Clinopyroxenes in high-P metaperidotites from Happo-O'ne, central Japan: Implications for wedge-transversal chemical change of slab-derived fluids

Lithos 119 (2010) 439–456 Contents lists available at ScienceDirect Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s Clinopyroxenes in high-P metaperidotites from Happo-O'ne, central Japan: Implications for wedge-transversal chemical change of slab-derived fluids Mohamed Zaki Khedr a,b,⁎, Shoji Arai a, Akihiro Tamura a, Tomoaki Morishita a a b Department of Earth Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan Department of Geology, Faculty of Science, Kafrelsheikh University, Egypt a r t i c l e i n f o Article history: Received 18 December 2009 Accepted 22 July 2010 Available online 3 August 2010 Keywords: Metaperidotites Slab fluids Clinopyroxenes Multi-stage metasomatism Happo-O'ne Japan a b s t r a c t The metaperidotites found in the Happo-O'ne region of central Japan are mostly lherzolites to harzburgites with subordinate dunites. Their protoliths originated as a series of refractory residues by near-fractional melting, 15–25% melting in the spinel field, which is confirmed by a HREE melting model of the apparently primary clinopyroxene. These metaperidotites display U-shaped primitive mantle (PM)-normalized REE patterns (0.02–0.5 times PM) and are highly enriched in LILE (0.2–20 times PM) relative to HFSE (b 0.2 times PM), providing evidence for mantle-wedge metasomatism. In-situ analyses confirmed that clinopyroxenes and tremolite are enriched in fluid-mobile elements (B, Sr, Pb, Li, Cs, Ba and Rb; 0.1–100 times PM) coupled with depletion of HFSE (Ta, Hf, Th, Zr, Ti and Nb; b 0.7 times PM) + U; these chemical features of clinopyroxenes and tremolite are similar to those of their whole rocks, and reflect slab-fluid metasomatism. We recognized three stages of clinopyroxenes that are different in morphology and REE patterns, but show the same major-element chemistry. Nearly euhedral clinopyroxene (Cpx1) displays a spoon-shaped REE pattern (0.2–3 times PM), reflecting the first stage (stage 1) of metasomatism. Acicular clinopyroxene (Cpx2) after tremolite +olivine shows a U-shaped REE pattern (0.06–1 times PM) (stage 2), whereas fine-grained clinopyroxene (Cpx3) derived from orthopyroxene shows very low concentrations of REE (stage 3). These three successive stages of retrogressive clinopyroxene formation possibly denote a retrogressive chemical change of the involved fluids during multi-stage metasomatism. The fluid of stage 1 is highly enriched in LREE (0.2–2 times PM), MREE, Pb, Sr, Li and Rb relative to the stage-3 fluid that is very low in MREE, Pb, LREE and Sr, but has high Na, Mn, Ba, B and Cs. The stage-2 fluid, which is low in LREE (0.06–1 times PM), Li and Pb, and very low in Cs, Rb and Hf relative to the fluid of stage 1, is considered to be a transitional fluid between stages 1 and 3. The retrogressive change of the fluid composition is possibly equivalent to a transversal change of the slab-derived fluid within the mantle wedge due to a change of metamorphic condition, depth of the subducting slab and an episode of metasomatism during the exhumation of the Happo-O'ne peridotites. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The slab-derived components are thought to be H2O-rich melts or aqueous fluids, which are the main metasomatic agents in the mantle wedge (Bizimis et al., 2000; Kawamoto, 2006; Malaspina et al., 2006). They would be aqueous fluids in cold subduction zones or partial melts in warm subduction zones (Widom et al., 2003; Kawamoto, 2006). These fluids are expected to metasomatize the overlying mantle-wedge peridotites by precipitating new phases as amphiboles, ⁎ Corresponding author. Department of Earth Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan. Tel.: +81 76 264 6513; fax: +81 76 264 6545. E-mail address: khedrzm@yahoo.com (M.Z. Khedr). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.07.021 phlogopite and clinohumite (e.g., Tatsumi et al., 1986; Peacock, 1990; Bizimis et al., 2000), modify mantle-wedge peridotites, result in partial melting in the supra-subduction zones, and recycle lithophile elements back into the mantle wedge (Tatsumi, 1986; Plank and Langmuir, 1993; Kessel et al., 2005; Malaspina et al., 2006). Experimental results verified that the slab-derived fluids are depleted in HFSE (high field-strength elements) and enriched in both LREE (light rare earth elements) and LILE (large-ion-lithophile elements) (Tatsumi et al., 1986; Kessel et al., 2005). Many recent studies have confirmed that mantle-wedge peridotites hydrated by slab-released fluids are depleted in HFSE relative to fluid-mobile elements (such as LILE and LREE) (e.g., McCulloch and Gamble, 1991; Keppler, 1996; Widom et al., 2003; Gaetani et al., 2003; Scambelluri et al., 2004; Kessel et al., 2005; Savov et al., 2005, 2007; Scambelluri et al., 2006; Marocchi et al., 2007; Khedr and Arai, 2009). The trace-element 440 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 characteristics reflect not only the type and nature of the metasomatic agents, but also the chemical changes that take place during fluid/ melt–mantle rock interactions (Maury et al., 1992; Ayers, 1998). Few studies, however, refer to changes in fluid compositions in the mantle wedge. The composition of the slab-derived components depends on the thermal structure of the slab–wedge interface and the composition of the slab (Pearce and Parkinson, 1993). Savov et al. (2005) reported the change in the slab-fluid compositions at greater depths, while Zack and John (2007) found that the degree of change depends on the amount of fluid–mineral surface interaction. Recently, Marocchi et al. (2007) stated that the character of the fluid phase changes with the changing metamorphic condition of the mantle wedge. The Happo-O'ne metaperidotites metasomatized by slab-released fluids under low-T, high-P conditions are accompanied by the formation of new metasomatic minerals such as secondary clinopyroxenes (Cpx2 and Cpx3), tremolite and chlorite. They have recorded mineralogical and chemical signatures of metasomatic processes, which resulted from chemical changes of the metamorphic fluids in forearc regions. We recognized not only secondary clinopyroxenes, but also the apparently primary Cpx1 that can be used in petrogenetic modeling and in determining the degree of melt extraction and the nature of melting condition (e.g., Johnson et al., 1990; Dick and Natland, 1996; Hellebrand et al., 2001). The aim of this study is to investigate the main type and origin of metasomatic agents, which are possibly slab-derived fluids. What is the retrogressive chemical change of these fluids? The retrogressive change of metasomatic fluids imposed on the Happo-O'ne metaperidotite is possibly related to a spatial difference of the slab-derived fluids, if we take its P–T trajectory for the exhumation history into account (Khedr and Arai, 2010). We will discuss a wedge-transversal chemical change of slabderived fluids in this article. Also, the nature of partial melting and the interaction between mantle rocks and slab fluids will be investigated by using clinopyroxene generations. 2. Geological setting and petrography The metaperidotites from the Happo-O'ne area, which is situated in the northeastern part of the Hida marginal tectonic zone on Honshu Island in central Japan, form a serpentinite mélange (Khedr and Arai, 2010 and references therein) (Fig. 1). In this area, blocks of coarsegrained garnet glaucophane schist, garnet epidote amphibolite, garnet mica schist, metagabbro, eclogite and utramafic rock are included in a serpentinite matrix (= serpentinite mélange) (Nakamizu et al., 1989; Fig. 1. Geological map showing distribution of ultramafic rocks in different metamorphic mineral zones, the Happo-O'ne complex, central Japan (Nozaka, 2005 and references therein). Di, diopside zone (olivine+ antigorite + diopside): Tr, tremolite zone (olivine + tremolite + orthopyroxene): Tlc, talc zone (olivine+ talc+ tremolite) (Nakamizu et al., 1989). M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Nozaka, 2005). Slices of mantle-wedge peridotites, which underwent serpentinization and high-P, low-T metamorphism, have been incorporated into the subduction complex (=the Renge metamorphic rocks) during the exhumation of the Renge high-P/T metamorphic rocks (Khedr and Arai, 2010). The Happo-O'ne metaperidotites from the tremolite and diopside zones, which were subjected to retrogressive metamorphism, are accompanied by the formation of high-P, low-T minerals and retrogressive textures during their exhumation path (Khedr and Arai, 2010). They subsequently underwent prograde metamorphism (talc zone) due to contact metamorphism around a 441 Cretaceous granitic intrusion, leading to the formation of talc peridotite and peridotite hornfels (Fig. 1) (Nozaka, 2005). The field observations show that the examined peridotites or serpentinized equivalents have certainly travelled with the high-P metamorphic rocks (Renge high-P/T schists) from some mantle depths. No systematic field relations have been recognized between different blocks because of the mélange nature (Nakamizu et al., 1989). The petrographic observations verify that antigorite serpentinites composed of antigorite (N65%) + olivine (b30%) + clinopyroxene (b1%) + spinel (b4%) are restricted in a highly hydrated part of Fig. 2. Photomicrographs of the Happo-O'ne metaperidotites from both diopside and tremolite zones showing different in morphology of clinopyroxenes. Crossed-polarized light except for (f). (a) Two different morphologies of clinopyroxenes, the apparently primary clinopyroxene (Cpx1) as euhedral crystal with exsolved lamellae and the fibrous clinopyroxene (Cpx2) without lamella enclosed mainly in antigorite, forming the main rock foliation in serpentinized lherzolite–harzburgite (No. MM2) (diopside zone). (b) Typical euhedral Cpx1 oblique to the fibrous Cpx2 and antigorite in serpentinized lherzolite–harzburgite (No. MM2). (c) Olivine porphyroclast with kink band surrounded by antigorite and fibrous Cpx2 in antigorite serpentinite (No. NK4) (diopside zone). (d) Retrogressive fine-grained clinopyroxene (Cpx3) associated with Ti-rich chromian spinel (black euhedral crystals) within serpentinized orthopyroxene in tremolite–chlorite peridotite (No. RT3) (tremolite zone). (e) Massive tremolite–chlorite peridotite (No. RT1) with a mineral assemblage of olivine + orthopyroxene + tremolite + chlorite + chromian spinel (tremolite zone). (f) Reflected light image of spinel showing double zone of inner chromian spinel (grey color) and outer magnetite and/or Cr-magnetite (light grey color) in serpentinized lherzolite–harzburgite (No. Nk5) (diopside zone). Abbreviations; Cpx, clinopyroxene: Tr, tremolite: Olv, olivine: Chl, chlorite: Atg, antigorite: Cr-Spl, chromian spinel: and Mt, magnetite or chromian-magnetite. Note the Cpx1 is generally oblique to the principal rock's foliation or occurs as inclusions inside the secondary Cpx2. 442 Table 1 Major (wt.%) and trace-element (ppm) abundances in whole rocks of the Happo-O'ne metaperidotites. Met.Zone Diopside zone Tremolite zone Serpentinized lherzolites–harzburgites Dunite Average Nk-5 Kz3* MM1 MM3 MM4 SG4 SG1 SG3 KZ3 RT1 RT2 RT3 RT5 Kz1 Kz10 Eb6 EB10 Hp0 Hp4 Hp1 JB-2 SiO2 TiO2 Al2O3 Fe2Oa3 MnO MgO CaO Na2O K2O P2O5 Total Mg#b Mg#c-Olv 41.41 0.01 0.78 8.79 0.15 45.72 0.59 bdl bdl 0 97.45 0.90 0.89 43.66 bdl 1.45 7.1 0.11 44.13 1.49 bdl bdl 0 97.94 0.92 0.88 42.87 0 1.24 8.21 0.15 44.68 1.27 bdl 0.01 0 98.43 0.91 0.89 44 0 1.36 7.37 0.14 45.08 1.94 0.04 bdl bdl 99.93 0.92 0.91 43.78 0.01 1.53 8.01 0.15 45.35 1.46 0.04 0.01 bdl 100.34 0.91 0.89 41.89 0 1.3 8.35 0.13 44.98 1.57 0.04 0.01 0.01 98.28 0.91 0.90 40.15 0.01 1.32 9.35 0.14 46.01 0.46 0.05 0.04 0 97.53 0.90 0.90 40.24 0.01 0.7 10.76 0.18 47.31 0.36 0.01 bdl bdl 99.57 0.89 0.90 44.90 0.01 1.86 7.11 0.12 43.86 1.89 0.12 0.01 0.00 99.88 0.92 0.91 43.31 0.01 1.72 7.72 0.13 42.62 2.41 0.30 0.02 0.00 98.24 0.91 0.90 43.30 0.02 1.16 8.63 0.14 45.40 1.29 0.33 0.03 0.00 100.30 0.90 0.90 44.90 0.02 1.92 7.05 0.12 44.09 1.74 0.03 0.01 0.00 99.88 0.92 0.90 42.52 0.02 1.23 8.78 0.12 45.01 0.39 bdl 0.01 0.00 98.08 0.90 0.88 43.28 0.01 1.29 7.47 0.12 45.01 1.75 0.03 0 0.02 98.98 0.91 0.92 43.28 0.00 1.13 8.74 0.15 45.83 0.70 0.04 bdl 0.00 99.87 0.90 0.89 43.95 0.01 1.50 8.13 0.13 43.52 1.68 bdl bdl 0.00 98.92 0.91 n.a 42.87 0.01 1.44 8.84 0.13 42.6 1.88 0.02 bdl 0 97.79 0.90 n.a 42.60 0.00 1.38 8.54 0.14 43.20 2.22 0.06 bdl 0.00 98.14 0.90 0.91 43.6 0 1.44 9.24 0.15 43.62 1.6 0.05 0 0 99.7 0.89 0.92 41.76 0.02 0.71 9.00 0.15 47.13 0.50 bdl bdl 0.00 99.27 0.90 0.92 CIPW normd Olv Opx Cpx 87.14 10.01 2.83 72.61 20.59 6.80 78.79 15.39 5.77 76.05 15.27 8.67 77.41 15.65 6.86 83.72 8.93 7.26 92.90 3.70 2.77 96.91 1.79 1.79 68.80 21.76 9.37 73.96 13.02 12.88 82.72 8.91 8.15 68.29 23.47 8.14 80.76 16.65 1.97 77.92 14.22 7.80 80.91 15.40 3.70 71.90 20.56 7.52 74.56 16.90 8.53 77.88 11.95 10.12 75.54 16.97 7.49 90.11 7.47 2.38 0.123 0.174 3.46 bdl bdl 0.103 0.118 0.175 1.30 0.021 12.49 0.101 0.203 bdl 78.20 0.045 0.062 0.357 0.015 0.055 bdl 0.068 bdl 12.77 3233 100.4 2239 0.658 1.19 2.22 0.53 0.061 0.485 3.65 bdl bdl 0.064 0.093 0.085 0.89 0.009 17.91 bdl 0.184 bdl 60.42 bdl bdl 0.215 bdl 0.033 bdl 0.060 0.016 9.31 2683 115.5 2272 0.296 1.06 0.074 0.220 3.96 bdl bdl 0.080 0.101 0.092 0.88 0.009 25.20 0.045 0.255 bdl 69.50 bdl bdl 0.266 bdl 0.089 bdl 0.071 bdl 12.04 3165 112.4 2237 0.407 0.97 0.081 0.507 4.88 bdl bdl 0.074 0.102 0.107 0.60 0.014 15.51 0.078 0.460 bdl 95.07 0.078 bdl 0.281 bdl 0.046 bdl 0.067 0.016 11.44 3127 113.6 2314 0.509 1.04 1.10 0.94 0.122 0.471 7.94 0.019 bdl 0.068 0.142 0.214 1.44 0.027 15.33 0.090 0.173 bdl 70.25 0.034 0.050 0.354 0.017 0.043 bdl 0.076 0.015 9.57 2622 112.2 2192 0.708 1.27 3.51 0.36 0.204 0.751 6.78 bdl bdl 0.115 0.075 0.036 0.51 bdl 8.22 bdl 0.153 bdl 83.58 0.002 bdl 0.079 bdl bdl bdl bdl bdl 6.31 7923 124.0 2525 0.112 0.043 0.168 1.08 bdl bdl 0.082 0.082 0.029 1.34 bdl 8.09 bdl 0.100 bdl 101.66 bdl bdl 0.043 bdl bdl bdl bdl bdl 4.51 5480 136.6 2614 0.111 0.287 0.341 2.93 bdl bdl 0.100 0.163 0.292 1.23 0.039 23.12 0.176 0.247 0.022 108.41 0.057 0.103 0.624 0.023 0.083 bdl 0.112 0.019 15.99 3759 103.1 2311 1.089 0.096 0.375 10.45 bdl bdl 0.100 0.190 0.294 0.71 0.034 35.54 0.130 0.368 0.015 94.75 0.038 0.075 0.540 0.019 0.077 bdl 0.106 0.019 14.63 3141 101.6 2039 0.997 0.276 0.508 1.30 bdl bdl 0.095 0.210 0.327 2.72 0.032 20.58 0.139 0.248 0.013 65.87 0.052 0.066 0.456 0.016 0.064 0.011 0.085 0.016 12.25 2800 105.0 2192 1.031 1.37 3.38 0.49 0.382 0.309 2.53 bdl bdl 0.077 0.120 0.129 2.03 0.017 5.02 0.073 0.112 bdl 75.20 0.037 bdl 0.211 bdl 0.034 bdl 0.056 bdl 8.07 2648 123.2 2536 0.465 2.18 2.72 0.53 0.061 0.176 1.11 bdl 0.014 0.127 0.160 0.210 1.16 0.022 8.72 0.090 0.986 bdl 91.78 bdl 0.068 0.454 bdl 0.069 0.020 0.117 bdl 9.91 2765 111.0 2293 0.757 1.67 0.087 0.235 1.32 0.022 0.015 0.129 0.192 0.223 2.38 0.023 10.32 0.098 1.113 bdl 100.13 0.043 0.064 0.455 0.018 0.062 0.017 0.085 0.020 11.37 2681 104.4 2080 0.845 1.46 3.75 0.41 0.298 0.485 1.11 bdl bdl 0.095 0.128 0.161 0.47 0.020 24.66 0.085 0.345 0.012 78.38 0.046 0.074 0.494 bdl 0.069 0.012 0.092 0.034 12.94 2960 109.7 2165 0.732 0.93 2.33 0.41 0.322 0.580 1.61 bdl bdl 0.076 0.107 0.104 1.06 0.011 26.54 0.050 0.256 bdl 72.99 bdl 0.040 0.361 0.014 0.050 bdl 0.074 0.020 12.51 3012 115.8 2365 0.470 1.53 4.17 0.29 0.172 0.418 25.31 0.044 bdl 0.088 0.176 0.300 1.67 0.036 44.38 0.147 0.367 0.014 113.36 0.055 0.075 0.523 0.018 0.065 0.020 0.088 0.019 14.01 3369 102.6 2208 1.020 1.23 2.69 0.51 0.084 0.312 11.09 bdl bdl 0.088 0.128 0.174 0.61 0.018 14.81 0.073 0.930 bdl 135.97 0.023 0.033 0.185 bdl 0.028 bdl 0.040 bdl 10.32 3063 118.9 2431 0.518 2.40 0.41 0.095 0.437 8.46 bdl bdl 0.096 0.127 0.166 0.91 0.017 21.90 0.068 0.393 bdl 152.05 0.040 0.040 0.275 0.010 0.036 bdl bdl 0.009 10.16 2893 108.6 2183 0.513 0.99 2.66 0.208 0.375 1.84 bdl 0.020 0.142 0.130 0.154 1.59 0.019 2.44 0.086 0.553 bdl 172.65 0.039 0.047 0.212 bdl 0.027 bdl bdl bdl 5.79 6755 130.5 2641 0.501 0.95 2.81 Trace elements Cs Rb Ba Th U Nb La Ce Pb Pr Sr Nd Zr Eu Ti Gd Dy Y Ho Er Tm Yb Lu Sc Cr Co Ni ∑RΕΕ (La/Yb)N (La/Gd)N (Gd/Yb)N (ppm) 0.038 0.105 1.86 bdl bdl 0.116 0.113 0.105 0.72 0.009 8.58 bdl 0.251 bdl 48.34 bdl bdl 0.090 bdl bdl bdl 0.050 bdl 10.83 3354 118.1 2467 0.278 1.54 Dunite Tr–Chl peridotites with Ti-rich Cr-spinel Tremolite–chlorite peridotites 4.69 0.47 0.67 5.57 230.69 0.27 0.15 0.51 2.42 6.76 4.33 1.16 186.62 6.60 42.19 0.87 8,946 3.14 3.96 21.81 0.85 2.51 0.38 2.70 0.39 58.46 38.97 34.18 22.70 0.99 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Rock type Sample no. M.Z. Khedr et al. / Lithos 119 (2010) 439–456 the diopside zone (Fig. 2c). Whereas in a slightly hydrated part of this zone, serpentinized peridotites (lherzolites–harzburgites) are dominant rocks and composed of olivine (b50%) + clinopyroxene (b9%) +antigorite (N40%) + chlorite (b5%) + spinel (b4.5%) (Fig. 2a, b). Dunites consisting of olivine (N60%) + clinopyroxene (b1%) + antigorite (b30%) + chlorite (b6%) + opaque minerals (b4%) are scarce in the diopside zone. The textural evidence indicates that clinopyroxenes show two different morphologies (Fig. 2a, b); the apparently primary clinopyroxene (Cpx1) occurs as euhedral (0.7–1.0 mm) (Fig. 2a, b) to subhedral crystals (0.5–1.2 mm) and the secondary clinopyroxene (Cpx2) occurs as a prismatic crystal, 0.05–1.0 mm in length, fibrous shape (up to 2.5 mm), and/or a xenomorphic crystal (0.5–1.0 mm) embedded mainly in antigorite. Most euhedral clinopyroxene (Cpx1) crystals show magmatic twinning (Fig. 2a), and probably represent a residual Cpx phase during partial melting. The Cpx1 is generally oblique to the principal foliation (Fig. 2a, b). The prismatic or fibrous clinopyroxene (Cpx2) without lamellae is enclosed mainly in antigorite and/or associated with olivine (Fig. 2a–c), defining the rock foliation. The Cpx2 is partially to entirely surrounding the Cpx1. Most of the chromian spinel is altered to chlorite, or, less commonly, to ferritchromite and/or magnetite (Fig. 2f). Al spinel components have been consumed to form low temperature Al-bearing minerals such as chlorite. The tremolite zone is characterized by a mineral assemblage, olivine (N40%)+orthopyroxene (b12%)+tremolite (b11%)+chlorite (b8%)+ antigorite (b50%) + spinel (b4%) (= tremolite–chlorite peridotites) (Fig. 2e). Details of the tremolite zone have been described by Khedr and Arai (2010). Fine-grained clinopyroxene (Cpx3) (Fig. 2d) replaces primary orthopyroxene (Opx) or occurs along Opx cleavage traces and grain boundaries; it is probably derived from replacement of clinopyroxene lamellae within Opx. Neither chlorite clot as a pseudomorph after garnet nor garnet relic is reported. Dunites of this zone include primary chromian spinel. 3. Analytical and experimental methods All selected metaperidotites for bulk-rock chemical analyses were homogeneous, and lacked veins. Their powders were heated up to 1000 °C to remove structural water before preparing fused beads for major- and trace-element analyses. Major-element contents (Table 1) were determined by an X-ray fluorescence (XRF) spectrometer (System-3270, Rigaku) at 50-kV accelerating voltage and 20-mA beam current at Kanazawa University on fused beads of the rock powders (0.5 g) mixed with Li2B4O7 (5.0 g). Trace-element contents (Table 1) were determined on fused beads of the rock powders (0.3 g) mixed with Li2B4O7 (1.5 g) by laser-ablation (193 nm ArF excimer: MicroLas GeoLas Q-plus)-inductively coupled plasma mass spectrometry (Agilent 7500 S) (LA–ICP–MS) at Kanazawa University. For all analyses, the laser-spot diameter was 153 μm at 5 Hz with an energy density of 8 J/cm2 per pulse. Details of the analytical procedures have been described by Ishida et al. (2004) and Morishita et al. (2005). For data calibration of LA–ICP–MS analysis, NIST 612 glass was used as an external standard, assuming the composition given by Pearce et al. (1997), and 29Si was used as an internal standard based on SiO2 concentrations measured by XRF for bulk rocks. Fused bead of JB-2 (GSJ reference material) was measured for estimation of the analytical quality (Table 1). The reproducibility is better than 5% for most elements, except Ta (low concentration) for which it is better than 16%. The accuracy of measurements based on JB-2 is acceptable and agrees with previous values in the literature 443 (Makishima and Nakamura, 1999) at the 78–105% confidence level with the exception of Nb and Ta (b119%). The major-element compositions of minerals from the Happo-O'ne metaperidotites were determined by JEOL wavelength dispersive electron probe X-ray micro-analyzer (JXA 8800) at Kanazawa University. Accelerating voltage, beam current, and beam diameter for the analyses were 20 kV, 20 nA, and 3 μm, respectively. Representative mineral compositions are reported in Table 2. Trace-element abundances in silicates (olivine, orthopyroxene, tremolite, chlorite, and clinopyroxenes) (Table 3) in the examined metaperidotites were in-situ determined by LA–ICP–MS at Kanazawa University. Analyses were performed by ablating 60-μm diameter spots for tremolite, chlorite and pyroxenes, whereas the spot diameter for olivine was 100 μm. All analyses were performed at 6 Hz with an energy density of 8 J/cm2 per pulse. Calibration was carried out by the same procedures as for the bulk-rock analyses; NIST 612 glass was used as an external standard and 29Si was used as an internal standard based on SiO2 concentration obtained by the electron microprobe. NIST 614 glass (secondary standard) was measured for quality control of each analysis. Precision or reproducibility is better than 5% for most elements, except Cr and Ni for which it is better than 10%. The accuracy and data quality based on the reference material (NIST 614) are high, and were described by Morishita et al. (2005). Mg# is Mg/(Mg + total Fe) atomic ratio for bulk rocks and silicates, and Mg/(Mg + Fe2+) atomic ratio in chromian spinel, calculated assuming spinel stoichiometry. Cr# is Cr/(Cr + Al) atomic ratio. 4. Geochemical characteristics 4.1. Whole-rock compositions The metaperidotites from the diopside zone, which were classified based on the normative abundance of clinopyroxene (Niu, 1997) (Table 1), are mainly serpentinized lherzolites to harzburgites with subordinate dunites; their bulk-rock Mg# ranges from 0.89 to 0.92 and matches with that of their olivine (Mg#, 0.88–0.91) (Table 1). Lherzolites to harzburgites are depleted in A12O3 (0.8–1.5 wt.%) and CaO (0.6–1.9 wt.%) relative to fertile lherzolites (3 to 4 wt.% A12O3 and CaO) from the Horoman peridotite complex (Takazawa et al., 2000). The dunites are highly depleted in CaO (0.4–0.5 wt.%) and Al2O3 (0.7– 1.3 wt.%), and high in MgO (46–47 wt.%) (Table 1; Fig. 3). Lherzolites to harzburgites (=tremolite–chlorite peridotites) from the tremolite zone are slightly enriched in A12O3 (1.1–1.9 wt.%) and CaO (0.4–2.4 wt.%) relative to those from the diopside zone (Table 1; Fig. 3). The chemistry of these peridotites has been described in detail by Khedr and Arai (2009). We noticed that Sc, HREE, CaO and Al2O3 decrease systematically as MgO increases, whereas Ni and Co show good positive trends with MgO (Figs. 3 and 4). Further, some HFSE (i.e., Nb, Zr) (Fig. 4g, h) and LILE (not shown here) exhibit no correlations with MgO. Total Fe as Fe2O3 (7.0–10.8 wt.%) (Table 1) in most Happo-O'ne peridotite samples is higher than that of the primitive mantle (PM) (Fig. 3e), possibly reflecting Fe-enrichment during subduction-related metasomatism. Iron may be added to mantle-wedge peridotites by a soluterich hydrous fluid (Parkinson and Arculus, 1999). The PM-normalized REE (rare earth elements) patterns are portrayed in Fig. 5a, c. The Happo-O'ne metaperidotites display Ushaped REE patterns (0.02–0.5 times PM); their PM-normalized multi-element patterns show high concentrations of LILE (0.2–20 times PM) with spikes at Pb, Cs, Sr and Ba, and are depleted in HFSE Notes to Table 1 Fea, total Fe as Fe2O3; Mg#b = Mg/(Mg+Fe2+) of whole rocks and Fe2+ as total iron; Mg#c-Olv, an average Mg# of olivines from EPMA analyses; CIPWd norm calculated following the scheme by Niu (1997). bdl, below detection limits; n.a, not analyzed. Tr, tremolite: Chl, chlorite: Met.Zone, metamorphic zone. Analyses of sample numbers: RT1, RT3, Kz3, Hp0, Hp1 and Eb6 are from Khedr and Arai (2009, 2010). Ta, Hf, Sm and Tb analyses (not listed in Table) of all samples are below detection limits . Note the detection limits (ppm) of Th (0.01), U (0.01), Ta (0.01), Hf (0.04), Sm (0.04), Eu (0.01), Gd (0.04), Tb (0.01), Ho (0.01), Tm (0.01), Yb (0.05), and Lu (0.01). Note average analysis of JB-2 for n = 15. 444 Table 2 Representative microprobe analyses of silicates and spinel in the Happo-O'ne metaperidotites. Met.Zone Diopside zone Sample No. MM1 Mineral Olv Cpx1 Cpx1 Atg Cpx1 Cpx1 Cpx2 Olv Cpx2 Chl Cpx2 Mt Olv Cpx1 Cpx2 Olv Cpx2 Cpx2 Chl Cr-Spl Cpx2 Chl Cr-Spl SiO2 TiO2 Al2O3 Cr2O3 FeOc MnO MgO CaO Na2O K2O NiO Total Mg# Cr# Wo En 40.51 0.00 0.00 0.00 11.06 0.22 48.56 0.02 0.00 0.00 0.37 100.7 0.89 55.02 0.00 0.10 0.06 0.80 0.03 18.45 25.12 0.12 0.01 0.04 99.7 0.98 55.97 0.00 0.01 0.05 1.10 0.06 17.97 25.48 0.06 0.02 0.02 100.7 0.97 43.20 0.01 2.51 0.75 2.62 0.00 38.37 0.02 0.08 0.01 0.15 87.7 0.96 55.64 0.00 0.04 0.07 1.21 0.03 18.00 25.10 0.08 0.00 0.03 100.2 0.96 55.71 0.00 0.07 0.02 1.18 0.02 17.93 26.25 0.10 0.01 0.03 101.3 0.96 54.76 0.01 0.09 0.00 0.69 0.06 18.49 26.05 0.14 0.00 0.03 100.3 0.98 40.40 0.00 0.00 0.01 11.01 0.21 49.38 0.02 0.00 0.01 0.37 101.4 0.889 54.83 0.00 0.06 0.08 0.83 0.03 18.71 25.60 0.12 0.00 0.03 100.3 0.98 32.64 0.03 13.26 3.01 3.10 0.01 33.41 0.01 0.09 0.00 0.21 85.8 0.95 54.63 0.03 0.20 0.14 0.86 0.02 18.65 25.23 0.25 0.01 0.02 100.0 0.97 1.00 0.07 0.00 0.00 90.53 0.09 1.05 0.10 0.02 0.00 0.62 93.5 0.06 1.00 40.68 0.02 0.01 0.00 11.13 0.24 49.23 0.03 0.02 0.00 0.37 101.7 0.89 54.78 0.00 0.13 0.43 1.42 0.05 17.81 25.44 0.37 0.00 0.00 100.4 0.96 54.77 0.04 0.35 0.22 1.31 0.01 18.27 24.98 0.43 0.00 0.00 100.4 0.96 40.09 0.00 0.00 0.00 9.68 0.17 49.46 0.02 0.00 0.00 0.37 99.8 0.90 54.27 0.01 0.09 0.10 0.88 0.01 18.56 25.81 0.12 0.01 0.01 99.9 0.97 55.36 0.01 0.07 0.05 0.89 0.01 18.70 25.90 0.14 0.01 0.02 101.2 0.97 32.60 0.04 13.53 3.23 2.78 0.02 32.98 0.01 0.11 0.00 0.23 85.5 0.95 0.33 0.66 3.58 54.04 34.06 0.71 3.48 0.03 0.00 0.01 0.06 96.9 0.19 0.91 54.52 0.00 0.10 0.04 0.81 0.03 19.21 25.07 0.17 0.01 0.02 100.0 0.98 33.45 0.03 12.88 3.18 2.96 0.01 33.64 0.00 0.01 0.00 0.20 86.3 0.95 0.06 0.77 2.33 58.02 33.01 0.68 2.89 0.02 0.03 0.01 0.05 97.9 0.16 0.94 48.83 49.91 49.59 48.66 49.12 48.99 50.35 47.85 49.76 49.13 49.52 48.24 48.97 49.42 49.32 49.35 49.21 49.44 MM2 Diopside zone Sample No. SG3 NK5 Mineral Olv Olv Cpx2 Chl Cr-Spl SiO2 TiO2 Al2O3 Cr2O3 FeOc MnO MgO CaO Na2O K2O NiO Total Mg# Cr# Wo En 41.11 0.00 0.02 0.03 9.67 0.18 48.52 0.02 0.00 0.00 0.31 99.9 0.90 40.89 0.00 0.00 0.00 10.57 0.27 48.67 0.02 0.00 0.00 0.42 100.8 0.89 55.93 0.00 0.07 0.03 1.20 0.04 18.08 25.80 0.15 0.00 0.04 101.3 0.96 32.91 0.00 14.52 2.81 3.44 0.02 33.53 0.00 0.02 0.00 0.20 87.4 0.95 0.00 0.09 4.94 50.53 40.16 0.77 3.58 0.00 0.02 0.00 0.06 100.2 0.19 0.87 MM4 48.94 49.78 KZ3* SG1 48.65 50.03 SG4 47.79 50.95 Tremolite zone 49.69 48.45 NK3 RT1 RT3 CrMt Fe-ch Cpx3 0.00 0.55 0.07 15.87 76.90 0.86 1.44 0.01 0.00 0.00 0.55 96.3 0.08 0.99 0.03 0.52 1.51 45.56 44.06 2.44 3.03 0.01 0.08 0.00 0.14 97.4 0.17 0.95 54.83 0.00 0.09 0.02 1.18 0.08 18.91 24.48 0.35 0.03 0.00 100.0 0.97 55.7 0.00 0.07 0.06 1.44 0.13 18.3 24.7 0.20 0.01 0.00 100.67 0.96 47.29 50.80 48.14 49.49 Hp0 CrSpla Olv Opx Tr Chl Atg 0.00 5.27 2.67 44.23 43.33 0.73 3.08 0.02 0.02 0.00 0.26 99.6 0.16 0.92 40.16 0.00 0.02 0.00 9.87 0.17 49.16 0.01 0.02 0.00 0.37 99.8 0.90 58.41 0.02 0.17 0.15 6.24 0.22 35.39 0.18 0.00 0.00 0.05 100.8 0.91 58.86 0.03 0.75 0.11 1.64 0.08 23.91 12.40 1.43 0.06 0.12 99.4 0.96 33.99 0.04 13.65 2.36 2.81 0.01 34.02 0.01 0.06 0.01 0.21 87.2 0.96 43.08 0.01 0.26 0.06 2.51 0.06 38.96 0.11 0.03 0.00 0.02 85.1 0.97 Cpx3 55.6 0.02 0.09 0.13 1.07 0.08 19.4 23.2 0.70 0.02 0.01 100.28 0.97 56.1 0.02 0.09 0.07 1.15 0.19 18.5 23.2 0.40 0.01 0.02 99.65 0.97 55.0 0.01 0.13 0.03 1.58 0.13 18.6 24.2 0.26 0.01 0.03 99.91 0.95 45.36 52.88 46.41 51.50 47.09 50.31 Hp1 Olv Tr Olv CrSplb 40.68 0.01 0.00 0.00 8.02 0.17 50.61 0.00 0.00 0.00 0.36 99.9 0.92 57.80 0.03 1.05 0.36 2.04 0.07 24.07 12.35 0.38 0.07 0.08 98.3 0.95 40.72 0.00 0.00 0.00 7.86 0.13 51.25 0.02 0.00 0.00 0.40 100.4 0.92 0.00 0.28 14.28 52.74 23.69 0.42 8.72 0.01 0.05 0.00 0.04 100.2 0.43 0.71 Cr-Spla, Ti-rich chromian spinel: Cr-Splb, primary chromian spinel: FeOc, total iron as FeO; Olv, olivine: Opx, orthopyroxene: Cpx, clinopyroxene: Tr, tremolite: Chl, chlorite: Atg, antigorite: Mt, magnetite: Cr-Mt, chromian magnetite: Fe-ch, ferritchromite. Rock types and numbers as in Table 1. M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Met.Zone MM3 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 (Ta, Hf, Zr, Nb, Ti and Th; b0.2 times PM). These metaperidotites are similar in chemical features to peridotites from the Izu–Bonin– Mariana (IBM) forearc (Parkinson and Pearce, 1998) and Mariana forearc Conical Seamount (Savov et al., 2005) (Fig. 5b, d). Their Th, Ta, Hf and U (below detection limits) are very low compared to those in the Horoman and IBM peridotites. The studied metaperidotites show high depletion in HFSE + U relative to chlorite harzburgites from SE Spain (Garrido et al., 2005). 4.2. Mineral compositions 4.2.1. Major elements Olivine from both diopside and tremolite zones has the same major-element compositions, and has Fo88–Fo92 (Fo90, on average) (Table 2). It is similar in chemistry and morphology to mantle olivine in ordinary primary peridotites, and lies in the field of mantle olivine (Khedr and Arai, 2010 and references therein). Clinopyroxenes in both diopside and tremolite zones are diopside with high Mg#s (0.95– 0.98) and have low contents of Al2O3 (0.0–0.35 wt.%), Cr2O3 (0.0– 0.43 wt.%), TiO2 (0.0–0.1 wt.%) and Na2O (0.03–0.42 wt.%) (Table 2; Fig. 6). They show a pronounced intra-grain chemical homogeneity as a result of low-T equilibration. Clinopyroxene phases in both diopside (Cpx1 and Cpx2) and tremolite zones (Cpx3) share the same majorelement composition, which is independent of grain morphology and size, and mode of occurrence, except high MnO content in the Cpx3 (Fig. 6). All clinopyroxene types show low Al2O3 and Cr2O3 values compared to the primary clinopyroxene from forearc peridotites (e.g., Ishii et al., 1992; Parkinson and Pearce, 1998; JunBing and ZhiGang, 2007; Murata et al., 2009) due to their low-T equilibration (b800 °C); they lie mainly in the secondary clinopyroxene field (e.g., Kimball et al., 1985; Peacock, 1987; Nozaka, 2005; JunBing and ZhiGang, 2007; Murata et al., 2009) (Fig. 6). Orthopyroxene, found only in the tremolite zone, has high Mg#s of 0.90–0.91 (Khedr and Arai, 2010). Tremolite, found only in the tremolite zone, has a high Mg# (up to 0.97) and appreciable amounts of Al2O3 (up to 2.9 wt.%), Na2O (up to 3.0 wt.%) and Cr2O3 (up to 1.2 wt.%) (Khedr and Arai, 2010). Chlorite in both diopside and tremolite zones falls mainly in the field of penninite with an average Mg#, 0.95, but some Al rich chlorite is clinochlore (Hey, 1954) (Table 2). Serpentine minerals in both diopside and tremolite zones are mainly antigorite, confirmed by TG-DTA. Antigorite exhibits a high Mg# (up to 0.98). (Table 2). Chromian spinel, ferritchromite, and magnetite are recognized in both diopside and tremolite zones. Ti-rich chromian spinel (Cr#s, 0.90–0.97; TiO2, 0.45– 5.7 wt.%) and primary chromian spinel with high Cr#, 0.72 (Table 2) (Khedr and Arai, 2010) are restricted only in the tremolite zone. They are absent in the diopside zone due to lack of orthopyroxene (host of Ti-spinel) and severe hydration of this zone. 4.2.2. Trace elements The PM-normalized REE and multi-element patterns of peridotitic silicates from the diopside zone are portrayed in Fig. 7a, b. Clinopyroxenes in serpentinized lherzolites–harzburgites from this zone exhibit two different REE patterns. One is a spoon-shaped REE pattern (0.2–3 times PM) (Fig. 7a), which is displayed by the apparently primary clinopyroxene (Cpx1); the Cpx1 has high REE concentrations, ΣREE = 5.4 to 12.7 ppm, CeN = 2.0–5.7 and YbN = 2.5– 7.7 (Table 3). Another is a U-shaped REE pattern (0.06–1 times PM) (Fig. 7a) exhibited by the secondary clinopyroxene (Cpx2) with moderate REE contents, ΣREE = 1.4–4.9 ppm, CeN = 0.4–3.2; YbN = 0.3–1.5. All clinopyroxene phases are enriched in fluid-mobile elements (B, Sr, Pb, Li, Ba and LREE; 0.1–100 times PM) and depleted in HFSE (Ta, Hf, Th, Zr, Ti and Nb; b0.7 times PM) + U, which are mostly below detection limits (Table 3; Fig. 7b), similar to those in their whole rock (Fig. 5b). Most of bulk-rock trace elements reside in clinopyroxene phases that form up to 9% of the rocks. Olivine is nearly free of MREE and LREE except La and Ce, and exhibits a spoon-shaped 445 REE pattern (Fig. 7a). Chlorite has very low HREE, which are below detection limits (Table 3; Fig. 7a). However, both olivine and chlorite are enriched in fluid-mobile elements (B, Pb, Cs, Li, Rb and Ba; 0.01–20 times PM) and transitional metals (Cr, V, Co, Ni and Sc; 0.001–25 times PM) relative to other elements, which are below detection limits (Table 3; Fig. 7b). The PM-normalized REE and multi-element patterns of metaperidotitic silicates from the tremolite zone (Fig. 7c, d) have been described by Khedr and Arai (2009). Tremolite with a U-shaped REE pattern is strongly enriched with fluid-mobile elements and Sc relative to HFSE. The secondary clinopyroxene (Cpx3), nearly free from MREE, is very low in trace-element concentrations (ΣREE = 0.4– 1.3 ppm) compared to coexisting tremolite (ΣREE = 4.4–8.0 ppm) (Table 3). Tremolite and olivine are the main hosts of Li compared with coexisting minerals such as the Cpx3 and chlorite. The studied olivine shows positive spikes at Li (Fig. 7d) in agreement with Lienriched olivine in chlorite peridotites (Scambelluri et al., 2006; Marocchi et al., 2007) infiltrated by slab-released fluids. Moreover, the mass balance calculation based on in-situ analyses (Table 3; Fig. 7) and volume% of peridotitic silicates (samples RT1, 2, 5) from the tremolite zone indicates that tremolite contributes greatly for all trace elements to the whole-rock budget, while some elements such as Co, Ni, B, Li and Pb are hosted by olivine. Chlorite chiefly accounts for Cs, Cr, V, Nb and Ti enrichment, whereas the Cpx3 accounts for Ba and Sr enrichment in the bulk rocks. In addition, the mass balance calculation of some samples (MM1, SG1, NK5, Kz3*) from the diopside zone indicates that the primary Cpx1 and secondary clinopyroxene (Cpx2) are the main host for all trace elements in their rock, except elements hosted by olivine such as Co, Li, Ni, B and Zr. Chlorite mainly serves as a host for Cs, Rb, Cr, V, Nb and Ti. In this respect, we noticed that the calculated concentrations of Cs, Rb, Ba, Pb, Sr, Ti and Cr are very low compared to the actual concentrations of these elements in the bulk rocks (Table 1). This discrepancy is noted because both Ti and Cr are hosted by Ti-rich chromian spinel, and LILE (Cs, Rb, Ba, Sr and Pb) are possibly concentrated along grain boundaries (e.g., Hiraga et al., 2004; Ishimaru et al., 2007). 5. Discussion 5.1. Metasomatism imposed on the Happo-O'ne peridotites The Happo-O'ne metaperidotites from the tremolite zone are stable at low-T (650–750 °C) and high-P (16–20 kbar) conditions, whereas those from the diopside zone are stable at very low-T, from 400 to 600 °C, at P b 20 kbar (Khedr and Arai, 2010). The slab-fluid metasomatism is associated with subduction of colder and older oceanic crusts (Widom et al., 2003; Kawamoto, 2006), which is similar to the oceanic slab represented by the metamorphic rocks of the Happo-O'ne area (Tsujimori, 2002; Nozaka, 2005 and references therein). Consequently, the oceanic-slab components in the study area are probably aqueous fluids rather than melts. In addition, based on field and petrographic observations, no textural evidence for previous melt–rock interaction was recognized. The Happo-O'ne metaperidotites display U-shaped REE patterns and are extremely depleted in HFSE (Fig. 5) relative to LILE. They are highly enriched in LILE (Cs, Pb, Ba, Sr and Rb), which can be concentrated in low-T (700–800 °C) aqueous fluids that are derived from the subducting slab (Kessel et al., 2005). Savov et al. (2005) stated that forearc serpentinized peridotites from Mariana forearc Conical Seamount with a U-shaped REE pattern are enriched in LILE and highly depleted in U, Th and HFSE due to fluid metasomatism from the subducting Pacific slab. Also, Downes (2001) reported that peridotitic xenoliths (Massif Central, France) fluxed by subduction fluids display U-shaped REE patterns and have negative anomalies at Nb, Zr and Hf, but great enrichment in Rb and Ba. These features, resembling those of the Happo-O'ne peridotites (Fig. 5), are related to 446 Table 3 Representative trace-element compositions (ppm) of silicates in the Happo-O'ne metaperidotites. Diopside zone Sample no. MM1 MM2 MM3 KZ3* SG1 NK5 Mineral Olv Olv Cpx1 Cpx1 Cpx1 Cpx1 Cpx1 Cpx1 Cpx2 Cpx2 Cpx2 Chl Olv Cpx1 Cpx2 Cpx2 Chl Olv Cpx2 Cpx2 B Li Cs Rb Ba Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sc V Cr Co Ni ∑RΕΕ (La/Yb)N (La/Gd)N (Gd/Yb)N 10.3 1.8 bdl bdl 0.25 0.040 0.002 0.006 n.a bdl 0.67 bdl 0.120 bdl bdl bdl 15.4 bdl bdl bdl 0.066 0.003 0.015 0.005 0.040 0.012 3.5 2.7 76.7 196.0 2587 0.08 0.04 5.6 1.5 bdl bdl 0.13 0.023 0.002 0.004 n.a bdl 0.83 bdl 0.065 bdl bdl bdl 11.52 bdl bdl bdl 0.040 0.001 0.008 0.003 0.030 0.011 2.9 0.8 12.7 190.0 2813 0.06 0.05 6.1 1.1 bdl bdl 1.28 0.033 0.476 1.595 1.288 0.171 321.87 0.590 0.248 bdl 0.108 0.071 57.7 0.244 0.070 0.685 5.070 0.186 0.733 0.117 0.789 0.102 39.3 20.7 210.5 13.5 262 5.94 0.41 1.63 0.25 5.8 1.9 bdl bdl 0.77 0.020 0.435 1.527 1.439 0.179 295.12 0.745 0.183 bdl 0.182 0.100 43.2 0.387 0.100 0.994 6.308 0.257 0.852 0.114 0.722 0.080 59.9 28.1 744.0 15.3 253 6.67 0.41 0.94 0.44 5.3 2.6 bdl bdl 0.99 0.019 0.396 1.240 1.082 0.144 257.84 0.560 0.194 bdl 0.145 0.080 40.10 0.288 0.074 0.756 5.329 0.213 0.678 0.102 0.605 0.064 55.8 22.9 596.4 16.7 259 5.35 0.45 1.15 0.39 9.3 2.2 bdl bdl 7.97 0.054 0.847 3.466 1.093 0.438 287.42 1.884 1.352 bdl 0.482 0.159 123.4 0.757 0.148 1.367 9.105 0.338 1.229 0.193 1.237 0.140 46.4 25.0 252.0 14.3 251 12.69 0.47 0.94 0.50 3.51 2.22 bdl bdl 0.27 0.01 0.67 2.83 0.81 0.41 302.7 1.89 0.73 bdl 0.52 0.18 95.39 0.87 0.18 1.59 10.18 0.44 1.41 0.22 1.22 0.14 65.14 30.31 370.0 13.79 217 12.55 0.37 0.64 0.58 2.84 1.84 bdl bdl 0.14 0.01 0.63 2.77 0.87 0.41 302.2 1.85 0.37 bdl 0.55 0.18 80.57 0.80 0.17 1.49 9.38 0.37 1.22 0.17 1.05 0.12 55.87 22.38 414.0 14.10 253 11.79 0.41 0.67 0.62 17.6 1.9 bdl bdl 3.40 0.052 0.852 1.951 0.846 0.204 293.82 0.756 0.932 bdl 0.155 0.044 62.1 0.181 0.027 0.212 1.130 0.043 0.148 0.027 0.244 0.032 13.2 4.6 334.0 15.3 139 4.88 2.37 3.96 0.60 29.52 2.47 bdl bdl 7.84 0.03 0.60 1.46 0.72 0.15 346.5 0.48 0.32 bdl 0.07 0.03 28.32 0.08 0.01 0.08 0.46 0.02 0.05 bdl 0.07 0.02 7.41 6.65 902.3 14.82 156 3.12 6.04 6.42 0.94 26.51 2.29 bdl bdl 7.74 0.03 0.59 1.31 0.65 0.13 327.9 0.44 0.23 bdl 0.07 0.02 23.12 0.07 0.01 0.07 0.40 0.02 0.05 bdl 0.08 0.01 7.48 5.54 938.4 14.34 154 2.87 4.69 6.89 0.68 5.40 bdl 0.033 0.061 0.53 0.319 bdl bdl bdl bdl 0.354 bdl bdl bdl bdl bdl 175.12 bdl bdl bdl 0.011 bdl bdl bdl bdl bdl 9.265 187.07 18778 54.653 1811 5.6 1.3 bdl bdl 0.29 0.025 0.006 0.008 n.a bdl 0.09 bdl 0.076 bdl bdl bdl 10.28 bdl bdl 0.007 0.087 0.002 0.014 bdl 0.043 0.010 3.1 2.3 100.1 190.6 2632 0.09 0.10 26.4 2.2 bdl bdl 2.75 0.213 0.680 2.152 0.713 0.286 324.41 1.391 1.153 bdl 0.385 0.185 106.05 0.468 0.089 0.709 4.222 0.170 0.505 0.074 0.401 0.056 77.3 40.6 1260 12.5 146 7.55 1.15 1.22 0.95 31.6 1.9 bdl bdl 5.56 0.123 0.349 0.704 0.638 0.063 207.95 0.210 0.685 bdl 0.041 bdl 43.98 0.041 0.010 0.086 0.601 0.022 0.086 0.016 0.137 0.030 63.1 14.8 1068 18.3 124 1.80 1.73 7.16 0.24 26.9 1.5 bdl bdl 5.41 0.028 0.308 0.653 0.768 0.062 256.62 0.239 0.588 bdl bdl 0.016 19.85 0.037 0.011 0.074 0.557 0.019 0.076 0.021 0.142 0.029 46.8 8.2 1013.9 17.9 85 1.69 1.47 7.05 0.21 2.9 1.2 bdl 0.197 1.24 0.435 bdl bdl 0.380 bdl 0.19 bdl 0.347 bdl bdl bdl 43.97 bdl bdl bdl bdl bdl bdl bdl bdl bdl 5.2 169.3 32286 51.3 1806 30.66 9.897 bdl bdl 0.16 0.023 0.008 0.017 0.088 bdl 0.270 bdl 0.010 bdl bdl bdl 3.89 bdl bdl bdl 0.008 bdl bdl bdl bdl bdl 2.828 0.251 2.7 204.66 3527 0.02 20.73 0.216 bdl bdl 2.13 0.038 0.187 0.644 0.152 0.073 196.6 0.308 0.589 bdl bdl 0.036 31.64 0.046 bdl 0.108 0.835 0.032 0.117 bdl 0.147 0.027 28.5 21.3 55.5 15.5 287 1.72 0.87 3.39 0.26 30.201 0.038 bdl bdl 0.78 0.016 0.142 0.561 0.157 0.064 199.7 0.242 0.347 bdl bdl 0.029 29.65 0.044 bdl 0.101 0.792 0.032 0.100 bdl 0.109 0.017 33.3 27.5 60.6 16.7 313 1.44 0.89 2.69 0.33 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Met.Zone Met.Zone Diopside zone Sample no. MM1 Mineral Olv MM2 Olv Cpx1 Cpx1 Cpx1 Cpx1 MM3 Cpx1 Cpx1 Cpx2 Cpx2 KZ3* Cpx2 Chl Olv Tremolite zone Sample no. RT1 Mineral Olv Opx* Cpx3 Cpx3 Tr* Chl Opx Tr Chl Olv Cpx3 Cpx3 Tr B Li Cs Rb Ba Nb La Ce Pb Pr Sr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu Sc V Cr Co Ni ∑RΕΕ (La/Yb)N (La/Gd)N (Gd/Yb)N 5.11 1.758 bdl bdl 0.11 0.054 0.012 0.020 0.069 bdl 0.063 bdl 0.011 bdl bdl bdl 11.81 bdl bdl bdl 0.010 bdl bdl bdl bdl bdl 2.33 0.190 2.8 169.55 3113 0.03 1.91 3.81 0.03 0.026 0.77 0.032 0.009 0.014 bdl bdl 1.09 bdl 0.047 bdl bdl bdl 94.50 bdl bdl bdl 0.038 bdl bdl bdl bdl bdl 5.14 16.57 1272 53.62 520 0.02 24.93 1.41 0.03 0.05 20.63 0.03 0.09 0.24 bdl 0.02 135.3 0.06 bdl bdl bdl 0.02 83.94 bdl bdl bdl 0.15 bdl 0.03 bdl 0.05 bdl 4.28 9.56 419 13.97 413 0.50 1.23 23.43 1.52 0.03 0.05 18.53 0.01 0.08 0.20 bdl 0.02 133.6 0.05 bdl bdl bdl 0.02 83.81 bdl bdl bdl 0.17 bdl 0.02 bdl 0.04 bdl 4.79 10.83 543 12.60 413 0.43 1.40 11.88 63.789 0.062 0.374 6.93 0.377 0.959 2.659 0.916 0.278 135.764 1.064 3.060 0.070 0.185 0.119 323.48 0.277 0.063 0.589 4.647 0.159 0.635 0.117 0.878 0.134 76.85 103.93 6559 27.67 769 8.11 0.74 2.91 0.26 4.33 bdl 0.028 0.077 0.82 0.447 bdl bdl bdl bdl 0.285 bdl 0.329 bdl bdl bdl 230.58 bdl bdl bdl bdl bdl bdl bdl bdl bdl 14.257 272.58 22414 53.019 1700 0.00 2.01 3.62 bdl bdl 0.30 0.01 bdl bdl 0.20 bdl 0.22 bdl bdl bdl bdl bdl 213.87 bdl bdl bdl 0.03 bdl bdl bdl bdl bdl 5.65 10.62 449 50.34 483 0.00 16.37 46.64 bdl 0.12 3.47 0.07 0.42 1.34 0.81 0.16 93.55 0.66 1.25 0.09 0.17 0.07 364.38 0.25 0.05 0.38 2.44 0.09 0.32 0.05 0.42 0.07 52.67 56.00 970 26.81 691 4.45 0.69 1.42 0.48 4.25 0.26 0.05 0.09 1.10 0.52 bdl bdl 0.10 bdl 0.11 bdl 0.42 bdl bdl bdl 301.77 bdl bdl bdl bdl bdl bdl bdl bdl bdl 16.48 298.66 25764 52.98 1829 0.00 3.568 1.686 bdl bdl 0.03 0.024 bdl 0.004 0.072 bdl 0.022 bdl bdl bdl bdl bdl 14.80 bdl bdl bdl bdl bdl bdl bdl bdl bdl 2.432 0.853 57.9 168.11 3375 0.00 30.721 1.961 bdl 0.059 34.86 bdl 0.083 0.299 bdl 0.033 187.6 0.104 bdl bdl 0.034 0.022 116.87 0.026 bdl bdl 0.246 0.010 0.028 bdl 0.066 0.010 5.6 10.0 464.1 12.3 385 0.71 0.85 2.70 0.31 27.953 1.059 bdl 0.051 36.79 0.043 0.222 0.599 bdl 0.063 215.2 0.210 0.036 bdl bdl 0.020 91.09 bdl bdl 0.041 0.300 bdl 0.048 bdl 0.063 bdl 9.1 8.7 445 9.7 377 1.27 2.41 10.195 40.459 bdl 0.049 2.53 0.072 0.557 1.863 0.690 0.229 86.9 0.917 1.221 0.078 0.228 0.071 237.93 0.241 0.042 0.361 2.535 0.087 0.347 0.059 0.493 0.074 48.2 49.3 1091 27.8 735 5.57 0.77 1.94 0.40 RT2 RT3 Cpx2 NK5 Cpx2 RT5 KZ3 Hp0 Chl* Tr Tr Olv* 6.58 0.56 bdl 0.129 1.13 0.291 bdl bdl bdl bdl 0.676 bdl 0.238 bdl bdl bdl 220.25 bdl bdl bdl bdl bdl bdl bdl bdl bdl 9.4 228.7 22131 49.8 1697 14.11 53.65 0.02 0.11 7.37 0.42 1.05 2.98 0.91 0.35 123.40 1.44 3.44 0.15 0.41 0.16 794.6 0.51 0.10 0.88 5.76 0.22 0.77 0.13 1.06 0.18 102.16 168.65 6219 28.56 756 10.25 0.67 1.72 0.39 21.01 97.83 bdl 0.14 0.21 0.04 0.31 1.26 0.85 0.17 176.87 0.79 0.38 bdl 0.19 0.10 162.6 0.23 0.05 0.35 2.33 0.09 0.29 0.05 0.36 0.06 42.02 37.87 484 25.32 757 4.29 0.60 1.15 0.52 8.46 2.86 bdl bdl 0.09 0.012 0.002 0.003 0.133 bdl 0.020 bdl bdl bdl bdl bdl 8.62 bdl bdl bdl 0.006 bdl bdl bdl bdl 0.004 2.788 0.201 14.2 172.76 3162 0.01 Chl Olv Cpx2 Cpx2 Hp4 Detection limits (ppm) Tr Chl Tr-PyxChl Olv 16.271 1.903 bdl 0.189 0.61 0.046 0.170 0.414 0.909 0.049 166.7 0.212 0.635 bdl 0.081 0.035 166.01 0.139 0.026 0.279 2.107 0.077 0.293 0.049 0.370 0.062 45.5 54.5 413.0 27.0 745 2.25 0.31 1.03 0.31 4.78 4.11 0.20 0.20 bdl 0.30 bdl bdl 0.11 bdl 0.26 bdl 0.22 bdl bdl bdl 103.26 bdl bdl bdl bdl bdl bdl bdl bdl bdl 11.17 115.99 13656 51.79 2067 1.689 0.558 0.018 0.052 0.12 0.021 0.009 0.009 0.066 0.006 0.011 0.030 0.023 0.043 0.057 0.014 0.79 0.032 0.013 0.036 0.012 0.010 0.024 0.012 0.038 0.021 0.113 0.067 2.751 0.053 3.543 0.397 0.124 0.008 0.014 0.02 0.004 0.004 0.001 0.021 0.001 0.002 0.010 0.011 0.024 0.015 0.003 0.33 0.013 0.003 0.013 0.005 0.003 0.008 0.020 0.013 0.003 0.033 0.023 0.815 0.015 1.272 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Met.Zone SG1 Cpx1 *Opx, *Tr, *Chl, and *Olv are mineral analyses from Khedr and Arai (2009). bdl, below detection limits: n.a, not analyzed. Note U (dl = 0.014 ppm), Th (dl = 0.025 ppm) and Ta (dl = 0.03 ppm) analyses (not listed in Table) for all silicates are below detection limits. Rock types and numbers as those in Table 1. Silicate mineral abbreviations as in Table 2. 447 448 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Fig. 3. Major oxides versus MgO variation diagrams for the bulk-rock chemistry of the Happo-O'ne metaperidotites from both diopside and tremolite zones, compared with Horoman peridotites from Japan (Takazawa et al., 2000), Izu–Bonin–Mariana (IBM) forearc peridotites (Parkinson and Pearce, 1998), and chlorite harzburgites derived from breakdown of antigorite serpentinites from SE Spain (Garrido et al., 2005). Note systematic correlations of oxides with MgO, reflecting partial melting between 15 and 25% by either isobaric batch melting (broken line) or near-fractional polybaric melting (solid line) (e.g., Niu, 1997). This is consistent with partial melting degrees (15 to b30% melting) (b), assuming a fertile lherzolite as a source (Ishiwatari, 1985 and references therein). Total Fe as Fe2O3 is higher than that of the primitive mantle (PM), suggesting Fe-enrichment during metasomatism; also, Na enrichment is related to metasomatism. PM compositions are after McDonough and Frey (1989), McDonough and Sun (1995) and Niu (1997). The major-element concentrations are recalculated to 100% on LOI-free basis. mantle metasomatism by slab-derived fluids (e.g., Keppler, 1996; Widom et al., 2003; Scambelluri et al., 2004; Savov et al., 2005, 2007; Marocchi et al., 2007; Khedr and Arai, 2009). This result is consistent with the presence of slab-derived fluid inclusions enriched in LREE and LILE relative to HFSE within garnet peridotites from Dabie Shan (Eastern China) (Malaspina et al., 2006, 2009). The depletion of HFSE in the Happo-O'ne peridotites (Fig. 5) is possibly inherited from their protoliths that were derived from the mantle wedge (Khedr and Arai, 2009). The chemical evidence indicates that the metasomatizing agent is hydrous fluids rather than melts due to the inability of the hydrous fluids to transport HFSE (e.g., Maury et al., 1992). The Happo-O'ne metaperidotites show large spikes at Ba and Pb (Fig. 5b, d), which are highly concentrated in slab-derived fluids (McCulloch and Gamble, 1991). Also, they are depleted in U relative to IBM forearc peridotites and depleted in HFSE + U relative to chlorite harzburgites from SE Spain (Garrido et al., 2005). The mobility of U is enhanced by the presence of chloride, fluoride and carbonate (e.g., Keppler and Wyllie, 1990; Bailey and Ragnarsdottir, 1994) that may be low in content in the Happo-O'ne peridotites. Further, the mobility of uranium depends on its oxidation state (Tatsumi et al., 1986); U+ 4 is immobile and insoluble in water, but U+ 6 is mobile (Parkinson and Pearce, 1998; Niu, 2004). Kessel et al. (2005) experimentally reported that aqueous fluids enriched in LILE + Pb at low-T (700–800 °C) are depleted in U and Th relative to high-T fluids or hydrous melts, and low-T aqueous fluids are not capable of transporting U and Th from the slab (Hermann et al., 2006). We think that the depletion of U and Th in the Happo-O'ne peridotites and their minerals is possibly due to the limited mobility of these elements M.Z. Khedr et al. / Lithos 119 (2010) 439–456 449 Fig. 4. Conservative-trace elements (ppm) versus MgO (wt.%) variation diagrams for the bulk-rock chemistry of the Happo-O'ne metaperidotites. HREE, Sc, Co and Ni show systematic correlations with MgO due to partial melting. Clear negative correlations of HREE and Sc versus MgO reflect partial melting in the spinel field. Symbols and fields of Horoman peridotites, IBM forearc peridotites and chlorite harzburgites are the same as those of Fig. 3. from the slab by high-P/low-T fluids and the low valence of U during slab-fluid metasomatism. The enrichment of fluid-mobile elements (B, Sr, Pb, Li, Ba and LREE; up to 100 times PM), coupled with high depletion of HFSE (Ta, Hf, Th, Zr, Ti and Nb; b0.7 times PM) + U (Fig. 7a, b) in clinopyroxene generations in the Happo-O'ne metaperidotites, is attributed to the addition of these elements from the subducting slab. These features resemble those of clinopyroxene from SSZ peridotites affected by slab-derived fluids (Bizimis et al., 2000; Choi et al., 2008). The high depletion of Nb and Ta (HFSE), which are not easily transported by fluids, is due to Al-poor nature of clinopyroxene in the Happo-O'ne metaperidotites and the low solubility of these elements in slab- 450 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Fig. 5. PM-normalized REE and trace-element patterns (spider diagram) for bulk-rock compositions of the Happo-O'ne metaperidotites from both diopside (a, b) and tremolite (c, d) zones. Bulk-rock compositions show U-shaped REE patterns and enrichment in fluid-mobile elements with spikes for Cs, Pb, Sr, and Ba (LILE) coupled with depletion of HFSE (Ta, Hf, Zr, Nb, Ti and Th). Note Hf (b 0.04 ppm), Sm (b 0.04 ppm), and Ta (b0.01 ppm) are below detection limits. The studied metaperidotites are similar to peridotites from IBM forearc (Parkinson and Pearce, 1998), Mariana forearc Conical Seamount (Savov et al., 2005) and Horoman complex, Japan (Takazawa et al., 2000), but differ from chlorite harzburgites from SE Spain (Garrido et al., 2005). They represent residues of 15–25% melting from a PM source by using a HREE fractional melting model (Niu, 2004). Note c and d panels (tremolite zone) obtained from Khedr and Arai (2009) are used for comparison. Normalized primitive mantle (PM) values are from McDonough and Sun (1995). The symbols are the same as those used in Fig. 3. M.Z. Khedr et al. / Lithos 119 (2010) 439–456 451 Fig. 7. PM-normalized REE and multi-element patterns for silicates in the Happo-O'ne metaperidotites from both diopside (a, b) and tremolite (c, d) zones. (a) Two different REE patterns, a spoon-shaped pattern for the apparently primary clinopyroxene (Cpx1) and a U-shaped pattern for the secondary clinopyroxene (Cpx2). Clinopyroxene in a suprasubduction zone (SSZ) (Ishii et al., 1992; Parkinson et al., 1992; Bizimis et al., 2000; Grégoire et al., 2001; JunBing and ZhiGang, 2007) is shown for reference. Note olivine with a slightly U-shaped REE pattern is nearly free of MREE. (b) Multi-element patterns of clinopyroxenes, olivine and chlorite showing the enrichment in fluid-mobile elements (B, Sr, Pb, Li and Rb) and transitional elements (Sc, Cr and V), but depletion in HFSE + U, mostly below detection limits. (c, d) Tremolite (Tr), orthopyroxene (Opx), olivine (Olv) and chlorite (Chl) after Khedr and Arai (2009) for comparison. Tremolite showing U-shaped REE patterns and spikes for B, Li, Pb, Sr and Sc, similar in chemistry to retrograde hornblende and tremolite in the mantle-wedge peridotites from Ulten Zone, Italy (Marocchi et al., 2007; Sapienza et al., 2009). Th (b 0.025 ppm) and U (b 0.014 ppm) are below detection limits. Normalized values are obtained from McDonough and Sun (1995). derived fluids (Baier et al., 2008). Ta is very low relative to Nb (Fig. 7b) because of the relatively lower solubility of the former in fluids and clinopyroxene than the latter (Baier et al., 2008). The Happo-O'ne clinopyroxene generations are similar in fluid-mobile element concentrations to clinopyroxene in forearc peridotites (Ishii et al., 1992; Parkinson et al., 1992; Bizimis et al., 2000; Grégoire et al., 2001; JunBing and ZhiGang, 2007) (Fig. 10). The apparently primary clinopyroxene (Cpx1) is depleted in Ti and Zr because of the high degree of melting and no addition of these elements from slab fluids (e.g., Bizimis et al., 2000; Sepp and Kunzmann, 2001). Not only the Cpx1 but also all coexisting major minerals are depleted in Ti and Zr; this depletion of Ti and Zr reflects depletion in HFSE for the whole rock (see Rampone et al., 1991). Orthopyroxene in the Happo-O'ne peridotites is highly depleted in Ti and Zr (Fig. 7d), providing no evidence for metasomatism by slab-derived melts. Sr is high concentration in the examined clinopyroxene generations because of the secondary addition of Sr from slab-derived fluids; this is consistent with high Sr content in peridotites (Alpe Arami) metasomatized by slab fluids (Paquin and Altherr, 2002). In contrast to the Happo-O'ne clinopyroxenes, metasomatic clinopyroxene precipitated from or modified by slab-derived melts exhibits positive spikes for U, Th, Zr, Hf and Ti (Grégoire et al., 2001; McInnes et al., 2001; Malaspina et al., 2009). Tremolite shows strong enrichment in fluid-mobile elements (B, Cs, Sr, Pb and Li; 1–100 times PM) and Sc (3–8 times PM) relative to HFSE (Ta, Zr, Nb, Hf, Th and Ti; b0.8 times PM) + U. This result is consistent with chemical similarity between the Happo-O'ne peridotites and mantle-wedge peridotites (Ulten, Italy), which are metasomatized by fluids from the subducting slab (Marocchi et al., 2007; Sapienza et al., 2009) (Fig. 7c, d). In general, high Sr, Pb, Li and U in amphiboles are related to fluids (Scambelluri et al., 2006). Further, other silicates such as olivine, orthopyroxene and chlorite are enriched in LILE (e.g., B, Sr) + Li relative to HFSE (Table 3; Fig. 7) due to slab-fluid metasomatism. This result is in agreement with high Li, B and Sr contents, which resulted from fluids derived from the subducting oceanic crust or serpentinized peridotites, in ultrahigh-P peridotite minerals from Alpe Arami (Central Swiss Alps) (Paquin and Altherr, 2002; Paquin et al., 2004). In summary, the textural and chemical evidence indicate the involvement of metasomatic fluids rather than melts. 5.2. Trace-element signatures of clinopyroxenes (Implication for partial melting) HREE in clinopyroxene, which are immobile during alteration, are a useful tool in petrogenetic modeling and in determining the Fig. 6. Major oxides versus Al2O3 variation diagrams for clinopyroxenes in metaperidotites, the Happo-O'ne complex. (a) Cr2O3, (b) Mg#, (c) Na2O, and (d) MnO. Fields for the primary clinopyroxene from Izu–Bonin–Mariana (IBM) forearc peridotites (Ishii et al., 1992; Parkinson and Pearce, 1998; JunBing and ZhiGang, 2007; Murata et al., 2009) and the secondary or retrograde clinopyroxene (Kimball et al., 1985; Peacock, 1987; Li et al., 2004; Nozaka, 2005; JunBing and ZhiGang, 2007; Murata et al., 2009) are used for comparison. Note the Happo-O'ne clinopyroxene phases (Cpx1, Cpx2 and Cpx3) are low in Al and Cr and no differences in their major-element compositions, except high MnO content in the Cpx3. 452 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 roxene in abyssal peridotites from the normal ridge segments (Johnson et al., 1990; Johnson and Dick, 1992) to indicate characteristics of the Happo-O'ne clinopyroxenes. The Cpx1 lies mainly in the field of clinopyroxene in abyssal peridotites except for its strong enrichment of LREE (Fig. 8b), suggesting that the Happo-O'ne metaperidotites represent residues of high degrees of partial melting that have been refertilized by fluid metasomatism. Elements Ti, Zr and HREE (i.e., Dy), which are relatively immobile during an alteration process (Tatsumi et al., 1986; Kogiso et al., 1997; Bizimis et al., 2000), are used to characterize clinopyroxenes in metaperidotites from the Happo-O'ne complex (Fig. 9). All clinopyroxene phases lie in the compositional space of SSZ peridotites that are affected by fluids. The apparently primary clinopyroxene underwent partial melting (b20% melting) in the spinel field (Fig. 9); thus, the Happo-O'ne protoliths (lherzolite–harzburgite) are residues after ~20% of partial melting in the spinel field. It is remarkable that HREE contents of the Happo-O'ne clinopyroxenes and tremolite (Fig. 7) are relatively high and do not reflect equilibration with garnet, which would lead to much lower HREE contents in clinopyroxene (e.g., Scambelluri et al., 2006). Chlorite is nearly free from HREE (Fig. 7), suggesting that this chlorite has never been formed after garnet. On the other hand, the HFSE and HREE contents of the whole rock may be used to determine the composition of the pre-metasomatized Fig. 8. Fractional melting models of clinopyroxenes from the Happo-O'ne metaperidotites. (a) Depleted mantle (DM)-normalized REE of the primary clinopyroxene (Cpx1) following a fractional melting model in the spinel stability field, possibly hydrous melting. Calculation method and parameters used in this model, following Johnson et al. (1990), are from Jean et al. (2010). The best fit of HREE in the primary clinopyroxene requires 5–20% melting in the spinel field with reflection in LREE-enrichment due to slab-fluid metasomatism. Note the Happo-peridotite protoliths were formed by polybaric hydrous fractional melting up to 20% melting. Normalized values of DM are from Salters and Stracke (2004). (b) Chondrite-normalized REE of clinopyroxenes following non-modal fractional melting in the spinel field. The calculation method and parameters used in this model, following Johnson et al. (1990), are from Sano and Kimura (2007). A MORB source is from Salters and Stracke (2004); chondrite data are from Anders and Grevesse (1989). This fractional melting model based on clinopyroxene-HREE contents (Cpx1) reflects 1–15% melting in the spinel field. Note the primary clinopyroxene (Cpx1) resembles the Cpx in abyssal peridotites with strong enrichment of LREE. The secondary Cpx2 and Cpx3, having low REE values, and the Cpx in abyssal peridotites (Johnson et al., 1990; Johnson and Dick, 1992) are plotted for comparison. Symbols as in Fig. 7. degree of melt extraction and the nature of melting conditions (e.g., Johnson et al., 1990; Dick and Natland, 1996; Hellebrand et al., 2001). We noted that the behavior of REE in clinopyroxene generations in the Happo-O'ne metaperidotites differs from that of major elements. Hence, we can use HREE of the apparently primary clinopyroxene (Cpx1) to provide reliable chemical information about the magmatic processes before subduction metasomatism (e.g. Johnson et al., 1990). Depleted mantle (DM)-normalized REE abundances (Fig. 8a) indicate the degree and nature of partial melting assuming hydrous fractional melting. The method and parameters used in this model, following Johnson et al. (1990), are from Jean et al. (2010). This model shows that HREE concentrations in the residual clinopyroxene (Cpx1) are precisely matched 1–20% (mainly 5%–15%) fractional melting in the spinel field. Hence, the Cpx1 (low in Na2O and TiO2) underwent near-fractional melting (e.g., Barth et al., 2003). Furthermore, by using non-modal hydrous fractional melting (Johnson et al., 1990; Sano and Kimura, 2007) (Fig. 8b), the best fit of HREE in the primary clinopyroxene (Cpx1) requires 5–15% melting in the spinel field. We compared chondrite-normalized REE in the examined clinopyroxene generations (Fig. 8b) with those in clinopy- Fig. 9. Ti, Dy and Zr (ppm) in clinopyroxenes. The primary clinopyroxene (Cpx1) underwent fractional melting between 15%–20% melting. Note all clinopyroxene phases in the Happo-O'ne metaperidotites are far away the trend of melt percolation, and lie in the filed of clinopyroxene (Cpx) in SSZ peridotite that was metasomatized by fluids. Dry melting is residual clinopyroxene compositions during dry melting (incremental batch melting at 0.1% increments) of a MORB source; amphibole melting is melting of a MORBdepleted source in the presence of amphibole; hydrous melting is melting in the presence of fluids, and exsolved Cpx is after orthopyroxene (Bizimis et al., 2000). Fields for clinopyroxene in SSZ (Parkinson et al., 1992; Batanova et al., 1994; Bizimis et al., 2000) and clinopyroxene in abyssal peridotites (Johnson et al., 1990; Johnson and Dick, 1992) are used for comparison. M.Z. Khedr et al. / Lithos 119 (2010) 439–456 453 Fig. 10. PM-normalized REE (a) and multi-element patterns (b) for all clinopyroxenes in the Happo-O'ne metaperidotites showing different clinopyroxene phases during multi-stage metasomatism (stages 1, 2 and 3). The examined clinopyroxenes were compared to the primary clinopyroxene (Cpx in Ulten Zone) metasomatized by slab-derived melt/fluid in high-P peridotites, Ulten zone, Italy (Scambelluri et al., 2006) and clinopyroxene in SSZ (Ishii et al., 1992; Parkinson et al., 1992; Bizimis et al., 2000; Grégoire et al., 2001; JunBing and ZhiGang, 2007). Note the apparently primary clinopyroxene (Cpx1) with a spoon-shaped REE pattern is similar in REE to the primary clinopyroxene from Ulten Zone, Italy, whereas the secondary clinopyroxenes (Cpx2 and Cpx3) with U-shaped REE patterns lie mainly in the compositional space of SSZ clinopyroxene. The retrograde clinopyroxene (Cpx3) derived from the primary orthopyroxene has very low REE concentrations. The Happo-O'ne clinopyroxene phases are nearly free of Hf, Ta, Th, U and Sm (below detection limits), and are low in Zr and Rb relative to clinopyroxene from Ulten zone (Italy). mantle (PMM) (Maury et al., 1992), and to estimate the nature and degree of melt extraction (e.g. Niu, 2004). We noticed that subduction-conservative elements such as Sc, Ni, Co, HREE, Ca and Al show systematic variations with MgO, in accordance with proposed melting trends (Niu, 1997), suggesting that they are residues after 15 to b30% melting (Figs. 3 and 4). The Happo-peridotite protoliths originated as a series of refractory residues by near-fractional melting (mainly 15%–25% melting) in the spinel field; this result is confirmed by using a HREE melting model of Niu (2004) (Fig. 5a, c). In summary, the chemistry of clinopyroxenes and their bulk rocks confirmed that the Happo-O'ne peridotites are residues after ~20% of partial melting for lherzolites to harzburgites and 20% to 25% melting for dunites in the spinel stability field. This result is supported by textural evidence for spinel facies. 5.3. Three metasomatic stages of clinopyroxene formation All clinopyroxenes generated in the Happo-O'ne metaperidotites share the same major-element compositions, and are low in Al2O3 and Cr2O3 contents compared to the primary clinopyroxene in IBM forearc peridotites (Fig. 6). This depletion of Al and Cr is due to a low-T equilibration of the Happo-O'ne clinopyroxenes in the corner of the mantle wedge during the reaction between peridotites and aqueous fluids (Khedr and Arai, 2010). This result is in agreement with Al content in pyroxenes, which decreases with gradually decreasing temperatures in the lherzolite system (Gasparik, 2000); hence Al is depleted in the Happo-O'ne pyroxenes (Khedr and Arai, 2010). We recognized three different phases of clinopyroxenes based on the morphology and REE pattern. The Cpx1 from the diopside zone is partially to completely surrounded by the Cpx2; hence, the former was formed before the latter. The clinopyroxene (Cpx1) is similar in the morphology and HREE pattern to primary mantle clinopyroxene (residual Cpx), and may be modified and/or recrystallized at the beginning of metamorphism in the mantle wedge at slightly high-T/P. The Cpx1 is somewhat similar in REE concentration and pattern to the primary clinopyroxene in high-P peridotites from Ulten Zone, Italy (Scambelluri et al., 2006) (Fig. 10a). We suggest that the apparently primary clinopyroxene (Cpx1) was metasomatically enriched in LILE and LREE (CeN =2.0–5.7) (Figs. 7 and 10) by the fluid (stage 1). All primary clinopyroxene from the tremolite zone was converted to tremolite and chlorite at the same metasomatic stage (stage 1) by slab-derived fluids according to the reaction, clinopyroxene + orthopyroxene + spinel + H2O =tremolite+ chlorite (e.g., Khedr and Arai, 2010 and references therein). It is remarkable that the tremolite produced from this conversion has the same REE concentrations (ΣREE= 2.3–10.3 ppm) with spikes of fluid-mobile elements as the precursor clinopyroxene (Cpx1) from the diopside zone (ΣREE= 5.4–12.7 ppm) (Table 3; Fig. 7a). The stage-1 fluid, at low-T (650–750 °C)/high-P (16–20 kbar) condition, is highly enriched in LREE (0.2–2 times PM), MREE, Pb, Sr, Li and Rb and highly depleted in Ta, Th, U and Nb (Figs. 7 and 10–12). Moreover, across the subducting slab at the amphibolite-facies condition, LREE, Pb and Sr are extremely released relative to U and Th from the slab to the mantle wedge at low temperature during the early stage of subduction (Hattori and Guillot, 2003; Hermann et al., 2006), similar to the stage-1 fluid in the Happo-O'ne area (Figs. 10–12). The secondary clinopyroxene (Cpx2) from the diopside zone was formed by the fluid–peridotite interaction in the mantle-wedge corner during low-T metasomatism (Khedr and Arai, 2010). In the metasomatic stage 2, the secondary clinopyroxene (Cpx2) and antigorite have been formed after tremolite and olivine according to the reaction, 18 olivine + 4 tremolite + 27 H2O = 8 diopside (Cpx2) + antigorite, at 400–600 °C, P b 20 kbar (Trommsdorff and Evans, 1980), in which diopside is stable instead of tremolite. This is supported by the absence of tremolite from the diopside zone and by the general association of the Cpx2 with antigorite (Fig. 2a–c). The Cpx2 is equivalent to the secondary clinopyroxene after Nozaka (2005), which was formed by the hydration reaction of tremolite and olivine as mentioned above. It is similar in REE content to clinopyroxene in forearc peridotites (Ishii et al., 1992; Parkinson et al., 1992; Bizimis et al., 2000; Grégoire et al., 2001; JunBing and ZhiGang, 2007) (Fig. 10). The Cpx2 displays the same trace-element concentrations and U-shaped REE and multielement patterns as those of precursor tremolite from the tremolite zone (Fig. 7). The low trace-element concentrations (ΣREE = 1.4– 4.9 ppm) in the Cpx2, relative to the apparently primary Cpx1 and clinopyroxene in abyssal peridotites (Fig. 8b), are mainly related to both slightly diluted fluids from the slab in a metasomatic stage 2 (Figs. 10–12) and the low trace-element contents in its source (tremolite + olivine). The Cpx2 is low in LREE (CeN = 0.4–3.2) relative to the Cpx1. The stage-2 fluid is diluted in LREE (0.06–1 times PM), Li and Pb, and nearly free of Cs, Rb and Hf relative to the stage-1 fluid (Figs. 10–12). Furthermore, in the last metasomatic stage (stage 3), the third generation of fine-grained clinopyroxene (Cpx3) was formed after primary orthopyroxene as a retrogressive phase during the exhumation of the Happo-O'ne metaperidotites with the Renge high-P/T metamorphic rocks (Khedr and Arai, 2010). The Cpx3 was retrogressively formed according to the reaction, orthopyroxene + olivine + spinel + H2O (possibly Ca-bearing) = clinopyroxene (Cpx3) +chlorite (Obata and Thompson, 1981). It is closely associated with chlorite (Fig. 2d) and metasomatic calcic-amphiboles (tremolite, edenite and 454 M.Z. Khedr et al. / Lithos 119 (2010) 439–456 Fig. 11. Fluid-mobile elements (Ce, Sr, Ba, B, Li and Pb ppm) versus immobile element (Yb ppm) in clinopyroxenes. The apparently primary Cpx1 modified in a metasomatic stage 1 reflects a composition of the stage-1 fluid that is highly enriched in LREE, Pb, Sr and Li. The secondary Cpx2 and Cpx3 formed in metasomatic stages 2 and 3, respectively, reflect fluid compositions of these stages. The stage-2 fluid is low in LREE, Li and Pb relative to the stage-1 fluid. The stage-3 fluid is nearly free of MREE and Pb, and has very low Sr and LREE concentrations, but has high Ba and B compared to the fluids of stages 1 and 2. Note that the variation in values of conservative element (Yb ppm) is due to various origins and/or source of clinopyroxene generations, while the variation in mobile-element values is related to diluted fluids during the exhumation of the Happo-O'ne peridotites. Note that the depth of the subducting slab and temperature (T) decrease from the stage-1 fluid to the stage-3 fluid (grey arrow). Symbols as in Fig. 7. richterite) (Khedr and Arai, 2010). The Cpx3 is distributed only in tremolite–chlorite peridotites (tremolite zone), which were affected by Na metasomatism (Fig. 3f). It is slightly higher in MnO and Na2O contents than the other clinopyroxene phases (Cpx1 and Cpx2) (Fig. 6c, d); the enrichment of Mn and Na in the Cpx3 is related to its association with Ti-rich chromian spinel and chlorite (Fig. 2d), and Na metasomatism in the later stage, respectively. The Cpx3 is somewhat similar in its U-shaped REE pattern to the secondary clinopyroxene (Cpx2). It shows low trace-element concentrations because of its derivation from primary orthopyroxene by very low-T fluid metasomatism (e.g., Khedr and Arai, 2010). Therefore, this clinopyroxene is nearly free of MREE and Pb, and has very low Sr, LREE (b0.3 times PM; CeN b 1.0) and HREE concentrations, but has high Ba, Na, Mn, B and Cs values compared to the previous two clinopyroxenes (Cpx1 and Cpx2) (Fig. 7 and Figs. 10–12). At a shallower level of the subducting slab (shallow subduction), the stage-3 fluid is poorer in MREE, LREE, Sr and Pb, but highly enriched in Na, Mn, Ba, B and Cs (Figs. 11 and 12). This is in agreement with low LREE and high Cs, As, Sb, B and Li discharged into the mantle wedge at a very low temperature during shallow subduction (Hattori and Guillot, 2003; Savov et al., 2005). This observation provides the evidence that the metasomatizing slabderived fluids became strongly diluted in the late-stage metasomatism during the exhumation of the Happo-O'ne peridotites (e.g., Marocchi et al., 2007). Hence, the metasomatizing fluid compositions from the subducting slab transversely change according to the depth levels of the subducting slab. Hattori and Guillot (2003) studied serpentinites in the forearc mantle wedge and recognized the compositional change (e.g., in LREE, Pb, Sr, B, As and Sb) of slab-derived fluids along the subducting slab by using bulk-rock chemistry. We plot the fluid-mobile elements versus immobile element (i.e.,Yb) of the Happo-O'ne clinopyroxene phases to investigate the retrogressive chemical change of the fluids across M.Z. Khedr et al. / Lithos 119 (2010) 439–456 455 References Fig. 12. The modified schematic illustration (Khedr and Arai, 2010) of the Happo-O'ne subduction zone showing the change of fluid compositions from the oceanic slab. The retrogressive chemical change of the fluid compositions is possibly equivalent to a transversal (nearly vertical) change of the slab-derived fluids within the mantle wedge; the Happo-O'ne metaperidotites during subduction and their exhumation path underwent metasomatism by fluids of different compositions from the deepest part of the subducting slab to the shallow subduction (decreasing slab depth and T). Note the stage-1 fluid, touched the Cpx1 and tremolite at T = 600–800 °C, has high LREE (up to 2 times PM), MREE, Pb, Sr, Li and Rb. The stage-2 fluid, affected on the Cpx2 at T = 400–600 °C, has moderate LREE (b1 times PM), Li and Pb and is very low in Cs, Rb and Hf relative to the stage-1 fluid. The stage-3 fluid, affected on Cpx3 at T = b400 °C, is nearly free of MREE and Pb and has very low Sr and LREE (b0.3 times PM) concentrations, but has high Na, Mn, Ba, B and Cs values compared to the previous two stages 1 and 2. the subducting slab (Fig. 11). LREE, Sr, Pb and Li decrease by decreasing the subducting slab level and T during the exhumation of the Happo-O'ne metaperidotites, while Ba and B increase at shallow-slab depth (Fig. 11), consistent with a compositional change of the slab fluids in the mantle wedge (Hattori and Guillot, 2003) (Fig. 11). Hence, the studied clinopyroxene generations reflect a chemical change of the slab-derived fluids during multi-stage metasomatism (stages 1, 2 and 3) of the Happo-O'ne peridotites (Fig. 12). The chemical change of slab fluids is related to a change of metamorphic condition (P, T), depth of the subducting slab, and an episode of metasomatism. The retrogressive chemical change of the fluid compositions is possibly equivalent to a transversal (nearly vertical) change of slab-derived fluids within the mantle wedge in which slices of the Happo-O'ne metaperidotites have left their original place to shallow depth during the subduction and exhumation path; these metaperidotites were metasomatized by slab-derived fluids with different compositions across the subducting slab (Fig. 12). This is in agreement with a chemical change of the fluid phase recorded by metasomatized high-P peridotites (Ulten Zone, Italy) as well as with the changing metamorphic condition of the mantle wedge (Marocchi et al., 2007). In summary, we suggest three metasomatic stages of clinopyroxene formation (Cpx1, Cpx2 and Cpx3) during subduction metasomatism. 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