Dehydration Breakdown of Antigorite and the Formation of B-Type Olivine CPO

Earth and Planetary Science Letters 387 (2014) 67–76

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Earth and Planetary Science Letters

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Dehydration breakdown of antigorite and the formation of B-type olivine CPO

Takayoshi Nagaya a, Simon R. Wallis a, Hiroaki Kobayashi a, Katsuyoshi Michibayashi b, Tomoyuki Mizukami c, Yusuke Seto d, Akira Miyake e, Megumi Matsumoto d a Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan b Institute of Geosciences, Shizuoka University, Shizuoka 422-8529, Japan c Department of Earth Science, Graduate School of Environmental Studies, Kanazawa University, Kanazawa 920-1192, Japan d Department of Earth and Planetary Science, Kobe University 1-1, Rokkoudai, Nada-ku, Kobe 657-8501, Japan e Department of Earth and Planetary Science, Faculty of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto 606-8502, Japan article info abstract

Article history: Peridotite formed by contact metamorphism and dehydration breakdown of an antigorite schist from Received 23 April 2013 the Happo area, central Japan shows a strong olivine crystallographic preferred orientation (Ol CPO). The Received in revised form 4 October 2013 lack of mesoscale deformation structures associated with the intrusion and the lack of microstructural Accepted 12 November 2013 evidence for plastic deformation of neoblastic grains suggest that olivine CPO in this area did not form Available online 4 December 2013 as a result of solid-state deformation. Instead, the good correspondence between the original antigorite Editor: L. Stixrude orientation and the orientation of the newly formed olivine implies the CPO formed by topotactic Keywords: growth of the olivine after antigorite. Ol CPO is likely to develop by a similar process in subduction subduction zones zone environments where foliated serpentinite is dragged down to depths where antigorite is no longer microstructure stable. The Happo Ol CPO has a strong a-axis concentration perpendicular to the lineation and within B-type olivine CPO the foliation—commonly referred to as B-type Ol CPO. Seismic fast directions parallel to the ocean trench antigorite are observed in many convergent margins and are consistent with the presence of B-type Ol CPO in the topotaxy mantle wedge of these regions. Experimental work has shown that B-type CPO can form by dislocation creep under hydrous conditions at relatively high stresses. There are, however, several discrepancies between the characteristics of natural and laboratory samples with B-type Ol CPO. (1) The formation conditions (stress and temperature) of some natural examples with B-type CPO fall outside those predicted by experiments. (2) In deformation experiments, slip in the crystallographic c-axis direction is important but has not been observed in natural examples of B-type CPO. (3) Experimental work suggests the presence of H2O and either high shear stress or relatively low temperatures are essential for the formation of B-type CPO. These conditions are most likely to be achieved close to subduction boundaries, but these regions are also associated with serpentinization, which prevents strong olivine CPO patterns from forming. We show B-type Ol CPO can form as a result of static topotactic growth of olivine after high-temperature breakdown of foliated serpentinite. These results resolve the discrepancies between experimental and natural examples of B-type CPO and show the need to rethink the formation process of olivine CPO in convergent margins. Topotactic growth of olivine after antigorite can account for the inferred distribution of B-type Ol CPO in the mantle wedge more successfully than dislocation creep. © 2013 Elsevier B.V. All rights reserved.

1. Introduction on olivine has revealed strong similarities between the CPO developed by dislocation creep in laboratory samples and natural Rocks commonly show some degree of crystallographic pre- mantle rocks (e.g., Avé Lallemont and Carter, 1970; Zhang and ferred orientation (CPO) of their constituent minerals. This crystal- Karato, 1995; Jung and Karato, 2001a; Katayama and Karato, 2006; lographic alignment causes anisotropy to develop in the physical Skemer et al., 2012). These studies show that one of the dominant properties of the rock including, for instance, heat conduction and inﬂuences on the CPO is the kinematic framework of deformation. the polarization of transverse seismic waves. In many ductilely deformed rocks—and in particular mantle tectonites—the dominant This means that if the CPO of a region of seismically anisotropic cause of CPO is assumed to be the lattice rotations caused by glide mantle is known or can be inferred, then measurements of seismic of dislocations and imposed grain shape changes, which lead to anisotropy can be used to infer the geometry of mantle ﬂow (e.g., stable CPO patterns. In support of this idea, experimental work Kneller et al., 2005).

Fig. 1. Geological sketch map of the Happo-O’ne complex in central Japan. The locations of the two samples as indicated by their mineral assemblages is shown bythe ellipsoidal areas including star symbols. The metamorphic zones are divided into Tremolite (Tr = olivine + tremolite), Diopside (Di = olivine + antigorite + diopside), and Talc (Tlc = olivine + talc + tremolite) zones. Serpentinite mylonites are dominantly exposed in the Diopside zone but are also locally present in the Tremolite zone. Modiﬁed after Nozaka (2005) (metamorphic zone boundary and trend of foliation from Nakamizu et al., 1989 and lithofacies distribution from Nakano et al., 2002).

In this work we document field and microstructural character- XA-8800R) at Nagoya University, at 20 kV accelerating voltage and istics of mantle rock in the Happo region of central Japan and 12 nA specimen current on the Faraday cup. The beam diameter show that the olivine CPO is due to growth in a preferred orienta- was 2–3 μm. Minerals were also identified using a laser Raman tion rather than ductile deformation. This previously unrecognized spectrometer (Thermo Nicolet Almega XR) equipped with a confo- mechanism for CPO development in peridotite is potentially impor- cal microscope (Olympus BX51) at Nagoya University. The 25 mW, tant in subduction zone settings and can account for discrepancies 532 nm Nd–YAG laser produces an irradiation power of 10 mW between the microstructural characteristics of natural examples of on the samples. The Rayleigh scattering was removed using an subduction–related ‘B-type’ olivine CPO and those of laboratory ex- edge filter. The Raman signal was dispersed by a 2400 grooves − periments. mm 1 grating and detected by a Peltier-cooled CCD camera (Andor Technology, 256 × 1024 pixels). The room temperature was kept at ◦ 22 ± 1 C. The spectral resolution is about 1 μm, and a wavenum- 2. Previous studies on olivine CPO in the Happo area − ber accuracy of approximately ±0.3–0.4cm 1 was determined by −1 In the Happo region of the Hida Gaien belt in central Japan checking the position of the silicon Raman band at 520 cm be- olivine CPO has been reported from a peridotite body that formed fore each measurement. in a metamorphic aureole around a granite body due to the de- 3.1. HP5 (antigorite mylonite) hydration of foliated antigorite schist (Nozaka, 2005). There is no significant change in the orientation of the regional mesoscopic fo- HP5 is a strongly foliated antigorite schist containing olivine- liation and mineral lineation towards the granite contact implying rich patches and veins that cross cut the foliation. In addition to intrusion was not associated with significant deformation of the antigorite, the matrix also consists of porphyroclasts of olivine, surrounding rocks (Fig. 1). These field observations provide good tremolite and Cr-magnetite (Table 1). The veins and patches con- evidence that the peridotite around the granite in the Happo area sist of olivine + antigorite + talc + chlorite + Cr-magnetite + and its associated CPO formed in the absence of plastic deforma- tremolite (Fig. 2a–b; Table 1). The talc and chlorite are generally tion and some process other than dislocation creep is responsible developed along the grain boundaries (Fig. 2c–d). The presence of for the development of the CPO. The CPO reported by Nozaka fine-grained talc (<10 μm) was confirmed using EPMA and laser (2005) is a B-type CPO with the a-axis maximum perpendicular to Raman spectroscopy. the stretching lineation and within the regional foliation (Fig. 6a). For our studies it is important to distinguish between old Such B-type CPO patterns have been highlighted as important con- olivine with a porphyroclastic micrsotructure and younger post- tributors to seismic anisotropy in the mantle of convergent mar- deformation olivine. In HP5 neoblastic metamorphic grains of gins. olivine are clear with straight grain boundaries and occur in patches or veins cross cutting the antigorite foliation. In contrast, 3. Sample description the old partially serpentinized olivine has irregular outlines, is wrapped around by the foliation and is commonly cleaved (Fig. 2e). Here we present detailed descriptions of two key samples There is also a difference in chemistry: neoblastic olivine shows from the Happo area. The chemical compositions of minerals a distinctly higher Fo (forsterite) content (>92 mol.%) than the were analyzed using a electron-probe microanalyzer (EPMA) (JEOL porphyroclastic olivine (P-olivine) (88–90 mol.%) (Nozaka, 2005; T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 69

Fig. 2. Photomicrographs of sample HP5 (antigorite schist) in sections cut parallel to the foliation (a–d and f) and in a section cut parallel to the mineral lineation and normal to the foliation (e). The arrow shows the direction of the lineation. (a) Olivine vein surrounded by antigorite schist (one-nicol). (b) Same view as (a), crossed-nicols. (c) Detailed view of (b) showing talc and chlorite associated with vein olivine. (d) Same view as (c), crossed-nicols. (e) Cleaved olivine, crossed-nicols. (f) Neoblastic olivine associated with talc and chlorite, crossed-nicols. Abbreviations: V-Ol = Vein Olivine, P-Ol = Porphyroclastic Olivine, N-Ol = Neoblastic Olivine, Atg = Antigorite, Tr = Tremolite, Mag = Magnetite, Tlc = Talc, Chl = Chlorite.

Table 1 Representative microprobe analyses (wt.%) of silicates and spinel in the Happo-O’ne antigorite mylonite (Sample no. HP5).

Mineral P-Ol N-Ol V-Ol Atg Chl Chl Tr Tlc Cr–Mag

SiO2 39.75 41.65 41.56 43.89 41.18 41.82 58.58 60.45 0.07 TiO2 0.03 nd nd 0.02 0.04 0.04 0.06 0.04 0.12 Al2O3 0.01 nd nd 0.08 5.16 3.00 0.28 0.24 0.02 Cr2O3 nd nd nd nd nd nd nd nd 5.03 FeO 10.45 6.86 7.05 4.69 4.29 2.78 1.31 1.71 28.94 Fe2O3 –– ––––––60.24 MnO 0.20 0.17 0.13 0.05 0.09 0.03 0.03 0.05 0.26 NiO 0.46 0.44 0.28 0.12 0.35 0.21 0.21 0.23 0.78 MgO 48.15 51.36 50.98 40.23 37.88 38.56 24.61 31.30 0.38 CaO 0.03 0.01 nd 0.03 0.03 0.01 13.04 0.02 0.02 Na2O nd nd nd 0.02 0.02 nd 0.27 0.11 0.03 Total 99.11 100.48 99.99 89.11 89.05 86.45 98.39 94.15 95.90

Cations/O = 44 477723224 Si 0.99 1.00 1.01 2.02 1.90 1.96 7.92 7.83 0.00 Ti 0.00 nd nd 0.00 0.00 0.00 0.01 0.00 0.00 Al 0.00 nd nd 0.00 0.28 0.17 0.04 0.04 0.00 Cr nd nd nd nd nd nd nd nd 0.16 + Fe2 0.22a 0.14a 0.14a 0.18a 0.17a 0.11a 0.15a 0.19a 0.97b + Fe3 –– –––––– 1.81b Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 Ni 0.01 0.01 0.01 0.00 0.01 0.01 0.02 0.02 0.03 Mg 1.79 1.84 1.84 2.76 2.60 2.70 4.96 6.04 0.02 Ca 0.00 0.00 nd 0.00 0.00 0.00 1.89 0.00 0.00 Na nd nd nd 0.00 0.00 0.00 0.11 0.04 0.00 Total 3.01 3.00 2.99 4.98 4.96 4.95 15.11 14.17 3.01

Mg/(Mg + Fe) 0.89 0.93 0.93 0.94 0.94 0.96 0.97 0.97 0.02

Abbreviations: nd, not determined; Ol, Olivine; P-, Porphyroclastic; N-, Neoblastic; V-, Vein; Atg, Antigorite; Chl, Chlorite; Tr, Tremolite; Tlc, Talc; Cr-Mag, Cr-bearing Magnetite. a Total iron as FeO. b FeO and Fe2O3 were calculated assuming spinel stoichiometry.

Khedr and Arai, 2012). In addition, neoblastic olivine (N-olivine) The stability range of the mineral assemblage olivine + talc + is associated with magnetite inclusions (e.g., Arai, 1975; Nozaka, antigorite ± tremolite ± chlorite or olivine + talc + tremolite sug- 2005); a common feature of olivine developed as a result of de- gests that metamorphism in Happo area took place at relatively hydration of serpentinite. The neoblastic olivine is also associated low pressures (Khedar and Arai, 2012) (phase relation line (4) with ﬁne-grained talc and chlorite developed along grain bound- shown in Fig. 3). Tremolite was probably formed where there was aries and locally tremolite (Fig. 2f). some original diopside in the rock prior to contact metamorphism Petrological studies in the Happo area (e.g. Khedar and Arai, (Khedar and Arai, 2012). The release of the minor but signiﬁcant 2012) show that neoblastic olivine formed by the following dehy- amounts of Al in antigorite leads to the formation of chlorite that dration reaction of antigorite: otherwise has a similar chemical composition to antigorite. The neoblastic olivine shows no clear evidence of undulose ex- Antigorite = 18 Forsterite + 4Talc+ 27 H2O tinction, kink band or subgrain formation. 70 T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76

Table 2 Representative microprobe analyses (wt.%) of silicates and spinel in the Happo-O’ne peridotite (sample no. HP11).

Mineral: C-Ol F-Ol Chl Tr Tlc Cr-Mag

SiO2 41.72 41.65 31.06 58.80 62.19 0.03 TiO2 nd 0.02 nd 0.01 nd 0.27 Al2O3 0.00 nd 17.10 0.60 0.48 0.09 Cr2O3 nd 0.02 1.03 0.10 0.04 11.64 FeO 6.75 6.78 3.41 1.49 1.24 28.47 Fe2O3 – – – – – 55.49 MnO 0.14 0.16 0.01 0.07 nd 0.42 NiO 0.40 0.34 0.16 0.13 0.16 0.68 MgO 51.00 51.12 32.50 24.01 31.98 1.23 CaO nd 0.00 nd 13.05 0.01 nd Na2O 0.01 0.02 0.04 0.36 0.20 0.01 Total 100.01 100.11 85.31 98.61 96.29 98.32

Cations/O = 4 4 7 23224 Si 1.01 1.01 1.50 7.93 7.84 0.00 Ti nd 0.00 nd 0.00 nd 0.01 Al 0.00 nd 0.97 0.10 0.07 0.00 Cr nd 0.00 0.04 0.01 0.00 0.36 + Fe2 0.14a 0.14a 0.14a 0.17a 0.13a 0.92b + Fe3 – – –––1.61b Mn 0.00 0.00 0.00 0.01 nd 0.01 Ni 0.01 0.01 0.01 0.01 0.02 0.02 Mg 1.84 1.84 2.34 4.83 6.01 0.07 Ca nd 0.00 nd 1.89 0.00 nd Na 0.00 0.00 0.01 0.15 0.08 0.00 Fig. 3. P –T diagram showing the metamorphic zones in Happo area (after Khedr Total 2.99 2.99 5.00 15.09 14.16 3.01 and Arai, 2010, 2012, and references therein). The temperature is mainly from 500 ◦ to 650 C(P < 7 kbar) in the Talc zone. The serpentinized retrograde metaperi- Mg/(Mg + Fe) 0.93 0.93 0.94 0.97 0.98 0.07 dotites (protoliths) from the Diopside zone were subjected to thermal metamorphism to form the prograde peridotites. Reaction lines (2) and (4) correspond to the Abbreviations: C-, Coarse-grained; F-, Fine-grained. Other abbreviations are the breakdown of antigorite and olivine is stable on the high-T side of these reactions. same as in Table 1. a Total iron as FeO. HP5 from the Talc zone preserves the reactants and products associated with reac- b FeO and Fe O were calculated assuming spinel stoichiometry. tion (4). HP11 from the Talc zone has undergone complete antigorite dehydration to 2 3 form peridotite. Star symbols with no. 5 and no. 11 shows supposed approximate metamorphic conditions of HPA5 and HPA11, respectively. Abbreviations: Tlc = Talc, dislocation creep such as strong undulose extinction, kink bands = = = = = Fo Forsterite, Tr Tremolite, Anth Anthophyllite, En Enstatite, Di Diop- and subgrains. side, Atg = Antigorite, Opx = Orthopyroxene, Cpx = Clinopyroxene, Chl = Chlorite, Gt = Garnet, Spl = Spinel, Brc = Brucite, Ctl = Chrysotile, An = Anorthite. 4. TEM observations and olivine dislocation density 3.2. HP11 (peridotite hornfels) Micro- and mesostructural observations imply that the neoblas- HP11 consists mainly of olivine with smaller amounts of chlo- tic olivine has not undergone any significant plastic deformation. rite, talc, tremolite and Cr-magnetite (Table 2). Serpentine minerals However, deformation features such as kink bands and undulose are rare. Tremolite in HP11, similar to that in HP5, shows the extinction can be erased by recrystallization both during and af- compositional characteristics of tremolite in the prograde peri- ter deformation. Measurements of dislocation density are a more dotite reported in Happo area: relatively low contents of Al2O3 rigorous test of whether or not samples have been affected by dis- (<1.0 wt.%), Cr2O3 (<0.35 wt.%), and Na2O(<0.6 wt.%) and high location creep. contents of SiO2 (up to 59.9 wt.%) and Mg/(Mg + Fe) (up to TEM measurements of dislocation densities of the vein olivine 0.98) relative to the retrograde tremolite (Nozaka, 2005; Khedr in HP5 and both the coarse- and fine-grained olivine in HP11 was and Arai, 2010, 2012). This feature is also seen in similar pro- carried out. A focused ion beam (FIB) system (FEI, Quanta 200 3DS) grade tremolitic amphibole in thermally decomposed metaserpen- at Kyoto University (Fig. S1) was used to extract samples of vein tinites (Trommsdorff and Evans, 1972; Springer, 1974; Frost, 1975; olivine in HP5 and coarse and fine olivine in HP11 from thin sec- Pinsext and Hirst, 1977; Nozaka and Shibata, 1995). The chemi- tions. A predefined area (30–40 μm2) was coated with Pt and the + cal compositions (Table 2) and microstructure of the constituent surrounds cut out to a depth of ∼10 μm using a Ga ion gun. minerals (Fig. 4) are very similar to the minerals in the veins of The resulting leaf was broken off at its base mechanically and then HP5 suggesting that both formed by the same process of antig- mounted on a TEM grid. The extracted sample leaves were thinned + orite decomposition and that HP11 represents a more completely to a thickness of 100–200 nm using a Ga ion beam at 30 kV reconstituted peridotite than HP5. and 0.1–30 nA. The samples were studied using a JEOL JEM-2100F HP11 consists of two microstructurally distinct domains: a dom- TEM, operated at 200 kV at Kyoto University. TEM images were inant course-grained domain (Fig. 4a–b) and a more restricted recorded using a CCD camera (Gatan, Orius 200D). fine-grained domain (Fig. 4c–d). The coarse-grained domains have Very few dislocations were observed in any of the samples. The vein or patch morphologies similar to the veins and patches of estimated dislocation densities in vein olivine of HP5 and coarse − HP5. The grain-size of olivine, d, was measured using the linear in- olivine and fine olivine of HP11 are 1.3 × 106 [cm 2], 4.3 × 106 − − tercept method with a correction factor C = 1.5(Gifkins, 1970). In [cm 2] and 6.7 × 106 [cm 2], respectively (Table S1). Such val- sample HP11 this method gave d = 164±22 μm (2σ ) in the coarse ues of dislocation density are comparable to gem quality peridot − grained domains and d = 25 ± 2μm(2σ ) in the fine-grained do- (3.2 × 104–3.2 × 106 [cm 2]; Blake, 1976) grown in the absence of mains. The same method gives an average grain size of the vein deformation and two orders of magnitude less than olivine with olivine in HP5 as 168 ± 23 μm (2σ ) (Fig. 2a–b). Neither the fine- B-type olivine CPO formed by plastic deformation experiments − nor the coarse-grained olivine shows microstructural evidence for (2.58–4.42 × 108 [cm 2]; (Katayama and Karato, 2006). Confidence T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 71

Fig. 4. Photomicrographs of sections cut normal to the foliation and parallel to mineral lineation of sample HP11. The arrow shows the direction of the lineation in each section. (a) Coarse-grained olivine domain, one-nicol. (b) Same view as (a) with crossed-nicols. (c) Fine-grained olivine domain, one-nicol. (d) Same view as (c) with crossed nicols. Abbreviations: N-Ol = Neoblast Olivine, Atg = Antigorite, Tr = Tremolite, Chl = Chlorite, Cr-Mag = Chromian Magnetite, Tlc = Talc. intervals (CI) for the estimated dislocation densities can be ob- 5.1. Electron-backscatter diffraction (EBSD) analysis tained by assuming a Poison distribution. Even at the 99% CI, all estimates are still at least an order of magnitude lower than those CPOs of antigorite and olivine grains are measured in thin sec- reported for olivine deformed by dislocation creep. See Supple- tions that were polished using a series of diamond pastes with mentary Material (Table S1) for more details. decreasing grain sizes down to 1/4 micron and further treatment Dislocation density can be lowered during long time-scale an- with colloidal silica to remove the surface damage zone. The crys- nealing processes, and there is the possibility that the present tal orientations were determined using a scanning electron mi- dislocation density in natural samples does not reflect the orig- croscope equipped with an electron-backscatter diffraction (EBSD) inal density. Some dislocation orientations are more susceptible system (HITACHI S-3400N with Oxford HKL Channel5), at 20 kV to recovery and annihilation. The present study revealed very few accelerating voltage and a working distance of 28 mm at Shizuoka internal dislocations (screw and edge) and no dislocation struc- University. EBSD patterns were collected in low vacuum (30 Pa) tures such as tilt boundaries which are known to be very stable allowing uncoated samples to be used. Over 200 crystal orienta- (Fig. S1). tions were measured per sample and the computerized indexation The very low dislocation densities and lack of any observed sta- of the diffraction pattern was visually checked for each measure- ble dislocation structures recorded in the present study suggest ment. Additional measurements including detailed mapping were that the olivine has not undergone dislocation creep. This con- carried out at Nagoya University using a JEOL JSM-6510LV with clusion is in good agreement with the meso- and microstructural Oxford HKL Channel5, a 20 kV accelerating voltage and a working observations that also indicate a lack of ductile deformation of the distance of 27 mm. At Nagoya EBSD measurements were made in neoblastic olivine domains. a low vacuum of 10 Pa and the samples were not coated. 5. Crystallographic preferred orientation 5.1.1. Antigorite CPO The antigorite schist (HP5) and the peridotite (HP11) have a The CPO of the antigorite bordering the olivine vein shows a strong foliation defined mainly by the grain shape of minerals. strong CPO with the b-axis parallel to the mineral lineation and the A well-defined mineral lineation is shown both by the grain shape c-axis normal to the foliation (Fig. 6a). This type of antigorite CPO of antigorite and/or the direction of boudinaged opaque mineral is common in serpentinite tectonites (Hirauchi et al., 2010; Soda grains including Cr-magnetite, which is associated with Cr-spinel and Takagi, 2010; Jung, 2011; Nishii et al., 2011) and is similar to (Khedr and Arai, 2010, 2012).Theseareusedastheframeworkfor that previously reported for the Happo region by Watanabe et al. discussing olivine and antigorite CPO patterns. (2011). 72 T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76

the mineral lineation and an a-axis concentration perpendicular to the foliation. The olivine CPO patterns of the neoblastic olivine (Fig. 6a) were measured in three separate veins with different orientations (Fig. 5). All three show very similar B-type Ol CPO patterns when plotted with respect to the mesoscopic foliation and mineral lineation. In the peridotite sample (HP11), the fine-grained olivine domains have a weak CPO. In contrast, the coarse-grained part shows a strong CPO that has a B-type geometry when plotted with respect to the orientation of the mesoscopic tectonic fabric. Sta- tistical analyses of the CPO in the coarse- and fine-grained domains based on the normalized eigenvalues from the orientations of all three crystallographic axes in sample HP11 (Anderson and Stephens, 1972; Woodcock and Naylor, 1983; Nishii et al., 2011) show the fine-grained domains do not have significant point concentrations whereas the concentrations of the coarse-grained domains are clearly non-random. The volume ratio between coarse- Fig. 5. Sketch of the micro- and mesostructures of HP5 (antigorite schist) show- and fine-grained olivine based on 1000 counted points is about ing the three sub-perpendicular olivine veins used in this study that have different : orientations with respect to the foliation and/or lineation. X, Y and Z show the 87 13 and the elastic anisotropy of the sample is, therefore, dom- principal axes of finite strain. inated by the coarse-grained domains (Fig. 6b).

5.2. Origin of B-type Ol CPO in the Happo area 5.1.2. Olivine CPO The antigorite schist, HP5, contains two distinct types of olivine: The neoblastic olivine grains have straight grain boundaries and pre-mylonitic olivine and neoblastic olivine formed in veins and do not show microstructural evidence of ductile deformation such patches. The porphyroclastic olivine shows only a weak CPO pat- as kink bands and undulose extinction (Fig. 2f, 4a–d). The vein tern (Watanabe et al., 2011)withac-axis concentration parallel to boundaries of sample HP5 outlines are locally irregular but no

Fig. 6. (a) Pole figures for the crystallographic axes of the vein olivine ((A), (B) and (C)) corresponding to the olivine veins (A), (B) and (C) (Fig. 5) and neighboring antigorite of the mylonitic matrix in HP5, secondary olivine in HP11 and olivine neoblasts from Nozaka (2005). There is a clear relationship between the crystallographic orientations of the vein olivine and neighboring antigorite. Antigorite grains clearly in contact with secondary vein olivine or located next to the vein olivine were measured. The CPO are presented in equal-area, lower-hemisphere projections with the foliation oriented vertically east–west and the lineation horizontal. Contours show multiples of uniform distribution. The CPOs were constructed by determining orientations for individual grains using the EBSD technique in this study and using a universal stage in Nozaka (2005). The original data of Nozaka (2005) were replotted and contoured to enable a direct comparison with the present study. N represents the number of grains measured. (b) Seismic anisotropy corresponding to the fabric of the peridotite hornfels HP11 in (a). Plots are all lower-hemisphere equal-area and are oriented so the foliation is horizontal and the lineation is east–west. The contours show multiples of uniform density. Vp (km/s) gives the 3D distribution of the P-wave velocity. This anisotropy − [ + ] of P-waves (%) is 100(Vpmax Vpmin)/ (Vpmax Vpmin)/2 . AVs (%) gives the 3D distribution of the polarization anisotropy of S-waves owing to S-wave splitting. Vs1 Polarizations are the polarization direction of the fast S-wave passing through the sample in various directions, and each small segment on the figure represents the trace of the polarization plane on the point at which S1 penetrates the hemisphere. Color shading for AVs is also shown on this figure. The figures were prepared using software of D. Mainprice (Mainprice, 1990). T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 73 clear folds, boudinage or necking were observed (Fig. 2a–b). In ad- type 2 topotaxy recognized by Boudier et al. (2010),butinourcase dition, TEM observation of vein olivine of HP5 and fine and coarse the relationships developed as the result of antigorite breakdown. olivine of HP11 shows very low dislocation densities (Table S1) and This result suggests that topotaxial growth of olivine in antigorite a lack of any observed stable dislocation structures (Fig. S1). These schist is a potentially important mechanism for forming B-type Ol observations suggest that the neoblastic olivine has not undergone CPO. significant ductile deformation and some other process is needed The fine-grained olivine does not show a significant CPO and to explain the formation of the CPO. represents an anomaly in our proposed scheme. Our studies have Veins commonly show strong CPO development as the result shown that the fine olivine in HP11 has a very low dislocation of competitive growth leading to the development of crystal fi- density and formed under relatively static conditions. However, bres elongate in the crystallographic orientation associate with the in addition to the difference in grain size, olivine in the finer- fastest growth rate—‘the survival of the fastest’ (Nollet et al., 2005). grained domains has no magnetite as inclusions suggesting the In HP5, the Ol-bearing grains are not elongate at high angles to fine-grained domains of HP11 formed by a different process to the vein wall (Fig. 2) and do not show increasing grain size to- the dominant coarse-grained domains. The fine-grained olivine is wards the center of the veins suggesting competitive growth is not associated with Cr-magnetite in contact with the coarse-grained the cause of the CPO in this case. In addition, the same CPO is olivine. One possibility is that fine-grained olivine, Cr-magnetite present in all three measured veins despite their different orien- and Al-rich chlorite (clinochlore) in HP11 formed by the break- tations (vein olivine (A)–(C) shown in Fig. 6a). The high angles down of chlorite with a lower Al content (low Al chlorite) such as between the veins suggests that not all the veins opened in the that observed in the vein of HP5. The maximum temperature in ◦ same direction (Fig. 5) and, therefore, that the olivine CPO is not the contact aureole is around ∼700 C which is sufficient for the controlled by the vein opening direction. These observations imply breakdown of low Al chlorite (Deer et al., 2009). The grain size that a CPO formation mechanism is required that is not controlled difference may be due to the greater availability of fluid for olivine by ductile deformation, competitive growth or vein opening direc- forming by antigorite breakdown. tions. 6. Discussion 5.3. Relationship of the crystallographic orientation between antigorite and olivine 6.1. B-type Ol CPO: a comparison of experimental results and natural examples In addition to dislocation creep, CPO patterns are also known to develop as the result of preferential growth in a specific orientation controlled by the orientation of preexisting minerals— B-type Olivine CPO has been the focus of attention for re- topotactic growth. Several studies have highlighted the importance search largely due to its potential to account for the seismic of topotactic replacement of olivine by antigorite (Boudier et al., anisotropy observed in many convergent margins (e.g., Jung and 2010; Brownlee et al., 2013). Here we investigate the inverse pos- Karato, 2001a). High T deformation experiments on water-present sibility that there is a topotactic relationship between neoblastic peridotite show that B-type Ol CPO patterns can be formed by dis- olivine and pre-existing antigorite. The CPO patterns of olivine and location creep under high differential stresses (Jung and Karato, antigorite show good general agreement in the orientations of the 2001a; Jung et al., 2006; Katayama and Karato, 2006). Extrapola- axes (Fig. 6). However this approach does not enable us to ex- tion of the experimental conditions using an exponential flow law amine the closer relations that are expected for individual grains. has been used to suggest that B-type olivine may also form un- To investigate the crystallographic relationships between the two der the lower temperatures and lower differential stress conditions mineral species, EBSD crystallographic orientation mapping of the (Katayama and Karato, 2006) expected close to subduction zone vein olivine and antigorite in HP5 (Fig. S2a-c) was carried out at boundaries (Kneller et al., 2005). However, a comparison with nat- Nagoya University using a step size of 1.0–2.3μm. ural examples of B-type Ol CPO shows three significant problems As a result of the EBSD mapping, we were able to confirm with this proposal. Firstly, mantle regions adjacent to the subduc- that grains of olivine within the olivine vein are locally in con- tion boundary are expected to be both water-rich and relatively tact with antigorite with very similar orientations of the crystal- cold, which leads to the formation of a hydrated mantle mineral- lographic axes. The recorded relationships are: (100)atg, (010)atg ogy including serpentine minerals such as antigorite. Recent work and [001]atg sub-parallel to [100]ol, [001]ol and [010]ol, respec- on antigorite-bearing forearc mantle shows that the presence of tively (Fig. S2d–e). This type of relationship has also been reported antigorite suppresses the development of strong Ol CPO and re- for antigorite replacing olivine and is referred to as type 2 topotac- duces the strength of any preexisting CPO (Wallis et al., 2011). tic relationship by Boudier et al. (2010). The reduction in CPO strength is thought to be due to grain- ◦ The small misorientation (1–2 for α, β, γ ) between the crys- boundary sliding that occurs between olivine and antigorite. Sec- tallographic axes of antigorite and olivine (Fig. S2e) may reflect ondly, extrapolation of experimental results suggests that B-type ◦ both measurement error (∼1 ) and the incompatibility of the or- CPO in the mantle forms under low-T or high shear stress condi- ◦ thorhombic olivine and monoclinic (β = 91.4 ) antigorite crystal tions, but these are incompatible with the formation conditions of systems. some natural examples of B-type Ol CPO (Shuguang and Li., 1998; We suggest the best explanation for the observed CPO pat- Hidas et al., 2007)(Fig 7). Thirdly, transmission electron micro- terns is topotactic growth of olivine on the preexisting antigorite scope (TEM) observations of B-type olivine formed by deformation grains that have a CPO. There is experimental evidence to show experiments show dislocations with Burgers vectors parallel to the that the breakdown of antigorite can result in the formation of c-axis are important in the formation of the CPO (Jung et al., 2006; olivine with crystallographic axes controlled by the orientation of Katayama and Karato, 2006). In contrast, there is no report of c-slip the preexisting antigorite (Souza Santos and Yada, 1983). There in natural B-type olivine. From this review of the available data, we is also a documented example of antigorite that grew topotaxi- conclude that present ideas about the formation of B-type olivine ally with respect to pre existing olivine (Boudier et al., 2010): in CPO and the explanation of seismic anisotropy in subduction zones both cases [010]atg is parallel [001]ol but [010]ol may be paral- do not adequately account for observed characteristics of natural lel to either [100]atg (type 1) or [001]atg (type 2). Our results samples and alternatives such as the topotaxial growth after antig- from the antigorite and olivine are in good agreement with the orite reported here, should be investigated. 74 T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76

6.2. Importance of growth-related olivine CPO in subduction zones

Antigorite-bearing mantle is predicted to be a widespread com- ponent of forearc mantle and as this material is dragged down by the traction of the downgoing slab, it will become deformed and foliated until it reaches sufficiently high T and P conditions to undergo dehydration. This type of dehydration reaction is thought to be an important part of the material transport in subduction zones (John et al., 2011). Our results imply that when this dehydration occurs, the newly formed olivine may develop a B-type CPO. The kind of topotactic olivine CPO that we report here may form anywhere that dehydration reaction of antigorite schist occurs, probably the most likely location for this to occur is in the mantle wedge away from the cold part immediately adjacent to the subduction boundary (Fig. 8). The development of B-type fabric in this region within the mantle wedge is compatible with recent seismic observations in NE-Japan (e.g., Nakajima et al., 2006; Wang and Zhao, 2008). In NE-Japan, seismic stations located in the forearc show trench- parallel anisotropy, whereas stations located in the back-arc show trench-normal anisotropy. Thermal modeling predicts that antig- Fig. 7. Formation conditions for natural B-type olivine CPO (abbreviations: HA = Hi- orite should break down at depths equivalent to or greater than gashi Akaishi, Japan (Mizukami et al., 2004); IMO = Imono, Japan (Tasaka et al., the location of the subducting plate beneath the volcanic arc (e.g. 2008); VM = Val Malenco, Italy (Jung, 2009); CDG = Cima di Gagnone, Switzer- Wada and Wang, 2009). However, the resulting domain of topotac- = land (Skemer et al., 2006); AK Almklovdalen, Norway (Cordellier et al., 1981); tic B-type olivine CPO should become progressively thinner toward BB = Bakony–Balaton, Hungary (Hidas et al., 2007); NQM = North Qilian Shan Mountains, China (Shuguang and Li, 1998)) and the proposed boundary line in the backarc region and other olivine CPO patterns such as A-, C- temperature and stress space between the two ranges for B- and C-type olivine and E-type (Katayama and Karato, 2006) are likely to dominate CPO developed by dislocation creep in deformation experiments (Katayama and in the backarc mantle. These CPO types are all associated with Karato, 2006). Deformation experiments were performed at a constant pressure trench-normal anisotropy (Fig. 8). of 2.0 GPa under water-saturated conditions and the B–C transition conditions for low T were extrapolated from the experimental results assuming an exponential Formation of Ol CPO by topotactic growth can account for the flow law derived for high stress conditions (Katayama and Karato, 2006). The dif- presence of B-type CPO in microstructural domains of natural sam- ferential stresses of natural B-type olivine were estimated from grain sizes (Jung ples that lack direct evidence for dislocation creep. The maximum and Karato, 2001b) measured from photomicrographs. This plot shows some ex- density (in terms of multiples of uniform distribution) of the pole amples of natural B-type CPO clearly formed under conditions incompatible with figures for B-type olivine CPO formed by plastic deformation is the extrapolated experimental data. The present plot assumes wet conditions. For deformation under dry conditions but at similar high temperatures and low about 2–3 (e.g., Jung and Karato, 2001a; Holtzman et al., 2003; stresses, the corresponding olivine CPO fabric is an A-type (Zhang et al., 2000; Katayama and Karato, 2006; Ohuchi et al., 2011) and up to about Jung et al., 2006). 6atthemost(Jung et al., 2006), the concentrations in this study

Fig. 8. Illustration of possible distribution region of B-type Olivine CPO developed by topotactic growth in subduction zones. Purple indicates the zones of antigorite stability. The green region shows the possible parts where B-type olivine forms by the breakdown of antigorite. The boundary lines (2) and (4) between purple and green domains are drawn following the reaction curves (2) and (4) shown in Fig. 3. The sketch on the right summarizes the suggested relationship between crystal orientations of antigorite and olivine and the formation of B-type olivine after dehydration of antigorite schist. The yellow numbers in the right sketch correspond to the same numbers in the left diagram. The brown dashed arrow represents the direction in which shallow antigorite is dragged down. The pink region shows possible parts of the mantle where A-, C- or E- type olivine CPO could form by plastic deformation related to mantle flow (e.g., Kneller et al., 2005). Brown and light blue indicate the continental crust and subducting lithosphere, respectively. Calculated thermal structure of a subduction zone is based on data for NE-Japan (adapted from (Wada and Wang, 2009)). Triangles and arrows on the surface indicate the volcanic arc and shear-wave splitting observed in many convergent margins, respectively (e.g., Nakajima and Hasegawa, 2004). T. Nagaya et al. / Earth and Planetary Science Letters 387 (2014) 67–76 75 show generally higher values (about 5–10) and therefore have a Holtzman, B.K., Kohlstedt, D.L., Zimmerman, M.E., Heidelbach, F., Hiraga, T., Hus- stronger effect on seismic anisotropy. toft, J., 2003. Melt segregation and strain partitioning: Implications for seismic The CPO present in a rock developed as a result of plastic defor- anisotropy and mantle flow. Science 29, 1227–1230. John, T., Scambelluri, M., Frische, M., Barnes, J.D., Bach, W., 2011. Dehydration of mation will be influenced by the presence of any preexisting CPO subducting serpentinite: Implications for halogen mobility in subduction zones (e.g. Skemer et al., 2012).ThepresenceofaB-typeOlCPOinsam- and the deep halogen cycle. Earth Planet. Sci. Lett. 308, 65–76. ples that demonstrate evidence for crystal plastic deformation (e.g. Jung, H., 2009. Deformation fabrics of olivine in Val Malenco peridotite found Mizukami et al., 2004) can be accounted for by the non-random in Italy and implications for the seismic anisotropy in the upper mantle. Lithos 109, 341–349. distribution of crystallographic orientations before the onset of de- Jung, H., 2011. Seismic anisotropy produced by serpentine in mantle wedge. Earth formation. Planet. Sci. Lett. 307, 535–543. Jung, H., Karato, S., 2001a. Water-induced fabric transitions in olivine. Science 293, 7. Conclusion 1460–1463. Jung, H., Karato, S., 2001b. Effects of water on dynamically recrystallized grain-size of olivine. J. 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