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Contrib Mineral Petrol (2015) 169:35 DOI 10.1007/s00410-015-1129-4

ORIGINAL PAPER

On topotaxy and compaction during antigorite and chlorite dehydration: an experimental and natural study

José Alberto Padrón‑Navarta1 · Andréa Tommasi1 · Carlos J. Garrido2 · David Mainprice1

Received: 27 December 2014 / Accepted: 6 March 2015 © Springer-Verlag Berlin Heidelberg 2015

CPO at a larger scale in natural samples to compaction and associated fluid migration. Microstructural features that might be related to compaction in the natural samples include: (1) smooth bending of the former foliation, (2) gradual crystallographic misorientation (up to 16°) of pris-matic orthopyroxene due to buckling by dislocation creep, (3) inversion of enstatite to low clinoenstatite (P21/c) along lamellae and (4) brittle fracturing of prismatic orthopyrox-ene enclosed by plastically deformed chlorite. The coexist-ence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction. An lower bound for the strain rates in the order of 10−12 to 10−13 s−1 and shear stresses of 60–70 MPa are estimated based on creep data. Lower shear stresses (20–40 MPa) are retrieved using a theoretical approach. These data point to slow compaction (and fluid extraction) in nature if the system is not perturbed by exter-nal forces, with rates only marginally higher than the visco-plastic deformation of the solid matrix.

Keywords Topotaxy · Serpentinite · Compaction · Fluids · Texture · CPO · Subduction zones

Introduction

Mineral replacement reactions play a fundamental role in the chemistry and the strength of a major part of the lith-osphere and are an inexorable consequence of the Earth’s dynamics. When fluids are present, these reactions occur by interface-coupled dissolution–precipitation (e.g. Put-nis 2009; Putnis and John 2010; Ruiz-Agudo et al. 2014). Dehydration reactions are an outstanding case of min-eral replacement reactions because they produce signifi-cant transient fluid-filled porosity. Under poorly drained

Abstract Dehydration reactions result in minerals’ replacement and a transient fluid-filled porosity. These reactions involve interface-coupled dissolution–precipita-tion and might therefore lead to fixed crystallographic ori-entation relations between reactant (protolith) and product phases (i.e. topotaxy). We investigate these two phenomena in the dehydration of a foliated antigorite (atg) serpentinite by comparing the crystallographic preferred orientation (CPO) developed by olivine (ol), orthopyroxene (opx) and chlorite (chl) during high-pressure antigorite and chlorite dehydration in piston-cylinder experiments and in natural samples recording the dehydration of antigorite (Cerro del Almirez, Betic Cordillera, Spain). Experiments were per-formed under undrained conditions resulting in fluid-filled porosity and in strong CPO of the prograde minerals, con-trolled by the pre-existing antigorite CPO in the reactant foliated serpentinite. The orientation of aol,opx and c

∗

chl is parallel to c∗

atg from the protolith. The Cerro del Almirez samples show similar, locally well-developed topotactic relations between orthopyroxene, chlorite and antigorite, but the product CPOs are weaker and more complex at the thin section scale. In contrast to the experiments, olivine from natural samples shows a weak correlation between bol and the former c∗

atg. We relate the strengthening of local topotactic relations and the weakening of the inherited

Communicated by Othmar Müntener.

* José Alberto Padrón-Navarta padron@gm.univ-montp2.fr

1 Géosciences Montpellier, CNRS & Université Montpellier 2, Place E. Bataillon, 34095 Cedex 5, Montpellier, France

2 Instituto Andaluz de Ciencias de la Tierra, CSIC & UGR, Avenida de las Palmeras 4, 18100 Armilla, Granada, Spain

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conditions, they generate fluid pressure gradients that evolve in time and space eventually controlling fluid migration (Connolly 1997, 2010). Such reactions have profound implications for the chemical and rheological evolution of the middle–lower crust and of the subduct-ing lithosphere (e.g. Connolly 2010). Among the later, the dehydration of serpentinite is of special relevance because it is the major source of fluids in subduction zones and might correspond to the loci of deep earth-quakes in these settings (Ulmer and Trommsdorff 1995; Peacock 2001).

One of the microstructural features of interface-coupled dissolution–precipitation processes is the development of fixed arrangements of crystallographic axes across the interfaces between reactant and product phases (topotaxy) as a result of strain minimization during epitaxial growth on the boundary layer (e.g. Putnis 2009). Two topotac-tic relationships between olivine and antigorite (the stable serpentine mineral at high-temperature and high-pressure conditions) have been reported in partially hydrated man-tle wedge xenoliths: [100]atg||[010]ol; (001)atg||(100)ol (TR1) and <100> atg|| <100> ol; (001)atg|| (010)ol (TR2) expressed as common direction and planes in contact, respectively (Boudier et al. 2010; Morales et al. 2013). These topo-tactic relations can eventually result in development of a strong crystallographic preferred orientation of serpent-inite minerals during hydration under static conditions. Because hydrous phases such as antigorite or chlorite have an exceptionally strong seismic anisotropy (Mainprice and Ildefonse 2009; Bezacier et al. 2010, 2013; Mookherjee and Mainprice 2014), the development of oriented hydrous minerals might have important consequences for the recon-struction of the mantle wedge flow based on seismic ani-sotropy data (e.g. Katayama et al. 2009; Mookherjee and Stixrude 2009; Boudier et al. 2010; Bezacier et al. 2010, 2013 Jung 2011; Mookherjee and Capitani 2011; Brownlee et al. 2013; Morales et al. 2013; Mookherjee and Mainprice 2014; Soda and Wenk 2014).

Nagaya et al. (2014a, b) recently proposed that the opposite reaction (i.e. antigorite dehydration) might also result in topotactic relations producing a strong olivine crystallographic preferred orientation (CPO) in prograde peridotites from the subducting slab and the mantle wedge. However, direct application of topotactic relationships is not straightforward as olivine is not the only product of antigorite dehydration. The growth of prograde olivine is necessarily coeval to the growth of enstatite + chlo-rite ± tremolite at high-pressure conditions (>1.6 GPa) or of talc + chlorite ± tremolite at lower pressures (Fig. 1, Trommsdorff et al. 1998; Padrón-Navarta et al. 2010a and references therein). In addition, a more fundamental prob-lem arises when transient fluid-filled porosity is produced during the mineral replacement reaction. Compaction and

fluid migration result in local strain fluctuations, which may significantly modify the inherited CPO. Without the consideration of these complexities, the development of CPO in prograde peridotites from foliated serpentinites and its potential implications for the seismic anisotropy in sub-duction zones can only be speculated.

Here, we directly address these questions by the analy-sis and comparison of CPO of olivine, enstatite and chlo-rite formed during high-pressure antigorite dehydration in piston-cylinder experiments and in a natural antigorite dehydration front (Cerro del Almirez, Betic Cordillera, Spain; Fig. 1). Whereas the experiments result in mineral replacement reactions with preservation of the fluid-filled porosity (undrained conditions), the natural case offers a unique opportunity to study the effects of strain produced by porosity compaction and fluid migration, which are only accessible at geological time scales. We show that com-paction results in distinctive microstructural features and a weaker CPO that undrained dehydration experiments and

wet solidus (G

rove et al. 2006)

wet

sol

idus

(Gre

en 1

973)

1000900800700600

0.5

1.0

1.5

2.0

2.5

3.0

P (G

Pa)

T (°C)

C4445

C4436

CdA

Atg Ol Chl

Atg Ol

Ol Opx Chl

FMASH (+ H2O) Sample Al06-44

Ol Opx Grt

Ol Opx Spl

Ol Opx Crd

Atg Ol

OpxChl

Ol Tl

Chl

Ol AnthChl

Atg OpxOl Chlorite dehydration

Antigorite dehydration

Fig. 1 Location of the antigorite and chlorite dehydration reactions in the FMASH system for a representative serpentinite and the pres-sure (P) and temperature (T) conditions of the natural example (Cerro del Almirez, Spain, CdA) and the experiments (C4436 and C4445) investigated in this work. Dashed lines are the wet solidi for the MOR pyrolite model with 6 wt% H2O (Green 1973) and for a model man-tle composition HZ1 with 14.5 wt% H2O (Grove et al. 2006). The bulk water content in Al06-44 is 11.47 wt% H2O (see Padrón-Nav-arta et al. 2013 for further details of the pseudosection computation, Perple_X 6.6.8, Connolly 2009)

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models based on the direct application of topotactic laws fail to reproduce.

Geological background

Samples used in the experimental study and for the charac-terization of the CPO evolution in nature come from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, SE Spain). This outcrop is notorious for providing a well-preserved record of the antigorite dehydration reaction at high-pressure conditions (ca. 1.6–1.9 GPa and 630–710 °C, Fig. 1) that took place during the Alpine subduction of the Nevado–Filábride complex (Trommsdorff et al. 1998; López Sánchez-Vizcaíno et al. 2001, 2005, 2009; Garrido et al. 2005; Padrón-Navarta et al. 2010a, b, 2011, 2012; Marchesi et al. 2013; Jabaloy et al. 2015). At the dehydra-tion front, a strongly foliated antigorite serpentinite consist-ing of antigorite + magnetite ± olivine is transformed to chlorite-bearing harzburgite (olivine + enstatite + chlo-rite ± tremolite, i.e. metaperidotite) with a granoblastic tex-ture (hereafter referred as granofels) through a metre-scale transitional lithology (Fig. 2), which contains all phases (Atg–Chl–Opx–Ol) in apparent textural and chemical equi-librium (Padrón-Navarta et al. 2011). The dehydration front crosscuts the serpentinite foliation, suggesting that min-eral replacement reactions took place under quasi-static conditions. The protolith foliation is preserved in transi-tional lithologies (Fig. 2), but it is very weak or lacking in

the granofels. Metaperidotites with the same mineralogy but with a very contrasting texture (spinifex-like texture) are interspersed with the granofels and were described in detailed elsewhere (Trommsdorff et al. 1998; Padrón-Nav-arta et al. 2011). This texture has been interpreted as due to metamorphic growth under strong disequilibrium as a consequence of local fluid pressure gradients (but see also Evans and Cowan 2012 for an alternative interpretation). The inferred origin of the spinifex-like texture precludes a simple comparison with the microstructure of the granofels, and therefore, it was not investigated in this work.

In the sampled area (western domain of the massif), the dehydration front has locally an apparent angle of 15°–20° with respect to the foliation (which has an orienta-tion of N10°E/28°E, Fig. 2, see also Jabaloy et al. 2015). A detailed oriented sampling was done in the dehydration front (numbers given in parentheses in Fig. 2 indicate the approximate distance of the sample normal to the closest dehydration front with positive distances indicating sam-ples above it). All samples were cut in the antigorite serpen-tinite structural frame: parallel to the lineation X, marked by the elongation of magnetite aggregates, and perpen-dicular to the foliation Z, marked by the orientation of the antigorite sheets (Padrón-Navarta et al. 2012; Jabaloy et al. 2015). The sampling includes two antigorite serpentinites: Al06-44 (+80 cm) and Al10-08 (+5 cm); two intermediate atg–chl–opx–ol bearing rocks: Al10-09 (−5 cm) and Al10-10 (−70 cm); and one chl-harzburgite granofels: Al10-11 (−2.8 m). Two non-oriented granofels samples were also

Fig. 2 Sketch of the high-pressure antigorite dehydration front in Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera, Spain) with the location of the samples investigated in this study.

Scans of the hand samples cut in the serpentine reference frame (ZX) are also shown. Samples Al10-06 and Al09-16 were collected at ca. 10 and 25 m from the dehydration front

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studied: Al10-06 (−10 m) and Al09-16 (−25 m). The thin section of sample Al10-06 was prepared perpendicular to a rough macroscopic banding defined by olivine aggregates.

Analytical methods

Microstructural features at the thin section scale were first investigated under polarized light petrographic micro-scope. Observations under crossed polarizers mode with an inserted compensator (λ-plate) were particularly use-ful to identify different sets of orientations in antigorite. This observation mode has also the advantage to allow easy identification of chlorite and antigorite in fine-scale intergrowths because of their contrasting sign of elonga-tion (negative and positive, respectively, Tröger 1979; Deer et al. 1992). The CPO of antigorite, olivine, enstatite and chlorite was measured by indexing of electron back-scattered diffraction (EBSD) patterns using the CamScan X500FE CrystalProbe equipped with a Oxford Instruments Nordlys Nano digital CCD camera at Géosciences Mont-pellier (CNRS-Université de Montpellier 2, France). Ana-lytical conditions during EBSD acquisition were optimized by using low vacuum conditions (4 Pa) in a nitrogen atmos-phere to prevent charging. This allowed working on non-coated samples and hence recording high-quality EBSD patterns at operating conditions of 15 kV, 3.5 nA, with a working distance of 25 mm. Crystallographic parameters used to index Kikuchi patterns were taken from Capitani and Mellini (2004, 2006) for antigorite with a polysome number of 16 and 17, respectively, and from Eggleton and Bailey (1967) for chlorite. Data for olivine and enstatite were taken from the Oxford Instrument HKL database. The crystal structure of the P21/c clinoenstatite from Ohashi (1984, a = 9.6060, b = 8.8131, c = 5.1700; β = 108.35°, α = γ = 90°) matched the Kikuchi pattern of the (100) lamellar features found in enstatite with a low mean angu-lar deviation (MAD less than 1°).

Detailed measurements on the natural samples were performed on selected areas of 1200–2400 μm × 1000–2000 μm in high-resolution automatic mapping with a step size of 4 or 7 μm. Maximum accepted angular deviation for measurements was 1.3°. Post-acquisition data treatment that fills non-indexed pixels based on the orientations of, successively, 8, 7 and 6 neighbouring pixels and removal of wild spikes was performed using the CHANNEL 5 software. Antigorite patterns are particularly well indexed in the serpentinite above the dehydration front (samples Al10-08 and Al06-44) with raw indexation rates of 66 and 68 % (87 and 89 % after post-acquisition treatment). These indexation rates are excellent, considering the small grain size of antigorite (the crystals measure on average 5 × 50 μm). The high indexation rates allow the correction

of systematic errors due to pseudo-hexagonal symmetry around [001] in antigorite and around [100] in olivine.

In contrast, the indexation of chlorite was challenging because of micron-scale deformation of the cleavage on the surface during sample preparation (which are present even after gentle chemical–mechanical polishing with colloidal silica, Inoue and Kogure 2012). As a result, in most of the cases, it was not possible to correct the pseudo-hexagonal symmetry along [001]. Poor indexation was also observed for antigorite from transitional lithologies despite their much larger grain sizes (up to 500 × 100 μm) and sharp Kikuchi patterns. In this case, the problem is not related to polishing, but to a high degree of disordering (modulation dislocation, twins, stacking faults and polysomatic faults) in these samples as already noted by TEM observations (Padrón-Navarta et al. 2008). Therefore, the [001] direc-tion of antigorite and chlorite is correctly indexed but 3 or 6 equally possible solutions are given for [100] and [010]. A 5-axis universal stage (E. Leitz Wetzlar hosted in Géo-sciences Montpellier) was used in an attempt to discrimi-nate between these possible solutions and also to measure the orientation of cleavage planes, grain boundaries and dihedral angles.

CPO of olivine and enstatite in the metaperidotites was obtained by automatic mapping of the whole thin section (35 mm × 20 mm) using a regular grid step of 25–30 μm in a JEOL JSM 5600 scanning electron microscope with an attached Nordlys HKL-Oxford’s camera at Géosciences Montpellier (CNRS-Université de Montpellier 2, France).

Piston‑cylinder experiments

Two small prisms of sample Al06-44 (ca. 1.5 × 1.5 × 3 mm3) were prepared with the foliation plane oriented perpendicular to the length of the prism and one face perpendicular to the foliation and parallel to the line-ation. One long edge of the prism was bevelled to mark the prism orientation after the experiment. Each prism was placed in an Au capsule with an outer diameter of 2.3 mm. The prism was packed in the capsule with powders of the same sample to avoid mechanical damage and to reduce the possible effect of small deviatoric stresses on the CPO during the run (see below). Experiments were performed in 0.5-in. (12.7 mm) end-loaded piston-cylinder appara-tuses at the Research School of Earth Science (The Aus-tralian National University) using a vessel with a 32-mm-length bore, employing pure NaCl, low-friction assemblies. Experiments were run during 168 h (7 days) at conditions beyond the thermal stability field of the antigorite (750 °C and 2 GPa, run C4436) and beyond the thermal stability of chlorite and amphibole in the garnet stability field (1000 °C and 3 GPa, run C4445, Fumagalli and Poli 2005). Escape

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of liquid with some bubbling was noted during pierc-ing of each capsule after the experiment, indicating fluid saturation.

Capsule C4436 was first cut with a low-speed diamond saw perpendicular to the long axis to locate the bevelled edge. Then, two sections perpendicular to the former folia-tion were prepared: one parallel to the former lineation and the other perpendicular to it. The exposed surface after cutting was impregnated with epoxy in a vacuum cham-ber and then mounted in epoxy for final polishing in the standard way for EBSD measurements. The orientation of the former lineation in run C4445 could not be recovered; the polished section was prepared perpendicular to the long axis. High-resolution automatic EBSD mapping of selected areas from the experimental samples was performed using the CamScan X500FE CrystalProbe as described before with a step size of 0.3 and 0.5 μm. In order to prevent charging on the epoxy mount during the EBSD analyses, the surface around the gold capsule was coated with a car-bon powder suspension.

CPO in experimentally dehydrated serpentinite

The original fabric of the serpentinite is dominated by an apparent shape-preferred orientation (SPO) of antigorite blades with high aspect ratios (3.2 ± 1.9, values obtained using the grain recognition algorithm from Channel 5). This SPO is accompanied by a strong CPO, with a con-centration of [001] normal to the foliation and girdle dis-tributions of (100) and (010) in the foliation plane, with maxima parallel and normal to the lineation, respectively (Fig. 3a). The average equivalent diameter (defined as 2 * √(area/π), where area is the measured grain section area) is 3.0 ± 2.4 μm. Locally, the long axes of some antig-orite blades depart from the general orientation of the folia-tion by up to 40°–50° (Fig. 3a).

Remnants of this fabric can still be recognized after the complete antigorite breakdown at 750 °C and 2.0 GPa (run C4436, Fig. 3b). Olivine, enstatite and chlorite crys-tals are elongated subparallel to the former foliation (aspect ratios range from 1.8 to 2.4). These reaction products have, however, smaller grain sizes (2.6 ± 1.1 and 2.0 ± 0.7 μm equivalent diameter for olivine and enstatite, respectively). Olivine tends to form aggregates of subautomorphic crys-tals aligned in the former foliation, whereas chlorite and enstatite grow as aggregates of acicular crystals. Locally, these acicular aggregates are at high angle to the former foliation in the same fashion as previously described for antigorite blades. Previously, fluid-filled porosity is con-centrated in the enstatite–chlorite domains. All product phases show a relatively strong CPO (Fig. 3b). In order to test the effect of deviatoric stress during piston-cylinder

experiments, the olivine CPO from the external part of the capsule filled with the randomly oriented antigorite pow-der was also measured. This olivine CPO can be taken as a reference for the fabric strength: It is close to a ran-dom distribution (Fig. 3b, top, pfJ = 1.04; pfJ is the pole figure strength index J). For olivine, [100] displays the strongest concentration (pfJ = 1.25), forming a point-like maxima normal to the former foliation and therefore paral-lel to the former [001]atg. [001]ol has a girdle distribution within the previous foliation plane. [010]ol has a higher dispersion (pfJ = 1.04), but forms weak maxima parallel to the lineation. The measurement of the CPO of enstatite was partially biased, since crystals elongated parallel to the thin section surface were better indexed. To circum-vent this issue, the data for opx plotted in Fig. 3b are the sum of the data obtained on two perpendicular sections (the section XZ and the section YZ rotated by 90° around Z). Enstatite fabric partially mimics the one observed for olivine although [100]opx shows a higher dispersion within the XZ plane reflecting the dispersion of the orientation of the acicular crystals elongation (Fig. 3b). The [001]opx axes are also concentrated in the former foliation plane, but they have a bimodal distribution with a maximum parallel and another perpendicular to X. This bimodality is likely a measurement bias related to the preferential indexation of crystals elongated parallel to the analysed surface. Chlo-rite displays a CPO very similar to the precursor antigor-ite CPO, although its [100]chl and [010]chl axes form more homogeneous girdles along the former foliation plane.

The higher-temperature run C4445 (1000 °C and 3.0 GPa) produced two compositional domains probably as the result of an important fluid circulation and/or fluid chemi-cal stratification inside the capsule due to the high amount of water released (ca. 12 wt%). This is a common feature of fluid-rich experiments at relatively high temperature as previously described in detail by Stalder and Ulmer (2001) that can be overcome if the press is mounted in a turning device (Schmidt and Ulmer 2004; Melekhova et al. 2006). In our experiments, the centre of the capsule corresponds to a very coarse-grained (58.0 ± 33 μm) olivine-rich domain with high porosity. Quench textures were locally observed at the contact with the gold capsule suggesting the pres-ence of small amounts of melt. The second domain (shown in Fig. 3c) is composed by olivine and enstatite in a ratio of 6:4. It has a finer and more homogeneous grain size (olivine = 9.9 ± 4.6 μm and enstatite = 8.1 ± 3.0 μm). Apparent dihedral angles close to 120° are common, and porosity is homogeneously distributed. Both olivine and enstatite crystals show lower aspect ratios (1.6 ± 0.5 and 1.8 ± 0.6 μm) than run C4436. Although it is not possible to recognize any evidence of the former antigorite foliation, olivine and enstatite show a clear CPO in both domains of the capsule, similar to the one previously described for

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dehydration at 750 °C and 2.0 GPa. This CPO is charac-terized by a concentration of [100] axes normal to the pre-existing foliation and of [001] axes in the foliation plane, with a weak maximum normal to the former lineation. The distribution of [010] is close to random for olivine and enstatite (Fig. 3c).

Textural evolution during antigorite dehydration in Cerro del Almirez

The sampling across the dehydration front in Cerro del Almirez enabled a detailed description of the textural

evolution during the antigorite dehydration at high pressure in a natural environment. Serpentinite above the dehydra-tion front is characterized by a strong antigorite SPO and CPO (cf. Padrón-Navarta et al. 2012); this antigorite folia-tion encloses magnetite aggregates and is crosscut by oli-vine porphyroblasts (Figs. 2, 4a). The first sign of the reac-tion in the transitional lithology is the individualization of domains rich in olivine and domains rich in antigorite–chlorite (and occasionally enstatite) intergrowths (Figs. 2, 4b). As the reaction progresses, the antigorite–chlorite intergrowths are replaced by enstatite–chlorite leading to the development of chl-harzburgites with granofels texture (Fig. 4c, d). Samples located at ≥10 m from the dehydration

Fig. 3 Orientation contrast images (left) and pole figures (right) in experimentally dehydrated serpentinite (piston-cylinder experiments). a Core of the well-foliated serpentinite (sample Al06-44) used as reactant in the experiments. b Serpentinite dehydrated at 750 °C and 2 GPa, i.e. slightly above the antigorite dehydration reaction roughly

corresponding to the PT conditions of the studied natural samples (cf. Fig. 1). c Serpentinite dehydrated at 1000 °C and 3 GPa, i.e. above the thermal stability of antigorite, chlorite and amphibole. All sam-ples were cut in the structural frame except for c where only the Z direction could be recovered

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front (Fig. 4e, f) show significant grain coarsening but sim-ilar textures. It is important to note that despite the coarsen-ing of olivine, enstatite and chlorite, the key textural and crystallographic relationships between products phases are developed at the early stages of dehydration. In the follow-ing, we describe these textural relationships.

Chlorite–antigorite

Millimetre scale intergrowths of chlorite and antigorite replace the antigorite foliation at the dehydration front. Fig-ures 4b and 5a, b show these intergrowths where chlorite occurs in the cores and antigorite at the rims in apparent crystallographic continuity. The crystallographic orienta-tion of the aggregate shown in Fig. 5a was mapped, and the results are plotted in pole figures showing near-parallel [001] axes. EBSD data alone cannot be used to constrain the parallelism between the other axes because of the hex-agonal pseudosymmetry around [001] observed during the

indexation of the Kikuchi patterns. Antigorite at the rims of the intergrowth in Fig. 5a shows two sets of characteristic cleavages. The angle between the cleavage planes meas-ured by U-stage is ca. 19°. This matches the expected angle between (101) and (10-1) cleavage in antigorite (18.96°, using lattice parameters for antigorite m = 17, Capitani and Mellini 2004), when looking close to the [010]-axis, which is the case as seen from the pole figure (Fig. 5a). This cleavage might correspond to the crystal forms {h0l} listed by Tröger (1979) (see also Fig. 9 in Amiguet et al. 2014). These observations would imply that the [010] axis of this grain is parallel to the Y structural direction (Fig. 5a) as indicated by the cleavage measurements. The orientation of [100] and [010] axes in the associated chlorite, however, cannot be determined. Sections with the cleavage planes of chl–atg aggregates roughly parallel to the XZ plane are not uncommon (Fig. 5b, grain 1) and can be used to investi-gate the relationship between the optical axes of chlorite and antigorite in the aggregates using the U-stage. We

Fig. 4 Thin sections scans and enlargements of the studied samples. a Antigorite serpentinite, b, c antigorite–chlorite–enstatite–olivine rock, d–f chlorite–enstatite–olivine granofels. All samples were cut in the structural frame (X horizontal and Z vertical) except for e and f

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recall that chlorite (clinochlore) with less than 6 % Cr2O3 (Tröger 1979; Deer et al. 1992), which is the case here, is optically positive with a||∼ α, b||β, c

∗|| ∼ γ, where a, b, c∗

are orthogonal crystal axes, α, β and γ are least, intermedi-ate and greatest orthogonal refractive indices, respectively, and optical axial plane (OAP) is normal to b. The situation for antigorite is more complex as Tröger (1979) reports a||γ , b||β, c

∗|| ∼ α and OAP is normal to b, whereas Deer

et al. (1992) have a||β, b||γ , c∗|| ∼ α and OAP is normal

to a, which are rotated 90° about c∗ with respect to those reported Tröger (1979). The optical direction γ must be in the basal plane for Tröger (1979) and Deer et al. (1992) conventions but being parallel to a and b, respectively. The optically investigated antigorite section (Fig. 5b, grain 1) is in contact with a chlorite grain. The orientation of this con-tact is compatible with being the (100) plane for antigorite and chlorite; the pole of the contact plane (dashed lines in Fig. 5b) coincides with one of the possible orientations for

Fig. 5 Local crystallographic relationships between antigorite, chlo-rite and olivine in sample Al10-09. Crossed polarizers mode with an inserted compensator. Sample structural frame has X horizontal and Z vertical. a Chlorite–antigorite aggregate and corresponding pole figures corresponding to an EBSD map of that aggregate (changes of Euler colour coding indicates change in orientation). The inferred ori-entation corresponds to the encircled points (see text for details). b The cleavage of the aggregate in grain 1 is approximately contained

in the ZX plane, whereas in grain 2 is contained close to the YX plane (solid line in pole figures). For grain 1, the optical axes (α, β and γ) are indicated in the pole figures by position of the Greek letters. The dashed black lines indicate the trace of the (100) plane, and solid black lines indicate the trace of the (001) plane. Note the occurrence of olivine intergrowth with grain 2. Numbers adjacent to each mineral are the number of analyses plotted (where k means thousands)

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the poles for (100)atg,chl. The β direction in this antigorite is perpendicular to the contact plane, whereas this direction corresponds to α for the associated chlorite. Interestingly, this observation would be in agreement with the antigorite optical properties reported by Deer et al. (1992), rather than those reported by Tröger (1979, see “Appendix”).

Olivine–antigorite

The growth of prograde olivine is closely associated with antigorite and chl–atg aggregates. Grain 2 in Fig. 5b shows, at its borders, an intergrowth of olivine (ol1) and antigor-ite sharing a parallelism between [001]atg,chl and [010]ol. It is possible that a full topotactic relationship exists between these two crystals with [001]ol||[010]atg and [100]ol||[100]atg (the second topotactic relation in Boudier et al. 2010 deduced by TEM observations, referred to as TR2 in the following) although it cannot be unequivocally con-strained by the EBSD data alone. It is interesting to note

that another olivine grain (ol2) spatially related to the anti-gorite–chlorite grain 2 and to ol1 is rotated ca. 30° around their common [010] axis (Fig. 5b).

Figure 6 shows that in the sites where olivine grows, antigorite blades are arranged in mutually perpendicular sets oriented at high angles (ca. 45°) to the former antigorite foliation. Figure 6a shows two orthogonal sets of antigorite blades related to one olivine single crystal. Olivine shows a topotactic relation with only one of these sets (the ones with blue interference colours in Fig. 5a; and set 2 on pole fig-ures, with [001]atg||[100]ol being well correlated in orienta-tion). However, the correlation of [010]atg||[001]ol necessary for first topotactic relation (TR1) in Boudier et al. (2010) in the pole figures is rather scattered for [010]atg. An alterative interpretation would be the correlation of [010]atg||[010]ol, where both axes are tightly grouped and better correlated. Figure 6b shows a more complicated situation where four orthogonal sets of antigorite (oriented N–S/E–W and NE–SW/NW–SE) are intergrown with three olivine crystals.

Fig. 6 Local crystallographic relationships between antigorite and olivine in sample Al10-09. Crossed polarizers mode with an inserted compensator. a Single olivine crystal intergrowth with three sets of antigorite orientations. The cleavage plane for each set is shown as a dashed line in pole figures. Note the similarities in the orientation

of the [100]ol and [001]atg from set 2 (encircled). b Several sets of antigorite orientations intergrowth with three olivine crystals. Note that some relations can be inferred for ol2 and ol3 and sets 1 and 3, respectively, but not for ol1

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In this case, topotactic relationships are difficult to estab-lish unambiguously. The orientation of one olivine crystal (ol2) seems to be controlled by the orientation of antigorite set 1 via the relation TR1 with [001]atg|| [100]ol being well correlated and [010]atg||[001]ol being more dispersed. The orientation of olivine (ol3) is apparently controlled by the orientation of set 3 through the topotactic relation TR2 with [001]atg|| [010]ol being correlated, although the orientations of [001]atg are separated into two closely related groups, which may be due to post-transformation deformation (Fig. 6b). However, the orientation of the biggest olivine crystal (ol1) does not show any particular relation to any of the antigorite sets. The crosscutting relations between the two orthogonal antigorite sets (Fig. 6b) suggest that all of them were formed simultaneously.

Oriented and decussated talc aggregates commonly occur in the ol–atg domains. Occasionally, enstatite can be observed at the core of these talc aggregates, suggest-ing that talc is a retrograde phase after enstatite (see also

Trommsdorff et al. 1998). When preserved, the enstatite [100] axis is normal to the chlorite (001) cleavage. This crystallographic relation is further explored in the next section.

Chlorite–enstatite–olivine

The two most outstanding textural features of the granofels chl-harzburgites are the recurrent arrangement of elongated single crystals of chlorite and enstatite and the development of triangular shapes as the result from their intersections (Fig. 7a, c, d). Enstatite devel-ops prismatic or tabular shapes with the shorter axis corresponding to the [100] direction. One or most com-monly two {100} faces of enstatite are in direct contact with two single crystals of chlorite of equivalent size (Fig. 7c). Invariably, these contacts are parallel to {001} in chlorite. The parallelism between axes in enstatite and chlorite is [001]opx || [100]chl and [010]opx || [010]chl.

Fig. 7 Local crystallographic relationships between chlorite, enstatite and olivine in the granofels. Crossed polarizers mode with an inserted compensator. a, b Sample Al10-06, c, d Sample Al09-16. The orientation of the low-index interface is indicated with a continu-ous line in the pole figures. The intersection of these interfaces (red

stars in the pole figure with nets in a, c and d) defines the orienta-tion of the long axis of the triangular prism formed by enstatite and chlorite aggregates. The approximate angles between the interfaces are indicated

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Although olivine generally shows xenomorphic shapes, some low-index faces in olivine (commonly {010}) have been observed in contact with enstatite (e.g. Fig. 7a) or with chlorite (Fig. 7b, c). However, except for a paral-lelism between the [010] axis of olivine and the shortest axis of the chlorite–enstatite aggregates, olivine does not show a clear crystallographic relationship with the other phases.

The interior of the triangular domains developed by the intersection of three enstatite (Fig. 7a) or chlorite–enstatite aggregates is occupied either by olivine (Fig. 7a, c) or by randomly oriented aggregates of chlorite (Fig. 7d). Using the U-stage, it is possible to infer the angles of these triangular domains that range between 45° and 65° (Fig. 7a, c, d).

Crystallographic preferred orientation at the thin section scale

In previous sections, we described the occurrence of recur-rent crystallographic relationships between the differ-ent prograde minerals. Although these relationships are widespread, it remains to be proven that they lead to a strong CPO at a larger scale as in the case of the dehydra-tion experiments (cf. Fig. 3). EBSD mapping at the scale of the thin section was conducted on the various reaction products to explore this issue. In this analysis, we use as a reference the orientation of the serpentine sample Al10-08 (Fig. 8a) outcropping 5 cm above the dehydration front (Fig. 2), which has a well-developed foliation, marked by

3.32

.13

310k

1.54

3.98

.08 1.63

7.88

.112.88

2.18

.10

29k

1.23

2.39

.271.21

3.29

.311.38

2.65

.16

42k

1.29

2.00

.201.14

3.24

.081.65

2.54

.40

55k

1.18

2.90

.201.33

1.92

.171.15

3.89

.25

233k

1.30

2.59

.241.21

3.39

.071.40

5.07

.051.79

5.02

.191.52

6.06

.04

18k

1.74

3.07

.20

176 k

1.27

.5

2.46

.191.21

2.31

.181.21

2.96

.27

32k

1.24

3.77

.201.33

3.25

.191.33

5.38

.09

168k

1.74

4.02

.091.53

5.83

.111.49

2.54

.22

80k

1.22

2.48

.321.16

2.62

.181.25

Al1

0-08

Atg Atg

Ol Opx

ChlOl

Al1

0-09

Al1

0-10

Al1

0-11

Al1

0-06

(100) (010) [001]Z

X

Z

X

Z

X

Z

X

Z

X’

Y’

(100) (010) [001]

(100) (010) [001]

[100] [010] [001]

[100] [010] [001]

[100] [010] [001]

Ol

Ol

Opx

Opx

[100] [010] [001][100] [010] [001]

[100] [010] [001][100] [010] [001]

(a)

(b)

(c)

(d)

(e)

Fig. 8 Crystallographic preferred orientation (CPO) at the scale of the thin section (cf. Fig. 4). All pole figures are displayed in the same reference frame: The foliation (XY plane) is oriented E–W vertical and the lineation (X direction) is E–W horizontal except for sample

Al10-06 (e) where only Z is constrained. Pole figures are represented using all measurements. Contours at 0.5 multiple intervals of an uni-form distribution (m.u.d.). N: number of grains measured; pfJ: fabric strength for each crystallographic axis

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the alignment of enstatite, and a lineation, marked by elon-gated magnetite aggregates.

In the atg–chl–opx–ol rocks below the dehydration front, development of different sets of antigorite oblique to the pre-existing foliation observed during the first stages of the reaction (Figs. 5b, 6a, b) results in a much weaker antigorite CPO (Fig. 8b). The orientation of [001] normal to the foliation plane observed in the reference serpentinite (pfJ[001] = 2.88) is still present, but the concentration is much weaker (pfJ[001] = 1.38) in the partially dehydrated Al10-09 sample. Chlorite mimics the antigorite orientation; it shows therefore the same dispersion (Fig. 8b). Moreover, the [001]chl,atg maximum is displaced towards the structural direction Y.

Olivine and enstatite CPO in atg–chl–opx–ol rocks are always weak. The strongest dispersion is observed in Al10-11, which has been collected farther from the dehy-dration front. However, some inferences can be made based on the two oriented samples (Al10-10 and Al10-11, Fig. 8c, d). The [010]ol and the [100]opx maxima in these two samples are roughly normal to the former foliation, that is, subparallel to [001]atg, in agreement with local observations of topotaxy between these phases (Figs. 5b, 6). In addition, the weak [001]opx maximum in Al10-10 and Al10-11 is roughly parallel the lineation in the pre-cursor serpentinite.

In the coarse-grained sample Al10-06 (geographically not oriented sample, but showing a rough macroscopic banding of olivine-rich domains, cf. Fig. 2), the [010]ol maximum is normal to the banding, suggesting that the banding might be parallel to the former antigorite foliation. The enstatite CPO in this sample is much weaker, close to random.

Deformation‑related microstructures

The antigorite dehydration front in Cerro del Almirez cross-cuts the foliation in antigorite (Fig. 2), which is preserved to some extent in the metaperidotite as a banding resulting from olivine-rich domains and chlorite- and enstatite-rich domains. This suggests that the reaction took place under static conditions (no large-scale tectonic stresses). How-ever, microstructures related to plastic or brittle deforma-tion are observed locally. At the dehydration front, open folds locally deflect the former serpentinite foliation (e.g. Al10-11, Fig. 9a). Prograde olivine crystals do not show intracrystalline deformation features, such as undulose extinction or subgrain boundaries. However, enstatite and chlorite occasionally show buckling of the (100) and (001) planes, respectively (Fig. 9b, c). The associated distortion of the lattice can reach up to 16° (Fig. 9c). In enstatite, larger distortions result in fractures normal to (100) where

it is replaced by talc. Chlorite with bended cleavage sur-rounds the distorted and fractured enstatite.

In some cases, buckling of enstatite is associated with formation of thin lamellae (up to 50 μm in width) of cli-noenstatite. These lamellae are best expressed at the edges of large (>1 cm in length) elongated enstatite crystals (sam-ple Al09-16, Figs. 2, 9d, e). EBSD measurements reveals that these lamellae correspond to low-temperature clinoen-statite with a space group P21/C. Clinoenstatite lamellae are also relatively abundant in medium-grained samples close from the dehydration front, where they crosscut the entire crystals (23 out of 306 crystals in sample Al10-10 and 113 out of 487 in Al10-11).

Discussion

Topotaxial relationships between antigorite and olivine

Two topotaxial relations between antigorite and olivine (TR1 and TR2, Fig. 11a) have been proposed by Boudier et al. (2010) based on a detailed TEM study on local atg–ol contacts in a partially hydrated mantle xenolith from the Moses Rock dike (Colorado Plateau). Based on EBSD analysis, Morales et al. (2013) showed that the antigorite fabric at the scale of the thin section in these samples could be partially explained by a combination of these two topo-tactic relations with a predominance of TR1.

Our experimental and natural observations suggest that similar topotaxial relations control the growth of olivine at the expenses of antigorite. Among these observations, we may highlight the marked difference in CPO between the experimentally prograde minerals developed after the foli-ated serpentinite and the ones developed after the serpent-inite powder (reference texture, Fig. 3b). The olivine CPO in the experimentally dehydrated samples is stronger than in the natural samples, in particular for the concentration of [100]ol normal to the former foliation (Figs. 3, 10b). This CPO suggests predominance of the TR1 topotaxial relation during dehydration. The other two axes for olivine are dis-tributed in a girdle along the former foliation plane despite the relatively point-like maxima for [100] and [010] in the reactant serpentinite. In nature (Fig. 8c, d), despite the weak olivine CPO in the chl-harzburgites, the orientation of [010]ol normal to the pre-existing serpentinite foliation sug-gests that the topotaxial relation TR2 likely controls the oli-vine growth, giving rise to an olivine CPO that is frequently described in the literature as B-type fabric (Jung and Karato 2001). A dominant TR2 topotactic relation between olivine and antigorite may also explain the CPO of olivine in equilibrium with talc–chlorite–tremolite produced by the antigorite dehydration at low pressure (<0.7 GPa and 650 °C), which was reported by Nagaya et al. (2014a)

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Fig. 9 Macroscopic and microscopic textural features potentially related to compaction and fluid-filled porosity migration (see text for “Discussion”). a Macroscopic deflection of the former foliation plane in a chl–opx–ol rock close to the dehydration front (sample Al10-11). b Buckling and fracturing of elongated enstatite crystals. Numbers indicate the angles between the traces of (100) planes relative to left end of the crystal. Note that chlorite accommodates this deformation by a continuous buckling of the cleavage planes and in some cases by extension normal to it (filled by talc and magnetite). White arrows

indicate domains where talc replaces enstatite along presumably pre-vious fractures. Sample Al09-16 (c) Relative misorientation profiles for two bended enstatite crystals. Sample Al10-06 (d) Large enstatite showing low clinoenstatite lamellae at one end (enlarged in e). The angle between the (100) traces from the central part of the crystal and the traces of the lamellae planes is indicated. e Detailed EBSD map showing the alternation between orthoenstatite Oen (Pbca) and low clinoenstatite LCen (P21/c)

c*atg

aatgbatg

c*chl

achlbchl

aopx

copxbopx(100)opx

(001)atg,chl

c

(100

) chl

(100

) atg

opx

ChlAtg

Natural observations(compacted)

Dehydration experiments(uncompacted)

(001)atgAtg

c*atg

aatgbatg

Prot

olith

aol

aopx

c*chl

opx

ol

Chl

Prod

uct p

hase

s

(b) (c)

(001)atgAtg

c*atg

aatgbatg

aol

bolcol

bol

aolcol

TR1 TR2

Boudier et al., 2010(a)

Fig. 10 Summary of topotaxial relationships between antigorite, chlorite, enstatite and olivine derived by a Boudier et al. (2010), b based on dehydration experiments (this study, Fig. 3) and c based on natural observations (this study, Figs. 5, 6, 7)

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(but see concerns by Nozaka 2014 and further discussion by Nagaya et al. 2014b). Soda and Wenk (2014) investi-gated local crystallographic relationship between antigor-ite and metamorphic olivine porphyroblast in serpentinite from Southwest Japan. Over four investigated relations, the TR2 was found in two cases (the two remaining topotac-tic relationships not being previously described). This is in further agreement with our observations, indicating that several relationships might exist at the thin section scale. This eventually weakens the fabric even in the absence of deformation.

Topotactic relationships between antigorite and chlorite and enstatite

Experimental (Fig. 3) and natural observations at the local scale (Figs. 5, 7) indicate the following orientation relation-ships between antigorite, chlorite and enstatite (Fig. 10b, c):

These relations are in agreement with the CPO of these minerals at the thin section scale (Fig. 8a–d) although also in this case the CPOs in the natural samples are weaker than in the experiments. Grobéty (2003) observed equiva-lent relationships during the replacement of antigorite by chlorite although the contacts between the crystals were highly irregular. The observed relationship between chlo-rite and enstatite is also in agreement with the relations between phyllosilicates and pyroxenes inferred by Padrón-Navarta et al. (2008).

In antigorite–chlorite aggregates (e.g. Fig. 5a, b), con-tact interfaces parallel to (001) are less common than nor-mal to it. This is in contrast to the common occurrence of (001) interstratifications between amesite (planar serpen-tine) and chlorite (c.f. Banfield et al. 1994; Banfield and Bailey 1996). We interpret this as the consequence of the modulated structure of the antigorite that results in the periodic inversion of the tetrahedral layers making the cou-pling along (001) planes energetically unfavourable. Cou-pling along (100) interfaces (cf. Figs. 5a, b, 10c) seems to be more stable, due to the minor lattice misfit along the b (0.2 % at ambient pressure) and along the c∗ axes (the mis-fit between 2c

∗

Atg and c∗

Chl is 1.8 %) between antigorite and chlorite.

Compaction and topotaxy

A reconstructive topotactic reaction (Figlarz et al. 1990; Mainprice et al. 1990), where the product phases use the crystal lattice of the precursor phase (antigorite) as sub-strate for their oriented growth, would lead to a strong CPO of chlorite-bearing product assemblage. Dehydration

(1)aatg,chl

∣

∣

∣

∣

copx, batg,chl

∣

∣

∣

∣

bopx, c ∗atg,chl ||aopx

experiments show, however, that this is not the case as only one direction, namely [100]ol,opx and [001]chl, is inherited from the precursor phase. This can be accounted for by the fact that a significant fluid-filled porosity is produced during the reaction (ca. 20 vol%, cf. Padrón-Navarta et al. 2010b). We hypothesize that during the dissolution–precip-itation process the fluid-filled porosity disturbed oriented growth on the foliation plane, but not normal to it.

Natural samples represent the final result of this process, which includes not only the reaction, but also the compac-tion of this fluid-filled porosity and grain coarsening. This local reinforcement of the opx–chl topotaxy during com-paction is associated with weakening of the CPOs at the thin section scale (Fig. 8c–e). We relate this CPO weaken-ing to the compaction in response to the migration of the fluids. However, compaction does not fully erase the crys-tallographic inheritance from the protolith. This is agree-ment with dehydration experiments in gypsum where a strong CPO of the reaction product (bassanite) was inher-ited from the orientation of the original gypsum c-axis ori-entation (Hildyard et al. 2011).

The time scale for compaction during antigorite dehy-dration is difficult to constrain. Numerical simulations sug-gest that the draining times of a dehydration front by com-paction in crustal lithologies (metapelite) are in the order of 10–100 ky (Connolly 1997, 2010). These time scales allow for grain growth to play a significant role during compaction. The reinforcement of local topotactic relations between chlorite and enstatite and their arrangement in tri-angular prisms with relatively constant interfacial angles is, for instance, an indication of minimization of interfa-cial energies during growth. Different growth rates might also explain the apparent modification of the topotaxial relations controlling olivine orientation (from TR1 in the experiments to TR2 in nature). If so, the predominance of TR2 in the natural samples may suggest that development of olivine (010) faces in contact with (001) planes of chlo-rite is energetically more favourable than growth of (100) faces, being favoured during equilibrium grain growth and compaction.

Origin of the clinoenstatite lamellae

Local overpressure may form in porous materials during dehydration reactions when the fluid pressure is not equal to the lithostatic pressure (e.g. Wheeler 1987; Llana-Fúnez et al. 2012). In addition, compaction generates strain incompat-ibilities at the grain scale and, hence, local deviatoric stresses. Coeval grain growth will tend to relax these heterogeneous stresses, but a perfect balance between these two processes (compaction and mineral growth) at the grain scale is rather unlikely (Wheeler 1987). Brittle and plastic deformation features (fracturing, buckling and possibly shear-induced

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clinoenstatite inversion) observed in enstatite and chlorite (Fig. 9) might be related to the accommodation of these com-paction-related strain incompatibilities. Alternatively, these features might have formed in response to fluid overpressure in nearby (at the metre scale) domains, as in the model sug-gested by Padrón-Navarta et al. (2011). If this were the case, we would expect, however, a heterogeneous (metre scale) dis-tribution of deformation features. Such deformation gradients have not been observed in Cerro del Almirez, but more work is needed to fully discard this hypothesis.

The inversion of orthoenstatite to low clinoenstatite has been well documented experimentally (see Coe and Kirby 1975 and reference therein). The transformation involves macroscopic shearing parallel to [001] on (100) leading to a particular crystallographic relationship between the host orthoenstatite and the clinoenstatite lamellae. It is dif-fusionless and also called martensitic transformation (e.g. Mainprice et al. 1990). The occurrence of clinoenstatite lamellae in nature is nevertheless strikingly rare, a fact that has been used to argue for a lack of true clinoenstatite hydrostatic stability field (Raleigh et al. 1971). Natural occurrence of clinoenstatite (closely associated with con-jugated kinks in highly strained bronzite crystal) was first described by Trommsdorff and Wenk (1968) in a major mylonite zone in central Australia. The microstructural fea-tures investigated by Trommsdorff and Wenk (1968) clearly differ from the ones reported here where low clinoenstatite inversion is relatively widespread and does not involve kinking of enstatite. A close similarity exists, however, between the clinoenstatite lamellae microstructure in Cerro del Almirez and the ones described by Frost et al. (1978, their Fig. 2) in a metaserpentinite (olivine–enstatite–antho-phyllite–tremolite–chlorite) from the Mount Stuart Batho-lith contact aureole in the Central Cascades. These authors proposed that the clinoenstatite lamellae were produced by deformation in a shear zone that post-dated the emplace-ment of the batholith. The preservation of the undisturbed dehydration front in Cerro del Almirez (Fig. 2) indicates that the clinoenstatite transformation studied here did not result from large-scale tectonic processes. We hypothesize therefore that the inversion to low clinoenstatite in Cerro del Almirez was coeval to porosity compaction after anti-gorite dehydration. It seems likely that the inversion to low clinoenstatite is related to the flexural deformation of elon-gated enstatite crystals (cf. Fig. 9d, e).

The coexistence of plastic deformation within enstatite crystals (gradual crystal misorientation, accommodated by a rotation around [010], Fig. 9c) and of clinoenstatite inver-sion (9d, e) in Cerro del Almirez can be used as a geopi-ezometer to constrain a lower bound for the local shear stresses and strain rates that can be developed during com-paction. Raleigh et al. (1971) experimentally demonstrated that dislocation glide (and polygonization) in enstatite and

clinoenstatite transformation (by shearing half unit cell in enstatite) produces very contrasting creep rates. Disloca-tion glide results in:

and for the clinoenstatite transformation

where ε̇ is the strain rate in s−1, σ = σ1 − σ3 is the dif-ferential shear stress in kbar, R is the gas constant and T is temperature in K. Because neither diffusion nor translation is required (Coe 1970; Coe and Kirby 1975), the clinoen-statite transformation has significantly lower activation energy than dislocation glide. Formation of clinoenstatite lamellae is therefore favoured under relative low-temper-ature conditions and/or high strain rates compared with deformation by slip. The transition between these two rate competing processes defines a plane in the − log ε̇ − σ − T space. This plane can be found by solving Eqs. 2 and 3 for ε̇ and σ (Fig. 11a, b),

The temperature of the antigorite dehydration in Cerro del Almirez is well constrained (630–680 °C, López Sánchez-Vizcaíno et al. 2005; Padrón-Navarta et al. 2010a, 2013). The coexistence of the two deformation processes in enstatite constrains therefore the local (at grain scale) strain rates and/or shear stress to 10−12 to 10−13 s−1 and 60–70 MPa, respectively (arrows in Fig. 11a, b). Although the power law form of Eq. 3 (that assumes diffusion-con-trolled creep) correctly reproduces the experimental data of Raleigh et al. (1971) at 1100 °C, an exponential fitting (i.e. assuming a glide-controlled dislocation mechanism) could be also equally appropriate as originally noted by the same authors. This would indicate that at high stress levels, the stress dependence of the creep rate of clinoen-statite is greater than the predicted by the power law (i.e. the power law breaks down, e.g. Tsenn and Carter 1987). Indeed, recent single-crystal rheological data obtained by Ohuchi et al. (2011) in the clinoenstatite field support this view. The use of a power or exponential law form for the low-temperature deformation regime has, however, lit-tle relevance when calculating the transition between the two mechanisms (Tsenn and Carter 1987). The transition in clinopyroxenite (Kirby and Kronenberg 1984) is a clear example of that, where a very high-n-value (83) is used to reproduce a mathematically equivalent exponential law function. This is no longer true when the n-value is only marginally higher than what it is assumed to correspond

(2)ε̇ = 104.2σ 2.4 exp

(

−294 kJ

RT

)

(3)ε̇ = 10−8.5σ 8.0 exp

(

−42 kJ

RT

)

(4)log ε̇ = 9.78 − 20917(1/T)

(5)log σ = 229 − 2342(1/T)

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to diffusion-controlled creep (from 3 to 5, Tsenn and Carter 1987) as in the case of Raleigh et al. (1971) where n = 8.0 ± 0.3. In this case, extrapolation to natural con-ditions of low stress will highly depend on the functional exponential form used to describe the experimental data corresponding to Eq. 3. A full analysis of the functional

form is, however, prevented given the limited experimen-tal data provided by Raleigh et al. (1971). Therefore, the above-proposed values for the strain rate and shear stress of compaction in Cerro del Almirez could be over estimated. This reinforces the conclusion that fluid expulsion is sig-nificantly slow and close to the deformation of the solid matrix if the system is not perturbed by external forces (Padrón-Navarta et al. 2011).

The thermodynamic approach from Coe (1970) to esti-mating the effect of shear stress on the temperature of the LCen–Oen transition may also be used to have an inde-pendent constrain of the shear stresses associated with the formation of clinoenstatite in Cerro del Almirez. Based on the geometry of the transformation and assuming a negligi-ble volume change, the variation in temperature due to the resolved shear stress (τr) on (100) parallel to [001] is given by

where v is the molar volume of Oen/LCen (31.3 cm3 mol−1 or 3.13 J bar−1), ϕ is the shear transformation angle (13.3° after Coe and Kirby 1975) and Δs = sOEn − sLCen is the entropy molar change in the transformation (2.93 J/mol K, Coe and Kirby 1975). Coe and Kirby (1975) proposed a lower bound for dT/dσ of around 3.00 K per MPa of shear stress on (100) planes in the [001] direction. Although as pointed out by Raleigh et al. (1971) dT/dr is not linearly independent of the temperature, the theoretical value is in fairly good agreement (3.47 K/MPa) with the dT/dσ (Fig, 11b) constrained by experimental deformation of pyroxene aggregates between 500° and 700°. This relation provides another lower bound estimation for the local shear stress magnitude in the natural samples, provided that the differ-ence in the transformation temperature (T) and the equi-librium (hydrostatic) temperature (T0) of the LCen/OEn (ΔT) is known. The equilibrium hydrostatic phase bound-ary between LCen and OEn has been experimentally con-strained by several workers (see Ulmer and Stalder 2001 for a comprehensive review). The LCen/Oen phase bound-ary can be linearly expressed as P = a + bT0 where pres-sure is in GPa and temperature in K. The Fe–Mg exchange seems to have a minor effect on the location of the equi-librium boundary resulting in a good agreement between the different experimental results. The intercept parameter a in GPa is −18,73, −24,71, −34,00 and −24,07, whereas the slope b in GPa K−1 is 0.022, 0.030, 0.038 and 0.028 for Grover (1972), Sclar et al. (1964), Boyd and England (1965) and Ulmer and Stalder (2001), respectively. All experimental results except those from Boyd and England (1965) indicate that the stable phase after antigorite dehy-dration in Cerro del Almirez at 660–680 °C and 1.6–1.9 GPa is the orthorhombic phase. Estimated ΔT ranges from 20 to 40 K (Grover 1972; Ulmer and Stalder 2001) to

(6)dT/dτr = ν tan ϕ/∆s

shear stress σ (σ1-σ3) MPa

0 40 80 120 160 200 240 280

Tem

pera

ture

(ºC

)

400

500

600

700

800

900

1000

1100

1200

1300

(a)

- log (s-1)

4 6 8 10 12 14 16 18 20

Tem

pera

ture

(ºC

)

400

500

600

700

800

900

1000

1100

1200

1300

Equilibrium boundary (Eq. 4 & 5)

Creep rate by slip and polygonization in Oen (Eq. 2)Creep rate by inversion to LCen (Eq. 3)

(b)

Temperature of antigorite dehydration at 1.6-1.9 GPa (cf. sample Al10-10 & Al10-11)

˙

Slip in enstatite

10-8 s-1

10-10 s-1

10-12 s-1

10-14 s -1

10-16 s -1

10-12 s -1

10-8 s -110

-10 s -1

Transformation to Low clinoenstatite

Slip in enstatite

10 MPa

50 MPa

50 MPa

100 MPa

200 MPa

300 MPa

500 MPa

Transformation to Low clinoenstatiteTransformation to Low clinoenstatite

ε

Fig. 11 a Calculated log (strain rate) (steady state) versus tempera-ture with contours of shear stress and b shear stress versus tempera-ture with contours of strain rate for clinoenstatite transformation (dashed lines) and slip in enstatite (thin solid lines). Experimental parameters are from Raleigh et al. (1971). The equilibrium bound-ary between the two processes is the intersection of the contouring (solid line). Temperature conditions for Cerro del Almirez are indi-cated with the white box, and the arrows suggest possible variations in strain rate and/or shear stress during compaction

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70–80 K (Sclar et al. 1964). This constrains the minimum τr needed for clinoenstatite transformation to be either in the order of 5–20 MPa or of 20–40 MPa, respectively. For comparison, Coe and Kirby (1975) reported τr values of about 100 MPa were more than sufficient to cause inver-sion at 0.5 GPa confining pressure and temperatures up to 850 °C.

Estimated local differential stresses during compac-tion after antigorite dehydration (60–70 MPa using the experimental data of Raleigh et al. 1971 or 5–40 MPa after Coe 1971 and Coe and Kirby 1975) are slightly higher or on the same order as those inferred by Etheridge (1983) (20–40 MPa, where the lower limit corresponds to fluid saturated conditions) for regional deformation and meta-morphism in the crust and from paleopiezometric analysis based on subgrain spacing recrystallized grain sizes in nat-urally deformed peridotites from subduction environments (3–40 MPa, Soustelle et al. 2010). They are also consistent with the stresses predicted by palaeowattmeter from Aus-tin and Evans (2007) for the range of olivine grain sizes observed in naturally deformed peridotites (10 µm to 1 cm), which are 2–100 MPa. The estimated strain rates are also only slightly lower (by 1–2 orders of magnitude) than the tectonic ones. Together, these data suggest that fluid extrac-tion and compaction are very slow in Cerro del Almirez, pointing to a very low permeability of the reaction products (the metamorphic peridotites).

Conclusions

Antigorite and chlorite dehydration reactions have the potential to produce the oriented growth of prograde min-erals in a (transient) fluid-filled porosity network. Experi-mentally reproduced reactions in undrained piston-cylinder apparatus resulted in a strong crystallographic preferred orientation for certain directions: [100] for olivine and enstatite and [001] in chlorite parallel to the reactant [001] in antigorite and the preservation of porosity. Natural observations of antigorite dehydration products suggest that compaction can locally enhance topotaxial relation-ships between prograde minerals while weakening the CPO at a larger scale. We ascribe this effect to the development of local stress incompatibilities during mineral growth and coarsening of the prograde minerals. The resulting CPO is probably the result of the same initial topotaxial relation-ships as observed in the experiments but disturbed by sur-face energy minimization during near equilibrium grain coarsening. This produces the dispersion of chlorite and enstatite CPOs and a possible change in the orientation of [010] axis in olivine. Microstructural features that might be coeval to compaction in the natural samples include: (1) smooth bending of the former foliation, (2) gradual

crystallographic misorientation (up to 16°) of prismatic enstatite due to buckling, (3) localized orthoenstatite(Pbca)/low clinoenstatite (P21/c) inversion and (4) brittle fractur-ing of prismatic enstatite wrapped by plastically deformed chlorite. The coexistence of enstatite buckling and cli-noenstatite lamellae allows to estimate a lower bound for the strain rates and local shear stresses generated during the grain growth and coeval compaction. Estimated values based on experimental creep rates on pyroxene aggregates (Raleigh et al. 1971) result in strain rates in the order of 10−12 to 10−13 s−1 and shear stresses of 60–70 MPa. Lower shear stress values (20–40 MPa) are retrieved using the thermodynamic model of Coe (1971) in combination with the hydrostatic high-pressure experimental data of Grover (1972) and Ulmer and Stalder (2001). These data suggest that, under low deviatoric stress, fluid extraction and com-paction near equilibrium in natural systems are only mar-ginally higher than the strain rate of the solid matrix.

Acknowledgments We gratefully acknowledge constructive reviews by I. Katayama, and an anonymous reviewer and comments by the editor O. Müntener. We are indebted to V. López-Sánchez-Vizcaíno and M.T. Gómez-Pugnaire for continuous support, sampling and dis-cussion on natural samples. C. Nevado and D. Delmas supplied high quality polished thin sections for EBSD measurements. We thank F. Barou for his technical assistance in the EBSD-SEM national facility at Geosciences Montpellier. U-stage measurements would not have been possible without the kind help and patience of F. Boudier. J. Her-mann is also thanked for his insightful hints in the design of the experi-ments at the Australian National University. The research leading to these results has been funded by HISLa-DR, a Marie Curie Action under grant agreement PIOF-GA-2010-273017 from the European Union Seventh Framework Programme (FP7/2007-2013). Grants from the Ministerio de Economía y Competitividad (CGL2012-32067 and CGL2013-42349-P) and Junta de Andalucía (research groups RNM-145 and RNM-131; Proyecto de Excelencia P09-RNM-4495) are also acknowledged. This research has benefited from EU Cohesion Policy funds from the European Regional Development Fund (ERDF) and the European Social Fund (ESF) in support of human resources, innova-tion and research capacities and research infrastructures.

Appendix

It has been considered convenient to include here a brief summary of the optical properties antigorite due to a cer-tain degree of confusion in the existing literature [c.f. Soda and Takagi (2010) corrected in Soda and Wenk (2014)]. There is a general agreement in that the α-direction is closely parallel to c, whereas two possible orientations of the other optical directions have been proposed. This ambi-guity dates from early work on the antigorite structure (Zussman 1954) and its origin might be found in the ori-entation of the parting planes other than cleavage, parallel either to (100) or to (010). Appendix Table 1 and Fig. 12 summarize the optical properties of clinochlore and antigo-rite. The recent work of Soda and Wenk (2014) correlates

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optical and EBSD data, with synchrotron X-ray diffraction in antigorite. This work strongly suggests that the orienta-tion of the β-direction is closely parallel to a (i.e. the OAP corresponds to (100) planes), which is in agreement with our observations.

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β 1.581 1.551–1.603 1.5660 1.563 ca. 1.566

γ 1.586 1.552–1.604 1.5670 1.564 1.562–1.574

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