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The influence of hydrous phases on the microstructure and seismic properties of ahydrated mantle rock

Luiz F.G. Morales a,⁎, David Mainprice b, Françoise Boudier b

a Deutsches GeoForschungsZentrum (GFZ), Section 3.2 Telegrafenberg, 14473, Potsdam, Germanyb Géosciences Montpellier and Université Montpellier 2, Place Eugène Bataillon, Batîment 22, 34095, Montpellier, France

a b s t r a c ta r t i c l e i n f o

Article history:Received 23 May 2012Received in revised form 26 February 2013Accepted 15 March 2013Available online 24 March 2013

Keywords:Subduction zoneMantle wedgeMicrostructuresHydrous phasesSeismic properties

To better understand themicrostructural evolution of a “serpentinized”mantle rock and the influence of varioushydrous phases on the seismic properties of the mantle wedge, we have conducted the detailed microstructuralanalyses of a sample of tremolite–chlorite–antigorite schist collected from the Moses Rock dike (central part ofthe Colorado Plateau). We performed differential effective media (DEM) modelling to study the effect of threehydrous phases forming two-phase aggregates with olivine, considering the crystallographic preferred orienta-tion (CPO) of each phase and the shape ratio of the hydrous phases. We have demonstrated that in a partiallyserpentinized peridotite, the olivine CPO characteristic of [100](010) dislocation glide is still preserved, andthe high-temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist. Thetransformation of olivine into antigorite occurs predominantly (~75%) by the relationship (100)ol || (001)atgwith [001]ol || [010]atg, with the (010)ol || (001)atg and [001]ol || [010]atg relationship observed in areas ofweak antigorite CPO. Chlorite results from the phase transformation of olivine in a relatively static environment,as shown by the correlation between the olivine–chlorite CPOs with (100)ol || (100)ch, (010)ol || (001)ch and(001)ol || (010)ch. The fluid percolation that caused the localized metasomatism and partial hydration of themantle occurred possibly along trans-lithospheric shear zones. The presence of chlorite induces the most impor-tant drop on the P-wave velocities and may help to explain some local low velocities in the fore-arc mantlewedges, but is unlikely to be of global importance due to its very high Vp/Vs ratio ~ 1.9. On the other hand,antigorite is the only phase that causes important modification on the propagation directions of P andS-waves, and the only phase to explain the polarization of the fastest shear waves parallel to the subductiontrench.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Many of the key-processes that occur in subduction zones arelinked to hydration/dehydration metamorphic reactions, which areof extreme importance to our understanding of the dynamics of sub-duction zones (e.g., Hacker et al., 2003a, 2003b; Hirth and Kohlstedt,2003; Kneller et al., 2005; Van Keken et al., 2002). The hydration ofthe lithospheric mantle may occur in different environments, suchas: (i) slow spreading ridges where “mantle rocks” are actually partof the oceanic crust, (ii) along faults and fracture zones where mantlerocks behave in a brittle manner and (iii) in the fractures due to thebending of the subducting plate, locally leading to the hydration ofthe mantle rocks (e.g., Escartín et al., 1997; Faccenda et al., 2009;Hyndman and Peacock, 2003).

In the last few years, antigorite, lizardite, chrysotile (serpentinegroup) have been receiving increasing attention as the direct expres-sion of hydration of the upper mantle and the water cycle in the deep

Earth (e.g., Boudier et al., 2010; Christensen, 2004; Hacker et al.,2003a, 2003b; Hirauchi et al., 2010; Hyndman and Peacock, 2003;Katayama et al., 2009; Nishii et al., 2011; Soda and Takagi, 2010). Al-though they are important phases because they may store ~13 wt.%of water (e.g., Bromiley and Pawley, 2003) and therefore representan important water reservoir in mantle rocks, they are not the onlyhydrous phases in subduction zones. Talc, amphiboles, chlorite,brucite and clinohumite are some of the hydrous minerals commonlyassociated with alteration of peridotites at different P–T conditions.Many of these phases, when present in altered peridotites, maydramatically change the physical properties of the hydrated mantle(e.g., Mainprice and Ildefonse, 2009). Nevertheless there are rela-tively few papers that address the modifications of the physical pro-perties in the mantle wedge caused by different hydrous minerals(e.g., Christensen, 2004; Connolly and Kerrich, 2002; Hacker et al.,2003a, 2003b; Hyndman and Peacock, 2003; Mainprice andIldefonse, 2009; Mainprice et al., 2008; Peacock and Hyndman,1999). Besides acting as a chemical-softening reaction usually facili-tating the deformation accommodation during the dynamic processesrelated to the subduction, the presence of these phases may change

Tectonophysics 594 (2013) 103–117

⁎ Corresponding author. Tel.: +49 331 288 28710.E-mail address: [email protected] (L.F.G. Morales).

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the elastic properties of the mantle wedge, and by consequence, theiranisotropic seismic properties (e.g., Hirauchi et al., 2010; Katayama etal., 2009; Mainprice and Ildefonse, 2009; Nishii et al., 2011). As an ex-ample, the distribution of seismicity along the subduction zonesseems to result from the dehydration processes and embrittlementof the subducted hydrated oceanic crust (e.g., Green, 2007;Kawakatsu and Watada, 2007; Kita et al., 2006). The water releasedduring this process then migrates and is recycled in the mantlewhere it controls the deformation and partial melting in the mantlewedge (e.g., Hilairet et al., 2007; Hirauchi et al., 2010). Some ofthese hydrated phases can be stable over pressure and temperatureconditions at depths of up to 100 km (Fumagalli and Poli, 2005;Iwamori, 2004; Ohtani et al., 2004; Schmidt and Poli, 1998, also seeFig. 2 of Mainprice and Ildefonse, 2009).

To better understand the microstructural evolution of a“serpentinized” mantle rock and the role of hydrous phases in alteringthe seismic properties of the mantle wedge, we have conducted the de-tailed analysis of a rare sample of tremolite–chlorite–antigorite schist. Al-though the assemblage of olivine + antigorite + chlorite + tremoliteis common in the hydrated mantle rocks in subduction zones (e.g.,

Hirauchi et al., 2010; Mizukami and Wallis, 2005), the study of partlytransformed rock preserving a significant fraction of the original olivinenetwork (~40%) within ~60% of the new hydrous phases is exceptional.Previous studies presented in the last few years make use of completelytransformed aggregates in which it is not possible to observe therelationships between the protolith (mantle) mineralogy and thetransformed (hydrous) phases (e.g., Katayama et al., 2009; Nishii etal., 2011; Soda and Takagi, 2010). In addition, this study presents re-sults of a microstructural study of all hydrous phases present ratherthan just antigorite, which allows an analysis of the impact of eachmin-eral on the modification of physical properties in the mantle wedge.

2. Geological setting

The sample of tremolite–antigorite–chlorite schist studied here(MR-1) was collected from the Moses Rock dike, which is exposedin the central part of the Colorado Plateau (Fig. 1) and belongs tothe Navajo Volcanic Field (NVF), with eruptive ages between 25 and31 Ma. The NVF is located in the “Four Corners” (SW of USA —

Fig. 1) and is dominated by potassic, mica-bearing lamprophyres.

Fig. 1. Location of the study area within the Navajo Volcanic Field in the Colorado Plateau region with the Google Earth satellite image showing the Moses Rock dike running fromNW–SE and crosscutting sedimentary sequences.Location map modified from Smith and Griffin (2005).

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The Moses Rock dike itself belongs to a series of kimberlitic andlamprophyric breccia composed of mantle and crustal xenoliths ofvariable compositions (Smith, 1995, 2010). These rocks were formedby the breakdown of the mantle peridotites while in contact with afluid phase and subsequently incorporated as vent wall fragmentsduring the ascension of this gas–solid eruptive mix (McGetchin andSilver, 1970). The mantle fragments include metaperidotites richin hydrous phases, jadeite clinopyroxenites and eclogites, spinelwebsterite and spinel lherzolites and were formed at depths between50 and 150 km and temperatures ~900 °C according to clinopyroxenethermometry (Hunter and Smith, 1981). These fragments representmantle specimens that were transported from the plate boundary(trench zone) to about 700 km to the NE during the low angle subduc-tion of the Farallon Plate (Dickinson and Snyder, 1978; Humphreys,2009; Saleeby, 2003; Sigloch et al., 2008). However the relations of therock fragments with the Farallon subduction are debated (e.g., Smith,2010) as they can represent either the hydrated mantle wedge or partof the Farallon slab. The former interpretation is coherent with the factthat the eclogite and garnetite inclusions observed in these diatremesrepresent the lithospheric mantle wedge below the Mojave Province af-fected by the Farallon subduction (Helmstaedt and Schulze, 1991). Thepresence of amphibole, chlorite, clinohumite and antigorite suggeststhat the mantle hydration occurred in depths of 45–60 km and temper-atures between 600 and 700 °C, with all the phases forming prior to theeruption of these rocks to the surface (Smith, 1979, 2010).

3. Analytical methods

The CPO of olivine, antigorite, chlorite and tremolite were acquiredvia indexation of electron backscatter diffraction patterns (EBSD) in ascanning electron microscope (SEM) (e.g., Engler and Randle, 2009).The sample of antigorite schist was prepared by standard methods,first via mechanical polishing using diamond paste of various grainsizes down to 0.25 μm and then chemically–mechanically polishedusing an alkaline solution saturated in colloidal silica (25 nm) to re-move any damage caused by the previous mechanical processes. TheEBSDmeasurements were carried out using the CamScan X500FE Crys-tal Probe equippedwith a HKL Nordlys camera and HKL Channel 5 suiteof programs installed at Géosciences Montpellier. The presence ofserpentine, chlorite and amphibole produced a strong electron chargingeffect on the sample surface and prevented reliable EBSD measure-ments. For this reason we performed the measurements using theSEM low-vacuum mode, using a 5 Pa pressure of nitrogen inside thevacuum chamber. We performed the automatic mapping of the entirethin-section (~600 mm2) and due to the relative small grain sizesof the hydrous phases, we have used a step size of 10 μm, using a work-ing distance of 24 mm, an accelerating voltage of 17 KeV and a beamcurrent of 3 nA. The minimum and maximum numbers of bands to bedetected in the EBSD patterns were set to be between 5 and 7, and themaximum accepted angular deviation for the measurements (MAD) ≤1.3°. Due to the relatively low-crystallinity of the hydrous phases, andconsidering the “large” mapping scale, the raw indexation rate wasaround 60%. Post-acquisition processing of the orientation map includedthe removal of wild spikes and the extrapolation of well-indexed pointsto zero-solutions (non-indexed points) with 5 or more neighbour pointshaving similar orientations. The systematic errors in the indexation of ol-ivine due to the pseudo-hexagonal symmetry along the [100] axis, anddue to a 2-fold rotation around the [010] in antigorite axis were alsocorrected. The orientation distribution functions, pole figures and theseismic properties were calculated considering all the points presentin themap as themeasured volume fractions are required for quantita-tive texture analysis. Texture analysis was carried out using the MTEX,the open-source MATLAB toolbox for texture analysis (Hielscher andSchaeben, 2008), which has been extended for the calculation of 2ndand 4th rank tensors of anisotropic physical properties (Mainprice et

al., 2011). Backscattered imaging and EDS analyses of the main phaseswere acquired using a FEI Quanta 3D FEG installed at GFZ-Potsdam.

4. Microstructures of an antigorite peridotite xenolith(Moses Rock dike)

The sample MR-1 is a rare sample where the mineral hydration andformation of tremolite, antigorite and chlorite (composing ~60% of therock), but still preserves the olivine microstructural framework(Fig. 2). The thin section studied here was cut normal to the antigoritefoliation and parallel to the dominant lineation. The identification ofthe hydrous phases is very challenging by optical means uniquely be-cause of the fine scale inter-layering of the hydrous phases and olivine.For complete phase identification, we had additional information frombackscattered electron (BSE) images (Fig. 3a–d) and local energy-dispersive X-ray spectroscopy (EDS) analyses. The dominant structureof this sample is a relatively strong foliationmarked by the dimensionalalignment of antigorite, with occasional chlorite usuallywrapping aroundmillimetre to centimetre-scale olivine crystals (Fig. 3c). Antigorite lamel-lae usually have an average aspect ratio of about 10:1 and normally thelongest axes are ofmillimetre-scale, whereas chlorite crystals have small-er aspect ratios. The backscattered images show that chlorite normallyoccurs associated with olivine crystals, but does not occur with themain antigorite foliation. The contact between chlorite and olivine grainsis straight and sharp; some of the larger grains of chlorite contain smallinclusions of olivine (Fig. 3a and b). The contacts between chlorite andantigorite are blurred in the BSE images and are marked by a small varia-tion in grey level caused by lower Al content in the antigorite (Fig. 3b).While antigorite and chlorite are widespread in the whole thin section,tremolite only occurs in isolated patches in contact with olivine, andsurrounded by the antigorite foliation (Fig. 3c). The lamellae of tremoliteare orientated at a high angle to the antigorite foliation (Fig. 3c) with asharp contact of the other phases. The individual crystals of tremoliteare usually larger than chlorite and antigorite lamellae and usually haveundulose extinction and rare sub-grain boundaries. In some places,antigorite crosscuts the tremolite crystals (Figs. 2, 3c).

The olivine crystals have an elongated shape with average aspectratios of 4:1 (Figs. 2 and 3) with the longest axes also parallel to thetrace of the antigorite foliation. These crystals are intensely dismem-bered by the development of antigorite (Fig. 4a–d) and the dominantparting direction is parallel to the antigorite foliation, with a second-ary plane normal to themain foliation (Fig. 4b,c). Sporadically the olivinecrystals may have narrow tips (Fig. 4c) and the phase boundaries be-tween olivine and hydrous phases are straight and sharp. The olivinemi-crostructural network is observed in the thin section cut normal to theantigorite foliation, where the olivine crystals present well-developed(100) sub-grains (Fig. 4a). In detail the antigorite lamellae present adual orientation relationship with the olivine crystals, which is revealedby the insertion of a gypsum plate on the optical microscope (Fig. 4d). Inthe dominant first group, the lamellae are parallel to the olivine (100)sub-grain boundaries and responsible for antigorite foliation (Fig. 4a,b).In the second group, the lamellae are parallel to the (010) planes ofolivine, which are the dominant parting planes observed in this sample(Fig. 4d). Boudier et al. (2010) documented and described in detailthese relationships via TEM analyses in the contact between olivineand antigorite. By knowing the phase transformation relationships, wecan assure that the “old” mantle flow plane and flow direction markedby the olivine is normal to the antigorite structure (Fig. 5).

5. Crystallographic orientations

The pole figures are plotted in the antigorite schist reference systemwith the trace of the foliation E–W (Fig. 6) and the lineation alsotrending E–W (marked X).

The olivine presents a classical orthorhombic distribution for theCPO, with a strong concentration of [100] close to the normal of the

105L.F.G. Morales et al. / Tectonophysics 594 (2013) 103–117


antigorite foliation (marked Z). The [010] axes show a double maxi-mum concentrated in the antigorite schist foliation plane. The [001]axes show the same tendency with a maximum in the antigoriteschist foliation plane normal to X and Z.

The CPO of the dominant hydrated phase antigorite is relativelyweak, with maximum densities between 2.1 and 3.3 times a uniformdistribution (Fig. 6). The poles to (001) planes form a point maximumnear Z. The poles of (100) and (010) form diffuse girdles normal to

Fig. 2. General overview of the microstructure of the serpentinized mantle xenolith with a relatively strong foliation marked by antigorite and chlorite wrapping around olivine“porphyroclasts”. The olivine crystals are intensely fractured normal to the foliation and some of these crystals have an elongation parallel to the antigorite foliation. mag = magnetite,tr = tremolite, ol = olivine atg = antigorite. Xatg and Zatg refer to the antigorite lineation direction and the normal of the antigorite foliation.

Fig. 3. Backscattered electron images from the sample MR-1 illustrating (a) the different phases present in the sample, showing relic crystals of olivine (ol) surrounded by theaggregates of antigorite (atg) and chlorite (chl). The antigorite foliation is horizontal E–W. (b) The contrast between chlorite and antigorite is very weak but the former usuallyshows a lighter grey in comparison to the very dark grains of antigorite, reflecting the higher Al content in the chlorite? (c) Tremolite (tr) occurs in isolate patches and it is crosscutby the antigorite lamallae, suggesting that the amphibole breaks down to antigorite. (d) A very common feature in this sample is the dual-orientation of antigorite crosscutting theolivine crystals, sometimes interleaved with chlorite. Chlorite appears more closely related to the olivine rather than to the antigorite. Xatg and Zatg refer to the antigorite lineationdirection and the normal of the antigorite foliation.



the [001] point maximum. It is clear from the textural relationships(Figs. 2–4) that the dominant fabric corresponds to the antigoritebasal plane (001) close to the foliation of antigorite schist. There is a sta-tistical correlation between the chlorite and olivine CPO, where (100)ol|| (100)chl, (010)ol || (001)chl and (001)ol || (010)chl. The chlorite CPOis weaker than olivine, the pole figure maximum densities being be-tween 1.9 and 2.6 times uniform. Tremolite CPO is also characterizedby a strong (001) point maximum near Z and discontinuous girdle of(100) and (010) slightly oblique to the antigorite foliation. (100)poles have a weak double maximum near X and (010) poles strongermaximum normal to X. Geometrically it is possible that there is somecorrelation between antigorite and tremolite CPOs, but tremolite has arelatively small volume fraction of 5% (Fig. 6).

6. Seismic properties and DEM modelling

Seismic properties of the sampleMR-1were estimated by averagingthe elastic constant of the aggregate considering the crystallographicorientations of olivine, antigorite, chlorite and tremolite (the dominantphases), their individual single crystal elastic constants, modal propor-tions in the sample and the densities of individual phases. Modal pro-portions of different phases were derived directly from the EBSD mapand the calculationswere carried out considering all themeasurementswith MAD ≤ 1.3°. The modal proportions of each phase are as follows:olivine 51%; antigorite 23.5%; chlorite 21.5%; tremolite 5%. The calcula-tions were carried out using the experimentally determined elasticconstants of olivine (Abramson et al., 1997), antigorite (Bezacier et al.,2010a). Alexandrov and Rhyzova (1961) report C11, C33, C44 and C66for clinochlore end-member of the chlorite series, whichwill be referred

to as “chlorite”. However, the authors did not manage to measure C12 fortheir hexagonal approximation of the monoclinic elastic symmetry ofchlorite. C12 for chlorite was then estimated by Mainprice and Ildefonse(2009) by using the systematics of C12 for muscovite, phlogopite andbiotite versus density. As there are no single crystal elastic constantsfor tremolite, we have adopted the one for another amphibole(glaucophane — Bezacier et al., 2010b). Principal seismic properties ofthe anisotropic single crystals used here in relation to the crystal refer-ence frame are presented in Fig. 7. The elastic constants of the aggre-gates were averaged using the Voigt–Reuss–Hill scheme and theseismic properties are plotted in the same reference frame as the CPOpole figures (Fig. 8) with the antigorite foliation EW.

The pole figures of the distribution of P- and S-waves for the wholesample resemble in many aspects that of a typical mantle peridotite(Fig. 8). The high P-wave velocity (Vp) is parallel to the maximumof [100] of olivine, and the fastest shear wave (Vs1) polarizationplanes are parallel to the north–south “mantle” foliation (dashed linein Fig. 8), but the maximum shear wave splitting is not parallel to anysimple direction. The highest anisotropies of Vp/Vs1 and Vp/Vs2 areparallel to the [100] maxima olivine, and the high Vp/Vs2 values trendparallel to the north–south “mantle” foliation. The magnitudes of veloc-ities and anisotropies differ from standard mantle values, where themaximum Vp is 7.59 km/s and the maximum Vs1 is 4.17 km/s, whilethe respective anisotropies are 5.7 and 2.8%. The maximum shear wavesplitting anisotropy is 5.24%, and the anisotropies of Vp/Vs1 and Vp/Vs2are around 4.4 to 4.9%. For comparison, the anisotropy of S-waves variesbetween 18% in olivine to 76% in the chlorite single crystals (Fig. 7).

To test the effect of different volume fractions of hydrous phaseson the seismic properties of the hydrated peridotites in a general

Fig. 4. (a) Microstructural details observed under polarized light, crossed nicols, include antigorite lamellae growing parallel to the (100) sub-grain boundaries in olivine (whitedashed lines). (b) In detail, antigorite crystals' growth in two directions within the olivine grains, one trending N–S on the image, and a secondary orientation (E–W) normal tothe foliation, parallel to the fractures. (c) Some olivine crystals exhibit very thin microfracture-like antigorite lamellae, normal and parallel to the antigorite schist foliation (N–Sin the image); (d) The same picture as (c) but using the gypsum plate, where the fast vibration direction (F) is parallel to antigorite schist foliation in antigorite bands. Xatg andZatg refer to the antigorite lineation direction and the normal of the antigorite foliation.



situation of subduction zone,we carried out a differential effectivemedia(DEM) modelling (Mainprice, 1997). First we calculated the elasticconstants for pure end-member aggregates of olivine, antigorite, chloriteand tremolite, using their CPO measured by EBSD, their single-crystalelastic constants and respective densities. To take into account the ob-served grain shape, we have used the self-consistent effective mediamethod (e.g., Mainprice, 1997). We assumed that the olivine grainswere spherical and the hydrous phases were 10 times longer in thebasal plane than normal to it, parallel to the antigorite foliation. Theself-consistent elastic constants were then used as end-members for atwo-phase DEM composite model of olivine + antigorite, olivine +chlorite and olivine + tremolite. The DEM model was constructed byprogressively adding the inclusions of the different hydrous phaseswith constant aspect ratio to the olivine background medium, andrecalculating the new effective media properties at each step. The tensorequation for differential effectivemedium calculations has been given byMcLaughlin (1977) as follows:

dCDEM

dV¼ 1

1−Vð Þ Ci−CDEM� �

Ai: ð1Þ

In this equation the term Ai = [I + G(Ci-CDEM)]−1 is the strainconcentration factor coming from the Eshelby equation for the elasticinclusion problem, where I is the 4th rank unit tensor, G the symmet-rical tensor of Green's function, Ci are the elastic constants of theself-consistent antigorite, chlorite or tremolite and CDEM is the

background material. As a last step, the seismic anisotropy of differentmixtures up to 100% of hydrous phase was calculated by solving theChristoffel equation for each volume fraction of interest. For that wehave used the elastic constants generated by the DEM model and theappropriate densities for each assemblage. Details of the anisotropicseismic property calculations and numerical methods are given byMainprice (1990, 2007).

The replacement of olivine by antigorite, chlorite or tremolite inducesa progressive decrease of compressional and shear wave velocities, withVp varying from 8.7 km/s in the pure olivine aggregate to ~8, 7.2 and6.7 km/s in the aggregateswith 100% of tremolite, antigorite and chloriterespectively. The velocity of the fastest shear waves (Vs1) vary from~5 km/swith 100% of olivine, to ~4.7, 4.1 and 3.4 km/swith the additionof tremolite, antigorite and chlorite respectively (Fig. 9a,b). The anisotro-py of P and Swaves on the other handhas amore complex evolution. Theaddition of antigorite to the olivine background leads to the strongestanisotropy of P-waves (AVp) and induces a linear increase from 7.5 to~13% (Fig. 9c). On the other hand, the addition of chlorite first inducesan increase of AVp up to ~10% (volume fraction of Xchl ~ 0.4), followedby a progressive decrease up to ~7% (Xchl ~ 0.85) and a final increasefor the “pure” chlorite aggregate. The addition of tremolite induces asmaller modification of AVp, first decreasing its value about 1% then in-creasing constantly up to ~8.2%, when Xtr = 1.0 (Fig. 9c). In the case ofthe maximum shear wave splitting anisotropy (AVs), the behaviour iseven more complex. The addition of both chlorite and antigorite to theolivine background induces a sinusoidal behaviour of AVs magnitudes,

Fig. 5. Schematic figure showing the relative orientations of the antigorite foliation and “asthenospheric” flow foliation based on the orientation of (100) sub-grain boundaries andon the crystallographic orientation of olivine, indicated by the [100] and [010] axes. Taken together, these information show that the olivine elongation parallel to the antigoritefoliation is apparent and results from the dominant development of antigorite parallel to (100) of olivine, compared to the secondary orientation parallel to (010) of olivine.



Fig. 6. Pole figures for olivine, antigorite, chlorite and tremolite for the sample MR-1, demonstrating a strong crystallographic preferred orientation for olivine, but relatively weakCPOs for antigorite and chlorite. Note the correlation between the olivine–antigorite CPO, as well as the link between the olivine and chlorite preferred orientations. Although thetremolite CPO is also relatively strong, it has a limited occurrence in the thin section. The pole figures were plotted using the antigorite reference frame, indicated in the pole figuresby the vertical E–W plane, with a horizontal lineation also E–W (marked by X), with Z indicating the pole of the foliation. Equal area stereonets, lower hemisphere, multiples ofuniform distribution, MAD ≤ 1.3°.



Fig. 7. Seismic properties of the single crystals anisotropic elastic constants used in our calculations and modelling. Vp: compressional wave velocities; AVs: percentage of shear wave splitting; Vs1 and Vs2: fastest and slowest shear wavevelocities; Vp/Vs1 and Vp/Vs2: ratio between compression and shear wave velocities. Seismic properties were calculated using the programs Single_Anis and Single_VpVs derived from Mainprice (1990). The elastic constants used herewere: olivine — Abramson et al., 1997; antigorite — Bezacier et al., 2010a; and chlorite — Alexandrov and Rhyzova, 1961. In the lack of tremolite single crystal elastic constants, we have used the glaucophane of Bezacier et al., 2010b.Stereographic projections of the main crystallographic axes were calculated using the MTEX toolbox for MATLAB (Hielscher and Schaeben, 2008, and available at http://code.google.com/p/mtex/). All the seismic properties are plottedin the upper hemisphere.

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with amaximumAVs of 11% caused by the addition of Xchl ~ 0.4 chlorite(Fig. 9d). After this critical value, the shear wave splitting starts to de-crease again up to Xchl ~ 0.7, finally reaching 9.9% anisotropy at Xchl =1.0. The addition of tremolite to the olivine aggregate first induces asmall decrease of AVs, followed by a progressive increase in magnitude,finally reaching 6.7% at Xtre = 1.0 (Fig. 9d).

The addition of tremolite induces a relatively small decrease on theVp/Vs1 and Vp/Vs2 ratios while a strong increase is observed whenthe volume fraction of chlorite increases, reaching a maximum valueof 2.17 (Fig. 10a). The increase of modal proportions of antigorite in-duces a small drop of Vp/Vs1 and an increase of Vp/Vs2. The magnitudethe anisotropy of Vp/Vs ratios has a complex evolution (Fig. 10b). Theaddition of antigorite results in an approximately constant anisotropyof Vp/Vs1 up to Xatg ~ 0.6, then a small increase up to 10%, but inducesa constant increase on the anisotropy of Vp/Vs2 up to 16%. When chlo-rite is added, the anisotropy of Vp/Vs1 remains constant up to Xchl ~ 0.6and then decreases to about 4%, but the Vp/Vs2 first increases up to~11% (Xchl ~ 0.4) followed by a small decrease and a final increasereaching an anisotropy of 10%.

With the addition of antigorite, the maximum Vp changes from aposition parallel to the [100] maxima of olivine (Z structural directionnormal to antigorite foliation) to parallelism with the lineation in theantigorite foliation (rotation of 90°), with the development of a planeof low velocity parallel to the foliation (Fig. 11). When chlorite isadded to the olivine background medium, the Vp maxima remainsin a constant position parallel to [100] axes of olivine up to chloriteXchl ≥ 0.75, when it moves towards the centre of the net and tendsto develop a plane low velocity normal to the reference foliation(Fig. 11). On the other hand the addition of tremolite does not signif-icantly modify the direction of maximum P-wave velocity (Fig. 11).

The maximum shear wave splitting (blue shading in Fig. 12) doesnot change position and remains parallel to X the lineation with the

addition of antigorite (Fig. 12). The addition of chlorite (up to 0.45)reinforces the S-wave anisotropy normal to the north–south “mantle”foliation plane (Fig. 12), then progressively themaximumbecomes par-allel to the north–south “mantle” foliation plane at Xchl = 1.00 chlorite.The addition of tremolite has an important effect on the maximumshear wave-splitting direction, which moves from X to nearer thecentre of the net between Y and Z. The addition of either chlorite ortremolite does not cause anymodification on the polarization directionsof the fastest shear wave S1, which remains parallel to the olivinenorth–south “mantle” foliation (Fig. 13). Nevertheless, when antigoriteis added the polarization direction becomes parallel to the referenceantigorite foliation (XY plane), and perpendicular to themantle foliation(YZ plane).

7. Discussion

7.1. Microstructural evolution of a serpentinized mantle rock

The studied sample represents a rare case of a mantle rock inwhich the partial hydration and the transformation of the mantlephases into hydrous minerals (antigorite, chlorite and tremolite)still preserves the framework of olivine grains and their internal struc-tures (e.g., (100) sub-grain boundaries) form during high-temperaturemantle deformation. In such a situation, we can extract informationfrom both the mantle microstructures and their relations with the hy-drous phases. It is important to remember that although the presenceof hydrous-phase bearing peridotites with antigorite, chlorite and trem-olite is a common feature in the area where this sample was collected,they are unlikely to occur in othermantlewedge assemblages as demon-strated byArai and Ishimaru (2008) and seems to be related to low-anglesubduction zone, as pointed out by Smith (2010).

Fig. 8. Calculated seismic properties for the sample MR-1, based on the CPO of olivine, antigorite, chlorite and tremolite, their respective modal proportions, the single crystal elasticconstants for each phase and their densities. Note the strong control exercised by the olivine CPO, principally on the P-wave seismic wave distribution. Averaging scheme wasVoigt–Reuss–Hill and the figures follow the same reference frame as Fig. 5, with the antigorite foliation as a vertical E–W plane (black solid horizontal line). The north–south “mantle”foliation vertical N–S plane (black dashed vertical line).



Fig. 9. Results from differential effective medium (DEM) modelling considering a background of spherical olivine grains and pancake-shaped inclusions of the hydrous phasesparallel to the antigorite and chlorite foliations. Compressional (Vp), fast shear wave (Vs1) velocities, anisotropy of P-waves (AVp) and shear wave splitting as a function ofmodal content between olivine and the three hydrous phases (antigorite, chlorite and tremolite).

Fig. 10. Results from differential effective medium (DEM) modelling considering a background of spherical olivine grains and pancake-shaped inclusions of the hydrous phasesparallel to the antigorite + chlorite foliation. Ratios and ratio anisotropy between compressional (Vp), and shear waves (Vs1 and Vs2) as a function of modal content betweenolivine and the three hydrous phases (antigorite, chlorite and tremolite).



7.1.1. OlivineThe olivine primary microstructure is still preserved in the form of

well-developed, widely separated sub-grain boundaries normal to thegrain elongation (Fig. 4a). These sub-grains are parallel to the (100)planes of olivine and record the asthenospheric flow (e.g., Nicolas andPoirier, 1976) prior to the hydration of these rocks. Themicrostructuralobservations are confirmed by the strong orthorhombic olivine CPO(Fig. 6 and Boudier et al., 2010) preserved in these rocks,which suggests

an asthenospheric flow dominated by the activation of [100] (010) slipsystems (e.g., Carter and Avé Lallemant, 1970; Gueguen and Nicolas,1980;Mercier, 1985). This is themost frequently occurring fabrics in pe-ridotites (e.g., Ben Ismaïl andMainprice, 1998;Mainprice, 2007) and typ-ical of high temperature, low stress deformation of dry olivine. In such aconfiguration, the foliation compatible with the olivine (100) sub-grainboundaries is normal to the antigorite EW foliation and the “mantle”lineation and foliation is vertically N–S. The apparent grain shape elonga-tion created by segmentation of olivine by the growth of antigorite isnow horizontal (see Fig. 5 and the inset in Fig. 6). Therefore, the elonga-tion of olivine crystals observed in the studied specimen is an apparentfeature that may lead to the incorrect interpretation of the mantle flowmicrostructure as being parallel to the dominant micro-layering ofantigorite. The orientation dispersion in the olivine polefigures, resultingin a CPO that resembles the characteristic fabric for the activation of[100]{0kl} slip systems, are probably produced by the combination ofphase transformation and antigorite deformation dismembering theolivine grains.

7.1.2. AntigoriteAs pointed out by Boudier et al. (2010), the replacement of olivine

by antigorite is topotactically controlled according to the followingtwo relationships: relation 1 (100)ol || (001)atg with [001]ol || [010]atg and relation 2 (010)ol || (001)atg and [001]ol || [010]atg. Thesetransformation relationships are confirmed at a larger scale by the indi-vidual orientation EBSD measurements along the phase boundariesbetween olivine and antigorite with an apparent predominance of(100)ol || (001)atg and subordinate (010)ol || (001)atg (Fig. 4). To quanti-fy the impact of the two transformation laws on the measuredantigorite CPO we have calculated a model antigorite CPO based onthemeasured olivine CPO. The orientation of “n” symmetrically equiva-lent antigorite crystals is given by

gAntigoriten¼1to4 ¼ Δg:SOlivinen ⋅gOlivine ð2Þ

whereΔg is the rotation between the olivine and new antigorite miner-al, Snolivine are the 4 rotational point group symmetry operations for oliv-ine and golivine is the measured orientation of olivine (see Mainprice etal., 1990 for further details of the method). Δg is determined by the ori-entation relationships 1 and 2 between olivine and antigorite. Becauseof the two-fold symmetry axis parallel to the [010]-axis of antigoriteonly 2 unique orientations are generated rather than the 4 predicted

Fig. 11. Results from differential effective medium (DEM) modelling considering abackground of spherical olivine grains and pancake-shaped inclusions of the hydrousphases parallel to the antigorite and chlorite foliations. Variations on the propagationdirections of compressional shear waves (Vp) as a function of modal content betweenolivine and the hydrous phases (antigorite, chlorite and tremolite), for modal proportionsof 40, 75 and 100% of the later. Note the rotation of 90° on the maximum Vp with theaddition of antigorite. Although the addition of chlorite and tremolite causes minormodifications on the Vp maximum propagation direction, it has different significances interms of the microstructural arrangement for an aggregate with 100% olivine or chlorite.See the inset for the reference frames of olivine and antigorite. Scale bar on the left is forthe aggregate with 100% of olivine.

Fig. 12. Results from a differential effective medium (DEM) modelling considering abackground of spherical olivine grains and pancake-shaped inclusions of the hydrousphases parallel to the antigorite + chlorite foliation. See the inset for the referenceframes of olivine and antigorite. Variations on the shear wave splitting as a functionof modal content between olivine and the hydrous phases (antigorite, chlorite andtremolite), for modal proportions of 40, 75 and 100% of the later. Note the rotation of90° on the maximum shear anisotropy with the addition of high contents of chlorite.The addition of antigorite does not modify the direction of maximum anisotropy buthas a significant effect in terms of the microstructural arrangement of the sample.The addition of tremolite produces a shear wave splitting patterns of complex naturethat cannot be compared with any of the observed microstructural or CPO aspects ofthis sample. Scale bar on the left is for the aggregate with 100% of olivine.

Fig. 13. Variation on the fast shear wave polarization directions as a function of the modalcontent of olivine and the hydrous phases (antigorite, chlorite, tremolite). Note that themain variation occurs when antigorite is added, resulting in a rotation of ~90° of thepolarization planes. Results from a differential effective medium (DEM)modelling consider-ing a background of spherical olivine grains and pancake-shaped inclusions of the hydrousphases parallel to the antigorite and chlorite foliations. See the inset for the reference framesof olivine and antigorite.



by the equation above for the general case. The pole figures predictedfrom the phase transformation of olivine to antigorite are shown inFig. 14. Fig. 14a shows the original olivine CPO and the resulting antigorite

CPO applying either 100% relation 1 (Fig. 14b) or 100% relation 2(Fig. 14c). In this case the pole figure densities are the same the originalolivine CPO, but (hkl) values of the antigorite pole figures are givenaccording to relations 1 and 2. Note that the resulting antigorite pole fig-ure (010) is the same for transformation relations 1 and 2. Neither rela-tion 1 or relation 2 gives a good match to the observed antigorite polefigures (Fig. 14d). To determine if a linear mixing law of relations 1 and2 could give better agreement with the observed antigorite pole figureswe programmed the following equation using MTEX,

f gð Þmixture ¼ Vf gð Þ1 þ 1−Vð Þf gð Þ2 ð3Þ

where V is volume fraction of relation 1 in the mixture, f(g)mixture, f(g)1and f(g)2 are the orientation distribution functions of the antigorite mix-ture, 100% relation 1 and 100% relation 2 respectively. From f(g)mixture thepole figures and their texture indiceswere calculated. The polefigure tex-ture index Tpf is defined as

Tpf ¼ ∫P2h yð Þdy y ¼ α;βf g dy ¼ sinα:dα:dβ ð4Þ

where Ph2 is the square of the pole figure density for crystal direction

h and sample direction y, where y can be given in spherical polar anglesα, β and dy is the incremental area. A mixture with 75% relation one(Fig. 14e) has pole figure densities for (100) and (001) similar to the ob-served antigorite pole figures (Fig. 14d). Nevertheless the model (010)has much higher density as well as pole figure texture index and it is aconstant for all volume fractions (Fig. 15). Themodel with equal amountsof mixture 1 and mixture 2 has identical pole figures (100) and (001)(Fig. 14f), but clearly does not agree with the observed CPO. Hence thetransformation lawmodelwith about 75% relation 1 gives the best agree-ment with the observed CPO densities.

Fig. 14.Measured pole figures of olivine and antigorite and model predicted pole figures of antigorite for 100% phase transformation relations 1 and 2, plus models with mixtures of50%:50% and 75%:25% ratios of relation 1 to relation 2. The pole figures were plotted using the antigorite reference frame, indicated in the pole figures by the vertical E–W plane(solid horizontal black line on measured pole figures), with a horizontal lineation also E–W (marked by X), with Z indicating the pole of the foliation. Equal area stereonets, lowerhemisphere, and multiples of uniform distribution.

Fig. 15. Phase transformation mixing law model results for antigorite pole figures. Thepole figure texture index for the 3 measured pole figures (100), (010) and (001) areshown as dashed horizontal lines. The pole figure texture index for the 3 transformationmixing law model pole figures (100), (010) and (001) are shown by solid lines. The model(010) does not varywith the volume fractionof antigorite relation 1 and is hence ahorizontalline; no agreement is foundwithmodel for this pole figure. The black dots indicatewhen thepolefigure texture of the observed andmodel polefigures agree, this only occurs in relation 1more than 50% of the volume fractions. In the range 70–75% relation 1 both (100) and (001)pole figures are in agreement between observed and model values.



The major differences are:

a) The highest observed density pole figure is the (001) with a pointmaximum (3.3) about 20° from Z, whereas themodelled pole figurehas a point maximum (3.2) exactly at Z;

b) The weakest observed density pole figure is the (100) with a diffusegirdle, this girdle is approximately present in the modelled polefigure;

c) The intermediate observed density pole figure is the (010), which ispoorly reproduced in terms of density, with the model havingmuchhigher density. Also, the observed girdle is weakly developed in themodel, where the strong maximum in the model is a weak maxi-mum in the pole figure from the sample.

These disagreements between the modelled and observed polefigures caused the basic model assumptions:

i) The model assumes the transformation of 100% of olivine toantigorite, but only a part of the original olivine remained to bemeasured in the sample. The measured olivine volume fraction isonly 51% and antigorite is 23.5%, where as many peridotites havevolume fractions of olivine as high as 70% or more, suggestingthat as much as 20% of the olivine has already been transformedin agreement with the measured antigorite volume fraction;

ii) From the microstructural images (Figs. 2–4) the observed antigoritefoliation has started to rotate and localizes the deformation producinga CPO that has two different origins, transformation and deformation.

With these facts in mind it is not too surprising that the model doesnot fit perfectly with the pole figures measured for antigorite. Thepoorest fit is the (010) pole figure, the discrepancies for (100) and(001) could be accounted for by some external rotation about theX-axis due to the localization of the deformation in the relativelyweak antigorite layers. The observed (010) pole figure has a moderateconcentration of 2.4 near the X-axis, which could indicate the activationof [010] slip in antigorite as previously suggested by CPOmeasurementsin naturally deformed antigorite (e.g., Hirauchi et al., 2010, Soda andTakagi, 2010). If so, this could explain the depopulation of the high den-sitymodel point maximum in favour of [010] slip in the direction of thelineation (X). In this case we can speculate that the slip processes inantigorite are mainly responsible for the disagreement of themeasuredand the modelled antigorite CPO. However, the model does indicatethat relation 1 is the dominant transformation mechanism needed toachieve the high pole figure density of (001) (Fig. 14).

7.1.3. Tremolite and chloriteAsmentioned previously tremolite only occurs in isolated patches in

direct contactwith olivine and is often crosscut by lamellae of antigoriteand has a volume fraction of only 5%. Chlorite is also associated withstraight grain boundary contacts with olivine as well as more diffuseboundaries with antigorite and has a volume fraction of 21.5%. Tremo-lite most probably represents the breakdown product of pyroxene(e.g., Evans, 1977), which would have been present in the anhydrousmantle peridotite. Tremolite is clearly not stable as it is now crosscutby antigorite and transforming to antigorite lamellae (Fig. 3c).

Chlorite association with olivine is shown in Fig. 3b, c, and d, as wellas its association with antigorite in Fig. 3a, b, c and d. Chlorite is crosscutby antigorite in Fig. 3c and d, suggesting that chlorite is breaking down toantigorite, as expected by the experimental phase diagram presented byFumagalli and Poli (2005) for hydrated lherzolite and harzburgite withdecreasing temperature. However, chlorite also has more blurred con-tacts in Fig. 3a and b suggesting a diffuse boundary smeared by varyingAl contents, which ismore in agreementwith chlorite and antigorite sta-bility being mitigated by the presence of Al-rich fluid (e.g., Spear, 1993).Chlorite also occurs in contact at the extremity of olivine grains (Fig. 3c,d) in a morphology similar to the one described by Padrón-Navarta etal. (2011) for chlorite harzbugite with a volume fraction of 20% chlorite.There is clear, but a weak correlation between the CPO of olivine and

chlorite, which presumably developed before and at a higher tempera-ture to the formation of the crosscutting antigorite. It is possible thatthe formation of tremolite and chlorite occurred at the same time andin the same temperature range.

According to Smith (1995) the chlorite in the inclusions from theNavajo Volcanic field diatremes were formed at the vicinity of garnetperidotites and other mantle rocks in response to fluid flow alongfault zones under pressures > 2 GPa. The percolation of metasomaticfluid may have localized the hydration of the mantle rocks that oc-curred along trans-lithospheric shear zones in the vicinity of garnetperidotites (Smith, 1995). We suggest that the composition of thisfluid may have also migrated the relative stability of chlorite andantigorite (e.g., Spear, 1993) and the formation of chlorite and trem-olite in a relatively static environment. The observed correlation be-tween the olivine and the chlorite CPOs suggests the preservation ofhydrostatic environment during the development of chlorite.

7.2. The role of some hydrous phases on the seismic anisotropy ofsubduction zones

If we take into account the discussion of the timing of the phasechanges given above then the role of tremolite and chlorite shouldbe considered to operate together at a higher temperature andantigorite at a lower temperature range. The reduction in P-wave ve-locity and S-wave velocity of tremolite plus chlorite will result inhigher velocity, but not so dramatically high if we consider only thevariation of tremolite (Fig. 9a,b). The combination of tremolite pluschlorite P-wave will induce significantly weaker anisotropies thanantigorite (Fig. 9a,b). The situation for S-wave anisotropy will bemore extreme, but the tremolite plus chlorite will be similar toantigorite with increasing anisotropy with volume fraction.

When antigorite is added to the pure aggregate of olivine, the anisot-ropy of P-waves increases progressively and possesses a different behav-iour to the models of Boudier et al. (2010), where the initial addition ofantigorite induces a decrease of both Vp and Vs. An important differencebetween these two models is that the low volume fraction of antigorite(>20%) in the olivine CPO results in a destructive relationship for anisot-ropy of Vp and Vs in the models of Boudier et al. (2010). This can beexplained by the fact that in Boudier et al. (2010) the seismic anisotropywas modelled considering the main topotactic relationship betweenolivine and antigorite, where (100)ol//(001)atg. Our calculations on theother handwere based onEBSDmeasurements of crystallographic orien-tations of antigorite, where the microstructural dictated frequency ofphase transformation relationships is added to the deformation inducedchanges of antigorite CPO. For the same reason, the values of anisotropymodelled by Boudier et al. (2010) are considerably higher than thevalues calculated for our sample.

Although the effect of chlorite on the seismic properties of hydrat-ed mantle rocks is rarely considered, it is expected to occur in meta-morphosed mantle slabs in fertile peridotites with relatively highcontents in Al (e.g., Christensen, 2004) and the dehydrated mantlewedge (e.g., Padrón-Navarta et al., 2011). As observed in our results,the presence of chlorite induces the most important drop on the Pand S velocities from the three hydrated phases analysed here andcan possibly explain the low seismic velocities observed in someforearc mantle wedges, an idea postulated by Christensen (2004).

The weak drop on seismic velocities caused by the addition oftremolite possibly results from the higher density of this phase incomparison to antigorite and chlorite. The increase of volume fractionof tremolite does not cause a strong effect on the anisotropy of P and Swaves mainly because the single crystals of amphibole in general arenot strongly anisotropic (e.g., Mainprice and Ildefonse, 2009). Theweak drop on the anisotropy when Xtre b 0.6 results from a destructiverelationship between the anisotropies of olivine and tremolite, is easilyvisualized by the directional differences on themaximumVp and AVs ofolivine and amphibole single crystals.



It is standard practice in seismology to calculate the Poisson's ratio tointerpret the data from the hydrated mantle wedge in terms of mineral-ogy. As shown by Mainprice and Ildefonse (2009), there is no physicalreason to use Poisson's ratio to characterize seismic properties in an an-isotropic media. In an isotropic media we have the Vp/Vs ratio, and theanisotropicmedia there are three velocities (Vp, Vs1 and Vs2) and there-fore two ratios (Vp/Vs1 and Vp/Vs2), which are also directionally depen-dent (Fig. 10). If we consider that tremolite and chlorite act together dueto petrological arguments given, the ratios will be very close together fortremolite plus chlorite and antigorite (Fig. 10). The situation for theanisotropy will be more complex, but values will globally be higher forantigorite and increase with the volume fraction.

In the low temperature subduction setting, antigoritewill be the sta-ble hydrous phase. The velocity ratio Vp/Vs calculated from experi-ments carried out at 1GPa and 500 °C by Christensen (2004) for anisotropic antigorite rock was 1.84. This value is close to the average ofVp/Vs1 and Vp/Vs2 for anisotropic aggregate. The value of 1.85 is a typ-ical value of the forearc mantle wedge in Costa Rica (DeShon andSchwartz, 2004) and can be compared to 1.81 for an unaltered perido-tite, although values as low as 1.65 have been reported (Rossi et al.,2006). If chlorite is the high temperature hydrous phase, then the ratioswould be significantly higher (around 2.1), but to our knowledge, seis-mologists studying subduction zones never reported values of Vp/Vsabove 1.9. This would be in agreement the global study by Brudzinskiet al. (2007) which concluded that the breakdown of antigorite wasthe cause of double Benioff zones rather than chlorite.

8. Conclusions

The analysis of the microstructures and crystallographic orienta-tion of olivine, antigorite, chlorite and tremolite in a rare sample ofantigorite schist where the olivine network still preserved, allowedus to draw the following conclusions:

(i) The network of olivine grains preserved in this sample recordthe high-temperature asthenospheric flow, dominated by the[100](010) slip, which is the most frequently observed fabricin natural deformed peridotites;

(ii) The asthenospheric flow plane and direction is actually normalto the main foliation formed by antigorite. The elongation of ol-ivine grains in this sample is an artefact caused by segmenta-tion of the olivine by transformation to antigorite in twotopotactical orientations and may lead to incorrect interpreta-tion of partially serpentinized peridotites as it does not repre-sent the mantle flow direction;

(iii) Transformation of olivine to antigorite occurred predominantly(~75%) along the (001)atg//(100)ol and subordinate (001)atg//(010)ol and the CPO of antigorite. The final observed CPO ofantigorite is the product of transformation and localization ofdeformation in the relatively weak antigorite foliation withprobably the activation of [010] slip in antigorite;

(iv) Chlorite results from the phase transformation of olivine in arelatively static environment, as shown by the correlationthat is preserved between olivine and chlorite CPOs. The perco-lation of metasomatic fluid may have localized hydration of themantle rocks that occurred along trans-lithospheric shearzones in the vicinity of garnet peridotites (Smith, 1995). Thecomposition of this fluid may have also controlled the relativestability of chlorite and antigorite (e.g., Spear, 1993).

Differential effective media modelling to test the effect of differenthydrous phases on the seismic properties of an aggregate of pure olivineconsidering the observed crystallographic orientations shows that:

(v) Progressive addition of three studied hydrous phases in an olivinebackground induced a considerable (and almost linear) decreaseof the seismic velocities of compressional and shear waves;

(vi) The evolution of P and S wave anisotropy is non-linear and differ-ent for each phase. The addition of antigorite results in an increaseof anisotropy, whereas for chlorite it first increases, followed by adecrease when the content of chlorite is higher than 50%;

(vii) The addition of chlorite induces an increase on Vp/Vs1 and Vp/Vs2ratio to higher values than observed in subduction zones, whereastremolite induces a small decrease of these ratios. Antigorite addi-tion on the other hand causes a small decrease of Vp/Vs1, but anincrease of Vp/Vs2 to values observed in subduction zones;

(viii) Antigorite on the other hand is the only hydrous phase that causesmajor modifications of P and S-wave properties. With the weakCPO of our sample, a high volume fraction of antigorite is neces-sary to change the fastest S-wave polarization of typical A-type ol-ivine CPO to one in a perpendicular orientation characteristic of anantigorite schist CPO.

Acknowledgements

We are grateful to C. Nevado and D. Delmas for the high qualitypolished sections for the EBSD, to Professor Shaocheng Ji andDr. Ken-ichi Hirauchi for the detailed and thoughtful review and toProfessor F. Storti for the Editorial comments.

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