Sylvain Deville

Antoni P Tomsia

Sylvain Meille

Complex Composites Built through Freezing

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crack propagation, enhancing the damage resistance of the materials, offering an interesting tradeoff of strength and toughness.

Freezing as a path to build complex composites has turned out to be a rich line of research and development. Understanding and controlling the freezing routes and associated phenomena has become a multidisciplinary endeavor. A step forward in the complexity was achieved with the use of anisotropic particles. The ice-induced segregation and alignment of platelets can yield dense, inorganic composites (nacre-like alumina) with a complex architecture and microstructure, replicating several of the morphological features of nacre. Now, a different class of complex composites is quickly arising: engineered living materials, developed in the soft matter and biology communities. The material-agnostic nature of the freezing routes, combined with the use of an aqueous system, the absence of organic solvents, and the low temperatures being used, are all strong assets for the development of such complex composites. More complex building blocks, such as cells or bacteria, can be frozen. Understanding the fundamental mechanisms controlling the interactions between the ice crystals and the objects, as well as the interactions between the soft objects themselves and ther fate, are essential in this context.

In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of nacre-like composites and understand the associated processes and properties. We describe the unique hierarchical and chemical structure of nacre, and the fabrication strategies for processing nacre-like composites. We also try to explain why natural materials work so well and see how we can implement these lessons in synthetic composites. Finally, we provide an outlook on the new trends and ongoing challenges in this field. We hope this Account will inspire future developments in the field of ice-templating and bioinspired materials.

■ INTRODUCTION

Nacre, the biocomposite covering the inside of many species of shellfish, is probably the flagship model of complex composites in materials science. Since scientists first realized how damageresistant nacre is, hundreds if not thousands of studies have addressed the structure/property relationship of nacre and synthesized damage-resistant materials inspired by the structure of nacre.

Nacre, like many natural composite materials, has toughness values that far exceed those of its constituents and their homogeneous mixtures, defeating the rule of mixtures. Nacre's microstructure evolved over 400 million years ago in the world's oceans, suggesting that an optimum was reached through evolution. Nacre is an assembly of aragonite (CaCO3) bricks separated by a protein layer (Figure 1). Nacre can be classified as a brick-and-mortar material. The bricks, made of an assembly of nanoparticles, are fairly complex, with variations of thickness and mineral bridges linking adjacent bricks. Each of nacre's microstructural features induces energy dissipation mechanisms when a crack propagates through the structure.

Inspired by this complex, hierarchical architecture, many processing routes have been proposed to replicate one or several of these structural features: top-down (gluing layers together), or bottom-up (layer-by-layer deposition). No single route can address all the requirements, and each one comes with limitations in terms of complexity, sample size, processing time, scalability, and possible materials combinations. In this context, we introduced freezing techniques as a path to build such complex composites in 2006. [START_REF] Deville | Freezing as a Path to Build Complex Composites[END_REF] The concept, initially demonstrated with ceramic/polymer or ceramic/metal composites, was later extended to ceramic/ceramic composites. [START_REF] Bouville | Tough and Stiff Bioinspired Ceramics from Brittle Constituents[END_REF] We now focus our research on the fundamental understanding of the physics at play during freezing, 3 laying the foundations for the processing of engineered living materials.

■ FREEZING AS A PATH TO BUILD COMPLEX COMPOSITES

When a suspension of colloidal particles is frozen, particles are rejected by the growing ice crystals and concentrate in the spacing in between. This phenomenon is well-known and documented in very different contexts, ranging from the freezing of soils in cold regions to the preparation techniques of soil samples in geophysics studies, or freezing-based treatments of sludges [START_REF] Deville | Freezing Colloids: Observations, Principles, Control, and Use: Applications in Materials Science[END_REF] . In materials science, freezing could be harnessed to prepare porous materials. After the ice is freezedried, the resulting scaffold is a porous body, with porosity templated by the ice (the approach that is now referred to as ice-templating 5 ).

In 2006, in a proof-of-concept study 1 , we took advantage of the ice-templating approach to build complex composites. We infiltrated ice-templated porous ceramics with a second phase (metal or polymer). This approach provided multi-layered, lamellar complex composites with a microstructure reminiscent of nacre (Figure 1). The resulting composites exhibited a marked crack deflection during crack propagation, enhancing the damage resistance (toughness) of the materials, offering an interesting trade-off of strength and toughness.

Freezing became a new approach for making brick-and-mortar materials, with several advantages. The first is the versatility of the approach: the ice-induced segregation is based on physical interactions and is thus in principle material-agnostic. The second main advantage is the number of levers available to control the morphology and the microstructure of the materials, inheriting features defined by ice physics and the interaction of particles with the ice. Many of these features (surface roughness, mineral bridges) are, to some extent, encountered in nacre, and provided hope that hierarchical synthetic materials could be prepared with this approach.

The technical requirements for the processing are low, and can easily be set up in a new lab. Unidirectional freezing can be easily performed with a source of cold, which can be achieved by a standard cooler, a source of liquid nitrogen, or a Peltier module. Additional control can be gained by controlling the freezing rates or gradients. [START_REF] Deville | Ice-Templating, Freeze-Casting: A Practical Guide to Get Started[END_REF] A standard freeze-dryer is then needed to sublimate the ice before further densification treatments.

The freezing technique filled a gap compared to the other processing routes for brick-andmortar materials. Because self-organization occurs in the bulk during unidirectional freezing, the processing time (typically minutes) is much shorter than the layer-by-layer deposition approaches, and cm-sized samples can be prepared easily. Finally, the typical layer thickness achieved with this approach (typically 5-100 µm) is neither in the nanoscopic range (nm), nor in the macroscopic range (100 µm-several mm), a thickness range critical to achieving an optimal damage resistance of the materials. This initial proof of concept opened a new line of research for the development of complex composites.

■ UNDERSTANDING AND CONTROLLING FREEZING

Control of the microstructure and process parameters

We initially devoted our research efforts to understanding the freezing process and particle redistribution, which ultimately control the microstructure and thus the properties of the materials.

Cracks, sometimes encountered in the final structure, affect mechanical performance. In situ investigations by X-ray radiography and computed tomography revealed that these cracks appear during freezing and are the result of particles' accumulation at the freezing front and instabilities of the front [START_REF] Deville | Metastable and Unstable Cellular Solidification of Colloidal Suspensions[END_REF][START_REF] Lasalle | Particle Redistribution and Structural Defect Development during Ice Templating[END_REF] , which can be controlled by adjusting process parameters such as the front velocity, the particle size, and the added polymer binders.

The control of ice growth to adjust the final microstructure was quickly demonstrated.

When water turns into ice, lamellar ice crystals with dendritic surfaces are formed (see Fig. 3c for example), because of the anisotropic growth rate of the ice crystals. A simple approach to modify the growth morphologies of the ice crystals was the addition of solutes (salt, sugar, glycerol) [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics through Additives and Templating[END_REF] .

Although the approach is versatile, it is very hard to use it predictably. A more consistent approach was introduced later, with a specific additive (zirconium acetate) that binds to the ice surface and thus controls its growth morphology in a controllable manner, independently of the nature of the particles used [START_REF] Deville | Ice Shaping Properties, Similar to that of Antifreeze Proteins, of a Zirconium Acetate Complex[END_REF] .

Freezing is unidirectional. If nucleation is not controlled, a spatially random, heterogeneous nucleation occurs, resulting in domains with locally aligned ice crystals, but no long-range order. If homogeneous anisotropic materials are sought, it is thus essential to control the short and long-range alignment of the ice crystals. We initially tested surface patterning, [START_REF] Munch | Architectural Control of Freeze-Cast Ceramics through Additives and Templating[END_REF] before developing a freezing under flow approach 2 . More robust reproducible alternatives have since been proposed and in particular, the so-called wedge technique, [START_REF] Pot | Versatile Wedge-Based System for the Construction of Unidirectional Collagen Scaffolds by Directional Freezing: Practical and Theoretical Considerations[END_REF] which has since been reused in several groups. It should also be noted that a fine control of the rheology of the suspensions is not critical. The typical solid loading appropriate for the freezing route are low (10-20 vol%), and thus the suspensions are almost always very fluid.

Improvement of mechanical properties

Building upon this initial proof of concept, in later studies we and other groups focused on exploring the concept and improving the microstructure and mechanical properties of such composites (Figure 2). Several compositions and approaches were explored to produce metal/polymer and metal/ceramic composites exhibiting the highest toughness to date, including for example alumina/PMMA and alumina/aluminum. At the same time, the control of several microstructural features and their impact on the properties were investigated, including the role of platelets surface roughness 14 and the addition of bridges. [START_REF] Grossman | Quantifying the Role of Mineral Bridges on the Fracture Resistance of Nacre-like Composites[END_REF] Model materials were developed to investigate each feature separately, providing guidelines for the design of complex composites. The mechanical properties were dramatically improved with a simple trick: pressing the porous body unidirectionally before infiltrating it with a second phase [START_REF] Munch | Bio-Inspired Hybrid Materials[END_REF] . This pressing step reduces the amount of porosity before infiltration, and thus shifts the ceramic/polymer ratio to higher ceramic content (60-80 vol%). Both the damage resistance of the composite and its strength were improved, with an R-curve toughness of to 30

MPa.m 1/2 and a yield strength up to 200 MPa. However, due to the difficulty of infiltrating liquid metals into small pores, this approach is limited to ceramic/polymer composites.

Exploration and control of the underlying physics

The sequence of solidification events translates directly into the microstructure and must be understood and controlled. Several studies, based on in situ observations techniques, investigated how the physics of freezing takes place in these particular systems (Figure 3). The initial events, nucleation and growth, lead to a steady-state growth of the ice crystals, and result, for example, in a gradient of the sample 17 , which can be either removed to maintain a constant open porosity or used, for example in membrane filtration applications. One of the keys to understanding the phenomena (crystal growth, particle redistribution) taking place during freezing is the ability to image them in situ, during solidification. This requires ideally 3D imaging at submicronic resolutions of ice crystals growing relatively fast (5-20 µm/s).

We initially used X-ray radiography [START_REF] Deville | In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particle Suspensions-Part I: Initial Instants[END_REF][START_REF] Deville | In Situ X-Ray Radiography and Tomography Observations of the Solidification of Aqueous Alumina Particles Suspensions. Part II: Steady State[END_REF] , followed by complete 3D imaging of the frozen sample.

Further developments provided in situ 3D imaging of the growing ice in a colloidal suspension [START_REF] Deville | Three-Dimensional in Situ Imaging of Ice Crystal Growth in a Colloidal Silica Suspension[END_REF] , although the spatial resolution was insufficient to image the colloidal particles. At this point, the interaction of the X-ray beam with the ice, very sensitive to small changes of temperature, also became an issue that prevented its further use up until this point in our research.

Physics provided the path to understanding the freezing of suspensions, and the fundamentals of particle interactions with the freezing front were investigated under carefully controlled conditions. We demonstrated, in particular, why a layer of concentrated particles accumulated at the tip of the ice crystals that appear in the steady-state of solidification [START_REF] Saint-Michel | Interaction of Multiple Particles with a Solidification Front: From Compacted Particle Layer to Particle Trapping[END_REF] , and the role of the boundary conditions (e.g., the mold and its characteristics) on the development of this layer [START_REF] Saint-Michel | Boundary-Induced Inhomogeneity of Particle Layers in the Solidification of Suspensions[END_REF] . This layer is one of the keys to understanding how microstructural defects develop in the final materials. More recently, phase field modeling has been used to predict the microstructural morphology maps [START_REF] Huang | Phase-Field Modeling of Microstructural Evolution by Freeze-Casting[END_REF] .

Limits of the initial approach

These initial developments faced several limits. First, the ceramic layers in these complex composites are polycrystalline. The strength of a polycrystalline brick is much lower than that of a single crystal. The second limit is the typical thickness of layers that can be achieved with this approach, slightly smaller than 10 µm. The brick thickness in natural nacre, considered an ideal target, is twenty times smaller. Bricks (or layers) too thick or too thin will only provide a limited crack deflection, although this depends on the composition and individual properties of the bricks and of the mortar [START_REF] Begley | Micromechanical Models to Guide the Development of Synthetic "brick and Mortar[END_REF] . Because nature had millions of years to optimize the thickness of platelets, there is a consensus that the nacre platelets' size is an optimum with respect to the constituent properties. Strangely, this has not been demonstrated (e.g by the fracture mechanics community).

Another issue is the (still) relatively high fraction of ductile phase (20-60 vol%, depending on the processing route), which limits the maximum strength of the composites. The best approaches rely on pressing the ice-templated structures. Reaching higher densities before the infiltration of a second phase requires a smooth wall's surface and long-range order of the lamellar walls (and thus crystals). Finally, the development of brick-and-mortar composites relies on an optimal adjustment of the interfacial strength between the bricks and the mortar. If the mortar strength exceeds the strength of the ceramic bricks, the bricks will fracture; this will eliminate the extrinsic toughening generated by crack bridging and brick pull-out, resulting in a catastrophic fracture. However, it remains extremely difficult to engineer precisely the interfacial strength and the mortar strength in these complex composites, the optimization of the properties still relies on trial and error approaches, as there are no simple guidelines to follow.

■ FROM ICE-INDUCED SEGREGATION TO SELF-ORGANIZATION AND SELF-ASSEMBLY

The growth of ice crystals and corresponding segregation of particles was initially performed with particles with uncontrolled and approximately isotropic morphologies. When such particles are segregated, concentrated, and packed between the crystals, no particular organization is observed.

Alignment of anisotropic particles

A step forward in the complexity was achieved with the use of anisotropic particles. They can be used to further control the microstructure of the materials. The platelets in nacre are typically 10-20 µm long (diameter) and 0.5 µm thick, although these values depends on the nacre type [START_REF] Loh | Nacre Toughening due to Cooperative Plastic Deformation of Stacks of Co-Oriented Aragonite Platelets[END_REF] . We tested two anisotropic systems in this size range: alumina and boron nitride. The platelets suspensions can be frozen under conditions where the platelets align with short-range order. The alignment also depends on the local spacing between the crystals [START_REF] Bouville | Self-Assembly of Faceted Particles Triggered by a Moving Ice Front[END_REF] , and either compact (with all particles aligned through the thickness) or sandwich structures (with dense outer layers and less dense cores) can be achieved. Discrete elements modeling was used to understand these behaviors and successfully guide the choice of the ideal processing conditions. Two types of porous materials were obtained with walls made of aligned platelets: alumina [START_REF] Bouville | Lightweight and Stiff Cellular Ceramic Structures by Ice Templating[END_REF] and boron nitride [START_REF] Bouville | Self-Assembly of Faceted Particles Triggered by a Moving Ice Front[END_REF] , with anisotropic thermal conductivity resulting from the texture of the aligned platelets. The ultimate objective was nevertheless the use of this phenomenon to align platelets and obtain dense, textured composites, discussed later in this paper.

This approach was extended afterward to other types of anisotropic objects and in particular fibers [START_REF] Ferraro | Light and Strong SiC Networks[END_REF] . Because of their aspect ratios, fibers percolate into a 3D structure at a low concentration.

Ice-templated porous materials made of fibers present an interesting combination of rigidity, high porosity, and low pressure drop, which could be useful in, for example, filtration applications.

Self-assembly

The alignment of platelets or fibers is not a self-assembly phenomenon. Proper ice-induced self-assembly was demonstrated later, by freezing block copolymers aqueous systems (Figure 4).

The segregation of monomers by the growing crystals increases their local concentration progressively, eventually triggering their self-organization into micelles, which themselves organize into crystalline phase upon further increase of the concentration [START_REF] Albouy | Freezing-Induced Self-Assembly of Amphiphilic Molecules[END_REF][START_REF] Dhainaut | Freezing-Induced Ordering of Block Copolymer Micelles[END_REF] . Although the process was not used to create complex composites as discussed in this paper, it nevertheless added selfassembly to the microstructure control toolbox. Harnessing such self-assembly could be used to further increase the complexity of the composites obtained by freezing, which is unexploited to date.

Concluding remarks on this section

All these results demonstrated the interest in and the versatility of freezing to obtain complex composites. From a physical point of view, the approach could be described as just another way to concentrate building blocks, but with more possible refinements (e.g. phase separation patterns). Another specificity of these approaches is the low temperature of the process (below 0°C). If there are no obvious specific advantages when working with ceramic particles, this temperature range could be a strong asset when engineering complex composites for biomedical applications, where drugs, cells, or other living elements are incorporated. These aspects are discussed in the final section of this paper.

■ NACRE-LIKE ALUMINA

The commercial availability of alumina platelets was a game-changer in the development of complex composites by freezing.

A new class of bioinspired brick-and-mortar materials

We took advantage of the ice-induced segregation and alignment of platelets to obtain dense, inorganic composites with a complex architecture and microstructure, replicating several of the morphological features of nacre 2 . These materials were thus naturally named nacre-like alumina. Using these alumina platelets permitted us to simultaneously solve three problems:

-Because each brick is made of a single crystal, the strength of the brick, and thus of the final composites (470 MPa), is much higher.

-The bricks' thickness (around half a micrometer) is an order of magnitude smaller than with the previous approaches. This lengthens the crack path upon crack propagation and is believed to further improve the damage resistance of the material.

-Because it is easier to pack platelets with a smooth surface, materials with a higher fraction of platelets are obtained after pressing and sintering. The final brick-and-mortar composites have a final phase repartition closer to that of nacre, with 95-98% ceramic platelets, and 2-5% of mortar (here a glass phase). This is another major source of strength increase in the final composite.

-The surface roughness of platelets and the presence of mineral bridges between adjacent platelets are essential toughening features of nacre. Small amount of nanoparticles can be added to the platelets suspension. The heteroaggregation of the platelets and nanoparticles can in principle provide both the desired surface roughness and the formation of bridges. The beneficial role of these nanoparticles was confirmed later in separate experimental [START_REF] Grossman | Quantifying the Role of Mineral Bridges on the Fracture Resistance of Nacre-like Composites[END_REF] or modeling studies [START_REF] Radi | Strength and Toughness Trade-off Optimization of Nacre-like Ceramic Composites[END_REF] .

However, this initial development came with limitations, including a complex processing route (freezing-under-flow, not easily transferable to another lab or scaled up) and small sample thickness (mm). This forced the development of alternative processing routes to achieve the same microstructure. If the ice crystals can induce a local alignment of the platelets, a long-range order is required to enhance the packing and densification of the microstructures. Several solutions (surface patterning, freezing under flow, wedge technique) have been proposed to control the ice nucleation and thus create long-range order. The wedge technique 11 seems to be the most appropriate for scale-up. It was also difficult to precisely control the homogeneity of the microstructure (unlike processes where layers are deposited one by one, for example), leading to spatially varying mechanical properties. The role of such heterogeneities may not be critical for all applications but needs to be carefully assessed.

Mechanical properties of nacre-like alumina

These composites exhibit interesting toughness values, better than that of alumina. The damage resistance of these composites can be defined by several parameters. The first (and easier to measure) is the fracture toughness (KIc), which is approximately 6 MPa.m ½ , and thus higher than that of a standard alumina (4 MPa.m ½ ). The main feature of interest, however, is their R-curve (increase of toughness) upon crack propagation. Nacre-like alumina reveals a constantly increasing R-curve, meaning that propagating a crack becomes more and more difficult. The measure is in principle valid up to a given crack length, and the maximum toughness measured at this point has been often used as a toughness measurement.

The trade-off between strength and toughness in nacre-like alumina fits a fairly unique spot in ceramic composites: because these complex composites are made of mineral only (typically 95% alumina platelets, with the remaining 5% alumina nanoparticles and a glass phase), the strength (470 MPa) is typical of a standard ceramic (but much higher than that of metal/ceramic or polymer/ceramic brick-and-mortar composites), but its toughness is much higher. Combined with their hardness, corrosion resistance, and preservation of properties up to several hundred degrees, these complex composites could thus make their way to structural applications in demanding environments.

These characteristics raise many questions, both fundamental and practical. Which toughening mechanisms contribute to the damage resistance observed? What is their relative contribution? How critical is it to achieve a homogeneous microstructure? Does the toughness of these composites depend on more than simple crack deflection at the brick/mortar interface?

We evaluated the properties of these composites at different length scales (Figure 5).

Macroscopic tests (single-edge notched beam and R-curve) on large (cm-sized) samples [START_REF] Saad | A Simple Approach to Bulk Bioinspired Tough Ceramics[END_REF] validated the properties at a larger scale while revealing the limitations of measurements performed previously on small (mm-sized) samples. Microscopic bending tests, performed on focussed ion beam-machined cantilevers, were used to evaluate the properties depending on the relative orientation of the cracks and the platelets [START_REF] Henry | Interface Failure in Nacre-like Alumina[END_REF] . Nanoindentation, performed at the surface of the samples [START_REF] Abando | Anisotropy Effect of Bioinspired Ceramic/ceramic Composites: Can the Platelet Orientation Enhance the Mechanical Properties at Micro-and Submicrometric Length Scale?[END_REF] , was used to evaluate the influence of the orientation of the platelets on the micromechanical properties.

Discrete elements modeling (DEM) was implemented to model crack propagation in these complex composites [START_REF] Radi | Elasticity and Fracture of Brick and Mortar Materials Using Discrete Element Simulations[END_REF] . Although many micro-mechanical models had been proposed previously to understand the properties of brick-and-mortar materials [START_REF] Bekah | The Micromechanics of Biological and Biomimetic Staggered Composites[END_REF] , DEM can naturally implement topological modifications such as crack propagation, crack branching, or multiple cracking, and is thus useful to understand the extrinsic toughening mechanisms acting during crack propagation.

DEM can guide the microstructural design such as the relative mechanical properties of the brick and the mortar, the role of adhesion between them, or the density of inorganic bridges between the platelets [START_REF] Radi | Strength and Toughness Trade-off Optimization of Nacre-like Ceramic Composites[END_REF] . These studies revealed that the mechanical properties of these materials were probably not at their optimum yet. The interface strength between the platelet and the mortar, for example, is currently too low [START_REF] Radi | Elasticity and Fracture of Brick and Mortar Materials Using Discrete Element Simulations[END_REF] . The models have since been made more realistic by generating real microstructures, using actual micrographs of nacre-like alumina [START_REF] Radi | Effect of Microstructure Heterogeneity on the Damage Resistance of Nacre-like Alumina: Insights from Image-Based Discrete Simulations[END_REF] . It should also be noted that this is a complex, multi-parameters optimization problem. Because several parameters and features can be adjusted, only local optimum can be achieved [START_REF] Radi | Strength and Toughness Trade-off Optimization of Nacre-like Ceramic Composites[END_REF] . The biggest challenge now is to implement such fine adjustments of the microstructure and composites properties in the actual processing routes of these composites.

Fracture mechanics have also been helpful to understand the mechanical properties of these composites and measure properties difficult to access otherwise. The combination of large-scale and small-scale experiments, for example, can provide a measure of the interface tensile and shear strength (between the bricks and the mortar), extremely difficult to access otherwise [START_REF] Doitrand | Determination of Interface Fracture Properties by Micro-and Macro-Scale Experiments in Nacre-like Alumina[END_REF] , as well as a more reliable fracture toughness measurement taking into account the anisotropy in the properties of these materials. This fracture mechanics analysis relies on the application of the so-called coupled criterion: both stress and energy conditions must be fulfilled for a crack to nucleate and propagate in the material.

Improving the microstructure and properties

Several strategies have been demonstrated to improve the microstructure and/or the mechanical properties, taking advantage of ceramic processing and control of the microstructure.

Zirconia, for example, can be added as secondary particles to increase both the strength and the toughness [START_REF] Henry | Nacre-like Alumina Composites Reinforced by Zirconia Particles[END_REF] . The improvements are due to both residual stresses (due to the mismatch of thermal expansion coefficients) and transformation toughening. A better control of the spatial repartition of the building blocks (alumina nanoparticles and platelets) can be achieved by heteroaggregation [START_REF] Muňoz | Nacre-like Alumina Composites Based on Heteroaggregation[END_REF] . Finally, a key feature of these composites is the interface between the platelets and the mortar. Adjusting its composition [START_REF] Pelissari | Nacre-like Ceramic Refractories for High Temperature Applications[END_REF] can bring noticeable improvements to the mechanical properties.

Nacre-like alumina without freezing

Alternatives to freezing have thus been reported, including pressing (platelets align under the uniaxial pressure [START_REF] Evers | Nacre-like Alumina with Unique High Strain Rate Capabilities[END_REF] ), sedimentation (platelets align at the top of the sediment [START_REF] Cerbelaud | Self-Organization of Large Alumina Platelets and Silica Nanoparticles by Heteroaggregation and Sedimentation: Toward an Alternative Shaping of Nacre-Like Ceramic Composites[END_REF] ), or magnetic field-assisted slip casting (platelets are functionalized with magnetic nanoparticles and the application of a magnetic field forces the alignment of the platelets [START_REF] Le Ferrand | Magnetically Assisted Slip Casting of Bioinspired Heterogeneous Composites[END_REF] ). These routes are much simpler and scalable and enable in particular the processing of samples of several centimeters in diameter and thickness. If freezing was initially used as a mean (align platelets locally) to an end (nacre-like microstructure), it is clear today that some of these alternative processing routes, and in particular uniaxial pressing, are easier to implement than freezing techniques at both lab and industrial scales. The quality of the texture (grain orientation, quantified by EBSD) is sufficiently close to yield similar mechanical properties [START_REF] Bouville | Strong and Tough Nacre-like Aluminas: Process-structure-performance Relationships and Position within the Nacre-Inspired Composite Landscape[END_REF] .

Concluding remarks on nacre-like alumina

The state of the art of these materials was recently reviewed by F. Bouville 47 . Among the many questions raised in this review and our paper, two are particularly intriguing and critical.

First: have we reached the optimum of these materials? The toughening mechanisms result from the interplay of several mechanisms and properties of the microstructure (adhesion strength between the platelets and the glassy phase, relative Young's modulus, dimensions of the glass phase, presence and characteristics of the mineral bridges, etc). Although some of these parameters have been explored separately, it is unlikely that we have reached the optimum yet. Since we try to optimize several mechanical properties at once, it is likely that only local optimums exist, with trade-offs to be made. This is in particular supported by discrete elements modelling [START_REF] Radi | Strength and Toughness Trade-off Optimization of Nacre-like Ceramic Composites[END_REF] . The second intriguing question is the optimal dimension of the platelets. All the developments of nacre-like alumina were enabled by the availability of alumina platelets of the same dimensions as that of the aragonite bricks in nacre. We can thus wonder whether this should be the optimal size. Micromechanical models [START_REF] Begley | Micromechanical Models to Guide the Development of Synthetic "brick and Mortar[END_REF] have shown that the optimal size and aspect ratio of platelets may depend not only of the nature of the materials used, but also on the mortar strength. Testing the validity of such models would require access to platelets of the same material but of different dimensions.

■ CURRENT TRENDS AND POSSIBLE FUTURES

Nacre-like alumina

Questions of interest can be classified into four categories:

-Morphology of the platelets: all experimental studies rely on the availability of platelets.

Although several dimensions and aspect ratios are available and have been tested, all platelets share a common characteristic: their thickness is constant. In nacre, the platelets exhibit a certain waviness, which enhances the mechanical properties when the structure is loaded [START_REF] Al-Maskari | Modeling of a Biological Material Nacre: Waviness Toughness Model[END_REF][START_REF] Wei | Optimal Length Scales Emerging from Shear Load Transfer in Natural Materials: Application to Carbon-Based Nanocomposite Design[END_REF] . Engineering such morphologies into monocrystalline platelets may nevertheless be extremely challenging.

-Composition: previous studies, in particular on brick-and-mortar materials, revealed the excellent properties of metal/ceramic composites. Nacre-like alumina with a metal interphase would certainly be interesting to test. We performed preliminary testing of nacre-like alumina with Ti interlayers [START_REF] Marcinkowska | Elaboration and Characterization of Mechanical Properties of Ceramic Composites with Controlled Architecture[END_REF] , although it turned out to be a poor choice and the mechanical properties were disappointing. Ni seems to be a better choice [START_REF] Garnier | Ni-Al2O3 Nacre-like Composites through Hot-Pressing of Freeze-Cast Foams[END_REF] . Although several types of platelets have been used

(alumina, BN, glass), most of the work has been done on alumina platelets, because of their appealing morphologies and interesting mechanical properties. Having also platelets stronger than alumina, such as silicon carbide or zirconia, would most likely improve the strength and toughness of these composites.

-Process: several processes have been reported to align the platelets. There is a strong interest in processing routes that would be able to control the local alignment of the platelets (critical for densification and local crack deflection) while also enabling other microstructural features (variations of long-range alignment, complex shapes of pieces). Additive manufacturing routes are of course of interest here, with shear-induced alignment of platelets upon deposition through a nozzle [START_REF] Barki | A. 3D Printed Boehmite Based Objects[END_REF] .

-Characterization: both the microstructure (defined at several levels) and the properties need to be thoroughly characterized. This characterization includes small-scale analysis [START_REF] Henry | Nacre-like Alumina Composites Reinforced by Zirconia Particles[END_REF] and fracture mechanics analysis adapted to materials showing extensive crack deviation. We also recently applied Raman imaging to image in situ the stress fields during crack propagation [START_REF] Saad | Microstructural and Mechanical Characterization of Nacre-like Alumina[END_REF] , which is helpful to understand the relative contributions of the various toughening mechanisms.

Beyond nacre-like materials: other freezing-based processes

Freezing is not used in the latest nacre-like alumina developments. The main steps of the freezing-based processing routes available are shown in Figure 6. Several freezing-based ideas nevertheless await further developments. Two main ideas could, in particular, be fruitful:

-Taking advantage of freezing-induced self-assembly [START_REF] Albouy | Freezing-Induced Self-Assembly of Amphiphilic Molecules[END_REF] to achieve more complex microstructures. Self-assembly has been limited so far to block copolymers systems (to template mesoporosity), but any type of self-assembly could in principle be triggered by freezing.

-One of the strengths of freezing is the possibility to combine it with other processes. A

few recent examples include additive manufacturing 51 , salting out [START_REF] Hua | Strong Tough Hydrogels via the Synergy of Freeze-Casting and Salting out[END_REF] , or biomineralization [START_REF] Mao | Synthetic Nacre by Predesigned Matrix-Directed Mineralization[END_REF] . Such combinations are useful to achieve complex compositions and microstructures.

Laying the foundations for soft matter complex composites

The developments discussed so far in this review were strongly anchored in the engineering materials communities, and in particular ceramic and metals processing. A different class of complex composites is quickly gaining interest: engineered living materials [START_REF] Sankaran | Engineered Living Biomaterials[END_REF] , developed in the soft matter and biology communities. The material-agnostic nature of the freezing routes, combined with the use of an aqueous system, the absence of solvents, and the low temperatures being used, are all strong assets for the development of such complex composites. More complex building blocks, such as cells or bacteria, can be frozen [START_REF] Christoph | Cellularized Cellular Solids via Freeze-Casting[END_REF] . Understanding the fundamental mechanisms controlling the interactions between the crystals and the objects, as well as the fate of soft objects is essential in this context. . We recently focused on two related aspects of these problems:

-Understanding the physics of ice interacting with soft objects and the various components added to the initial mix (solutes, surfactants, cryoprotectants, etc.). Cryoconfocal microscopy 3 is particularly appropriate for such studies, revealing self-assembled objects in a temperature gradient [START_REF] Dedovets | A Temperature-Controlled Stage for Laser Scanning Confocal Microscopy and Case Studies in Materials Science[END_REF] , the key role of solute [START_REF] Tyagi | Solute Effects on Dynamics and Deformation of Emulsion Droplets during Freezing[END_REF] , or the importance of the relative thermal conductivity of the particles and the melt [START_REF] Tyagi | Objects Interacting with Solidification Fronts: Thermal and Solute Effects[END_REF] . These studies contribute to a better understanding of the formation of the microstructure and the parameters that control it.

-The behavior of soft objects at the freezing front. Soft objects can deform (and heal or not), coalesce, grow, shrink, or lose their integrity (cells) upon interacting with the front. Their survival and functionality in the final complex composites are directly related to their physicochemical environment during freezing, as we recently reported with yeast cells 59 frozen in the presence of alginate. The list of questions that must be addressed expands rapidly.

Applications

Despite hundreds (or thousands) of papers and many claims of possible applications, no applications of these materials exist so far. Bringing a lab innovation to the market is a much more complex endeavor than most people realize. Reproducibility, for example, is often paramount and yet rarely reported (probably because it involves tedious work). Intellectual protection through patents is also one of the early keys. The number of patents related to freezing-based approaches for such materials has steadily increased over the years. To succeed, novel composites must present either an enormous gap in terms of properties (to justify their adoption), or be able to match the properties of existing solutions at a much lower cost. Either way, having scalable, cost-effective routes is often a key to success. Novel materials are generally more likely to succeed if they are obtained using already industrialized tools and processes.

After 20 years of research on the topic, we now have a much clearer view of what can be achieved with these approaches, in terms of materials, microstructure, properties, and processes.

Most likely, applications may appear first where an important value is added, or where the cost is less critical. A first application analysis was recently reported [START_REF] Chan | Assessment of Nacre-like Ceramics in Replacement to Ni Superalloys in Aircraft's Engines[END_REF] , evaluating the potential of nacrelike alumina as a replacement for Ni superalloys in aircraft's engine. The authors concluded that although similar properties, engine performance, and life cycle sustainability could be achieved, the reliability, shaping, repair, and governance of the production process remain limiting and require further progress and improvements. There are usually and unfortunately little incentives for academics to investigate such aspects. Alternatively, lessons can certainly be learned from a domain where freezing-based routes are routinely used at very large scales to process relatively cheap materials: food engineering and the frozen food industry.

■ CONCLUSIONS

Freezing as a path to build complex composites turned out to be a rich line of research and developments and is now a formidable toolbox with many advantages, both in terms of process (low temperature, relative material independence) and microstructures. As with any tool, future progress will likely emerge from the combination with other tools, such as additive manufacturing.

Understanding and controlling the freezing routes and associated phenomena is a multidisciplinary endeavor. Knowledge and techniques from materials science, physics, chemistry, physical chemistry, and biology are required, making it an appealing (albeit complex) topic for many groups worldwide, with both great science and potential applications in the future.

■ AUTHOR INFORMATION

Corresponding Author: Sylvain Deville, Université de Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, 69622 Villeurbanne, France Email: sylvain.deville@univ-lyon1.fr, orcid/org/0000-0002-3363-3184

Figure 1 .

 1 Figure 1. The complex structure of nacre in abalone shell and the first complex composites obtained by freezing. Only a part of the microstructural complexity was initially reproduced. The alumina composites shown here were obtained during the study reported in 1 . Reproduced from 10.6084/m9.figshare.18741092 © (2022) S. Deville, CC BY 4.0 license https://creativecommons.org/licenses/by/4.0/.

Figure 2 :

 2 Figure 2: Microstructures of different types of composites built through freezing. Alumina-epoxy

Figure 3 :

 3 Figure 3: Observation techniques and modeling approaches. The phenomena must be addressed at different scales to fully comprehend the underlying physics. Optical microscopy (reproduced with permission from 18 © 2017 American Chemical Society), X-ray radiography and tomography (reproduced with permission from 19 © 2020 Elsevier), 2D and 3D confocal microscopy 20 (reproduced from 10.6084/m9.figshare.5722675 © 2020 S Deville et al,), discrete elements modeling (reproduced with permission from 21 , © 2014 American Chemical Society), and phase field modeling (reproduced with permission from 22 © 2017 WILEY-VCH).

Figure 4 .

 4 Figure 4. From ice-induced segregation to ice-induced alignment and self-assembly. Objects such as ceramic particles or monomers are segregated and concentrated between the ice crystals (left). Anisotropic objects, such as fibers or platelets, can get aligned during ice growth (center). Monomers of block copolymers can be concentrated between the ice crystals, leading to their self-assembly into micelles and more complex architectures. (Reproduced from 10.6084/m9.figshare.18741092, © 2022 S. Deville ).

Figure 5 :

 5 Figure5: Understanding the mechanical properties of these complex composites. Complementary analysis at different scales is required, from macroscopic tests (SENB[START_REF] Saad | A Simple Approach to Bulk Bioinspired Tough Ceramics[END_REF] and R-curves measurements) to smaller scale (nanoindentation[START_REF] Abando | Anisotropy Effect of Bioinspired Ceramic/ceramic Composites: Can the Platelet Orientation Enhance the Mechanical Properties at Micro-and Submicrometric Length Scale?[END_REF] and microcantilever[START_REF] Henry | Interface Failure in Nacre-like Alumina[END_REF] ), modeling (discrete element modeling[START_REF] Radi | Effect of Microstructure Heterogeneity on the Damage Resistance of Nacre-like Alumina: Insights from Image-Based Discrete Simulations[END_REF] ) and Raman spectroscopy[START_REF] Saad | Microstructural and Mechanical Characterization of Nacre-like Alumina[END_REF] . SENB, microcantilever: Reproduced from 10.6084/m9.figshare.11907276, 10.6084/m9.figshare.11999985.v1 © (2020) S. Deville. Nanoindentation: Reproduced with permission from 35 © 2020 Elsevier. Discrete element modeling: Reproduced with permission from 38 © 2020 Acta Materialia Inc. Published by Elsevier Ltd. Raman spectroscopy: reproduced with permission from 40 © copyright The Authors.

Figure 6 :

 6 Figure 6: Schematic of the main steps of the freezing-based processing routes available. Reproduced from 10.6084/m9.figshare.18741092, © 2022 S. Deville.

Authors

■ REFERENCES


Sébastien Potel

Very low-grade metamorphism in the para-autochtonous sedimentary cover of the

Keywords: Illite crystallinity, very low-grade metamorphism, Western Alps, external crystalline massif

. Based on Ferreiro Mählmann (2001), this can supposed a long stady state heat flow corresponding to a long term metamorphic event like in the eastern Alps.

Introduction

In the western Alps, the illite "crystallinity" method has been used in order to differentiate the anchizone from diagenesis and epizone domains according to [START_REF] Kübler | Evaluation quantitative du métamorphisme par la cristallinité de l'illite. Etat des progrès réalisés ces dernières années[END_REF]. The general trend of the metamorphic grade, increasing from W to E, from diagenesis to epizone, is perturbed by tectonic duplexes and the influence of crystalline basement [START_REF] Desmons | Alpine metamorphism of the Western Alps: II. High-P/T and related pre-greenschist metamorphism[END_REF]).

In the northern part of the Pelvoux massif, [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] studied the low-grade metamorphism in the region of the Frontal Penninic units (FPU) of the Western Alps between the Arc and Isère Valleys (Fig. 1a-b). The results of the study allowed relating the metamorphism to the different stages of deformation.

Previous metamorphic studies described south of the Pelvoux massif a laumontite-prehnitepumpellyite subfacies (zeolite facies) in the "grès de Champsaur" formation, with rockforming laumontite (Aprahmian 1988) and pumpellyite-prehnite in veins [START_REF] Waibel | Evolution of volcaniclastic foreland turbidites in the external Western Alps[END_REF]). On the metamorphic map of the Alps (Frey et al. 1999) and the map of metamorphic structure of the Alps [START_REF] Oberhänsli | Metamorphic structure of the Alps (1:1'000'000[END_REF]) the sub-greenschist facies (epizone) is found in the tiny basement of Fournel and Dormillouse valleys (cf. Fig 1b).

The aim of this contribution is to present the results of a study on metamorphic grade in the southern footwall of the Penninic Basal Contact (PBC, [START_REF] Ceriani | Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs[END_REF], within the Dauphinois cover of the southeastern rim of the Pelvoux Massif, and to compare the results with those from [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF]. The metamorphic grade was investigated in the globigerina metamarls level and in the Mesozoic cover by mineral assemblages, simultaneous measurements of illite and chlorite "crystallinity" using the KI-and ÁI-methods, K-white mica b cell dimension and K-white mica polytypes content. Within the Rocher de l'Yret shear zone (RYSZ, Fig. 1), results are completed by a fluid inclusion study. Quartz precipates found along the strike slip fault planes dissecting the uppermost basement units from the RYSZ were analysed. All the methods were used for a better P-T estimate to get a higher precision on the metamorphic evolution in this region and to conclude better with some geodynamic implications. The results of this metamorphic study are compared to those from [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] north of the Pelvoux massif, to the FT data available in the region [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF][START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF] and previous studies describing mineral assemblage in the "grès de Champsaur" [START_REF] Aprahamian | Cartographie du métamorphisme faible à très faible dans les Alpes françaises externs par l'utilisation de la cristallinité de l'illite[END_REF][START_REF] Waibel | Evolution of volcaniclastic foreland turbidites in the external Western Alps[END_REF]).

Regional Geology

The Dauphinois domain is part of the European margin and composed of Variscan basement carrying a thick Permian to Tertiary sedimentary cover. During Mesozoic rifting, the eastern margin of the European plate was substantially thinned. Formation of major tilted crustal blocks allowed for the deposition of thick Mesozoic sedimentary series in asymmetric half grabens intensively described in literature [START_REF] Tricart | Tectonique synsédimentaire triasico-jurassique et rifting téthysien dans la nappe briançonnaise de Peyre-Haute (Alpes occidentales)[END_REF][START_REF] Davies | Interaction of thrusts and basement faults in the French external Alps[END_REF][START_REF] Gillcrist | Structural inversion and its controls; examples from the Alpine Foreland and the French Alps[END_REF][START_REF] Butler | The influence of pre-existing basin structure on thrust system evolution in Western Alps[END_REF][START_REF] Coward | Extensional structures and their tectonic inversion in the Western Alps[END_REF][START_REF] Huyghe | A comparison of inverted basins of the southern North Sea and inverted structures of the external Alps[END_REF][START_REF] Lazarre | Héritage téthysien et polyphasage alpin; réinterprétation tectonique du "synclinal" de l'aiguille de Morges (massif du Pelvoux, Alpes occidentales, France)[END_REF][START_REF] Sue | Raccourcissement polyphase dans le massif du Pelvoux (Alpes occidentales) : exemple du chevauchement de socle de Villard-Notre-Dame[END_REF].

Mesozoic strata are discordantly overlain by a conglomerate formation [START_REF] Gupta | Tectonic control on paleovalley incision at the distal margin of the early Tertiary Alpine foreland basin, southeastern France[END_REF], followed by the classical "Priabonian trilogy" [START_REF] Ravenne | Sédimentation et tectonique dans le bassin marin Eocene supérieur-Oligocene des Alpes du Sud[END_REF][START_REF] Apps | The structural setting and palaeogeographical evolution of the Gres d'Annot Basin. Deep-water sedimentation in the Alpine Basin of SE France; new perspectives on the Gres d'Annot and related systems[END_REF]), which comprises, from base to top: (1) shallow water nummulitic limestone of variable thickness (0 -50 m), (2) hemipelagic globigerina marls (0 -50 m), (3) the "grès de Champsaur" formation, a regular alternance of turbiditic sandstones and shales [START_REF] Perriaux | Quelques données sur la sédimentologie des grès du Champsaur (Hautes-Alpes)[END_REF][START_REF] Waibel | Evolution of volcaniclastic foreland turbidites in the external Western Alps[END_REF]), whose thickness varies from 700 to 1200 meters. In some areas immediately south of the Pelvoux Massif, the silici-clastic deposits contain up to 50% of volcanic detritus [START_REF] Debrand-Passard | Synthèse géologique du Sud-Est de la France[END_REF][START_REF] Bürgisser | Deformation in foreland basins of the Western Alps (Pelvoux massif, SE France); significance for the development of the Alpine arc[END_REF].

Locally, three Alpine deformation phases (D 1 , D 2 and D 3 ) can be recognized [START_REF] Trullenque | Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera Massifs[END_REF]. D 1 , first phase of deformation that affected the Dauphinois units corresponds to the third phase of deformation described by Ceriani et al. 2001, i.e Oligocene to lower Miocene out of sequence thrusting along the Roselend thrust (RT). It is the major deformation phase encountered in the investigated area since it is related to the thrusting along the RT [START_REF] Ceriani | A combined study of structure and metamorphism in the frontal Penninic units between the Arc and the Isère valleys (Western Alps): Implications for the geodynamics evolution of the Western Alps[END_REF]. D 1 deformation structures constantly show WNW-directed kinematic indicators measured directly along tectonic contacts, i.e. consistent with activity along the RT.

The second D 2 deformation phase recorded within the investigated area results in SWdirected movements that overprint the top-WNW D 1 deformation features and which are missing N of the Pelvoux massif. [START_REF] Trullenque | Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera Massifs[END_REF] proposed that the D 2 top-SW deformation phase is linked to the formation of the Apeninnes chain, as a consequence of the opening of the Thyrenian basin during Langhian times. D 3 structures, normal faults related to the Durance fault system (High Durance Faulted Zone, Tricart 2004) are the latest deformation features found in the investigated area. These normal faults are notably responsible for a strong geothermal gradient and seismicity recorded along the Durance valley. The timing of the onset of normal faulting, also recorded north of Pelvoux, remains a matter of debate in literature (see Fügenschuh et al. 1999;[START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF][START_REF] Tricart | La dénudation tectonique de la zone Ultradauphinoise et l'inversion du front Briançonnais au Sud Est du Pelvoux (Alpes Occidentales); une dynamique Miocène á actuelle[END_REF]. However, in this region, fission track data on zircon [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF]) reveal that maximum temperatures were well below greenschist facies conditions. [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF] interpreted the heating event as being due to the emplacement of internal thrust sheets, which thin out westwards.

Material and methods

In marine pelites and carbonates, no diagnostic minerals and mineral assemblages form at conditions of very low-grade field. In these rocks, the transitions from non-metamorphic to very low-grade and from very low-grade to low-grade metamorphic domains take place through the diagenetic zone, the anchizone and the epizone, each zone being characterized by specific values of the illite Kübler Index (Árkai et al. 2003). The illite "crystallinity" (IC or basal reflection in XRD patterns (Frey 1987;[START_REF] Guggenheim | Report of the association internationale pour l'étude des argiles (AIPEA) nomenclature committee for 2001: order, disorder and crystallinity in phyllosilicates and the use of the 'crystallinity index[END_REF]. [START_REF] Guggenheim | Report of the association internationale pour l'étude des argiles (AIPEA) nomenclature committee for 2001: order, disorder and crystallinity in phyllosilicates and the use of the 'crystallinity index[END_REF] recommended that the use of a "crystallinity index" should be avoided, although it may be placed within quotation marks when referring in a limited way to previously referenced work. They also recommended to refer to an index by relating it to the author describing the procedures necessary to define the value, regardless of what the index may actually be describing. Therefore, we will refer for K-white mica to the illite "crystallinity" for raw data and to Kübler index after calibration against Kübler's scale.

Illite "crystallinity" is considered to be a function of crystallite thickness, the number of lattice defects [START_REF] Merriman | A transmission electron microscope study of white mica crystallite size distribution in a mudstone to slate transitional sequence, North Wales[END_REF]. Temperature is thought to be the main factor controlling the illite "crystallinity", but other parameters like lithology, time, tectonic stress and fluid/rock ratio may probably have important effect (see Frey 1987). Árkai (1991) and Árkai et al. (1995) proposed a similar index the chlorite "crystallinity" (ChC) or Árkai index (ÁI) to monitor the reaction's progress.

Twenty-four metamarls samples were collected within the Priabonian and Mesozoic cover formations of the Dauphinois. Sampling was restricted to these formations in order to minimize the petrological effect on the IC. The outcrops density also limited the sampling area, around the Dormillouse and Fournel valleys outcrops are poor. Moreover, westward the Selle Fault cut out the Priabonian formation. The nature, age and tectonic setting of this fault are poorly constrained leading to uncertainties on its effects on the metamorphic pattern. Therefore, samples were taken in the Dormillouse, Fournel, Beassac and Chambran valleys and in the Yret zone (Figs. 1 and2).

In the different samples, the main schistosity is related to the D 1 phase [START_REF] Trullenque | Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera Massifs[END_REF], the D 2 phase only affects the area south of an E-W line of Beassac valley. The clay fraction analysed is generally related to D 1 .

Samples with detrital mica visible in hand specimen and/or weathered specimens were avoided as far as possible to eliminate detrital contamination. Mineral abbreviations used are from [START_REF] Kretz | Symbols for rock-forming minerals[END_REF].

X-ray diffraction

Clay mineral separation was conducted using techniques described by [START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF].

Carbonate removal was done using a 5% acetic acid (C 2 H 4 O 2 ) and washed after with deionised water. To minimize the effect of possible detrital clay minerals, we avoided long grinding processes (< 15 sec) and repeated the settling procedure for the ≤2µm fraction 5 times. Illite and chlorite crystallinity was measured at the University of Giessen on air-dried preparates, using a D501 Bruker-AXS (Siemens) diffractometer, CuKα radiation at 40 kV & 30 mA and divergence slits of 0.5° with a secondary graphite monochromator. Two slices of each sample were prepared and each measured two times as air-dried and one time glycolated.

The range of measurement, the time counting and the step size were as follow: for whole-rock paragenesis between 2 and 70 ∆°2Θ with 1 sec and 0.02° step, for air-dried preparate between 2 and 70 ∆°2Θ with 2 sec and 0.01° step.

IC was calculated using the software DIFFRACPlus (evaluation/release 2001 by ©Bruker AXS) and MacDiff 4.25 (written by R. Petschick, 17 May 2001). IC measured in Giessen (IC Giessen ) values were transformed into KI values using a correlation with the SW standards (CIS standards) of [START_REF] Warr | Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data[END_REF] (KI CIS = 1.2702 * IC Giessen -0.0314) (Table 1). The Kübler index was used to define the limits of anchizone, and the transition values were chosen as follows: KI = 0.25 ∆°2Θ for the epizone to high anchizone boundary, KI = 0.30 ∆°2Θ for the high to low anchizone boundary and KI = 0.42 ∆°2Θ for the low anchizone to diagenetic zone. However, the use of the CIS standards is not universally accepted as giving Kübler-equivalent zone limits. [START_REF] Kisch | On the calibration of illite Kübler index (illite 'crystallinity')[END_REF] show that the CIS standard values are much broader than those obtained by all other laboratories and that the high-and low-grade boundaries of the anchizone of the raw values of [START_REF] Warr | Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data[END_REF] are much broader than the Kübler-equivalent. This discrepancy is likely to reflect errors in the conversion of the IC values into Kübler equivalent [START_REF] Kisch | On the calibration of illite Kübler index (illite 'crystallinity')[END_REF]Ferreiro Mählmann and Frey this volume;Ferreiro Mählmann et al. this voluume). Therefore, following the recommendations done by [START_REF] Kisch | On the calibration of illite Kübler index (illite 'crystallinity')[END_REF], we published in the Table 1 the IC Giessen and ChC to allow comparison with other laboratories. The same experimental conditions were also used to determine chlorite "crystallinity" on the (002) peak of the second (7 Å) basal reflections of chlorite (Table 1). The ChC measurements were calibrated with those of [START_REF] Warr | Interlaboratory standardization and calibration of clay mineral crystallinity and crystallite size data[END_REF] and expressed as the Árkai index (ÁI) [START_REF] Guggenheim | Report of the association internationale pour l'étude des argiles (AIPEA) nomenclature committee for 2001: order, disorder and crystallinity in phyllosilicates and the use of the 'crystallinity index[END_REF]: ÁI = 0.8775 * ChC + 0.0239.

The anchizone boundaries for the Árkai index were defined by correlation with the Kübler index and are given as 0.24 ∆°2Θ for the epizone to anchizone boundary and 0.30 ∆°2Θ for the anchizone to diagenetic zone.

Randomly oriented samples for K-white mica b cell dimension and polytype determination of K-white mica were prepared using wood glue on quartz sample holder. The K-white mica b cell dimension is based on the d 060,331 spacing and on the increasing celadonite substitution that occurs with pressure increase in white mica [START_REF] Ernst | Significance of phengitic micas from low grade schists[END_REF][START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M 1 Muscovites[END_REF]). [START_REF] Guidotti | Compositional controls on the a and b cell dimensions of 2M 1 Muscovites[END_REF] presented linear regression equations that quantify the changes in the K-white mica b cell dimensions of muscovite 2M 1 that result from cation substitutions on the interlayer and octahedral sites. The K-white mica b cell dimension value was determined by measurement of the (060) peak of the potassic white mica, if present [START_REF] Sassi | The b 0 of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF], by using the cell-refinement program WIN-METRIC V.3.0.7 (©Bruker AXS).

Illite-muscovite polytype determination was done using a curve of 2M 1 /(2M 1 +1M) peaks ratio calibrated using different mixture of illite polytype following the technique described by Dalla [START_REF] Dalla Torre | Determination of white K-mica polytype ratios: comparaison of different XRD methods[END_REF]. [START_REF] Merriman | Very low-grade metapelites: mineralogy, microfabrics and measuring reaction progress[END_REF] recognized that the wealth of data on variations in white mica polytypism as a function of temperature is generally consistent with predictable trends. However, they recommended that polytype sequences should not be used other than as indicators of reaction progress.

Microthermometry

Quartz-calcite veins were sampled along the Rocher de l'Yret shear zone (RYSZ) (Figs. 1 & 2), in order to substantiate P-T estimation and to reconstruct the fluid evolution history during metamorphism [START_REF] Frey | Very low-grade metamorphism in external parts of the Central Alps: Illite crystallinity, coal rank and fluid inclusion data[END_REF]). Microthermometry investigations were done at the University of Giessen on double polished thin sections and performed using a Linkam THM 600/S/Geo heating-freezing stage coupled to a TMS 94 temperature controller with an error of ± 1°C. The stage is mounted on an Olympus microscope with a 100X Objective. The heating and cooling stage was calibrated using synthetic fluid inclusion calibration standard: CO 2 and H 2 O from ©Bubbles Incorporation. Two-phase (consisting of vapour and liquid at room temperature) fluid inclusions were identified. No dissolved volatile phase was observed, either by melting of CO 2 at or below its triple point of -56.6°C, nor by formation or dissolution of clathrate or liquid-vapor equilibrium of a volatile component such as higher hydrocarbons (HHC), CH 4 , CO 2 , N 2 or H 2 S. Thus, microthermometry was restricted to measuring the melting temperature of ice (Tm ice ) and the bulk homogenization temperature of the fluid inclusions (Th i ). As none of the investigated fluid inclusions contained any observable gas component, salinity was derived from the ice melting temperature in NaCl-equivalence after [START_REF] Hall | Freezing point depression of NaCl-KCl-H 2 O solutions[END_REF]. Density for fluid inclusions with homogenisation temperatures less than 200°C are from the equations of state given by [START_REF] Brown | P-V-T properties of fluids in the system H2O±CO2±NaCl: New graphical presentations and implications for fluid inclusion studies[END_REF] and for temperatures greater than 200°C are from [START_REF] Zhang | Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl-KCl-CaCl2-H2O using synthetic fluid inclusions[END_REF]. The isochores were calculated from the equation of state given by [START_REF] Zhang | Determination of the homogenization temperatures and densities of supercritical fluids in the system NaCl-KCl-CaCl2-H2O using synthetic fluid inclusions[END_REF].

Raman microspectrometry

Gas, liquid and solid phases were investigated at the University of Frankfurt am Main using a Raman microprobe Leica/Renishaw and the software Renishaw WiRE TM 2.0 and GRAMS. An Argon laser (green laser: λ = 514.5 nm -1 ) was used as excitation laser radiation.

The Raman microspectrometry was used to identify the presence of CH 4 , CO 2 , HHC (C 2 H 6 or C 3 H 8 ) and N 2 in fluid inclusions. The relevant peak is positioned at 2917cm -1 for CH 4 , 1285 and 1388 cm -1 for CO 2 , 2890 and 2954cm -1 for C 2 H 6 or C 3 H 8 respectively and 2331cm -1 for N 2 [START_REF] Burke | Raman microspectrometry of fluid inclusions[END_REF]. The small size of most fluid inclusions (< 10µm) and the large vertical dimension of the laser beam focus (4µm in diameter) cause the presence of Raman lines of the enclosing quartz (1160cm -1 ) in the spectra of fluids. This line does not interfere at all with the CH 4 , CO 2 and N 2 lines.

Results

Mineralogy

The mineralogy of the studied samples is given in Table 1.

In the Priabonian and Mesozoic meta-marls, the mineral assemblages consist of quartz + K-white micas + chlorite and calcite with minor amounts of albite. Gypsum is detected in some samples from the Fournel and Dormillouse valleys (Do2, Fo3a and Fo3c). In the fraction <2 µm, illite-muscovite predominates, chlorite and quartz are significant, and feldspar, if detected, is only present in small quantities.

Characteristics of the phyllosilicates

Figure 2 shows the distribution of the KI data, the values of which are listed in Table 1.

The values in the Dormillouse valley vary between uppermost diagenetic conditions (0.43∆°2Θ) in the south and low anchizonal values (0.36 ∆°2Θ) in the north. In the Fournel, Beassac and Chambran Valleys, the KI data indicate low anchizonal conditions (0.35 to 0.41 ∆°2Θ). In the Yret Zone, illite crystallinity values correspond to the diagenesis/anchizone boundary (0.37 to 0.43∆°2Θ) (Fig. 2). No trend can be observed between KI and elevation of the sample. However, a trend can be observed in each valley or zone, with an increase from south to north of KI values. This is similar to that Aprahamian (1988) described.

Figure 2 presents the distribution of ÁI values in the studied area. The pattern of very lowgrade metamorphism shown by AÍ agrees well with the pattern of the KI (Fig. 2). A positive linear correlation (R 2 = 0.64) is found between KI and ÁI values (Fig. 3a). This gives confidence on the fact that illite and chlorite "crystallinities" actually refer to the same P-T conditions and thus the same experienced T-t history.

The percentage of 2M 1 illite-muscovite polytype relative to the KI in the studied area shows a positive trend with increasing metamorphic grade (Fig. 3b). [START_REF] Sassi | The b 0 of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF] determined a semi quantitative relationship between the K-white mica b cell dimension and the metamorphic pressure gradient under greenschist facies condition. They plotted the K-white mica b-values as cumulative frequency curves in order to compare them with other metamorphic belts. [START_REF] Guidotti | Classification and correlation of Metamorphic Facies Series by Means of Muscovite b 0 data from Low-Grade Metapelites[END_REF] collated published Kwhite mica b cell dimension data for greenschist and blueschist facies rocks. They presented a qualitative plot of b cell dimension as a family of curves in P-T space. The values presented by [START_REF] Guidotti | Classification and correlation of Metamorphic Facies Series by Means of Muscovite b 0 data from Low-Grade Metapelites[END_REF] do not extend to the sub-greenschist facies. However, the considerable amount of K-white mica b cell dimension data from very low-grade metapelites accumulated in different metamorphic belt corroborate the groupings found by [START_REF] Guidotti | Classification and correlation of Metamorphic Facies Series by Means of Muscovite b 0 data from Low-Grade Metapelites[END_REF], as shown by [START_REF] Merriman | Very low-grade metapelites: mineralogy, microfabrics and measuring reaction progress[END_REF]. K-white mica b cell dimensions were calculated for 21 samples from the anchizone (three samples were not integrated due to interferences and poor intensity signal), all fall in the same range with a minimum 8.998 and a maximum 9.028 Å values (Table 1). Specimens from the Mesozoic cover have lower values (at around 9.000 Å), while Priabonian are centred around 9.020 Å. However, both value groups are in the intermediate pressure facies (between 9.000 and 9.040 Å) as shown in Figure 4 (the curve obtained is similar to the reference curve from Ryoke), suggesting an inferred geothermal gradient of 25-35°C/km [START_REF] Guidotti | Classification and correlation of Metamorphic Facies Series by Means of Muscovite b 0 data from Low-Grade Metapelites[END_REF].

Vein mineralogy, textural relationships and microthermometry

Samples were collected within the RYSZ, previously decribed by [START_REF] Butler | Thrust zone kinematics in a basement-cover imbricate stack; eastern Pelvoux Massif, French Alps[END_REF], and consisting in an imbricate of basement slices and para-autochtonous sedimentary cover. This shear zone, lying directly below the frontal penninic nappe stack, has been considered as the map trace of the RT in this area [START_REF] Trullenque | Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera Massifs[END_REF]. All kinematic indicators measured in the field or deduced from microfabric analysis of calcite ultramylonites [START_REF] Trullenque | Microfabrics of calcite ultramylonites as records of coaxial and non-coaxial deformation kinematics: Examples from the Rocher de l'Yret shear zone (Western Alps)[END_REF], consistently show a WNW directed sense of transport, consistent with activity along the RT.

No evidence of D 2 structures are found in this area.

Samples have been collected along strike slip fault planes dissecting basement slices and indicating a clear WNW directed sense of transport. These faults are clearly related to the first deformation phase D 1 . The chronology of the different fluid inclusion populations (Table 2) is a relative one with respect to their host mineral and their overgrowth [START_REF] Mullis | Das Wachstumsmilieu der Quarzkristalle im Val d´Illiez (Wallis, Schweiz)[END_REF].

Veins are mainly composed of small quartz crystals associated sometimes with calcite crystals. Only two-phase fluid inclusions (consisting of vapour and liquid at room temperature) were identified with the microscope. Fibre quartz is observed in the sample F72.

In F92, two fluid inclusion assemblages are identified. The first one is a two-phase type in the host quartz and probably of primary type, i.e. trapped during mineral growth. The second one is overprinting the first assemblage and probably of pseudo-secondary type. The true temperature of entrapment of the fluid inclusions, i.e. mineral growth, was constrained by intersecting the isochores of primary two-type phase fluid inclusions with the inferred geothermal gradient deduced from the K-white b cell dimension (Fig. 5). Isochores were calculated for the lower and upper homogenization temperature (Th i ) with an average salinity of 5.6 and 13.8wt.% NaCl equivalent for samples F72 and F92, respectively.

Discussion

The structural metamorphic map of the Alps [START_REF] Oberhänsli | Metamorphic structure of the Alps (1:1'000'000[END_REF] indicates lower subgreenschist facies E of the RT and N of the Pelvoux massif. North of the Pelvoux massif, [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] described an E-W metamorphic gradient on the E side of the RT. In the Briançonnais, they reported an increase from anchizonal to epizonal conditions from E to W.

In the same area, the Dauphinois domain (in the footwall of the RT) shows epizonal metamorphic conditions. The Dauphinois domain in this area is only affected by one deformation phase, corresponding to the third deformation phase in the FPU [START_REF] Ceriani | A combined study of structure and metamorphism in the frontal Penninic units between the Arc and the Isère valleys (Western Alps): Implications for the geodynamics evolution of the Western Alps[END_REF][START_REF] Ceriani | Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs[END_REF]) and equivalent to D 1 in our studied area [START_REF] Trullenque | Tectonic and microfabric studies along the Penninic Front between Pelvoux and Argentera Massifs[END_REF]. The K-white b cell dimensions obtained by [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] in the Dauphinois Domain are ranging between 9.000 and 9.030Å similar to those obtained in our study, indicating similar geothermal metamorphic gradient between the two areas. Frey (1987) and [START_REF] Essene | Clay mineral thermometry -a critical perspective[END_REF] showed that KI cannot be used as a precise geothermometer and only provide an approximate temperature range. In spite of this, attempts have been made to relate KI values to absolute temperatures. This was done in different lowgrade metamorphic units by comparing the KI data with other geothermometers [START_REF] Ferreiro Mählmann | Zur Bestimmung von Diagenesehöhe und beginnender Metamorphose-Temperaturgeschichte und Tektogenese des Austroalpins und Süpenninikums in Vorarlberg und Mittelbünden[END_REF][START_REF] Ferreiro Mählmann | The pattern of diagenesis and metamorphism by vitrinite reflectance and illite-"crystallinity" in Mittelbünden and in the Oberhalbstein. Part2: Correlation of coal petrographical and of mineralogical parameters[END_REF]Merriman and Frey 1999;[START_REF] Mullis | Correlation of fluid inclusion temperatures with illite "crystallinity" data and clay mineral chemistry in sedimentary rocks from the external part of the Central Alps[END_REF][START_REF] Potel | Very low-grade metamorphic evolution of pelitic rocks under high-pressure/low-temperature conditions, NW New Caledonia (SW Pacific)[END_REF]. [START_REF] Mullis | Correlation of fluid inclusion temperatures with illite "crystallinity" data and clay mineral chemistry in sedimentary rocks from the external part of the Central Alps[END_REF] have suggested a diagenesis -anchizone boundary in the Alps on the basis of a comparison of IC versus FI data. This allows to better define the range of temperature that the SE of the Pelvoux massif was subjected during tectonic evolution.

The KI values indicate that the Dauphinois domain in the SE of the Pelvoux massif underwent up to low anchizonal metamorphic conditions (Table 1; Fig. 2). According to [START_REF] Mullis | Correlation of fluid inclusion temperatures with illite "crystallinity" data and clay mineral chemistry in sedimentary rocks from the external part of the Central Alps[END_REF], the boundary is linked to a temperature of around 230-240°C (similar to the compilation results from Ferreiro Mählmann ( 1994)). This can be compared to the temperature estimates obtained west of our studied area in the "grès de Champsaur" (W of Prapic). [START_REF] Waibel | Evolution of volcaniclastic foreland turbidites in the external Western Alps[END_REF] described quartz-prehnite±calcite veins in the "grès de Champsaur".

The country rock is dark green in colour, enriched in pumpellyite of variable composition, and to a lesser extent prehnite, at the expense of laumontite. [START_REF] Aprahamian | Cartographie du métamorphisme faible à très faible dans les Alpes françaises externs par l'utilisation de la cristallinité de l'illite[END_REF] described also mottled sandstone (faciès moucheté), the pale-coloured mottles being enriched in laumontite as cement and as partial replacement product of albitized clastic plagioclase and the darker areas being relatively enriched in chlorite cement. According to [START_REF] Frey | A new petrogenetic grid for low-grade metabasites[END_REF] and [START_REF] Potel | Composition of pumpellyite, epidote and chlorite from New Caledonia -How important are metamorphic grade and whole-rock composition?[END_REF], the coexistence of zeolites (laumontite in this case) and prehnitepumpellyite is restricted to a rather small P-T area below 240°C and therefore in the range obtained by using the correlation from [START_REF] Mullis | Correlation of fluid inclusion temperatures with illite "crystallinity" data and clay mineral chemistry in sedimentary rocks from the external part of the Central Alps[END_REF].

Zircon FT data were gained from the turbiditic "grès de Champsaur" and the underlying Pelvoux crystalline basement [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF]. These data provide important information on the peak temperatures reached during Alpine metamorphism. For the zircon FT system, several estimates on the temperature range of the partial annealing zone exist (e.g. [START_REF] Yamada | Annealing kinetics of fission tracks in zircon: an experimental study[END_REF][START_REF] Tagami | Revised annealing kinetics of fission tracks in zircon and geological implications[END_REF][START_REF] Yamada | Statistical modelling of annealing kinetics of fission-tracks in zircon; reassessment of laboratory experiments[END_REF]). Tagami et al. (1996) suggested a temperature range between 200 and 320°C to be appropriate, which covers the temperature range relevant to the metamorphic zonation based on illite "crystallinity" (e.g. [START_REF] Mullis | Correlation of fluid inclusion temperatures with illite "crystallinity" data and clay mineral chemistry in sedimentary rocks from the external part of the Central Alps[END_REF]. The combination of both methods therefore provides additional constraints on the maximum temperature reached to the SE of the Pelvoux massif. In the Glarus Alps, the comparison between fluid inclusion data and zircon FT data suggests that zircon FT annealing in the Taveyanne sandstone (a lateral equivalent of the "grès de Champsaur") only become detectable above 250 °C [START_REF] Rahn | The metamorphic and exhumation history of the Helvetic Alps, Switzerland, as revealed by apatite and zircon fission tracks[END_REF]). The measured K-white mica b cell dimension values inferred intermediate geothermal gradient with 25-35°C/km [START_REF] Merriman | Very low-grade metapelites: mineralogy, microfabrics and measuring reaction progress[END_REF].

This gradient seems to be a very conservative approach, however this result is similar to that we can found in the Dauphinois domain north of the Pelvoux Massif [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF].

Combining this information with the fluid inclusion data obtained in the Yret zone, a trapping temperature for the fluid inclusions of 220 ± 10°C can be obtained (Figs. 5a & 5b).

Consequently, the comparison of KI, FT and fluid inclusion leads to the conclusion that the SE of the Pelvoux massif was subjected to temperatures higher than 200°C, but lower than 250°C. This is higher than the estimation of temperature by [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF] who estimated that the burial temperature of the "grès de Champsaur" was not higher than 200°C. This implies metamorphic burial between 7 and 9 km. Neither our KI nor the FT data [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF] reveal the presence of a metamorphic gradient from E to W in the SE of the Pelvoux massif as described by [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux[END_REF]. The absence of epizonal KI values in the Vallouise valley [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux[END_REF][START_REF] Aprahamian | Cartographie du métamorphisme faible à très faible dans les Alpes françaises externs par l'utilisation de la cristallinité de l'illite[END_REF], moves the anchizone-epizone boundary to the N of the Pelvoux massif, where epizonal metamorphic conditions are observed [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF]. As mentioned by Ferreiro Mälhmann and [START_REF] Ferreiro Mählmann | Standardisation, calibration and correlation of the Kübler-Index and the vitrinite/bituminite reflectance measure: a inter-laboratory and field related study[END_REF], one of the reasons of the lack of epizonal values, is that the KI values obtained by CIS calibration led to a broadening of the anchizone area.

The available metamorphic data from the south of the Pelvoux massif combined with the available deformation data allow us to deduce some conclusions concerning the tectonometamorphic evolution in this area. A first important result provided by the KI, fluid inclusion and FT data is the fact that the Dauphinois domain exhibits lower metamorphic conditions compared to the Penninic domain. According to the metamorphic map of the Alps, the Penninic domain west of the RT is metamorphosed under lower greenschist facies [START_REF] Oberhänsli | Metamorphic structure of the Alps (1:1'000'000[END_REF]. Field evidences SE of the Pelvoux massif (this paper, Trullenque 2005), indicate that D 1 corresponds to the first penetrative schistosity, marked by very finegrained K-white mica, chlorite, quartz and albite. The metamorphic gradient observed and the formation of the fluid inclusions studied are related to this deformation phase and the burial heating (with a maximum of 9 km burial) was generated by the overthrusted Penninic domain along the RT onto the Dauphinois domain. As mentioned above, the K-white mica b cell dimension values in our studied area SE of the Pelvoux massif, are similar to those obtained in the north by [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF]. Therefore, the metamorphic gradient observed in the Penninic domain must be established before D 1 , as observed by [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] further to the N (the D 1 observed SE of the Pelvoux massif correspond to the D 3 , see above). So, we have no trace of an older metamorphic event.

North of the Pelvoux massif, the metamorphic grade is higher with epizonal conditions and may be explained by thicker Penninic units on top of the Dauphinois domain leading to higher degree of burial and therefore show a regional metamorphic gradient increasing from S to N. This is corroborated by the same pressure gradient observed north and south of the Pelvoux massif in the Dauphinois domain (same range of K-white mica b cell dimension).

Ferreiro [START_REF] Ferreiro Mählmann | Correlation of very low-grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps[END_REF] in the eastern Alps showed that in rocks submitted to a long metamorphic event (20 Ma) and a stady state heat flow, the smectite content in the anchizone is between 0 and 5% (90% of the samples without smectite), and that under stabile thermal conditions (equilibrium), KI can be used as a thermometer. In this region, [START_REF] Henrichs | Sedimentpetrographische Untersuchungen zur Hochdiagenese in der Kössen-Formation (Ober Trias) der westlichen Ostalpen und angrenzenden Südalpengebiete[END_REF] reported K-white mica b cell dimension values between 9.000 and 9.022 Å and the compilation of the Ferreiro Mählmann et al. (2012) shows a normal to low geothermal gradient (30°C/km). Comparison of our results with KI and K-white mica b cell dimension values will suggest that the Dauphinois Domain has been submitted to a relative long heating time like in the eastern Alps.

Conclusion

The tectono-metamorphic history of the Dauphinois domain in the SE of the Pelvoux is similar that described by [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] in the north. Peak metamorphic conditions in the Dauphinois domain are reached during the overthrusting of the Penninic units during D 1 top-WNW directed thrusting along the RT. The metamorphism is characterized by temperatures below 250°C and higher than 200°C and a metamorphic gradient between 25 and 35°C, and a long time event. This range of temperature is compatible with the FT data published by [START_REF] Seward | Preliminary report on fission tracks studies in the Pelvoux area, SE France[END_REF] indicating that the burial temperature in the region was lower than 250°C. Based on our KI values and the mineral paragenese observed in the W of the "grès de Champsaur", no E-W metamorphic zonation as described by [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux[END_REF][START_REF] Aprahamian | Cartographie du métamorphisme faible à très faible dans les Alpes françaises externs par l'utilisation de la cristallinité de l'illite[END_REF]) is observed. But, in the studied area, the grade of metamorphism in the Dauphinois Domain is lower than in the northern of the Pelvoux massif [START_REF] Ceriani | The tectono-metamorphic evolution of the Frontal Penninic units of the Western Alps: correlation between lowgrade metamorphism and tectonic phases[END_REF] confirming the N-S metamorphic zonation described by [START_REF] Aprahamian | La cristallinité de l'illite et les minéraux argileux en bordure des massifs cristallins externes de Belledonne et du Pelvoux[END_REF][START_REF] Aprahamian | Cartographie du métamorphisme faible à très faible dans les Alpes françaises externs par l'utilisation de la cristallinité de l'illite[END_REF]. This is also outlined by the zircon FT ages indicating an increase in age primarily from the north to the south [START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF] and supports their hypothesis that the more southerly located areas were lees deeply buried and hence less exhumed. Like proposed by [START_REF] Ceriani | Multi-stage thrusting at the "Penninic Front" in the Western Alps between Mont Blanc and Pelvoux massifs[END_REF] and [START_REF] Fügenschuh | Late stages of deformation and exhumation of an orogen constrained by fission-track data: A case study in the Western Alps[END_REF], the overthrust along the RT of the Penninic units may caused burial and metamorphism in the external Dauphinois units. [START_REF] Sassi | The b 0 of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists[END_REF] and [START_REF] Guidotti | Classification and correlation of Metamorphic Facies Series by Means of Muscovite b 0 data from Low-Grade Metapelites[END_REF]. The thick curve represents the liquid-gas reaction curve of water. CP: critical point of water. "crystallinity" values of the <2μm grain-size fraction samples (data in Δ°2θ), percent of 2M 1 polytypes and K-white mica b cell dimension data.

Dauphinois domain

Nummulitic

Fig. 1 :

 1 Fig. 1: (a) Simplified geological overview of the arc of the western Alps (A: Argentera

Fig. 2 :

 2 Fig. 2: Distribution and values of Kübler index (KI) and Árkai index (ÁI) in the studied area.

Fig. 3 :

 3 Fig. 3: Correlation between Kübler index (KI) and Árkai index (AÍ) (a) and the 2M 1 /1M d

Fig. 4 :

 4 Fig. 4: Cumulative curves of K-white mica b cell dimensions for 21 anchizonal samples from

Fig. 5 :

 5 Fig. 5: Depth-Temperature diagrams with isochores representing the higher and the lower

  Figure1

  Figure 2

Table 1 :

 1 Samples identification, stratigraphic age, Illite Kübler index and chlorite

Table 2 :

 2 Fluid inclusion data of two quartz samples from the Yret Zone, Western French Alps.

Acknowledgements

This manuscript benefited greatly from constructive and helpful reviews and comments from M. Rahn and R. Ferreiro Mählmann.