Geochemistry of metasomatized peridotites above subducting slab: a case study of hydrous metaperidotites from Happo-O'ne complex, central Japan

313Geochemistry of metasomatized peridotites above subducting slabJournal of Mineralogical and Petrological Sciences, Volume 104, page 313─318, 2009

doi:10.2465/jmps.090611M.Z. Khedr, [email protected] Corresponding author S. Arai, [email protected]

LETTER

Geochemistry of metasomatized peridotites above subducting slab: a case study of hydrous metaperidotites from

Happo-O’ne complex, central Japan

Mohamed Zaki Khedr and Shoji ArAi

Department of Earth Sciences, Kanazawa University, Kanazawa 920-1192, Japan

Hydrous peridotites from the Happo-O’ne area in central Japan, which form a serpentinite mélange in the high-P/T Renge metamorphic belt, vary from depleted lherzolites to harzburgites with subordinate dunite that contains primary chromian spinel with Cr# = 0.72 on average. Systematic variations of major and trace ele-ments of bulk-rock compositions possibly resulted from various degrees of partial melting experienced by their protoliths (15% to <30% fractional melting). Some hydrous peridotites from the tremolite zone are representa-tives of the mantle peridotite facies with Ti-rich chromian spinel (up to 5.7 wt% TiO2), and these peridotites have the same compositions as other peridotites except for enrichment in Na2O, Ba, and Ti. Their bulk rocks show U-shaped PM (primitive mantle)-normalized REE (rare earth elements) patterns (0.05-0.5 times PM) and are enriched with Cs, Pb, Sr, Ba, and Rb (0.2-20 times PM), indicating metasomatism by fluids derived from the subducting slab. The hydrous peridotites contain low amounts of Zr, Hf, Ta, and Nb (HFSEs), which possi-bly indicate that metasomatizing melts are not involved. These results are confirmed by in-situ analysis of trem-olites that show U-shaped REE patterns (0.1-3 times PM) and are highly enriched with fluid-mobile elements (B, Li, Cs, Sr, and Pb; 1-100 times PM) and Sc relative to HFSEs.

Keywords: Bulk-rock compositions, Hydrous metaperidotites, Tremolites, Happo-O’ne, Japan

INTRODUCTION

The characteristics of trace elements of the mantle wedge reveal the nature of metasomatic fluids or melts derived from the subducting slab (e.g., Maury et al., 1992). Man-tle-wedge peridotites are expected to be metasomatized by slab fluids/melts according to the temperature distribu-tion of subduction zones (e.g., Kawamoto, 2006). Mantle-wedge peridotites hydrated by slab-derived fluids are en-riched in LILEs (large ion-lithophile elements) and depleted in HFSEs (high-field-strength elements) (e.g., Scambelluri et al., 2004; Marrochi et al., 2007). Perido-tites from Happo-O’ne, central Japan, which have been metamorphosed to various degrees along with associated high-P/low-T metamorphic rocks, are a good analogy to mantle-wedge peridotites just above the slab. We focus mainly on peculiar hydrous peridotites with Ti-rich chro-mian spinel from the tremolite zone (cf., Khedr and Arai, 2009), which are possible representatives of some part of the mantle wedge above the slab. The aim of this study is

to further unravel the petrogenesis of these hydrous peri-dotites with Ti-rich chromian spinel and to provide im-portant information about the nature of slab-derived fluids and enrichment processes occurring within the mantle wedge.

GEOLOGICAL SETTING AND PETROGRAPHY

The Happo-O’ne complex is located in the northeastern part of the Hida Marginal Tectonic Zone on Honshu Is-land, central Japan (Nozaka, 2005 and references therein). Happo-O’ne peridotites, which are hydrated to various degrees, are partly massive in the central part and foliated in the marginal part, forming a serpentinite mélange (No-zaka, 2005). In this mélange, garnet-mica/garnet-glauco-phane schists, garnet-epidote amphibolites, and eclogites are included in the serpentinite matrix (Nakamizu et al., 1989). Nozaka (2005) stated that the metamorphism is retrogressive in both tremolite and diopside zones, with the exception of a later contact metamorphism (talc zone) caused by a Tertiary granitic intrusion. We studied only hydrous peridotites (12 samples) from the tremolite zone (Nozaka, 2005).

314 M.Z. Khedr and S. Arai

The tremolite zone is characterized by a mineral as-semblage of olivine (>40%) + orthopyroxene (<12%) + tremolite (<11%) + chlorite (<8%) + spinels (<4%) + an-tigorite (<50%) (Fig. 1), and some of the spinels are re-markably Ti-rich (peridotites with Ti-rich spinel) (Khedr and Arai, 2009). Some peridotites in the tremolite zone having olivine (>40%), tremolite (<8%), chlorite (<5.5%), spinels (<3.5%), and antigorite (<50%) are free of ortho-pyroxene and Ti-rich spinel. The olivines are free of opaque inclusions and are similar to primary mantle oliv-ines. The orthopyroxene crystals, up to 8 mm across, with thin clinopyroxene exsolution lamellae (Fig. 1b) resemble primary mantle orthopyroxenes, and have been partly al-tered to diopside and amphibole (Figs. 1a and 1b). These crystals include small (<0.1 mm across) euhedral to sub-hedral grains of Ti-rich chromian spinel having a deep reddish-brown color (Figs. 1a and 1b); this spinel is also enclosed by tremolites in contact with orthopyroxenes. Chlorite flakes do not form clot-like aggregates as a pseu-domorph after garnet (cf., Marocchi et al., 2007). A proto-granular to porphyroclastic texture in dunites with prima-ry chromian spinels is considered as a primary texture (Khedr and Arai, 2009).

CHEMISTRY

Powders of the hydrous peridotite samples were heated up to 1000 °C before preparing fused glasses for major- and trace-element analyses, which are presented in Table 1 (available online from http://www.jstage.jst.go.jp/browse/jmps) and Figures 2 and 3a, 3b. Major-element contents were determined using fused glasses (0.5 g powder + 5.0 g Li2B4O7) by XRF (X-ray fluorescence, System-3270, Rigaku) at a 50-kV accelerating voltage and 20-mA beam current, at Kanazawa University. Further, trace-element

contents were determined using fused glasses (0.3 g pow-der + 1.5 g Li2B4O7) by laser ablation (MicroLas GeoLas Q-plus) coupled with an ICP-MS (inductively coupled plasma mass spectrometry) (Agilent 7500S) (LA-ICP-

MS). For all analyses, the spot diameter was 153 μm at 5Hz with an energy density of 8 J/cm2 per pulse.

The minerals were analyzed for major elements by using an electron microprobe (JXA-8800, JEOL) at Kana-zawa University. The representative mineral compositions are listed in Table 2 (available online from http://www.jstage.jst.go.jp/browse/jmps). The accelerating voltage, beam current, and beam diameter for the analyses were 20 kV, 20 nA, and 3 µm, respectively. The trace-element ab-undances of the representative minerals were determined in-situ by LA-ICP-MS at Kanazawa University; the rep-resentative analyses are listed in Table 3 (available online from http://www.jstage.jst.go.jp/browse/jmps). Analyses were carried out by ablating 60-μm-diameter spots for tremolites, chlorites, and orthopyroxenes. All analyses were carried out at 6 Hz with an energy density of 6 J/cm2 per pulse. An NIST 612 standard glass and SiO2 measured by an electron microprobe were used for calibration.

Bulk-rock compositions

The Happo-O’ne hydrous peridotites from the tremolite zone are mainly lherzolites to harzburgites (= tremolite-chlorite peridotites) with subordinate dunite (Khedr and Arai, 2009). These peridotites have low contents of TiO2 (≤0.02 wt%) relative to the PM (primitive mantle) (Table 1). Lherzolites to harzburgites are slightly depleted with A12O3 (1.1-1.9 wt%) and CaO (0.4-2.4 wt%) relative to the most fertile lherzolite (3-4 wt% A12O3 and CaO) from the Horoman peridotite complex (Takazawa et al., 2000), whereas the dunite is highly depleted with Al2O3 (0.7

Figure 1. Photomicrographs (crossed-polarized light) of Happo-O’ne hydrous peridotites with Ti-rich chromian spinel from the tremolite zone. (a) Tremolite-chlorite peridotite (No. RT1) with a diagnostic mineral assemblage of olivine + orthopyroxene + tremolite + chlorite + chromian spinel. Note the Ti-rich chromian spinel (black euhedral) enclosed by orthopyroxene. (b) Orthopyroxene with thin exsolution lamellae of clinopyroxene (No. RT1). Cr-Spl, Ti-rich chromian spinel; Di, diopside; Am, amphibole; Atg, antigorite; Olv, olivine; Chl, chlorite; Tr, tremolite; Opx, orthopyroxene.

315Geochemistry of metasomatized peridotites above subducting slab

Figure 3. REE and primitive mantle-normalized multi-element patterns for bulk-rock compositions and tremolites from Happo-O’ne hydrous perido-tites. (a), (b) Bulk-rock compositions showing U-shaped REE patterns and spikes in Cs, Pb, Sr, and Ba. Hf (<0.04 ppm), Sm (<0.04 ppm), and Ta (<0.01 ppm) are below detection limits. It should be noted that the Happo-O’ne peridotites represent residues of 15-25% melting from a PM source by using an HREE fractional-melting model (Niu, 2004). The symbols are the same as those used in Figure 2. (c), (d) In-situ analyses of tremolites, orthopyroxenes, olivines, and chlorites; tremolites show U-shaped REE patterns and spikes for B, Li, Pb, Sr, and Sc. Normalized values are obtained from McDonough and Sun (1995). Th (<0.025 ppm) and U (<0.014 ppm) are below the detection limits. Open symbols, peridotites with Ti-rich chro-mian spinel; solid symbols, other peridotites.

Figure 2. (a)-(e) Relations between MgO and major to trace elements in bulk rocks of Happo-O’ne hydrous peridotites (tremolite-chlorite peridotites) from the tremolite zone, compared with Horoman peridotites from Japan (Takazawa et al., 2000), Izu-Bonin-Mariana (IBM) forearc peridotites (Parkinson and Pearce, 1998), and chlorite-harzburgites of deserpentinization origin, from SE Spain (Garrido et al., 2005). The Happo-O’ne peridotites have the same residual trend as those of Ho-roman and IBM peridotites and differ from the SE Span-ish peridotites. The solid and broken lines (a)-(d) indi-cate possible residual trends after polybaric near-fractional (1% melt porosity) melting and isobaric batch melting, respectively (Niu, 1997).The melting curves were calculated for residual spinel and garnet peridotite after Niu (1997) and Takazawa et al. (2000). The primi-tive-mantle (PM) compositions are obtained from Mc-Donough and Sun (1995) and Niu (1997). Note the good negative correlations between Yb (HREE), Sc, and MgO, indicating partial melting in the spinel stability field. (f) Ti-Yb (ppm) subduction-conservative element (Parkinson and Pearce, 1998) of the Happo-O’ne peri-dotites showing degrees of melting around 15-20% in the spinel stability field. FMM: fertile MORB mantle (Pearce and Parkinson, 1993). The element concentra-tions are recalculated to 100% on LOI-free basis. The abbreviations are the same as those shown in Figure 1.

316 M.Z. Khedr and S. Arai

wt%) and CaO (0.5 wt%) and is high in MgO (47 wt%) (Table 1 and Fig. 2). Ti-rich chromian-spinel-bearing pe-ridotites show the same compositions as other peridotites, except for their enrichment in Na2O, Al2O3, CaO (Fig. 2), Ba, Ti, and Sc (Fig. 3b). The Mg# [i.e., Mg/(Mg + total Fe) atomic ratio] of the examined peridotites ranges from 0.90 to 0.92 and is concomitant with that of olivines (Mg# = 0.90-0.92) and orthopyroxenes (Mg# = 0.90-0.91) in these peridotites, indicating neither removal of Mg nor addition of Fe during the metamorphism (Khedr and Arai, 2009). From Table 1 and Figure 2, it can be observed that CaO, Al2O3, Ti, Yb (HREEs, heavy rare earth elements), and Sc decrease but Ni and Co increase with an increase in MgO; the examined peridotites are similar to both Ho-roman peridotites (Takazawa et al., 2000) and Izu-Bonin-

Mariana (IBM) forearc peridotites (Parkinson and Pearce, 1998) and differ from chlorite harzburgites from SE Spain (Garrido et al., 2005).

The hydrous peridotites from Happo-O’ne show U-

shaped PM-normalized REE patterns (0.05-0.5 times PM) (Fig. 3a); their PM-normalized trace-element patterns are enriched in LILEs (Cs, Sr, Ba, Rb, and Pb; 0.2-20 times PM) and depleted in HFSEs (Ta, Hf, Zr, Nb, and Ti) + Th, similar to those in the IBM peridotites (Fig. 3b). Hydrous peridotites have both low trace-element concentrations and high depletion rates of HFSEs, Th, and U relative to chlorite harzburgites from SE Spain (Garrido et al., 2005).

Mineral compositions

Olivines show Fo88-Fo91 except for Fo91-Fo92.5 in dunite (Table 2); they are similar in chemistry to primary mantle olivines in orogenic peridotites (e.g., Arai, 1980). Ortho-pyroxenes show high Mg#s—0.90-0.91. Chromian spi-nels with high TiO2 contents (3.1 wt% on an average, up to 5.7 wt%), which are enclosed mainly by orthopyrox-enes, have high Cr#s of 0.90-0.97 and low Mg#s of 0.1-

0.24 (Table 2) (Khedr and Arai, 2009). The primary chro-mian spinel in dunite has a high Cr# of 0.74 and low Mg# of 0.38, which is similar to spinels of forearc peridotites. Tremolites with Mg#s of 0.95-0.96 have high contents of Al2O3, Na2O, and Cr2O3 (= retrograde tremolite, Nozaka, 2005) relative to the prograde tremolite (Trommsdorff et al., 1998).

The tremolites in the Happo-O’ne hydrous perido-tites show slightly U-shaped PM-normalized REE pat-terns (0.1-3 times PM) (Fig. 3c), which are similar to those displayed by the whole rock; in addition, their PM-nor-malized multi-element patterns (Fig. 3d) are highly en-riched with fluid-mobile elements (B, Li, Cs, Sr, and Pb; 1-100 times PM) and Sc relative to HFSEs and Th (below detection limits) (Table 3), which are similar to those of

their host rocks. Hence, most trace elements in bulk rocks reside principally in the tremolites, which comprise 11% (average = ~ 8%) of the rocks. We cannot neglect the roles of other silicates (Table 3) in the enrichment of flu-id-mobile elements in the whole-rock composition; the chlorites (~ 4.6% on average), orthopyroxenes (<12%), and olivines (>40%) are almost free of REEs (Table 3 and Fig. 3c) except for La and Ce (0.001-0.2 times PM). The PM-normalized multi-elements of the other silicates (Fig. 3d) are enriched with fluid-mobile elements (B, Cs, Li, Pb, and Rb; 0.1-30 times PM) and transition metals (V, Cr, Sc, Co, and Ni; 0.1-20 times PM) relative to other ele-ments that are below detection limits (Table 3). Thus, all the silicate minerals in these peridotites are characteristi-cally enriched with the fluid-mobile elements relative to other elements.

DISCUSSION

Partial melting to form residual peridotite protoliths

From Table 1 and Figure 2, subduction-conservative ele-ments (e.g., HREEs, Al, Ni, Co, Ti, and Sc) (Pearce and Parkinson, 1993) in the peridotites from Happo-O’ne show good correlations with MgO and undergo varying degrees of melt depletion from a fertile-mantle source by either isobaric batch melting or near-fractional polybaric melting (e.g., Niu, 1997); hence, protoliths of the Happo-

O’ne hydrous peridotites were formed as refractory resi-dues by varying degrees of partial melting from 15% to <30% (mainly around 20% fractional melting) in the spi-nel stability field, which is similar to the case of Horoman and IBM peridotites (Fig. 2). Although Sc and Yb (HREEs) are highly compatible with garnet, they are incompatible during melting; thus, the good negative Yb-MgO and Sc-MgO trends (Fig. 2) indicate the residual nature in the spinel stability field (e.g., Niu, 2004). The HREEs are a good measure of the melting degree; an REE fractional melting model (Niu, 2004) suggests that the Happo-O’ne peridotites represent residues of 15-25% melting from the PM source (Fig. 3a).

Subduction-zone metasomatism

Similar to Horoman and IBM forearc peridotites, the Happo-O’ne hydrous peridotites show U-shaped REE patterns (Fig. 3a) and are enriched with LILE (Cs, Pb, Ba, Sr, and Rb) and depleted in HFSEs (Hf, Ta, Zr, and Nb) + Th (Fig. 3b). These characteristics are due to the mantle metasomatism by slab-derived fluids (e.g., Scambelluri et al., 2004; Savov et al., 2005; Marocchi et al., 2007). It is well known that HFSEs are relatively immobile in aque-

317Geochemistry of metasomatized peridotites above subducting slab

ous fluids but more mobile in slab melts (Audétat and Keppler, 2005 and references therein). HFSEs and Th, which behaves like HFSEs (Hawkesworth et al., 1997), are sensitive to melt infiltration and melt-residue interac-tions (Pearce and Parkinson, 1993; Marchesi et al., 2006), and their depletion in the Happo-O’ne peridotites is pos-sibly attributed to their protolith formed in the sub-arc mantle. Hence, we adopted hydrous fluids as the metaso-matizing agents instead of melts, because of the low-tem-perature regime of the metamorphism (e.g., Nozaka, 2005; Khedr and Arai, 2009) and lack of evidence for melt inva-sion.

These results are confirmed by in-situ analysis of sil-icate minerals (Table 3); the tremolites show U-shaped REE patterns and possibly act as a sink of fluid-mobile elements derived from the slab (Figs. 3c and 3d). Because these tremolites are retrogressive after primary clinopy-roxenes (e.g., Nozaka, 2005) in the presence of slab-de-rived fluids, they are enriched with Sc and HREEs that are inherited from clinopyroxenes, and LREEs and LILEs relative to MREEs and HFSEs + Th that are inherited from the slab (oceanic-crust-type) fluids. The enrichment in LILEs and LREEs coupled with depletion of HFSEs + Th (possibly inherited from the Happo-O’ne protolith) is due to the involvement of an aqueous fluid infiltrated in the cooled mantle-wedge peridotites (e.g., Marocchi et al., 2007 and references therein). This result is consistent with the presence of fluid-mobile elements in olivines, chlorites, and orthopyroxenes (Table 3, Figs. 3c and 3d).

Comparison with prograde metaperidotites

The Happo-O’ne hydrous peridotites differ in chemistry from chlorite harzburgites (Figs. 2 and 3) produced by high-pressure breakdown of antigorite serpentinite from SE Spain (Garrido et al., 2005); owing to the prograde or-igin and mineral assemblages of chlorite harzburgites, they are free of Na2O, very low in CaO, and enriched with HFSEs, REEs, Th, and U (Trommsdorff et al., 1998). Their antigorite-serpentinite precursor is enriched with fluorine and chlorine (Garrido et al., 2005), which results in a high mobility of Ti in these harzburgites (Audétat and Keppler, 2005) to form Ti-clinohumite; this is not the case with the Happo-O’ne peridotites.

The TiO2 content of the Happo-O’ne bulk rocks is very low (≤0.02 wt%) relative to PM, indicating that TiO2 was not added from any external source during the meta-morphism; Ti released from primary pyroxenes during the retrograde metamorphism resides mainly in the chromian spinel (Khedr and Arai, 2009). Ti-rich chromian-spinel-bearing peridotites are enriched with Na, Ba, and Ti rela-tive to other hydrous peridotites (free of orthopyroxenes)

because orthopyroxenes, which are sometimes altered to edenite and richterite (enriched with Na) in addition to di-opside and tremolite, were found only in the former peri-dotites.

ACKNOWLEDGMENTS

We thank the Ministry of the Environment and the Forest-ry Agency of Japan for giving us permission to collect samples from the Chubu Sangaku National Park in the Happo-O’ne complex. We are highly grateful to the Egyptian Government (sponsor) for providing financial assistance to the first author. We also thank Dr. A. Tamura for assistance with our LA-ICP-MS. Further, we are grateful to reviewers Dr. E. Takazawa and Dr. T. Kogiso for fruitful comments and to Prof. T. Hirajima for sugges-tions and editorial assistance.

DEPOSITORY AND SUPPLEMENTARY MATERIALS

Tables 1, 2, 3 and color version of Figure 1 are available online from http://www.jstage.jst.go.jp/browse/jmps.

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Manuscript received June 11, 2009Manuscript accepted August 20, 2009

Manuscript handled by Takao Hirajima