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Tectonophysics

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Lattice-preferred orientation of amphibole, chlorite, and olivine found inhydrated mantle peridotites from Bjørkedalen, southwestern Norway, andimplications for seismic anisotropy

Hyunsun Kang, Haemyeong Jung⁎

School of Earth and Environmental Sciences, Seoul National University, Seoul 08826, Republic of Korea

A R T I C L E I N F O

Keywords:AmphiboleChloriteOlivineLattice-preferred orientationSeismic anisotropyBjørkedalen

A B S T R A C T

Understanding lattice-preferred orientations (LPOs) of olivine is important in the study of seismic anisotropy andmantle flow of the upper mantle in the earth. Although olivine is the major mineral of the upper mantle, bothamphibole and chlorite in a deformed peridotite may develop LPO and thus affect the seismic anisotropy inwater-rich environments. However, it is not known whether the coexistence of LPO of amphibole and chloritecontributes to seismic anisotropy constructively or destructively. Therefore, the LPOs of amphibole, chlorite, andolivine in hydrated mantle peridotites from Bjørkedalen, southwestern Norway, were examined. In this study,the hydrated mantle peridotites showed three types of the LPOs of olivine, classified as A type LPO, B type-likeLPO, and mixed LPO between the two types. Most of the olivines showed weak LPOs but amphibole and chloriteshowed strong LPOs. High water content in olivine (440–690 ppm H/Si) and numerous hydrous inclusions inolivine and orthopyroxene indicated that water induced the fabric change of olivine from A type to B type-likeLPO. The seismic velocity and seismic anisotropy were calculated on the basis of the LPOs of the studied mi-nerals. It is found that the LPOs of both amphibole and chlorite can contribute to a strong trench-parallel seismicanisotropy in the subduction zone constructively, depending on the dip angle of flow. The results suggest that thedevelopment of the LPO of amphibole and chlorite can contribute significantly to seismic anisotropy of thesubduction zone.

1. Introduction

Subduction zones are unique areas in which hydrous phases trans-port water deep into the earth (Fumagalli and Poli, 2005; Ohtani, 2005;Wada et al., 2012). A subducting hydrous slab releases water into theoverlying mantle wedge through dehydration reaction as it goes downdeeper to a higher pressure environment. Mantle hydration includeshydrogen incorporation in nominally anhydrous phases such as olivineand pyroxenes and the formation of hydrous phases such as amphibole,chlorite, serpentine, talc, brucite, and clinohumite. Deformation in themantle results in a lattice-preferred orientation (LPO) of the constituentminerals and can cause anisotropic seismic properties (Ben Ismail andMainprice, 1998; Cao et al., 2015, 2017; Hansen et al., 2014; Jung andKarato, 2001a; Jung et al., 2006; Karato et al., 2008; Michibayashi andOohara, 2013; Nicolas and Christensen, 1987; Ohuchi et al., 2011;Skemer and Hansen, 2016; Soustelle et al., 2010). In this regard,knowledge of the LPO and the anisotropic seismic properties of bothanhydrous and hydrous minerals such as olivine, amphibole and

chlorite is important for understanding the seismic anisotropy, mantleflow patterns, and geodynamics in subduction zones (Jung, 2017; Kimand Jung, 2015; Ko and Jung, 2015; Long and Silver, 2008; Long, 2013;Mainprice and Ildefonse, 2009; Savage, 1999; Wagner et al., 2013).

The water content of olivine, the most abundant mineral of theupper mantle, is one of the key factors of its LPO (Jung and Karato,2001a). Previous experimental studies have shown that five differentLPOs of olivine—types A, B, C, D, and E—develop on the basis ofvarious water content and stress conditions (Jung et al., 2006) duringthe mineral's deformation at high pressure and high temperature(Bystricky et al., 2000; Demouchy et al., 2012; Hansen et al., 2014;Jung and Karato, 2001a; Karato et al., 2008; Katayama et al., 2004;Ohuchi et al., 2011; Soustelle and Manthilake, 2017). Studies of naturalrock samples have also shown the water and stress dependence of oli-vine fabrics and slip systems (Behr and Smith, 2016; Frese et al., 2003;Jung, 2009; Jung et al., 2009a, 2013; Kaczmarek et al., 2016; Katayamaet al., 2005; Kim and Jung, 2015; Linckens et al., 2011; Mehl et al.,2003; Michibayashi and Oohara, 2013; Mizukami et al., 2004; Park

https://doi.org/10.1016/j.tecto.2018.11.011Received 9 May 2018; Received in revised form 5 October 2018; Accepted 19 November 2018

⁎ Corresponding author.E-mail address: hjung@snu.ac.kr (H. Jung).

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et al., 2014; Park and Jung, 2015; Park and Jung, 2017; Précigout andAlmqvist, 2014; Précigout et al., 2017; Puelles et al., 2012, 2016;Sawaguchi, 2004; Skemer et al., 2006, 2013; Soustelle et al., 2010;Tasaka et al., 2008; Yamamoto et al., 2017). Several other factors thataffect the development of olivine LPO were reported: pressure (Junget al., 2009b; Ohuchi et al., 2011; Raterron et al., 2007), changes indeformation mechanisms (Miyazaki et al., 2013; Soustelle andManthilake, 2017; Sundberg and Cooper, 2008), temperature (Carterand Ave'Lallemant, 1970; Goetze, 1978; Karato et al., 2008; Katayamaand Karato, 2006; Liu et al., 2018), and the presence of melt (Holtzmanet al., 2003; Qi et al., 2018).

A type LPO of olivine typically occurs under dry and low-stressenvironments and has been defined by the alignment of its [100] axessubparallel to the shear direction and [010] axes subnormal to the shearplane (Jung and Karato, 2001a; Zhang and Karato, 1995). On thecontrary, LPO types B, C, and E of olivine can develop under wet en-vironments and specific stress conditions (Jung and Karato, 2001a;Jung et al., 2006; Katayama et al., 2004). The B type LPO of olivine ischaracterized by the alignment of its [001] axes subparallel to the sheardirection and [010] axes subnormal to the shear plane. There are alsoreports that B type LPO of olivine can be generated by diffusion creep(Soustelle and Manthilake, 2017; Sundberg and Cooper, 2008), pressureincrease (Jung et al., 2009b; Ohuchi et al., 2011), grain boundarysliding (Précigout and Hirth, 2014), and deformation in the presence ofmelt (Holtzman et al., 2003; Qi et al., 2018).

Both amphibole and chlorite are stable in a broad range of hightemperature and high pressure conditions in hydrated mantle wedgesand subducting slabs (Fumagalli and Poli, 2005; Mainprice andIldefonse, 2009; Pawley, 2003). The hydrous phases such as amphiboleand chlorite are elastically very anisotropic (Almqvist and Mainprice,2017; Kim and Jung, 2015; Mainprice and Ildefonse, 2009; Mookherjeeand Mainprice, 2014). However, there are few studies that have re-ported chlorite fabrics (Kim and Jung, 2015; Mainprice and Ildefonse,2009; Morales et al., 2013; Padrón-Navarta et al., 2015; Wallis et al.,2015). Only few studies on the LPO of chlorite have reported its im-portance of producing strong seismic anisotropy in slab-mantle inter-face at subduction zones (Kim and Jung, 2015; Mainprice and Ildefonse,2009; Morales et al., 2013). Recent experimental study on the LPO ofamphibole in simple shear at the pressure of 1 GPa (Ko and Jung, 2015)has shown that the LPO of amphibole can induce a strong seismic

anisotropy (up to P-wave velocity anisotropy (AVp) of 14.6% andmaximum S-wave velocity anisotropy (max AVs) of 12.1%) in the lowercrust and shallow subduction zones. Recent studies of natural rocksamples have also shown the importance of amphibole LPO in inter-preting crustal seismic anisotropy (Ji et al., 2013, 2015). However, eventhough a peridotite in a mantle wedge can contain up to ~15–40%amphibole (Cao et al., 2016), no study exists on how the LPO of am-phibole can modify seismic signatures of the mantle rock in a hydratedlower part of mantle wedge. It is also not known whether the coex-istence of LPO of amphibole and chlorite in a deformed peridotitecontributes to seismic anisotropy constructively or destructively.

In this paper, we present a microstructural study of the Bjørkedalenperidotites located within the Western Gneiss Region of southwesternNorway. The Bjørkedalen peridotites provide rare natural samples ofhydrated mantle assemblages containing olivine, amphibole, andchlorite. This study investigates the LPO formation of olivine and hy-drous minerals and their effects on seismic anisotropy under water-richconditions such as a subduction zone. Our results suggest that waterinduces fabric change of olivine and the LPO of amphibole and chloritecan contribute significantly to the seismic anisotropies of the hydratedmantle wedge.

2. Geological setting of the study area

The Western Gneiss Region (WGR) of Norway is an elongated areathat extends 300 km in length and 150 km in width (Beyer et al., 2006).The WGR is the lowest exposed tectonic unit of the Scandinavian Ca-ledonides and is structurally overlain by the Caledonian allochthons(Fig. 1). The WGR consists of various rock types, mostly amphibolite- togranulite-facies gneisses, eclogites, and pods and lenses of orogenicperidotites (Hacker et al., 2010; Lapen et al., 2009). The WGR is gen-erally known as the western margin of the Baltica that subducteddeeply under the Laurentian lithospheric mantle during the Scandianphase of the Caledonian orogeny (425–380Ma) (Krogh, 1977). Thepresence of ultra-high-pressure minerals such as coesite and diamond ineclogites and garnet in peridotites in the WGR indicate the deep sub-duction of Baltic crust into the Laurentian mantle (Smith, 1984; Terryet al., 2000; Van Roermund et al., 2000).

The WGR experienced a sequence of metamorphic events during theCaledonian orogeny, including (1) the Proterozoic granulite facies

Fig. 1. Geological map of Bjørkedalen,southwestern Norway; the study area ismarked by a red star. The distributionof geological features are from Younget al. (2011) and Brueckner andCuthbert (2013); the distribution ofchlorite and garnet peridotite bodiesare after Lapen et al. (2009); the eclo-gite isobar and the peak metamorphictemperature are after Hacker et al.(2010) and Kylander-Clark et al.(2008). (For interpretation of the re-ferences to colour in this figure legend,the reader is referred to the web ver-sion of this article.)

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condition (950Ma), (2) an ultrahigh-pressure condition associated withthe continental collision between Baltica and Laurentia (425–400Ma),and (3) the amphibolite-facies condition during exhumation to ashallow crustal depth (400–380Ma) (Hacker et al., 2010). The ex-humation of the subducted Baltic crust is related with the extensionalong detachment shear zones, such as the Nordfjord-Sogn DetachmentZone (Young et al., 2011). Most shear senses of the WGR are top-W/SWimprinted on an earlier top-E/SE thrust fabric, consistent with ex-humation following a continental collision (Brueckner and Cuthbert,2013; Hacker et al., 2010).

The peridotites in the WGR are known as fragments of Archeanmantle enclosed within the Proterozoic Baltic continental crust duringsubduction of Baltica beneath Laurentia (Beyer et al., 2012). Sub-sequent exhumation of the peridotites was associated with retrogressivemetamorphism under amphibolite and greenschist facies conditions, aswell as extensive hydration (Carswell, 1986; Cordellier et al., 1981;Kostenko et al., 2002). The peridotites typically contain garnet- orspinel-bearing assemblages that have undergone various degrees ofretrogression. The garnet-bearing peridotites show evidence of re-fertilization during the Proterozoic melt percolation event, whereassome of the spinel peridotites have extremely depleted compositionformed by melt extraction during the Archean (Beyer et al., 2006).

Peridotites rich in amphibole and chlorite are distributed as lens-shaped body enclosed within gneiss in Bjørkedalen, in the central-western part of the WGR (Fig. 1). They are metamorphosed underretrograde amphibolite facies condition and are associated with ex-tensive hydration (Carswell, 1986). The hydrous minerals amphiboleand chlorite in the peridotites are regarded as a result of interactionbetween peridotites and hydrous crusts during the Caledonian orogeny(Brueckner, 1998; Carswell, 1986).

3. Sample description

Seven peridotites from Bjørkedalen were selected for study. Thesesamples are fine-grained and well-foliated spinel (chromite)-bearingdunites with porphyroclastic textures (Fig. 2a). The modal compositionsof the samples were quantified based on mineral counting (vol%) in theelectron backscattered diffraction (EBSD) operation. These sampleswere mostly composed of olivine (59–99%), amphibole (0–41%), andchlorite (0–10%) with minor orthopyroxene and spinel (Table 1). Theperidotites did not show evidence of pervasive serpentinization. Amongthe studied samples, olivine was the most abundant in sample 1190(99%), amphibole was the most abundant in sample 1196-2 (41%), andchlorite was the most abundant in sample 1194-2 (10%). Two samples1194-2 and 1196-3 contained considerable amounts of both amphibole(10%) and chlorite (10%).

4. Methods

4.1. Measurement of LPOs and calculation of seismic anisotropy

The foliation of the Bjørkedalen peridotites was determined by well-developed schistosity with parallel alignment of flattened olivine andflakes of chlorite and amphibole. The lineation was determined bymeasuring the shape-preferred orientations of olivine and orthopyr-oxene on foliation by using the projection-function method (Panozzo,1984). Next, thin sections of the X–Z plane were prepared, where Xrepresents lineation, and Z is normal to foliation. They were coatedwith carbon to prevent charges in the scanning electron microscope(SEM) operation. The LPOs of the minerals were measured at the Schoolof Earth and Environmental Sciences (SEES) in Seoul National Uni-versity (SNU), Korea, by using an EBSD system with Oxford Channel 5software attached to the SEM (JSM-6380, JEOL). The EBSD analysiswas conducted under the conditions of 20 kV accelerating voltage, 60spot size, and 15mm working distance on samples tilted 70°. The Ki-kuchi band patterns of individual grains were manually indexed for

accurate solution. The Kikuchi pattern was measured one point pergrain because grain size distributions were more or less homogeneous.

The fabric strength of each mineral was calculated by using both M-index (Skemer et al., 2005) and J-index (Bunge, 1982). The M-index isdefined as

∫≡ −M 12

|R (θ) R (θ)| dθT 0(1)

where RT (θ) denotes the theoretical distribution of misorientationangles for a random fabric, and R0 (θ) denotes the distribution of ob-served misorientation angles. It ranges from 0 in random fabric to 1 in asingle crystal (Skemer et al., 2005). The J-index is defined as

∫≡ f gJ [ ( ) ]dg2(2)

where f(ϕ1,ψ,ϕ2) represents the orientation distribution functions(ODFs), ϕ1, ψ, ϕ2 dictates the Euler angles of the orientation data, anddg is the volume element. The J-index ranges from 1 (in a randomfabric) to infinity (single crystal fabric), and describes the intensity ofthe ODF (Bunge, 1982).

Seismic velocity and anisotropy of P- and S-waves were calculatedby using the measured LPOs, density, and elastic stiffness of olivine(Abramson et al., 1997), amphibole and chlorite (Aleksandrov andRyzhova, 1961). The bulk density of peridotites was calculated as theweighted average of single-crystal density of constituent minerals withtheir respective volume fractions. The Voigt-Reuss-Hill averagingscheme was used to calculate macroscopic elasticity stiffness tensorsfrom the LPOs and the elastic stiffness of the constituting minerals. Thecalculation was processed by the petrophysics software developed byMainprice (1990) (available at following website: http://www.gm.univ-montp2.fr/PERSO/mainprice/W_data/CareWare_Unicef_Programs/). P-wave velocity anisotropy (AVp) was calculated as

= − + × ×AVp [(Vp Vp )/[(Vp Vp ) 0.5]] 100.max min max min (3)

S-wave velocity anisotropy (AVs) was calculated as

= − + × ×AVs [(Vs1 Vs2)/[(Vs1 Vs2) 0.5]] 100, (4)

where Vs1 and Vs2 represent fast and slow S-wave velocity, respec-tively.

4.2. Observation of dislocation microstructures

The oxygen decoration technique was applied to observe disloca-tions in the olivine (Jung and Karato, 2001b; Karato, 1987; Kohlstedtet al., 1976). Two representative samples, 1196-3 and 1194-2, werefashioned into chips and were heated in air for 1 h at 800 °C for oxi-dization. A thin oxide layer produced by the oxidation of the samplesurface was removed by SYTON (colloidal silica) polishing. The sampleswere then carbon-coated for the SEM operation. The dislocation mi-crostructures of olivine were captured in the backscattered electron(BSE) images by the JEOL JSM-6380 SEM at the SEES, SNU. The SEMworking conditions include 15 kV acceleration voltage, 20 nA beamcurrent, 10mm working distance, and 60 spot size.

The dislocation density of olivine was measured following a ‘firstprinciple’ method (Jung and Karato, 2001b; Karato and Lee, 1999):

∑=ρ l/V, (5)

where ∑l is the total length of dislocations in a volume (V). The dis-location density measurements of 24 grains of each sample wereaveraged to account for the heterogeneous distribution of dislocationdue to local stresses caused by grain-grain interactions.

4.3. Measurement of water content and identification of hydrous inclusionsby FTIR

Fourier transformation infrared (FTIR) spectroscopy was used to

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measure water content in olivine and to identify the hydrous inclusionsin the olivine and orthopyroxene in two representative specimens 1196-3 and 1194-2. Each sample was polished on both sides with a thicknessof 90 ± 15 μm and 100 ± 15 μm, respectively, for 1196-3 and 1194-2.The polished samples were soaked in acetone for 24 h to eliminateadhesives and were heated at 120 °C for 24 h afterward to dry waterexisting on the surface or along the grain boundary. The unpolarizedFTIR absorption spectra were obtained at room temperature and pres-sure with a Nicolet 6700 FTIR spectrometer equipped with a continuumIR microscope housed at the Tectonophysics Laboratory at SNU. Weused an unpolarized transmitted light with a KBr beam splitter and aMercury-Cadmium-Telluride (MCT) detector. Moisture in the air was

removed by N2 purging on the sample chamber. The aperture size wasset to 50×50 μm. Each FTIR spectrum, with a resolution of 4 cm−1,was averaged in a series of 128 scans to reduce noise. The water contentof olivine was measured by focusing the IR beam through olivineswithout any visible inclusions in an optical microscope. The watercontent of orthopyroxene was not measured because there were manycracks and inclusions. The calibration method of Paterson (1982) wasused to quantify the amount of water from IR absorptions in a wave-number range of 3400–3730 cm−1, dominated by OH stretching vi-brations. The average water contents of the samples were calculated bytaking the average measurements of five different olivines. In addition,micrometer-scale hydrous inclusions within olivine and orthopyroxene

Fig. 2. Optical photomicrograph showing micro-structures of peridotite samples from Bjørkedalen.(a), (b) Cross-polarized light image showing Ol-richsamples (1190 and 1193). (c) Ol porphyroclast em-bedded within the matrix of Ol and Opx (sample1190). (d) Plane-polarized light image showing dis-seminated chromites (Chr) occurring within Chl(sample 1193). (e) Elongated Ol and Amp alignedsubparallel to lineation (sample 1196-2). The imagewas captured with the λ-plate inserted. (f) ElongatedOl and Chl aligned subparallel to lineation (sample1194-2). The horizontal and vertical directions in allimages are parallel to lineation and normal to folia-tion of the specimen, respectively. Yellow arrowsindicate undulose extinction. Ol: olivine, Opx: or-thopyroxene, Amp: amphibole, and Chl: chlorite.(For interpretation of the references to colour in thisfigure legend, the reader is referred to the web ver-sion of this article.)

Table 1Modal composition, fabric type, and fabric strength of Bjørkedalen peridotites.

Sample Modal composition (%) Fabric type Fabric strengthc

Ol Amp Chl Opx Spl Ola Ampb Ol Amp Chl

M J M J M J

1190 99.4 0.0 0.2 0.3 0.1 A – 0.088 2.75 – – – –1193 97.7 0.1 1.0 1.0 0.2 Mixed – 0.039 1.75 – – – –1196-1 79.8 11.0 4.0 4.7 0.6 Mixed I 0.032 1.44 0.114 7.55 – –1196-2 58.5 40.8 0.2 0.0 0.5 Mixed I 0.067 3.55 0.179 9.97 – –1196-3 76.9 9.5 9.3 3.7 0.6 Mixed I 0.041 1.44 0.129 9.35 0.137 5.681194-1 87.5 10.4 2.0 0.0 0.1 B-like I 0.033 1.27 0.166 6.69 – –1194-2 77.5 10.2 9.9 2.1 0.3 B-like I 0.033 1.31 0.163 7.84 0.232 8.70

aFabric type of olivine (Jung and Karato, 2001a). bFabric type of amphibole (Ko and Jung, 2015). cFabric strength is shown as M-index (Skemer et al., 2005) and J-index (Bunge, 1982). J-index was calculated using a de la Vallee Poussin kernel and half-width of 10°. Mineral abbreviations are used after Whitney and Evans(2010).

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hosts were also identified by focusing the IR beam on the inclusion-richarea of the minerals.

4.4. Identification of small inclusions by using micro-Raman spectroscopy

Micrometer- to submicrometer-scale inclusions within olivine hostswere identified by using micro-Raman spectroscopy. A confocal dis-persive DXR Raman microscope equipped with an Ar laser with 532 nmwavelength and 10mW power housed at the Tectonophysics Laboratoryat SNU was used with a 50× objective lens and 0.7 μm laser size. Foreach Raman spectrum, each sample was exposed 64 times to reducenoise on the resulting spectrum.

4.5. Analysis of mineral compositions

The mineral compositions of the Bjørkedalen peridotites were ana-lyzed by using a SHIMADZU 1600 electron probe micro-analyzer(EPMA) at the Korea Basic Science Institute (KBSI) Jeonju Center. Majorconstituent elements of olivine, orthopyroxene, amphibole, chlorite,and spinel were quantitatively analyzed as oxide weight percentages.The EPMA was operated under 15 kV accelerating voltage, 20 nA beamcurrent, and 1 μm beam size conditions. The results were processedthrough Minpet 2.02 program (Richard, 1995) to obtain the chemicalformulas of the given minerals.

5. Results

5.1. Microstructure

The peridotites from Bjørkedalen contain porphyroclasts of olivineand orthopyroxene enclosed in a fine-grained matrix of olivine andorthopyroxene (Fig. 2a). The sizes of the olivine and orthopyroxeneporphyroclasts ranged up to 5000 μm, whereas those of recrystallizedolivine and orthopyroxene in the matrix were 100–200 μm. Some oli-vine porphyroclasts were highly strained with large aspect ratios andshowed evidence of undulose extinction (Fig. 2c); subgrain boundarieswere well developed indicating an intense dynamic recrystallization(Fig. 2b). The recrystallized olivines were tabular and slightly elongatedbecause of the deformation process. The amphibole existed as column-or needle-shaped crystals with a relatively large grain size (up to2000 μm at their long axis, Fig. 2e). The chlorite occurred as discreteflakes or ribbons (Fig. 2e, f) and was often associated with disseminatedfine-grained chromite (Fig. 2d). The amphibole and chlorite werealigned subparallel to the lineation defined by the elongated olivine(Fig. 2e, f). The grain size of chlorite or amphibole was large, up to2000 μm at their long axis, so that the pinning effect by those hydrousminerals appeared to be negligible.

5.2. Mineral chemistry

The representative compositions of the minerals of the Bjørkedalen

peridotites are summarized in the supplementary Tables S1–S5. Botholivine and orthopyroxene show very depleted compositions. The for-sterite in olivine is 92.4–93.1% and no zoning is observed. The ortho-pyroxene is enstatite with Mg number (Mg#) of 92.3–93.2 and lowAl2O3 contents (0.05–0.12 wt%), which gives high Cr number (Cr#:16.7–40.0). Two different varieties of amphibole are identified in theseperidotites based on classification of Hawthorne (1981). The first is richin CaO content and is defined as tremolite, whereas the second is re-latively rich in MgO and FeO contents and is classified as magnesio-cummingtonite. The chlorite composition varies from clinochlore topenninite. Its high MgO and low FeO contents, at 34–36wt% and1–3wt% respectively, result in high Mg# of 96.1–97.8. Spinel is char-acterized by its low Mg# and high Cr# at 16.3–34.6 and 84.4–90.8,respectively, and can be classified as chromite.

5.3. Dislocation microstructure of olivine

Dislocation microstructures of olivine in the two representativesamples showing mixed LPO (1196-3) and B type-like LPO (1194-2)were observed in the BSE images after oxygen decoration of the spe-cimens (Fig. 3). Dislocations were shown as bright dots and straight orcurved lines in the images. Both curved and straight dislocations wereobserved in sample 1196-3 (Fig. 3a), whereas straight dislocations weregenerally observed in sample 1194-2 (Fig. 3b). In both samples, dis-locations were distributed heterogeneously among grains as well as in asingle grain and were aligned in a line to form subgrain boundaries. Thedislocation density of sample 1196-3 was 4.6× 1011m−2, and that ofsample 1194-2 was 3.6×1011 m−2 (Table 2).

5.4. Temperature-pressure-stress estimation of specimens

The equilibrium temperatures of the Bjørkedalen peridotites werecalculated using the geothermometer which was based on the solubilityof Cr and Al in the orthopyroxene of the spinel peridotites (Witt-Eickschen and Seck, 1991). Equilibrium temperature estimates rangedfrom 650 °C to 700 °C. At these temperatures and taking into accountthe presence of the spinel, the equilibrium pressure range would be1.5–0.4 GPa (Gasparik, 1984; O'Neill, 1981). The differential stresses ofthe samples were estimated from (1) the recrystallized grain size of theolivine under wet and dry conditions (Jung and Karato, 2001b) and (2)the olivine dislocation density piezometer of Karato and Jung (2003).The two-dimensional (2-D) recrystallized grain size of the olivine wasmeasured by using the linear intercept method, and the three-dimen-sional (3-D) grain size was obtained by multiplying the 2-D size by 1.5(Gifkins, 1970). The recrystallized grain size of the olivine, rangingfrom 101 μm to 134 μm (Table 2), indicates that the studied peridotitesunderwent differential stress of 31–42 ± 15MPa in dry conditions orthat of 74–100 ± 15MPa in wet conditions. Stress estimates from thedislocation densities showed that samples 1196-3 and 1194-2 experi-enced stresses of 72 ± 20MPa and 61 ± 20MPa, respectively. Thestress estimates from the dislocation density of olivine were consistent

Fig. 3. Backscattered electron images showing dislocation microstructures of olivine in the two representative samples: (a) 1196-3 (mixed LPO) and (b) 1194-2 (Btype-like LPO). Bright lines and dots are dislocations in olivine.

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with the stress estimates from the recrystallized grain size of olivine andwere in agreement within an error bar (Table 2).

5.5. LPO of minerals

Most of the olivines showed weak LPOs. Representative LPOs ofolivine are shown in Fig. 4a–c (see all LPOs of olivine in Fig. S1a) andthey are classified as A type LPO, B type-like LPO, or mixed LPO of thetwo types. In sample 1190, the [100] axes of the olivine were alignedsubparallel to lineation, and the [010] axes subnormal to foliation,which is A type LPO. The [010] axes of olivine in the other two samples,

1194-1 and 1194-2, were aligned subnormal to foliation, and the [001]axes were subparallel to lineation, which is close to B type LPO (Jungand Karato, 2001a). Four other samples, 1193, 1196-1, 1196-2, and1196-3, showed an intermediate LPO between A type and B type, calledmixed LPO. The fabric strength (M-index) of olivine was low in therange of M=0.032–0.088 (J= 1.27–3.55, Table 1). There were dif-ferences in olivine fabric strength (M-index) between samples withvarying mineral mode. For example, the fabric strength of the sample(1190) with the highest content of olivine was the highest (M=0.09),whereas that of samples with less olivine contents were, in general, low(M=0.03–0.07) (Figs. 4a–c, S1a, and Table 1).

Table 2Recrystallized olivine grain size, dislocation density, water content of olivine, and estimated stress of Bjørkedalen peridotites.

Sample Recrystallized grain size of Ol (μm) Dislocation density of Ol (m−2) Water content of Ol (ppm H/Si)a Stress (MPa)

Dryb Wetc Dislocationd

1190 130 – – 32 ± 15 80 ± 15 –1193 101 – – 42 ± 15 100 ± 15 –1196-3 105 4.6×1011 600 ± 70 41 ± 15 92 ± 15 72 ± 201194-2 134 3.6×1011 520 ± 50 31 ± 15 74 ± 15 61 ± 20

aAverage water content is shown for five individual olivines (Section 5.7). Paterson calibration (1982) was used. bStress was estimated under a dry condition by usingrecrystallized grain-size piezometer (Jung and Karato, 2001b). cStress was estimated under a wet condition by using recrystallized grain-size piezometer (Jung andKarato, 2001b). dStress was estimated based on dislocation density (Karato and Jung, 2003).

Fig. 4. Representative pole figures of olivine (a–c), (d) amphibole, and (e) chlorite plotted in the lower hemisphere using equal area projection. Foliation is indicatedas a white line, and lineation is indicated as a red dot. A half scatter width of 20° was used. S: foliation, L: lineation, N: number of grains, and M: M-index. J-index ofsample is shown in Table 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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The LPOs of amphibole in the five samples were strong (Figs. 4d,S1b) and were characterized as the alignment of [001] axes subparallelto lineation, whereas the [100] axes were aligned subnormal to folia-tion. This LPO of amphibole corresponds to type I LPO, which wasproduced through the deformation of an amphibolite at high pressure,as part of a recent experimental study (Ko and Jung, 2015). The fabricstrength of amphibole was high in the range of M=0.114–0.179(Table 1, Fig. S1b). Chlorite LPOs in the two analyzed specimens were

very strong (Figs. 4e, S1c), with the [001] axes aligned subnormal tofoliation. Chlorite LPOs showed a girdle component on the [100] axes,(110) and (010) poles along the foliation. This LPO of chlorite is con-sistent with that found in chlorite peridotites from the Almklovdalen,southwestern Norway (Kim and Jung, 2015). The fabric strength ofchlorite was very high, in the range of M=0.137–0.232 (Table 1, Fig.S1c).

Fig. 5. Representative seismic anisotropy of olivine, amphibole, chlorite, and the entire rock (Ol+Amp+Chl) in the peridotite samples. The x- and z-directionsrepresent the lineation and the direction normal to the foliation, respectively. The P-wave velocity (Vp), S-wave anisotropy (AVs), and polarization direction of thefast shear wave (Vs1) are plotted.

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5.6. Seismic velocity and anisotropy

The seismic velocity and anisotropy of P- and S-waves calculated forolivine, amphibole, chlorite, and the entire rock are shown in Figs. 5,S2–S3, and the magnitude of the seismic anisotropies for each sample issummarized in Table 3. The olivine-rich sample (1190) with 99% oli-vine has a AVp of 7.5% and a max AVs of 5.2%. For the amphibole-richsample (1196-2) with 59% olivine and 41% amphibole, the AVp ofthese minerals is 5.5% and 15.2%, respectively, and the max AVs is4.4% and 11.9%, respectively (Fig. S2a, b). The AVp and the max AVsof the entire rock for this sample 1196-2 are 9.0% and 6.3%, respec-tively (Fig. S3). For the chlorite-rich sample (1194-2) with 78% olivine,10% amphibole, and 10% chlorite, the AVp of these minerals is 1.8%,14.0%, and 25.2%, respectively (Fig. 5); that of the entire rock is cal-culated to be 6.0% (Fig. 5) using the modal ratio of those three mi-nerals. The max AVs of olivine, amphibole, and chlorite in the sample(1194-2) is 1.1%, 9.5%, and 46.2%, respectively (Fig. 5); that of theentire rock is 8.3% (Fig. 5). Our data show that hydrous minerals,amphibole and chlorite, produce significantly higher seismic P- and S-wave anisotropies than olivine (Fig. 5, Table 3). We found that seismicanisotropy patterns of S-waves of amphibole and chlorite are similar(Fig. 5). The anisotropy of olivine seems to be nearly negligible whencompared to the anisotropy resulting from the amphibole and chlorite.

Fig. 6 shows the variation of P- and S-wave anisotropy with in-creasing modal content of both amphibole and chlorite in sample 1194-2 considering the LPOs of olivine, amphibole, and chlorite (Figs. 4c, d,e, S4). We chose sample 1194-2 because this sample contains highcontent of both amphibole (10.2%) and chlorite (9.9%) among theseven analyzed samples. The abundant hydrous mineral content mayreflect mineral assemblage in the hydrated portion of mantle wedge. Apositive linear relationship exists between the modal proportions of thehydrous minerals (amphibole and chlorite) and both AVp and AVs. Theamphibole has relatively higher values of AVp compared with the maxAVs (Fig. 6a), whereas the chlorite has much higher max AVs than AVp(Fig. 6b). The gap between AVp and max AVs increases as the content ofamphibole and chlorite increases. The values of AVp and max AVs as afunction of combined amphibole and chlorite content are shown inFig. 6c, assuming the same fraction of the two minerals. In the presenceof both amphibole and chlorite, the magnitudes of AVp and max AVsare between the results for amphibole (Fig. 6a) and chlorite (Fig. 6b).We also found an almost similar result for sample 1196-3, which has amixed LPO of olivine (Fig. S5).

The effect of dip angle of flow on the seismic anisotropy of P- and S-wave and the polarization direction of the fast S-wave (Vs1) was cal-culated by using the LPOs of olivine, amphibole, and chlorite in sample1194-2 (Figs. 7, S6–S9). For the horizontal flow (Fig. 7a), the magni-tude of P- and S-wave anisotropy for vertically propagating seismicwave is very low (AVs= 0.5% for olivine, 1.6% for amphibole, and 5%for chlorite) and the polarization direction of the Vs1 is subparallel to x-direction. When the dip angle of the subducting slab changed from 0° to

60°, the magnitude of P- and S-wave anisotropy for vertically propa-gating seismic waves became very high for amphibole (AVs= 7%) andchlorite (AVs=20%), and the Vs1 polarization direction of the verti-cally propagating S-wave changed from subparallel to x (trench-normal) to subparallel to y (trench-parallel) for amphibole, chlorite,and the entire rock (Ol+Amp+Chl) (Fig. 7b). However, the criticaldip angle of change for the Vs1 polarization direction differs amongminerals. Olivine shows little variation in the Vs1 polarization directionof vertically propagating S-waves when the dip angle of flow is changedfrom θ=45° to θ=60°, showing a consistent oblique Vs1 polarizationdirection tilted 45° to the trench (Fig. S6). For amphibole, the Vs1polarization direction of the vertically propagating S-waves changes totrench-parallel (y) direction at θ=45° (Fig. S7). However, that ofchlorite changes to trench-parallel at a little steeper flow dip angleθ=50° than that of amphibole (Fig. S8). The S-wave anisotropy patternand polarization direction of vertically propagating fast S-waves aresimilar, in general, in amphibole and chlorite when the dip angle offlow is high (50–60°) (Figs. 7b, S7–S8). The constructive effect of am-phibole and chlorite on those seismic properties was found in the entirerock at high dip angles of flow (50–60°) (Figs. 7b, S9).

The result of the same analysis on sample 1196-3 with mixed LPO of

Table 3Seismic anisotropy of Bjørkedalen peridotites. AVp: seismic anisotropy of P-wave; Max AVs: maximum seismic anisotropy of S-wave.

Sample Ol Amp Chl Entire rock

AVp (%) MaxAVs(%)

AVp (%) MaxAVs(%)

AVp (%) MaxAVs(%)

AVp (%) MaxAVs(%)

1190 7.5 5.22 – – – – 7.5 5.221193 4.4 2.98 – – – – 4.4 2.981196-1 2.8 1.97 10.0 9.45 – – 3.4 2.331196-2 5.5 4.35 15.2 11.91 – – 9.0 6.341196-3 4.6 2.87 11.5 7.54 22.3 31.56 7.4 6.701194-1 1.9 1.14 15.0 10.47 – – 3.3 1.931194-2 1.8 1.06 14.0 9.50 25.2 46.23 6.0 8.32

Fig. 6. Relationship between hydrous mineral content (%) and seismic aniso-tropy (%) using the lattice-preferred orientations (LPOs) of olivine, amphibole,and chlorite in the sample 1194-2. (a) Relationship between relative amphibolecontent (%) and seismic anisotropy (%) assuming a peridotite composed of thetwo minerals olivine and amphibole. (b) Relationship between chlorite content(%) and seismic anisotropy (%) assuming a peridotite composed of the twominerals olivine and chlorite. (c) Relationship between amphibole + chloritecontent (%) and seismic anisotropy (%) assuming a peridotite composed of thethree minerals olivine, amphibole, and chlorite. The amphibole and chloritecontents were assumed to be equal. AVp: P-wave anisotropy; Max AVs: max-imum S-wave anisotropy.

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olivine is shown in Figs. S10–S13. We found that the effect of hydrousminerals on the seismic anisotropy of the entire rock did not changedepending on types of olivine LPOs when we considered B type andmixed type. Mixed LPO of olivine in sample 1196-3 produced relativelyhigher seismic anisotropy (~2.0% AVs) for the vertically propagatingseismic wave (Fig. S10) than sample 1194-2 with B type-like LPO ofolivine (~0.5% AVs; Fig. S6). However, the polarization direction of theVs1 was subparallel to the trench-normal direction and did not changegreatly depending on the dip angle of the mantle flow (0–60°). Theeffect of the dip angle on the seismic anisotropy of amphibole andchlorite (Figs. S11–S12) were similar to those described in sample1194-2 (Figs. S7–S8), which produced a strong trench-parallel Vs1polarization direction in the high dip angles. The result also remainedthe same for the whole rock of sample 1196-3. Amphibole and chloritewere found to produce a constructive effect on the seismic anisotropiesof the whole rock at high dip angles (50–60°; Fig. S13). From the resultsof the above analysis, LPOs of hydrous minerals were found to producea dominant effect on the strong trench-parallel seismic anisotropy of theentire rock at a high dip angle (60°), whereas the contribution of olivineLPO was rather minor.

5.7. Measurement of water content and identification of hydrous inclusionsby FTIR

FTIR analysis was conducted on the two representative sampleswith mixed (1196-3) and B type-like (1194-2) LPO of olivine.Representative IR spectra of olivine without visible inclusions in theoptical microscope are shown in Fig. 8. IR absorption peaks of olivinewere found at and near the wavenumbers 3526, 3545, 3566, 3587,3613, 3619, 3628, 3649, 3670, 3675, 3689, and 3710 cm−1. The IRpeaks at 3526, 3545, 3566, 3587, 3613, 3619, 3628, and 3675 cm−1

were related to structurally bound –OH in olivine (Khisina et al., 2001;Kitamura et al., 1987; Kurosawa et al., 1997; Miller et al., 1987). Insample 1196‐3, olivine had a water content in the range of370–780 ppm H/Si and an average of 600 ± 70 ppm H/Si for five in-dividual olivines (Table 2). In sample 1194-2, olivine contained440–690 ppm H/Si with an average of 520 ± 50 ppm H/Si.

Hydrous inclusions were identified within olivine and orthopyr-oxene. Representative IR spectra of hydrous inclusions are shown inFigs. 9 and S14. The IR absorption bands were located at wavenumbers3690, 3689, 3688, 3687, 3685, 3684, 3682, 3677, 3673, 3662, and3661 cm−1. Peaks at 3690, 3689, 3688, 3687, 3685, and 3684 cm−1

Fig. 7. Effect of dip angle (θ) of flow on the seismic anisotropy of olivine, amphibole, chlorite, and entire rock considering the lattice-preferred orientations (LPOs) ofthe three minerals within sample 1194-2. (a) Seismic anisotropy for the horizontal flow. (b) Seismic anisotropy for the flow dipping at 60° to the east from thehorizontal flow. In the schematic diagram, seismic signatures from entire rock (sample 1194-2, Ol+Amp+Chl) are shown in the two different mantle flow regime.The blue and green arrows represent vertically propagating S-wave and the direction of mantle flow, respectively. The polarization direction of the fast S wave (Vs1)is shown as blue lines. The seismic signature of the center of the plot (surrounded by yellow circle) shows the direction of the vertically propagating S-wave. The x-and z-directions represent the lineation and the direction normal to the foliation, respectively. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

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were indicative of intrinsic hydroxide (–OH) in serpentine (Jung et al.,2013; Miller et al., 1987), whereas peaks at the wavenumbers 3677,3673, 3662, and 3661 cm−1 indicated the existence of amphibole ortalc (Miller et al., 1987; Skogby and Rossman, 1991).

5.8. Identification of small inclusions in olivine by using micro-Ramanspectroscopy

Raman analysis on the two representative samples 1196-3 (mixedLPO) and 1194-2 (B type-like LPO) revealed inclusions such as anti-gorite, anthophyllite, talc, and magnesite within the olivine hosts. Therepresentative micro-Raman spectra are shown in Figs. 10 and S15. Thecharacteristic Raman spectra of antigorite correspond to Raman shiftsat 688, 686, 681, 384, 381, 230, 229, 200, and 199 cm−1 (Fig. S15a, b,d) (Auzende et al., 2004; Jung et al., 2014; Rinaudo et al., 2003). Talcpeaks are positioned at the Raman shifts at 1051, 1049, 676, 673, 362,359, 194, 191, 111, and 108 cm−1 (Fig. S15b, c) (Fumagalli et al.,2001). Anthophyllite was also identified by spectra at 678, 676, 436,and 194 cm−1 (Fig. S15a, c) (Bard et al., 1997; Jung et al., 2013;Rinaudo et al., 2004). The peaks at 1095, 741, 327, and 212 cm−1

indicate the presence of magnesite (Fig. 10b) (Gillet, 1993). Those in-clusions are commonly produced by the serpentinization of olivine,indicating that substantial amounts of water (–OH) existed in Bjørke-dalen peridotites.

6. Discussion

6.1. Origin of the Bjørkedalen peridotites

The mineral chemistry of olivine and spinel in peridotites providesinformation on their origins (Arai, 1994; Clos et al., 2014). The high Mgnumber (92.4–93.1) in olivine and high Cr number (84.4–90.8) inspinel indicate that peridotites from Bjørkedalen have cratonic origin(Fig. S16). Previous studies on the peridotites in WGR revealed a mantlewedge origin from ultra-depleted Archean lithospheric mantle beneathLaurentia (Beyer et al., 2012). The extremely depleted chemical com-positions of the Bjørkedalen peridotites correspond strongly with thoseof the Almklovdalen dunites in the WGR (Beyer et al., 2006). Therefore,it can be interpreted that the Bjørkedalen peridotites are fragments ofLaurentian mantle that was trapped in the Baltic crust during the

Fig. 8. Fourier transformation infrared (FTIR) spectroscopy spectra of olivine without visible inclusions in the representative samples 1194-2 (B type-like LPO) and1196-3 (mixed LPO).

Fig. 9. Fourier transformation infrared(FTIR) spectroscopy spectra of olivineand orthopyroxene in the re-presentative sample 1194-2 (B type-like LPO). Wavenumbers shown in bluerepresent serpentine; those shown inred represent amphibole or talc. Whitesquares indicate the area where infra-red beam was transmitted and analyzedby FTIR. (For interpretation of the re-ferences to colour in this figure legend,the reader is referred to the web ver-sion of this article.)

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Scandian orogeny (425–380Ma).Several studies on the WGR peridotites showed that significant late

stage deformation and fluid infiltration occurred in their evolutionhistory. Carswell (1986) suggested seven stages of metamorphic evo-lution for Mg-Cr type peridotites in WGR. According to the study, theassemblage of Bjørkedalen peridotites corresponds to stage 6, and arerelated to strong late-stage deformation and increased water activity.Kostenko et al. (2002) also showed that the chlorite peridotites in WGRexperienced pervasive multistage fluid infiltration and deformationduring the late Caledonian uplift.

Although the origin of the Bjørkedalen peridotite is Archean, the P/T conditions, altered composition, and flow stresses of the Bjørkedalenperidotites inferred from this study imply that the peridotites wereretrograded and that their microstructures reflect late-stage deforma-tion. The temperature estimates (650–700 °C) were in agreement withthe peak metamorphic temperature, but the estimated pressure range(1.5–0.4 GPa) was lower than the eclogite isobar (> 2.4 GPa) in thearea (Fig. 1). This result could be in agreement with a decompressing P-T-t path. Abundant hydrous minerals contained in the Bjørkedalenperidotites (Fig. 9) imply the presence of water during retrograde me-tamorphism. The hydrous minerals amphibole and chlorite definedfoliation and lineation consistent with olivine, suggesting that thoseminerals were deformed at the same time. Under such a wet condition,the peridotites are considered to have experienced stress of74–100 ± 15MPa (Table 2) and resulted in deformation fabrics ofconstituent minerals. The deformation of olivine at temperature as lowas 650–700 °C appears to be ductile as shown in the deformation mi-crostructures in Fig. 2.

6.2. Mechanisms for fabric transition in olivine

The peridotites from Bjørkedalen show changes in olivine LPOsfrom A type to B type-like (Fig. 4). A type LPO of olivine is commonlyobserved in various upper mantle geodynamic environments such asophiolites, subduction zones, and kimberlites (Ben Ismail andMainprice, 1998). This fabric can be interpreted as a remnant from along residence in the mantle wedge under low-water and low-stressconditions. Previous studies have proposed several mechanisms for theolivine fabric transition from A type to B type LPO. Although an in-crease in confining pressure ≥3 GPa in dry olivine may induce suchfabric change (Jung et al., 2009b; Ohuchi et al., 2011; Soustelle andManthilake, 2017), no mineral index of high pressure peridotites, suchas garnet, diamond, or coesite were identified in the analyzed rocksamples. Indeed, our samples show a spinel-bearing assemblage that

requires equilibrium pressure< 1.5 GPa. Although the eclogite isobar(> 2.4 GPa) in the area may suggest deformation of the peridotitesunder high pressure conditions (> 3GPa), it is difficult to consider thatsuch pressure-induced fabrics can remain unchanged during significantlate-stage deformation (Carswell, 1986; Kostenko et al., 2002). There-fore, pressure-induced slip transition can be excluded for the me-chanism of B type-like fabric of the Bjørkedalen peridotites.

An additional explanation for B type fabric development is thediffusion creep mechanism enhanced by a large amount of secondaryphases such as pyroxene in the specimen (Jung et al., 2014; Sundbergand Cooper, 2008). However, this alternative seems to not apply for thestudied Bjørkedalen peridotites because (1) they contain a limitedamount of secondary phases (up to 23.1%, except for one sample (1196-2) with 41.5%); and (2), the high dislocation density observed in olivinefor the two representative samples with both mixed and B type-like LPO(Fig. 3), suggests that the olivine fabric changed from A type to B typein a dislocation creep regime.

Water is also an important factor for B type fabric development(Jung and Karato, 2001a; Jung et al., 2006). A large amount of evidencedemonstrates a water-rich environment in the studied samples. First,hydrous minerals such as amphibole and chlorite are distributed in thesamples as stable mineral assemblages. These minerals show consistentfoliation and lineation with olivine (Fig. 2e, f), indicating their con-temporary deformation. Second, a high amount of water was found inolivine (approximately 440–690 ppm H/Si, Fig. 8). The solubility ofwater in olivine is known to increase with pressure (Kohlstedt et al.,1996). Thus, the water content of olivines measured in this study couldbe less than the original content because the water could have been lostduring the exhumation process. Nevertheless, the water content de-termined in this study (440–690 ppm H/Si) is sufficient to form a B typefabric (Jung and Karato, 2001a). Third, numerous hydrous mineralssuch as serpentine, talc, and amphibole were detected within the oli-vine and orthopyroxene anhydrous minerals by FTIR spectroscopy(Fig. 9). Those hydrous inclusions are indicative of a hydrous en-vironment under which the Bjørkedalen peridotites were deformed andmetamorphosed. The Raman spectroscopic study on the samples iden-tified sub-micrometer-scale inclusions of antigorite, anthophyllite, talc,and magnesite within the olivine (Fig. 10). Our data suggest that water-induced fabric transition from A type to B type-like LPO of olivine oc-curred in the Bjørkedalen peridotites. Kim and Jung (2015) reportedwater-induced B type fabric of olivine in a neighboring chlorite peri-dotite in Almklovdalen, southwestern Norway. It is considered that thetwo adjacent peridotites underwent olivine fabric change with the si-milar mechanism.

Fig. 10. Representative micro-Raman spectra of host mineral (olivine) and inclusions in the sample showing B type-like LPO (1194-2). Ath: anthophyllite, Tlc: talc,Mgs: magnesite, and Atg: antigorite.

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Although the stress estimates of the Bjørkedalen peridotites are ra-ther low (Table 2), it is known that B type fabric of olivine can beformed under both high and low stress conditions depending on thetemperature. An initial experimental study by Jung and Karato (2001a)showed that B type olivine fabrics are formed under high stress con-ditions above 340MPa, at a high temperature of ~1470 °C. Anotherexperimental study by Katayama and Karato (2006) showed that B typeolivine fabrics can be formed under lower stress conditions than thosereported in Jung and Karato (2001a), when olivine was deformed underlow temperature conditions. Studies on natural peridotites have alsoreported B type olivine fabrics in low temperature and low stress con-ditions (Skemer et al., 2006). Considering the equilibrium temperatureof the Bjørkedalen peridotites (650–700 °C), the relatively low stressestimates of the peridotites (74–100 ± 15MPa under wet conditions)may induce the fabric change to the B type olivine (Katayama andKarato, 2006).

6.3. LPO and seismic anisotropy of amphibole and chlorite

Our data show that the LPO, fabric strength (M-index), and seismicanisotropy of both amphibole and chlorite differ considerably fromthose of olivine (Figs. 4, 5, and Tables 1, 3). The amphibole shows ahigher M-index (0.114–0.179) than that of olivine (0.032–0.088)(Table 1), resulting in strong seismic anisotropy (Table 3), with AVpvalues up to 15.2% and max AVs values up to 11.9% (sample 1196-2,Fig. S2b). In addition, the chlorite developed a very strong LPO; thehighest M-index (0.232), appeared in sample 1194-2 (Fig. 4e). Conse-quently, this chlorite produced the strongest seismic anisotropy, withAVp and max AVs of 25.2% and 46.2% (Table 3, Fig. 5). The M-indexand seismic anisotropy of chlorite found in this study is much higherthan those of chlorite (M=0.099, max AVs= 31.7%) reported inneighboring chlorite peridotites from Almklovdalen, Norway (Kim andJung, 2015).

The strong seismic anisotropy of amphibole and chlorite also de-pends on the intrinsic anisotropy of those minerals. Previous studiesreported that both P- and S-wave anisotropies of single crystal amphi-bole and chlorite (Mainprice and Ildefonse, 2009) are higher than thatof olivine. Ben Ismail and Mainprice (1998) reported AVp and max AVsvalues of 24.6% and 18.2%, respectively, in a single olivine crystal,while Ko and Jung (2015) recently reported AVp and max AVs values of27.1% and 30.7%, respectively, in a single hornblende crystal, and Kimand Jung (2015) reported AVp and max AVs values of 35.5% and76.2%, respectively, in a single chlorite crystal.

In this study, we also found that type I LPOs of amphibole producedhigher AVp than AVs, although the values are lower than those ofchlorite (Figs. 6, S4, and S5). The chlorite LPO found in this studyyielded the highest values of AVp and AVs among the three mineralswith significantly higher AVs than AVp values. The combined effect oftype I LPO of amphibole and LPO of chlorite on the S-wave anisotropyof the entire rock was constructive, as anticipated from their generallysimilar AVs patterns (Figs. 5 and S2). The constructive effects of hy-drous phases on seismic anisotropy were found to be associated withboth B type-like and mixed type LPO of olivine, which are the mostlikely LPO types in a wet mantle environment. The olivine LPO isprobably not an important factor in producing large seismic anisotropyin a hydrated mantle wedge if the LPO of those hydrous phases arepresent at least above a threshold volume fraction.

In addition, we found that the presence of amphibole and chlorite inthe peridotites can contribute to change the polarization direction ofthe Vs1 (Fig. 7). The dip angle of flow can reflect the angle of thedownward movement of the mantle wedge, considering 2-dimensionalcorner flow. Our results may be an oversimplification to represent allseismic anisotropy for the slabs that are subducting at different angleswhere complex corner flows and more complicated LPO would develop(Faccenda and Capitanio, 2012; Li et al., 2014). Additionally, de-formation history can also play an important role in LPO evolution

(Boneh et al., 2015) and the real scenario is expected to be much morecomplicated than presented here. However, our simplified model of 2-Dcorner flow can contribute to first-order understanding of seismic ani-sotropy in the subduction zone.

For a horizontal flow, olivine with B type-like fabric (1194-2)showed a Vs1 polarization direction subparallel to lineation or flowdirection (trench-normal) for a vertically propagating S-wave (Fig. 7a).This result is contrary to those reported in previous studies showing atrench-parallel Vs1 polarization direction of vertically propagating S-waves for the B type olivine LPO (Jung and Karato, 2001a; Lee andJung, 2015). This disparity is attributed to the weak scattered LPO of[100] axes of olivine in the sample 1194-2 compared with typical Btype olivine LPOs that show strong concentration of [100] axes in thedirection normal to lineation on the foliation (Jung et al., 2006). Al-though the trench-normal polarization direction of the Vs1 for olivinechanged gradually to trench-parallel by increasing the dip angle fromθ=45° to θ=60°, the magnitude of the AVs of olivine remained verysmall, at ~0.3% at θ=60° (Fig. S6). Thus, olivine alone in this case isunable to contribute to the strong trench-parallel seismic anisotropyobserved in subduction zones.

Amphibole and chlorite produced strong trench-parallel seismicanisotropy in high dip angles> 45° for amphibole and>50° forchlorite (Figs. S7–S8). The angle calculated in this paper, in which theVs1 polarization direction of the vertically propagating S-wave changesto trench-parallel, is nearly consistent with the results of previous stu-dies on amphibole deformed at high pressure (Ko and Jung, 2015) andchlorite in chlorite peridotite (Kim and Jung, 2015). The change in theVs1 polarization direction was more dramatic for chlorite than am-phibole when the dip angle changed (Figs. S7–S8). At high dip angles offlow, the overall S-wave anisotropy pattern and polarization directionof vertically propagating fast S-wave became similar for both amphi-bole and chlorite.

The constructive effect of seismic signatures of amphibole andchlorite is reflected in the S-wave anisotropy pattern and polarizationdirection of vertically propagating fast S-wave of the entire rock (Fig.S9). The trench-normal component of the Vs1 polarization direction ofthe vertically propagating S-wave, shown in the dip angle of up to 50°,may reflect that of olivine, the most abundant mineral at ~80%. Inaddition, the magnitude of the AVs of the vertically propagating S-waves for the entire rock changed from small at ~1% at θ=0° to largeat ~3% at θ=60° with an increase in the dip angle (Fig. S9). Themagnitude of the AVs at θ=60°, ~3%, is significantly higher than thatof ~0.3% in the olivine-only case.

6.4. Implications for seismic anisotropy in subduction zones

Although the Bjørkedalen peridotites represent retrograde meta-morphic conditions and late-stage deformation, it is known that bothamphibole and chlorite are stable in broad P-T conditions (amphiboleup to 2.5 GPa and 900 °C, and chlorite up to 6.0 GPa and 700 °C) in thehydrated mantle wedge (Fumagalli and Poli, 2005; Mainprice andIldefonse, 2009; Pawley, 2003). It is also believed that the way in whichthe lattice preferred orientation of constituent minerals are formed andthe resultant microstructures are similar regardless of whether theperidotites are deformed in the prograde stage or retrograde stage.Furthermore, prograde peridotites present at the depth of the mantleare difficult to obtain because core drilling to such depth is very costly(Teagle and Ildefonse, 2011). In their place, retrograded peridotites arecommonly used to study and to infer deformed microstructures andseismic anisotropy in the mantle (Kim and Jung, 2015; Skemer et al.,2006). Therefore, the mineral fabrics of the retrograded Bjørkedalenperidotites can be considered analogous to those deformed in the en-vironment of a hydrated mantle wedge. The olivine fabrics in Bjørke-dalen peridotites dominantly show mixed and B type-like LPOs, whichis consistent with that expected in hydrous mantle environments (Jungand Karato, 2001a). Similarly, the amphibole and chlorite LPO found in

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this study is also expected to exist as a similar type within wet mantleconditions. Despite current limitations, we expect future work on hy-drous peridotites directly taken from the hydrated mantle wedge willprovide a more accurate understanding of LPO development of mi-nerals.

In many subduction zones, a range of delay times of S waves hasbeen reported (Long and Silver, 2008). However, anomalously longdelay times (1–4 s) observed in several subduction zones, such asRyukyu, Izu-Bonin, and Tonga-Kermadec arc (Anglin and Fouch, 2005;Greve et al., 2008; Long and van der Hilst, 2006; Smith et al., 2001)raise considerable questions on the thickness of the anisotropic layerbecause the magnitude of AVs of olivine is known to be relatively small(~5%) (Jung and Karato, 2001a). Alternatively, LPO of hydrous phasessuch as serpentine, amphibole and chlorite can be the source of theobserved strong anisotropy. Among the hydrous phases, serpentine isstable at cold mantle conditions, which is likely in Izu-Bonin and Tonga,thus it has limitations in explaining the strong seismic anisotropy ob-served in a warm mantle condition. Instead, LPO of amphibole andchlorite, which are stable at high temperature in the mantle wedge(Fumagalli and Poli, 2005), may explain the long delay time of S-wavesobserved in warm mantle conditions, such as in the Ryukyu arc. Ourstudy showed that formation of LPO of amphibole and chlorite con-tributes constructively to produce strong seismic anisotropy. Therefore,the combined effect of deformed olivine, amphibole, and chlorite in ahydrated mantle wedge may provide a likely explanation for the in-terpretation of anomalously strong seismic anisotropy observed inwarm mantle conditions.

Fig. 11 shows the relationship between the delay time (dt) and thethickness of the anisotropic layer (L) applying the AVs and<Vs>values calculated from both experimentally deformed B type olivine at1.9 GPa (Jung and Karato, 2001a) and constituent minerals of the re-presentative sample 1194‐2 (olivine, amphibole, and chlorite). Therelationship between dt and L was calculated using the followingequation (Silver and Chan, 1991),

= × < >dt L AVs/ Vs (6)

where<Vs> stands for (Vs1+Vs2) / 2, an average of fast and slowsplit S-wave velocities. When we consider the previously reported value

of max AVs of the experimentally deformed B type olivine, an aniso-tropic layer thickness of 100–200 km, which is unrealistically thick, isneeded to explain the long delay time (1–2 s) observed in the Ryukyuarc (Long and van der Hilst, 2006). The required anisotropic layerthickness decreases to 40–75 km and 5–15 km when the seismic ani-sotropy of amphibole and chlorite is considered, respectively (Fig. 11).The combined effect of deformed olivine, amphibole, and chlorite insample 1194-2, with respective modal composition of 77.5%, 10.2%,and 9.9%, requires an anisotropic layer thickness of 50–110 km toproduce a delay time of 1–2 s. Our result show that only ~20% of thecombined amount of amphibole and chlorite reduce the layer thicknessinto half the thickness required for the olivine experimental data. Al-though the 50–110 km thick hydrated mantle layer would be an over-estimation considering the relatively dry set of peridotite xenoliths fromsubduction zones, higher concentration of those minerals in a relativelythin, localized hydrated layer may partly explain the long delay time ofS waves observed in subduction zones.

A number of magnetotelluric studies have reported anomalouslyhigh electrical conductivities of up to 1 S/m in a mantle wedge ofsubduction settings that cannot be solely explained by the presence ofhydrated olivine and pyroxene (Evans et al., 2014; McGary et al., 2014;Soyer and Unsworth, 2006; Worzewski et al., 2011). Experimentalstudies on electrical conductivity suggest that the high conductivityanomalies in these regions can be explained by the presence of freefluids released by the dehydration of hydrous minerals, including am-phibole (Wang et al., 2012) and chlorite (Manthilake et al., 2016). Suchevidence further indicates that hydrous minerals are very important inunderstanding the mechanical properties of a subduction zone mantlewedge.

The trench-parallel seismic anisotropy observed in several subduc-tion zones can be attributed to the presence of B type LPO of olivinecombined with the LPOs of amphibole and chlorite in a hydratedmantle wedge. McCormack et al. (2013) suggested B type olivine fabricof mantle wedge to be a possible source of seismic anisotropy observedin the Ryukyu arc. The seismic anisotropy calculated from olivine withB type fabric supports the interpretation. Wagner et al. (2013) reporteda trench-parallel seismic anisotropy in a localized region of Cascadiasubduction zone and suggested chlorite to be the most probable mantlehydrous phase to be stable and cause such an anisotropy there. It isinteresting to note that dip angle of mantle flow in the region is high,consistent with the result of this study that shows the trench-parallelseismic anisotropy of chlorite for a high dip angle of mantle flow (Fig.S8). In addition, our study suggests that formation of type I LPO ofamphibole could also be another source producing trench-parallelseismic anisotropy in Cascadia, based on its similar dip angle de-pendency with chlorite, in which the polarization direction of fast S-wave changes from trench-normal to trench-parallel in high dip flow(Fig. S7). Trench-parallel anisotropy was also reported in Tonga andMariana (Pozgay et al., 2007; Smith et al., 2001), where slabs aredipping at a high angle. In contrast, a trench-normal seismic anisotropywas observed in a shallowly dipping slab (θ < 45°) in northeast Japan(Huang et al., 2011). Above mentioned observations are further sup-ported by the result of this study which showed the dip angle depen-dence of polarization direction of seismic anisotropy of a hydratedmantle peridotite containing olivine, amphibole, and chlorite (Fig. S9).

Overall, our results showed that the existence of strong LPOs inhydrous minerals such as amphibole and chlorite produced strongseismic anisotropy in hydrated mantle peridotites. The constructiveeffect of combined LPO of amphibole and chlorite implies that thestrong seismic anisotropy can be produced by the LPOs of those hydrousminerals in the mantle wedge where water is transported by the de-hydration of the subducting slab. A subduction zone where seismicanisotropy appears to be weak may potentially be associated with lesshydration of the mantle wedge. The strong S-wave anisotropy observedin many subduction zones (Long and Silver, 2008; McCormack et al.,2013; Smith et al., 2001) can be at least partly attributed to the strong

Fig. 11. Relationship between delay time and thickness of anisotropic layer ofolivine (experimental data), amphibole, chlorite, and entire rock(Ol+Amp+Chl; sample 1194-2). The shaded area shows the long delay time(1–2 s) observed in the Ryukyu arc (Long and van der Hilst, 2006).

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LPOs of hydrous minerals, amphibole and/or chlorite.

7. Conclusion

Change in olivine LPO from A type to B type-like was observed inBjørkedalen peridotites. High water content in olivine (440–690 ppmH/Si) and many hydrous inclusions found in olivine and orthopyroxenesuggest that olivine fabric was changed from A type to B type-like LPOdue to water. The samples contain varying amounts of amphibole andchlorite at 0–41% and 0–10%, respectively, in modal composition.Amphibole LPOs were characterized by [001] axes aligned subparallelto lineation and [100] axes running subnormal to foliation (type I LPO).Chlorite LPOs were characterized by [100] axes, (110) and (010) poleshaving a girdle component along the foliation, and [001] axes stronglyaligned subnormal to foliation. The hydrated mantle peridotites showedweak LPO of olivine but strong LPOs of amphibole and chlorite.

The calculated seismic anisotropy of the studied peridotites showedthat the amphibole and chlorite in the peridotite had a profound effecton the seismic anisotropy of the entire rock. In this study, it is foundthat the actual LPO of olivine is probably not that important for theresulting seismic anisotropy when hydrous phases are present and whatreally matters is LPO of phases like amphibole and chlorite, at leastabove a threshold volume fraction. The P- and S-wave anisotropiesreached respective values of 15.2% and 11.9% for amphibole and up to25.2% and 46.2% for chlorite. In addition, both amphibole and chloritein the hydrated peridotites changed the polarization direction of thefast S-wave (Vs1) for vertically propagating S-waves from trench-normal to trench-parallel, depending on the dip angle of the mantleflow. It was found that the LPO of amphibole and chlorite can con-tribute to seismic anisotropy of the entire rock constructively. It isconcluded that although olivine is the major mineral in the peridotites,relatively small amounts of amphibole and chlorite, at ~10%, cancontribute significantly to the seismic anisotropy of the entire rock in asubduction zone. A more complex numerical simulation should becarried out to access anisotropy near a subducting slab in a more rea-listic case (Li et al., 2014; Nagaya et al., 2016). However, our simplifiedpresentation of corner flow in a subduction zone may provide a basis fora further understanding of the effect of hydrous minerals on seismicanisotropy in a hydrated mantle wedge.

Acknowledgments

H.J. thanks Prof. Håkon Austrheim and Prof. Youngwoo Kil for theirkind help in the field at Bjørkedalen in Norway. The authors also thankmembers of the Tectonophysics laboratory of the SEES at SNU for theirassistances. Three anonymous reviewers and editor are acknowledgedfor insightful and constructive comments on this manuscript. This re-search was supported by the Mid-career Research Program throughNational Research Foundation of Korea (NRF: 2017R1A2B2004688) toHaemyeong Jung.

Declarations of interest

‘None’.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.tecto.2018.11.011.

References

Abramson, E., Brown, J., Slutsky, L., Zaug, J., 1997. The elastic constants of San Carlosolivine to 17 GPa. J. Geophys. Res. Solid Earth 102, 12253–12263.

Aleksandrov, K.S., Ryzhova, T.V., 1961. The elastic properties of rock forming minerals,pyroxenes and amphiboles. Bull. Acad. Sci. USSR Geophys. Ser 9, 871–875.

Almqvist, B.S., Mainprice, D., 2017. Seismic properties and anisotropy of the continentalcrust: predictions based on mineral texture and rock microstructure. Rev. Geophys.55, 367–433.

Anglin, D.K., Fouch, M.J., 2005. Seismic anisotropy in the Izu-Bonin subduction system.Geophys. Res. Lett. 32.

Arai, S., 1994. Characterization of spinel peridotites by olivine-spinel compositional re-lationships: review and interpretation. Chem. Geol. 113, 191–204.

Auzende, A.-L., Daniel, I., Reynard, B., Lemaire, C., Guyot, F., 2004. High-pressure be-haviour of serpentine minerals: a Raman spectroscopic study. Phys. Chem. Miner. 31,269–277.

Bard, D., Yarwood, J., Tylee, B., 1997. Asbestos fibre identification by Raman micro-spectroscopy. J. Raman Spectrosc. 28, 803–809.

Behr, W.M., Smith, D., 2016. Deformation in the mantle wedge associated with Laramideflat-slab subduction. Geochem. Geophys. Geosyst. 17, 2643–2660.

Ben Ismail, W., Mainprice, D., 1998. An olivine fabric database: an overview of uppermantle fabrics and seismic anisotropy. Tectonophysics 296, 145–157.

Beyer, E.E., Griffin, W.L., O'Reilly, S.Y., 2006. Transformation of Archaean lithosphericmantle by refertilization: evidence from exposed peridotites in the Western GneissRegion, Norway. J. Petrol. 47, 1611–1636.

Beyer, E.E., Brueckner, H.K., Griffin, W.L., O'Reilly, S.Y., 2012. Laurentian provenance ofArchean mantle fragments in the Proterozoic Baltic crust of the NorwegianCaledonides. J. Petrol. 53, 1357–1383.

Boneh, Y., Morales, L.F., Kaminski, E., Skemer, P., 2015. Modeling olivine CPO evolutionwith complex deformation histories: implications for the interpretation of seismicanisotropy in the mantle. Geochem. Geophys. Geosyst. 16, 3436–3455.

Brueckner, H.K., 1998. Sinking intrusion model for the emplacement of garnet-bearingperidotites into continent collision orogens. Geology 26, 631–634.

Brueckner, H.K., Cuthbert, S.J., 2013. Extension, disruption, and translation of an oro-genic wedge by exhumation of large ultrahigh-pressure terranes: examples from theNorwegian Caledonides. Lithosphere 5, 277–289.

Bunge, H.-J., 1982. Texture Analysis in Materials Science: Mathematical Models.Butterworths, London.

Bystricky, M., Kunze, K., Burlini, L., Burg, J.-P., 2000. High shear strain of olivine ag-gregates: rheological and seismic consequences. Science 290, 1564–1567.

Cao, Y., Jung, H., Song, S., Park, M., Jung, S., Lee, J., 2015. Plastic deformation andseismic properties in fore-arc mantles: a petrofabric analysis of the Yushigou harz-burgites, North Qilian suture zone, NW China. J. Petrol. 56, 1897–1944.

Cao, Y., Song, S., Su, L., Jung, H., Niu, Y., 2016. Highly refractory peridotites inSongshugou, Qinling orogen: insights into partial melting and melt/fluid–rock reac-tions in forearc mantle. Lithos 252, 234–254.

Cao, Y., Jung, H., Song, S., 2017. Olivine fabrics and tectonic evolution of fore-arcmantles: a natural perspective from the Songshugou dunite and harzburgite in theQinling orogenic belt, central China. Geochem. Geophys. Geosyst. 18, 907–934.

Carswell, D., 1986. The metamorphic evolution of Mg-Cr type Norwegian garnet peri-dotites. Lithos 19, 279–297.

Carter, N.L., Ave'Lallemant, H.G., 1970. High temperature flow of dunite and peridotite.Geol. Soc. Am. Bull. 81, 2181–2202.

Clos, F., Gilio, M., van Roermund, H.L.M., 2014. Fragments of deeper parts of the hangingwall mantle preserved as orogenic peridotites in the central belt of the Seve NappeComplex, Sweden. Lithos 192, 8–20.

Cordellier, F., Boudier, F., Boullier, A., 1981. Structural study of the Almklovdalenperidotite massif (southern Norway). Tectonophysics 77, 257–281.

Demouchy, S., Tommasi, A., Barou, F., Mainprice, D., Cordier, P., 2012. Deformation ofolivine in torsion under hydrous conditions. Phys. Earth Planet. Inter. 202, 56–70.

Evans, R.L., Wannamaker, P.E., McGary, R.S., Elsenbeck, J., 2014. Electrical structure ofthe central Cascadia subduction zone: the EMSLAB Lincoln Line revisited. EarthPlanet. Sci. Lett. 402, 265–274.

Faccenda, M., Capitanio, F., 2012. Development of mantle seismic anisotropy duringsubduction-induced 3-D flow. Geophys. Res. Lett. 39.

Frese, K., Trommsdorff, V., Kunze, K., 2003. Olivine [100] normal to foliation: latticepreferred orientation in prograde garnet peridotite formed at high H2O activity, Cimadi Gagnone (Central Alps). Contrib. Mineral. Petrol. 145, 75–86.

Fumagalli, P., Poli, S., 2005. Experimentally determined phase relations in hydrousperidotites to 6.5 GPa and their consequences on the dynamics of subduction zones. J.Petrol. 46, 555–578.

Fumagalli, P., Stixrude, L., Poli, S., Snyder, D., 2001. The 10 Å phase: a high-pressureexpandable sheet silicate stable during subduction of hydrated lithosphere. EarthPlanet. Sci. Lett. 186, 125–141.

Gasparik, T., 1984. Two-pyroxene thermobarometry with new experimental data in thesystem CaO-MgO-Al2O3-SiO2. Contrib. Mineral. Petrol. 87, 87–97.

Gifkins, R.C., 1970. Optical Microscopy of Metals. American Elsevier.Gillet, P., 1993. Stability of magnesite (MgCO3) at mantle pressure and temperature

conditions-a Raman-spectroscopic study. Am. Mineral. 78, 1328–1331.Goetze, C., 1978. The mechanisms of creep in olivine. Phil. Trans. R Soc. Lond. A 288,

99–119.Greve, S.M., Savage, M.K., Hofmann, S.D., 2008. Strong variations in seismic anisotropy

across the Hikurangi subduction zone, North Island, New Zealand. Tectonophysics462, 7–21.

Hacker, B.R., Andersen, T.B., Johnston, S., Kylander-Clark, A.R.C., Peterman, E.M.,Walsh, E.O., Young, D., 2010. High-temperature deformation during continental-margin subduction & exhumation: the ultrahigh-pressure Western Gneiss Region ofNorway. Tectonophysics 480, 149–171.

Hansen, L.N., Zhao, Y.-H., Zimmerman, M.E., Kohlstedt, D.L., 2014. Protracted fabricevolution in olivine: implications for the relationship among strain, crystallographicfabric, and seismic anisotropy. Earth Planet. Sci. Lett. 387, 157–168.

Hawthorne, F.C., 1981. Crystal chemistry of the amphiboles. Rev. Mineral. Geochem. 9,

H. Kang, H. Jung Tectonophysics 750 (2019) 137–152

150

1–102.Holtzman, B., Kohlstedt, D., Zimmerman, M., Heidelbach, F., Hiraga, T., Hustoft, J., 2003.

Melt segregation and strain partitioning: implications for seismic anisotropy andmantle flow. Science 301, 1227–1230.

Huang, Z., Zhao, D., Wang, L., 2011. Shear wave anisotropy in the crust, mantle wedge,and subducting Pacific slab under northeast Japan. Geochem. Geophys. Geosyst. 12.

Ji, S., Shao, T., Michibayashi, K., Long, C., Wang, Q., Kondo, Y., Zhao, W., Wang, H.,Salisbury, M.H., 2013. A new calibration of seismic velocities, anisotropy, fabrics,and elastic moduli of amphibole-rich rocks. J. Geophys. Res. Solid Earth 118,4699–4728.

Ji, S., Shao, T., Michibayashi, K., Oya, S., Satsukawa, T., Wang, Q., Zhao, W., Salisbury,M.H., 2015. Magnitude and symmetry of seismic anisotropy in mica-and amphibole-bearing metamorphic rocks and implications for tectonic interpretation of seismicdata from the southeast Tibetan Plateau. J. Geophys. Res. Solid Earth 120,6404–6430.

Jung, H., 2009. Deformation fabrics of olivine in Val Malenco peridotite found in Italyand implications for the seismic anisotropy in the upper mantle. Lithos 109, 341–349.

Jung, H., 2017. Crystal preferred orientations of olivine, orthopyroxene, serpentine,chlorite, and amphibole, and implications for seismic anisotropy in subduction zones:a review. Geosci. J. 21, 985–1011.

Jung, H., Karato, S., 2001a. Water-induced fabric transitions in olivine. Science 293,1460–1463.

Jung, H., Karato, S., 2001b. Effects of water on dynamically recrystallized grain-size ofolivine. J. Struct. Geol. 23, 1337–1344.

Jung, H., Katayama, I., Jiang, Z., Hiraga, T., Karato, S., 2006. Effect of water and stress onthe lattice-preferred orientation of olivine. Tectonophysics 421, 1–22.

Jung, H., Mo, W., Choi, S., 2009a. Deformation microstructures of olivine in peridotitefrom Spitsbergen, Svalbard and implications for seismic anisotropy. J. Metamorph.Geol. 27, 707–720.

Jung, H., Mo, W., Green, H.W., 2009b. Upper mantle seismic anisotropy resulting frompressure-induced slip transition in olivine. Nat. Geosci. 2, 73–77.

Jung, H., Lee, J., Ko, B., Jung, S., Park, M., Cao, Y., Song, S., 2013. Natural type-C olivinefabrics in garnet peridotites in North Qaidam UHP collision belt, NW China.Tectonophysics 594, 91–102.

Jung, S., Jung, H., Austrheim, H., 2014. Characterization of olivine fabrics and mylonitein the presence of fluid and implications for seismic anisotropy and shear localiza-tion. Earth Planets Space 66, 46.

Kaczmarek, M.-A., Reddy, S.M., Nutman, A.P., Friend, C.R., Bennett, V.C., 2016. Earth'soldest mantle fabrics indicate Eoarchaean subduction. Nat. Commun. 7.

Karato, S., 1987. Scanning electron microscope observation of dislocations in olivine.Phys. Chem. Miner. 14, 245–248.

Karato, S., Jung, H., 2003. Effects of pressure on high-temperature dislocation creep inolivine. Philos. Mag. 83, 401–414.

Karato, S., Lee, K., 1999. Stress-strain distribution in deformed olivine aggregates: in-ference from microstructural observations and implications for texture development.In: Proceedings of the Twelfth International Conference on Textures of Materials, pp.1546–1555 ITOCOM-12.

Karato, S., Jung, H., Katayama, I., Skemer, P., 2008. Geodynamic significance of seismicanisotropy of the upper mantle: new insights from laboratory studies. Annu. Rev.Earth Planet. Sci. 36, 59–95.

Katayama, I., Karato, S., 2006. Effect of temperature on the B-to C-type olivine fabrictransition and implication for flow pattern in subduction zones. Phys. Earth Planet.Inter. 157, 33–45.

Katayama, I., Jung, H., Karato, S., 2004. New type of olivine fabric from deformationexperiments at modest water content and low stress. Geology 32, 1045–1048.

Katayama, I., Karato, S., Brandon, M., 2005. Evidence of high water content in the deepupper mantle inferred from deformation microstructures. Geology 33, 613.

Khisina, N., Wirth, R., Andrut, M., Ukhanov, A., 2001. Extrinsic and intrinsic mode ofhydrogen occurrence in natural olivines: FTIR and TEM investigation. Phys. Chem.Miner. 28, 291–301.

Kim, D., Jung, H., 2015. Deformation microstructures of olivine and chlorite in chloriteperidotites from Almklovdalen in the Western Gneiss Region, southwest Norway, andimplications for seismic anisotropy. Int. Geol. Rev. 57, 650–668.

Kitamura, M., Kondoh, S., Morimoto, N., Miller, G.H., Rossman, G.R., Putnis, A., 1987.Planar OH-bearing defects in mantle olivine. Nature 328, 143.

Ko, B., Jung, H., 2015. Crystal preferred orientation of an amphibole experimentallydeformed by simple shear. Nat. Commun. 6, 6586.

Kohlstedt, D., Goetze, C., Durham, W., Vander Sande, J., 1976. New technique for dec-orating dislocations in olivine. Science 191, 1045–1046.

Kohlstedt, D., Keppler, H., Rubie, D., 1996. Solubility of water in the α, β and γ phases of(Mg, Fe)2 SiO4. Contrib. Mineral. Petrol. 123, 345–357.

Kostenko, O., Jamtveit, B., Austrheim, H., Pollok, K., Putnis, C., 2002. The mechanism offluid infiltration in peridotites at Almklovdalen, western Norway. Geofluids 2,203–215.

Krogh, E.J., 1977. Evidence of Precambrian continent–continent collision in WesternNorway. Nature 267, 17.

Kurosawa, M., Yurimoto, H., Sueno, S., 1997. Patterns in the hydrogen and trace elementcompositions of mantle olivines. Phys. Chem. Miner. 24, 385–395.

Kylander-Clark, A., Hacker, B., Mattinson, J., 2008. Slow exhumation of UHP terranes:titanite and rutile ages of the Western Gneiss Region, Norway. Earth Planet. Sci. Lett.272, 531–540.

Lapen, T.J., Medaris, L.G., Beard, B.L., Johnson, C.M., 2009. The Sandvik peridotite,Gurskøy, Norway: three billion years of mantle evolution in the Baltica lithosphere.Lithos 109, 145–154.

Lee, J., Jung, H., 2015. Lattice-preferred orientation of olivine found in diamond-bearinggarnet peridotites in Finsch, South Africa and implications for seismic anisotropy. J.

Struct. Geol. 70, 12–22.Li, Z.H., Di Leo, J.F., Ribe, N.M., 2014. Subduction-induced mantle flow, finite strain, and

seismic anisotropy: numerical modeling. J. Geophys. Res. Solid Earth 119,5052–5076.

Linckens, J., Herwegh, M., Müntener, O., Mercolli, I., 2011. Evolution of a polymineralicmantle shear zone and the role of second phases in the localization of deformation. J.Geophys. Res. Solid Earth 1978–2012, 116.

Liu, W., Zhang, J., Barou, F., 2018. B-type olivine fabric induced by low temperaturedissolution creep during serpentinization and deformation in mantle wedge.Tectonophysics 722, 1–10.

Long, M.D., 2013. Constraints on subduction geodynamics from seismic anisotropy. Rev.Geophys. 51, 76–112.

Long, M.D., Silver, P.G., 2008. The subduction zone flow field from seismic anisotropy: aglobal view. Science 319, 315–318.

Long, M.D., van der Hilst, R.D., 2006. Shear wave splitting from local events beneath theRyukyu arc: trench-parallel anisotropy in the mantle wedge. Phys. Earth Planet. Inter.155, 300–312.

Mainprice, D., 1990. A FORTRAN program to calculate seismic anisotropy from the latticepreferred orientation of minerals. Comput. Geosci. 16, 385–393.

Mainprice, D., Ildefonse, B., 2009. Seismic Anisotropy of Subduction ZoneMinerals–Contribution of Hydrous Phases, Subduction Zone Geodynamics. Springer,pp. 63–84.

Manthilake, G., Bolfan-Casanova, N., Novella, D., Mookherjee, M., Andrault, D., 2016.Dehydration of chlorite explains anomalously high electrical conductivity in themantle wedges. Sci. Adv. 2, e1501631.

McCormack, K., Wirth, E.A., Long, M.D., 2013. B-type olivine fabric and mantle wedgeserpentinization beneath the Ryukyu arc. Geophys. Res. Lett. 40, 1697–1702.

McGary, R.S., Evans, R.L., Wannamaker, P.E., Elsenbeck, J., Rondenay, S., 2014. Pathwayfrom subducting slab to surface for melt and fluids beneath Mount Rainier. Nature511, 338.

Mehl, L., Hacker, B.R., Hirth, G., Kelemen, P.B., 2003. Arc-parallel flow within the mantlewedge: evidence from the accreted Talkeetna arc, south central Alaska. J. Geophys.Res. Solid Earth 1978–2012, 108.

Michibayashi, K., Oohara, T., 2013. Olivine fabric evolution in a hydrated ductile shearzone at the Moho Transition Zone, Oman Ophiolite. Earth Planet. Sci. Lett. 377,299–310.

Miller, G.H., Rossman, G.R., Harlow, G.E., 1987. The natural occurrence of hydroxide inolivine. Phys. Chem. Miner. 14, 461–472.

Miyazaki, T., Sueyoshi, K., Hiraga, T., 2013. Olivine crystals align during diffusion creepof Earth's upper mantle. Nature 502, 321.

Mizukami, T., Wallis, S.R., Yamamoto, J., 2004. Natural examples of olivine lattice pre-ferred orientation patterns with a flow-normal a-axis maximum. Nature 427,432–436.

Mookherjee, M., Mainprice, D., 2014. Unusually large shear wave anisotropy for chloritein subduction zone settings. Geophys. Res. Lett. 41, 1506–1513.

Morales, L.F.G., Mainprice, D., Boudier, F., 2013. The influence of hydrous phases on themicrostructure and seismic properties of a hydrated mantle rock. Tectonophysics594, 103–117.

Nagaya, T., Walker, A.M., Wookey, J., Wallis, S.R., Ishii, K., Kendall, J.-M., 2016. Seismicevidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone. Sci.Rep. 6.

Nicolas, A., Christensen, N.I., 1987. Formation of anisotropy in upper mantle peridotites-areview. In: Composition, Structure and Dynamics of the Lithosphere-AsthenosphereSystem. 16. pp. 111–123.

Ohtani, E., 2005. Water in the mantle. Elements 1, 25–30.Ohuchi, T., Kawazoe, T., Nishihara, Y., Nishiyama, N., Irifune, T., 2011. High pressure

and temperature fabric transitions in olivine and variations in upper mantle seismicanisotropy. Earth Planet. Sci. Lett. 304, 55–63.

O'Neill, H.S.C., 1981. The transition between spinel lherzolite and garnet lherzolite, andits use as a geobarometer. Contrib. Mineral. Petrol. 77, 185–194.

Padrón-Navarta, J.A., Tommasi, A., Garrido, C.J., Mainprice, D., 2015. On topotaxy andcompaction during antigorite and chlorite dehydration: an experimental and naturalstudy. Contrib. Mineral. Petrol. 169, 1–20.

Panozzo, R., 1984. Two-dimensional strain from the orientation of lines in a plane. J.Struct. Geol. 6, 215–221.

Park, Y., Jung, H., 2015. Deformation microstructures of olivine and pyroxene in mantlexenoliths in Shanwang, eastern China, near the convergent plate margin, and im-plications for seismic anisotropy. Int. Geol. Rev. 57, 629–649.

Park, M., Jung, H., 2017. Microstructural evolution of the Yugu peridotites in theGyeonggi Massif, Korea: implications for olivine fabric transition in mantle shearzones. Tectonophysics 709, 55–68.

Park, M., Jung, H., Kil, Y., 2014. Petrofabrics of olivine in a rift axis and rift shoulder andtheir implications for seismic anisotropy beneath the Rio Grande rift. Island Arc 23,299–311.

Paterson, M., 1982. The determination of hydroxyl by infrared absorption in quartz, si-licate glasses and similar materials. Bull. Mineral. 105, 20–29.

Pawley, A., 2003. Chlorite stability in mantle peridotite: the reactionclinochlore+ enstatite= forsterite+ pyrope+H2O. Contrib. Mineral. Petrol. 144,449–456.

Pozgay, S., Wiens, D., Conder, J., Shiobara, H., Sugioka, H., 2007. Complex mantle flow inthe Mariana subduction system: evidence from shear wave splitting. Geophys. J. Int.170, 371–386.

Précigout, J., Almqvist, B.S., 2014. The Ronda peridotite (Spain): a natural template forseismic anisotropy in subduction wedges. Geophys. Res. Lett. 41, 8752–8758.

Précigout, J., Hirth, G., 2014. B-type olivine fabric induced by grain boundary sliding.Earth Planet. Sci. Lett. 395, 231–240.

H. Kang, H. Jung Tectonophysics 750 (2019) 137–152

151

Précigout, J., Prigent, C., Palasse, L., Pochon, A., 2017. Water pumping in mantle shearzones. Nat. Commun. 8, 15736.

Puelles, P., Ibarguchi, J.G., Beranoaguirre, A., Ábalos, B., 2012. Mantle wedge de-formation recorded by high-temperature peridotite fabric superposition and hydrousretrogression (Limo massif, Cabo Ortegal, NW Spain). Int. J. Earth Sci. 101,1835–1853.

Puelles, P., Ábalos, B., Ibarguchi, J.G., Sarrionandia, F., Carracedo, M., Fernández-Armas,S., 2016. Petrofabric and seismic properties of lithospheric mantle xenoliths from theCalatrava volcanic field (Central Spain). Tectonophysics 683, 200–215.

Qi, C., Hansen, L.N., Wallis, D., Holtzman, B.K., Kohlstedt, D.L., 2018. CrystallographicPreferred Orientation of Olivine in Sheared Partially Molten Rocks: the source of the“a-c Switch”. Geochem. Geophys. Geosyst. 19, 316–336.

Raterron, P., Chen, J., Li, L., Weidner, D., Cordier, P., 2007. Pressure-induced slip-systemtransition in forsterite: single-crystal rheological properties at mantle pressure andtemperature. Am. Mineral. 92, 1436–1445.

Richard, L., 1995. MinPet: Mineralogical and Petrological Data Processing System,Version 2.02. MinPet Geological Software, Québec, Canada.

Rinaudo, C., Gastaldi, D., Belluso, E., 2003. Characterization of chrysotile, antigorite andlizardite by FT-Raman spectroscopy. Can. Mineral. 41, 883–890.

Rinaudo, C., Belluso, E., Gastaldi, D., 2004. Assessment of the use of Raman spectroscopyfor the determination of amphibole asbestos. Mineral. Mag. 68, 455–465.

Savage, M., 1999. Seismic anisotropy and mantle deformation: what have we learnedfrom shear wave splitting? Rev. Geophys. 37, 65–106.

Sawaguchi, T., 2004. Deformation history and exhumation process of the HoromanPeridotite Complex, Hokkaido, Japan. Tectonophysics 379, 109–126.

Silver, P.G., Chan, W.W., 1991. Shear wave splitting and subcontinental mantle de-formation. J. Geophys. Res. Solid Earth 96, 16429–16454.

Skemer, P., Hansen, L.N., 2016. Inferring upper-mantle flow from seismic anisotropy: anexperimental perspective. Tectonophysics 668, 1–14.

Skemer, P., Katayama, I., Jiang, Z., Karato, S., 2005. The misorientation index: devel-opment of a new method for calculating the strength of lattice-preferred orientation.Tectonophysics 411, 157–167.

Skemer, P., Katayama, I., Karato, S., 2006. Deformation fabrics of the Cima di Gagnoneperidotite massif, Central Alps, Switzerland: evidence of deformation at low tem-peratures in the presence of water. Contrib. Mineral. Petrol. 152, 43–51.

Skemer, P., Warren, J.M., Hansen, L.N., Hirth, G., Kelemen, P.B., 2013. The influence ofwater and LPO on the initiation and evolution of mantle shear zones. Earth Planet.Sci. Lett. 375, 222–233.

Skogby, H., Rossman, G.R., 1991. The intensity of amphibole OH bands in the infraredabsorption spectrum. Phys. Chem. Miner. 18, 64–68.

Smith, D.C., 1984. Coesite in clinopyroxene in the Caledonides and its implications forgeodynamics. Nature 310, 641.

Smith, G.P., Wiens, D.A., Fischer, K.M., Dorman, L.M., Webb, S.C., Hildebrand, J.A.,2001. A complex pattern of mantle flow in the Lau backarc. Science 292, 713–716.

Soustelle, V., Manthilake, G., 2017. Deformation of olivine-orthopyroxene aggregates athigh pressure and temperature: implications for the seismic properties of the

asthenosphere. Tectonophysics 694, 385–399.Soustelle, V., Tommasi, A., Demouchy, S., Ionov, D., 2010. Deformation and fluid–rock

interaction in the supra-subduction mantle: microstructures and water contents inperidotite xenoliths from the Avacha Volcano, Kamchatka. J. Petrol. 51, 363–394.

Soyer, W., Unsworth, M., 2006. Deep electrical structure of the northern Cascadia (BritishColumbia, Canada) subduction zone: implications for the distribution of fluids.Geology 34, 53–56.

Sundberg, M., Cooper, R.F., 2008. Crystallographic preferred orientation produced bydiffusional creep of harzburgite: effects of chemical interactions among phases duringplastic flow. J. Geophys. Res. Solid Earth 113.

Tasaka, M., Michibayashi, K., Mainprice, D., 2008. B-type olivine fabrics developed in thefore-arc side of the mantle wedge along a subducting slab. Earth Planet. Sci. Lett. 272,747–757.

Teagle, D., Ildefonse, B., 2011. Journey to the mantle of the Earth. Nature 471, 437.Terry, M.P., Robinson, P., Ravna, E.J.K., 2000. Kyanite eclogite thermobarometry and

evidence for thrusting of UHP over HP metamorphic rocks, Nordøyane, WesternGneiss Region, Norway. Am. Mineral. 85, 1637–1650.

Van Roermund, H.L.M., Drury, M.R., Barnhoorn, A., De Ronde, A.A., 2000. Super-silicicgarnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep(> 6 GPa) origin. J. Metamorph. Geol. 18, 135–147.

Wada, I., Behn, M.D., Shaw, A.M., 2012. Effects of heterogeneous hydration in the in-coming plate, slab rehydration, and mantle wedge hydration on slab-derived H2Oflux in subduction zones. Earth Planet. Sci. Lett. 353, 60–71.

Wagner, L.S., Fouch, M.J., James, D.E., Long, M.D., 2013. The role of hydrous phases inthe formation of trench parallel anisotropy: evidence from Rayleigh waves inCascadia. Geophys. Res. Lett. 40, 2642–2646.

Wallis, D., Lloyd, G.E., Phillips, R.J., Parsons, A.J., Walshaw, R.D., 2015. Low effectivefault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone,NW India. J. Struct. Geol. 77, 45–61.

Wang, D., Guo, Y., Yu, Y., Karato, S., 2012. Electrical conductivity of amphibole-bearingrocks: influence of dehydration. Contrib. Mineral. Petrol. 164, 17–25.

Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. Am.Mineral. 95, 185–187.

Witt-Eickschen, G., Seck, H.A., 1991. Solubility of Ca and Al in orthopyroxene from spinelperidotite: an improved version of an empirical geothermometer. Contrib. Mineral.Petrol. 106, 431–439.

Worzewski, T., Jegen, M., Kopp, H., Brasse, H., Castillo, W.T., 2011. Magnetotelluricimage of the fluid cycle in the Costa Rican subduction zone. Nat. Geosci. 4, 108.

Yamamoto, T., Ando, J.-i., Tomioka, N., Kobayashi, T., 2017. Deformation history ofPinatubo peridotite xenoliths: constraints from microstructural observation and de-termination of olivine slip systems. Phys. Chem. Miner. 44, 247–262.

Young, D., Hacker, B., Andersen, T., Gans, P., 2011. Structure and 40Ar/39Ar thermo-chronology of an ultrahigh-pressure transition in western Norway. J. Geol. Soc. 168,887–898.

Zhang, S., Karato, S., 1995. Lattice preferred orientation of olivine aggregates deformedin simple shear. Nature 375, 774.

H. Kang, H. Jung Tectonophysics 750 (2019) 137–152

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