N Mtiraoui
email: mtiraoui_naima@yahoo.fr

N Dhouibi

J Dhahri

O Toulemonde
email: olivier.toulemonde@icmcb.cnrs.fr

Magnetic and magnetocaloric properties in Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11and 0.133) perovskite manganites

Keywords: Perovskite, Magnetization, Phase transition, Magnetic refrigeration, Modeling and simulation

A systematic investigation of structural, magnetic and magnetocaloric properties for Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) manganites is reported. X-ray powder diffraction (XRD) patterns suggest that our samples crystallize in the orthorhombic perovskite structure with a Pbnm space group. Magnetization temperature dependence under the field-cooled cooling (FCc), field-cooled warming (FCw) and zero-fieldcooling (ZFC) modes exhibit a ferromagnetic(FM)-paramagnetic(PM) phase transition at T C temperature increasing from 120K for x=0.11 to 145K for x=0.133. An analysis using the Banerjee's criterion curves near T C indicates that x=0.11 displays a second-order transition while x=0.133 displays a first order magnetic phase transition. The analysis is further confirmed by the phenomenological universal curves of the normalized entropy variation as a function of rescaled temperature. The maximum magnetic entropy change ( and relative cooling power (RCP) were estimated to 1.68 J /kg K and 89.45 J /kg for x=0.11 and 4.11 J/ kg K and 132.45 J /kg for x=0.133 under 3T, respectively. The experimental approach mixing three different alkaline earth on the A-site perovskite significantly improves the broadening of the magnetic entropy change temperature dependence and then the relative cooling refrigerant capacity. Such approach can be transferred to manganite showing ferromagnetic to paramagnetic phase transition near room temperature for magnetic refrigeration application.

Introduction

Perovskite type manganite with a general formula Ln 1-x A x MnO 3 , (Ln = rare earth element and A = divalent alkaline earth elements) have gained tremendous attention in the scientific community owing to their several fascinating physical properties such as colossal magnetoresistance (CMR), charge and / or orbital ordering state and large magnetocaloric effect (MCE) [START_REF] Dagotto | COLOSSAL MAGNETORESISTANT MATERIALS: THE KEY ROLE OF PHASE SEPARATION[END_REF][START_REF] Siwach | Low field magnetotransport in manganites[END_REF][START_REF] Phan | Review of the magnetocaloric effect in manganite materials[END_REF][START_REF] Debnath | Large magnetic entropy change near room temperature in La0. 7 (Ca0. 27Ag0. 03) MnO3 perovskite[END_REF][START_REF] Tishin | The Magnetocaloric Effect and Its Applications[END_REF]. Therefore, researchers are interested in these materials because of their exceedingly technological applications like magnetic refrigeration, magnetic field sensors, computer memory systems, etc.

Magnetic refrigeration (MR) systems based on the magnetocaloric effect (MCE) in magnetic materials is an auspicious refrigeration technology thanks to its many advantages over conventional gas compression (CGC) and expansion technique such as high or potential energy efficient, environmental friendliness, economic and ecological cleanliness [START_REF] Yu | Review on research of room temperature magnetic refrigeration Recherches sur les systèmes frigorifiques magnétiques à température ambiante : la littérature passée en revue[END_REF][START_REF] Gschneidner | Magnetocaloric Materials[END_REF].

The MCE is an intrinsic property, and results from the strong coupling of magnetic sub-lattices with a magnetic field. It is simply defined as the adiabatic temperature change (∆T ad ) and/or an isothermal change in magnetic entropy (-∆S M ) of a system due to a varying magnetic field. Generally, direct MCE comes into a play in the proximity of a magnetic phase transitions at the Curie temperature [START_REF] Tishin | The Magnetocaloric Effect and Its Applications[END_REF].

There are many materials for magnetic refrigeration application near room temperature.

Gadolinum (Gd) is one of them, it has been considered as the most active material in the magnetic refrigeration technology because it has a large magnetic entropy change ∆S M =10.2J/KgK at T=294K at 5T. However, the expensive price (4000 $/kg), easy oxidation and hard preparation of this rare earth prevents it from practical application [START_REF] Provenzano | Reduction of hysteresis losses in the magnetic refrigerant Gd 5 Ge 2 Si 2 by the addition of iron[END_REF][START_REF] Chennabasappa | A successful process to prevent corrosion of rich Gd-based room temperature magnetocaloric material during ageing[END_REF]. For this reason, it was necessary to find other cheap and safe alternatives with the same efficiency.

Recent studies have revealed that manganites with perovskite structure are attractive candidates for magnetic refrigerant technology owing to their large MCE close to their Curie temperature. The advantages of manganites are their lower cost, simple production, easily tunable transition temperature, extraordinary chemical stability [START_REF] Phan | Large magnetic entropy change above 300 K in a colossal magnetoresistive material La 0[END_REF] and high electrical resistivity, which are beneficial for reducing eddy-current heating [START_REF] Za | Giant magnetic entropy change in manganese perovskite La0. 67Sr0. 16Ca0. 17MnO3 near room temperature[END_REF]. Temperature span as large 5K has even been reported around room temperature for manganites showing different chemical composition [START_REF] Engelbrecht | Exprimetal results for a magnetic refrigerator using three different types of magnetocaloric material regenerators[END_REF][START_REF] Guillou | Development of a new magnetocaloric material used in a magnetic refrigeration device[END_REF].

The parent LaMnO 3 , as a typical compound, is an antiferromagnetic (AFM) insulator.

Its antiferromagnetic behavior is characterized via a super-exchange (SE) interaction between Mn 3+ sites. Partial Substitution of the trivalent rare earth ions (such as La 3+ , Nd 3+ , Pr 3+ etc.) by divalent alkaline earth ions causes the mixed valence states of Mn (Mn 3+ and Mn 4+ ) and can lead a transition from paramagnetic to ferromagnetic and insulator to metal states .

The magnetic properties of these substituted manganites are mostly explained on the basis of double exchange (DE) interactions between Mn 3+ and Mn 4+ ions in the Mn-O-Mn network and Jahn-Teller distorsion [START_REF] Zener | Interaction between the d shells in the transition metals[END_REF][START_REF] Millis | Double Exchange Alone Does Not Explain the Resistivity of La 1-x Sr x MnO 3[END_REF].

In this work, the structural, magnetic and magnetocaloric properties of Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) perovskite manganites are reported keeping in mind that mixing three different alkaline earth on the A-site perovskite improve the broadening of the temperature dependence of the maximum magnetic entropy change and then the relative cooling refrigerant capacity [START_REF] Mtiraoui | Critical behavior and magnetic entropy change in Nd0.7Sr0.1Ba0.1Ca0.1MnO3-δ perovskite[END_REF].

Experimental

The perovskite manganites Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) were prepared through the conventional solid-state reaction route using high-purity Nd 2 O 3 , CaCO 3 , BaCO 3 , SrCO 3 and MnO 2 powders. Homogeneous stoichiometric mixtures were heated in air at 900 °C for 48 hours for decarbonatation. Then these were pelletized at 1100°C for 24 hours. Finally, powders were reground, pressed into pellets form and then sintered at 1400°C during 24 h in air to assure homogenization. The morphology and composition were investigated by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray spectroscopy (EDX) at room temperature.

The phase purity and crystallographic structure of the materials were determined by XRD patterns with CuKα radiation (1.5406 Å) in the 18°≤2≤ 81° range with a step width of 0.02°and a count time of 18s/step. The diffraction patterns were carried out by the Rietveld refinement process using program FULLPROF software [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF]. The magnetization measurement as a function of temperature (field-cooled cooling (FCc), field-cooled warming (FCw) and zero-field cooling) were reported in the range 5-350 K under 0.02T using a superconductive quantum interference device. The isothermal magnetic entropy change of the material were calculated from M(H) isotherms up to 3T at various temperatures in steps of 5 K.

Results and discussions

Microstructure and structural analysis

The chemical composition were investigated by Energy Dispersive X-ray spectroscopy (EDX) technique. For x=0.133, traces of (Ca,Sr,Ba) 2 Mn 2 O 5 (at 2=25.6°and 32.3°) are found in the XRD pattern with the main peak corresponding to unknown phase despite the synthesis under air.

The XRD data was analyzed by using the Rietveld method with the Fullprof program [START_REF] Rietveld | A profile refinement method for nuclear and magnetic structures[END_REF]. The structure is indexed in the orthorhombic structure with a space group Pbnm.

The quality of the Rietveld refinement is assessed via the goodness of the fit factor χ 2 .

The detailed results pertaining to the refinement are gathered in Table 1.

As expected, the lattice parameters as well as the unit cell volume increase with x. This is associated with the larger ion radii of the alkaline earth replacing Nd 3+ at the A-site of the perovskite structure.

As first assumption, the distortion of the crystal structure is governed with Goldschmidt tolerance factor (t G ) [START_REF] Guiner | Theorie et technique de la Radiocristallographie[END_REF] defined by:

) ( 2 O B O A G r r r r t    (1)
Here, to unity [START_REF] Johnsson | Crystallography and Chemistry of Perovskites[END_REF], the lattice structure is defined as ideal cubic perovskite but any deviation from this value upward and downward is the signature of a distorted perovskite structure. In the region t G <0.96, an orthorhombic symmetry is suggested for the compounds, while for 0.96 <t G <1 is a rhombohedral one's that is expected.

The t G values for the present samples are mentioned in Table 1. The calculated tolerance factor t G of our samples are consistent with the XRD patterns and the resulting orthorhombic structure. showed nearly identical features, the M FC (M FCw and M FCc ) and M ZFC curves coincide in the paramagnetic phase above 150K. However as the temperature is reduced further, the curves do not overlap and start splitting and subsequently a sharp rise in both M FC and M ZFC is seen with decrease in temperature, which may be attributed to the existence of magnetic domain nucleation in the compounds [START_REF] Mtiraoui | Critical behavior and magnetic entropy change in Nd0.7Sr0.1Ba0.1Ca0.1MnO3-δ perovskite[END_REF].

Magnetic properties

Transition temperatures (T C ) can be deduced accurately from the minimum of the first derivative magnetization curves (dM/dT) for the ZFC curve versus temperature. They are shown as inset in Fig. 3(a-b) and their values are listed in Plots of 1/χ (T) for x=0.11 and 0.133 compounds, are presented in the inset of Fig. 3a-b. They obeys the Curie-Weiss (CW) law [START_REF] Kittel | Introduction to Solid state Physics[END_REF]:

  CW T C T     ) (
(2), in the PM region (T>T C ), θ CW is the Curie-Weiss temperature representing the molecular mean field between the magnetic moments and C is the Curie constant defined as: The and values are listed in Table 2. Interestingly, our paramagnetic moments are on the same range than those already reported in La 0.7 Ca 0.3 MnO 3 sample [START_REF] Kumar | Evidence for Jahn-Teller polaron formation and spin-cluster-assisted variable-range-hopping conduction in[END_REF] or Nd 0.6 Sr 0.4 MnO 3 [START_REF] Kittel | Introduction to Solid state Physics[END_REF]. Our analysis of the data collected up to 350K do not allow us to extract a close to the ionic theoretical one's suggesting the Mn 3+ -Mn 4+ spin clusters already at such temperature. Finally, the deviation seen in our inverse susceptibility for sample x=0.133 around 320K may likely due to the traces of (Ca,Sr,Ba) 2 Mn 2 O 5+d even if air atmosphere was used during the synthesis process.

However, the paramagnetic behavior observed above 320K still deviate from the ionic one's.

3.3.Magneto-caloric effect

To investigate the MCE properties, magnetization isotherms were measured around T C at temperature intervals of 5K. Fig. 4 shows the magnetization isotherms (M-H) of the samples Nd [START_REF] Ho Thi | Influence of Sr Doping on Magnetic and Magnetocaloric Properties of Nd0.6Sr0.4MnO3[END_REF], the S-shape near the Curie temperature has never been reported yet. Above T C , M-H curves for x=0.11 exhibits an almost linear behavior, which is typical for paramagnetic materials.

For deeper understanding the type of the magnetic phase transition for the Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) compounds, the Arrot plots H/M versus M 2 based on the M-H data are constructed (Fig. 5).

According to the Banerjee's criterion [START_REF] Banarjee | On a generalized approach to first and second order magnetic transitions[END_REF], the nature of magnetic transition can be determined from the slope of the straight line in the Arrott plots (H/M-M 2 ), a negative or positive slope of these plots respectively implies a first-order or second-order transition.

For compounds under investigation, it can be seen Fig. 5(a) for x=0.11, that all the Arrot curves H/M versus M 2 clearly exhibit positive slope in a wide temperature range revealing that this transition is a second order phase transition (SOPT). However, as seen in Fig. 5(b) for x=0.133 sample, the slopes of some H/M vs M 2 plots at high temperatures (above T c ) are negative, the magnetic transition is then first order (FOPT). The first order nature of the transition is further supported by the narrowing of the first derivative magnetization curve (dM/dT) versus temperature in comparison with the other sample as shown in insets in Fig. 3(a-b). Similar behavior have already been reported in other manganites [START_REF] Zhang | Effect of Li doping on the magnetic and magnetocaloric properties of Pr 0.5 Sr 0.5-x Li x MnO 3 (0≤x≤0.3)[END_REF][START_REF] Foldeaki | A powerful tool in magnetic refrigerator design[END_REF].

The isothermal magnetic entropy change, (∆S M ), is an important parameter to characterize the (MCE). From the magnetization isotherms M (H, T) reported in Fig 4, we can estimate the MCE from the entropy change.

On the basis of the thermodynamic theory, the isothermal-field induced magnetic entropy change (∆S M ) from 0 to H max can be calculated through the formula [START_REF] Wang | Magnetocaloric effect in perovskite manganites La 0.7-x Nd x Ca 0.3 MnO 3 and La 0.7 Ca 0.3 MnO 3[END_REF]:

The relationship between entropy and magnetization can be obtained using the following Maxwell equation:

From Eq [START_REF] Gschneidner | Magnetocaloric Materials[END_REF] and ( 8) one can get:

where H max is the maximum external field. 3 and plotted as a function of H (see Fig 7). It is easy to see that is likely proportional to magnetic field.

For H=3 T, the maximum values of (-∆S M ) are found to vary between 1.68 and 4.1 J/kg K for x=0.11and 0.133, respectively, A comparison of our extracted values with others reported for neodymium bases perovskites manganites, are shown in Table 3.

In general, the large magnetic entropy change in manganites mainly originate from the double exchange interaction between Mn 3+ and Mn 4+ ions [START_REF] Guo | Large Magnetic Entropy Change in Perovskite-Type Manganese Oxides[END_REF]. Furthermore, the spin-lattice coupling in the magnetic ordering process also plays an important role [START_REF] Radaelli | Simultaneous Structural, Magnetic, and Electronic Transitionsin in La 1-x Ca x MnO 3 with x=0.25 and 0.50[END_REF].

Due to strong coupling between spin and lattice, significant lattice change accompanying the magnetic transition in perovskite manganites has been observed [START_REF] Toulemonde | Changes in the Jahn-Teller distortion at the metal-insulator transition in CMR manganites[END_REF]. Consequently, the abrupt change in magnetization near T C for the x= 0.133 compound would be due to a more significant change of the electron/phonon coupling at the phase transition with respect to the one's for the x=0.11 compounds [START_REF] Mira | Drop of magnetocaloric effect related to the change from first-to second-order magnetic phase transition in La 2/3 (Ca 1x Sr x ) 1/3 MnO 3[END_REF]. It would resulting in the larger magnetic entropy change as seen in La2/3(Ca1-xSrx)1/3MnO3 [START_REF] Alvarez-Alonso | On the broadening of the magnetic entropy change due to Curie temperature distribution[END_REF].

The relative cooling power (RCP) is related to the refrigeration efficiency of a magnetocaloric material. It is estimated using the following equation: [START_REF] Chennabasappa | A successful process to prevent corrosion of rich Gd-based room temperature magnetocaloric material during ageing[END_REF] where is the full width at half maximum of the ∆S M peak assuming a gaussian shape. [START_REF] Fan | Magnetocaloric effect of half-doped manganite Nd 0.5 Ca 0.25 Sr 0.25 MnO 3[END_REF]. However, in our compounds, it is most likely the alkaline earth statistic distribution on the perovskite A-site at the origin of the Curie temperature distribution.

To deeply understand the origin of the MCE near the transition temperature, we have modeled MCE's experimental data in the frame of Landau theory taking into account magnetoelastic couplings and electronic interactions. The Gibbs free energy of the system can be written as:

A and B are the temperature dependent of Landau coefficients. For energy minimization ( , the magnetic equation of state is transformed as:

Landau's coefficients A and B were determined from the linear fitting region of the Arrot plots (Fig. 5).

The corresponding magnetic entropy change (∆S M ) is determined from differentiation of the magnetic part of the free energy: [START_REF] Engelbrecht | Exprimetal results for a magnetic refrigerator using three different types of magnetocaloric material regenerators[END_REF] where and are the temperature derivatives of the Landau coefficients.

  Fig. 1a-b show the EDX spectra for the Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) samples. These spectra show peaks corresponding to the elements of Nd, Sr, Ba, Ca, and Mn. The estimated atomic percentages are shown in the inset of Fig.1. It is a semi quantitative analysis and the given atomic % are consistent with the targeted chemical composition mixing the same content of Ca, Sr and Ba. The shape and grain size of the powder samples Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) were studied by Scanning Electron Microscopy (SEM) as presented in Fig. 1. The grain size was estimated from the SEM micrographs by considering as many particles as possible. The first observation shows that both samples are homogenous with average grains size ranging from 1.44 to 1.53 μm for x=0.11 and 0.133, respectively. Structural analysis of the Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) samples was carried out by powder X-ray diffraction. Fig.2(a) displays the XRD patterns collected for x=0.11 compound at room temperature whereas the one's for x=0.133 is shown in supplementary Information Figure 2(b)). In spite of the very small peaks associated with the phases of non-magnetic secondary alkaline earth metal carbonates (principal peak around 2=31°) because of CO 2 adsorption, the main phase is single for x= 0.11.

  ionic radius of the A and B cations and oxygen anion, respectively which values are given by Shannon[START_REF] Shannon | Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides[END_REF]. If t G is equal

Fig. 3

 3 Fig.3 represents temperature dependencies of magnetization in three various modes zero-field-cooled (Z FC ), field cooled cooling (FCc) and field cooled warming (FCw)

9 B

 9 =1.38016 ×10 -16 ergK -1 is Boltzmann constant, N A =6.023 ×10 23 mol -1 is Avogadro number, B  =9.274×10 -21 emu is Bohr magneton and the effective moment expressed in Bohr magneton. The Curie constant C and θ CW can be determined from a linear fit to paramagnetic phase of χ -1 (T) under ZFC modes. However, our inverse magnetic susceptibility can only be fitted for small temperature segments whose experimental paramagnetic moments continuously increase. Two arbitrary temperature ranges where choose to illustrate that phenomenon. As seen table 2, the positive values of θ CW , higher than T C indicates the presence of short range ferromagnetic interactions between the neighboring atomic spins. It illustrates magnetic inhomogeneity above T C . Furthermore, based on the obtained C value, the experimental paramagnetic moments (in unit of Bohr magneton μ B ) can be determined with the following equation: compared with, the theoretical effective moment μ cal of Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) calculated by the relation: (5) The theoretical values for the ionic magnetic moments in the spin only hypothesis for the manganese ions are μ eff (Nd 3+ )=3.62 B  , μ eff (Mn 3+ )=4.and μ eff ( Mn 4+ )= 3.8 B  .

Fig. 6

 6 Fig. 6 shows the temperature dependence of magnetic entropy change (-∆S M ) calculated under different applied magnetics fields change from 1 to 3T for Nd 1- 3x Ba x Ca x Sr x MnO 3 (x=0.11and 0.133) samples. These two compounds show similar behavior, i.e. the (-∆S M ) value reaches a maximum near the transition temperature (T C ) at low applied fields and it increases with increasing H which may be due to the improvement of FM interactions. Also as the alkaline earth content increases, maximum entropy change increase (see Table3). The calculated values of are given in Table3and plotted as a function of H (seeFig 7). It is

Fig. 8

 8 Fig.8 shows an almost linear behavior of the RCP data of both compounds (x=0.11 and 0.133) with respect to the applied magnetic field. This indicates that RCP is powerful field dependent. The obtained values of , T C and RCP at 3T magnetic field for Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) are summarized in Table 3 allowing a simple comparison with other reported neodymium based perovskite manganites. Mixing different alkaline earth on the A-site perovskite improves the broadening of the temperature dependence of the maximum magnetic entropy change and then the relative cooling refrigerant capacity. Such broadening due to Curie temperature distribution has already be pointed out in relation with microstructure change controlled in purpose in the intermetallic Pr 2 Fe 17[START_REF] Fan | Magnetocaloric effect of half-doped manganite Nd 0.5 Ca 0.25 Sr 0.25 MnO 3[END_REF]. However, in our

Fig. 9 shows

 9 Fig. 9 shows Landau magnetic entropy change (-∆S M ) at H=3T in comparison with the experimental results. We find a good agreement between the two approaches based on the Maxwell relation (dots) and Landau relation (lines).This agreement further support the impact of the magnetoelastic couplings and electron interactions on the magnetocaloric properties of the Nd 1-3x Ba x Ca x Sr x MnO 3 (x=0.11and 0.133) compounds.Finally, Franco and co-workers[START_REF] Franco | Scaling laws for the magnetocaloric effect in second order phase transitions: from physics to application for the characterization of materials[END_REF][40] have suggested the phenomenological universal curve for the temperature dependence ΔS M (T) as more effective criterion to discriminate the order of phase transition than the Banerjee's one's.The universal curve is estimated by normalizing all (-ΔS M ) curves with respect to their maxima, ΔS ' = ΔS M / ; and by rescaling the temperature axis by defining another variable θ as following:

Fig. 10

 10 Fig. 10 depicts the dependence of the entropy change ∆S ' versus θ for the two studied compounds measured at different applied fields. As shown in Fig. 10(a) all the experimental points collapse on to one universal curve in the all temperature interval indicating the validity of using the universal curve for x=0.11 with successive second order phase transition (SOPT). But, for x=0.133 sample (Fig.10 (b)), curves do not collapse onto a single one's and their divergence suggest the occurrence of a first order phase transition (FOPT), which is consistent with the Arrott plots and the sharp magnetization first derivative.

Table 2

 2

	. T C slightly

3 compounds

[START_REF] Chanda | Current-Driven Spin Precession in Bulk Nd0.6Sr0.4MnO3[END_REF]

.

  1-3x Ba x Ca x Sr x MnO 3 (x=0.11 and 0.133) around T C in the magnetic field range. Below Curie temperature, the (M-H) curves of both samples rises rapidly at low magnetic fields and afterwards tended to saturate at high fields, which confirms the ferromagnetic behavior. Interestingly, if the magnetization saturation found for x= 0.133 is in the range of those already reported for Nd 0.6 Sr 0.4 MnO 3

Summary

In summary, the structural, magnetic and magnetocaloric properties of


Geeth Manthilake
email: geeth.manthilake@uca.fr

Julien Chantel
email: julien.chantel@univ-lille.fr

Nicolas Guignot
email: nicolas.guignot@synchrotron-soleil.fr

Andrew King
email: king@synchrotron-soleil.fr

The Anomalous Seismic Behavior of Aqueous Fluids Released during Dehydration of Chlorite in Subduction Zones

Keywords: elastic wave velocity, subduction zone, dehydration, chlorite, seismic anomalies, high pressure

Dehydration and fluid circulation are integral parts of subduction tectonics that govern the dynamics of the wedge mantle. The knowledge of the elastic behavior of aqueous fluid is crucial to understand the fluid-rock interactions in the mantle through velocity profiles. In this study, we investigated the elastic wave velocities of chlorite at high pressure beyond its dehydrating temperature, simulating the progressive dehydration of hydrous minerals in subduction zones. The dehydration resulted in an 8% increase in compressional (Vp) and a 5% decrease in shear wave (Vs) velocities at 950 K. The increase in Vp can be attributed to the stiffening of the sample due to the formation of secondary mineral phases followed by the dehydration of chlorite. The fluid-bearing samples exhibited Vp/Vs of 2.45 at 950 K. These seismic parameters are notably different from the major mantle minerals or hydrous silicate melts and provide unique seismic criteria for detecting mantle fluids through seismic tomography.

Introduction

The dehydration of subducted hydrous minerals releases a flux of fluid into the wedge mantle [START_REF] Schmidt | Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation[END_REF]. The aqueous fluid entering the overlying mantle influences geochemical and geophysical processes, such as arc melting [START_REF] Hirschmann | Mantle solidus: Experimental constraints and the effects of peridotite composition[END_REF], mantle metasomatism [START_REF] Ishimaru | Melting and multi-stage metasomatism in the mantle wedge beneath a frontal arc inferred from highly depleted peridotite xenoliths from the avacha volcano, Southern Kamchatka[END_REF][START_REF] Scambelluri | The fate of B, Cl and Li in the subducted oceanic mantle and in the antigorite breakdown fluids[END_REF], intermediatedepth seismicity [START_REF] Hacker | Subduction factory 2. Are intermediate-depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?[END_REF][START_REF] Zhao | Influence of fluids and magma on earthquakes: Seismological evidence[END_REF], and the dynamics of the wedge-mantle flow [START_REF] Arcay | Numerical simulations of subduction zones. Effect of slab dehydration on the mantle wedge dynamics[END_REF]. The determination of the physical state of slab fluid can constrain the thermal state of the slab as melting, and dehydration reactions occur at different temperatures. Detection and differentiation of aqueous fluid and silicate melt and how they migrate and interact with the overlying mantle are therefore fundamental to the understanding of the subduction zone system [START_REF] Zhao | Seismological structure of subduction zones and its implications for arc magmatism and dynamics[END_REF].

Seismic velocities, anelastic attenuation, and seismic anisotropy are the key parameters that provide information on physical and chemical interactions between the slab and the mantle [START_REF] Zhao | Seismological structure of subduction zones and its implications for arc magmatism and dynamics[END_REF]. The low velocity and high attenuation (low-Q) in subduction zones have often been interpreted as an indication of the presence of a liquid phase [START_REF] Zhao | Seismological structure of subduction zones and its implications for arc magmatism and dynamics[END_REF]. This interpretation comes from the notion that the shearing motion cannot be transmitted through a liquid, as it may slow down the compressional (P) waves and hinder the propagation of shear (S) waves and attenuate [START_REF] Birch | Interpretations of the low-velocity zone[END_REF][START_REF] Spetzler | The Effect of Temperature and Partial Melting on Velocity and Attenuation in a Simple Binary System[END_REF][START_REF] Anderson | Partial melting in the upper mantle[END_REF]. The attenuation is often described by the seismic attenuation factor (Q -1 ), which depends on the temperature, composition, fluid/melt fraction, fluid/melt geometry, and grain size [START_REF] Faul | Shear wave attenuation and dispersion in melt-bearing olivine polycrystals: 2. Microstructural interpretation and seismological implications[END_REF][START_REF] Karato | Defect microdynamics in minerals and solid-state mechanisms of seismic wave attenuation and velocity dispersion in the mantle[END_REF], and provides robust constraints on subduction zone components [START_REF] Jang | Seismic attenuation structure of southern Peruvian subduction system[END_REF].

The seismic structures of most subduction systems have been well documented, and the high-resolution seismic tomography images [START_REF] Jang | Seismic attenuation structure of southern Peruvian subduction system[END_REF], receiver function method [START_REF] Kawakatsu | Seismic Evidence for Deep-Water Transportation in the Mantle[END_REF],

Minerals 2021, 11, 70 2 of 11 and seismic refraction methods allow the detection of seismic anomalies with great accuracy [START_REF] Allam | Ten kilometer vertical Moho offset and shallow velocity contrast along the Denali fault zone from double-difference tomography, receiver functions, and fault zone head waves[END_REF]. The prior knowledge of the elastic properties of major mantle mineral phases provides the basic framework for the interpretation of seismic parameters [START_REF] Anderson | Measurement of elastic constants of mantle-related minerals at temperatures up to 1800[END_REF] obtained through inversion methods. The available criteria allow the identification of solid from liquid phases [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Freitas | Experimental evidence supporting a global melt layer at the base of the Earth's upper mantle[END_REF][START_REF] Freitas | Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: Effect of evolving melt texture[END_REF][START_REF] Weidner | Ultrasonic Acoustic Velocities During Partial Melting of a Mantle Peridotite KLB-1[END_REF][START_REF] Fomin | Water, hydrous melting, and teleseismic signature of the mantle transition zone[END_REF] or distinguish hydrous minerals from nominally anhydrous minerals [START_REF] Kawakatsu | Seismic Evidence for Deep-Water Transportation in the Mantle[END_REF][START_REF] Fomin | Water, hydrous melting, and teleseismic signature of the mantle transition zone[END_REF][START_REF] Wang | Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges[END_REF][START_REF] Nakajima | Seismic evidence for thermally-controlled dehydration reaction in subducting oceanic crust[END_REF]. However, there are no robust seismic parameters to identify aqueous fluids. At present, the interpretation of the presence of fluid is based on indirect observations, mainly through comparison of the seismic anomaly with the depth of dehydration, temperature profile, and proximity to the volcanic activity [START_REF] Kawakatsu | Seismic Evidence for Deep-Water Transportation in the Mantle[END_REF][START_REF] Audet | Seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing[END_REF][START_REF] Hyndman | Serpentinization of the forearc mantle[END_REF][START_REF] Bostock | An inverted continental moho and serpentinization of the forearc mantle[END_REF]. Despite the importance of seismic properties of fluid-bearing systems that have long been identified [START_REF] Kawakatsu | Seismic Evidence for Deep-Water Transportation in the Mantle[END_REF][START_REF] Nakajima | Seismic evidence for thermally-controlled dehydration reaction in subducting oceanic crust[END_REF][START_REF] Saffer | Hydrogeology and mechanics of subduction zone forearcs: Fluid flow and pore pressure[END_REF], the elastic velocity data on aqueous fluids have not been determined experimentally, partly due to the challenging nature of investigating fluid-bearing systems at high pressure. With the notable absence of seismic parameters of aqueous fluids, the interpretation of seismic anomalies in subduction zones remains an uncertain exercise.

The present study investigated the elastic properties of the dehydration-induced fluid occurring in subduction zones. Using high-pressure ultrasonic measurements, we have determined elastic wave velocities of chlorite and dehydrating fluids at 1.8 and 3.8 GPa with increasing temperatures, simulating the progressive dehydration in the subducting slab. Chlorite (Mg, Fe) 5 Al(Si 3 Al)O 10 (OH) 8 , is one of the major hydrous phases stable in subducting ultramafic, mafic, and sediment lithologies [START_REF] Schmidt | Experimentally based water budgets for dehydrating slabs and consequences for arc magma generation[END_REF][START_REF] Tatsumi | Migration of fluid phases and genesis of basalt magmas in subduction zones[END_REF][START_REF] Grove | The Role of H2O in Subduction Zone Magmatism[END_REF][START_REF] Poli | Petrology of Subducted Slabs[END_REF]. The presence of chlorite in both the subducting slab and the hydrated mantle wedge presents a unique opportunity to investigate diverse dehydration scenarios in subduction zones.

Materials and Methods

Sample Characterization

In this study, we measured the elastic wave velocities of natural chlorite with (Mg 3.77 Fe 1.23 )Al (Si 3 Al)O 10 (OH) 8 stoichiometry at conditions representative to the mantle wedge and subduction zone settings. The natural chlorite sample used in this study was from the Ambatomainty region in Madagascar. The chemical composition of chlorite was determined by electron probe microanalysis (EPMA) using a Cameca SxFiveTactis electron microprobe, operating at an accelerating voltage of 15 kV and a beam current of 20 nA. The samples were further characterized by powder X-ray diffraction (XRD). After each experiment, cross-sections of run products were investigated using energy-dispersive X-ray spectroscopy (EDS) chemical mapping, using a Jeol JSM-5910 LV scanning electron microscope.

High-Pressure, High-Temperature Experiments

High-pressure and high-temperature conditions were generated using the 1200-ton DIA-type multi-anvil module at the PSICHE beamline in SOLEIL Synchrotron in France.

For experiments conducted at 1.8 and 3.8 GPa, we used an octahedral pressure medium composed of MgO + Cr 2 O 3 in a 10/4 multi-anvil configuration (Figure 1a). Cylindrical core samples of 1.2 mm diameter and 0.5 mm in length were placed in a hexagonal boron nitride (hBN) capsule, which electrically insulated the sample from the furnace during the measurements. The high-purity hBN, sintered at high temperature and pressure without a boron oxide (B 2 O 3 ) binder (BNHP-FINAL Advanced Materials), ensures that there were no B 2 O 3 forming reactions with the aqueous fluids. Two Ni disks (10 µm) placed at the top and bottom of the sample served as metal markers for X-ray radiography images, which were used for the measurement of the sample lengths using the radiography images (Figure 1b). The presence of Ni was expected to maintain the oxygen fugacity of the sample close to the Ni-NiO buffer. A W 95 Re 5 -W 74 Re 26 thermocouple junction was placed at one side of the sample, which monitored the temperature. A dense Al 2 O 3 buffer rod was placed between one of the tungsten carbide (WC) anvil truncations and the sample, to enhance the propagation of elastic waves and to provide sufficient impedance contrast to reflect ultrasonic waves at the buffer rod-sample interface. We placed a 0.3 mm thick alumina disc on the opposite end of the sample to maintain the sample geometry (Figure 1a). Both ends of the anvil, the alumina buffer rod, and the samples were mirror-polished to enhance mechanical contacts. All ceramic parts of the cell assembly, including the pressure medium, were fired at 1273 K prior to their assemblage, to remove any adsorbed moisture, and were kept at 400 K in a high-vacuum furnace (10 -2 Torr) before assembling the experiment.

between one of the tungsten carbide (WC) anvil truncations and the sample, to enhance the propagation of elastic waves and to provide sufficient impedance contrast to reflect ultrasonic waves at the buffer rod-sample interface. We placed a 0.3 mm thick alumina disc on the opposite end of the sample to maintain the sample geometry (Figure 1a). Both ends of the anvil, the alumina buffer rod, and the samples were mirror-polished to enhance mechanical contacts. All ceramic parts of the cell assembly, including the pressure medium, were fired at 1273 K prior to their assemblage, to remove any adsorbed moisture, and were kept at 400 K in a high-vacuum furnace (10 -2 Torr) before assembling the experiment.

Seismic Wave Velocity Measurements

Sound wave velocities of the samples were measured using the ultrasonic interferometry technique [START_REF] Li | Modern techniques in measuring elasticity of Earth materials at high pressure and high temperature using ultrasonic interferometry in conjunction with synchrotron X-radiation in multi-anvil apparatus[END_REF]. In this method, electrical signals of sine waves of 20-50 MHz (3-5 cycles) with Vpeak-to-peak of 5 V were generated by an arbitrary waveform generator (Tektronix AFG3101C); they were then converted to primary (Vp) and secondary (VS) waves by a 10° Y-cut LiNbO3 piezoelectric transducer attached to the mirror-polished truncated corner of a WC anvil. The resonant frequency of the transducer is 50 MHz for compressional waves (P-waves) and 30 MHz for shear waves (S-waves). Elastic waves propagated through the tungsten carbide (WC) anvil, alumina buffer rod (BR), and the sample, and were reflected at the interfaces between the anvil-BR, the BR-sample, and the sample electrode (Figure 1c,d). The reflected elastic echo waves were converted back to electrical signals by the transducer and captured by a Tektronix DPO 5140 Digital Phosphor Oscilloscope at a rate of 5 × 10 9 sample/s. Signals at 20, 30, 40, and 50 MHz were recorded at each temperature step. The two-way travel time for the sound waves propagating through the sample was determined by the time difference between the arrivals of the echoes from the BR-sample interface and the sample-backing material interface by the pulse-echo overlap method [START_REF] Li | Modern techniques in measuring elasticity of Earth materials at high pressure and high temperature using ultrasonic interferometry in conjunction with synchrotron X-radiation in multi-anvil apparatus[END_REF] Sample lengths at each temperature were determined using the X-ray radi-

Seismic Wave Velocity Measurements

Sound wave velocities of the samples were measured using the ultrasonic interferometry technique [START_REF] Li | Modern techniques in measuring elasticity of Earth materials at high pressure and high temperature using ultrasonic interferometry in conjunction with synchrotron X-radiation in multi-anvil apparatus[END_REF]. In this method, electrical signals of sine waves of 20-50 MHz (3-5 cycles) with V peak-to-peak of 5 V were generated by an arbitrary waveform generator (Tektronix AFG3101C); they were then converted to primary (Vp) and secondary (V S ) waves by a 10 • Y-cut LiNbO 3 piezoelectric transducer attached to the mirror-polished truncated corner of a WC anvil. The resonant frequency of the transducer is 50 MHz for compressional waves (P-waves) and 30 MHz for shear waves (S-waves). Elastic waves propagated through the tungsten carbide (WC) anvil, alumina buffer rod (BR), and the sample, and were reflected at the interfaces between the anvil-BR, the BR-sample, and the sample electrode (Figure 1c,d). The reflected elastic echo waves were converted back to electrical signals by the transducer and captured by a Tektronix DPO 5140 Digital Phosphor Oscilloscope at a rate of 5 × 10 9 sample/s. Signals at 20, 30, 40, and 50 MHz were recorded at each temperature step. The two-way travel time for the sound waves propagating through the sample was determined by the time difference between the arrivals of the echoes from the BR-sample interface and the sample-backing material interface by the pulse-echo overlap method [START_REF] Li | Modern techniques in measuring elasticity of Earth materials at high pressure and high temperature using ultrasonic interferometry in conjunction with synchrotron X-radiation in multi-anvil apparatus[END_REF] Sample lengths at each temperature were determined using the X-ray radiography images (Figure 1b). Experimental measurements of velocity are subject to uncertainties originating from the estimation of temperature, pressure, sample dimensions, and fitting errors. Errors have been estimated to be 2.0% (2σ).

Synchrotron Experiments

Energy-dispersive X-ray diffraction was used to determine the sample pressure and verify the sample state. Diffraction was performed in an energy-dispersive mode using a CAESAR-type diffraction detector system [START_REF] Wang | A new technique for angle-dispersive powder diffraction using an energy-dispersive setup and synchrotron radiation[END_REF]. The characteristics of the beam-line were discussed in previous studies [START_REF] Manthilake | Thermal conductivity of FeS and its implications for Mercury's long sustaining magnetic field[END_REF][START_REF] Xie | Formation of bridgmanite-enriched layer at the top lower-mantle during magma ocean solidificatio[END_REF][START_REF] Andrault | Deep and persistent melt layer in the Archaean mantle[END_REF]. Simultaneously, we acquired radiographic images for the determination of the sample lengths, yielding accurate quantitative determination of elastic velocity at each temperature (Figure 1b). The focused beam configuration (in vertical) allowed each image to be assembled by scanning the press in front of the beam. This had the advantage of producing almost flat-field corrected images, a feature usually not available when using multi-anvil modules.

Results

The ultrasound wave velocities of polycrystalline aggregates of natural chlorite were obtained in situ at pressures of 1.8 and 3.8 GPa and temperatures up to 1200 K (Figure 2). The increase in temperature incurred a gradual decrease in both Vp and vs. in chlorite, consistent with the softening of bulk and shear moduli with temperature. However, temperature appeared to have a more pronounced effect on Vp than it had on Vs. Compared to major mantle minerals, the Vp and vs. of chlorite appeared to be significantly lower, 5 km/s and 2.5 km/s respectively, at 900 K. We observed that the increase in pressure had a negligible effect on both compressional and shear wave velocities.

Discussion

Elastic Wave Velocities

The measured elastic wave velocities of Fe-bearing chlorite The dehydration of chlorite exhibited a unique velocity behavior which resulted in an increase in Vp and decrease in Vs. Upon dehydration, it was expected that both Vp and vs. would decrease at the dehydration and subsequent release of fluid, following the characteristic elastic wave velocity behavior for liquids [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Freitas | Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: Effect of evolving melt texture[END_REF][START_REF] Li | Effect of dynamic melting on acoustic velocities in a partially molten peridotite[END_REF]. The increase in temperature beyond 900 K prompted a marked change of velocities, as Vp increased from 5.1 to 5.5 km/s, and vs. decreased from 2.37 to 2.23 km/s at 950 K. The change in velocities corresponded to an 8% increase in Vp and a 5% decrease in Vs, relative to the normalized velocities in chlorite prior to dehydration (Figure 2b).

Analyses of recovered experimental run products after sound wave velocity measurements confirmed the partial dehydration of chlorite at high temperatures, which produced olivine, pyrope-garnet, hercynite spinel (Fe 2 (Mg, Al)O 4 ) at 1.8 GPa, and the stability of olivine and Mg-sursassit at 3.8 GPa was consistent with previous studies [START_REF] Cai | High-pressure and high-temperature stability of chlorite and 23-Å phase in the natural chlorite and synthetic MASH system[END_REF][START_REF] Fumagalli | The high-pressure stability of chlorite and other hydrates in subduction mélanges: Experiments in the system Cr2O3-MgO-Al2O3-SiO2-H2O. Contrib[END_REF] (Figure 3). We observed a higher proportion of Mg-sursassite at 3.8 GPa, which indicated partial dehydration of chlorite and stability of an intermediate hydrous phase [START_REF] Cai | High-pressure and high-temperature stability of chlorite and 23-Å phase in the natural chlorite and synthetic MASH system[END_REF]. The partially dehydrated chlorite retained the relict preferred orientation perpendicular to the compression axis.

Discussion

Elastic Wave Velocities

The measured elastic wave velocities of Fe-bearing chlorite (Mg3.77Fe1.23)Al(Si3Al)O10(OH)8 in this study indicated a ~20% decrease in Vp and ~38% decrease in Vs. relative to the ambient temperature velocities of Mg-endmember chlorite Mg5Al(Si3Al)O10(OH)8 obtained using static density functional theory (DFT) [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF]. The decrease in Vp and vs. with the incorporation of Fe in chlorite was in agreement with the effect that Fe had on elastic wave velocities of mantle minerals [START_REF] Fan | Elasticity of single-crystal Fe-enriched diopside at high-pressure conditions: Implications for the origin of upper mantle low-velocity zones[END_REF][START_REF] Chung | Effects of Iron/Magnesium Ratio on P-and S-Wave Velocities in olivine[END_REF][START_REF] Núñez-Valdez | Thermal elasticity of (Fex,Mg1-x) 2 SiO 4 olivine and wadsleyite[END_REF][START_REF] Tennakoon | Single crystal elasticity of natural topaz at high-temperatures[END_REF][START_REF] Speziale | Compositional Dependence of the Elastic Wave Velocities of Mantle Minerals: Implications for Seismic Properties of Mantle Rocks. Earth's Deep Mantle[END_REF]. The incorporation of Fe resulted in an increase in density from 2.60 gcm -3 of Fe-free verity [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF] to 2.761 gcm -3 in our sample, which represented a 6% increase in density at ambient conditions. While the incorporation of Fe affected both elastic moduli and the density, it has been shown that the increase in density due to the increase in Fe contents was responsible for the observed low velocities in Fe-bearing minerals compared to their Fe-free varieties [START_REF] Dorfman | Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle[END_REF].

The unique increase in Vp at the dehydration of chlorite reflected important mineralogical changes that occurred in the sample. Unlike melting, in which mineral phases are

Discussion

Elastic Wave Velocities

The measured elastic wave velocities of Fe-bearing chlorite (Mg 3.77 Fe 1.23 )Al(Si 3 Al)O 10 (OH) 8 in this study indicated a ~20% decrease in Vp and ~38% decrease in Vs. relative to the ambient temperature velocities of Mg-endmember chlorite Mg 5 Al(Si 3 Al)O 10 (OH) 8 obtained using static density functional theory (DFT) [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF]. The decrease in Vp and vs. with the incorporation of Fe in chlorite was in agreement with the effect that Fe had on elastic wave velocities of mantle minerals [START_REF] Fan | Elasticity of single-crystal Fe-enriched diopside at high-pressure conditions: Implications for the origin of upper mantle low-velocity zones[END_REF][START_REF] Chung | Effects of Iron/Magnesium Ratio on P-and S-Wave Velocities in olivine[END_REF][START_REF] Núñez-Valdez | Thermal elasticity of (Fex,Mg1-x) 2 SiO 4 olivine and wadsleyite[END_REF][START_REF] Tennakoon | Single crystal elasticity of natural topaz at high-temperatures[END_REF][START_REF] Speziale | Compositional Dependence of the Elastic Wave Velocities of Mantle Minerals: Implications for Seismic Properties of Mantle Rocks. Earth's Deep Mantle[END_REF]. The incorporation of Fe resulted in an increase in density from 2.60 gcm -3 of Fe-free verity [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF] to 2.761 gcm -3 in our sample, which represented a 6% increase in density at ambient conditions. While the incorporation of Fe affected both elastic moduli and the density, it has been shown that the increase in density due to the increase in Fe contents was responsible for the observed low velocities in Fe-bearing minerals compared to their Fe-free varieties [START_REF] Dorfman | Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle[END_REF].

The unique increase in Vp at the dehydration of chlorite reflected important mineralogical changes that occurred in the sample. Unlike melting, in which mineral phases are consumed to produce melt, dehydration stabilizes a suite of solid minerals together with the fluid phase. We attributed the unique increase in Vp during dehydration of chlorite to the increase in the stiffness of the sample due to the crystallization of secondary mineral phases, which affected the compressibility, compensating for the velocity decrease by the fluid phase. The increase in Vp can be explained by dehydration of only 10 vol.% of chlorite (Figure 4). Further dehydration increased the volume fraction of fluid in the sample, gradually masking the effect of nominally anhydrous minerals, garnet, olivine, spinel, as seen by the decrease in Vp with increasing temperature (Figure 4).

the increase in the stiffness of the sample due to the crystallization of secondary mineral phases, which affected the compressibility, compensating for the velocity decrease by the fluid phase. The increase in Vp can be explained by dehydration of only 10 vol.% of chlorite (Figure 4). Further dehydration increased the volume fraction of fluid in the sample, gradually masking the effect of nominally anhydrous minerals, garnet, olivine, spinel, as seen by the decrease in Vp with increasing temperature (Figure 4).

The Velocity Ratio (Vp/Vs) in Chlorite and Dehydrating Fluids

The velocity ratio Vp/Vs is frequently used to interpret composition, structure, and dynamic properties in the Earth's interior [START_REF] Afonso | On the Vp/Vs-Mg# correlation in mantle peridotites: Implications for the identification of thermal and compositional anomalies in the upper mantle[END_REF][START_REF] Brantut | Influence of fluids on V P /V S ratio: Increase or decrease?[END_REF][START_REF] Hacker | Subduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite[END_REF]. For major upper mantle minerals, the ratio varies within a narrow range of 1.7-1.8, with garnet showing the highest value around 1.8 [START_REF] Hacker | Subduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite[END_REF]. The velocity ratios of major mantle minerals show a general tendency to increase with increasing pressure and temperature. In this study, we observed a 13% increase in the Vp/Vs ratio, from 2.14 at 900 K to 2.45 at 950 K, as a result of the dehydration of chlorite. After dehydration, the value remained nearly constant, while further increases occurred in temperatures up to 1200 K (Figure 5). The unusually high Vp/Vs ratio of chlorite was consistent with the previous DFT calculations [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF]. The progressive decrease in velocity ratio in chlorite with increasing temperature prior to the dehydration may be

The Velocity Ratio (Vp/Vs) in Chlorite and Dehydrating Fluids

The velocity ratio Vp/Vs is frequently used to interpret composition, structure, and dynamic properties in the Earth's interior [START_REF] Afonso | On the Vp/Vs-Mg# correlation in mantle peridotites: Implications for the identification of thermal and compositional anomalies in the upper mantle[END_REF][START_REF] Brantut | Influence of fluids on V P /V S ratio: Increase or decrease?[END_REF][START_REF] Hacker | Subduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite[END_REF]. For major upper mantle minerals, the ratio varies within a narrow range of 1.7-1.8, with garnet showing the highest value around 1.8 [START_REF] Hacker | Subduction Factory 5: Unusually low Poisson's ratios in subduction zones from elastic anisotropy of peridotite[END_REF]. The velocity ratios of major mantle minerals show a general tendency to increase with increasing pressure and temperature. In this study, we observed a 13% increase in the Vp/Vs ratio, from 2.14 at 900 K to 2.45 at 950 K, as a result of the dehydration of chlorite. After dehydration, the value remained nearly constant, while further increases occurred in temperatures up to 1200 K (Figure 5). The unusually high Vp/Vs ratio of chlorite was consistent with the previous DFT calculations [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF]. The progressive decrease in velocity ratio in chlorite with increasing temperature prior to the dehydration may be linked to the decrease in the principal elastic moduli C33 component, due to the development of preferred orientation in chlorite perpendicular to the principal compression axis. Low elastic wave velocities and a high Vp/Vs ratio have also been reported for antigorite [START_REF] Wang | Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges[END_REF], another principal hydrous mineral in subduction zones.

While the velocity ratio of major mantle minerals varies in a narrow range, the parameter has been considered as a reliable seismic tool to distinguish liquid phases from solid minerals. The velocity ratio has been used to predict the melting in the asthenospherelithosphere boundary, at subduction zones, and at the D" layer at the core-mantle boundary region [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Revenaugh | Seismic Evidence of Partial Melt within a Possibly Ubiquitous Low-Velocity Layer at the Base of the Mantle[END_REF][START_REF] Revenaugh | Seismic evidance for silicate melt atop the 410-km mantle discontinuity[END_REF]. The iron-enriched minerals observed at the core-mantle boundary region appeared to have anomalous seismic velocities, while the velocity ratio has been used to predict chemical interactions between the metal-liquid outer core and the overlying solid mantle [START_REF] Williams | Seismic evidance for partial melt at the base of the Earth's mantle[END_REF].

Minerals 2021, 11, x 7 of 12 linked to the decrease in the principal elastic moduli C33 component, due to the development of preferred orientation in chlorite perpendicular to the principal compression axis. Low elastic wave velocities and a high Vp/Vs ratio have also been reported for antigorite [START_REF] Wang | Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges[END_REF], another principal hydrous mineral in subduction zones. While the velocity ratio of major mantle minerals varies in a narrow range, the parameter has been considered as a reliable seismic tool to distinguish liquid phases from solid minerals. The velocity ratio has been used to predict the melting in the asthenosphere-lithosphere boundary, at subduction zones, and at the D" layer at the core-mantle boundary region [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Revenaugh | Seismic Evidence of Partial Melt within a Possibly Ubiquitous Low-Velocity Layer at the Base of the Mantle[END_REF][START_REF] Revenaugh | Seismic evidance for silicate melt atop the 410-km mantle discontinuity[END_REF]. The iron-enriched minerals observed at the core-mantle boundary region appeared to have anomalous seismic velocities, while the velocity ratio has been used to predict chemical interactions between the metal-liquid outer core and the overlying solid mantle [START_REF] Williams | Seismic evidance for partial melt at the base of the Earth's mantle[END_REF].

In the Earth, melts or aqueous fluids occur in small quantities (a few vol.%), distributed along grain boundaries of the matrix minerals. The presence of a small amount of liquid shuts off the direct travel paths for wave propagations through shearing motion thereby effectively increasing the travel time and decreasing Vs. The compressional wave velocity is decreased due to the absence of the shear component; however, compressional waves could still travel through liquid media. Therefore, a general increase in Vp/Vs is expected for liquid phases due to the large drop in Vs. This tendency has been demonstrated by recent experimental studies [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Freitas | Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: Effect of evolving melt texture[END_REF][START_REF] Weidner | Ultrasonic Acoustic Velocities During Partial Melting of a Mantle Peridotite KLB-1[END_REF], showing a systematic increase in Vp/Vs with increasing melt fraction.

The elastic wave velocity of aqueous fluids has not been investigated prior to this study. In this study, we have shown for the first time that the Vp/Vs ratio of the fluidbearing sample increased sharply upon dehydration from 2.15 at 900 K to 2.45 at 950 K [START_REF] Zhang | Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle[END_REF]. Diamond symbols indicate antigorite [START_REF] Wang | Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges[END_REF] at 1.8 GPa (red) and 3.8 GPa (blue) at 300 K. The blue shaded line indicates the Vp/Vs of basaltic melt [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF] for increasing melt fraction. The faded blue area corresponds to the dehydrating fluid. The red and blue dashed-lines are the second-order polynomial fits through data points.

In the Earth, melts or aqueous fluids occur in small quantities (a few vol.%), distributed along grain boundaries of the matrix minerals. The presence of a small amount of liquid shuts off the direct travel paths for wave propagations through shearing motion thereby effectively increasing the travel time and decreasing Vs. The compressional wave velocity is decreased due to the absence of the shear component; however, compressional waves could still travel through liquid media. Therefore, a general increase in Vp/Vs is expected for liquid phases due to the large drop in Vs. This tendency has been demonstrated by recent experimental studies [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Freitas | Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: Effect of evolving melt texture[END_REF][START_REF] Weidner | Ultrasonic Acoustic Velocities During Partial Melting of a Mantle Peridotite KLB-1[END_REF], showing a systematic increase in Vp/Vs with increasing melt fraction.

The elastic wave velocity of aqueous fluids has not been investigated prior to this study. In this study, we have shown for the first time that the Vp/Vs ratio of the fluidbearing sample increased sharply upon dehydration from 2.15 at 900 K to 2.45 at 950 K (Figure 5). Despite the increase in temperature, the fluid fraction appears to have had a negligible influence on the ratio. A higher Vp/Vs ratio was observed for the hydrous silicate melt [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF][START_REF] Freitas | Simultaneous measurements of electrical conductivity and seismic wave velocity of partially molten geological materials: Effect of evolving melt texture[END_REF]. Nevertheless, the Vp/Vs of hydrous basaltic melt was strongly dependent on the melt fraction, and a Vp/Vs of less than 2 can be expected for the geologically relevant melt fractions in the Earth [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF]. Thus, the Vp/Vs ratio of about 2.4 could be a robust indicator of the presence of fluid in the mantle.

The Effect of Dehydration on the Amplitudes of P-and S-Wave Echoes

The amplitudes of both P-and S-wave echoes exhibited unusual behavior above 900 K, closely following the dehydration temperature of chlorite (Figure 6a,b). For example, the amplitude of the first echo of P-waves at 3.8 GPa was unaffected by the onset of dehydration at 900 K; however, an 8.5% decrease in the amplitude relative to the values normalized to the chlorite before dehydration was observed at around 1050 K. The amplitude of the first echo of P-wave of chlorite at 1.8 GPa, on the other hand, decreased by 50%, then recovered to the initial value and continued to decrease with increasing temperature. Amplitudes of the 1st echo of S-waves of 1.8 and 3.8 GPa samples followed a decreasing trend, with chlorite at 3.8 GPa exhibiting a steep change compared to the measurements at 1.8 GPa.

robust indicator of the presence of fluid in the mantle.

The Effect of Dehydration on the Amplitudes of P-and S-Wave Echoes

The amplitudes of both P-and S-wave echoes exhibited unusual behavior above 900 K, closely following the dehydration temperature of chlorite (Figure 6a,b). For example, the amplitude of the first echo of P-waves at 3.8 GPa was unaffected by the onset of dehydration at 900 K; however, an 8.5% decrease in the amplitude relative to the values normalized to the chlorite before dehydration was observed at around 1050 K. The amplitude of the first echo of P-wave of chlorite at 1.8 GPa, on the other hand, decreased by 50%, then recovered to the initial value and continued to decrease with increasing temperature. Amplitudes of the 1st echo of S-waves of 1.8 and 3.8 GPa samples followed a decreasing trend, with chlorite at 3.8 GPa exhibiting a steep change compared to the measurements at 1.8 GPa. The decrease in wave amplitudes mostly depends on the reflectance (and transmittance) of waves at the interfaces between sample and alumina. Reflectance and transmittance of waves at interface depend on the contrast of acoustic impedance. The sharp decrease in the amplitude of P-and S-waves can be correlated to the release of aqueous fluid into the sample, which may change the acoustic impedance of the fluid-bearing sample. The unique increase in amplitude after the initial drop may be related to the growth of olivine, garnet, and spinel crystals in the sample, which may increase the bulk density of the sample relative to chlorite (Figure 6a). However, the attenuation in hydrous minerals during progressive dehydration appears to be governed by the fine balance between the amount of aqueous fluid present in the sample and the increase in bulk density due to the appearance of nominally anhydrous mineral phases. The shear waves lose more energy compared to P-waves, due to scattering at grain boundaries, especially when the grain boundaries contain a liquid phase. The decrease in the amplitudes of S-waves observed in our study was consistent with the continuous release of fluid into the sample with increasing temperature (Figure 6b).

The 1st, 2nd, and 3rd echos observed for p-waves in our sample at 3.8 GPa exhibited a unique behavior, indicating a sudden drop in amplitude at three different temperatures (Figure 6c). The magnitude of the drop in amplitude and the temperature at which the The decrease in wave amplitudes mostly depends on the reflectance (and transmittance) of waves at the interfaces between sample and alumina. Reflectance and transmittance of waves at interface depend on the contrast of acoustic impedance. The sharp decrease in the amplitude of P-and S-waves can be correlated to the release of aqueous fluid into the sample, which may change the acoustic impedance of the fluid-bearing sample. The unique increase in amplitude after the initial drop may be related to the growth of olivine, garnet, and spinel crystals in the sample, which may increase the bulk density of the sample relative to chlorite (Figure 6a). However, the attenuation in hydrous minerals during progressive dehydration appears to be governed by the fine balance between the amount of aqueous fluid present in the sample and the increase in bulk density due to the appearance of nominally anhydrous mineral phases. The shear waves lose more energy compared to P-waves, due to scattering at grain boundaries, especially when the grain boundaries contain a liquid phase. The decrease in the amplitudes of S-waves observed in our study was consistent with the continuous release of fluid into the sample with increasing temperature (Figure 6b).

The 1st, 2nd, and 3rd echos observed for p-waves in our sample at 3.8 GPa exhibited a unique behavior, indicating a sudden drop in amplitude at three different temperatures (Figure 6c). The magnitude of the drop in amplitude and the temperature at which the maximum drop occurred decreased from the 1st echo to the 3rd echo. While we could not offer a definitive explanation, we propose that this observation could be due to the cumulative effect of sound waves traveling multiple times through the fluid-bearing media.

Geophysical Implications

Mantle wedges, where increased fluid activities can be expected, are often characterized by anomalous seismic properties. The low velocity and high Vp/Vs have often been interpreted as an indication of the presence of hydrous minerals such as serpentine and chlorite [START_REF] Mookherjee | Unusually large shear-wave anisotropy for chlorite in subductin zone settings[END_REF][START_REF] Bezacier | High-pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge[END_REF]. In light of the new data obtained in this study, we can explore low-velocity and high Vp/Vs regions to track fluid migration in mantle wedges.

The most compelling observation comes from the absolute Vp/Vs ratios obtained using the 3D seismic tomography at the central Chile-Argentina subduction zone [START_REF] Wagner | Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina[END_REF] and the south-central Chilean subduction zone [START_REF] Wagner | Upper mantle structure in the south central Chilean subduction zone (30 • to 36 • S)[END_REF], where they emphasize high Vp/Vs ratios at the mantle depth of about 50 km. It has been speculated that a Vp/Vs ratio of around two, may indicate the presence of frozen pooled-melt [START_REF] Wagner | Depleted lithosphere, cold, trapped asthenosphere, and frozen melt puddles above the flat slab in central Chile and Argentina[END_REF] in these subduction systems. However, the depth at which the anomaly is observed (~50 km) may not correspond to the arc melting. The depth at which the seismic anomaly has been observed also coincides with the high conductivity anomaly observed for the Andean subduction system [START_REF] Brasse | Electrical conductivity beneath the Bolivian Orocline and its relation to subduction processes at the South American continental margin[END_REF][START_REF] Brasse | The Bolivian Altiplano conductivity anomaly[END_REF]. Such high conductivities can be attributed to the presence of aqueous fluids [START_REF] Manthilake | Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges[END_REF]. We interpreted the low-velocity anomaly and the high Vp/Vs ratio observed in the Chile and Argentina subduction systems as some of the best examples of the presence of aqueous fluid in subduction systems. The increase in δVp, decrease in δVs, and the high Vp/Vs ratio around 2.4 can, thus, be used to define aqueous fluids in subduction zones. This study would lay the foundation for tracking dehydrating fluid in the mantle through a high-resolution seismic tomography method.

Figure 1 .

 1 Figure 1. (a) Schematic assembly configuration of the multi-anvil cell assembly (before compression) used for the sound wave velocity measurements. (b) The stacked radiography images of the sample with increasing temperature at 3.8 GPa. The red dashed lines show the thickness of the sample. Examples of the ultrasonic signals recorded at 40 MHz for P-waves (c), and S-waves (d) recorded at 900 and 1000 K, before and after the dehydration of chlorite.

Figure 1 .

 1 Figure 1. (a) Schematic assembly configuration of the multi-anvil cell assembly (before compression) used for the sound wave velocity measurements. (b) The stacked radiography images of the sample with increasing temperature at 3.8 GPa. The red dashed lines show the thickness of the sample. Examples of the ultrasonic signals recorded at 40 MHz for P-waves (c), and S-waves (d) recorded at 900 and 1000 K, before and after the dehydration of chlorite.

Minerals 2021, 11 , x 5 of 12 Figure 2 .

 11122 Figure 2. (a) Primary (Vp) and secondary (Vs) wave velocities as a function of temperature at 1.8 and 3.8 GPa. (b) The% drop in velocity (δVp, δVs) upon dehydration at 1.8 and 3.8 GPa compared to chlorite before dehydration. The lines are the linear fits through data points. Uncertainties in velocity estimations are estimated to be less than 2.0% (2σ), smaller than the symbol size.

Figure 3 .

 3 Figure 3. Scanning electron microscope backscattered electron images showing the chlorite sample after dehydration at (a) 1.8 GPa and (b) 3.8 GPa.

Figure 2 .

 2 Figure 2. (a) Primary (Vp) and secondary (Vs) wave velocities as a function of temperature at 1.8 and 3.8 GPa. (b) The% drop in velocity (δVp, δVs) upon dehydration at 1.8 and 3.8 GPa compared to chlorite before dehydration. The lines are the linear fits through data points. Uncertainties in velocity estimations are estimated to be less than 2.0% (2σ), smaller than the symbol size.

Figure 2 .

 2 Figure 2. (a) Primary (Vp) and secondary (Vs) wave velocities as a function of temperature at 1.8 and 3.8 GPa. (b) The% drop in velocity (δVp, δVs) upon dehydration at 1.8 and 3.8 GPa compared to chlorite before dehydration. The lines are the linear fits through data points. Uncertainties in velocity estimations are estimated to be less than 2.0% (2σ), smaller than the symbol size.

Figure 3 .

 3 Figure 3. Scanning electron microscope backscattered electron images showing the chlorite sample after dehydration at (a) 1.8 GPa and (b) 3.8 GPa.

Figure 3 .

 3 Figure 3. Scanning electron microscope backscattered electron images showing the chlorite sample after dehydration at (a) 1.8 GPa and (b) 3.8 GPa.

Figure 4 .

 4 Figure 4. P-wave velocity compared with the vol.% of chlorite dehydration. The Vp vs. temperature diagram showing the P-wave velocity for chlorite and dehydration products. The black dashed line indicates the Vp of chlorite before dehydration, extrapolated to high temperatures. The purple solid line indicates the P-wave velocity of a mixture of olivine [47], garnet [48], and spinel [49], computed for 3:1:1 proportion based on the dehydration reaction 2chl = 3ol + gt + spl + 8 H2O. Blue dashed lines indicate the linear mix between the velocity of chlorite and the (3ol + gt + spl) without taking into account the velocity effect of fluid. The numbers next to dashed lines indicate the fraction chlorite dehydrated producing the (ol + gt + spl). The red and blue solid lines indicate linear fits through the Vp data points of chlorite measured at 1.8 and 3.8 GPa, respectively.

Figure 4 .

 4 Figure 4. P-wave velocity compared with the vol.% of chlorite dehydration. The Vp vs. temperature diagram showing the P-wave velocity for chlorite and dehydration products. The black dashed line indicates the Vp of chlorite before dehydration, extrapolated to high temperatures. The purple solid line indicates the P-wave velocity of a mixture of olivine [47], garnet [48], and spinel [49], computed for 3:1:1 proportion based on the dehydration reaction 2chl = 3ol + gt + spl + 8 H 2 O. Blue dashed lines indicate the linear mix between the velocity of chlorite and the (3ol + gt + spl) without taking into account the velocity effect of fluid. The numbers next to dashed lines indicate the fraction chlorite dehydrated producing the (ol + gt + spl). The red and blue solid lines indicate linear fits through the Vp data points of chlorite measured at 1.8 and 3.8 GPa, respectively.

Figure 5 .

 5 Figure 5. The Vp/Vs ratios of chlorite and dehydrating fluid at 1.8 and 3.8 GPa. The Vp/Vs of major mantle phases, olivine, garnet, clinopyroxene (cpx), orthopyroxene (opx), are shown for comparison [47]. Diamond symbols indicate antigorite [23] at 1.8 GPa (red) and 3.8 GPa (blue) at 300 K. The blue shaded line indicates the Vp/Vs of basaltic melt[START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF] for increasing melt fraction. The faded blue area corresponds to the dehydrating fluid. The red and blue dashed-lines are the second-order polynomial fits through data points.

Figure 5 .

 5 Figure 5. The Vp/Vs ratios of chlorite and dehydrating fluid at 1.8 and 3.8 GPa. The Vp/Vs of major mantle phases, olivine, garnet, clinopyroxene (cpx), orthopyroxene (opx), are shown for comparison[START_REF] Zhang | Sound velocities of olivine at high pressures and temperatures and the composition of Earth's upper mantle[END_REF]. Diamond symbols indicate antigorite[START_REF] Wang | Anomalous Sound Velocities of Antigorite at High Pressure and Implications for Detecting Serpentinization at Mantle Wedges[END_REF] at 1.8 GPa (red) and 3.8 GPa (blue) at 300 K. The blue shaded line indicates the Vp/Vs of basaltic melt[START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF] for increasing melt fraction. The faded blue area corresponds to the dehydrating fluid. The red and blue dashed-lines are the second-order polynomial fits through data points.

Figure 6 .

 6 Figure 6. The% change of amplitude of P-(a) and S-(b) waves relative to the amplitude for the chlorite before dehydration. Blue and red lines are for 1.8 and 3.8 GPa respectively. The vertical dashed line indicates the dehydration temperature observed for chlorite. (c) The change of amplitudes in the 1st, 2nd, and 3rd echos of the P-waves at 3.8 GPa. The blue and red lines in panels (a) and (b) are the cubic-spline fits through data points.

Figure 6 .

 6 Figure 6. The% change of amplitude of P-(a) and S-(b) waves relative to the amplitude for the chlorite before dehydration. Blue and red lines are for 1.8 and 3.8 GPa respectively. The vertical dashed line indicates the dehydration temperature observed for chlorite. (c) The change of amplitudes in the 1st, 2nd, and 3rd echos of the P-waves at 3.8 GPa. The blue and red lines in panels (a) and (b) are the cubic-spline fits through data points.

Acknowledgments:

We thank Emmy Voyer for her assistance with the scanning electron microscope and Laurent Jouffret for help characterizing the starting chlorite samples. Denis Andrault, Ali Bouhifd, Nathali Bolfan Casanova, and J.-P. Itie are thanked for their involvement in the installation of the multi-anvil press at the PSICHE beamline.

Data Availability Statement:

The data presented in this study are contained within the article.

Author Contributions: G.M. designed the study. G.M. and J.C. performed the experiments and analyzed data. G.M. wrote the manuscript. N.G. and A.K. assisted with the synchrotron set-up at the PSICHE beamline. All authors have read and agreed to the published version of the manuscript. Funding: G.M. acknowledges funding from the INSU-CNRS. This research was financed by the French Government Laboratory of Excellence initiative n • ANR-10-LABX-0006, the Région Auvergne, and the European Regional Development Fund. This is ClerVolc contribution number 449.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Conflicts of Interest:

The authors declare no conflict of interest.