# Laboratory earthquakes triggered during eclogitization of lawsonite-bearing blueschist

Sarah Incel, Nadège Hilairet, Loïc Labrousse, Timm John, Damien Deldicque, Thomas P. Ferrand, Yanbin Wang, Jörg Renner, Luiz Morales, Alexandre Schubnel

► To cite this version:

Sarah Incel, Nadège Hilairet, Loïc Labrousse, Timm John, Damien Deldicque, et al. Laboratory earthquakes triggered during eclogitization of lawsonite-bearing blueschist. Earth and Planetary Science Letters, 2017, 459, pp.320-331. 10.1016/j.epsl.2016.11.047. insu-03747286

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Accepted Manuscript

Laboratory earthquakes triggered during eclogitization of lawsonite-bearing blueschist

Sarah Incel, Nadège Hilairet, Loïc Labrousse, Timm John, Damien Deldicque et al.

PII: S0012-821X(16)30681-1

DOI: http://dx.doi.org/10.1016/j.epsl.2016.11.047

Reference: EPSL 14211

To appear in: *Earth and Planetary Science Letters*

Received date: 2 August 2016

Revised date: 17 November 2016

Accepted date: 24 November 2016

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Please cite this article in press as: Incel, S., et al. Laboratory earthquakes triggered during eclogitization of lawsonite-bearing blueschist. *Earth Planet. Sci. Lett.* (2016), http://dx.doi.org/10.1016/j.epsl.2016.11.047

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## Highlights

* Brittle failure during the transition from Lws-blueschist to Lws-eclogite.

* Grain-size reduction is proposed to cause brittle failure at HP/LT conditions.

* Fast shear fuses the rock to produce lab-equivalents to natural pseudotachylites.
1 Laboratory earthquakes triggered during eclogitization of lawsonite-bearing blueschist

2 Sarah Incel¹, Nadège Hilairet², Loïc Labrousse³, Timm John⁴, Damien Deldicque¹, Thomas

3 Ferrand¹, Yanbin Wang⁵, Jörg Renner⁶, Luiz Morales⁷, Alexandre Schubnel¹

4 ¹Laboratoire de Géologie de l'ENS - PSL Research University - UMR8538 du CNRS, 24 Rue

5 Lhomond, 75005 Paris, France

6 ²CNRS - UMET, Université Lille 1, 59655 Villeneuve d'Ascq, France

7 ³Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France

8 ⁴Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany

9 ⁵GeoSoilEnviroCARS, University of Chicago, Argonne, IL 60439, USA

10 ⁶Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany

11 ⁷ScopeM - ETH Zürich. Auguste-Piccard-Hof 1, HPT D9, 8093, Zürich, Switzerland

12 Corresponding author: incel@geologie.ens.fr

13 Abstract

14 The origin of intermediate-depth seismicity has been debated for decades. A substantial

15 fraction of these events occurs within the upper plane of Wadati-Benioff double seismic zones

16 believed to represent subducting oceanic crust. We deformed natural lawsonite-rich blueschist

17 samples under eclogite-facies conditions (1.5 < *P* < 3.5 GPa; 583 K < *T* < 1121 K), using a D-

18 DIA apparatus installed at a synchrotron beamline continuously monitoring stress, strain,

19 phase changes, and acoustic emissions (AEs). Two distinct paths were investigated: i) heating

20 during deformation at pressures > 2.5 GPa to maximum temperatures ranging from 762-1073 K, during which lawsonite and glaucophane became gradually unstable while entering the

21 stability field of lawsonite-eclogite and the breakdown reaction of lawsonite was only crossed

22 in case of the highest final temperature; ii) heating while deforming at a pressure < 2 GPa to a

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24 maximum temperature of 1121 K associated with crossing the breakdown reaction of lawsonite
25 and successively entering the stability fields of epidote-blueschist and eclogite-amphibolite but
26 not of lawsonite-eclogite. Upon entering the Lws-Ecl stability field samples exhibited brittle
27 failure, accompanied by the radiation of AEs. In-situ X-ray diffraction and microstructural
28 analysis demonstrate that fractures are topologically related to the formation of omphacite.
29 Amorphous material was detected along the fractures by transmission-electron microscopy
30 without evidence for free-water. Since the newly formed omphacite crystals are small
31 compared to the initial grains, we interpret the observed mechanical instability as a
32 transformation-induced runaway under stress triggered during the transition from lawsonite-
33 blueschist to lawsonite-eclogite. In contrast, we find no microstructural evidence that the
34 breakdown of lawsonite, and hence the liberation of water leads to the fracturing in samples
35 that experienced the highest quench temperatures of 1073 and 1121 K, although some AEs
36 were detected during an experiment performed at 1.5 GPa. Our experimental results challenge
37 the concept of "dehydration embrittlement", which ascribes the genesis of intermediate-depth
38 earthquakes to the breakdown of hydrous phases in the subducting oceanic plate. Instead we
39 suggest that grain-size reduction (transformational faulting) during the transition from
40 lawsonite-blueschist to lawsonite-eclogite leads to brittle failure of the deviatorically loaded
41 samples.

42 **Keywords:** Intermediate-depth seismicity, eclogitization, transformational faulting, shear
43 heating, dehydration embrittlement, lawsonite-blueschist

44 **1. Introduction**

45 Intermediate-depth earthquakes occur between approximately 50 to 300 km
46 (Gutenberg and Richter, 1945; Hacker et al., 2003). Almost all intermediate-depth earthquakes
47 occur in subduction zones and follow two distinct planes – the so-called upper and lower
48 Wadati-Benioff planes of seismicity (Frohlich, 1989; Brudzinski et al., 2007). The upper plane
49 is believed to correspond to earthquakes generated within the subducting oceanic crust while
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the lower plane is ascribed to earthquakes located in the subjacent mantle (Hacker et al., 2003).

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For several decades, scientists have been puzzled by the occurrence of earthquakes at intermediate-depths at which rocks are expected to deform plastically under the prevailing pressures and temperatures (Raleigh and Paterson, 1965; Green and Burnley, 1989). A multitude of theoretical, experimental, and field studies addressed this problem (e.g., Kirby, 1987; Green and Burnley, 1989; Hirth and Tullis, 1994; Dobson et al., 2002; Austrheim and Andersen, 2004; Jung et al., 2004; Hacker et al., 2003; John and Schenk, 2006; Kelemen and Hirth, 2007; John et al., 2009; Gasc et al., 2011; Schubnel et al., 2013; Deseta et al., 2014; Thielmann et al., 2015). From these, three main hypotheses for the mechanisms of intermediate-depth earthquakes emerged: i) thermal runaway or shear heating (Hobbs and Ord, 1988; John et al., 2009; Prieto et al., 2013), ii) dehydration embrittlement (Green and Houston, 1995; Hacker et al., 2003; Jung et al., 2004), and iii) transformational faulting (Kirby, 1987; Green and Burnley, 1989; Schubnel et al., 2013). The term thermal runaway describes the feedback between shear heating and the temperature-dependent rock rheology. The dehydration embrittlement model postulates a change from ductile to brittle behavior due to an increase in pore-fluid pressure and hence a decrease in effective stress during the breakdown of hydrous phases in the rock. Finally, transformational faulting refers to mechanical weakening of the rock caused by volume and/or enthalpy changes but also by grain-size reduction during transformation.

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So far, most experimental work testing these hypotheses focused on serpentine-dehydration experiments in the pressure-temperature range of intermediate-depth seismicity. Investigating metamorphic reactions within metabasalts of the descending oceanic crust has been almost completely disregarded (except for Okazaki and Hirth, 2016), although the distribution of hypocenters within the subducting oceanic crust correlates very well with the thermodynamically expected stability fields of different paragenesis for basaltic rock compositions at elevated pressures and temperatures (Peacock and Wang, 1999; Hacker et

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al., 2003; Kita et al., 2006). Indeed, hypocenter locations in cold subducting oceanic crust such as below northern Japan (Kita et al., 2006) coincide with the predicted stability field of lawsonite-eclogite (Lws-Ecl), i.e. an intermediate facies between lawsonite blueschists (Lws-Bs), comprising glaucophane and lawsonite as hydrous phases, and eclogites (Ecl) whose paragenesis of garnet + omphacite is nominally dry. The lack of seismicity once entering the stability field of eclogite is striking (Kita et al., 2006) and this seismic boundary suggests that metamorphic reactions during the transition from Lws-Bs to Ecl transiently change the rheology of the rock and enable brittle failure.

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The key objective of the present contribution is to distinguish the mechanical impacts of glaucophane and lawsonite breakdown and their potential link with brittle failure. We performed deformation experiments with in-situ monitoring of phase content and acoustic emissions on lawsonite-bearing blueschist samples. The discussion focuses on the influence of metamorphic reactions during deformation on the micro- and nanostructures as well as on the record of acoustic emissions. Finally, we present a plausible mechanism for brittle failure of the samples during their transformation to eclogite.

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## 2. The Corsican blueschist

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### 2.1 Sample description

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A lawsonite-blueschist (Lws-Bs) from Alpine Corsica served as sample material. Having experienced peak metamorphic conditions of approx. 520 ± 20 °C and 2.3 ± 0.1 GPa (Vitale Brovarone et al., 2011) it mostly consists of glaucophane (Gln, mineral abbreviations follow Whitney and Evans, 2010) and lawsonite (Lws), where Lws often appears as lenses. The Gln:Lws ratio of approx. 3:2 is estimated from optical analyses of several thin sections. The minor phases actinolite (Act), garnet (Grt), phengite (Ph), titanite (Ttn), and omphacite (Omp1), with $X_{jd} \approx 0.4$ and $X_{di} \approx 0.6$, comprise less than 15 % of the rock's volume. Apatite (Ap), and albite (Ab) are also found in limited amounts (Vitale Brovarone et al., 2011). The

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mineral phases Gln, Omp1, and Act were chemically investigated (Figure 1) and our analyses are in good agreement with those previously reported (Vitale Brovarone et al., 2011).

## 2.2 Sample preparation

A fragment of the Lws-Bs, selected for its homogeneous distribution of Gln and Lws content, was crushed, ground and sieved to a grain size fraction < 38 µm. A Frantz isodynamic magnetic separator was used to segregate Gln and Lws. Three different powders with different Gln:Lws ratios were produced: i) bulk-rock, ii) glaucophane-rich (Gln-rich), and iii) lawsonite-rich (Lws-rich) (Figure 1). Powders were used rather than the intact rock, because of i) the heterogeneity in mineralogy (e.g. due to Lws lenses) and texture (schistosity), which would considerably hamper the sample-to-sample comparison and ii) the need of a small grain-size (< 38 µm) for the in-situ X-ray powder diffraction (XRD) during the deformation of the samples (more details in 3.4).

## 2.3 Characterization of the starting material

Powders were embedded in epoxy and afterwards characterized by element mapping of Ca and Na using a scanning electron microscope (SEM). Element ratios were calculated using the software ImageJ (Abramoff et al., 2004) and can be directly transformed to different Gln:Lws ratios (Figure 1). The bulk-rock, Gln-rich, and Lws-rich powders show Gln:Lws ratios of approx. 1:1, 2:1, and 1:3, respectively. As in the natural rock, minor and accessory phases comprise around 15 % of the powder volume. For these three different powders, three different P-T pseudosections were calculated in the NCFMASH system using the thermodynamic modeling software Theriak-Domino (De Capitani and Petrakakis, 2010). As expected for a divariant reaction, the position of the Gln-out reaction depends on the system's stoichiometry, hence on the Gln:Lws ratio (Figure 2). The slightly lower stability of amphibole (Amp) or rather Gln at HP/ LT of the Gln-rich powder relative to the bulk-rock powder (Figure 2) is related to an increase of Fe and Mg content with increasing Gln in the system stabilizing the almandine
component of Grt. The univariant Lws-breakdown curve (Lws = Ep) shifts with bulk chemistry due to the Ca balance between garnet (grossular component) and lawsonite. The calculated stability fields of the different metabasalts and the positions of the different Gln- and Lws-out curves for the three powders (Figure 2) are in agreement with previous findings (Vitale Brovarone et al., 2011).

## 3. Experimental methods

### 3.1 Experimental setup

Deformation experiments were performed using a deformation diamond-anvil (D-DIA) apparatus, attached to the GSECARS synchrotron beamline 13-BM-D at the Advanced Photon Source Chicago. During nominal isotropic pressurization, a main ram controls the advancement of four horizontal and two vertical anvils that compress the sample assembly until a prescribed load is reached. Differential rams, which move the vertical anvils independently, create a deviatoric stress field in the sample assembly (Wang et al., 2003). Anvils are made of tungsten carbide (WC), except for two horizontal anvils composed of sintered diamond, translucent to X-rays. The use of synchrotron X-rays allows for in-situ monitoring of strain, using radiographs of the sample, but also for recording powder diffraction patterns from which phase changes and stress state are deduced. Temperature is inferred from temperature-output power calibrations with an uncertainty below 10 %. Raterron et al. 2013 demonstrated the existence of non-negligible axial temperature gradients of about 155 K/mm with the sample's center colder than the edges.

### 3.2 Sample assembly

The powder was placed in a cylindrical BN sleeve surrounded by the graphite furnace and placed into 9 x 9 x 8 mm amorphous Boron-epoxy cuboids. The sample is sandwiched between two gold foils and two alumina pistons (Figure 3). The initial size of the powder charge is approx. 2.1 mm in diameter by 3 mm in height. All powders were hot-isostatically pressed
152 before triaxial deformation at temperatures of about 583 to 653 K for approx. 1 h (Table 1).
153 Sample BS_3_1073 was hot-pressed in a piston-cylinder apparatus at 3 GPa and 923 K for 1
154 day, then machined and loaded in the deformation cell assembly. This sample was kept at
155 isostatic conditions at 583 K in the D-DIA for only 30 min.

156 **3.3 Experimental procedure**

157 After the hot-pressing, deformation with a strain rate of about 10⁻⁵ s⁻¹ was started.
158 Syndeformational heating was performed by increasing the power in steps of 10 W
159 corresponding to a temperature increase of approx. 50 K. The heating steps were initiated at
160 10, 15, 20, 25, 30, and 35 % strain for the Gln-enriched and the Lws-enriched samples, at 10,
161 20, 30, and 40 % strain for the run BS_1.5_1121, and at 5, 12, 18, 20, 25, 30, and 35 % strain
162 for the run BS_3_1073.

163 **3.4 In situ data acquisition setup, stress and strain computations**

164 The X-ray transmission images and diffraction patterns of the samples were acquired
165 using a monochromatic synchrotron X-ray beam at 13-BMD (λ = 0.2431 Å, approx. 51 keV)
166 during the course of deformation runs (Figure 3). The typical acquisition time for images was
167 2 s and for the diffraction patterns 300 s. In image mode, the beam is typically 2 mm x 3 mm,
168 i.e., comparable to initial sample size, and the CCD camera is positioned to capture the
169 radiograph. In diffraction mode, the CCD camera is moved out of the diffraction path and slits
170 reduce the beam size to typically 200 x 200 microns. Therefore, only a limited volume in the
171 center of the sample is probed. The detector tilt and rotation relative to the incident beam were
172 calibrated using the diffraction pattern of a CeO₂ standard and the software Fit2D (Hammersley
173 et al., 1996).

174 The d-spacing of a crystal is sensitive to the orientation of the diffracting plane with
175 respect to an anisotropic stress field. For instance, the d-spacing of lattice planes oriented
176 perpendicular to the direction of the maximum principal stress σ₁ decreases and thus the
177 diffraction line shifts to higher 2θ angles. Therefore, in a powder with random crystal orientations, the different responses of lattice planes to anisotropic stress result in a distortion or pseudo-ellipticity of the Debye rings that are initially circular under hydrostatic conditions. 178 179 This distortion is called lattice strain and can be used as a proxy for the differential stress 180 effectively acting on the polycrystalline aggregate (Uchida et al., 1996, Singh et al., 1998). The 181 deviation of the diffracting plane from the normal to the maximum compression axis is 182 characterized by an angle ψ related to the diffraction angle θ according to: 183

$$ \cos \psi = \cos \theta \cos \delta \qquad (1) $$

184 where δ represents the projection of ψ on the detector plane. Calculation of differential stress 185 was performed using Fit2D (Hammersley et al., 1996) and the multifit-polydefix software 186 (Merkel and Hilairet, 2015). The relation between the d-spacing d<sub>(hkl)</sub> (for a lattice plane with 187 indices hkl) and the lattice strain Q<sub>(hkl)</sub> reads: 188

$$ \frac{d_{(hkl)}(\psi) - d_{P,(hkl)}(\psi)}{d_{P,(hkl)}(\psi)} = Q_{(hkl)}(1 - 3\cos^2 \psi) \qquad (2) $$

189 (Uchida et al., 1996; Singh et al., 1998) where d<sub>P,(hkl)</sub> denotes the d-spacing at “hydrostatic” or 190 isotropic stress and d<sub>(hkl)</sub>(ψ) corresponds to the d-spacing of the plane hkl with angle ψ to the 191 maximum principal stress. For each experiment, the d-spacings were extracted from the 192 diffraction patterns and fitted in a d<sub>(hkl)</sub> over ψ plot. 193 194

195 The refinement of the lattice strain relied on the d<sub>P,(hkl)</sub> of up to 5 crystallographic planes. 196 The planes (110), (-111), and (151) of glaucophane were always used but sometimes also the 197 (-202) and the (240) planes for the processing of experiments on the bulk-rock or the Gln-rich 198 powders. For tests on the Lws-rich powder, the 3 crystallographic planes (002), (201), and 199 (202) of lawsonite sufficed to refine the unit cell volume. We employed the second-order 200 equation of state for Gln and Lws (Comodi et al., 1991; Mao et al., 2007) and constraints on 201 their thermal expansion (Holland et al., 1996; Jenkins and Corona, 2006).
202 Mean stress $\sigma_m$ calculates as:

$$ \sigma_m = \frac{\sigma_1 + \sigma_2 + \sigma_3}{3} \qquad (4) $$

204 where $\sigma_1$ denotes the maximum principal stress, and $\sigma_2$ and $\sigma_3$ are considered identical to the 205 confining pressure $P_c$ determined based on the shift in peak position before and after 206 compression and heating. Differential stress $t_{(hkl)}$ was calculated for a specific plane using the 207 lattice strain $Q_{(hkl)}$ according to (2) and the effective moduli $G_{(hkl)}$:

$$ t_{(hkl)} = 6 Q_{(hkl)} G_{(hkl)}. \qquad (3) $$

209 The effective moduli $G_{(hkl)}$ derive from the elastic compliances $S_{ij}$ (inverted from the stiffness 210 matrix $C_{ij}$) for Gln (Bezacier et al., 2010) and Lws (Mao et al., 2007). In the following, the 211 differential stress is obtained by averaging the $t_{(hkl)}$ values observed for the two most intense 212 diffraction peaks. For the bulk-rock and the Gln-rich experiments, these corresponded to the 213 (110) and (151) planes of glaucophane and to the (002) and (201) planes of lawsonite for the 214 Lws-rich experiment.

215 The calculated differential stresses do not represent absolute values (see Hilairet et al., 216 2012, for a detailed discussion). Due to difficulties in calculating the effective moduli for low- 217 symmetry phases like Gln, the calculation relied on orthorhombic instead of monoclinic 218 symmetry. For a single experiment, it was possible to simultaneously calculate the differential 219 stresses from Gln and Lws crystals. While the calculated differential stresses lie in the same 220 order for the two phases, Lws appears to support more differential stress than Gln (Figure 4c), 221 in agreement with previous experimental studies on blueschist rheology (Kim et al., 2013).

222 The powder-diffraction patterns were also used to continuously monitor the phase 223 assemblage, using diffraction peak heights for individual phases integrated over the 360° of 224 the Debye rings. While this is a crude estimate and does not take into account the effects of 225 evolution in preferred orientations, the changes in intensities observed here are consistent with 226 the microstructural observations and with the calculated pseudosections.
Bulk axial strain was determined using the radiographs of the samples. The gold foils on top and below a sample act as strain markers due to their high X-ray absorption contrast compared to the sample and the ceramic pistons. The current length of the sample was determined by measuring the distance between the shadows of the gold foils and strain ε was calculated by comparing the change in sample length, ΔL, the initial length L before the deformation was started, i.e.:

$$\varepsilon = \frac{\Delta L}{L} \qquad (5)$$

Strain rate was calculated by relating the strain to the time increments between radiographs.

## 3.5 Acoustic emission setup and localization technique

Piezoceramic lead-zirconate transducers with a thickness of 0.5 mm, a diameter of 10 mm, and a resonance frequency of 2-4 MHz (see also Schubnel et al., 2013, for details of the setup) were placed behind each of the six anvils (Figure 3). Acoustic emission (AE) waveforms were acquired simultaneously from all six transducers by a multi-channel oscilloscope. Emissions were recorded over the entire duration of each experiment, including cold compression, hydrostatic heating, deformation, and syndeformational heating sequence, then quenching and decompression. In fact, during the first stage of compression many AEs were recorded, which are attributed to powder compaction. From ca. 1 GPa to the final confining pressure (Table 1), no further AEs were recorded demonstrating that the AEs subsequently recorded during triaxial deformation are not caused by the compaction. Furthermore, previous deformation experiments with the same acoustic setup and the same sample assembly (Gasc et al., 2011) demonstrated that except for the porous alumina, deformation of the cell materials does not result in AEs, thus excluding the assembly as a cause for the AEs recorded in the present study.

Locations of AEs were determined using a constant P-wave velocity of 8000 m/s determined by previous experiments with the same experimental setup (Schubnel et al., 2013).
It is assumed that the travel time within each anvil remains constant over the course of an experiment. The travel time in the sintered diamond anvils was calculated to be 0.78 µs lower than in the WC anvils. Picking of the first arrivals was performed manually as the total number of AEs recorded during deformation was limited. The travel time was first corrected for anvil type, then the difference in arrival times between each pair of facing anvils was interpreted as a difference in travel-time within the sample assembly. The precision of the location of AEs along each of the three axis between the 3 anvil pairs lies within the sensor thickness of around 0.5 mm. The exact localization of each hypocenter in the sample would require a more sophisticated AE data analysis which lies beyond the scope of the present study.

## 4. Results

Our study comprises six deformation experiments using the three powders at the experimental conditions shown in Table 1. The sample nomenclature uses letters to represent the powder type, i.e. BS for blueschist bulk-rock, GLN for Gln-rich, and LWS for the Lws-rich powder, and numbers to give confining pressure in GPa and the quench temperature in K on the second and third position, respectively.

### 4.1 Mechanical Data, in-situ XRD analysis, and AE records

Differential stress decreased during syndeformational heating in all tests, i.e., independent of sample composition (Figure 4a-f). The recorded diffraction patterns reveal that the intensity of the Omp (-221) peak, the most intense peak of Omp, first increases above approx. 573 K but decreases above 700 K all the way to the end of the deformation (Figure 4g- i). The sample, which was hot-pressed in a piston-cylinder, initially shows more Omp, but follows the same Omp (-221) intensity evolution as the other samples hot-pressed in the D-DIA (Figure 4i). The Lws (002) peak completely disappears in bulk-rock samples (BS_1.5_1121, BS_3_1073) passing 734 and 1020 K, respectively (Figure 4d, j, f, l). The intensity of this peak decreases but does not vanish for samples enriched with either Lws
277 (LWS_2.5_925) or Gln (GLN_2.5_927) quenched at around 830 K. The intensity remains constant in the lowest temperature experiment, quenched at 762 K well before the breakdown of Lws (GLN_2.5_762, Figure 4e, k).

280 Acoustic emissions tend to occur in two clusters – one at low temperatures (around 640 to 700 K) and one at higher temperatures (approx. 830 to 1121 K) (Figure 4m-o), but not every experiment actually yield both clusters. For example, the experiment on a sample with bulk-rock composition (BS_1.5_1121) conducted at the lowest confining pressure of 1.5 GPa exhibits only the high-temperature cluster (Figure 4m), while a sample with the same composition deformed at a confining pressure of approx. 3 GPa only shows the low-temperature cluster (Figure 4o).

287 **4.2 Microstructural Analyses – SEM and EMPA**

288 Observed microstructures are similar for the suite of experiments. Only the bulk-rock sample deformed at the lowest confining pressure of 1.5 GPa (BS_1.5_1121) exhibits no fracture related to deformation (Figure 5g). We focus here on presenting four representative experiments, one representing each powder composition and additionally the low-pressure bulk-rock experiment (BS_1.5_1121), which shows no macroscopic strain localization. The microstructure of the sample quenched at the lowest temperature (GLN_2.5_762) is shown in the supplementary material in Figure S1. The Gln-enriched sample (GLN_2.5_927) and the Lws-enriched sample (LWS_2.5_925) were both deformed at a confining pressure of approx. 2.5 GPa (Table 1) and quenched before the complete breakdown of Lws expected from the computed pseudosection (Figure 2). The tests on the bulk-rock powder (BS_1.5_1121, BS_3_1073) were performed at 1.5 and 3 GPa, respectively, and quenched after the breakdown of Lws.

300 **4.2.1 Microstructural and chemical analysis of the Gln-enriched sample**

301 The Gln-enriched sample (GLN_2.5_927) exhibits two fractures starting in the lower corners of the sample extending diagonally towards the sample center (Figure 5a). Several
narrow fractures, oriented parallel to the main fractures, dissect and offset grains of Lws and Omp1 (Figure 5b). The horizontal fracture is a typical decompression feature.

A narrow fracture parallel to a main fracture shows slightly brighter than average edges in backscattered electron (BSE) mode, indicating a change in chemistry (Figure 5c). Profiles across such a fracture show Ca enrichment in the brighter zones at the fracture edges (Figure 6b, d) related to the formation of a second generation of omphacite. In comparison to Omp1 of the starting material, this secondary omphacite, called Omp2 in the following, has a higher jadeite component of $X_{jd} \approx 0.7$. Mineral chemical data (wavelength-dispersive (WDX) and energy-dispersive X-ray spectrometry (EDX)) of Omp2 of all samples after deformation was plotted in a Ca-Al-Na ternary plot to demonstrate chemical variations of Omp2 (open circles in Figure 6c) and for the main phases Gln, Omp1, and Act of the natural blueschist (solid circles in Figure 6c). Strikingly, the composition of Omp2 plots between the Gln and Omp1 composition of the natural Lws-Bs (Figure 6c). These microstructural observations match well with the growth of the Omp (-221) peak during the first stage of the deformation (Figure 4g, h, i). The incipient breakdown of Lws is evidenced by its association with small epidote (Ep) needles - its dehydration product at low-pressure/high-temperature (LP/HT) conditions (Figure 5b).

### 4.2.2 Microstructure and mineral chemical analysis of the Lws-enriched sample

The Lws-enriched sample is cross-cut by two main fractures (Figure 5d). Several narrow fractures are oriented parallel or sub-parallel to these main fractures. Along one of these narrow fractures, Gln grains are observed that are dissected and offset (Figure 5e). The fracture edges exhibit a brighter than average contrast, resulting from Omp2 (Figure 5f). Most of the Omp2 crystals are located along fractures as was found for the Gln-enriched sample (GLN_2.5_927), too.

In most places, the fracture-filling material was lost during preparation of the samples, but some fractures are still filled by a material appearing dark in BSE mode (Figure 5e) implying that its average atomic number is smaller than the adjacent Lws and Gln crystals (Figure 5e).
These zones are so narrow that EDX or WDX measurements give mixed analysis of filling material and rock matrix. After the interaction with the electron beam, this dark material always shows holes implying a poor crystallinity.

### 4.2.3 Microstructure and chemical analysis of the bulk-rock samples

The samples prepared from bulk-rock powder (BS_1.5_1121, BS_3_1073) were performed at two different confining pressures, 1.5 GPa and 3 GPa, and quenched after the breakdown of Lws. Pseudomorphs after Lws appear as patches filled with Ep needles (Figure 5i, k). Their occurrence is in good agreement with the thermodynamically expected stability of Lws (Figure 2). After deformation, the lowest pressure experiment (BS_1.5_1121) only shows a horizontal decompression crack, but no macroscopic strain localization (Figure 5g). Rims of Ca-Amp formed around Gln grains (Figure 5i). The high-pressure sample (BS_3_1073) is completely cross-cut by sample-size fractures (Figure 5j). Brighter zones along fractures, pores, and rims around Gln crystals were identified as Omp2 (Figure 5k, l). This sample also shows crystals offset along shear fractures (Figure 5k). Garnet grains exhibit growth zonation with Ca enrichment in the rim relative to its core (Figure 6a). A filled fracture between two Gln crystals shows tiny crystals "floating" within a dark material filling the fracture (Figure 5l) of which the larger ones, with an idiomorphic crystal shape, were classified as Omp2. Holes left after EDX and WDX measurements within this zone suggest that this dark material is poorly crystalline or amorphous. Observations on samples prepared in a piston-cylinder apparatus (as the hot-pressed sample BS_3_1073, see 3.2) show no evidence that this amorphous material represents a remnant of an incomplete reaction (supplementary material Figure S2).

## 4.3 Nanostructural Analysis – Transmission electron microscopy (TEM)

The focused ion beam (FIB) sections for the Gln-enriched sample (GLN_2.5_927) were cut across a fracture with edges indicating Omp2 and Ca-Amp. The TEM images taken in bright field (BF) mode document the structure of the fracture at different scales (Figure 7). The sealed fracture appears brighter in the TEM images relative to the surrounding material
indicating that this area is either thinner, poor in heavy atoms or less crystalline than the surrounding phases. The adjacent grains were identified as Ca-Amp and Omp2 (Figure 7b).

The contact between the fracture filling-material and Omp2 and the absence of spots in the diffraction pattern reveal that the material filling out the fracture is amorphous (Figure 7c).

Grains next to the fracture exhibit 120° triple junctions (Figure 7c).

The FIB section in the sample prepared from the bulk-rock sample (BS_3_1073) was cut across a narrow, sealed fracture along which a pair of displaced garnet crystal was found.

The TEM images were also taken in BF mode. The most striking observation is the occurrence of a bright, according to diffraction analyses amorphous material appearing as bubbles (Figure 8a, d) or pockets in the matrix sometimes "trapped" within nests of Ep-needles (Figure 8e).

These bubbles and pockets are completely absent in the Gln-enriched sample (GLN_2.5_927) quenched before the Lws-breakdown. The fracture, across which this FIB section was cut, appears to be filled by idiomorphic shaped garnet crystals varying in size (Figure 8f), surrounded by a bright, amorphous groundmass (Figure 8c, f, g). Amorphous material thus occurs at two structurally different sites, bubbles and pockets in the matrix or filling material in fractures. The grains next to the fracture were in most cases found to be Omp2 (Figure 8c, d).

Unfortunately, it was not possible to obtain precise chemical analysis of the amorphous material in either FIB section.

## 5. Discussion

### 5.1 Comparison between experiments quenched before and after Lws breakdown

The P-T path experienced by a sample during deformation controls its microstructure, phase assemblage and the occurrence of AEs (Figure 9). At high Pc (> 3 GPa), experiments only show an early low-temperature AE cluster at around 638–701 K. Experiments at intermediate Pc (around 2.5 GPa) exhibit two successive clusters, one at low temperatures of 638–701 K and one at high temperatures of around 831 – 927 K. Finally, the experiment at
low $P_c$ (< 1.5 GPa) only exhibits a high-temperature AE cluster beginning at around 873 K and not ceasing until the end of the experiment at 1121 K. Thus, the pattern of AE occurrence during syndeformational heating strongly depends on confining pressure, regardless of the powder type used for the experiment (Figure 4m, n, o).

The samples quenched after the Lws breakdown (BS_1.5_1121, BS_3_1073) differ from those quenched before (GLN_2.5_927, LWS_2.5_925) by the occurrence of pseudomorphs after Lws and bubbles composed of amorphous material interpreted as a quenched Lws-breakdown derived fluid. The appearance of Ep in the high pressure experiment (BS_3_1073) suggests either that the experiment was quenched at lower pressure than calculated, that Ep stability extends to higher pressures than thermodynamically predicted, or that it survives metastably (Figure 9). Except for the bubbles which are absent in the nanostructure of the Gln-enriched sample (GLN_2.5_927) micro- and nanostructures are similar for the Lws-enriched sample (LWS_2.5_925), the high-pressure bulk-rock sample (BS_3_1073) and the Gln-enriched sample (GLN_2.5_927). All three samples exhibit amorphous material filling fractures with adjacent Omp2 crystals (Figures 7c, 8c, d).

## 5.2 Transition from Lws-Bs to Lws-Ecl

Mean stress is used as a measure of thermodynamic pressure assuming that the change in slope of the equilibrium curves under non-hydrostatic conditions can be neglected (MacDonald, 1957; Kirby, 1987). During the first stage of deformation the calculated mean stress and the confining pressure differ by 0.7-1 GPa (Figure 9; Table 1). The difference decreases towards the end of the experiments due to the decrease in differential stress with increasing temperature (Figure 4a-f). The uncertainty regarding the relevant thermodynamic state variable does not affect the prognosis from the calculated P-T pseudosections that the experiments performed at $P_c$ > 2.5 GPa enter the Lws-Ecl stability field except of the experiment performed at $P_c$ < 2 GPa (Figures 2, 9).
The occurrence of the low-temperature AE cluster (shown as stars in Figure 9) matches the expected transition conditions for Lws-Bs to Lws-Ecl and the growth of the (-221) peak of omphacite (Figure 4). The powder diffraction pattern were taken from a small volume of the sample center and therefore do not quantify spatial stress heterogeneities in the samples, e.g., stress concentrations at the edges, that might lead to local variations in phase stability. Combined with the observation that Omp2 occurs along shear fractures, the correlation of formation of Omp2 with the low-temperature AE records suggests that its appearance at HP/LT triggered the brittle failure of the sample. The formation of Omp2 is related to the breakdown of Gln and the liberation of Ca derived from the successive disappearance of Lws at HP (Lws-Bs ≈ 19% Lws to Lws-Ecl ≈ 13% Lws, Figure 2).

The amorphous material filling the fractures suggests that slip along the fracture generated a melt (likely due to frictional heating) out of which idiomorphic crystals grew (Figures 8f, 7c). The absence of bubbles and pockets acts as evidence that the reaction initiating the brittle failure of the sample released no or only minor amounts of fluid in the Gln-enriched sample (GLN_2.5_927) quenched before the breakdown of Lws at HT (Figure 7). Therefore, we suppose that the grain-size reduction during the formation of Omp2 at HP/LT may have acted as weakening mechanism (Spray, 1995) and that nanocrystalline Omp2 at the edges of the fracture may represent remnants of the nucleation zones for failure (Figure 5c, f). Due to the small grain size the Omp2 crystals could only be distinguished in FIB sections at the TEM and therefore a significant quantification of the grain-size distribution was not possible. The notion of a rapid transformation to Omp2 is supported by the chemical and structural similarity between Omp2 and Gln and the recognition that Ca diffusion, derived from Lws, is fast and possibly even enhanced by a high grain-boundary density during the reaction (Putnis, 2014).

Our observations of a high Jd content of Omp2 ($X_{jd} \approx 0.7 \%$) texturally linked to Gln is in accord with the breakdown reaction for glaucophane, Gln $\rightarrow 2$ Jd + Tlc, quoted by Carman and Gilbert, 1983. This exothermic reaction with an enthalpy of reaction of about -8.9 kJ
433 (Holland, 1988) may facilitate the generation of a melt. However, HP talc (Tlc) was neither
434 found in the XRD nor in any of the samples after deformation in contrast to its occurrence in
435 MORB (Wei and Clarke, 2011). The absence of Tlc in our samples may result from (partial)
436 melting of the reaction products during fast shear. The amorphous material should show a
437 chemistry close to talc stoichiometry, but within this study it was not possible to precisely
438 analyze the amorphous material.

439 The sample prepared with bulk-rock composition (BS_3_1073), which entered the Lws-
440 Ecl stability field at HP/LT, showing only the low-temperature cluster of AEs is the only one
441 which exhibits newly grown Ca-rich Grt coronas (Figure 6a) and idiomorphically shaped Grt
442 crystals are “floating” within the amorphous fracture-filling materials (Figure 8). These newly
443 formed Grt crystals can be linked to the metamorphic reaction 4 Gln + 3 Lws → 8 Jd + 3 Di +
444 3 Prp + 7 Qtz + 7 H₂O (Evans, 1990). The absence of Grt in all other samples might be related
445 to a combination of its high nucleation barrier and the lack of initial Grt grains in the starting
446 material acting as nucleation sites (Rubie, 1998). In samples where Grt is absent, the
447 amorphous material filling the fractures might represent the sink for Fe and Mg liberated from
448 Gln.

449 **5.3 Breakdown of lawsonite**

450 The P-T domain in which the high-temperature AEs were recorded matches with the
451 thermodynamically expected breakdown of Lws (Figure 9). Therefore, we suppose that these
452 AEs were triggered by the associated dehydration reaction. The microstructure of the sample
453 deformed under the lowest confining pressure (BS_1.5_1121), that did not enter the stability
454 field of Lws-Ecl (Figure 9), reveals the absence of macroscopic strain localization (Figure 5g).
455 The recorded AEs are therefore either linked to fractures that are overprinted by ongoing
456 plastic deformation or to other processes during dehydration, e.g. the formation of
457 pseudomorphs after lawsonite or the growth of Ca-Amp rims around Gln (Figure 5h, i). The
458 observation that most of the high-temperature AEs do not coincide with bigger stress drops
and were mostly recorded while heating additionally strengthens the interpretation that these AEs were not caused by the brittle failure of the samples (Figure 9). In contrast to our observations, a recently published experimental study reported that dehydration of Lws leads to brittle fracturing accompanied by AEs (Okazaki and Hirth, 2016). Okazaki and Hirth 2016 used almost pure lawsonite samples and this difference in chemical composition of the starting material could explain the contrasting observations.

## 5.4 Link to natural observations

The relocalization of seismic events below Hokkaido, NE Japan, shows that they occur where the expected position of the subducting oceanic crust coincides with the upper Wadati-Benioff plane (Kita et al., 2006). Additional information from thermodynamic modeling implies a link between metamorphic reactions during the eclogitization of Lws-Bs and seismicity (Hacker et al., 2003; Kita et al., 2006). The involvement of the breakdown of hydrous phases (lawsonite and amphibole) during eclogitization leads to the proposition of dehydration embrittlement as possible mechanism for intermediate-depth seismicity (Hacker et al., 2003; Jung et al., 2004; Okazaki and Hirth, 2016). Yet, a recent study on source-parameter modelling and the energy budget of intermediate-depth earthquakes of the Bucaramanga Nest, Colombia (Prieto et al., 2013), concluded that the combination of narrow shear zones (less than 3 cm), rapid stress drops, and localized temperature increase is best explained by shear heating. A geochemical, petrological, and micro-textural study investigating the formation of pseudotachylites (PST) in metagabbros from Alpine Corsica comes to a similar conclusion (Deseta et al., 2014). The analyzed PSTs show different water contents ranging from 0–4 wt. % and the authors therefore suggest that the dehydration of hydrous phases is unlikely to trigger the formation of PST at high pressure. The authors explain the generation of melt due to fusion of the wall rock minerals. The amount of water present in the PST will therefore only depend on the availability of hydrous minerals in the fracture zone and their fusion during fast shear. Since PSTs are also be found in anhydrous rocks like lower crustal gabbros (Andersen
et al., 1990; John et al., 2009) the instability related to Lws breakdown may well result from strain localization due to aspects of the reaction other than the dehydration, e.g. density change, grain-size reduction, and liberation of heat during reaction. Numerical simulations show that grain-size reduction and shear heating are the most likely causes for the formation of HP-pseudotachylites (John et al., 2009; Thielmann et al., 2015).

## 6. Conclusions

We experimentally reproduced the transition from lawsonite-blueschist to lawsonite-eclogite, the expected metamorphic evolution of a lawsonite-bearing blueschist during cold subduction. We did not find any evidence for a straightforward link between the breakdown of Lws and the failure of the samples that would support a mechanism akin to dehydration embrittlement. Instead we showed that metamorphic reactions during the transition from lawsonite-blueschist to lawsonite-eclogite can cause fracturing accompanied by AEs matching observations from seismicity locations in cold subduction zones, numerical simulations, and field observations. Shear fractures are decorated with newly grown omphacite grains, enriched in Jd content and texturally linked to Gln, which suggests that the breakdown reaction for glaucophane, Gln → 2 Jd + Tlc may have played a role in triggering shear instabilities. We argue that grain-size reduction due to the formation of nanocrystalline secondary omphacite grains and latent heat release by the reaction act in conjunction as weakening mechanism triggering the brittle failure of the samples. Shear heating fuses the wall rock next to the fracture surface and the melt is later quenched as amorphous material filling the fractures – a possible laboratory equivalent to natural pseudotachylites.

## Acknowledgement

The authors would like to thank Christian Chopin for providing the blueschist sample for this study. The research leading to these results has received funding from the People Program (Marie Curie Actions) of the European Union's Seventh Framework Program FP7/2017-2019.
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<figure><img src="image_3.png" /></figure>

<table><thead><tr><th>Oxides</th><th>Gln</th><th>Lws</th><th>Act</th><th>Omp1</th><th>Ca [%]</th><th>Na [%]</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>58.42</td><td>38.34</td><td>58.36</td><td>56.07</td><td rowspan="2">46</td><td rowspan="2">54</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>10.81</td><td>32.56</td><td>0.75</td><td>8.79</td></tr><tr><td>CaO</td><td>0.61</td><td>17.39</td><td>11.93</td><td>13.59</td><td rowspan="2">33</td><td rowspan="2">67</td></tr><tr><td>Na<sub>2</sub>O</td><td>7.39</td><td>0.00</td><td>0.58</td><td>6.70</td></tr><tr><td>FeO</td><td>11.27</td><td>0.34</td><td>9.37</td><td>7.13</td><td rowspan="2">71</td><td rowspan="2">29</td></tr><tr><td>MgO</td><td>9.69</td><td>0.00</td><td>18.08</td><td>8.01</td></tr><tr><td>K<sub>2</sub>O</td><td>0.03</td><td>0.002</td><td>0.06</td><td>0.04</td><td rowspan="3">71</td><td rowspan="3">29</td></tr><tr><td>MnO</td><td>0.11</td><td>0.00</td><td>0.49</td><td>0.22</td></tr><tr><td>TiO<sub>2</sub></td><td>0.00</td><td>0.04</td><td>0.07</td><td>0.08</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.03</td><td>0.02</td><td>0.00</td><td>0.00</td><td rowspan="2">71</td><td rowspan="2">29</td></tr><tr><td>Total</td><td>98.36</td><td>88.70</td><td>97.71</td><td>100.64</td></tr></tbody></table>

646

647 **Figure 1:** Characterization of the three different powders and their Ca:Na ratios and chemical

648 analyses of major phases of the Corsican blueschist.
<figure><img src="image_4.png" /><figcaption>**Figure 2:** Summarized *P*-*T* pseudosection showing the Amp- and the Lws-out curves for the three different powders (Lws-Ecl: lawsonite-eclogite, Lws-Bs: lawsonite-blueschist, Ep-Bs: epidote-blueschist, Ecl-Amph: eclogite-amphibolite, Ep-Ecl: epidote-eclogite, Ecl: eclogite).</figcaption></figure>

649

650

651

652

653
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Schematic drawing of the experimental setup for the D-DIA experiments. For the sake of clarity only the upper vertical anvil is shown.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4:** Mechanical data and peak intensities (proxy for phase proportion) from in-situ XRD, and record of AEs. The columns represent the low pressure (1.5 GPa), the intermediate pressure (2.5 GPa), and the high pressure (3 - 3.5 GPa) experiments (from left to right). Color code: bulk-rock in blue, Gln-rich powder in violet, Lws-rich powder in yellow. For the run BS_3_1073 it was possible to calculate the differential stress on Gln and Lws.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5:** BSE images taken at the SEM using an acceleration voltage of 15 kV. The four rows represent the microstructure of the runs GLN_2.5_927 (a, b, c), LWS_2.5_925 (d, e, f), BS_1.5_1121 (g, h, i), and BS_3_1073 (j, k, l). The color represents the starting material used for the experiment (violet: Gln-rich, yellow: Lws-rich, and blue: bulk-rock). The BSE images a, d, g, and j are overview images of the samples GLN_2.5_927, LWS_2.5_925, BS_1.5_1121, and BS_3_1073, respectively, after deformation. The maximum compression direction $σ_1$ is oriented vertically on the images (parallel to the short edge).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6:** a) BSE image showing a brighter Grt core and a darker newly grown Grt corona. The white rectangle marks the position selected for an element distribution map, which is shown for Ca in the upper right corner. b) BSE image with the position of two line measurements, which correspond to the two profiles in the Ca p.f.u. over distance plot in d). c) Ca-Na-Al (a.p.f.u.) ternary plot for individual mineral compositions. Solid circles represent WDX analysis of Act (grey), Omp1 (green), and Gln (purple) from the natural Corsican blueschist. The open circles represent all WDX and EDX analyses on the deformed samples (blue: bulk-rock, violet: Gln-rich, yellow: Lws-rich). d) Two profiles corresponding to the two lines in b) showing Ca enrichment of the region on the fracture's wall.</figcaption></figure>

673
<figure><img src="image_9.png" /><figcaption>**Figure 7:** TEM images (BF mode) of the experiment GLN_2.5_927. a) The fracture appears brighter. b) Closer zoom onto the fracture. On the upper left side of the fracture a Ca-Amp was found, which shows a chemical composition close to actinolite. On the lower right side of the fracture Omp2 was detected. c) Triple junction between three grains identified as Omp2. The diffraction image in the upper right corner demonstrates that the material filling out the fracture is amorphous.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 8:** TEM images (BF mode) showing the nanostructure of the sample BS_3_1073. a) Omp2 crystal surrounded by fluid. b) Epidote needles, the dehydration product of Lws, trapping a fluid. The white rectangle shows the position of image e. c) Zoom onto the shear fracture across which the FIB section was cut. The fracture appears brighter and shows many idiomorphic Grt crystals "floating" within an amorphous material. In the upper right corner just next to the fracture, small idiomorphic crystals appear which were identified as Omp2. The white rectangle marks the position of the zoom shown in f. d) Closer zoom of an Omp2 crystal. In the lower left corner of this image a brighter bubble-like structure appears, which incorporates darker small crystals. The bright material is amorphous and the tiny crystals are garnets. e) Zoom of the fluid pocket of image b. f) Closer zoom of the area marked with the white rectangle in c. A brighter material surrounds idiomorphic garnet crystals. A mixture of this bright, amorphous material and this idiomorphic Grt grains fills out the whole shear fracture. g) Zoom of an idiomorphic Grt crystal "floating" within amorphous material.</figcaption></figure>

690
<figure><img src="image_11.png" /><figcaption>**Figure 9:** Mean stress versus temperature paths of all experiments superimposed to the same P-T pseudosection as in **Figure 2**. Acoustic emissions recorded during the deformation are represented as stars and often correlate with stress drops in the experimental mean stress versus T curves. Most of the low-temperature AEs occurred prior to stress drops. The high-temperature AEs were often recorded while heating and are not accompanied by bigger stress drops. Two clusters of AEs are observed – the first at low-temperature, and the second at high-temperature. The Amp- and the Lws-out reactions are indicated as in **Figure 2**. The low-temperature cluster of AEs plots within the transition area between Lws-Bs and Lws-Ecl. The second high-temperature cluster of AEs correlates with the breakdown of Lws.</figcaption></figure>

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<table><caption>716 Table 1: Experimental conditions of the six D-DIA experiments.</caption><thead><tr><th>Sample</th><th>Experiment #</th><th>Starting material</th><th>P<sub>c</sub> [GPa]</th><th>T range [K]</th><th>Strain rate [s<sup>-1</sup>]</th><th>Maximum strain [%]</th></tr></thead><tbody><tr><td>BS_3_1073</td><td>D1616</td><td>Bulk</td><td>3</td><td>583 - 1073</td><td>4·10<sup>-5</sup></td><td>40</td></tr><tr><td>BS_1.5_1121</td><td>D1661</td><td>Bulk</td><td>1.5</td><td>649 - 1121</td><td>7·10<sup>-5</sup></td><td>42</td></tr><tr><td>GLN_3.5_948</td><td>D1655</td><td>Gln-rich</td><td>3.5</td><td>659 - 948</td><td>1·10<sup>-5</sup></td><td>42</td></tr><tr><td>GLN_2.5_927</td><td>D1618</td><td>Gln-rich</td><td>2.5</td><td>654 - 927</td><td>2·10<sup>-5</sup></td><td>43</td></tr><tr><td>GLN_2.5_762</td><td>D1807</td><td>Gln-rich</td><td>2.5</td><td>656 - 762</td><td>4·10<sup>-5</sup></td><td>37</td></tr><tr><td>LWS_2.5_925</td><td>D1667</td><td>Lws-rich</td><td>2.5</td><td>656 - 925</td><td>3·10<sup>-5</sup></td><td>40</td></tr></tbody></table>