cropping systems are also responsible for rapid degradation of the soils of the delta as well as most of the abandoned private farms [START_REF] Breman | L'intensification agricole au Sahel[END_REF][START_REF] Dunia | Suivi de la salinisation dans les pe ´rime `tres irrigue ´s par te ´le ´de ´tection et syste `me d'information Ge ´ographique[END_REF]). [START_REF] Loyer | Degradation saline des sols induite par l'irrigation en domaine sahelien[END_REF] underlined that a converted system for irrigation without drainage and rice cultivation under permanent flooded conditions helps the desalinization process first for the soils and then for the groundwater. This induced and unexpected alkalinization of these soils due to a rise in the groundwater in which movements are not well controlled. This raises the issue of evacuating drainage water. [START_REF] Sadio | Pe ´doge `nese et potentialite ´s forestie `res des sols sulfate ´s acides sale ´s des tannes du Sine Saloum, Se ´ne ´gal[END_REF] described a similar process with the tanns of Sine Saloum, which are characterized by a remarkable heterogeneity due to their morphology and physical and chemical properties. Their characteristics are closely related to topography, type of material, and hydrology [START_REF] Sy | [END_REF]. Since 1971, their pedogenetic evolution has led to a severe drought all over the country, particularly in the fluviomarin fields [START_REF] Michel | Les bassins des fleuves Se ´ne ´gal et Gambie. Etude ge ´ographique[END_REF]. This has led to dramatic consequences for the social and ecological environment [START_REF]Enjeux de l'apre `s barrage valle ´e du Se ´ne[END_REF]. Moreover, insufficient rainfall induced by the aridness of the climate has triggered and amplified salt concentration and acidification of the soils all over the area. This salt concentration, which was spread out a little bit few decades ago, has quickly reached all soils, from the terrace up to the linked Photo. 2 a rice field under optimal management practices Colluviums. Data collected from the surveys conducted in several villages along the arm of the river showed that according to farmers, rice cultivation was the best management practice for restoring degraded salt affected lands because of its rooting system, its capacity to grow under flooded conditions, and its ability to be a salt tolerant crop (Photo.2). However, irrigation is often costly and technically complex, and it requires skilled management. Failure to apply efficient principles of water management may result in the wastage of water through seepage; over-watering and inadequate drainage create waterlogging and salinity problems that reduce the soil productivity, eventually leading to loss of cultivable land. The problems of salt-affected soils are old, but their magnitude and their intensity have been increasing fast due to large-scale efforts to irrigate additional areas in recent decades. The problems have been made worse by scaling up the irrigation systems without adequate provision for drainage, and they are being aggravated by poor water management practices and unsound reclamation procedures. The general characteristics and basic principles involved in the identification, reclamation, and management of salt-affected soils are the same throughout the world. In the delta of the Senegal River where the study took place, the salinity of the soils is a real issue with an anthropic origin. This issue touches the low and middle delta of the River Valley. The objective of this study was to (1) analyze the dynamics of salinization process as related to climate change and (2) compare different management practices to remediate saline soils for rice cropping.

Materials and Methods

The site was located in the upper limit of the delta of the Senegal River Valley. Over 900 ha of land with different soil orders identified as Inceptisol (18 %), Vertisol (36 %), Alfisol (14 %), and Aridisol (30 %) were severely affected by the salinization process (Soil Survey Staff 1992). To restore the degraded soils, different management practices were compared to determine the most efficient one that can be readily adopted by farmers. They consisted of (1) abandoning lands with no watering and no cropping (T1); (2) leaching followed by rice cropping (T2);

(3) leaching followed by continuous rice cropping (T3); (4) leaching followed by crop rotation with rice, sugarcane, and fallow (T4); and (5) leaching followed by continuous rice cropping over years (T5). The leaching process consisted of watering the plots with an amount of water that dissolved the accumulated salts in the soil to make them move downward to the root zone or shallowly wash the soil from the salt. The leaching process was the same as for all management practices except for the "abandoned plot." Plots receiving leaching treatments were arranged in bands of 0.7 ha separated by small flood banks. The dimensions of the bands differed from one plot to another. This way of turning plots into smaller bands ensured good coverage of the soil with a film of water and thus established the correct homogeneity of the plot. The watering by band consisted of having each band receive water up to a 40 cm height. In general, the frequency of watering was once every 25-30 days. After watering twice, the plot was dried out for 10 days before soil sampling at 0-20 and 20-30 cm for analysis in the laboratory. Once the soil sampling was done and the experimental design was set up, water was added to the plots. Each band was watered up to a 40 cm height. Watering was done while controlling the level of the water table with piezometers installed in the plots. This management could not proceed without knowing the physical and chemical characteristics of the soils. The idea was then to proceed with watering in a way that had one part of the salt percolate downward to be collected by the buried drains; the other part rose up by capillarity towards the shallow zone of the soil and was drained out towards the open drains. During leaching, water was sampled regularly from the plots, the buried drains, the open drains, and from irrigation. Changes in soil salinity concentration were measured by electrical conductivity (EC) at a 1:5 soil to water ratio. Likewise, soil acidity was measured by soil pH water at a 1:5 soil to water ratio.

Results and Discussion

In the delta of the Senegal River, salinization of the lands is one of the biggest constraints farmers are facing. This salinization process results in two phases that are characteristic of the evolution of the environment during the Nouakchotian transgression and the drought periods of the 1970-1980. During that time, marine transgression polluted the groundwater of the delta. Afterward, these waters became highly concentrated due to the fossilization of the salt. Indeed, the weakness of the flow strongly penalized traditional farming in the flooding zones as well as breeding in the falling areas. Moreover, such process favored the rising of the salty water in the bed of the river in more than hundred kilometers from the mouth of the river [START_REF] Guillaumie | La se ´cheresse au Sahel, un exemple de changement climatique[END_REF]. This rising action led to the construction of the Diama dam in 1986 to stop the saline intrusion and develop cropping systems under irrigation. However, if water softening is a major factor in the delta, the dam is causing the reverse process in zones in the drainage basins and in certain agricultural racks where irrigation practices have no drainage systems and are not well conducted. In the delta, itinerant agriculture reduces the potential for tenure, mainly in the Boudoum zone and Lake Guiers. From a social point of view, this work will cause collective awakening of the impact of these techniques on the environment and therefore on the socio-economic activities of the populations living around the lake. Through this awakening, they should reach self-sufficiency in rice and vegetable cropping in the Dieri upland zone. The physical factors influencing the salinization processes are rainfall, temperature, relative humidity, wind speed, solar radiation, total evaporation, and population. Therefore, a monitoring process was set up and run from 1997 to 2010. During the same period, mean annual temperatures were above 25 ! C. There were extreme temperatures from 26.2 ! C in 1999 to 29.1 ! C in 2007 (Fig. 3). An analysis of these data revealed that temperatures were higher than that usually recorded. Maximum temperatures were above 34 ! C, and the range was significant.

During that period, temperatures ranged from 14.5 to 17! C. The relative humidity was strongly influenced by the continental location and marine bangs (Ciss 2002). Analysis of the data over the last 12 years showed high Observations of all parameters seem to indicate that the period of sunshine determines evaporation. Indeed, the highest evaporation data were related to the maximum number of hours of sunshine, and this occurred during the years with high evaporation demand. This is the case for 2004 (Figs.5 and .6). With less rainfall and high temperatures over the period between 2005 and 2010, these 1972-1973 and 1984-1985. Thus, changes in the sedimentary regimen in the climatic context of the Sahelian zone raised fossil salts through thermocapillarity. Such salinity levels may be favored by the shallow depth of the groundwater, which is usually in contact with the plant roots due to inadequate irrigation and a maladapted drainage system. The amount of water added to better leach a saline rooting zone depends mainly on the initial level of salinity of the soil, the soil texture, and the leaching technique used. Nevertheless, findings from the work done by [START_REF] Charollais | Contribution a ` l'e ´tude du bilan hydrique de la canne a ` sucre dans les pe ´rime `tres irrigue ´s de la CSS, me ´moire de maitrise[END_REF] did not agree with previous diagnoses. They indicated a significant decrease in the (EC) at the surface and at depths for the cultivated soil, without significant changes in exchangeable Sodium percentage (ESP). This decrease in EC is illustrated by the changes in chlorides and sodium in solutions. In conjunction with this, the level of exchangeable sodium did not change significantly, but a significant decrease of the exchangeable potassium was noticed. This decrease in EC was also observed in the delta of Senegal. [START_REF] Wopereis | Soil fertility management in irrigated rice systems in the Sahel and Savanna regions and West Africa. Part 1: agronomic analysis[END_REF] confirmed that rice cultivation, whether it was drained or not, led to a desalination of the shallow layers. On irrigated rice, the presence of film water at the surface, with lower concentration, made a lateral evacuation of the salts possible along with the blocking the capillary increase in the saline in saline water and deep groundwater. In the fields alternating between cropping under dry conditions and rice cultivation, the crops grown for diversification, such as tomatoes or onions, were largely grown in irrigated fields for the villages or for private use, and most of these fields did not have a drainage system. The crops grown for diversification are mainly grown in the village's irrigated perimeters or in private perimeters, most of which do not have drainage systems. In spite of their filtering soils and unsuitability for rice cultivation-according to the extension agents and agents from development projects-farmers sometimes grow rice in these types of soils. This will allow for a leaching of the accumulated salts after few years of vegetable cropping. Before planting the crop, salt leaching was essentially done in the shallow water film and showed the effect of a flooding and evacuation cycle on changes in salinity of the surface horizon. Leaching by flooding showed that it was possible to obtain remarkable results. These results were encouraging and were obtained using a very rigorous soil and water management system for irrigation and drainage. By combining irrigation and cropping, convenient trends in the desalinization process were obtained. The set of management practices used produced different results with an overall decrease in salinity levels.

The abandoned plot treatment (Fig. 7) showed the highest salt content of 7.12 and 8.82 dS cm "1 at the soil surface (0-20 cm) and at the subsoil level (20-30 cm), respectively. Non-cultivated soils have a salinity level with chlorine and are totally saturated with sodium at deeper levels and are equally split into sodium, calcium, and magnesium at the surface. The first prospection [START_REF] Poitevin | Etude d'impact des techniques culturales sur les ame ´nagements hydro-agricoles dans la re ´gion de Podor (Se ´ne ´gal), me ´moire de quatrie `me anne ´e de l'ESAP, multigr. ORSTOM Dakar, p 53 et annexes Sadio S[END_REF] showed an increase in the electric conductivity (EC) in the irrigated fields in the studied zone. With the other management practices where irrigation, leaching, and cropping are combined, a gradual but significant decrease in soil salinity content was noticed (Fig. 7). At the shallow Fig. 7 Changes in electrical conductivity between management practices level (0-20 cm), the salinity level measured by EC 1/5 showed a decrease from 3.54 to 0.17 dS cm "1 in the four other management practices that went from leaching followed by rice cropping (T2) to rice cropping in the regular season (T5).

The mean concentration of the salinity in the plots under leaching with different croppings varied from 0.28 to 3.54.00 dS cm "1 for the 0-20 cm horizon (Fig. 7). For the 20-30 cm layer also, for all plots, a noticeable decrease in salt concentration was observed (3.24-0.17 dS cm "1 ) except for the abandoned plot (T1) where there was a saline rise through capillarity and an increase in salt concentration due to intense evaporation. At the shallow soil depth, this decrease was estimated to be 47.26 % between T1 and T2, 45.48 % between T2 and T3, 81.37 % between T3 and T4, and 6.67 % between T4 and T5. A comparison of these results with those from previous studies showed a noticeable decrease in the level of salinity. At a greater depth, the soil salinity decrease was estimated to be 63. 27 % between T1 and T2, 74.30 % between T2 and T3, 44.57 % between T3 and T4, and 84.78 % between T4 and T5. The study showed that it is possible to leach soluble salts initially present in saline profile up to 70 % by using an amount of water equivalent to 1/3 the thickness of the soil. This improves if the immersion continues with enough drainage water [START_REF] Hoffman | Guidelines for reclamation of salt-affected soils[END_REF]. Data have showed that with rice cultivation over years and a good drainage system, desalinization of the land can be achieved.

Leaching combined with different cropping systems, such as management practices to restore degraded soils affected by salinization processes, showed that from the soil surface to a shallow depth, soil acidity was close to neutral with a pH range of 6.75-7.27 (Fig. 8). At a greater depth (20-30 cm), data showed acidic soil. At any rate, soil pH at both depth levels presented optimal conditions for cropping after leaching processes were conducted.

Conclusion

Desalinization techniques used for 2 years showed convincing results not only for restoring degraded soils for better cropping but also for improving the socioeconomic environment through an increase of agricultural land tenure for food security. The positive impact of leaching saline soils was noticed in the gradual increase in arable land and thus potential for crop production and improvement in soil quality. Once recovered from salinization, these lands can represent the most suitable zones for cropping compared to the uplands. Globally, desalinization through rice cultivation presents an economic and ecological cost, which should be taken into account when it comes to restoring degraded salty soils for crop production.

Fig. 1

 1 Fig.1 Distribution of the affected zones by salinization processes (Source: National Institute of Pedology 2008)

Fig. 2

 2 Fig. 2 Changes in mean annual rainfall over time (Source: CSS Weather Station 2011)

Figure . 2

 2 shows that the geographical location of the delta in the Sahelian region has two seasons with contrasting rainfall patterns in the range of 200-400 mm. There is a short rainy season of 2-3 months and a dry season of 9-10 months.

Fig. 3 Fig. 4

 34 Fig. 3 Changes in annual mean temperature over time (Source: CSS Weather Station 2011)

Fig. 5 Fig. 6

 56 Fig. 5 Changes in total evaporation over time (Source: CSS Weather Station 2011)

Fig. 8

 8 Fig. 8 Changes in soil pH between management practices