Jean-Louis L Vigneresse
email: jean-louis.vigneresse@univ-lorraine.fr

Bénédicte Cenki
email: benedicte.cenki@umontpellier.fr

Géoressources

Multiphase rheology as a cause for stick-slip like melt extraction

Keywords: stick-slip, melt extraction, friction, hysteresis, pinning/depinning Running title Melt extraction: stick-slip motion

o Numerical model of melt extraction o Sporadic motion of melt and an analogue with dry friction (stick-slip) o Hysteresis and memory effects during changes in input parameters o Global expression of the free energy variation, with a term akin to velocity and a second term to acceleration o Interactions between strain, chemistry and entropy (order)

ABSTRACT

When partial melting is parametrized in numerical thermo-mechanical models in a straightforward manner, it directly relates to variations in temperature. However, melt extraction is, to our knowledge rarely taken into account in these models, even if melt extraction and escape have a key influence on the mechanical behavior of the entire system.

1.

Introduction

Temperature rules melt generation in the crust, most often through heat diffusion, or advection, which are both continuous processes. The extraction of melt out of its matrix is by contrast discontinuous, initiating from melt pockets that aggregate to form veins to escaping and feeding isolated plutons [START_REF] Kisters | Melt segregation and far-field melt transfer in the mid-crust[END_REF]. According to such field observations, the paradigm of melt extraction and transport turned from diapirism or dyking to discrete solitary waves [START_REF] Connolly | Temperature-dependent viscoelastic compaction and compartmentalization in sedimentary basins[END_REF][START_REF] Rabinowicz | Melt segregation under compaction and shear channeling: Application to granitic magma segregation in a continental crust[END_REF]) that propagate from themselves, driven by alternate changes in permeability. Such models of porous flow also apply to propagating fluids, melt or water within a stressed crust, with applications to magmatism, geothermic and water circulation into the basement. They form the basis of poroelasticity [START_REF] Cheng | Poroelasticity[END_REF]. The paradigm switch from a viscous material more mobile into a viscous matrix to a problem of permeability suggests that hydraulic parameters should rule crustal fluid transport.

Modelling melt extraction thus points to the rheology of multi-phase material, i.e. the differential response of a mobile fluid circulating into a deforming matrix. The viscosity contrast between the melt and its matrix i.e. a weak and a strong phase, may be apprehended from several points of view including estimates of an effective viscosity, intermediate between the two end-members [START_REF] Pistone | Rheological flow laws for multiphase magmas: An empirical approach[END_REF][START_REF] Agard | The subduction plate interface: rock record and mechanical coupling (from long to short timescales)[END_REF]. Three factors then add complexity to such a direct approach. The first is the dependence of the viscosity of the strong phase on the applied strain rate whereas the melt is simply Arrhenian. This makes the experimental studies hard to be transposed to natural conditions [START_REF] Pistone | Rheological flow laws for multiphase magmas: An empirical approach[END_REF] because the fast strain rate imposed by the duration of experiments. The second factor is strain partitioning [START_REF] Vigneresse | Strain partitioning during partial melting and crystallizing felsic magmas[END_REF] between the weak and the strong phase [START_REF] Goodwin | Competency contrast, kinematics, and the development of foliations and lineations in the crust[END_REF]. Their respective abundance determines which phase globally controls the effective rheology. When the weak phase is abundant, the rheology is governed by the distribution and geometry of rigid blocks [START_REF] Agard | The subduction plate interface: rock record and mechanical coupling (from long to short timescales)[END_REF][START_REF] Ioannidi | Effective rheology of a two-phase subduction shear zone: Insights from numerical simple shear experiments and implications for subduction zone interfaces[END_REF] as observed at the interface of subduction zones. In contrast, when the weak phase is in minor amount, the strong rigid matrix controls the bulk rheology (Vachon and Hyeronimus, 2017).

The two approaches can be considered as a variant of a direct problem, overdetermined, and always offering a solution. However, they do not tackle the third factor, which is the exchange of matter between the melt and its matrix, forming feedback loops. Hence melting occurs when adequate minerals are close together, thus loweringthe energy of melting. Conversely back reactions, especially involving ferro-magnesian minerals, precipitate elements, consuming energy to form incongruent minerals [START_REF] Cenki | Melt-producing and melt-consuming reactions in anatectic granulites: P-T evolution of the Achankovil cordierite gneisses, South India[END_REF][START_REF] Nyström | Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology[END_REF]. The two reactions act in modifying the free Gibbs energy budget of the reactions. However, their extent, i.e. the amount of material involved that rules the energy budget cannot be controlled exactly in time and space. It thus lowers the accuracy of a direct modelling.

To partly avoid and solve these ambiguities, a cellular automaton has been constructed and modified [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]Vigneresse et al., 2020), giving insights to the mechanisms of two-phase rheology in partially molten rocks (see below).

The process of melt fusion and extraction can be directly compared with frictional motion, that rules displacement of two materials in contact under an applied force [START_REF] Persson | Theory of friction: Stress domains, relaxation, and creep[END_REF][START_REF] Marone | Laboratory-derived friction laws and their application to seismic faulting[END_REF]. Friction characterizes the relative motion, usually between two solid phases [START_REF] Byerlee | Friction of rocks[END_REF]. It links the tangential (τ) to normal (σ) forces (Fig. 1). The former controls motion imposed by an applied force, whereas the second akin to the gravitational force resists to lateral motion. The static coefficient of friction (µ0) is a linear function of the applied forces, and it is constant for most materials [START_REF] Byerlee | Friction of rocks[END_REF] (Fig. 1a). The irregularities of the contact make lateral motion sporadic in time. A kinetic coefficient of friction [START_REF] Dieterich | Modeling of rock friction: 1. Experimental results and constitutive equations[END_REF][START_REF] Ruina | Slip instability and state variable friction laws[END_REF] quantifies the derivative of the displacement, akin to a velocity.

Irregularities, or asperities in the planar surface separating the two bodies resist to motion.

The frictional motion becomes transient owing to the change in the velocity of the motion (Fig. 1b). Stick-slip is a special case of motion in which a prolonged stagnation (the "stick" phase) is interrupted by short bursts of movement (the "slip" phase). The motion is not periodic in the sense that the time interval between the short bursts is not always identical, depending on the resisting forces (Fig. 1c). The amplitude of the bursts that vary around an average value are not identical neither. When the initial conditions locally vary, a transient motion develops with sudden acceleration in motion. Such transient effects are not well understood, though they represent the responses of the system to the initial parameters variations While the discontinuous nature of melt extraction could be easily been transposed from discrete permeability waves, the transient effects are not intuitively transposable to displacements between viscous bodies or to poroelasticity. Hence, the discontinuous nature of seismicity for example, yield to the concepts of asperity, small irregularity in the contact between materials. They represent pinning points restraining motion and the sudden change in contact area, or depinning leads to sliding. Following this model, frictional motion manifests by alternate sticking and sliding phases, known as stick-slip motion [START_REF] Baumberger | Solid friction from stick-slip down to pinning and aging[END_REF]Brace and Byerle, 2008).

Illustrating and interpreting frictional motion between a fluid, melt or water, and its matrix represents an example of alternate concepts between rheology and hydraulics applied to magmatism. The starting point of this study is a numerical simulation of melt extraction using a cellular automaton [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]. The origin of a sporadic motion is examined in relation with the initial parameters and internal changes in free energy through the interactions between its three components (entropy, strain and chemical potential).

2.

Numerical modelling set up

A numerical modelling using a Lagrangian formulation has been built from a cellular automaton modified from a preceding model (e.g. available code in [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]. The initial system consists in discrete cells, organized in a hexagonal pattern with dimension 64 by 38 cells (Fig. 1). Cells contain indifferently melt and solid in proportion varying from 0 to 100 [START_REF] Vigneresse | Melting by numbers: assessing the effective melt fertility of crustal rocks[END_REF]. The solid phase involves quartz and plagioclase, two micas as muscovite and biotite, and some incongruent or residual ferromagnesian minerals. The initial mineral and melt proportions are fixed, with a textural disposition, either random, in layers or small pockets [START_REF] Vigneresse | Melting by numbers: assessing the effective melt fertility of crustal rocks[END_REF]. A stoechiometric equation rules dehydration melting [START_REF] Gardien | Melting of biotite + plagioclase + quartz gneisses: the role of H2O in the stability of amphibole[END_REF]. Similarly, back reactions are introduced for the ferromagnesian [START_REF] Cenki | Melt-producing and melt-consuming reactions in anatectic granulites: P-T evolution of the Achankovil cordierite gneisses, South India[END_REF]. The melt sink is infinite, as well the heat source. Those two conditions assume a stationary condition for melt evolution. The remaining solid phase is replaced by the moving melt from one cell to the next, according to simple and/or pure shear geometry. Shear strain localization is possible by moving melt to more than one cell. Periodic lateral boundaries avoid melt accumulation. The initial melt content can be modulated, or by partial refilling in melt mimicking temperature burst. Superimposed to the initial conditions and geometric parameters, two rules control melt motion and extraction [START_REF] Vigneresse | Rheological transitions during partial melting and crystallization with application to felsic magma segregation and transfer[END_REF]. In addition to a liquid percolation threshold (LPT) a melt escape threshold (MET) controls extraction. The effect of stress and strain rate is also examined in both situations of pure (σ) and simple (τ) shear. However, the adopted 2D geometry biases the results since melt may escape the system in the third direction. The time is represented by an iterative process. As in most cellular automata, that are qualitative by essence, the time aspects are only apprehended through iterations, i.e. without exact calibration in time.

3.

Results concerning melt extraction

Results allow to identify the effects on melt extraction of the main parameters: i.e. the initial amount of melt (φ0) and the threshold for extraction (MET). The former is supposed to overcome melt percolation through the system. The latter is equivalent to fluid draining, thus renamed as ζ (Fig. 2d,e). All parameters vary on a scale from 0 to 100. The extreme values of met (> 50), i.e. with more melt than matrix, are not representative of nature, but allow a control on the evolution of the numerical model.

Bulk results show a stationary state when there is no significant change in the input parameters, but a spike in melt extraction occurs when the ζ value changes, and more commonly when sporadic extraction occurs. The three modes (stationary, spike and sporadic) show a good analogy with frictional motion, pointing to interpretation with the changes in friction, i.e. in terms analogue to a frictional law (e.g. [START_REF] Marone | Laboratory-derived friction laws and their application to seismic faulting[END_REF].

Preliminary assessment of the results

Melt extraction is scaled in relative quantity of melt extracted versus iterations, i.e. time, without possibility of more precise scaling (e.g. hours or years) yielding a relative time scale.

Similarly, the energy budget of the process is not quantitively available.

The amount of melt is another parameter used during the computation. The possible amount of melt in each cell ranges from 0 to 100. Consequently, the extracted melt fraction (σ) at each step also ranges from 0 to 100 (Fig. 3). The fraction of extracted melt per step shows, as expected, a spike at the start of the model, that rapidly converges toward a stationary value. The spike is positive when the initial melt fraction is high, negative when melt fraction is low. Curves with different initial melt fractions converge after about 70-80 steps. In a second stage, the percentage of melt extracted (σ) at each step is normalized to the initial melt (φ0) and the dimension of the model (Nx * Ny), providing the coefficient of melt extraction (µ) (Fig. 3). After normalization, the value converges after about 150 steps toward a value ranging 0 to about 1.5 (Fig. 3) increasing with the initial melt (φ0). In this example, the linear relation between µ and φ0 defines a bulk coefficient of melt extraction (µ0) that equals 0.17 ± 0.002 whatever the initial melt percentage, and using a value of 0.25 for ζ assuming a one order less variation of (µ0) as a function of ζ [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF].

A note of caution seems necessary, here, prior to interpreting the results. The coefficient of melt extraction reaches a value of 1.0 for φ0 larger than 50 and values higher correspond to more melt extracted than produced. It means that the model diverges for high values of initial melt, i.e. when there is more liquid than solid. Such a situation, mathematically possible, does not reflect reality. However, it allows examining the results in a larger range of parameters.

We have indicated on the figures 6 to 7 the range of values for melt extraction, as estimated in nature.

The coefficient of melt extraction (µ) shows two behaviours, regular or stationary, and transient (Fig. 4), depending on whether the other parameters are constant or not during a single run. Both cases are examined separately. The first case leads to a steady state regime.

Conversely, the transient mode due to varying ζ, or strain rate (ε° and γ°) induces a spike of amplitude ∆µ.

The return to the steady state values follows a classical damping effect reflecting an exponential decrease of the sharp increase with a characteristic time (Dc) about 70-80 steps (Fig. 4). The lack of definition in time (i.e. iterations) precludes a more detailed investigation on the variability of Dc. The spike is positive or negative, depending on the lack or enhanced quantity of melt. It thus, like a charge and discharge of a capacitor, requires using a logarithmic function, with the sharp increase normalized to the initial value. The function takes the form of a ln(φ/φ0). Such functions are applied to changes in ζ and strain rate (ε° and γ°)

Towards a law of melt extraction

The preceding diagrams allow quantifying the relative variations as a function of the input parameters. The results can be summarized as follow and interpreted by a simplified law of melt extraction that takes into account the linear and non-linear effects, associated to the potential barriers of melt extraction mimicking a frictional law [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]:

µ = µ0 + a (φ/φ0) + b (ζ/ζ0) + c (σ/σ0) + d ln (δφ/φ0) + e ln (δσ°/σ°0)
with the numerical values µ0 = .022, a = .0018, b = -.0015, and c = .016 (simple shear) or = .066 (pure shear) [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]. The two functions in δφ and stress rate (δσ°) variations are non-linear and could not be determined quantitatively. Globally, the effective production of melt ranges from 15 % to a maximum of 40 % depending on protolith composition [START_REF] Vigneresse | Melting by numbers: assessing the effective melt fertility of crustal rocks[END_REF]. The potential barriers represent variations one order of magnitude less than the coefficient of melt extraction µ0.

It should be compared to the suggested frictional laws [START_REF] Dieterich | Time-dependent friction in rocks[END_REF], with the modification about displacement [START_REF] Ruina | Slip instability and state variable friction laws[END_REF].

µ = µ0 + a ln(V/V0) + b ln(V0 θ /Dc) and dθ/dt = 1 -Vθ/Dc
Nevertheless, because of the poor access to time, the problems of strength recovery (healing) interpreted from the state variable θ, without more specification [START_REF] Ruina | Slip instability and state variable friction laws[END_REF], have not been considered in the actual study. The analogy points to the effect of velocity, either as the velocity of traction during dry friction, or melt flux under strain partitioning on the viscous melt, eventually enhanced by acting stress. In addition, a second term related to imposed strain is present in the equations, that may have an influence on the sporadic motion during friction or melt extraction. In a first approach, this term is a derivative of the motion, akin to acceleration. It suggests that melt motion is represented by the relative value of melt volume (φ/φ0), whereas the remaining other parameters (ζ, strain) act as enhancing/damping the flow.

Sporadic melt extraction

One difficulty in interpreting the irregular sporadic melt extraction patterns mostly links with the poor determination related to temporal calibration. It includes difficulties in a true scaling in time units, but also difficulty in interpreting a discontinuous diagram of irregular amount of extracted quantity of melt. It manifests during friction by a stick-slip motion (Fig. 2c), as a second order manifestation, interpreted by the hysteretic nature of friction. Hence, friction is a dissipative function of energy, that needs additional energy to recover the initial state. We provisionally adopt the term stick-slip behaviour for describing melt extraction.

4.1.

Variations in stick-slip amplitude (σµ)

Stick-slip manifests during friction by alternate periods of no or restricted motion, or stick phase, followed by slip at constant velocity (Fig. 2c). Similarly, the amplitude of melt extraction, varies with iterations, but remains constrained between two values defining a dynamic and static extraction modes (Fig. 2f). The difference is referred to as σµ. This value is examined as a function of the other parameters. The amplitude of σµ varies linearly between 0.0015 and 0.013 for an initial melt content varying from 5 to 85 %, though the last value is unrealistic in nature, but brackets the range of variables. It slightly varies by about 0.003 for a variation of 50 in the initial melt content (φ0), i.e. from dry to saturated melt. The values increase with ζ by about 0.035 for a change of 20 in ζ (Fig. 5). Thus, the effect is more pronounced depending on ζ rather than on φ0. The linear effect also excludes artifacts such as errors or noise. It correlates with a non-strictly periodic effect in amplitude.

The very moderate decrease, about one order less than the coefficient of melt extraction with the initial melt content points to an enhanced melt extraction with the total melt content (Fig. 5), suggesting a possible lubrication of the interface between melt and its matrix. Nevertheless, this effect remains weak compared to the effect of ζ. It also points to enhanced/damping effects occurring during transient changes compared to the variations induced by varying the initial parameters.

Variation of amplitude depending on the system history

The amplitude increase of the sporadic melt extraction has been tested against the other parameters evidenced in the melt extraction law (φ, ζ, ε°) [START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]. This has been realized under two modes. In a first set of runs, these parameters remain fixed along the whole run. The separate runs (previous sections) show moderate variations in σµ, with a bulk increasing linear trend whatever the change in all parameteers (Fig. 6).

Alternatively, a second set of runs, that are successive, allows abrupt changes in the parameters during iterations, indicated by enhanced melt extraction spikes before a progressive return to the previous extraction mode. Those spikes, with amplitude ∆µ, are positive or negative depending on charge or discharge of melt. By contrast, the amplitude of melt extraction (σµ) shows a pronounced decrease, for all parameters (φ, ζ, ε°), when one of them is kept constant (Fig. 6). The decrease in amplitude is significant (0.12 to 0.0), since σµ tends to zero when the changes in input parameters modify the coefficient of melt extraction by a factor 2 or 3. In addition the variation in amplitude for separate runs largely overcomes the variation for separate runs (Fig. 6). To further explore the situation, their amplitudes have been plotted during successive variations of the inputs (Fig. 7a). The amplitudes decrease with iteration (i.e. time) according to a law proportional to ζ -1 ln(ζ/ζ0) with a coefficient α and ζ0 the initial permeability (Fig. 7b). It points to a memory effect acting on the stick-slip amplitude, and decreasing with the system memory. Nevertheless, as stated before, the poor variability in the amplitude of Dc, i.e. some quantification of the time constant of the relaxation, precludes insights to the energetical aspects of melt extraction. An interpretation would be an enhanced permeability due to healing. Once porosity is enlarged, it should keep its high value, leading to enhanced melt extraction.

Return on the spike amplitudes

The fact that σµ decrease with the melt extraction has an impact on the amplitude of the enhanced melt extraction spikes (∆µ), but no real trend could be estimated from the spike amplitudes. Taking into account a possible memory effect allows to reconsider differently the spikes. From the various runs, the spikes amplitudes have been plotted as a function of ζ variations, distinguishing the positive from the negative variations. These plot shows a weak coherence, adopting a double curve, not symmetric to each other, but mimicking some kind of hysteresis cycle as a function of the quantity of melt refilling or escaping. It corroborates the asymmetric plot of ∆µ versus φ variations (Fig. 4).

Implications and discussion

The large variability observed in the modelled melt extraction has some direct consequences.

Hysteresis

Hysteresis is the dynamic time lag between input and output of the system, reflecting its dependence on its previous history. It is associated with the need to consume or deliver additional energy for the return to initial conditions. Examples of hysteresis in geological systems include remanent magnetism, plastic deformation, phase change during melting and crystallization, or surface tension [START_REF] Persson | Theory of friction: Stress domains, relaxation, and creep[END_REF][START_REF] Hill | The Mathematical Theory of Plasticity[END_REF]. A simplistic representative example is a drop of water on a glass. In case of a porous medium, with a two-phase flow (air and water) the curve of capillary pressure versus the wetting phase saturation also describes a hysteretic cycle, with a wetting stage, and return along drying curve. Due to the solubility of the gaseous phase, the hysteretic curve shrinks with successive cycles of drying and wetting.

Pinning and depinning

The sporadic melt extraction, with alternate bursts, links to variations in the melt velocity, i.e. de-/acceleration. As stated in friction, it relates to a state variable θ [START_REF] Ruina | Slip instability and state variable friction laws[END_REF], essentially defined by its derivative with time (dθ/dt). It is characterized by alternate episodes of lock (stick) and motion (slip). The frictional interpretation commonly relates to those stages to surface conditions. Indeed, they present asperities, or small stronger areas of relief, arresting the motion. If the interpretation is correct, the alternate stages of enhanced/reduced melt extraction should also be controlled by such points of resistance, i.e.

pinning and depinning, and their equivalent in terms of chemical thermodynamics is consuming/releasing Gibbs free energy.

Experimental studies on stick-slip, using small transducers and fast camera recording allow a better understanding of the phenomena traduces in a force vs time diagram during dry friction (Fig. 8a). The equivalent diagram in displacement vs time indicates periods of no motion corresponding to the stick stage followed by a sudden jump in velocity (Fig. 8a).

However, more sensible experiments from the group of Jay Fineberg (Hebrew University, Jerusalem) tackled the fine motion during the sticking stage in a bi-phase material interface.

They record high-speed measurements of the real contact area, whereas the strain field is measured in the region surrounding propagating rupture tips [START_REF] Svetlizky | Classical shear cracks drive the onset of dry frictional motion[END_REF].

They observe highly localized slip pulses accompanied by a large local reduction of the normal stress near the rupture tip. The pulses propagate along the direction of motion of the softer material at a maximal velocity, evolving while propagating. When the rupture front velocity overcomes the seismic S-waves velocity, it determines a supershear front [START_REF] Rosakis | Cracks faster than the shear wave speed[END_REF] that propagates in the opposite direction, suggesting crack-like rupture. Such results corroborate the ideas of dislocations steadily propagating along a material interface. In parallel, the transition from static to dynamic friction is quantitatively described by classical singular solutions for the motion of a rapid shear crack [START_REF] Rubinstein | Detachment fronts and the onset of dynamic friction[END_REF]. They observed propagations at sub-Rayleigh velocity, the sound speed along a free surface, but also a slower front that could be the trace of healing at contact area, before slip arrest. This is similar to ageing that immediately commences as contact area increases. Its characteristic timescale is determined by the local memory of the system, according to its effective contact time before slip arrest [START_REF] Ben-David | Slip-stick and the evolution of frictional strength[END_REF].

Such evolution at the microscale of dislocations has been surveyed in detail on a single

LiF crystal since it allows simultaneous observation of evolution of the dislocation structures through the Bauschinger effect, a piling-up of dislocations at grain boundaries. Indeed, the microscopic distribution of stress alters the stress/strain characteristics of the material [START_REF] Harea | Bauschinger's effect and dislocation structure under friction of LiF single crystals[END_REF]. This property is used to trace the dislocation pattern during deformation [START_REF] Perfilyev | Dislocation structure and stick-slip phenomenon[END_REF]. Observations show that the indenter velocity increases nonlinearly during the transition from stick to slip. in the mid-part of the slip phase, the indenter reaches its maximum velocity, while the friction force decreases abruptly (Fig. 8b). The average velocity of the indenter in the stick phase drops by about 300 times than the maximum velocity in the slip phase. It drastically affects the behaviour of the dislocation regime. Three stages of creep are evidenced between two slip events [START_REF] Perfilyev | Dislocation structure and stick-slip phenomenon[END_REF]. First, a primary creep links with the formation of bonds (pinning effect). Their number increases during the second long-time steady state. In contrast, the tertiary creep is characterized by the rupture of bonds (depinning effect).

The link between pinning/depinning and dislocations [START_REF] Anderson | Theory of Dislocations[END_REF] brings some highlights on the asperity model since it marks by acoustic emission that may be recorded during the stick-slip motion. Known as the Barkhausen noise in ferro-magnetism [START_REF] Santa-Aho | Barkhausen noise probes and modelling: A review[END_REF] it results from the rapid size variations of magnetic domains, with their magnetic spins equally oriented. The produced noise is recorded and is currently in use to detect defects in the alloys.

Implications on melt extraction processes

The preceding section provides mechanical examples involving the role of dislocations in pinning/depinning effects during frictional stick-slip motion (Fig. 9a).

Dislocations creep correspond to a change in the ordering of a crystal, i.e. a change in entropy. It may apply to solid-solid contacts but fails when a viscous material is present [START_REF] Marone | Laboratory-derived friction laws and their application to seismic faulting[END_REF]. Another model of stick-slip behaviour derived from poroelasticity [START_REF] Cheng | Poroelasticity[END_REF] takes into account the surface tension and the solubility between the components, solid, liquid and air (Fig. 9c). I corresponds to an interaction between strain of the flow and strain imposed by the matrix. In both cases a locking effect, or barrier, retards motion while an acceleration occurs when additional energy overcomes the resisting forces. In both cases, the processes are the sum of a static equivalent to the free energy of the system plus a term function of the its first derivative, equivalent to velocity, and a second derivative, equivalent to acceleration. The energy barrier linked to dislocations has a fixed amplitude, also linked to a progressive increase in pinning, thus favouring a limit to a dynamic friction coefficient.

In between those two end-members, melt extraction results from a viscous-viscous interface. There is no dislocation, by lack of a true crystalline network, and there is no nonpercolating third phase as air bubbles. Hence the matrix is a crystalline rock undergoing melting, but with a heterogeneous amount of solid phases either products or unreacted inherited minerals. The melt is a mixture of silicate polymers and ions in a supercritical state.

Metamorphic reactions between the melt and its matrix may be considered as a cause of for pinning/depinning effects (Fig. 9b). Retrograde, melt-consuming reactions, producing solid phase are the counterpart of prograde melting and may produce new minerals phases acting as asperities causing stick-slip or sporadic melt extraction. Both type of reactions are commonly observed in migmatites, rocks that have undergone partial melting [START_REF] Stüwe | Effective bulk composition changes due to cooling: a model predicting complexities in retrograde reaction textures[END_REF][START_REF] Kriegsman | Partial melting, partial melt extraction, and partial back reaction in anatectic migmatites[END_REF].

Dehydration of hydrous minerals occurs during prograde melting, mainly muscovite above 730 °C and the biotite around 800°C [START_REF] Clemens | The granulite-granite connexion[END_REF]. Hence the leucocratic portions of migmatites, or leucosomes, are mostly composed of quartz, feldspars and rare micas. These are interpreted as crystallized melt usually of granitic composition. The darker ferromagnesian minerals form the melanosome and may segregate aside the leucosomes by flow differentiation or reactive transport [START_REF] Kriegsman | Partial melting, partial melt extraction, and partial back reaction in anatectic migmatites[END_REF]. The minerals from the melanosome can either be relics of reactants, products of melt consuming reactions or newly formed incongruent mineral phases.

During muscovite breakdown with temperature increase, mineral phases disappear whereas melt-consuming reactions may form new solid phase upon cooling [START_REF] Kriegsman | Partial melting, partial melt extraction, and partial back reaction in anatectic migmatites[END_REF]. Those may be quartz, plagioclase and muscovite [START_REF] Nyman | Melt extraction during formation of Kfeldspar + sillimanite migmatites, west of Revelstoke, British Columbia[END_REF][START_REF] Konn | Dehydration melting and fluid recycling during metamorphism: Rangeley Formation, New Hampshire, USA[END_REF] followed by biotite, sillimanite, quartz and plagioclase [START_REF] Cenki | Melt-producing and melt-consuming reactions in anatectic granulites: P-T evolution of the Achankovil cordierite gneisses, South India[END_REF]. In contrast garnet and cordierite may grow as incongruent porphyroblasts [START_REF] Nyström | Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology[END_REF]. During growth, they may incorporate large quartz rounded inclusions indicating surface tension and immiscibility. Upon cooling, retrograde phases may form coronas around these porphyroblasts, surrounding by the leucosome, illustrating back reactions between the restitic phases and the melt. Ti-rich biotites also grow at the expense of ferro-magnesian incongruent phases [START_REF] Cesare | Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions[END_REF]. Melt consuming reactions include mica growth, finegrained intergrowths in micas, breakdown of incongruent mineral phases and coronas formation [START_REF] Cenki | Melt-producing and melt-consuming reactions in anatectic granulites: P-T evolution of the Achankovil cordierite gneisses, South India[END_REF][START_REF] Cesare | Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions[END_REF]. The amount of melt production, or consumption, depends on the ability of elements to enter such reactions. It thus has no bounding values, making the amplitude of stick slip non periodic.

Interestingly, the formation of garnet coronas through a process of diffusion is slow.

Thus, depending on the heat flow variations, a so-called tectonic "hysteresis" in the melt extraction may result [START_REF] Loomis | Kinetics of garnet granulite reaction[END_REF]. The retrograde reactions, added to changes in the pore size, determine zones of pinning and depinning (Fig. 9b).

The sporadic mode of melt extraction corroborates with the observation of segregated melt from its matrix in discrete pockets [START_REF] Kisters | Melt segregation and far-field melt transfer in the mid-crust[END_REF]. Such discrete pockets commonly form shrinking patterns with subvertical pipe-like bodies. They further connect together to evacuate a felsic melt into veins following the regional foliation planes. Discrete melt batches also corroborate numerical models of porosity waves [START_REF] Connolly | Temperature-dependent viscoelastic compaction and compartmentalization in sedimentary basins[END_REF][START_REF] Rabinowicz | Melt segregation under compaction and shear channeling: Application to granitic magma segregation in a continental crust[END_REF].

Another conclusion from such competition between melting and solidification (backreactions) during melt extraction may have implications on the interpretation of field samples.

Hence, magma crystallization is commonly coeval with fluids release, due to space shrinking, rapidly reaching fluid saturation (e.g. second boiling, [START_REF] Candela | A review of shallow, ore-related granites: textures, volatiles, and ore metals[END_REF]. The additional presence of a compressible fluid phase within the competing enhanced melting and crystallization (i.e.

external pressures variations on the system) is coherent with an variation in the extraction rate during solidification with sudden pressure changes [START_REF] Pistone | Felsic melt and gas mobilization during magma solidification: An experimental study at 1.1 kbar[END_REF]. The stick-slip like melt motion thus adds a supplementary positive feedback loop to the process. Conversely, during initial melting, the freed fluid from hydrous minerals does not develop in a separate phase since it immediately enters the melt because of its solubility. In addition, the poor volume change due to biotite dehydration [START_REF] Rushmer | Volume change during partial melting reactions: implications for melt extraction, melt geochemistry and crustal rheology[END_REF] does not alter the bulk volume of the system, thus avoiding pressure related loops.

Hysteresis and differences in melt motion increase the symmetry breaking between melting and solidification. In particular, the order of minerals crystallization, commonly accepted from ferro-magnesian, plagioclase and quartz, i.e. following a decrease in chemical potential, is perturbated when fluids (water-dominated) enter the process. It has implications on the segregation or not of incompatible elements that do not follow a similar trend compared to congruent minerals. The second important difference between melting and crystallization relates to the fate of water, inducing a large scale more or less independent hydrothermal activity during magma crystallization, whereas the processes of dry or hydrous melting imply changes in the melt final composition. However, developing such ideas is out of the scope of the present paper.

Analogy between dry friction and melt extraction

The dry friction coefficient (µd) characterises the relationship between a force (F) applied to a mass (m) pulled by a spring. The usual relation between the force applied on a mass and its acceleration applies. The force is proportional to the first derivative of the motion (dx/dt) through the coefficient of friction (µd). Thus m δ 2 x/δt 2 = -µd δx/δt

The solution to this differential equation is given by

δx(t)/δt = δx(0)/δt exp [-(µd /m) t]
It relates the mass-force relationship to the stress-strain relationship. It also points the double effect of the velocity and its variations (i.e. acceleration), thus providing an explanation to the "state variable" (θ) introduced in dry friction [START_REF] Ruina | Slip instability and state variable friction laws[END_REF]. In terms of melt extraction, it corresponds to variation in the melt flow, either due to enhanced motion under applied stress or enhanced/retarded melting.

Notes of caution

The ideas presented here are derived from the numerical model that examines the motion of melt in migmatites. As such, it should be considered as a model, i.e. either representing the effects of the identified parameters, or in contrast, as a way to bracket the values of the parameters to better fit field observations. This requires a note of caution on the way the results are interpreted. In its actual shape, all parameters have been varied in a full range. Secondly, chemical reactions develop between the matrix and the melt as prograde reactions, but also between the melt and its matrix, as retrograde reactions. They consume or produce melt, thus modifying the melt flow velocity. Such reactions of chemical exchange are important when considering the free energy balance of the system. the corresponding interactions between strain (flow velocity) and chemical potential (melting and backreactions) modify the bulk energy of the system, inducing sporadic variations. Because they were not considered in other forward models [START_REF] Ioannidi | Effective rheology of a two-phase subduction shear zone: Insights from numerical simple shear experiments and implications for subduction zone interfaces[END_REF], they resulted is a stationary motion, that certainly does not fully consider the exact nature of two-phase rheology. Similarly, variations due to the energy required to overcome dislocations induces instabilities due to strain (flow) and entropy due to ordering modification (dislocation creep) as in dry friction (Fig. 9a). In poroelastic flow, the interactions develop between strain (flow) and changes in the pore aperture (external strain) determine sporadic motion (Fig. 9c). The specific case of melt extraction implies chemical exchanges (melting and back reactions) between the melt and its matrix (Fig. 9b).

Thirdly, the transient character of the flow presents some memory effects, due to the irreversibility of the reactions and the thermodynamic metastability of some minerals phases. However, such memory effects should not be confused with a time-related effect. In particular, it does not implicitly to point any re-use of veins.

At last, because of the model is constructed from a cellular automata, the model cannot be scaled in time, but uses a proxy through iterations. It is therefore not aimed to prediction, e.g. for quantitative mush (crystal-melt) interaction, as described for magma chamber rejuvenation or in magma eruptability. Similarly, a purely chemical approach is no more predictive, and a dry friction modelling does not inform on the next earthquake to occur. In contrast, and we focus on the message here delivered, any modeling of a multi-phase material should incorporate the interactions between the three components of energy, i.e. the external variables such as entropy, strain and chemical potential.

Conclusions

The The applied force varies from a velocity V0 to V. The coefficient of friction presents a spike (∆µ) function of the relative change in velocity, with a return to stationary conditions after a delay Dc. The new coefficient of friction may be higher or lower than the initial µ0. c) The basal contact of the block is no more homogenous, with either two phases or asperities. An irregular motion, with successive sticking and sliding states is observed, with amplitude (σµ).

A stiction spike is also observed, as well as return to stick-slip when the velocity varies. d)

Melt extraction coefficient (µ) defined as the ratio of initial melt to permeability (z) taken as a melt escape threshold. e) Similar to friction, the initial melt (φ) varies, as well as the permeability (ζ) and added stress (τ). A spike is observed before return to a stationary state. f) irregular amount of melt is extracted mimicking stick-slip behaviour when the melt flow presents reactions damping its amount and velocity. Figure 7. Amplitude of the spikes (∆µ) when the permeability value varies along a single run. The successive changes are positive or negative, yielding similar changes in ∆µ.

The curves have been separated for increasing and decreasing permeability changes.

For

  instance, the initial amount of melt ranges 0 to 100, though such high values are obviously not observed in partially molten rocks. However, the extreme values are considered as providing the numerical trends that rule the underlying process. Similarly, the coefficient of melt extraction presents values greater than 1. It corresponds to the effect of an infinite source and sink for the melt. They are not realistic. Such large values correspond to an initial melt larger than 50. Such notes of caution being stated, what are the main results of the model.First, the model illustrates the role of sharp variations in the ability of melt to be extracted and move (ζ), formerly assimilated to a melt escape threshold[START_REF] Vigneresse | Simulation of crustal melt segregation through cellular automata: Insight on steady and non-steady state effects under deformation[END_REF]. These variations show a transient behaviour. It implies that the definition of the melt escape threshold as a purely geometric threshold corresponding to percolation is incorrect since it is essentially static. Hence, the ζ parameter is dynamic, determining stiction spikes (∆µ) and the amplitude of the stick-slip motion (σµ). It corresponds to a variation of the flux, i.e. an acceleration, positive or negative, of the flux.

  present model examines the transient flow of melt extraction depending on variations of the controlling parameters, mostly the changes in flux velocity (amount of melt). The occurrence of spikes reflecting sporadic melt extraction with a progressive return to a stationary state is remarkable. We interpret these results with an analogy to the stick-slip behaviour in mechanics that might develop resulting in an irregular, non-periodic amount of melt extracted. This effect shows hysteresis, essentially due to the presence of phases consumed during prograde (melting) or retrograde (garnet and sillimatite growth) metamorphic reactions. The interactions between strain, chemical potential and entropy modify the free energy of the system. They act as pinning and depinning during dry friction and other flows of a weaker phase within a stronger matrix. The model highlights a dynamic concept of melting and places melt extraction between dry friction and poroelasticity.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Schematic representation of frictional law and its analogue for melt

Figure 3 .

 3 Figure 3. Results of the numerical simulation for different values in the initial amount

Figure 4 .

 4 Figure 4. Two regimes, regular or stationary and transient, are observed depending on

Figure 5 .

 5 Figure 5. Variation of the amplitude of the stick-slip effects (σµ) as a function of the

  Figure 6.

Figure 8 .

 8 Figure 8. Experiments on stick slip on a single crystal of LiF, redrawn from Perfilyev et

Figure 9 .

 9 Figure 9. Cartoon illustrating the analogies between (a) frictional law, (b) melt

Acknowledgements

The actual paper started from a collaboration with Jean-Pierre Burg (ETH Zurich). At that time, we were mostly interested in the rheology of a two-phase material, with intrinsic feedback loops and non-linearity. More recent discussions focused on the role of factors that alter continuous melting due to heat diffusion toward a discontinuous motion of the melt. In particular, we focused on the transient behaviours, yielding an analogue to stick-slip motion, pointing to pinning/depinning processes that illustrate discontinuous motion. The analogies with dry friction and poroelasticity should bring insights on irregular motions observed in those disciplines. BC acknowledges the funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 793978.

We appreciated the constructive comments of one reviewer, focusing on the link back to field observations. The general comments also obliged us to focus more on the final message of the paper, i.e. interactions between the energy components.


Cedric Gil-Jardine
email: cedric.gil-jardine@chu-bordeaux.fr

Coralie Gil

Saint Jammes Juliane Tortes

Barbara Villoing

Hugues Lefort

Cédric Gil-Jardiné

Médecin Urgentiste

Saint Jammes Infirmière Juliane Tortes

Barbara Psychothérapeute

Villoing

Prise en charge préhospitalière d'une urgence psychiatrique

Le parcours de soins pour un patient présentant une urgence psychiatrique fait intervenir de multiples intervenants : proches aidants, libéraux médicaux ou paramédicaux, forces de sécurité intérieure, service départemental d'incendie et de secours, aide médicale urgente (AMU), coordonnées par les services d'aide médicale urgente (Samu), le centre de réception et de régulation des appels 15 (CRRA) et ses vecteurs médicalisés les services mobiles d'urgence et de réanimation (Smur), spécialistes hospitaliers et, bien sûr, les psychiatres et leur réseau de soins spécifiques au sein des établissements de santé mentale et des centres médico-psychologiques [START_REF][END_REF].

Les professionnels de santé non médicaux, tels les assistants de régulation médicale (ARM) et les infirmiers, jouent un rôle important dans l'identification de la situation, la prise en charge initiale et la délivrance de traitements adaptés à chaque situation sur protocole. Deux situations préhospitalières sont développées : les états d'agitation aiguë et les urgences d'allures psychiatriques franches exposées dans les Recommandations formalisées d'experts publiées en 2021 par la Société française de médecine d'urgence (SFMU) [START_REF] Rerbal | Organisation et modalités d'intervention des cellules d'urgence médicopsychologique. Recommandations de la Société française de médecine d'urgence (SFMU) en collaboration avec l'Association de formation et de recherche des cellules d'urgence médicopsychologique -Société française de psychotraumatologie (AFORCUMP-SFP)[END_REF]. La prise en charge des urgences psychiatriques dans un contexte de situations sanitaires exceptionnelles, impliquant ou non de multiples victimes, est détaillée dans les recommandations de la SFMU publiées en 2018 [START_REF] Cesareo | Recommandations de bonne pratique clinique concernant la prise en charge médicale des victimes d'une "tuerie de masse[END_REF].

T1 Une organisation régulée

Au niveau national, la prise en charge des patients présentant des troubles psychiatriques est sectorisée en fonction de leurs lieux de résidence [START_REF][END_REF]. Il est ainsi préférable de l'organiser dans le secteur dont chaque patient dépend. Le premier maillon de la chaîne des soins se réalise dès l'appel à la régulation médicale (CRRA du Samu). L'ARM doit donc non seulement être en mesure de localiser le patient, mais aussi de connaître le plus précocement possible son lieu de résidence. Les soins nécessaires doivent ensuite s'organiser en réseaux. Ils associent la régulation médicale (CRRA), les Smur, les structures des urgences et les services de psychiatrie. Cette organisation a pour objectif de limiter l'admission dans les structures d'urgences aux patients nécessitant une évaluation somatique.

T1 L'état d'agitation aiguë avec ou sans intoxication évidente

Cet état est un motif d'appel fréquent en régulation [START_REF] Flórez | Incidence of acute agitation and variation in acute agitation management by emergency services[END_REF]. Il se définit comme une perturbation du comportement moteur psychique et relationnel qui suscite souvent une relation d'intolérance des proches [START_REF] Lejonc | Conférence de consensus : "L'agitation en urgence (petit enfant excepté)[END_REF][START_REF] Niquille | Prise en charge des états d'agitation extrahospitaliers : le point de vue de l'urgentiste[END_REF]. Il nécessite une intervention urgente quand le comportement du patient met en avant des menaces d'auto-ou d'hétéro-agressivité [START_REF] Lejonc | Conférence de consensus : "L'agitation en urgence (petit enfant excepté)[END_REF][START_REF] Allen | The expert consensus guideline series. Treatment of behavioral emergencies 2005[END_REF]. Dès la prise d'appel, l'ARM doit identifier les situations d'urgence.

T2 Rechercher l'adhésion aux soins

Dans le contexte préhospitalier, le patient, les témoins et les professionnels de santé se retrouvent fréquemment exposés à de nombreux risques et notamment à des actes violents, qu'ils soient volontaires ou involontaires. Cette situation nécessite une réponse rapide et adaptée à chaque patient [START_REF] Lejonc | Conférence de consensus : "L'agitation en urgence (petit enfant excepté)[END_REF]. Cette prise en charge doit, autant que possible, être réalisée par une équipe pluriprofessionnelle impliquant des professionnels de santé et les forces de sécurité intérieure et l'Amu (ambulanciers, secouristes, pompiers).

TEG1 En premier lieu, il est nécessaire d'assurer la sécurisation de la situation, du ou des patients, des témoins et des professionnels présents sur place. Cette étape d'approche est fondamentale et conditionne le plus souvent toute la suite des soins : non seulement en préhospitalier, mais aussi le parcours clinique du patient. En effet, l'approche relationnelle, dans laquelle l'infirmier joue souvent un rôle essentiel, a un double objectif. Elle permet de prévenir les risques de violences et de passage à l'acte soit par une approche comportementale pure, soit par un traitement per os lorsqu'il peut être proposé. Elle aide aussi à réduire les alternatives d'issues pour le patient, en lui proposant d'adhérer pleinement à la nécessité de soins immédiats qu'un état d'agitation aiguë impose sans attendre.

TEG1 Même si l'agitation est au premier plan dans des pathologies authentiquement psychiatriques, comme la crise d'angoisse, la bouffée délirante aiguë et autres troubles psychotiques chroniques, il est nécessaire de rechercher une étiologie pour ne pas méconnaître une cause organique : neurologique, métabolique, infectieuse ou toxique. Les antécédents médicaux ou psychiatriques et le mode d'apparition devront être recherchés dès la prise d'appel par l'ARM et le médecin régulateur, mais également sur les lieux de l'intervention. Les proches aidants sont ici primordiaux. Il faut également veiller à rechercher le contexte avec notamment une prise de toxique [START_REF] Lejonc | Conférence de consensus : "L'agitation en urgence (petit enfant excepté)[END_REF][START_REF] Nassisi | ED management of delirium and agitation[END_REF]. Cette démarche préalable à l'orientation doit permettre d'identifier les causes accessibles à un traitement immédiat, telles l'hyperthermie, l'hypoglycémie et l'épilepsie. Le traitement est alors, et avant toute autre chose, étiologique quand il existe une cause somatique identifiée et rapidement curable. Repérer et traiter rapidement une hypoglycémie permet une résolution "miraculeuse" de la crise d'agitation et évite les autres conséquences telles la chute, les traumatismes crâniens, le coma.

T2 Sédater : soulager ou la main forcée ?

En complément, si l'approche relationnelle doit toujours être privilégiée, l'utilisation de médicaments sédatifs en association peut parfois s'avérer nécessaire. Le choix de la molécule sera fonction de la situation et du patient. Existe-t-il un péril imminent pour le patient ou pour autrui ? Quel est l'âge et quelles sont les comorbidités du patient ? TEG1 Trois classes thérapeutiques se dégagent pour la prise en charge de ces patients : les benzodiazépines, les neuroleptiques et les anesthésiques. La sédation pharmacologique d'un patient en urgence n'est pas dénuée de risque du fait de l'incertitude diagnostique. Le choix devra se porter sur une molécule que l'ensemble de l'équipe préhospitalière maîtrise bien. La substance idéale doit être facile d'administration, avoir un délai et une durée d'action courts pour permettre un retour à la pleine conscience rapide. Elle doit également avoir peu d'effets secondaires, et en particulier ne pas induire d'interaction avec d'autres médicaments [START_REF] Ostermann | Sedation in the intensive care unit: a systematic review[END_REF]. L'usage d'une molécule déjà utilisée par (ou sur) le patient est à privilégier car donnant l'assurance de sa bonne tolérance.

TEG1 La voie orale doit toujours être privilégiée, tant on sous-estime trop souvent le nombre de patients agités collaborant aux soins. Il est recommandé d'utiliser en première intention une benzodiazépine (lorazépam, midazolam) ou un neuroleptique (halopéridol) [START_REF] Claret | Recommandations de bonne pratique clinique. Prise en charge du patient adulte à présentation psychiatrique dans les structures d'urgences[END_REF].

Lorsque la voie per os est inenvisageable, l'alternative de choix est la voie intramusculaire. Elle permet une administration sûre et efficace du produit sédatif choisi. Dans la prise en charge de l'agitation aiguë, les classes thérapeutiques les plus évaluées sont les benzodiazépines (midazolam), les neuroleptiques (halopéridol) et les anesthésiques (kétamine). Les études comparant les benzodiazépines et les neuroleptiques montrent une efficacité et une sûreté similaires entre ces deux classes thérapeutiques [START_REF] Klein | Intramuscular midazolam, olanzapine, ziprasidone, or haloperidol for treating acute agitation in the emergency department[END_REF]. Le midazolam semble avoir un effet sédatif plus rapide que les neuroleptiques. Il faut donc probablement le privilégier dans les états d'agitation sévère. Ces situations les plus à risque peuvent également justifier l'utilisation d'une bithérapie associant neuroleptique et benzodiazépine sans qu'il n'existe apparemment de majoration du risque iatrogène [START_REF] Claret | Recommandations de bonne pratique clinique. Prise en charge du patient adulte à présentation psychiatrique dans les structures d'urgences[END_REF].

Récemment, la kétamine a été étudiée dans les états d'agitation aiguë. Les niveaux de preuve sont encore faibles. Cependant, les résultats vont globalement tous dans le même sens en montrant une supériorité de la kétamine à l'halopéridol en termes de sédation, mais expose à plus de complications [START_REF] Tran | Ketamine for pharmacological management of aggression and agitation in pre-hospital settings: a review of comparative clinical effectiveness, safety and guidelines[END_REF]. La plupart de ces travaux sont réalisés auprès de paramedics, dans un fonctionnement préhospitalier anglo-saxon.

En cas d'utilisation de neuroleptiques, la surveillance de la survenue d'éventuelles complications doit être étroite : survenue d'un syndrome extrapyramidal (dyskinésie, acathisie) et plus particulièrement l'apparition d'une dystonie aiguë dont le spasme laryngé [START_REF] Justic | Does "ICU psychosis" really exist?[END_REF]. D'autres neuroleptiques tel l'halopéridol (chlorpromazine, thioridazine, pimozide, dropéridol, ziprasidone, lévomépromazine) peuvent être utilisés dans l'indication d'une agitation aiguë, mais ils exposent à plus de risque de complications : trouble de la conduction cardiaque par allongement de l'intervalle QT visible sur l'électrocardiogramme (ECG), un effet anticholinergique, une atteinte hémodynamique ou un syndrome malin des neuroleptiques [START_REF] Claret | Recommandations de bonne pratique clinique. Prise en charge du patient adulte à présentation psychiatrique dans les structures d'urgences[END_REF].

TEG1 Quels que soient le traitement et la voie choisis, il est donc impératif de réaliser un ECG s'il n'a pas été possible de le faire avant l'administration du traitement sédatif, pour mesurer l'intervalle QT, en particulier si un neuroleptique a été utilisé.

T1 La décompensation aiguë d'allure psychiatrique

Toute première décompensation aiguë d'allure psychiatrique doit bénéficier d'une évaluation médicale à la recherche d'une étiologie somatique qui mimerait une présentation psychiatrique.

Parmi les patients déjà connus pour une pathologie psychiatrique, il reste fondamental de ne pas méconnaître une pathologie somatique se surajoutant à l'origine de ces situations de décompensation aiguë. Ceci est d'autant plus important que bon nombre de traitements psychiatriques ont un risque iatrogène important. Chez ces malades, il est important de rechercher précocement le caractère habituel ou non de la présentation. En cas de doute, une évaluation préalable dans une structure d'urgence peut se justifier.

Pour les patients suivis pour pathologies psychiatriques dans un réseau de soins chroniques, il faut rechercher une rupture de la dynamique de la pathologie psychiatrique, et cela dès la prise d'appel en régulation (CRRA). Cela permet de distinguer une vraie situation d'urgence de celles récurrentes sans réelle décompensation.

T2 Réguler pour la bonne direction hospitalière TEG1 Autant que possible, le transport des patients pour lesquels aucune pathologie somatique n'est suspectée par la régulation doit se faire vers des centres habilités par la loi à réaliser des hospitalisations sous contrainte. Si la situation le permet, les médecins de ville ou en réseau (type SOS Médecins) rédigeront ainsi le premier certificat pour réaliser un transport vers un centre habilité. Si ce n'est pas le cas, l'envoi d'un Smur pourrait se justifier. Par ailleurs, tout patient présentant un tableau psychiatrique peut être transporté sans certificat préalable sous la responsabilité médicale du médecin régulateur. TEG1 L'identification du risque suicidaire lors de la prise en charge préhospitalière du patient doit être la plus précoce possible. Elle est essentielle et doit impliquer l'ensemble des intervenants : ARM, ambulanciers, infirmiers, médecins et secouristes sur place [START_REF] Hunt | Signs of current suicidality in men: a systematic review[END_REF][START_REF] Marco | What calls for service tell us about suicide: a 7-year spatio-temporal analysis of neighborhood correlates of suicide-related calls[END_REF]. Dans la littérature, les facteurs associés sont un changement important de l'humeur, le désespoir concernant le futur, ainsi qu'une colère intense. Il est donc nécessaire de former les personnels des Samu à la détection de ces facteurs. Les tentatives d'autolyse avec passage à l'acte (intoxication, traumatisme, etc.) nécessitent une première évaluation dans les services d'urgences pour juger de la sévérité et réaliser d'éventuels examens complémentaires, biologiques ou imagerie cérébrale en coupe. Si la situation ne correspond qu'à une menace, sans passage à l'acte vrai, il faut orienter le patient pour une évaluation psychiatrique.

T1 Conclusion

L'approche en préhospitalier d'un patient présentant un état d'urgence psychiatrique ne s'improvise pas. Elle est cadrée par de récentes recommandations [START_REF] Rerbal | Organisation et modalités d'intervention des cellules d'urgence médicopsychologique. Recommandations de la Société française de médecine d'urgence (SFMU) en collaboration avec l'Association de formation et de recherche des cellules d'urgence médicopsychologique -Société française de psychotraumatologie (AFORCUMP-SFP)[END_REF][START_REF] Cesareo | Recommandations de bonne pratique clinique concernant la prise en charge médicale des victimes d'une "tuerie de masse[END_REF]. Elle met en sécurité l'entourage du patient, puis le patient lui-même. Il faut toujours rechercher une adhésion du sujet avec une approche relationnelle pour prendre soin de lui. Ceci facilite grandement le parcours préhospitalier, grâce à la connaissance des éléments ayant amené à cette crise, le repérage et la correction rapide de décompensations somatiques, et permet d'accompagner le patient et ses proches aidants avec douceur, en étant éventuellement soutenu par une médication adaptée.