EVOLUTION IN H 2 O CONTENTS DURING DEFORMATION OF POLYCRYSTALLINE QUARTZ: AN EXPERIMENTAL STUDY

Shear experiments were performed in a Griggs-type apparatus at 800°C and 1.5GPa, at a strain rate of 2.1 x 10 -5 s -1 using different starting materials: (i) Powder (grain size 6-10µm) of dry Brazil quartz with 0.15wt% added H 2 O, (ii) "dry" Brazil quartz porphyroclasts (grain size ~100-200µm), devoid of fluid inclusions embedded in the same fine grained powder, and (iii) "wet" porphyroclasts (grain size ~100-200µm), containing initially a high density of µm-scale fluid inclusions embedded in the same powder. After hot pressing, samples were deformed to large shear strains (γ~3 to 4.5), in order for the microstructures and H 2 O distribution to approach some state of "equilibrium". The H 2 O content and speciation in quartz were analyzed by Fourier Transform Infra-Red (FTIR) spectroscopy before and after the experiments. Mechanical peak strength is generally lower in experiments with 100% hydrated matrix, intermediate in experiments incorporating wet porphyroclasts (with a proportion of 30 or 70%) and highest in those with dry porphyroclasts. All experiments with porphyroclasts show pronounced strain weakening, and the strengths of most samples converge to similar values at large strain. Wet porphyroclasts are pervasively recrystallized during deformation, while dry porphyroclasts recrystallize only at their rims and remain weakly deformed. Recrystallization of the initially fluid-inclusion-rich porphyroclasts results in a decrease in inclusion abundance and total H 2 O content, while H 2 O content of initially dry clasts increases during deformation. H 2 O contents of all high strain samples converge to similar values for matrix and recrystallized grains. In samples with wet porphyroclasts, shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free-fluid phase. These high porosity sites are sinks for collecting H 2 O in excess of the storage capacity of the grain boundary network of the recrystallized aggregate. The H 2 O storage capacity of the grain boundary network is determined as a H 2 O-boundaryfilm of ~0.7 nm thickness.

INTRODUCTION

Quartz weakening by H 2 O is well known since the seminal papers by [START_REF] Griggs | Quartz: Anomalous weakness of synthetic crystals[END_REF] and [START_REF] Griggs | Hydrolytic weakening of quartz and other silicates[END_REF]. In natural rocks, it is tacitly assumed that quartz is sufficiently "wet", because quartz usually is one of the weakest components in plastically deformed rocks. However, some recent observations have described potentially drier natural rocks, which are (Fitzgerald et al., 2006[START_REF] A N U S C R I P T A C C E P T E D Accepted Manuscript Menegon | Dry and strong quartz during deformation of the lower crust in thepresence of melt[END_REF][START_REF] Kilian | Dislocation creep of dry quartz[END_REF]. On the other hand, there is clear evidence for shear zones involving material initially rich in H 2 O (principally in the form of fluid inclusions), such as metasediments along the subduction plate interface (Palazzin et al., 2016;[START_REF] Raimbourg | Fluid properties and dynamics along the seismogenic plate interface[END_REF]Raimbourg et al., 2015, Saffer and[START_REF] Saffer | Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure[END_REF], and the final deformed rocks do not necessarily indicate high H 2 O contents.

Thus, the introduction or reduction of H 2 O in quartz in natural rock deformation remains one of the great unknown parameters.

As pointed out by [START_REF] Griggs | Hydrolytic weakening of quartz and other silicates[END_REF], the H 2 O weakening effect in quartz may combine different elementary processes, which all contribute to enhance plastic deformation. One process is related to the enhanced intra-crystalline plasticity of quartz. It has been extensively studied in single crystals, most of them synthetic [START_REF] Griggs | Hydrolytic weakening of quartz and other silicates[END_REF][START_REF] Griggs | A model of hydrolytic weakening of quartz[END_REF][START_REF] Griggs | Quartz: Anomalous weakness of synthetic crystals[END_REF][START_REF] Kekulawala | Hydrolytic weakening in quartz[END_REF][START_REF] Kekulawala | An experimental study of the role of water in quartz deformation[END_REF][START_REF] Mclaren | The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz[END_REF][START_REF] Mclaren | Dislocation nucleation and multiplication in synthetic quartz: relevance to water weakening[END_REF]. The microscopic weakening phenomenon may eventually depend on the hydrolysis of Si-O bonds in the cores of dislocations [START_REF] Griggs | A model of hydrolytic weakening of quartz[END_REF], on enhanced climb of dislocations [START_REF] Cordier | Water solubility in quartz and its influence on ductility[END_REF][START_REF] Mainprice | A transmission electron microscopy study of experimentally deformed quartzite with different degrees of doping[END_REF][START_REF] Tullis | Hydrolytic weakening of quartz aggregates: The effect of water and pressure and recovery[END_REF], or involve the increased dislocation nucleation at nano-scale H 2 O clusters acting as dislocation sources [START_REF] Gerald | Microstructures in water-weakened single crystals of quartz[END_REF][START_REF] Mclaren | Dislocation nucleation and multiplication in synthetic quartz: relevance to water weakening[END_REF]. H 2 O, in the form of molecular H 2 O is most efficient to weaken quartz single crystals (e.g., [START_REF] Paterson | The interaction of water with quartz and its influence in dislocation flow: an overview[END_REF][START_REF] Kekulawala | Hydrolytic weakening in quartz[END_REF][START_REF] Kekulawala | An experimental study of the role of water in quartz deformation[END_REF]. However, the growth of nano-scale clusters of H 2 O to form larger fluid inclusions, resulting from heat treatment, increases quartz strength [START_REF] Kekulawala | Hydrolytic weakening in quartz[END_REF]: milky quartz containing a large concentration of H 2 O confined in µm-scale fluid inclusions is stronger than synthetic crystal with a much lower H 2 O concentration. The effect of OH structural defects on deformation is unclear but appears limited [START_REF] Doukhan | TrépiedPlastic deformation of quartz single crystals[END_REF][START_REF] Paterson | The interaction of water with quartz and its influence in dislocation flow: an overview[END_REF]Kronenberg, 1994;Cordier et al., 1994). However, in a given quartz material, the weakening effect increases with the H 2 O content [START_REF] Griggs | Hydrolytic weakening of quartz and other silicates[END_REF][START_REF] Blacic | Plasticity and hydrolytic weakening of quartz single crystals[END_REF], Kronenberg and Tullis, 1984[START_REF] Cordier | Water solubility in quartz and its influence on ductility[END_REF][START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF][START_REF] Den Brok | Fourier transform IR-determination of intragranular water content in quartzites experimentally deformed with and without added water in the ductile deformation field[END_REF], Stunitz et al., 2017).

Another elementary process, proposed by [START_REF] Griggs | Hydrolytic weakening of quartz and other silicates[END_REF] as a cause of quartz hydrolytic weakening, is recrystallization, which operates in polycrystals [START_REF] Jaoul | The effect of varying water contents on the creep behavior of Heavitree quartzite[END_REF][START_REF] Tullis | Hydrolytic weakening of quartz aggregates: The effect of water and pressure and recovery[END_REF][START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF]. Similar to single crystals, in quartz aggregates, there is an inverse correlation between the amount of added H 2 O and the assemblage strength [START_REF] Jaoul | The effect of varying water contents on the creep behavior of Heavitree quartzite[END_REF][START_REF] Kronenberg | Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening[END_REF]. In particular, in H 2 O-rich experiments, the small grain sizes decrease strength, suggesting some contribution of grain-boundary mediated process to weakening [START_REF] Kronenberg | Flow strengths of quartz aggregates: Grain size and pressure effects due to hydrolytic weakening[END_REF].

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The case of deformation of polycrystalline material is of prime interest to nature, as it directly applies to the strength of mylonites and ultramylonites. It involves the combination of intracrystalline and grain boundary processes, so that identifying the H 2 O-weakening effect is difficult. In addition, natural material may contain H 2 O concentrations varying over several orders of magnitude [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF]: microcrystalline cherts, non-metamorphic or of diagenetic grade, contain up to ~40,000 H/10 6 Si. In contrast, the threshold for the onset of weakening, in other words the boundary between wet and dry quartz in experiments on synthetic quartz, is ~100 H/10 6 Si [START_REF] Griggs | Quartz: Anomalous weakness of synthetic crystals[END_REF]Cordier andDoukhan, 1991, Den Brock, 1994;[START_REF] Christie | Experimental evidence of basal slip in quartz[END_REF][START_REF] Kilian | Dislocation creep of dry quartz[END_REF]. Not all of the H 2 O content of natural quartz (primarily in µm-scale fluid inclusions) has a weakening effect [START_REF] Kekulawala | Hydrolytic weakening in quartz[END_REF], but the mechanical role of large H 2 O concentrations is still to be determined.

In particular, considering natural quartz as a two-phase-system of crystal and fluid inclusions, the transfer of H 2 O from the inclusions into the crystal (and the associated weakening) is difficult because of the very low solubility and diffusive flux of H 2 O in quartz [START_REF] Kronenberg | Solubility and diffusional uptake of hydrogen in quartz at high water pressures: Implications for hydrolytic weakening[END_REF][START_REF] Paterson | The thermodynamics of water in quartz[END_REF], Gerretsen et al., 1989). As a consequence, micro-fracturing [START_REF] Kronenberg | Solubility and diffusional uptake of hydrogen in quartz at high water pressures: Implications for hydrolytic weakening[END_REF][START_REF] Law | Crystallographic fabrics: a selective review of their applications to research in structural geology[END_REF], Gerretsen et al., 1989, FitzGerald et al., 1991, Stunitz et al., 2017) and diffusion (along subgrain boundaries or dislocation cores; [START_REF] Post | The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening[END_REF]), as well as grain boundary migration [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF], have been proposed as mechanisms responsible for H 2 O uptake, because oxygen diffusion may be increased along dislocations or grain boundaries.

Finally, H 2 O concentration is not a fixed quantity and depends itself on deformation, as recrystallization redistributes H 2 O within the material. For example, in rock of low metamorphic grade from Japanese accretionary complexes, the H 2 O concentration decreases with increasing grade, reflecting the progress of recrystallization [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF].

In grains with a large initial H 2 O content stored in fluid inclusions, the migration of grain boundaries that accompanies recrystallization leads to the loss of most fluid inclusions and the decrease H 2 O concentration (Bakker and Jansen, 1994;[START_REF] Kilian | Dislocation creep of dry quartz[END_REF]Palazzin et al., 2016). Therefore, the "steady-state" H 2 O concentration (and the associated strength) in a deforming and recrystallizing aggregate is poorly known. FTIR measurements in natural mylonites formed at mid-greenschist and amphibolite facies conditions show low H 2 O contents, <100 to ~320 H/10 6 Si at grain boundary regions [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF] and in individual single grains [START_REF] Kilian | Dislocation creep of dry quartz[END_REF].

Given the wealth of existing experimental and natural studies on H 2 O weakening of quartz, this study specifically focuses on the strength of quartz as a function of

H 2 O-content. Another M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4
aspect of this study is the evolution of H 2 O-content (i.e. increasing and decreasing H 2 O contents in deforming polycrystals). "Wet" initial conditions (e.g., applicable to examples to sediments in subduction settings) and dry initial crystals are compared and their H 2 Oevolution studied. We have carried out high strain experiments in order to simulate natural shear zones but also to be as close as possible to steady-state conditions, both in terms of microstructures and H 2 O distribution. We focused on the role of H 2 O on recrystallization and the effect of H 2 O content and speciation.

METHODS

Experimental procedure

All experiments were performed in a Griggs-apparatus at 800°C, ~1500 MPa and a constant strain rate of ~2 x10 -5 s -1 . These conditions were chosen in order to activate dislocation creep in the samples (cf. [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF]. Pre-cut (at 45°) Al 2 O 3 pistons were inserted into a pre-annealed platinum jacket. A ~1mm thick layer of fine quartz matrix powder was placed between pistons, which contained a fixed proportion of larger grains ("porphyroclasts"). The matrix material (Brazil quartz) was the same in all experiments while two distinct types of porphyroclasts were used.

The quartz matrix comes from a dry Brazil single-crystal crushed in a stainless steel mortar.

After crushing, we applied repeated cycles of sedimentation in a H 2 O column to sort grains of a specific size. The powder we obtained consisted of grains with a diameter in the range of ~6-20µm, although a small fraction of the grains (~15% in volume), had larger, up to 110µm, diameter.

The first material chosen for the porphyroclast was dry Brazil quartz. A single euhedral crystal of ~4 cm long was cut, trying to avoid optically visible secondary fluid inclusions planes. Porphyroclasts were obtained by crushing the selected part of the crystal in a stainless steel mortar and separating a size in the range of 200-250 µm by sieving.

The second porphyroclast material was milky vein quartz from the Hyuga mélange, a lowgrade metamorphic unit from the Shimanto Belt in Japan [START_REF] Raimbourg | Long-term evolution of an accretionary prism: The case study of the Shimanto Belt, Kyushu, Japan[END_REF]Palazzin et al., 2016 for detailed description). Fragments from mm-wide veins were crushed in an agate mortar and quartz was separated from the remaining matrix by a Frantz magnetic separator and density (Sodium Polytungstate) techniques. The material was sieved to separate the 200-250 µm size fraction. A final hand-picking was carried out to avoid possible clay mineral contamination.

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The matrix and porphyroclasts were dried at 110°C for 48 hours before preparing the mixture.

Four experimental samples were prepared by mixing 30% and 70% porphyroclasts of either Hyuga or Brazil quartz with the pure Brazil quartz matrix. In addition, one sample with 100% pure Brazil matrix was deformed.

To ensure an homogenous distribution of clasts in the matrix, we added acetone to the clasts+matrix mixture in a glass beaker and stirred the slurry obtained in an ultrasonic bath, until complete acetone evaporation (cf. [START_REF] De Ronde | Reaction-induced weakening of plagioclase-olivine composites[END_REF]. The porphyroclasts and matrix quartz mixtures were dried at 110°C and then placed on the lower Al 2 O 3 -piston surface after adding distilled water with a micropipette. The material was wrapped in Ni-foil inside the Ptjacket. The amount of water was adjusted to the proportion of the matrix (100, 70 or 30%), corresponding to an H 2 O-content of 0.15 wt.% for matrix (Table 1). The second pre-cut piston was placed on top of the sample, and the Pt jacket was welded with a Lambert precision point welder. The sample was then placed in the center of a graphite furnace inside the solid confining medium (NaCl). The sample-piston assembly is illustrated in Figure 1a.

Pumping and heating were performed slowly to reach the desired temperature and pressure conditions (normally over ~9 hours). Confining pressure was initially increased to 150 MPa before heating. Temperature was increased in steps of 100°C at a rate of 25°C per minute at pressure intervals of several hundreds of MPa. When pressure and temperature were at 1.5 GPa and 800°C, axial piston movement was started at a constant displacement rate. The typical time to reach the hit point was ~20 to 24 hrs. During this stage, the sample was effectively subjected to hot isostatic pressing, leading to a denser aggregate.

The hit point, marked by a sharp increase in axial force, corresponds to the onset of sample loading and deformation. Axial force then rises rapidly to a maximum value ("peak stress").

The subsequent deformation of the sample, up to γ ~4-6, is achieved for near constant, or slowly decreasing axial force.

At the end of each experiment, the sample was quenched to a temperature of 200°C within 3 minutes. Subsequently, the confining pressure was lowered to room pressure over a period of several hours, keeping a differential stress of (initially) 100-200 MPa on the sample in order to minimize unloading cracks. Some samples were kept under hydrostatic conditions without deformation to obtain information of the microstructures during the initial steps of the experiments. All samples

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were cut along their long axis normal to the shear zone in order to provide thin and thick sections for microscopy and FTIR analysis, respectively.

FTIR measurements

To enable analysis of the H 2 O-content, slices of the single Brazil quartz crystal (cut in the direction normal to the [c]-axis) and isolated porphyroclasts of Hyuga quartz (both corresponding to the starting material) were firstly embedded in orthodontic acrylic resin (Vertex Orthoplast) and then manually doubly polished (thickness 120-140 µm). Deformed samples were cutted and directly polished to 120-140 µm thick sections. All samples were then accurately cleaned in ultrasonic bath with acetone to remove remaining resin. The H 2 Ocontent was determined by Fourier Transform Infrared (FTIR) spectroscopy with a microscopic FTIR continuum spectrometer (Nicolet-6700, Thermo Scientific) at the Institut des Sciences de la Terre d'Orléans (France). All the measurements were carried out at atmospheric temperature, after purging the optical path of the IR beam with dry air in order to avoid measurement of atmospheric water. A sodium chloride window was used to collect and subtract the background for each measurement. A 50×50 µm aperture window was used for all the analyses. 256 scans per spectrum were collected with 4 cm -1 resolution. We estimated the concentration of "molecular" H 2 O, using the Paterson (1982) calibration of the integral absorption band between ~2800 and 3780 cm -1 using the Beer-Lambert law, A= C * t * е where C is the concentration, t the thickness, and е the integrated molar absorption coefficient (0,8120 L cm -2 mol -1 ). Sample thicknesses t was obtained from the height of the peak at 1790 cm -1 corresponding to the Si-O band, again making use of the Beer-Lambert law (e.g., [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF]. H 2 O concentration (wt. ppm H 2 O/SiO 2 ) was obtained (Table 1) by dividing the H/10 6 Si-value of the Paterson (1982) calibration by a factor 6.67 given in [START_REF] Kilian | Dislocation creep of dry quartz[END_REF].

EBSD -CIP -Grain Size determinations

Crystallographic preferred orientation (CPO) of quartz was measured by electron backscatter diffraction (EBSD) [START_REF] Lloyd | Dynamic recrystallization of quartz under greenschist conditions[END_REF][START_REF] Prior | Problems in determining the misorientation axes, for small angular misorientations, using electron backscatter diffraction in the SEM[END_REF] and by computer integrated polarized (CIP) microscopy [START_REF] Heilbronner | Image Analysis in Earth Sciences[END_REF]. EBSD patterns were acquired with an Oxford AZTEC system and a Nordlys detector on a FE-Zeiss Merlin compact SEM at the University of Tromsø. We used thin sections with a thin carbon coat at 70° tilt angle, 20

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kV acceleration voltage, ∼10 nA beam current, and 11 to 14 mm working distances at a step size of 0.1 µm. Initial noise reduction was performed with CHANNEL 5 software by removing isolated points and replacing non-indexed points with the orientation of their neighbors (interactively filled starting with eight similar neighbors down to six or five similar neighbors). Thin sections were rotated 45° counterclockwise in order to align shear zone boundaries (forcing block interface) to reference frame direction. Data were analyzed with MTEX toolbox [START_REF] Heilbronner | The grain size(s) of Black Hills Quartzite deformed in the dislocation creep regime[END_REF]. Analysis of the grain boundary fraction was carried out using EBSD maps. Grains were segmented based on c-axis misorientation after converting EBSD images to CIP images using a procedure described in [START_REF] Heilbronner | The grain size(s) of Black Hills Quartzite deformed in the dislocation creep regime[END_REF].

RESULTS

Characterization of the starting material

Hyuga quartz has a milky appearance due to the great abundance of many tiny fluid inclusions (Palazzin et al., 2016). These inclusions range from a few to 10µm in diameter [START_REF] Raimbourg | Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan[END_REF] and are heterogeneously distributed in the porphyroclasts which look dark in thin section and grain mounts (Figure 1b-1). As shown by FTIR spectra, OH content is variable as a consequence of the distribution of fluid inclusions (Figure 1b-1). Mean measured values (listed in Table 1) are of the order of ~23,000 H/10 6 Si, or ~0.34wt% H 2 O, which is about twice the amount of H 2 O added to the sample matrix. After the rise in T and P up to 800°C and 1.5GPa, Hyuga porphyroclasts show a lower abundance of fluid inclusions than the starting material, so that they appear clearer (Figure 1b-2). The measured amount of OH is lowered to about half of the initial quantity (~12,000 H/10 6 Si), corresponding to ~0.18wt%, similar to the added H 2 O content of 0.15wt% (Table 1, Figure 1b-2).

Porphyroclasts of dry Brazil quartz are optically strain free and contain no fluid inclusions (Figure 1b-3). The mean measured H content in Brazil quartz is about 260 H/10 6 Si and confirms that this material can be considered as "dry" quartz for deformation (Table 1; [START_REF] Post | The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening[END_REF][START_REF] Paterson | The interaction of water with quartz and its influence in dislocation flow: an overview[END_REF]. [START_REF] Kohlstedt | Strength of the lithosphere: Constraints imposed by laboratory experiments[END_REF], all other samples are in the fully plastic regime. All curves are characterized by a first stage of a rapid increase in stress up to peak values (γ = 0 to ~0.5/1.5), followed by a weakening stage that may or may not reach steady state stress values.

Mechanical data

Moderate

Two samples with 100% matrix show a ~50% difference in peak stress (at γ ~0.8) and a smaller difference at γ ~3.5 (Figure 2). The sample with 30% Hyuga porphyroclasts shows a peak shear stress between that of the two matrix samples (Figure 2) and a final stress value that is almost identical to that of the matrix sample (~260 MPa) at γ ~3.2. The sample with 70% Hyuga porphyroclasts shows a considerably higher peak stress but weakens to a similar finite strength as the 30% Hyuga and matrix samples. Samples with Brazil porphyroclasts reach higher peak shear stresses than their Hyuga porphyroclasts counterparts: up to ~650 MPa for 70% clasts and ~420 MPa for 30% Brazil clasts. After the peak stress, all matrix + clasts assemblages display significant weakening with a similar slope, irrespective of the nature of the clasts. It is remarkable that the flow stresses of three assemblages, containing 70% and 30% of Hyuga porphyroclasts and 30% of Brazil porphyroclasts, converge towards a common flow stress of ~250-280 MPa at γ ~3.3 (Figure 2), similar to the 100% matrix, regardless of their peak stress values.

Microstructural observations

Hot pressed material

After hot-pressing, only little porosity is observed, typically at triple junctions of grains (Figure 3). Pores rarely exceed 1µ diameter, and total porosity is estimated to be ~ 2 to 4 % of the sample volume. Very few open grain boundaries are present, but these are approximately normal to the piston axis, so that they probably result from quenching and unloading (Figure 3).

Pure matrix deformation

The deformed samples with pure 100% matrix are characterized by pervasive recrystallization of the deformed parts of the samples. Strain is partitioned into narrow regions at the piston ends, extending into a broader region, which occupies almost the full width of the shear zone in the center (Figure 1a). The quartz grains in the regions of strain localization show a strong shape fabric, and individual grain boundaries are difficult to detect in the high strain regions in the light microscope because of their strong CPO (see paragraph 3.5.1).

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Matrix + Brazil porphyroclasts

In these samples, a foliation results from the elongation of matrix domains and recrystallized regions extending from porphyroclasts at an angle of about 18° to the shear zone boundaries (piston interfaces; Figure 4a,b). In the sample with 70% clasts, tails of recrystallized quartz (Figure 4b,d) extending from porphyroclasts define σ-type clasts (Hanmer, 1984b;[START_REF] Passchier | Porphyroclast systems as kinematic indicators[END_REF]. Porphyroclasts in the assembly with 30% clasts show a variable shape: some are elongated, while others have an equant shape, both of which are mainly inherited from their pre-deformation state (Figure 4a,c). Porphyroclasts contain few or no subgrains.

Their rims are commonly sutured and their cores are surrounded by small recrystallized grains (Figure 4c,d). Some undulatory extinction and deformation lamellae are common (Figure 5a,b). These microstructures are characteristic of dislocation creep regime 1 described by [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF]. In both Brazil clast samples the extent of recrystallization of porphyroclasts is limited, and relict clasts and recrystallized rims are easy to distinguish (Figure 4-5a,b).

Matrix + Hyuga porphyroclasts

Hyuga porphyroclasts (6a,b) invariably are strongly elongated, up to an aspect ratio of ~10.

Their shape fabric defines a foliation at about 10° to the shear zone boundaries. They are extensively recrystallized into elongated grain aggregates, with serrated boundaries, and subgrains, forming a microstructure of dislocation creep regime 2 of [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF].

Because of the penetrative recrystallization, former porphyroclasts are difficult to distinguish from the surrounding matrix ( Figs 5c,d and6c), except for the fact that porphyroclasts recrystallized and grown into larger grains and subgrains than the matrix ( Figs 5c,d and6c,d). The clasts in the 70% Hyuga porphyroclasts sample are more pervasively recrystallized than in the 30% clasts sample, possibly due to higher strain.

Shear Bands

In both samples containing Hyuga porphyroclasts, shear bands develop oblique to the shear zone boundaries in a synthetic orientation (c´-orientation) with the imposed shear sense (Figs 6c,d and7). The angle between these planes and the shear zone boundary varies between ~25 and ~35°. The stretched porphyroclasts define the S-planes of a SC' fabric (C-planes are absent). Plane polarized light images show the shear bands as trails of small fluid inclusions (Figure 7a). The shear bands are most ubiquitous in the sample containing 70% of Hyuga porphyroclasts, where the bands cut across the matrix and the stretched porphyroclasts, making an average angle of 27 degrees with porphyroclast long axes. SEM images (Figure

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show that the C' bands are zones of high porosity, composed of cube-shaped, idiomorphic grains, of size ~2-5 µm, surrounded by voids.

H 2 O content/distribution

Matrix

The original Brazil quartz crystal, from which the matrix was produced, contains a very low proportion of H 2 O, of the order of ~250 H/10 6 Si (Table 1 andFigure 1b-3). In each experiment, H 2 O had been initially added to the dry Brazil quartz matrix, with a proportion of 0.15wt%, or equivalently 10 000 H/10 6 Si.

After the experiments, the matrix displays a broad absorption band in the region between 3000 and 3800 cm -1 (Figure 8a), representing the molecular H 2 O in quartz and in the grain boundaries. From this broad absorption band, the H 2 O content after experiments can be estimated as ~900 to 2900 H/10 6 Si in deformed samples and ~4200 H/10 6 Si in an undeformed sample (464GP, Table 1). The water initially added has been retained to variable extent in the matrix during the application of pressure and temperature and later deformation.

The same initial material (Brazil quartz matrix + 30% Hyuga porphyroclasts) was brought to high P and T in one experiment and then retrieved (464GP). In another experiment it was deformed to high strain (428GP). The Brazil matrix in the deformed sample contains ~1500 H/10 6 Si, i.e. 2 to 3 times less than the corresponding experiment without deformation (~4200 H/10 6 Si; Table 1).

Comparing the different experimental materials, the H 2 O content of the matrix is significantly higher for experiments containing initially "wet" Hyuga porphyroclasts (~1500 for 30% and ~2900 H/10 6 Si for 70% porphyroclasts) than in experiments containing initially "dry" Brazil porphyroclasts (~950 and ~1300 H/10 6 Si for 30% and 70% of porphyroclasts) or experiments with 100% matrix (~800 H/10 6 Si) (Figure 8a, Table 1). Additionally, in experiments containing Hyuga porphyroclasts, the H 2 O -content of the quartz matrix and that of the recrystallized Hyuga clasts are similar after deformation, considering the large standard deviation: with 30% of clasts, the matrix contains ~1500 H/10 6 Si and porphyroclasts ~2300 H/10 6 Si, while for 70% clasts, the matrix contains ~2900 H/10 6 Si and porphyroclasts ~2350 H/10 6 Si (Table 1).

All spectra show discrete absorption bands at 3363, 3382, and 3595 cm -1 , which were not present in the original Brazil quartz spectra (cf. Figures 1D and8). The most prominent discrete absorption band in the deformed matrix measurements is at 3595 cm -1 .

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Porphyroclasts

Hyuga porphyroclasts display a broad absorption band in the region between 3000 and 3800 cm -1 (Figures 1b,c,d and10a), which is related to molecular H 2 O contained in the large number of fluid inclusions initially present. During hot pressing the broad band of molecular H 2 O shows a general decrease with respect to the as-is material. This reduction in water content is enhanced with deformation, as Hyuga porphyroclasts in deformed samples contain ~1/10 of their initial water content (Table 1, Figure 9a).

In addition to this evolution of the H 2 O content, the shape of the H 2 O spectrum is also modified by the application of P-T conditions and deformation from a broad triangular absorption centered around around 3400cm -1 that is charateristic of fluid H 2 O to a very broad flat flat shape absorption (Figure 9a) that is unlike absorption spectra of liquid H 2 O. The position of discrete absorption bands, related to structurally-bound H 2 O, are also modified.

The initial spectrum displays only a discrete absorption band at 3382 cm -1 . Application of P-T condition results in the development of a band at 3585cm -1 , while after deformation a discrete band around 3595cm -1 appears as well. As a result of this evolution in shape and integrated area, recrystallized Hyuga porphyroclasts and the fine grained matrix converge towards a similar IR spectrum (Figure 9b).

Initially, Brazil porphyroclasts contain virtually no H 2 O (~250 H/10 6 Si), resulting in a flat absorption spectrum (Figure 8b), where only one secondary absorption band at 3485 cm -1 is visible. The application of P-T conditions and deformation (Figure 10,Table 1) results first in the increase in H 2 O content in the cores of the porphyroclasts (~650 H/10 6 Si) and in their recrystallized rims (~1500 for 30% and ~900 H/10 6 Si for 70% of porphyroclasts). The very weakly deformed cores, showing only undulatory extinction, contain a smaller amount of H 2 O (Figure 10b,c,f) than observed for the recrystallized regions with their grain boundaries (Figure 10d,e,f). Furthermore, new discrete absorption bands appear at 3595 cm -1 and at 3363 cm -1 in the recrystallized region, whereas the core shows a discrete band at 3585 cm -1 , which is absent in the recrystallized region (Figure 10a,f).

Shear bands

In the experimental sample containing 30% of Hyuga porphyroclasts, the shear bands that cut across porphyroclasts and matrix contain a high H 2 O concentration of ~3700 H/10 6 Si (Table 1). This H 2 O content is significantly higher than either porphyroclasts (~2300 H/10 6 Si) or the matrix (~1500 H/10 6 Si) that hosts these bands. It corresponds to trails of fluid inclusions

Crystallographic fabrics

Grain boundary maps, orientation images and pole figures are derived from EBSD maps and shown for a pure matrix sample and for one sample containing 70% Hyuga and one with 70% Brazil clasts. For easier interpretation, the orientation images are recalculated as c-axis orientation images (see color look-up table on the right of Figs 11b-12b and 13b).

100% Matrix

In the 100% matrix experiment, the central region of pervasive recrystallization of the sample shows elongated grains, whose long axes are oriented at ~25 degrees (on average) from the shear plane (Figure 11a,b). Their CPO shows a well-defined maximum rotated synthetically with the sense of shear, and all crystallographic axes are concentrated in or near the X-Z plane (Figure 11c).

Recrystallized Hyuga porphyroclasts CPO

EBSD map shows that a large proportion of initial Hyuga porphyroclasts, strongly elongated after deformation, consists of recrystallized grains with small grain size (light blue domain,

Figure 12a,b).

The CPO shows a similar asymmetry with respect to the shear plane as the 100% matrix.

Parts of the recrystallized regions show many subgrain boundaries in a rather uniform CPO domain (Figure 12b).

Recrystallized tail of a Brazil porphyroclast

In the experiment with 70% Brazil quartz, the crystallographic fabric of the recrystallized tail is qualitatively similar as that of the 100% matrix, but the grain size distribution is broader (Figure 13a,b). The c-axis maxima are slightly weaker (Figure 13c). The recrystallized tail appears to have a better developed CPO than the matrix (orange domain, Figure 13b).

Shear bands and strain shadows

Shear bands are developed only in Hyuga clast samples. CIP analysis of these samples reveals that shear bands inside larger porphyroclasts have a completely different c-axis orientation from that of the surrounding material, as seen in light microscope images with the compensation plate inserted (Figure 7b). Strain shadows around Brazil porphyroclasts

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constitute other high-porosity domains where small idiomorphic grains are surrounded by voids, and the crystallographic orientation of these small grains, similar to that of shear bands, is very different from surrounding quartz. The strong contrast in orientation between the grains in shear bands and pressure shadows and the surrounding material, the high-porosity, and the grain shapes suggest that these grains formed as a result of precipitation in a fluid filled porosity.

DISCUSSION

Mechanical Data

After exhibiting an initial peak stress, all samples deform at flow stresses well below the values of the confining pressure (Figure 2; Goetze Criterion: [START_REF] Kohlstedt | Strength of the lithosphere: Constraints imposed by laboratory experiments[END_REF]. The inference of the Goetze Criterion that fractures opening and extension are suppressed when Pc > differential stress is consistent with the fact that none of our samples contained thoroughgoing fractures. The 100% Brazil quartz matrix samples show a higher flow stress than that of wet polycrystalline quartz of [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF] in coaxial experiments at the same temperature and confining pressure. The shear strain rate of our experiments is faster than the coaxial strain rate of [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF], which may explain the differences.

From the mechanical data it is clear that the originally dry Brazil quartz matrix material has become fully "wet" during pressurization and heating, as it has been achieved in experiments previously by [START_REF] Post | The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening[END_REF] and Rutter and Brodie (2004a).

All samples with porphyroclasts show a pronounced peak stress and subsequent weakening behavior, consistent with a higher initial strength of the large porphyroclasts, which subsequently weaken during recrystallization. Even the Hyuga quartz, which contains a lot of H 2 O in the form of aqueous fluid inclusions and thus may be expected to be weaker than the pure Brazil quartz matrix, shows this behavior. The mechanical evolution of the sample raises two points: (1) Porphyroclast material shows pronounced strain weakening, probably caused by recrystallization (2) Fine grained quartz material tends to be weaker than that with coarse grained porphyroclasts. The former point has already been addressed by [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF], who have observed strain weakening behavior for dislocation creep regime 1. The weakening is attributed to local grain boundary migration (bulging recrystallization) of more or less undeformed porphyroclasts. In our examples here, dynamic recrystallization takes place by progressive subgrain rotation (see below), so that the recrystallization mechanism does not seem to be the only controlling factor for the weakening.

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Steady state is reached by the 30% Hyuga material (428GP) after γ ≈ 1.5 (Figure 2). The 70% Hyuga sample (456GP) reaches similar stresses as the 30% sample, but at higher gamma values and after higher peak stress. The grain sizes of the 70% Hyuga sample are smaller, even though the finite stress values of the samples are identical. The findings are consistent with the results by [START_REF] Kidder | The influence of stress history on the grain size and microstructure of experimentally deformed quartzite[END_REF], who find that the recrystallized grain size is dependent on stress history.

Samples with dry Brazil porphyroclasts do not reach steady state after γ = 3.5 or 4 is consistent with Brazil clasts behaving as more rigid particles, and their recrystallization takes more strain than does recrystallization of the Hyuga clasts. The high peak strength of the 70% Brazil clast sample can be attributed to the high strength of the Brazil clasts themselves with low H 2 O content. The fact that they survive deformation as more or less rigid particles implies that they have not become sufficiently hydrated. Only limited recrystallization takes place at their margins, where tails of recrystallized quartz grains develop (Figs. 5-6a,b and14). The CPO of their recrystallized grains is oriented synthetically with respect to the imposed shear sense (dextral; Figure 14), i.e. similar to that of weaker material of other samples.

Applying the empirical quartz flow law by [START_REF] Hirth | An evaluation of quartzite flow laws based on comparisons between experimentally and naturally deformed rocks[END_REF], and the Paterson & Luan (1990) flow law (for silicic acid), we estimated the differential stress of our quartz matrix for strain rate values of 2*10 -5 s -1 . Both flow laws predict lower shear stresses (∆σ/2) than our experimental values for pure matrix (111 MPa and 178 MPa versus observed ~200 MPa) but the values are not far off (yellow star and red dot in Figure 2).

Effect of H 2 O on deformation

Our samples are deformed by crystal-plastic processes, so that they can be compared with the different deformation regime defined in [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF]. Even though Brazil and Hyuga porphyroclasts were deformed at the same P and T, the degree and mechanism of recrystallization was quite different in the two materials.

Dry Brazil porphyroclasts show undulatory extinction and abundant deformation lamellae as typical features for dislocation glide with limited recovery or recrystallization in quartz. These features usually occur together with small scale fractures, kinks or short wavelength misorientation bands, and dislocation tangles [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF], Trepmann and Stoeckhert, 2013, Stunitz et al., 2017) and can be compared to microstructures observed in the dislocation creep regime 1 of [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF]. The presence of small recrystallized grains limited to prophyroclasts rims suggests the onset of subgrain rotation recrystallization.

Recrystallized tails at porphyroclasts form a CPO similar to the matrix CPO (Figure 13),

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consistent with activation of slip on the basal <a> system (Schmidt and Casey, 1986;[START_REF] Law | Crystallographic fabrics: a selective review of their applications to research in structural geology[END_REF].

Deformed milky Hyuga quartz shows very different microstructures. The shape of highly elongated Hyuga porphyroclasts can be compared with natural quartz aggregates deformed at mid-greenschist conditions [START_REF] Law | Strain path partitioning within thrust sheets: Microstructural and petrofabric evidence from the Moine Thrust zone at Loch Eriboll, northwest Scotland[END_REF] or with experimentally deformed Black Hill Quarzite (BHQ) described in regime 2 by [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF] or by [START_REF] Hirth | An evaluation of quartzite flow laws based on comparisons between experimentally and naturally deformed rocks[END_REF]. These elongated grain aggregates are recrystallized to a great extent (Figs. 6 and12). Similar microstructures, and corresponding activated slip systems, are reported also for BHQ [START_REF] Heilbronner | Evolution of c axis pole figures and grain size during dynamic recrystallization: results from experimentally sheared quartzite[END_REF]) deformed at higher temperatures and higher strain rates.

According to our microstructural observations and CPO, recrystallization took place by subgrain rotation. The development of a common CPO for recrystallized porphyroclasts and matrix indicates that deformation is rather homogenously accommodated within the whole system and dominated by basal <a> slip.

The difference between Brazil porphyroclasts, deforming with limited recrystallization, and Hyuga porphyroclasts, pervasively recrystallized by subgrain rotation, can only be explained by their original difference in H 2 O content. Our observations further support the effect of H 2 O on enhancing microstructural development and recrystallization proposed by [START_REF] Jaoul | The effect of varying water contents on the creep behavior of Heavitree quartzite[END_REF], [START_REF] Tullis | Hydrolytic weakening of quartz aggregates: The effect of water and pressure and recovery[END_REF] and [START_REF] Hirth | Dislocation creep regimes in quartz aggregates[END_REF].

Most samples converge to a similar strength after a shear strain of ~3 and recrystallization of large volume fractions of the samples (Figure 2), even though the initial concentrations of H 2 O differed (Figs. 9 and 11, Table 1). Some of the main parameters controlling strength are therefore grain size and H 2 O distribution, and we can deduce that weakening in experiments with Hyuga porphyroclasts is due to grain size reduction and fluid redistribution as a consequence of dynamic recrystallization.

Evolution of H 2 O with deformation

H 2 O speciation

In addition to the "broad" band of molecular H 2 O in the range 2800-3800 cm -1 , our samples show a large set of secondary, discrete absorption bands distributed between 3660 and 3300 cm -1 (Figs 8-9 and 10 and Table 3). Most of these bands were not present in the original material and appear after deformation.

-Absorption band at 3595 cm -1 .

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The band 3595 cm -1 has been detected in different kinds of natural clear quartz and natural/synthetic amethyst (e.g. [START_REF] Kats | Hydrogen in alpha quartz[END_REF][START_REF] Aines | Water in minerals? A peak in the infrared[END_REF][START_REF] Rovetta | Experimental and spectroscopic constraints on the solubility of hydroxyl in quartz[END_REF]Kronenberg, 1994), and in natural deformed and then annealed quartzite [START_REF] Niimi | The infrared absorption band at 3596 cm-1 of the recrystallized quartz from Mt. Takamiyama, southwest Japan[END_REF].

The common interpretation is that H + charge compensates for Al 3+ substituting for Si 4+ . [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF] described this band in pegmatitic quartz and quartz ribbons, giving the same interpretation.

In our experiments, the band at 3595 cm -1 is not detected in all samples but it is strictly correlated with attaining P and T and most importantly with deformation. This band is first identified in all matrix analyses (Figure 8a), while it is absent in the IR spectra of undeformed Brazil quartz (Figure 8b). In Hyuga porphyroclasts, it is not observed at hotpressed conditions, but it develops after deformation, when porphyroclasts are pervasively recrystallized (Figure 9. The band is also observed in Brazil porphyroclasts, but exclusively in their recrystallized tail (Figure 10b,f). The 3595 cm -1 band is therefore closely associated with recrystallized aggregates of grains, suggesting that the OH species of this band are associated with grain or perhaps subgrain boundaries.

Several studies on quartz IR spectra in naturally-deformed rocks further substantiate the connection between a discrete absorption band near 3600 cm -1 and recrystallization microstructures. Quartz in mylonites from Sambagawa metamorphic rocks in Japan [START_REF] Nakashima | Infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks[END_REF] and the western Adirondack in the USA [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF] show a discrete absorption band around 3600 cm -1 . Such a peak is absent in other mylonites, such as granite mylonite in the vicinity of the Median Tectonic Line in Japan [START_REF] Nakashima | Infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks[END_REF][START_REF] Niimi | The infrared absorption band at 3596 cm-1 of the recrystallized quartz from Mt. Takamiyama, southwest Japan[END_REF]. However, in these samples it is not clear whether grain boundaries were analyzed by FTIR in addition to grain interiors. The speciation of OH in quartz at subgrain/grain boundaries may be related to silanols at the quartz surface, although the corresponding absorption bands are reported at slightly higher wavenumbers (e.g. 3627 and 3649 cm-1, see the review in (Kronenberg, 1994)) than the ~3595 cm -1 peak observed in our study. In summary, a strong connection between increased surface area by recrystallization microstructure and 3595 cm -1 absorption band is clear from this study, but the nature of this OH absorption band requires further work.

-Absorption band at 3585 cm -1

The absorption band at 3585 cm -1 exists in both, Hyuga and Brazil porphyroclasts after deformation (Figs. 9a,b and10). This band has been observed in different materials as

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amethyst (Kronenberg, 1994, Chakraborty and[START_REF] Chakraborty | Distribution of OH in synthetic and natural quartz crystals[END_REF], metamorphic cherts [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF], chalcedony [START_REF] Frondel | Structural hydroxyl in chalcedony (type B quartz)[END_REF][START_REF] Graetsch | The nature of water in chalcedony and opal-C from Brazilian agate geodes[END_REF][START_REF] Graetsch | Structural defects in microcrystalline silica[END_REF], flint [START_REF] Graetsch | Structural defects in microcrystalline silica[END_REF]), agate (Yamagishii et al., 1997) and synthetic quartz [START_REF] Stalder | OH defects in quartz in the system quartz-albite-water and granite-water between 5 and 25 kbar[END_REF][START_REF] Wood | Infrared absorption of defects in quartz[END_REF][START_REF] Kats | Hydrogen in alpha quartz[END_REF][START_REF] Aines | Water in minerals? A peak in the infrared[END_REF][START_REF] Cordier | Water solubility in quartz and its influence on ductility[END_REF][START_REF] Chakraborty | Distribution of OH in synthetic and natural quartz crystals[END_REF][START_REF] Paterson | The thermodynamics of water in quartz[END_REF][START_REF] Rovetta | Experimental and spectroscopic constraints on the solubility of hydroxyl in quartz[END_REF]. In experiments of hydrothermal growth of quartz, the intensity of the band increases with pressure and H 2 O activity in the surrounding fluid phase [START_REF] Stalder | OH defects in quartz in the system quartz-albite-water and granite-water between 5 and 25 kbar[END_REF]. A connection of this 3585 cm -1 absorption band to deformation was established in deformation experiments [START_REF] Stünitz | Water redistribution in experimentally deformed natural milky quartz single crystals-Implications for H2O-weakening processes[END_REF], where the development of 3585 cm -1 band is confined to deformed parts of single quartz crystals. In addition, a band at 3580 cm -1 was observed to develop after annealing of quartz at high temperature [START_REF] Cordier | Water speciation in quartz: a near infrared study[END_REF][START_REF] Rovetta | Solubility of hydroxyl in natural quartz annealed in water at 900°C and 1.5GPa[END_REF]). In the latter set of experiments, it was correlated with the formation and healing of cracks during annealing, while no cracks were reported in the former study. [START_REF] Stünitz | Water redistribution in experimentally deformed natural milky quartz single crystals-Implications for H2O-weakening processes[END_REF] suggest that the 3585 cm -1 band may record OH-defects associated with dislocations. The systematic association between this band and deformation microstructures is also apparent in our samples. This band, absent in "as-is" Hyuga milky quartz samples, appears after both, hot-pressing and deformation (Figure 10a), while in Brazil quartz it is observed only in unrecrystallized but deformed (e.g. lamellae) porphyroclasts cores (Figure 10f). This close connection to microstructures supports the conclusion by [START_REF] Stünitz | Water redistribution in experimentally deformed natural milky quartz single crystals-Implications for H2O-weakening processes[END_REF] that the band 3585 cm -1 is caused by structurally bond OH at dislocations.

-Absorption bands at 3382 cm -1 and at 3363 cm -1 :

The sharp absorption bands at 3382 cm -1 and at 3363 cm -1 are specific to the nature of the experimental material, either Hyuga or Brazil quartz.

The absorption band at ~3380 cm -1 is systematically observed in Hyuga milky quartz before and after deformation. It has been detected in quartz grains of gneiss [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF] and in cherts [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF] and is attributed by several authors to OH groups bonded with Al +3 substituting for Si [START_REF] Pankrath | Polarized IR spectra of synthetic smoky quartz[END_REF][START_REF] Stalder | OH defects in quartz in the system quartz-albite-water and granite-water between 5 and 25 kbar[END_REF]Konzett, 2012) Kronenberg, 1994;Suzuki and Nakashima, 1999;[START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF]. This interpretation is supported in our case by the high content of aluminum in Hyuga quartz [START_REF] Raimbourg | Fluid circulation in the depths of accretionary prisms: an example of the Shimanto Belt, Kyushu, Japan[END_REF].

The band at 3363 cm -1 is observed only in deformed and recrystallized Brazil quartz, either in the matrix (Figure 9a) or at recrystallized tails of porphyroclasts (e.g. Figure 11). While it is

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generally assigned to surface Si-OH groups [START_REF] Yamagishi | High temperature infrared spectra of hydrous microcrystalline quartz[END_REF], our study does not yield further clues to this attribution.

To summarize, we infer that three different species of H 2 O are observed after deformation.

The first one is molecular H 2 O incorporated in porphyroclasts on the form of fluid inclusions, represented by the broad absorption band between 2800 and 3800 cm -1 . The second type of H 2 O is stored in grain boundaries as a result of recrystallization processes. It is represented by the discrete band at 3595 cm -1 corresponding to structurally bond OH-defects. The third type of H 2 O is characterized by the discrete band at 3585 cm -1 and is probably due to OH-defects on dislocations.

Mechanisms of H 2 O incorporation and expulsion

H 2 O content in "as-is" Hyuga clasts is estimated at ~22,900 H/10 6 Si. It decreases first with the application of temperature and pressure to ~12,000 H/10 6 Si (hot-pressed conditions), and then further decrease to ~2,300 H/10 6 Si after deformation of γ~4 (Table 1 and figs. 1b,c,d and10). The expulsion of H 2 O during the application of pressure and temperature does not proceed through recrystallization, as the corresponding new grains were not observed (Figure 1c). In the absence of significant deformation and formation of grain boundaries through recrystallization, the expulsion of H 2 O is probably caused by micro-fractures draining fluids from inclusions. One driving force for fracturing during the rise in P and T conditions is the ∆P between the fluid pressure in the inclusions initially present (imposed by the fluid isochoric line) and the confining pressure (experimentally imposed). Decrepitation of fluid inclusions is observed for relatively low ∆P of less than a couple of 100 MPa in experiments [START_REF] Hall | Preferential water loss from synthetic fluid inclusions[END_REF][START_REF] Vityk | Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction[END_REF]. Furthermore, the initial assembly has a relatively large porosity and heterogeneous grain size distribution (clast + matrix). The application of P and T results in compaction, hence in local stress concentration, which may result in fracturing that may drain fluid inclusions. These fractures are not easily observable because they are systematically healed at high pressure and temperature [START_REF] Tarantola | Modification of fluid inclusions in quartz by deviatoric stress I: experimentally induced changes in inclusion shapes and microstructures[END_REF].

During subsequent plastic deformation, dynamic recrystallization (Figs. 6-7-13) involving formation and mobility of grain boundaries, provides an efficient process to further decrease the H 2 O concentration in the quartz porphyroclasts (Figure 14a). Grain boundary mobility has an important effect on modification of volume and distribution of fluid inclusions in experiments (Olgaard et FitzGerald, 1993;[START_REF] Schmatz | The interaction of fluid inclusions and migrating grain boundaries in a rock analogue: deformation and annealing of polycrystalline camphor-ethanol mixtures[END_REF]. Observations in natural

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samples (e.g. [START_REF] Kilian | Dislocation creep of dry quartz[END_REF] are consistent with experimental ones. In domains where the Hyuga quartz recrystallized in nature as a result of deformation, the number of fluid inclusions, hence the H 2 O content, is strongly decreased (Palazzin et al., 2016). Similarly, [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF] show a strong decrease in the total H 2 O content with increasing recrystallization. This implies that extensively recrystallized "dry" porphyroclasts in natural shear zones might have been initially wet.

In contrast to Hyuga porphyroclasts, "as-is" Brazil porphyroclasts have a very low initial H 2 O content (~260 H/10 6 Si). After deformation, both, slightly deformed cores and strongly recrystallized domains, show a significant increase in H 2 O: up to 640 and 950-1530 H/10 6 Si, respectively (Table 1, Figures 11-14a). Their final content is similar to the hydrated matrix, which indicates a transfer of H 2 O from the matrix to the porphyroclasts during deformation.

The process responsible for the increased H 2 O content of recrystallized tails of Brazil porphyroclasts is probably identical to that of Hyuga clasts inferred above. The transfer of H 2 O to the unrecrystallized cores of Brazil porphyroclasts is probably caused by microcracking as inferred by [START_REF] Kronenberg | Solubility and diffusional uptake of hydrogen in quartz at high water pressures: Implications for hydrolytic weakening[END_REF]; [START_REF] Rovetta | Solubility of hydroxyl in natural quartz annealed in water at 900°C and 1.5GPa[END_REF]; [START_REF] Gerretsen | The uptake and solubility of water in quartz at elevated pressure and temperature[END_REF] and Cordier andDoukhan (1989, 1991). The H 2 O uptake by non-recrystallized cores of Brazil porphyroclasts, has a limited weakening effect, as attested by the higher strength of samples with a high content of Brazil porphyroclasts (Figure 2). Their final H 2 O content, ~640H/10 6 Si, is nevertheless higher than the minimum amount of H 2 O in wet synthetic quartz (~200 H/10 6 Si) necessary for weakening of these crystals [START_REF] Kekulawala | Hydrolytic weakening in quartz[END_REF][START_REF] Kekulawala | An experimental study of the role of water in quartz deformation[END_REF].

H 2 O budget

Concentration and distribution of H 2 O

We measured relatively large and variable (from ~810 to ~3710 H/10 6 Si) concentrations of H 2 O in the deformed quartz assemblies (Table 1). The large variability reflects first the difference in initial H 2 O content: in deformation experiments with Hyuga porphyroclasts, the final H 2 O content is between ~1470 and 3710 H/10 6 Si, while in experiments with Brazil porphyroclasts, or pure matrix, the final H 2 O content is between ~640 and 1530 H/10 6 Si. In experiments in quartzite of a large grain size, carried out at the same P-T conditions as ours, a strain rate of 10 -6 s -1 and with 0.3 wt% H 2 O [START_REF] Post | The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening[END_REF], the H 2 O content in quartz interiors is ~800 H/10 6 Si, i.e. considerably lower than that in the finely recrystallized Hyuga material, in spite of the very large amount of initially added H 2 O (0.3 wt% H 2 O ≈ 20,000H/10 6 Si). In natural mylonite deformed at 800°C and 800MPa, the intracrystalline H 2 O content is ~320 H/10 6 Si [START_REF] Gleason | A natural example of crystal-plastic deformation enhancing the incorporation of water into quartz[END_REF]). A compilation of data of natural quartz deformed in nature and in experiments, shows that intragranular H 2 O is significantly lower than bulk measurements, which include the contribution of H 2 O at grain boundaries [START_REF] Kilian | Dislocation creep of dry quartz[END_REF]. This hypothesis is supported by direct FTIR transects across grains and their boundaries [START_REF] Post | The rate of water penetration in experimentally deformed quartzite: implications for hydrolytic weakening[END_REF], or by the inverse relationship, in various metamorphic rocks, between the grain size of a rock and the H 2 O volume that can be stored in the grain aggregate [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF].

The concentration of H 2 O stored in the grain boundaries of the recrystallized material varies systematically; in experiments with 30% or 70% of Hyuga porphyroclasts, the recrystallized porphyroclasts yield an approximately uniform content of ~2300 H/10 6 Si. The matrix contains similar amounts of H 2 O (~1500 and 2900 H/10 6 Si). These matrix H 2 O contents are systematically larger than in the 100% matrix experiment (~800 H/10 6 Si), despite the same amount of initially matrix-added water. Thus, H 2 O is redistributed from Hyuga porphyroclasts to the matrix producing a uniform range of H 2 O contents in Hyuga porphyroclasts and matrix.

The H 2 O concentration tends to some "equilibrium" concentration, similar to the microstructural evolution which tends to some final "equilibrium" state (Figure 14a).

Shear bands: evidence for excess H 2 O

Shear bands are primarily observed in experiments with Hyuga porphyroclasts and are composed of small faceted quartz grains surrounded by void space (Figure 7c). IR measurements show that they contain a significantly larger amount of H 2 O than the surrounding matrix (Table 1). Their CPO is also different from the surrounding quartz matrix (Figure 12). The grain shapes, presence of voids between crystals, and the different crystal orientations from host quartz suggest growth of these quartz grains in a pore space filled with

H 2 O stored in the grain boundary region

We observed that the deformed quartz aggregates develop towards a common microstructure (Figs. 11-12 and 13), strength (Figure 2), and H 2 O content of the matrix, clasts, and recrystallized material regardless of the initial starting material and relative clast contents (Figure 14a). The redistribution of H 2 O mainly takes place by recrystallization and formation of new grain boundary area. If it is assumed that the largest part of the H 2 O is distributed along the grain boundary regions (the grain interiors cannot store a large amount of H 2 O as fluid inclusions because the grain size is below 10 µm), then the volume fraction of H 2 O in the quartz aggregate can be plotted against the grain boundary area per analyzed volume (Figure 14b). The volume fraction of H 2 O is measured by FTIR (Table 1), and the grain boundary density (length per surface) is determined by EBSD in the deformed microstructures (Figs. 11-12 and 13, Table 4). A similar approach was taken by [START_REF] Ito | Water distribution in low-grade siliceous metamorphic rocks by micro-FTIR and its relation to grain size: a case from the Kanto Mountain region, Japan[END_REF], who used modeled average grain sizes instead of measured boundary fractions. As a result, the pure matrix and the matrix with Brazil clasts have the same nominal grain boundary widths of ~0.7 nm, whereas the matrix with Hyuga clasts has a grain boundary width which is more than twice this width (~1.7 nm; Figure 14b). Realistic grain boundary widths have been determined to be ~0.5 nm for feldspar [START_REF] Hiraga | Grain boundaries as reservoirs of incompatible elements in the Earth's mantle[END_REF]) and ~1nm for pyroxene [START_REF] Raimbourg | Crystal bending, subgrain boundary development, and recrystallization in orthopyroxene during granulite-facies deformation[END_REF], of similar magnitude to those determined here for quartz. The physical state of H 2 O stored at grain boundaries is not established, nor the precise nature of the grain boundary itself [START_REF] Raimbourg | Crystal bending, subgrain boundary development, and recrystallization in orthopyroxene during granulite-facies deformation[END_REF]. Two types of models are usually proposed for H 2 O along the grain boundary: (1) A thin molecular film or (2) A "channel-island" structure, where domains of thin film ("islands") are separated by channels where the thickness of H 2 O is larger (e.g. [START_REF] Brok | Effect of microcracking on pressure-solution strain rate: The Gratz grainboundary model[END_REF][START_REF] Nakashima | Diffusivity of ions in pore water as a quantitative basis for rock deformation rate estimates[END_REF]). Our results cannot resolve the details of H 2 O distribution along grain boundary, and the method employed here is neither appropriate nor intended for an exact determination of a grain boundary width. The "nominal boundary width" obtained here is merely a measure of the H 2 O storage in the grain boundary region under the assumption that the H 2 O is evenly distributed along the grain boundaries with a common grain boundary width (Figure 14a). is exceeded, shear bands may develop. This process accounts for the fact that shear bands develop principally in the material with the highest initial H 2 O content (i.e. in the samples with Hyuga porphyroclasts). Riedel shear bands, formed during deformation and acting as drains to collect the H 2 O from the surrounding matrix, have been similarly described in wet (1.3 wt% H 2 O) Dover flint experiments [START_REF] Schmocker | Granular flow and Riedel band formation in water-rich quartz aggregates experimentally deformed in torsion[END_REF].

The shear bands do not have a weakening effect on the bulk sample: The sample with 70% Hyuga clasts and the highest H 2 O content is not weaker than others (Figure 2). The lack of weakening by shear bands can be explained by a lack of simple connectivity of these bands on the sample scale [START_REF] Pec | Semi-brittle flow of granitoid fault rocks in experiments at mid-crustal conditions[END_REF][START_REF] Marti | Experimental investigation of the brittle-viscous transition in mafic rocks -Interplay between fracturing, reaction, and viscous deformation[END_REF].

If H 2 O is principally stored at grain boundaries, the storage capacity of the grain aggregate is controlled by the grain size distribution and evolution. The process of redistribution of H 2 O takes place by different processes of microcracking (including fluid inclusion decrepitation) and formation of grain boundaries by recrystallization. These processes can be observed in natural samples and the H 2 O storage capacity will be an important parameter to be determined for deformation in natural samples, too.

Conclusions

In this experimental study the mechanical behavior, microstructural evolution, and H 2 O content of different starting materials of "wet" quartz converge at higher shear strain at T=800°C, strain rate=5*10 -5 s -1 and Pc=1.5GPa. From these results we draw several conclusions:

• The high crystalline H 2 O content, initially present in the form of fluid inclusions in wet quartz, is responsible for enhancing the recrystallization rate, in contrast to initially dry crystals that do not recrystallize extensively.

• Sharp absorption bands at 3585 cm -1 and 3595 cm -1 develop during deformation. The occurrence of the 3585 cm -1 band is consistent with dislocation-bound OH, whereas the 3595 cm -1 band most likely indicates OH adsorbed at grain boundaries.

• Recrystallization results in a clear decrease in H 2 O concentration in quartz crystals initially rich in fluid inclusions, whereas H 2 O uptake in initially dry crystals takes place by microcracking.

• In the deformed and recrystallized assemblies, most H 2 O is stored in the grain boundary region. The amount that can be stored in the grain boundary (i.e. the storage capacity) is limited and corresponds to a nominally ∼1.7 nm thick grain boundary region. H 2 O in excess of this nominal thickness is drained into micro-veins, formed as dilatant synthetic shear bands.

• These dilatant shear bands do not control the strength of samples as they are not simply connected.

  peak shear stress and lower flow strength values (Figure 2, Table 2) indicate dominant plastic deformation in most samples at the applied strain rate of ~2.1 x 10 -5 s -1 . Only the 70% Brazil clasts sample reaches peak stress above the Goetze criterion (∆σ = 1600 MPa Figure 2) for plastic deformation (∆σ ≈ P conf ;

  ) and the presence of abundant voids (SEM observations) along the shear bands (Figure7a,c).

  Hyuga porphyroclasts, with their stored high H 2 O content in fluid inclusions, act as an additional source of H 2 O.The experiments were carried out to large shear strain in an attempt to reach some steady-state of the microstructure and H 2 O concentration. The flat portions of σ-Ɛ curves do not reach state, but the convergence of strengths of most samples indicates that some final stage of the microstructural development has been reached. In order to find a physical meaning of the H 2 O concentration in experiments with Hyuga porphyroclasts, we need to consider where the H 2 O is stored, as the IR analysis window size (50x50µm) is larger than the grain size, so that the analyses include grain interiors and grain boundaries.

  at high P and T. These shear bands appear to be the locations where excess free fluid phase is stored.

  The thicker nominal grain boundary width of the sample with Hyuga clasts and higher H 2 O content coincides with the occurrence of shear bands (absent in other samples), which are sites of porosity and thus H 2 O storage. It is likely that the nominal boundary width of ~1.7 nm corresponds to the H 2 O storage capacity of the grain boundary region. If this storage capacity

Table 1 -

 1 Water content

	Sample	Composition of	Condition	Measured	Thickness	Abs	SD	wt.	SD	wt. ppm	SD	Number of
	number	sample	of sample	material	(cm)	(cm²)		ppm		H/10 6 Si		analyses
								H 2 O
	457GP	100% matrix	deformed	Matrix	0.0120	11.1	2.90	121	28	807	189
	starting material	Hyuga porphyroclasts	as-is	Porphyroclast	0.0140	383.5	164.4	3446	1234	22973	8224
	464GP	Hyuga porphyroclasts	attained P-T conditions	Core of porphyroclast	0.0133	191.2	56.2	1809	516	12058	3448
	464GP	Matrix with Hyuga porphyroclasts	attained P-T conditions	Matrix	0.0127	63.9	34.7	633	344	4219	2296
	428GP	Hyuga porphyroclasts	deformed	Porphyroclast	0.005	13.6	5.61	342	141	2278	941	8
	428GP	Matrix with Hyuga porphyroclasts	deformed	Matrix	0.005	8.76	1.31	220	33	1469	220	9
	428GP	Matrix with Hyuga porphyroclasts	deformed	Shear Bands	0.005	22.1	5.66	556	142	3710	949
	456GP	Hyuga porphyroclasts	deformed	Recrystallized porphyroclast	0.0141	39,6	7.77	353	76	2355	509
	456GP	Matrix with Hyuga porphyroclasts	deformed	Matrix	0.013	44.8	14.8	430	129	2868	863	9
	starting material	Brazil porphyroclasts	as-is	Porphyroclast	0.0136	4.19	0.9	39	10	258	68	7
	426GP	Brazil porphyroclasts	deformed	Core of porphyroclast	0.0123	9.36	6.18	96	68	638	455
	426GP	Brazil porphyroclasts	deformed	Recrystallized porphyroclast	0.0118	21.5	14.9	229	131	1526	873
	426GP	Matrix in Brazil sample	deformed	Matrix	0.0131	14.8	5.03	142	45	947	299
	465GP	Brazil porphyroclasts	deformed	Recrystallized porphyroclast	0.0201	21.5	8.35	135	52	898	348
	465GP	Matrix in Brazil sample	deformed	Matrix	0.0201	31.9	9.39	200	59	1332	391

Table 2 -

 2 Mechanical Data

	Sample number	Composition of sample	% water added	ɣ̇ [s -1 ]	[MPa] Peak τ	shear stress ɣ at peak	[MPa] τ at ɣ = 3	[MPa] τ at end	ɣ final
	457GP	100% matrix	0.15	2.21 x 10 -5	192	0.61	164	201	4.59
	462GP	"	"	2.19 x 10 -5	307	0.78	247	256	3.54
	426GP	30% Brazil p.	0.10	2.12 x 10 -5	421	0.81	286	271	3.30
	428GP	30% Hyuga p.	0.10	2.08 x 10 -5	245	0.80	247	256	3.14
	465GP	70% Brazil p.	0.045	2.17 x 10 -5	645	1.55	553	445	4.31
	456GP	70% Hyuga p.	0.045	2.07 x 10 -5	398	0.69	277	292	3.87
	464GP	30% Hyuga p.	0.10	0.5	-	-		-	-
	459GP	100% matrix	0.15	18	-	-		-	-

Table 3 -

 3 Absorption bands

	Absorption [cm -1 ]

Table 4 -

 4 Grain boundary density

	Experiment	Composition of sample	Grain boundary density (1/m)
	457GP	100% matrix	4.7 10 5
	456GP	70% Hyuga porphyroclasts	6.7 10 5
	426GP	30% Brazil porphyroclasts	6.09 10 5
	465GP	70% Brazil porphyroclasts	7.75 10 5

Acknowledgements

We thank S. Janiec and G. Badin for thin and thick sections preparation, I. Di Carlo for help with the EBSD analyses, and S. Marti and B. Richter for their help in CIP and mechanical data processing. This work has received funding from (i) the European Research Council (ERC) under the seventh Framework Program of the European Union (ERC Advanced Grant, grant agreement No 290864, RHEOLITH) and (ii) the Labex VOLTAIRE (ANR-10-LABX-100-01), which are acknowledged. We gratefully thanks A. Kronenberg and L. Menegon for their very constructive reviews and the editor T. Takeshita for his help.