Melt-Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan

Melt^Peridotite Reactions and FluidMetasomatism in the Upper Mantle, Revealedfrom the Geochemistry of Peridotite and Gabbrofrom the Horoman Peridotite Massif, Japan

SANJEEWA P. K. MALAVIARACHCHIy, AKIO MAKISHIMA ANDEIZO NAKAMURA*THE PHEASANT MEMORIAL LABORATORY FOR GEOCHEMISTRY AND COSMOCHEMISTRY (PML), INSTITUTE FOR

STUDY OF THE EARTH’S INTERIOR, OKAYAMA UNIVERSITY AT MISASA, TOTTORI-KEN 682-0193, JAPAN

RECEIVED OCTOBER 26, 2009; ACCEPTED APRIL 27, 2010ADVANCE ACCESS PUBLICATION JUNE 11, 2010

An investigation of the petrology and geochemistry of peridotites and

gabbros in the Horoman massif, Hokkaido, Japan was undertaken

to constrain geochemical processes in the upper mantle.Two types of

sample were studied: one type comprises peridotites and gabbros

forming thin layers varying from a few millimeters to centimeters in

scale (thin-layer peridotites and gabbros); the other comprises thick

layers (41m scale; massive peridotites and gabbros). There is no

clear trace element evidence for metasomatism in the thin-layer peri-

dotites. Instead, they have melt^rock reaction textures interpreted in

terms of the formation of secondary pyroxene at the expense of pri-

mary porphyroclastic olivine and dissolution of primary porphyro-

clastic pyroxene to form secondary olivine. The thin-layer gabbros

also exhibit no metasomatic effects; they have incompatible element

depleted trace element characteristics and mid-ocean ridge basalt

(MORB)-like isotopic signatures consistent with the presence of a

new type of gabbro that previously has not been described from the

Horoman Massif.The whole-rock chemistry of the thin-layer perido-

tites and thin-layer gabbros can be explained by melt^peridotite

reactions between isotopically highly depleted MORB mantle

(represented by the thin-layer peridotites) and melt with geochem-

ical affinity to Pacific MORB (represented by the thin-layer gab-

bros). Sm^Nd and Lu^Hf isotope systematics suggest that these

reactions might have occurred at �300 Ma. Some of the plagioclase

lherzolites and all of the spinel lherzolites and harzburgites within

the massive peridotites show enrichment in incompatible trace

elements and more radiogenic Hf^Nd^Pb isotopic compositions

than the incompatible-element depleted thin-layer peridotites. The

analyzed massive gabbros are interpreted as subduction-related

magmas formed in a MORB-source mantle wedge, which have sub-

sequently interacted with a fluid or melt in the Hidaka subduction

zone. Hf^Nd^Pb isotope systematics reveal that this interaction

may have occurred at an age younger than �50 Ma. Melt- and

fluid-related processes occurring in the upper mantle are systematic-

ally identified from the samples of the Horoman Massif based on

petrography, major and trace element, and Sr^Nd^Pb^Hf isotope

geochemistry. These processes occurred in different tectonic settings

such as the convecting oceanic mantle and supra-subduction zone

mantle wedge and have variably modified the original chemistry of

residual mantle protolith, formed by partial melting of a depleted

MORB source mantle at �1 Ga.

KEY WORDS: Horoman peridotite; melt^rock reaction; metasomatism;

upper mantle

I NTRODUCTIONThe Earth’s upper mantle is composed of peridotite that isdepleted in incompatible elements relative to the primitivemantle as a result of a long history of melt extraction by

*Corresponding author. Present address: The Pheasant MemorialLaboratory for Geochemistry and Cosmochemistry (PML), Institutefor Study of the Earth’s Interior, Okayama University at Misasa,Tottori-Ken 682-0193, Japan.Telephone: þ81858 43 3745; Fax: þ81 85843 2184. E-mail: [email protected] address: Research School of Earth Sciences, The AustralianNational University, Building 61, Mills Road Acton, Canberra, Act0200, Australia

� The Author 2010. Published by Oxford University Press. Allrights reserved. For Permissions, please e-mail: [email protected]

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partial melting (Hofmann, 1988; Workman & Hart, 2005);this depletion is reflected in its radiogenic isotope compos-ition. Fragments of the upper mantle are found as perido-tite xenoliths trapped in oceanic and continental basalts,abyssal peridotites on the ocean floor and Alpine-typeperidotite massifs in orogenic belts. Compared withmantle xenoliths or abyssal peridotites, Alpine-type massifperidotites can provide broader windows into the Earth’smantle because they occur on relatively larger scales andare generally less altered.The Horoman peridotite massif is an Alpine-type ultra-

mafic body (orogenic peridotite) located in the Hidakametamorphic belt at the southern tip of Hokkaido innorthern Japan (Fig. 1). This massif covers an area of�80 km2 and is 43 km thick (Niida, 1974, 1984). It com-prises a continental or island arc crust^mantle section anda metamorphosed oceanic crust^mantle section (Komatsuet al., 1994). As one of the least altered orogenic peridotitecomplexes (Niida, 1974, 1984), the Horoman Massif has

the potential to provide direct geochemical informationabout the upper mantle.Niida (1974, 1984) divided the Horoman Massif into two

stratigraphic zones: the Upper Zone and the Lower Zone(Fig. 1). Takahashi (1991) identified three peridotite suitesincluding plagioclase lherzolites, spinel lherzolites, harz-burgites and dunites. The massif is considered to haveformed by partial melting followed by melt extraction, be-cause whole-rock TiO2, Al2O3, CaO and Na2O contentsin the peridotites decrease with increasing whole-rockMgO content, similar to typical partial melting residues(e.g. Obata & Nagahara, 1987; Frey et al., 1991; Takahashi,1991; Takazawa et al., 1992, 2000; Yoshikawa & Nakamura,2000). Takazawa et al. (1996) and Yoshikawa & Nakamura(2000) showed that the plagioclase lherzolites are geo-chemically similar to the estimated mantle source formid-ocean ridge basalt [MORB; depleted MORB mantle(DMM)], as in many other peridotite complexes (e.g.Frey et al., 1985; Bodinier et al., 1988).

Fig. 1. Lithological map of the Horoman peridotite massif (modified after Takahashi, 1992). Sample localities along the Horoman River areindicated. The bold dotted line shows the division into Upper and Lower Zones after Niida (1984).

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Based on Sm^Nd isotope systematics, Yoshikawa &Nakamura (2000) concluded that melt extraction from aDMM source occurred at �850 Ma. The ReÔs isotopedata of Saal et al. (2001) gave an apparent melting age of�900 Ma. Malaviarachchi et al. (2008) reported Sm^Ndand Lu^Hf isochron ages of �1 Ga as the partial meltingage for the Horoman Massif. From the highly depletedNd, Hf and Pb isotopic compositions and geochemicalsimilarities to abyssal peridotites, it is suggested that theHoroman Massif did not originate from subcontinentallithospheric mantle below the Hidaka crustal section, butrepresents a fragment of ancient oceanic mantle(Yoshikawa & Nakamura, 2000; Shimizu et al., 2006;Malaviarachchi et al., 2008). At �23Ma, the Horomanperidotites were affected by fluid metasomatism, whichformed phlogopite veins in the mantle wedge above theHidaka subduction zone, followed by exhumation to thesurface (Yoshikawa et al., 1993).To understand the geochemical history of the Horoman

peridotite massif, in this study we combine trace elementgeochemistry with multi-isotope systematics (Pb, Sm^Nd,Lu^Hf and Rb^Sr) to deduce the history of the melt^peri-dotite reactions and fluid-metasomatism processes, whichare preserved in the Horoman peridotites. Further, weextend our studies to the origin of gabbros in theHoroman Massif (Type I and II gabbros, Shiotani &Niida, 1997; Takazawa et al., 1999). During the course ofthis study a new gabbro type, which is considered to beisotopically similar to Pacific MORB, was discovered.

ANALYZED SAMPLES ANDANALYTICAL METHODSField samplingTwo types of peridotite and gabbro sample were collectedaccording to their field occurrence (Malaviarachchi et al.,2008). One comprises thin-layer peridotites and gabbros(TLP and TLG, respectively), and the other massive peri-dotites and gabbros (MSP and MSG, respectively). TLPand TLG were taken from a single section of �2m thick-ness along the Horoman River, exhibiting alternatinglayers of peridotite and gabbro, a few millimeters to severalcentimeters in thickness. The section is within a large(�1km) plagioclase lherzolite layer in the Upper Zone(Niida, 1984), as shown in Figs 1 and 2. These peridotiteand gabbro layers have sharp contacts between the layerswhich are concordant with their foliation. The sampleswere collected by drilling to avoid mutual contaminationof the layers. The studied TLP consists of nine plagioclaselherzolite and one harzburgite layer. The five TLG consistof olivine- and pyroxene-rich gabbros.Seven MSP, four Type I MSG and three Type II MSG

(Type I is more titaniferous and less chromium-rich thanType II in whole-rock composition; Niida, 1984; Shiotani

& Niida, 1997; Takazawa et al., 1999), with layer thicknessesof a few to several hundred meters, were collected alongthe Horoman River, from the base of the Lower Zone tothe top of the Upper Zone (Fig. 1). The MSP range fromplagioclase lherzolite and spinel lherzolite to harzburgiteto dunite. Five pelitic schist or gneiss samples from the sur-rounding Hidaka metamorphic belt were also sampled.

Sample preparation and analytical methodsThe samples were split into two portions. One portion wasused for thin sections and the other for whole-rockpowder. All sample preparation and analyses were per-formed at the Pheasant Memorial Laboratory forGeochemistry and Cosmochemistry (PML) at theInstitute for Study of the Earth’s Interior (ISEI),Okayama University at Misasa, Japan (Nakamura et al.,2003).Major element mineral chemical analyses were carried

out using a Horiba EMAX- 7000 energy dispersive X-rayspectrometer attached to a Hitachi S-3100H scanning elec-tron microscope. X-ray elemental mapping (Al, Fe, Mg,Ca, Na, Cr, Si and S) of thin sections were performed byelectron probe microanalysis (EPMA) using a JEOL JXA8500 system. In situ trace element compositions of clinopyr-oxene (Cpx) were analyzed by secondary ion mass spec-trometry (SIMS) using a Cameca ims-5f system followingthe procedures outlined by Nakamura & Kushiro (1998).The rock samples were crushed in a jaw crusher, from

which clean fragments devoid of any surface contamin-ation or alteration were selected, washed with Milli-Q

�

water in an ultrasonic bath for 1h, and subsequently driedovernight at 1108C. For the TLP and TLG, the dried frag-ments were ground to a powder in a silicon nitride mortarin a clean laboratory. For MSP and MSG, the dried frag-ments were ground using an alumina pack mill.Whole-rock major element, Cr and Ni contents were

determined in duplicate on fused disks using a PhillipsPW 2400 X-ray fluorescence spectrometer on 10-timesdiluted glass beads made with a lithium tetraborate flux(Takei, 2002). Replicate analyses gave50·5 relative % dif-ference. Li, Rb, Sr, Y, Cs, Ba, rare earth elements (REE),Pb,Th and U were determined by the isotope dilutionîn-ternal standardization (IDÎS) method (Makishima &Nakamura, 2006) by inductively coupled plasma quadru-pole mass spectrometry (ICP-QMS; Agilent 7500cs) using50mg of sample. Another 50mg was used for determin-ation of B, Nb, Ta, Zr and Hf after the IDÎS method (Luet al., 2007a) by ICP-QMS using a Teflon bomb with Aladdition (Tanaka et al., 2003).The analyses from sample di-gestion were done in duplicate, and an average value is re-ported. Analytical reproducibility (RSD) was 510% formost elements.Pb isotope analysis of unleached samples of the MSP was

performed after HF^HBr decomposition, applying thetwo double-spike method of Kuritani & Nakamura (2003)

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by thermal ionization mass spectrometry (TIMS;Finnigan-MAT 262 equipped with five Faraday cups), instatic multi-collection mode. Samples of 5^10 ng of Pbwere analysed and the data were normalized to the NBS981 standard; the analytical reproducibility was 0·008%,0·006% and 0·006% for 206Pb/204Pb, 207Pb/204Pb and208Pb/204Pb, respectively. Total procedural blanks weretypically 15^20 pg and are negligible. UnleachedTLP sam-ples were analysed for Pb isotopes by multicollector(MC)-ICP-MS (Neptune) applying the same HF^HClO4

decomposition and Pb purification method (Makishimaet al., 2007) as for the leached samples described below.For Pb isotope analysis on the leached samples, 500mg ofsample was leached with 6M HCl for 1h in an ultrasonicbath. These leaching conditions were determined afterleaching with 6MHCl for 20, 40, 60 and 80min of samples0902-2 and 0902-10, which have depleted and enriched Pbisotope signatures respectively. Both samples showed no

significant shifts (50·1%) in Pb isotope ratios in eachleaching experiment, indicating the freshness of the sam-ples. After leaching, the samples were decomposed withHF^HClO4, and the Pb was purified and analysed usingthe simple double-spike method (Makishima et al., 2007),by MC-ICP-MS (Neptune, Thermo Electron). Totalamounts of Pb in the analyses were40·8 ng; the reproduci-bility in the Pb isotope composition at this level is50·05% (Makishima et al., 2007). The total blank levelswere �12 pg of which the effects to the result were50·1%and therefore negligible. The U/Pb and Th/Pb of theleached sample solutions were determined by ICP-QMS.The Pb isotope compositions of unleached and leachedsamples were compared (leached ratio/unleached ratio)and it was found that the average difference in magnitudeof a given Pb isotope ratio is not significant, which indi-cates the freshness or unaltered character of these perido-tites. The Pb isotope compositions of the unleached

Fig. 2. Sketch section (left) and field photograph (right) of a 2m section of TLP and TLG. The peridotites and gabbros are interlayered on ascale of a few millimeters to centimeters within the �2m section. Samples were collected as �2·5 cm diameter drill cores. The photographshows the outcrop after drilling just before picking out the samples. The positive or negative numbers indicate that the sample is above orbelow the horizon HR 0 and the distances (cm) from that horizon.


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gabbro and pelitic schist samples were determined usingthe HF^HClO4 decomposition and Pb purificationmethod, as described above, by MC-ICP-MS (Neptune).For Lu^Hf and Sm^Nd isotopic analysis, samples were

digested using a similar process to that used for trace elem-ent determinations, with Lu^Hf and Sm^Nd spikes. TheTi-addition method (Makishima & Nakamura, 2008) wasused for purification of Hf and Lu, and the isotope ratiosof Hf and Lu were determined using the methods of Luet al. (2007b) and Makishima & Nakamura (2008) byMC-ICP-MS (Neptune). Purification of Nd and Sm wascarried out using the method of Nakamura et al. (2003).Analyses were performed by TIMS (Finnigan MAT 262)and Triton for Nd and MC-ICP-MS (Neptune) for Sm.Normalizing values of the isotope fractionation correctionfor 143Nd/144Nd and 176Hf/177Hf were 0·7219 and 0·7325respectively.For TLP and TLG, the Sr isotopic composition together

with the Rb content were determined by ID-TIMS(Finnigan MAT 262), normalized to 86Sr/88Sr¼ 0·1194.Details of the chemistry and mass spectrometry methodsfor Rb and Sr isotope determinations have been given byYoshikawa et al. (1993).

RESULTSPetrographyRepresentative mineral modes determined by point count-ing (41000 points for MSP) and X-ray elemental mappingby EPMA (for TLP and TLG) are shown in Table 1. Thedominant mineral assemblage of both TLP and MSP isolivine (Ol), clinopyroxene (Cpx), orthopyroxene (Opx),spinel (Spl) and plagioclase (Pl). The average abundanceof Cpx in the TLP is �10 vol. %, whereas that in theMSP is �5%, indicating that the TLP are relatively morefertile than the MSP (Table 1). In all samples, Ol occurs asanhedral porphyroclasts (�1^3mm) and fine-grained neo-blasts (51mm). Porphyroclastic Opx and Cpx with anhe-dral shapes are mainly 51^4mm in size. However, insome TLP samples (HR þ12, HR þ20, HR þ55 and HR^90), Cpx occurs as elongate grains. Exsolution lamellaeof Opx in Cpx porphyroclasts are common, providing evi-dence for sub-solidus cooling from high temperatures(Fig. 3a); Cpx lamellae in Opx porphyroclasts are also pre-sent. A two-pyroxene^spinel symplectite with vermicularSpl was observed in HR þ12 (Fig. 3b). In general,51mmSpl occurs interstitially between Ol and pyroxene grainsin the matrix (Fig. 3c and d). Most Pl are anhedral andgenerally50·5mm in size (Fig. 3c and d); they appear tobe derived from sub-solidus decompression reactionsduring the spinel- to plagioclase-facies transition, as alsoobserved in previous studies (e.g. Takahashi, 1991, 1992;Takazawa et al., 1992, 1996, 2000; Yoshikawa & Nakamura,2000). Some Opx and Ol porphyroclasts exhibit highly

strained textures, such as the development of sub-grainboundaries in Opx and kink bands in Ol (Fig. 3d).The petrographic characteristics of MSP are similar to

those described in previous studies (e.g. Niida, 1984;Takahashi & Arai, 1989; Takahashi, 1991; Yoshikawa et al.,1993; Takazawa et al., 1996, 2000; Morishita & Arai, 2003;

Table 1: Representative mineral modal compositions of the

samples

Rock type, Cpx Opx Ol Plg Spl

sample no.

Massive peridotites*

Pl. lhz

0901-4 14·1 18 65 1·8 1

0902-2 3·3 8·5 85·2 1·7 1·3

0902-8 4 8·4 82·1 3·4 1

0902-8a 5·3 6·4 80·5 1·2 2·5

0902-8b 4·3 12·4 77·4 4·1 1

0902-8c 4·6 6·4 79·3 4·9 2·7

0904-1 9·6 24·1 64·5 0·7 0·6

Hrz

0901-2 2·4 10·1 86·1 0·5

0902-3 0·6 13·9 84·3 1·2

Thin-layer peridotitesyz

Plg. lhz

HR þ55 6·5 19·1 67·7 5·3 1·4

HR þ20 8·1 24·5 59·7 6·4 1·3

HR þ12 4·9 20·3 68·7 5·4 0·7

HR 0 24·7 11·2 54·4 9 0·7

HR –37 7·6 19·3 68·3 3·2 1·6

HR –50 6·2 25·3 64·8 2·8 0·9

HR –60 15·1 13·8 59 11·4 0·7

HR –70 15·8 15·1 61·6 6 1·5

HR –90 7·4 25·2 56·4 9·7 1·3

Hrz

HR þ5 3 26 67·5 3 0·5

Thin-layer gabbrosyz

HR –10 24·5 20·6 34·6 19·9 0·4

HR –20 8·6 28·9 44 17·1 1·4

HR –30 17·2 22·7 27·6 30·3 2·2

HR –40 26·2 28·8 21 23 1

HR –80 30·5 9·3 32 26·3 1·9

Pl. lhz, plagioclase lherzolite; Spl. lhz, spinel lherzolite; Hrz,harzburgite.*Modal abundances calculated by point counting (41000points).yThe numbers of the samples indicate the distance (þ/�)from the standard-horizon sample HR 0 in Fig. 2.zModal abundances calculated by EPMA mapping.


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Fig. 3. Petrography and evidence for melt^peridotite reactions in theTLP. (a) Photomicrograph of a Cpx porphyroclast with Opx exsolutionproviding evidence of sub-solidus cooling from high temperatures. (b) Two-pyroxene^spinel symplectite after garnet in sample HR þ12 indi-cates a decompressional history from the garnet peridotite facies. (c) Back-scattered electron image showing the typical peridotite mineral as-semblage in the TLP. (d) Photomicrograph illustrating highly strained peridotite textures such as the development of subgrain boundaries inOpx and kink bands in Ol, and showing sub-solidus plagioclase. (e) Photomicrograph of HR 0 in cross-polarized light highlighting the pyrox-ene dissolution reaction texture (marked with yellow lines) forming secondary olivine. (f) Photomicrograph of HR þ5 in cross-polarizedlight. Reaction rims of Opx around the primary porphyroclastic olivine are highlighted with yellow lines. (g) Crystallization of fine aggregatesof Cpx, Opx and Ol interstitial to pyroxene porphyroclasts, formed as a result of melt^rock reaction. (h) Presence of melt ribs between Cpxporphyroclasts indicating melt migration pathways.


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Sawaguchi, 2004). Briefly, the plagioclase lherzolites ofthe MSP have porphyroclastic textures consisting of Ol,Opx, Cpx, Pl and Spl. Olivine is generally anhedral.Usually, Opx and Cpx contain exolution lamellae. Someof the Ol and pyroxenes in the sample 0908-1 have beenslightly serpentinized (�1% of volume) along fractures.Sample 0901-4 contains two-pyroxene^Spl symplectites.In all samples, very fine-grained (5100 mm) aggregates ofPl, Ol, Spl and rare Opx are observed. Samples 0902-8,-8a, -8b, and -8c contain fluid inclusions in Ol. The spinellherzolites of the MSP contain Ol, Opx, Cpx andSpl. These samples also have porphyroclastic (3^10mm)textures and contain two-pyroxene^Spl symplectites.Exsolution lamellae in pyroxenes and symplectitic, fine-grained (5100 mm), aggregates of Opx, Cpx and Spl arecommon. Fluid inclusions are also present in Ol in thesesamples. Porphyroclastic harzburgite samples in the MSPare composed of Ol, Opx, Spl and rare Cpx, with exsolu-tion lamellae in pyroxene porphyroclasts. Two-pyroxene^Spl symplectites and fine-grained aggregates of Opx, Cpxand Spl are observed. There are fluid inclusions in Ol.Dunites in the MSP have equigranular textures. The Oland Opx are generally �3^5mm in size, but rarelyelongated grains up to 30mm in length also occur. Thesesamples also contain abundant fluid inclusions in Ol.Most TLG contain 1^1·5mm Pl, commonly preserving

resorbed textures. In HR ^30, HR ^40 and HR ^80, sub-hedral Pl with a cumulate texture is observed. The pyrox-enes (Opx and Cpx) are irregular in shape and vary insize from 1 to 4mm. They often preserve exsolution fea-tures. Olivines are51 to 3mm in size and some of the Olgrains are partly included in Cpx, whereas others showinter-cumulus textures. Spinel is fine-grained (51mm), es-pecially in HR ^40, HR ^30 and HR ^80, but also occursas larger anhedral grains, sometimes43mm, and is oftenassociated with plagioclase.Four samples (0902-7, 0902-9, 0903-8 and 0903-11) of the

MSG are Type I and three samples (0903-1, 0903-2 and0903-5) are Type II gabbros. Typical mineral proportionsfor both types of sample are 35^55% Pl, 10^45% Cpx,6^40% Ol and minor amounts of Opx, amphibole, greenSpl and opaque phases.They show equigranular to gneissictextures in thin section. Plagioclase has a cumulus texture.Tiny Ol grains occur as inter-cumulus phases at Pl grainboundaries. Amphibole is interstitial to Cpx and greenSpl occurs in Pl-rich zones.

Reaction textures preserved inTLP

In the TLP, inter-mineral grain boundaries have partlybeen distorted, creating wide, irregular interfacial angles,where primary mineral porphyroclasts (e.g. Ol1, Cpx1and Opx1) are fully or partially dissolved as a consequenceof melt^rock reaction (Fig. 3e and f). Textures suggestingdissolution of pyroxene porphyroclasts (Opx1, Cpx1) and

new growth of fine-grained (50·5mm) secondary Ol(Ol2) are observed in all samples (Fig. 3e):

Cpx1þOpx1ðþmeltÞ ! Ol2ðþmeltÞ: ð1Þ

Newly formed Ol2 forms reaction rim on the partiallydissolved or ‘corroded’ pyroxene grains. Secondary Ol2 isalso found interstitial to primary pyroxene porphyroclasts.The boundaries of the Opx porphyroclasts have becomeirregular as a result of the formation of Ol2 grainembayments.Conversely, there is evidence for dissolution of primary

Ol porphyroclasts (Ol1) forming fine-grained (50·5mm)pyroxene rims around Ol1, most prominently in HR þ5(Fig. 3f). New growth of fine grained Opx and Cpx aggre-gates (Opx2 and Cpx2) interstitial to Ol1 are alsoobserved:

Ol1ðþmeltÞ ! Cpx2þOpx2ðþmeltÞ: ð2Þ

Fine aggregates of Cpx2Ôpx2Ôl2 are found as ‘meltribs’, especially in HR þ20 (Fig. 3g and h). These meltribs have tapering edges and are intergranular to primarypyroxene porphyroclasts. However, all the samples showremnant Ol-rich parts with polygonal grain boundariesdefining an interlocking granoblastic texture free of anyreaction textures.

Major element composition of mineralsIn the TLP, the Mg-number of Ol [Mg/(Mg þ Fe)� 100]varies between 89·5 and 91. Spinel Cr-number [Cr/(CrþAl)� 100] varies between 20 and 35. Plagioclase inthe TLP have anorthite [Ca/(CaþNa)� 100] contentsbetween 62·5 and 73.The TLG have Ol Mg-number between �87 and 89·5.

Spinel Cr-number varies between 25 and 35 and theMg-number for Cpx and Opx ranges between 87·5 and90. Plagioclase in theTLG has anorthite contents between65 and 70.Clinopyroxenes in theTLP were analysed in more detail

than the other phases (Table 2). They are richer in Al2O3

than Cpx in the MSP (Fig. 4). The Cpx porphyroclastsare Cr- and Ti-bearing diopsides (maximum �1wt %Cr2O3 andTiO2). A maximum Al2O3 content of 7·1wt %was observed in the core of the Cpx in sample HR 0. TheAl2O3 content of its rim is �4·9 wt %. Generally, theAl2O3 content shows a decrease from the core to rim ofeach pyroxene porphyroclast.

REE patterns in CpxThe REE patterns of Cpx in the TLP are highly depletedin the light REE (LREE) to middle REE (MREE)region (Table 3, Fig. 5a), broadly comparable with thoseof abyssal peridotites (Johnson et al., 1990; Johnson &Dick, 1992; Hellebrand et al., 2001, 2002). All the sampleshave pronounced Eu negative anomalies except for thesymplectite-bearing sample HR þ12. Compared with the


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Cpx REE patterns of the MSP in this study and previousstudies (e.g. Takazawa et al., 1992, 1996, 1999; Yoshikawa &Nakamura, 2000; Morishita et al., 2004), the REE abun-dances of Cpx in the TLP are 2^3 times higher, and aremuch higher than those of Cpx in average abyssal perido-tites, mantle xenoliths and orogenic peridotites. There isno LREE enrichment in the Cpx of theTLP. Most Cpx ex-hibit weak to moderate heavy REE (HREE) depletion.There is no significant REE zoning observed in the Cpx

of the TLP, except for sample HR 0, which shows REEenrichment at the rim compared with the core (Fig. 5b).Clinopyroxenes in the two-pyroxene^spinel symplectites

of the TLP have different REE patterns from those of theCpx of the host-rock (Fig. 5c and d). The REE contents ofthe symplectite Cpx (A and B in Fig. 5c) are lower thanthose of the host-rock Cpx (C in Fig. 5c) but significantlyhigher than those of the symplectite Cpx in the MSPof this study (see line with black squares in Fig. 5d).

Table 2: Major element mineral chemical compositions of Cpx of the Horoman peridotites by EDS

Rock type Thin-layer peridotites Massive peridotites

Plg. lhz Hrz Plg. lhz

Sample no.: HR þ55 HR þ20 HR þ12 HR 0 HR –37 HR –50 HR –60 HR –70 HR –90 HR þ5 BZ-262* XXy 1072305z 1072403Cz

SiO2 50·52 51·35 51·48 50·42 51·02 50·22 50·36 50·03 51·97 51·58 53·11 53·20 53·17 53·54

TiO2 0·65 0·84 0·07 0·43 0·71 0·47 1·03 0·82 0·73 0·64 0·24 0·3 0·39

Al2O3 4·94 4·95 5·41 4·24 6·00 5·70 6·06 6·11 5·48 5·08 3·88 3·66 3·26 2·67

FeO 2·79 3·21 2·75 3·78 3·16 2·85 3·19 3·11 3·07 2·55 3·03 2·1 3·45 4·35

MnO 0·28 0·05 0·05 0·07 0·10 0·10 0·24 0·24 0·08 0·10 0·05 0·1 0·12

MgO 15·80 17·26 15·99 15·01 14·87 15·85 16·06 15·96 16·68 16·72 18·11 16·95 17·03 20·56

CaO 23·06 21·02 23·28 23·99 21·86 22·73 21·15 22·07 20·41 20·97 20·38 22·82 20·16 15·79

Na2O 0·10 0·73 0·27 0·22 0·92 0·37 1·01 0·28 0·91 0·82 0·64 0·41 0·75 0·57

K2O 0·02 0·02 0·02 0·02 0·01 0·01 0·01 0·01 0·02 0·01

Cr2O3 0·64 0·65 0·57 1·02 0·63 0·73 0·92 0·50 0·62 0·69 1·12 1·03 1·28 1·44

NiO 0·26 0·25 0·05 0·19 0·04 0·08 0·03 0·01 0·02 0·12 0·05 0·06

Total 99·06 100·33 99·94 99·39 99·32 99·11 100·06 99·14 99·99 99·28 100·55 100·28 99·56 99·49

Site assignments (cations on O¼ 6)

T¼ 2 Si4þ 1·862 1·863 1·875 1·868 1·869 1·848 1·835 1·838 1·881 1·882 1·911 1·922 1·936 1·935

Al3þ 0·138 0·137 0·125 0·132 0·131 0·152 0·165 0·162 0·119 0·118 0·089 0·078 0·064 0·065

M1¼ 1 Mg2þ 0·868 0·933 0·868 0·829 0·812 0·870 0·872 0·874 0·900 0·909 0·971 0·912 0·921 1·098

Al3þ 0·076 0·074 0·107 0·054 0·128 0·096 0·095 0·102 0·115 0·100 0·075 0·078 0·076 0·049

Ti4þ 0·018 0·023 0·002 0·012 0·020 0·013 0·028 0·023 0·020 0·018 0·007 0·008 0·011

Cr3þ 0·019 0·019 0·016 0·030 0·018 0·021 0·026 0·015 0·018 0·020 0·032 0·029 0·037 0·041

M2¼ 1 Fe2þ 0·086 0·097 0·084 0·117 0·097 0·088 0·097 0·096 0·093 0·078 0·091 0·063 0·105 0·130

Ca2þ 0·910 0·817 0·908 0·952 0·858 0·896 0·826 0·869 0·791 0·820 0·785 0·883 0·789 0·621

Naþ 0·007 0·051 0·019 0·016 0·065 0·026 0·071 0·020 0·064 0·058 0·044 0·029 0·053 0·041

Kþ 0·001 0·001 0·001 0·001 0·0005 0·0005 0·0005 0·0005 0·001 0·0005

Mn2þ 0·007 0·001 0·001 0·002 0·003 0·003 0·006 0·006 0·002 0·003 0·001 0·003 0·004

Ni2þ 0·008 0·007 0·001 0·006 0·001 0·002 0·001 0·0003 0·001 0·004 0·001 0·002

Total 4·000 4·024 4·008 4·019 4·003 4·015 4·023 4·004 4·004 4·008 4·006 3·994 3·993 3·997

Al/Si 0·115 0·114 0·124 0·099 0·139 0·134 0·142 0·144 0·124 0·116 0·086 0·081 0·072 0·059

Mg-no. 91·0 90·6 91·2 87·6 89·4 90·8 90·0 90·1 90·6 92·1 91·4 93·5 89·8 89·4

Ca-Tschermakite 0·868 0·856 0·879 0·879 0·868 0·855 0·833 0·843 0·869 0·874 0·898 0·919 0·925 0·905

Plg. lhz, plagioclase lherzolite; Spl. lhz, spinel lherzolite; Hrz, harzburgite.*Takazawa et al. (1996).yMorishita et al. (2003).zOzawa (2004).


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The REE pattern of the host-rock Cpx has no Eu anomalyand is slightly enriched in all REE relative to the Cpx inthe symplectite. Cpx in the core and rim of the symplectitehave nearly similar compositions.All Cpx in MSP, except the Cpx in the plagioclase lher-

zolites, show LREE-enriched patterns (Fig. 5e). LREE-depleted patterns and negative Eu anomalies are observedin the Cpx of the plagioclase lherzolites. Spinel lherzolitesand harzburgites show HREE depletion in Cpx (Fig. 5e).The REE patterns of the TLG Cpx are similar to those

of the TLP, and have negative Eu anomalies (Fig. 5f). Cpxin sample HR ^80 shows strong HREE depletion similarto Cpx from a garnet-bearing source (e.g. Johnson et al.,1990; Hellebrand et al., 2002). Abundances of LREE inCpx of the TLG are higher than those of Cpx in the TypeI and II gabbros, but HREE abundances are nearly equalto those of Cpx in theType I and II gabbros.Type I gabbros have LREE- and HREE-depleted Cpx

patterns (Fig. 5f). These samples also show strong negativeEu anomalies. The clinopyroxene of the Type II gabbrosalso has a LREE-depleted pattern with a slight negativeEu anomaly and a flat HREE pattern. The REE abun-dances in the Cpx of the Type I gabbros are higher thanthose of Type II.

Whole-rock major element compositionA description of the whole-rock major element compos-itions of the TLP and MSP has been given byMalaviarachchi et al. (2008) and is reported inthe Supplementary Data (available for downloading at

http://www.petrology.oxfordjournals.org/). We report hereadditional data for the TLG, MSG and Hidaka meta-sediments.The variation of Al2O3, CaO, Na2O, TiO2, NiO and

Cr2O3 in the TLG vs MgO is illustrated in Fig. 6, com-pared with the compositions of the TLP and MSP. TheMgO, TiO2, NiO and Cr2O3 contents of the TLG arehomogeneous. Although the Al2O3 and CaO contents arelower and MgO, NiO and Cr2O3 contents are higherthan those of the MSG (both Type I and II gabbros), theNa2O and SiO2 contents are in the same range as those ofthe MSG. The FeO and TiO2 contents are distinctlyhigher than those of the Type II gabbros, but close tothose of theType I gabbros.The analyzed MSG consist of both Type I and II gab-

bros; their major element compositions are consistent withthose reported in previous studies (e.g. Takazawa et al.,1999), except for higher Na2O contents (Table 4 andFig. 6d). Type I gabbros show relatively large variations inAl2O3,TiO2, Na2O3 and CaO at almost constant MgO.Major element compositions of the Hidaka meta-

sediments (pelitic schist samples) are given in Table 4.Their average composition is close to that of the globalsubducting sediment (GLOSS) of Plank & Langmuir(1998).

Whole-rock trace element chemistryWhole-rock trace element data for theTLP and MSP havebeen given by Malaviarachchi et al. (2008); these dataare provided in the Supplementary Data Electronic

Fig. 4. The Ca-Tschermaks component of Cpx (Al in the tetrahedral site, AlT) inTLP and MSP vs Ca per formula unit (p.f.u).The grey shadedarea represents a typical mantle Cpx composition from Gaetani & Grove (1995). œ, data for the Horoman peridotites from previous studies(Takazawa et al., 1999; Morishita et al., 2003; Ozawa, 2004).


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Appendix. We report here data for the TLG, MSG andHidaka meta-sediments.The TLG have LREE-depleted chondrite-normalized

REE patterns and rather smooth, incompatible elementdepleted, primitive mantle normalized trace element pat-terns, with no evidence for metasomatism (Fig. 7a and b).

The trace element patterns are similar to those of normal(N)-MORB (Hofmann, 1988), but the abundances areabout a factor of four lower than those of N-MORB. Thepatterns exhibit Li and Zr negative and slight Pb and Srpositive anomalies; however, no alkali element or B enrich-ments are observed. Abundances of REE in these samples

Table 3: Trace element compositions (mg/g) of the Cpx of Horoman peridotites and gabbros measured by SIMS

Lithology, Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Hf

sample no.

Thin-layer peridotites

Pl. lhz

HR þ55 7·94 37·2 74·7 0·025 0·552 1·09 5·33 1·10 8·17 3·76 1·12 4·57 5·73 3·34 2·87 0·455 2·21

HR þ20 9·59 34·8 51·9 0·036 0·045 1·00 4·14 0·919 5·92 3·14 1·00 4·13 5·80 3·37 2·86 0·406 1·63

HR þ12 5·36 32·6 40·7 0·021 0·743 3·37 0·852 5·35 2·23 0·919 3·58 5·24 3·17 2·68 0·379 1·06

HR þ12 (symplectite core) 6·24 23·4 23·0 0·018 0·021 0·582 2·70 0·676 3·91 1·98 0·752 3·09 3·39 2·07 1·95 0·339 0·988

HR þ12 (symplectite rim) 6·33 21·6 21·4 0·024 0·230 0·614 2·52 0·624 3·74 1·94 0·594 2·89 3·32 1·88 1·91 0·290 0·854

HR 0 (core) 16·6 35·0 30·0 0·126 3·48 0·993 3·94 0·890 5·57 2·74 0·926 4·55 5·52 3·52 3·77 0·514 1·45

HR 0 (rim) 10·1 54·0 80·2 0·021 0·365 1·22 5·95 1·32 9·78 4·02 1·32 6·27 8·14 5·20 5·02 0·739 2·23

HR –37 8·14 33·5 77·8 0·028 0·136 1·55 6·66 1·52 9·51 4·03 1·21 5·53 5·33 3·08 2·70 0·406 1·57

HR –50 7·15 34·9 92·4 0·033 0·338 1·66 7·60 1·51 9·85 4·24 1·41 5·01 5·61 3·36 3·00 0·363 1·79

HR –60 11·1 48·8 64·4 0·022 0·144 1·82 8·11 1·65 9·93 4·52 1·54 5·54 8·07 4·97 4·44 0·711 1·89

HR –70 9·32 55·0 114 0·017 0·442 1·42 7·32 1·64 12·2 6·08 1·78 7·18 9·11 5·14 4·61 0·616 2·61

HR –90 9·17 54·6 108 0·026 0·347 1·67 7·90 1·95 12·1 4·84 1·73 7·38 9·08 4·92 4·56 0·735 2·36

Hrz

HR þ5 16·5 24·2 53·5 0·036 0·077 1·67 5·98 1·35 8·17 3·23 1·14 4·53 4·47 2·32 1·96 0·293 1·23

Massive peridotites (representative)

Pl. lhz

0901-4 7·25 33·5 51·0 0·175 0·94 4·50 1·03 6·3 2·70 1·00 4·00 5·30 3·00 2·85 0·415

0902-8 6·20 42·0 71·0 0·405 0·84 4·25 1·06 6·95 3·35 0·91 4·60 6·65 3·85 3·50 0·480

Spl. lhz

0904-2 10·5 6·02 2·38 0·873 0·699 0·341 0·864 0·108 0·762 0·240 0·116 0·632 1·19 0·499 0·361 0·023 0·209

0901-1 7·40 23·7 6·05 1·11 0·564 0·218 0·551 0·085 0·637 0·588 0·255 2·36 4·37 2·05 1·54 0·105 0·367

0901-1 (symplectite) 0·546 2·59 0·493 0·148 2·73 0·017 0·039 0·005 0·029 0·032 0·016 0·164 0·352 0·165 0·212 0·020 0·017

Hrz

0901-3 9·18 1·07 0·990 1·00 0·280 0·237 0·601 0·078 0·481 0·317 0·090 0·395 0·168 0·148 0·075 0·006 0·064

Thin-layer gabbros

HR –10 9·67 40·4 65·3 0·018 0·579 1·43 6·39 1·37 9·79 4·05 1·26 5·42 6·47 3·87 3·38 0·481 1·77

HR –20 9·98 53·9 119 0·018 0·106 1·56 7·48 1·82 11·9 5·04 1·78 7·28 8·79 5·28 4·35 0·706 2·49

HR –30 11·3 62·8 136 0·020 2·27 1·72 8·51 1·91 12·4 6·13 1·58 8·37 9·69 5·39 4·88 0·704 3·62

HR –40 7·98 44·8 82·6 0·019 2·17 1·41 6·91 1·53 9·77 3·95 1·34 6·14 7·01 3·92 3·81 0·557 1·98

HR –80 8·73 40·8 74·5 0·202 0·194 1·31 6·37 1·35 8·66 3·28 1·14 4·31 6·24 3·26 1·64 0·171 3·86

Massive gabbros (representative)

Type I

0902-7 4·11 69·6 50·8 0·173 1·22 0·638 3·09 0·604 5·37 4·97 1·10 11·7 13·5 5·38 4·48 0·344 2·58

Type II

0903-1 0·229 2·67 4·00 0·060 0·266 0·006 0·043 0·010 0·077 0·048 0·014 0·206 0·490 0·285 0·329 0·026 0·203

Plg. lhz, plagioclase lherzolite; Hrz, harzburgite; Spl. lhz, spinel lherzolite.


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Fig. 5. Rare earth element patterns of Cpx from the Horoman samples. (a) TLP; dotted line indicates average Cpx composition from mantlexenoliths and orogenic massifs (references are too numerous to be mentioned, but can be obtained from the authors on request); dashed line in-dicates average Cpx composition from abyssal peridotites (Johnson et al.,1990; Hellebrand et al., 2002, 2005). (b) The core and rim REE patternsof HR 0 (TLP) (filled and open symbols, respectively). (c) A back-scattered electron image showing the analysis points of the Cpx in TLPHR þ12 (symplectite shown in Fig. 3b). The core and rim parts of the symplectite and the host-rock Cpx are indicated as A, B and C, respect-ively. (d) REE patterns of Cpx at A, B and C in (c). The line with black squares shows the symplectite Cpx in MSP of this study (Spl lherzolite,0901-1). Single-dot^dash line and double-dot^dash line show REE patterns of symplectite Cpx and bulk symplectite (Morishita & Arai, 2003),respectively. (e) REE patterns of Cpx in the plagioclase and spinel lherzolites and harzburgite of the MSP. Dotted and dashed lines are as in(a). (f) Bold red lines show REE patterns of Cpx inTLG. Average Cpx composition of the Type I andType II gabbros of this study are shownby dashed lines.


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are within the range of some less-abundant MORB-typegabbros (Fig. 7b) found elsewhere (e.g. Zimmer et al., 1995;Miller & Tho« ni, 1997).Type I MSG have weak to strongly depleted LREE^

MREE patterns and HREE abundances close to

N-MORB values (Table 5 and Fig. 7c and d). They showalkali, alkaline earth element (Cs, Rb and Ba) and Pb en-richments with high field strength element (HFSE; Nb,Ta, Zr and Hf) depletions (Fig. 7d). Boron, Pb and Srshow large positive anomalies. One sample (0902-7) shows

Fig. 6. Whole-rock major element compositions of the Horoman peridotites and gabbros. Shaded grey and red^brown fields represent typicalresidual lherzolite and harzburgite compositions of orogenic peridotites, respectively, including previous Horoman data (Frey et al., 1991;Takazawa et al., 2000; Yoshikawa & Nakamura, 2000). Data for TLP and MSP are from Malaviarachchi et al. (2008). Open and crossed circlesrepresent Type I and II gabbros, respectively, fromTakazawa et al. (1999) and Morishita et al. (2003).


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extreme depletion of the LREE (Fig. 7c). Type II MSGhave relatively flat patterns in the MREE^HREE atlower abundance levels than those of the Type I gabbros(Fig. 7e and f). They also have alkali and alkaline earthelement and Pb enrichments and are characterized byprominent positive B, Li, Sr and Pb anomalies like theType I MSG. These samples also show negative HFSEanomalies (Fig. 7f).Trace element compositions of the Hidaka pelitic schist

samples are given in Table 5, and show slightly lower (onaverage) values than GLOSS (not shown).

Whole-rock isotope geochemistryWhole-rock isotope data for the TLP and MSP have beengiven by Malaviarachchi et al. (2008) and in the

Supplementary Data Appendix. We report here data fortheTLG, MSG and Hidaka meta-sediments.The Pb^Nd^Hf isotopic compositions of the gabbros

and Hidaka meta-sediments are reported in Table 6. ThePb isotopic range of the TLG is 206Pb/204Pb¼17·50^18·17,207Pb/204Pb¼15·33^15·44, 208Pb/204Pb¼ 37·33^37·79, ofwhich the most depleted sample coincides with the DMMend-member (Fig. 8a and b). TheTLG define a linear cor-relation with the TLP samples trending towards averagePacific MORB.The TLG have more or less homogeneous Sm^Nd

isotope compositions with a range in 143Nd/144Ndof 0·51302^0·51306 and in 147Sm/144Nd of 0·231^0·247(Table 6 and Fig. 8c). These samples define an isochronwith an age of 257�60 Ma (MSWD¼ 30; initial143Nd/144Nd¼ 0·5126�9, 2SD). The 176Hf/177Hf of the

Fig. 6. Continued.


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TLG ranges from 0·28297 to 0·28312 and 176Lu/177Hf isvariable in the range 0·032^0·049 (Table 6 and Fig. 8d).In the Lu^Hf isochron diagram (Fig. 8d) the TLG definean isochron with an age of 313�64 Ma (MSWD¼10; ini-tial 176Hf/177Hf¼ 0·2828�5, 2SD), which is consistentwith the age derived from the Sm^Nd systematics, al-though the uncertainties of the age are larger. The TLPdo not define a clear isochron, but half of the samples plotalong an �1 Ga isochron defined by the MSP for bothSm^Nd and Lu^Hf systematics (Malaviarachchi et al.,2008).The isotope compositions of the rest of theTLP sam-ples plot along the �300 Ma isochron defined by theTLG(for both Nd and Hf) (Fig. 8c and d).Both Type I and II MSG have Pb isotope compositions

plotting in the MORB field, close to the (average) arcmagma composition (Nakamura et al., 1985; Shibata &Nakamura, 1997) in NE Japan (Fig. 9a and b). The Pbisotopic compositions are: 206Pb/204Pb¼17·90^18·41,207Pb/204Pb¼15·42^15·55, 208Pb/204Pb¼ 37·58^38·34 forType I MSG, and 206Pb/204Pb¼17·77^18·33,

207Pb/204Pb¼15·42^15·51, 208Pb/204Pb¼ 37·44^38·25 forType II MSG (Table 6). These values are comparablewith those reported by Takazawa et al. (1999).Characteristically, the MSG isotope compositions (Figs 9and 10) are collinear with the average compositions ofdepleted MORB, NE Japan arc magmas and GLOSS(Fig. 9a and b). The 143Nd/144Nd varies in the range0·51313^0·51333 for the Type I and 0·51308^0·51326 for theType II gabbros, respectively (Table 6). For 147Sm/144Nd,this range is 0·278^0·495 for the Type I and 0·224^0·280for the Type II gabbros. The eNd(0) is within the range of9·6^13·5 and 12^13 for the Type I and Type II gabbrosrespectively (Table 6). Both gabbro types show trendssignificantly different from those of the TLG. The Type Igabbros and Type II gabbros have 176Hf/177Hf¼ 0·28321^0·28342 and 0·28332^0·28339 respectively. The 176Lu/177HfforType I is 0·062^0·119 and that forType II is 0·029^0·103.For the Rb^Sr isotope compositions of theTLP, 87Sr/86Sr

varies in the range 0·70207^0·70276 and 87Rb/86Sr0·003^0·018 (Table 7). For the TLG, 87Sr/86Sr varies

Table 4: Major element compositions of the Horoman gabbros and Hidaka meta-sediments measured by XRF

Rock type, Major elements (wt %)

sample no. SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO NiO CaO Na2O K2O P2O5 Total

Massive gabbros

Type I

0902-7 46·1 0·475 14·1 0·047 12·4 0·208 9·75 0·016 13·0 1·87 0·138 0·019 98·1

0902-9 48·5 0·358 11·4 0·059 11·7 0·185 11·0 0·024 12·1 2·09 0·310 0·010 97·6

0903-8 50·0 1·02 16·0 0·076 8·29 0·149 8·12 0·017 11·1 2·53 0·775 0·083 98·1

0903-11 49·4 0·340 16·8 0·074 6·89 0·123 10·8 0·024 14·2 1·73 0·166 0·006 100·7

Type II

0903-1 47·5 0·216 18·4 0·093 6·22 0·103 11·1 0·035 12·3 2·22 0·076 0·003 98·3

0903-2 48·9 0·247 18·5 0·110 5·27 0·098 9·18 0·020 13·8 2·45 0·191 0·001 98·7

0903-5 48·9 0·326 16·6 0·041 7·24 0·123 8·78 0·010 15·0 1·45 0·100 0·007 98·5

Thin-layer gabbros

HR –10 44·2 0·282 7·63 0·298 10·2 0·161 30·0 0·192 6·31 0·675 0·011 0·013 100·1

HR –20 46·5 0·287 6·47 0·355 9·37 0·140 31·5 0·197 5·05 0·597 0·015 0·014 100·5

HR –30 46·4 0·295 7·30 0·325 8·65 0·143 29·8 0·175 5·96 0·741 0·010 0·012 99·9

HR –40 46·7 0·317 6·24 0·292 8·64 0·154 30·8 0·186 6·04 0·598 0·029 0·011 100·0

HR –80 45·8 0·310 7·85 0·339 7·78 0·132 29·5 0·191 7·37 0·908 0·024 0·008 100·2

Hidaka meta-sediments

Pelitic schist

0903-6 59·8 0·585 18·3 0·004 6·12 0·098 2·78 0·001 6·12 3·04 1·63 0·137 98·5

0903-7 58·9 1·05 15·2 0·007 9·54 0·147 3·43 0·001 6·01 2·44 1·89 0·176 98·8

0903-9 64·6 0·759 15·2 0·006 7·58 0·090 1·94 0·002 2·78 2·23 3·58 0·173 98·9

0903-10 63·8 0·656 16·2 0·003 6·08 0·089 1·45 0·001 2·80 2·75 4·58 0·143 98·5

0903-12 63·5 0·758 15·5 0·004 8·09 0·137 2·17 0·001 3·86 2·23 2·31 0·181 98·7

*Total Fe as FeO.


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between 0·70245 and 0·70265 and 87Rb/86Sr from 0·005 to0·009. None of theTLP andTLG samples define isochronsin Rb^Sr isotope diagrams. In Sr^Hf, Sr^Nd, Sr^Pb andPb^Nd isotope diagrams (Fig. 8e^h), the TLG show a sys-tematic relationship with the TLP, whereas the TLG havecompositions close to those of Pacific MORB.

Pb^Nd^Hf isotopic compositions of the Hidakameta-sediments (pelitic schists) are reported in Table 6and Fig. 9. Their Pb isotope compositions overlap withsome of the Type I gabbros but trend towards GLOSS,whereas Nd and Hf isotope compositions plot along theterrestrial array (Fig. 9a^c).

Fig. 7. Rare earth element (a, c and e) and trace element (b, d and f) patterns for the Horoman gabbros normalized to chondrite and primitivemantle values, respectively (McDonough & Sun, 1995). The dotted line in (b), (d) and (f) shows the N-MORB composition of Hofmann(1988). A and B in (b) represent MORB-type gabbroic melts from Zimmer et al. (1995) and Miller & Tho« ni (1997), respectively.


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Table 5: Trace element compositions of the Horoman samples measured by ICP-QMS

Rock type, Trace elements (ppm)

sample no. Li B Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd

Massive gabbro—Type I

0902-7 6·05 2·27 7·71 12·1 23·0 9·43 0·19 2·12 2·98 0·06 0·25 0·07 0·75

0902-9 9·28 7·33 10·9 56·4 10·6 4·81 0·17 4·72 20·17 0·21 1·04 0·20 1·23

0903-8 7·96 7·59 18·2 323 21·7 12·1 0·29 0·79 112 1·61 5·88 1·06 6·49

0903-11 9·25 11·6 4·27 148 7·46 6·43 0·10 0·42 14·9 0·64 1·81 0·29 1·77

Massive gabbro—Type II

0903-1 5·68 5·39 1·28 154 5·09 2·82 0·10 0·92 10·9 0·21 0·74 0·15 0·98

0903-2 5·63 13·7 3·12 149 5·72 5·17 0·096 1·96 27·6 0·38 1·04 0·20 1·16

0903-5 6·62 4·09 1·51 119 7·42 5·45 0·10 1·23 15·1 0·27 1·01 0·20 1·40

Thin-layer gabbro

HR –10 1·66 0·08 0·04 32·9 5·37 9·1 0·21 0·001 1·10 0·36 1·32 0·25 1·56

HR –20 1·62 0·09 0·07 30·6 5·60 12·2 0·27 0·001 1·10 0·34 1·10 0·22 1·32

HR –30 1·63 0·08 0·06 31·1 6·52 10·4 0·16 0·0003 0·71 0·36 1·33 0·25 1·59

HR –40 1·65 0·08 0·07 26·6 6·92 11·2 0·18 0·001 1·04 0·34 1·40 0·29 1·78

HR –80 1·71 0·08 0·10 38·8 8·48 11·5 0·14 0·001 1·96 0·44 1·62 0·31 1·92

Hidaka meta-sediments—pelitic schist

0903-6 19·4 5·01 32·3 368 15·8 98·6 2·448 0·625 265 8·33 19·6 2·56 11·3

0903-7 20·6 3·62 38·2 289 22·3 136 3·988 0·688 380 12·6 28·5 3·65 16·1

0903-9 31·9 7·82 39·7 314 15·4 121 4·14 2·61 149 16·5 37·1 4·25 17·0

0903-10 20·5 3·02 47·6 402 16·1 128 5·206 2·62 269 21·5 45·5 5·40 21·3

0903-12 25·4 21·3 43·0 383 16·6 122 4·907 1·47 384 13·9 31·9 3·86 16·4

Rock type, Trace elements (ppm)

sample no. Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U


0902-7 0·76 0·36 1·92 0·44 3·71 0·92 3·02 0·46 3·16 0·50 0·44 0·02 0·44 0·02 0·02

0902-9 0·56 0·28 1·22 0·25 1·90 0·44 1·32 0·20 1·32 0·20 0·26 0·02 1·05 0·01 0·05

0902-8 2·36 0·95 3·41 0·60 4·20 0·90 2·55 0·37 2·39 0·35 0·65 0·04 1·84 0·04 0·02

0902-11 0·71 0·40 1·13 0·21 1·49 0·32 0·92 0·13 0·87 0·13 0·29 0·01 1·88 0·05 0·01


0903-1 0·43 0·30 0·74 0·14 1·00 0·22 0·62 0·09 0·59 0·09 0·15 0·01 0·43 0·001 0·001

0903-2 0·47 0·30 0·81 0·15 1·06 0·23 0·66 0·10 0·61 0·09 0·20 0·01 0·48 0·02 0·01

0903-5 0·64 0·35 1·05 0·21 1·45 0·32 0·91 0·13 0·86 0·13 0·24 0·01 0·51 0·006 0·004

Thin-layer gabbro

HR –10 0·58 0·25 0·847 0·152 1·03 0·22 0·63 0·09 0·59 0·09 0·28 0·02 0·20 0·01 0·003

HR –20 0·50 0·22 0·799 0·150 1·06 0·23 0·67 0·10 0·65 0·10 0·35 0·02 0·13 0·01 0·003

HR –30 0·61 0·26 0·932 0·179 1·23 0·27 0·81 0·12 0·75 0·12 0·35 0·01 0·23 0·01 0·003

HR –40 0·68 0·27 1·03 0·191 1·36 0·30 0·86 0·13 0·85 0·13 0·38 0·01 0·30 0·01 0·004

HR –80 0·70 0·29 1·11 0·218 1·55 0·35 1·04 0·16 1·04 0·16 0·39 0·005 0·23 0·02 0·004

Hidaka meta-sediments—pelitic schist

0903-6 2·72 0·79 3·16 0·49 3·11 0·64 1·83 0·26 1·74 0·262 2·57 0·16 6·26 1·63 0·544

0903-7 3·77 1·09 4·30 0·68 4·37 0·89 2·58 0·37 2·42 0·374 3·98 0·27 7·51 2·56 0·916

0903-9 3·50 1·03 3·53 0·53 3·22 0·65 1·87 0·27 1·84 0·275 3·45 0·28 2·21 6·01 1·47

0903-10 4·03 1·00 3·93 0·55 3·20 0·64 1·82 0·27 1·78 0·279 3·71 0·36 3·45 7·54 2·31

0903-12 3·53 0·99 3·64 0·55 3·45 0·71 2·05 0·31 2·05 0·310 3·55 0·34 11·33 4·43 1·39


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Table 6: Pb, Nd and Hf isotopic compositions of the Horoman gabbros and Hidaka meta-sediments

Rock type, 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb Nd (ppm) Sm (ppm) 143Nd/144Nd*

sample no.


0902-7 18·349 15·542 38·334 0·93 0·76 0·513332� 6

0902-9 18·343 15·542 38·327 1·01 0·63 0·513131� 6

0903-8 18·244 15·519 38·201 6·47 2·40 0·513255� 6

0903-11 18·271 15·522 38·239 1·74 0·71 0·513269� 6


0903-1 17·711 15·407 37·432 0·94 0·44 0·513308� 4

0903-2 18·020 15·438 37·715 1·18 0·51 0·513274� 6

0903-5 17·848 15·418 37·569 1·42 0·65 0·513298� 6

Thin-layer gabbros

HR –10 17·908 15·426 37·631 1·59 0·62 0·513050� 6

HR –20 18·008 15·416 37·629 1·35 0·55 0·513050� 7

HR –30 18·080 15·440 37·704 1·55 0·61 0·513061� 4

HR –40 18·169 15·444 37·799 1·77 0·69 0·513035� 6

HR –80 17·511 15·337 37·344 1·96 0·75 0·513020� 7


0903-6 18·308 15·525 38·252 11·9 2·92 0·512827� 6

0903-7 18·373 15·537 38·330 15·9 3·90 0·512807� 4

0903-9 18·405 15·551 38·389 17·8 3·75 0·512646� 5

0903-10 18·401 15·541 38·354 21·7 4·25 0·512666� 6

0903-12 18·346 15·540 38·330 16·9 3·77 0·512695� 4

Rock type, 147Sm/144Nd eNd(0) Hf (ppm) Lu (ppm) 176Hf/177Hf* 176Lu/177Hf eHf(0)

sample no.


0902-7 0·495 13·5 0·44 0·37 0·283418� 4 0·119 22·8

0902-9 0·376 9·6 0·33 0·17 0·283206� 5 0·073 15·3

0902-8 0·224 12·0 1·75 0·35 0·283319� 3 0·029 19·3

0902-11 0·247 12·3 0·30 0·22 0·283335� 8 0·103 19·9


0903-1 0·280 13·0 0·16 0·09 0·283389� 9 0·078 21·8

0903-2 0·262 12·3 0·24 0·09 0·283344� 9 0·054 20·2

0903-5 0·278 12·8 0·27 0·12 0·283360� 4 0·062 20·8

Thin-layer gabbros

HR –10 0·234 8·0 0·35 0·08 0·283080� 5 0·032 10·9

HR –20 0·247 8·0 0·42 0·10 0·282966� 4 0·034 6·9

HR –30 0·239 8·2 0·41 0·11 0·283028� 6 0·037 9·1

HR –40 0·236 7·7 0·42 0·13 0·283090� 5 0·043 11·3

HR –80 0·231 7·4 0·45 0·16 0·283118� 4 0·049 12·3


0903-6 0·149 3·7

0903-7 0·149 3·1 4·1 0·37 0·282958� 5 0·013 6·6

0903-9 0·127 0·2 4·6 0·28 0·282883� 6 0·009 3·9

0903-11 0·118 0·6

0903-12 0·134 1·2

*Uncertainties are in-run analytical errors (2SE) in last decimal place.Standards are as follows. NBS 981 (n¼ 30): 206Pb/204Pb¼ 16·942� 0·001; 207Pb/204Pb¼ 15·500� 0·001;208Pb/204Pb¼ 36·726� 0·001 (1SD). PML Nd (n¼ 13) by MAT262 (M): 143Nd/144Nd¼ 0·511725� 15 (2SD); correspondingLa Jolla ratio¼ 0·511861. eNd(0) was calculated based on 143Nd/144Nd CHUR(0)¼ 0·512638 at La Jolla Nd¼ 0·511860;total blanks for Nd and Sm were 1·2 pg (n¼ 4) and 1·3 pg (n¼ 3), respectively; Nd concentration was determined sim-ultaneously with Nd isotope ratio by TIMS; Sm concentration was determined by ID-MC-ICP-MS (Neptune). eHf(0) wascalculated based on 176Hf/177Hf CHUR(0)¼ 0·282772; total blanks for Hf and Lu were 22·4 pg (n¼ 4) and 2·9 pg (n¼ 4),respectively; Hf concentration was determined simultaneously with Hf isotope ratio by MC-ICP-MS (Neptune) and Luconcentration was determined by ID-MC-ICP-MS (Neptune).


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Fig. 8. Isotopic compositions of the TLG and TLP. (a) and (b) show variation of 206Pb/204Pb vs 207Pb/204Pb and 208Pb/204Pb, respectively.Linear regression line for theTLP is from Malaviarachchi et al. (2008). (c) 147Sm/144Nd^143Nd/144Nd isochron plot for theTLG andTLP show-ing an isochron of 257�60 Ma (2SD) for theTLG. An isochron with an age of 1 Ga for the MSP (Malaviarachchi et al., 2008) is also shown.(d) 176Lu/177Hf^176Hf/177Hf isochron plot for the TLG showing an isochron of 313�64 Ma (2SD) for the TLG plotted withTLP. An isochronwith an age of 1 Ga for MSP (Malaviarachchi et al., 2008) is also shown. The cross-hatched areas in (a)^(d) indicate the compositions of theMSP. (e) 87Sr/86Sr vs 176Hf/177Hf, (f) 87Sr/86Sr vs 143Nd/144Nd, (g) 87Sr/86Sr vs 206Pb/204Pb and (h) 206Pb/204Pb vs 143Nd/144Nd forTLG andTLP.


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Fig. 9. Present-day isotopic compositions (a^c) and Th/La ratios (d) of the MSG and Hidaka meta-sediments. (e, f) Sm^Nd and Lu^Hf iso-chron diagrams. Symbols for MSG are as in Fig. 6; yellow hexagons represent Hidaka meta-sediments. Red open square in all panels andopen circle in (a)^(c) indicate the most depleted MORB value and highly depleted Horoman mantle composition, respectively(Malaviarachchi et al., 2008). (a) and (b) show 206Pb/204Pb vs 207Pb/204Pb and 208Pb/204Pb, respectively. Star indicates the arc magma compos-ition from NE Japan (Nakamura et al., 1985; Shibata & Nakamura, 1997). (c) 176Hf/177Hf vs 143Nd/144Nd. Dashed line represents the terrestrialarray (Chauvel et al., 2008). (d) Th/La vs 206Pb/204Pb, (e) Sm^Nd and (f) Lu^Hf isochron plots for the MSG. None of the samples define ameaningful isochron (‘pseudochrons’ shown as regression lines), showing a scattering of data points in both diagrams. GLOSS indicates theglobal subducting sediment composition (Plank & Langmuir, 1998) and the dotted fields in (a) and (b) are the global MORB compositionfrom the PetDb database (http://www.petdb.org/petdbWeb/).


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Fig. 10. Modelling the Pb, Hf and Nd isotopic compositions of the MSG to evaluate the influence of a subduction component (slab-derivedfluid or melt). Symbols for MSG are as in Fig. 6. Red open square in all panels indicates the most depleted MORB value (Malaviarachchiet al., 2008). Plots of age-corrected (a^h) and present-day (i, j) 143Nd/144Nd and 176Hf/177Hf vs 206Pb/204Pb for the MSG with model binarymixing curves between highly depleted MORB (red square; Malaviarachchi et al., 2008) and a subduction component with a similar isotopiccomposition to the average Hidaka meta-sediment (grey hexagon). Initial ratios at450 Ma are too scattered to provide a good fit for themixing relationship, suggesting that the age of interaction of the MSG with a subduction component is550 Ma. The per cent increments oneach curve indicate the proportion of the subduction component added to the MSG. Calculation parameters for highly depleted MORB:206Pb/204Pb¼17·3, Pb¼ 0·18 ppm; 143Nd/144Nd¼ 0·51335, Nd¼11·17 ppm; 176Hf/177Hf¼ 0·28340, Hf¼ 2·97 ppm. Average Hidaka metasediment:206Pb/204Pb¼18·41, Pb¼ 2·21ppm; 143Nd/144Nd¼ 0·51273, Nd¼17 ppm; 176Hf/177Hf¼ 0·28292, Hf¼ 4·35 ppm (Malaviarachichi et al., 2008).


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DISCUSSIONPartial melting of Horoman peridotitesThe two-pyroxene^spinel symplectite textures observed inboth theTLP and MSP indicate the former presence of re-sidual garnet and subsequent sub-solidus breakdownduring re-equilibration from garnet- to spinel-facies condi-tions. Interstitial plagioclase found in the plagioclase lher-zolites provides evidence of a decompression history fromspinel to plagioclase facies. LREE-depleted REE patternssimilar to those of abyssal peridotites observed in the Cpxof theTLP and massive plagioclase lherzolite samples pro-vide evidence for their residual character (Fig. 5a and e).The presence of garnet during partial melting can also beinferred from the HREE-depleted REE patterns displayedby the Cpx of the TLP and the massive spinel lherzolitesand harzburgites. Negative Eu anomalies in the Cpx ofthe TLP and the massive plagioclase lherzolites may indi-cate subsequent plagioclase-facies equilibration. The (flat)

LREE-enriched REE patterns of the refractory spinellherzolites and harzburgites of the MSP indicate latermetasomatic effects (Fig. 5e).Whole-rock major element variation trends (Fig. 6) are

consistent with the data of previous studies (Obata &Nagahara, 1987; Frey et al., 1991; Takahashi, 1991; Takazawaet al.,1992, 2000;Yoshikawa & Nakamura, 2000), other oro-genic peridotites (Frey et al., 1985; Bodinier, 1988; Bodinieret al., 1988) and abyssal peridotites (Niu, 1997, 2004; Niuet al., 1997; Baker & Beckett, 1999), implying extraction ofpartial melts from a relatively homogeneous source.Four massive plagioclase lherzolite samples (0904-1, -1a,

-1b and -1c), which have strong negative Zr and Hf anoma-lies, HREE depletion and the most enriched 176Hf/177Hfisotope compositions among the MSP whole-rocks(Malaviarachchi et al., 2008) exhibit behaviour that isdecoupled from the crust^mantle array (Fig. 11a and b).Similar compositions have also been reported by Salters

Fig. 10. Continued.


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& Zindler (1995) and Bizimiz et al. (2004) from theHawaiian (Salt Lake Crater) peridotite xenolith suite.Generally the Hf isotope compositions of mantle materialsare well correlated with their Nd isotope compositions asa result of the similar behaviour of Sm/Nd and Lu/Hfduring mantle differentiation processes; these parent^daughter ratios are not sensitive to modification byshallow-level processes but are changed during meltingprocesses (Salters & Hart, 1989). Therefore, we suggestthat the decoupling of Nd from Hf in some of our samplescould possibly be due to selective and probably extensivemelting of a garnet-rich source, during partial melting.Many of the plagioclase lherzolites of theTLP and MSP

have high positive eNd(0) and eHf(0) values (Table 6,Fig. 11b). These samples plot considerably above theDMM evolution curve (Goldstein et al., 1984; Vervoortet al., 1999; not shown). This implies that the Horoman

residual mantle has evolved with higher Sm/Nd and Lu/Hf, from the �1 Ga partial melting event that formed theHoroman DMM, compared with those of MORB. TheHoroman peridotite massif is interpreted to have beenformed beneath a paleo-Pacific ultraslow-spreadingridge segment (e.g. Shimizu et al., 2006) whereas the�1 Ga age has been interpreted to correspond to the timeof break-up of the Rodinia supercontinent to formthe proto-Pacific Ocean (e.g. Hoffman, 1991; Li et al.,1999; Torsvik, 2003; Cawood, 2005; Ishikawa et al., 2007),which is also evidenced in some ophiolite samples (e.g.Pearson et al., 2007).Thus, from this perspective, the forma-tion of the Horoman Massif might be related to intense(proto) Pacific mantle melting associated with thebreak-up of Rodinia at �1 Ga, resulting in a large degreeof depletion in the residual mantle, which eventuallyevolved to the high positive present-day eHf and eNd

values (Table 6).

Melt^peridotite reactions in TLP andorigin of TLGTheTLP have more fertile modal compositions comparedwith the MSP, with increased pyroxene and plagioclaseabundances (Table 1). Dissolution of primary pyroxeneand olivine (Opx1, Cpx1, Ol1) to form their secondarycounterparts (Opx2, Cpx2, Ol2) is the main feature of themelt^peridotite reactions in the TLP [reactions (1) and(2)]. Melt^peridotite reaction processes have apparentlycaused the depleted TLP to become enriched in pyroxeneand Pl, and overall incompatible elemental abundances.In contrast, melt^peridotite reactions are not observed inthe MSP.Petrographic data suggest that the melt^peridotite reac-

tions may have occurred at a fairly deep level in the re-sidual mantle, at least in the spinel peridotite facies,because the sample with the two-pyroxene^spinel symplec-tite after garnet (HR þ12) also shows the evidence for re-action (1). This melt^rock reaction process may havecontinued as the melt composition changed during inter-action with the surrounding peridotite along its pathway.The melt^peridotite reactions can convert the host perido-tite towards either more refractory [reaction (1)] or morefertile [reaction (2)] compositions. Furthermore, the min-eral textures suggest that the melt migration observed inthe TLP may not be pervasive, because there are melt-unaffected zones with equilibrium grain boundariespresent.Potential evidence for possible remnants of the reacted

melt occurs in the harzburgite sample (HR þ5) in theform of ‘melt pods’ associated with reaction rims ofOpx2 (Fig. 3f). However, importantly, these melt pods arenot glass, but composed solely of plagioclase. A relict Olgrain found within a melt pod (Fig. 3f) suggests thatthe melt has reacted with pre-existing Ol1, implyingthat it was silica-saturated and that dissolution of olivine

Table 7: Sr isotopic compositions* of the Horoman

peridotites and gabbros

Rock type, Rb (ppm) Sr (ppm) 87Sr/86Sry 87Rb/86Sr

sample no.

Thin-layer peridotites

Pl. lhz

HR þ55 0·036 28·2 0·702178� 8 0·004

HR þ20 0·009 9·4 0·702065� 7 0·003

HR þ12 0·012 5·5 0·702205� 11 0·006

HR 0 0·040 18·4 0·702360� 8 0·006

HR –37 0·036 5·8 0·702476� 10 0·018

HR –50 0·030 7·8 0·702443� 8 0·011

HR –60 0·043 19·8 0·702528� 8 0·006

HR –70 0·033 17·3 0·702477� 7 0·006

HR –90 0·026 10·4 0·702763� 10 0·007

Hrz

HR þ5 0·028 7·0 0·702306� 9 0·011

Thin-layer gabbros

HR –10 0·054 32·2 0·702450� 8 0·005

HR –20 0·056 30·6 0·702470� 8 0·005

HR –30 0·063 28·9 0·702481� 8 0·006

HR –40 0·077 25·6 0·702456� 9 0·009

HR –80 0·083 38·3 0·702647� 8 0·006

Standards

JB-2 (n¼ 3) 6·22 177·2 0·703597� 8 0·102

NIST (n¼ 10) 0·710196� 6

*Isotope ratios were measured by TIMS (MAT 262).yUncertainties are in-run analytical errors (2SE) in last deci-mal place; total blanks for Rb and Sr were 2·4 pg and17 pg, respectively (n¼ 3).Pl. lhz, plagioclase lherzolite; Hrz, harzburgite.


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had started [i.e. reaction (2)]. Characteristically, the‘melt pod-plagioclase’ has a lower anorthite and aluminacontent (An¼ 52, Al2O3 �29wt %) compared with theinterstitial Pl (An¼ 71, Al2O3 �33wt %) in the samesample.Melt ribs may represent zones of considerable tem-

perature contrast between the melt and the host

peridotite, resulting in chilling of the contact of the meltagainst the peridotite, so that the inner part of the meltis isolated in a ‘protective cover or sheath’, preventingany further reaction with the host peridotite. Suchshielded melt can behave as a closed system and laterfractionate fine aggregates of CpxÔpxÔl with coolingas observed in theTLP (e.g. Fig. 3g and h).

Fig. 11. Variation of 143Nd/144Nd vs 176Hf/177Hf (a) and eHf vs eNd (b) forTLP (filled green circles) and MSP (filled light blue circles). The Hf^Nd decoupled samples in (a) (see text) plot with the field of HFSE-depleted Hawaiian xenoliths (hatched area). Dashed lines in (a) and (b)show the terrestrial array (Chauvel et al., 2008). The lines labeled BSE indicate the present-day Bulk Silicate Earth values. MORB (stippledfield) and OIB (diagonal-shaded area) data are from the PetDb database (http://www.petdb.org/petdbWeb/).

Fig. 12. (a) Variation of whole-rock CaO vsYb forTLP (filled green circles),TLG (filled red circles) and MSG (Type I gabbros, red triangles;Type II gabbros, purple triangles). TLP and TLG samples are collinear; the dashed line depicts the trend of melt^peridotite reaction. The starindicates the DMM composition (Workman & Hart, 2005). (b) Variation of B/Nb vs Ce/Pb. The symbols are as in (a). TLP andTLG plot col-linearly, implying interaction of a TLG melt with TLP similar to that in (a). Hatched area in (a) and (b) indicates the composition of theMSP from this study.


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The average Al2O3 content of the Cpx porphyroclasts(Table 2) in the TLP (5·4wt %) is higher than that ofCpx in the MSP (3·4wt %) and also higher than that oftypical mantle Cpx (�3wt %, McKenzie & Bickle, 1988;McKenzie & O’Nions, 1991; Gaetani & Grove, 1995). TheAlT content of the Cpx in the TLP ranges from 0·118 to0·165 (Table 2 and Fig. 4), which is about twice that ofCpx in the MSP. In contrast, the Si4þ content of Cpx inthe TLP ranges from 1·835 to 1·882, which is lower thanthat of Cpx in the MSP (1·911^1·936; Table 2).Petrographic observations in theTLP, such as the presenceof primary pyroxene porphyroclasts rimmed by fine(late-stage) Ol2 (Fig. 3a), suggest that Cpx1 has reactedwith a SiO2-undersaturated silicate melt [i.e. reaction (1)].The REE concentrations in Cpx in the TLP are signifi-

cantly higher than those in plagioclase lherzolites fromorogenic peridotite massifs, mantle xenoliths, abyssal peri-dotites and ophiolitic peridotites (Fig. 5a). The maximumREE contents are about four times those of Cpx in DMM(Workman & Hart, 2005); consequently this large enrich-ment of REE inTLP Cpx cannot be explained by partialmelting of a source similar to DMM or even primitivemantle. Experimental data suggest that the partition coef-ficients of the REE increase with increasing AlTcontent ofCpx (Gaetani & Grove, 1995; Hill et al., 2000; Blundy& Wood, 2003). Therefore, we suggest that interactionwith a melt may have caused Al enrichment in the Cpx oftheTLP, and as a consequence the partition coefficients ofthe REE between Cpx and melt may have increased,resulting in unusually high REE partitioning into theCpx (e.g. compare with abyssal and other peridotite Cpxin Fig. 5a).Because melt^peridotite reactions can cause grain

boundary enrichment, whole-rock data that include grainboundary phases must be taken into account in discussingthe mass balance of exchanged elements.Whole-rock com-positions suggest that the highly and moderately incompat-ible element abundances (e.g. Ce and Zr, respectively) inthe TLP are disturbed and show considerable scatteringwhen plotted versus Al2O3, reflecting the effects of melt^peridotite reactions (not shown).Whole-rock CaO andYbvariations are indicated in Fig. 12a, showing collinear vari-ation among the TLP and TLG, in strong contrast to theMSP. Melt^peridotite reactions, but not partial melting,can explain the trend in the TLP, because the Yb concen-trations in some TLP are considerably higher than thoseof DMM. The collinearity of the TLP and TLG stronglysuggests that the TLG represent the melts that reactedwith theTLP.Ce/Pb and B/Nb ratios do not change significantly

during partial melting or fractional crystallization andtherefore are widely used to distinguish melt-related pro-cesses from fluid-related processes (Ryan & Langmuir,1987; Hofmann, 1988; Ishikawa & Nakamura, 1994;

Workman & Hart, 2005). The systematic correlation ofCe/Pb with B/Nb for theTLP andTLG (Fig. 12b) indicatesmelt^peridotite reaction. The MSP compositions do notfollow this trend and therefore have a differentpetrogenesis.The linear trend between the TLP and TLG towards

Pacific MORB in the Pb isotope diagrams also supports amodel of melt^peridotite reaction between the isotopicallydepleted TLP and the Pacific MORB-like TLG (Fig. 8aand b). Similar systematic correlations are observedamong Hf^Sr, Nd^Sr, Pb^Sr and Nd^Pb isotopes(Fig. 8e^h), consistently suggesting melt^peridotite reac-tion between the TLP and TLG. In the Sm^Nd and Lu^Hf isochron diagrams (Fig. 8c and d, respectively) theTLG define an �300 Ma isochron. The Sm^Nd and Lu^Hf isotope ratios of those TLP samples that deviate fromthe 1 Ga peridotite isochron in Fig. 8c and d also plotalong the �300 Ma isochron defined by the TLG.However, in terms of their Rb^Sr systematics, the TLPand TLG do not define a distinct isochron and exhibitsome scatter, probably indicating that their Rb/Sr ratioshave been more disturbed compared with their Sm/Nd orLu/Hf ratios during melt^peridotite reaction. This sugges-tion (for Rb/Sr) is supported by the absence of petro-graphic evidence for low-temperature fluid alteration andthe whole-rock trace element patterns of the TLP, inwhich Sr and Rb show highly disturbed behaviour(Malaviarachchi et al., 2008). Thus we suggest that theTLG^TLP reactions may have occurred at �300 Ma.

Metasomatism in MSPMantle metasomatism causes decoupling of highly incom-patible elements from major and compatible trace elements(Hellebrand et al., 2001, 2002; Luth, 2003; Pearson et al.,2003). Highly incompatible elements, once lost from theresidue during partial melting, are re-enriched during sub-sequent metasomatism, and therefore this process is alsoknown as refertilization (Hellebrand et al., 2002). Fluidmetasomatism has affected the MSP, enriching theirfluid-mobile incompatible element contents and resultingin crystallization of hydrous minerals such as amphiboleand/or phlogopite. In Fig. 5e, the LREE-enriched REEpatterns of the Cpx provide evidence for metasomatism.In addition, alkali element (e.g. Rb) enrichment in Cpxand whole-rocks (e.g. Malaviarachchi et al., 2008) clearlyindicates that fluid-metasomatic processes have occurredin the MSP. These observations are consistent with thoseof previous studies (e.g. Takazawa et al., 1992, 2000;Yoshikawa & Nakamura, 2000); the metasomatic eventhas been dated to �23 Ma (Yoshikawa et al., 1993).

Composition and origin of the MSGThe LREE-depleted REE patterns of both Cpx (Fig. 5f)and whole-rock (Fig. 7c^f) and the isotopic compositions(Fig. 9a^c) of both types of MSG indicate that they could


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be formed from MORB-type mantle melts. HighlyHREE-depleted Cpx grains in Type I MSG indicate meltderivation from a garnet-bearing source (Fig. 5f). NegativeEu anomalies in the Cpx REE patterns but not in thewhole-rock trace element patterns suggest a subsequenthistory in the plagioclase stability facies. However, theCpx of the Type II MSG have low REE abundances com-pared with those of Type I MSG (Fig. 5f) and whole-rocksamples with positive Eu and Sr anomalies (Fig. 7f) indi-cating derivation from a plagioclase-rich source.Previous studies have suggested that Type I and Type II

MSG are the products of melting of garnet- andplagioclase-bearing sources, respectively (Niida, 1984;Shiotani & Niida, 1987; Takazawa et al., 1999). This inter-pretation is consistent with the above observations.However, the trace element patterns of both types ofMSG show strong B, Pb, Sr and alkaline element enrich-ments together with HFSE (Nb, Ta, Zr, Hf) depletions(Fig. 7d and f) that were not reported in these earlier stu-dies. The presence of HFSE negative anomalies is a typicalgeochemical feature of arc magmas (e.g. Mu« nker et al.,2004, and references therein). Thus, the alkaline elementenrichment and HFSE depletion together with theMORB-like REE patterns suggest that Type I and IIMSG were formed from arc magmas generated in aMORB-source mantle wedge. This suggestion is furthersupported by evidence for the presence of a MORB-source mantle wedge beneath NE Japan (Nakamura et al.,1985; Shibata & Nakamura, 1997). The trace elementratios (e.g. Th/La) and Pb isotope compositions of theMSG show a mixing trend towards the Hidaka metasedi-ment composition (Fig. 9d), confirming that the MSGhave been affected by slab-derived fluids or melts in theHidaka subduction zone. This is further supported by thefact that none of the MSG define meaningful isochrons ineither Sm^Nd or Lu^Hf isotope diagrams (e.g. Fig. 9eand f) as a result of disturbance of their Nd and Hf isotoperatios by interaction with a slab fluid or melt.To investigate the nature and timing of the mixing rela-

tionship we modeled and compared the compositions ofthe MSG with a possible slab-derived fluid or melt (as themixing agent) using the age-corrected Nd^Hf^Pb isotopecompositions (Fig. 10). The end-members used are thedepleted MORB (Malaviarachchi et al., 2008) and a hypo-thetical slab-derived fluid or melt with a similar Nd^Hf^Pb isotopic composition to the average Hidaka subductedsediments. Initial ratios at 100 Ma or older are too scat-tered to provide a good fit for the mixing relationship(e.g. Fig. 10a^d); however, the mixing curves (curves withper cent increments) at ages550 Ma agree well with theage-corrected MSG data (Fig. 10e^j). Thus, model curvesthat are younger than 50 Ma give a clear fit to theobserved Nd^Hf^Pb isotopic variations of the MSG, sug-gesting that the Type I and Type II gabbros were

influenced by mixing with a slab-derived fluid or melt atan age550 Ma. The amount of the subduction fluid com-ponent was calculated to be up to �5% and �25% forthe Type I and Type II gabbros respectively (Fig. 10iand j). Therefore, we suggest that the originalarc-magmatic compositions of the MSG have recently(550 Myr ago) been modified by a slab-derived fluid ormelt in the Hidaka subduction zone.

Geodynamic implicationsThe data from this study are consistent with suggestionsfrom previous studies that the Horoman peridotites wereformed as residues of partial melting in the garnet stabilityfield (e.g. Takazawa et al., 2000; Yoshikawa & Nakamura,2000), perhaps at a mid-ocean ridge setting at �1 Ga, andwere later metasomatized in a supra-subduction zonemantle wedge at �23 Ma (e.g. Yoshikawa & Nakamura,2000; Malaviarachchi et al., 2008).The highly unradiogenicPb isotope characteristics of both the TLP and MSP havebeen interpreted to reflect a considerable decrease in theirU/Pb and Th/Pb ratios (Malaviarachchi et al., 2008).Decreased U/Pb and Th/Pb ratios within the residualmantle may be the consequence of hydrothermal processesin the sub-oceanic mantle that increase the Pb concentra-tion of the peridotite residues. When such residues arestored for a long time-span they can eventually evolve tohighly unradiogenic Pb isotope compositions at the pre-sent day (e.g. Kelemen et al., 2007; Malaviarachchi et al.,2008) and are sampled as highly depleted peridotites indifferent tectonic settings (e.g. Liu et al., 2008;Malaviarachichi et al., 2008)A melting column beneath a mid-ocean ridge system

comprises both melting residues and migrating partialmelts. Within this partially molten upper mantle, theliquid phase moves upward relative to the residual solidphase, mainly driven by its density contrast. As shownschematically in Fig. 13a, partially molten materials (i.e.residual mantle) advance in the melting regime and grad-ually become isolated from the partial melting zone,whereas the partial melts escape as MORB.During mantle upwelling and partial melting the re-

sidual peridotites may be recycled back into the mantleand re-processed geodynamically at several different ridgesystems (Fig. 13b). This process may take a few hundredmillion years for one cycle to be completed. As a conse-quence, these recycled residues are highly refractory andcannot contribute to further melting (Liu et al., 2008;Malaviarachichi et al., 2008). Some of the recycled residuesreact with a melt at a subsequent time, depending on themelt composition, velocity and liquidus temperature, asobserved in theTLP at �300 Ma (Fig. 13b). Our data indi-cate that the reactive melts can be recognized geochemi-cally as TLG (e.g. Figs 8 and 12), with a probable originwithin the garnet stability zone in the mantle (see discus-sion on TLG). However, not all the peridotite samples


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Fig. 13. A schematic illustration of the history of the Horoman peridotites, showing adiabatic upwelling under a mid-ocean ridge forming resi-dues and melts by partial melting. (a) At �1 Ga, formation of the Horoman peridotites as residues of partial melting of a MORB source(along paths shown by the green arrows) and MORB melts (red dashed arrows). Some of the residues that have moved to the topmost regionsof the oceanic mantle or abyssal surfaces are affected by hydrothermal processes (see text). The residues are recycled back into the meltingregion by mantle convection. (b) Recycled residues, because of their refractory nature, cannot contribute to further melting but some have inter-acted with partial melts of MORB mantle at �300Ma (red arrows). Their chemistry is modified by melt^peridotite reactions (e.g. TLP; seetext) and they are isolated from the melting region together with other residues that survived the melt-interaction process (e.g. MSP and hydro-thermally modified residues). (c) The residues isolated from the MORB setting are subsequently transported into a hydrous mantle-wedge set-ting by mantle convection forces and plate tectonics. At �50 Ma, MSG (already formed as arc magmas in the Hidaka arc setting) interactwith a subduction component (slab-derived fluid or melt). In this arc setting the MSP were selectively metasomatized by fluids released fromthe subducted sediments; however, the TLG and TLP are not geochemically modified (see Discussion). The mantle residues formed at 1 Gamight still exist as refractory residues ubiquitously in the mantle.


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show evidence of these melt^rock reactions, suggesting thatthey have escaped this process (e.g. MSP).As the Horoman peridotites exhibit convincing geo-

chemical evidence of ancient oceanic mantle characteris-tics, a ‘lithospheric origin’ for these peridotites can beruled out (e.g. Yoshikawa & Nakamura, 2000; Shimizuet al., 2006; Malaviarachchi et al., 2008). Thus, theHoroman peridotites could not already be part of thelithosphere overlying the subduction zone. The residualperidotites may have been moved into the mantle-wedgesetting of the Hidaka subduction zone through transportby plate tectonics and mantle convection forces, and wereeventually emplaced above the Hidaka subducting plate.Arc magmas derived from the Hidaka MORB-sourcemantle wedge may have intruded into the Horoman peri-dotites as MSG (Fig. 13c). The MSG are clearly differentfrom the pyroxenites, which occur on a minor scale in theHoroman Massif, both in terms of their petrography andgeochemistry (e.g. Niida, 1984; Takazawa et al., 1999). Thearc magma characteristics of the MSG are clearly reflectedin their HFSE depletion in normalized trace element pat-terns (see Fig. 7d and f). Subsequent interaction of theMSG with a slab-derived fluid or melt may have startedat an age of550 Ma (see discussion on MSG and Fig. 10).In the arc environment, the MSP were infiltrated by an

aqueous metasomatic fluid at �23 Ma (Yoshikawa et al.,1993; Yoshikawa & Nakamura, 2000). However, our datashow that the TLP and TLG were not affected by any ofthe fluid-related processes in the mantle wedge (see discus-sion on TLP and TLG). The reason is probably the highabundance of pyroxene; the pyroxene-rich matrix mayhave prevented efficient fluid or melt percolation throughthe rocks as a result of an increase of the dihedral angle atmineral triple junctions (4608) compared with anolivine-rich matrix (Watson & Lupulescu, 1993; Mibeet al., 1998; Yoshikawa & Nakamura, 2000).

CONCLUSIONSThe thin peridotite layers (TLP) of the Horoman perido-tite massif underwent melt^peridotite reactions at�300 Ma with a melt that has a geochemical affinity toPacific MORB. The geochemical characteristics of thethin gabbro layers (TLG) imply derivation from agarnet-bearing source and they are plausible candidatesfor the reactive melt in theTLP, especially with respect totheir Pb^Nd^Hf and Sr isotope compositions. Themassive-scale peridotite layers (MSP) are not influencedby melt^rock reactions, but show evidence for aqueousfluid metasomatism. The massive-scale gabbros (Type Iand II MSG) have a geochemical affinity with arcmagmas derived from a MORB-source mantle wedge;their subsequent interaction or mixing with a slab-derivedfluid or melt appears to have occurred at ages youngerthan �50 Ma.

ACKNOWLEDGEMENTSWe thank K. Kobayashi and T. Kunihiro, R. Tanaka, andT. Moriguti, C. Sakaguchi and S.Tokeshi for the analyticalhelp in SIMS and SEM/EDX, EPMA and XRF, andTIMS respectively. Other colleagues in the PheasantMemorial Laboratory are thanked for analytical assistanceand fruitful discussions. We thank K. Ozawa, K.Kunugiza, T. Kuritani, T. Usui and T. Moriyama for theirhelp in collecting samples, and M. Tanimoto for prelimin-ary analysis of the MSP. B. Mysen and F. Jenner areacknowledged for improving the manuscript. In-depthreviews by D.G. Pearson and two anonymous reviewersare greatly appreciated. Editorial handling and construc-tive comments by SimonTurner and Marjorie Wilson aregratefully acknowledged.

FUNDINGThis study was supported by the program of the ‘Centre ofExcellence for the 21st Century in Japan’ from theJapanese Ministry of Education, Culture, Sports, Scienceand Technology (MEXT) to E.N. and Grants-in-aid fromJapan Society for the Promotion of Science ( JSPS) toA.M., and was a part of the PhD research project of thefirst author.

SUPPLEMENTARY DATASupplementary data for this paper are available at Journalof Petrology online.

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Melt-Peridotite Reactions and Fluid Metasomatism in the Upper Mantle, Revealed from the Geochemistry of Peridotite and Gabbro from the Horoman Peridotite Massif, Japan