magma genesis beneath northeast japan arc: a …...magma genesis beneath northeast japan arc: a new...

Gondwana Research 16 (2009) 446–457

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Magma genesis beneath Northeast Japan arc: A new perspective on subductionzone magmatism

Tetsu Kogiso a,⁎, Soichi Omori b, Shigenori Maruyama b

a Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-nihonmatsu, Sakyo, Kyoto 606-8501, Japanb Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan

⁎ Corresponding author. Fax: +81 75 753 2919.E-mail address: [email protected] (T. Kogi

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.05.006

a b s t r a c t
a r t i c l e i n f o
Article history:Received 28 February 2009Received in revised form 6 May 2009Accepted 8 May 2009Available online 18 May 2009

Keywords:SubductionArc magmaDehydrationSlab meltingPhase equilibrium

It is being accepted that earthquakes in subducting slab are caused by dehydration reactions of hydrousminerals. In the context of this “dehydration embrittlement” hypothesis, we propose a new model to explainkey features of subduction zone magmatism on the basis of hydrous phase relations in peridotite and basalticsystems determined by thermodynamic calculations and seismic structures of Northeast Japan arc revealedby latest seismic studies. The model predicts that partial melting of basaltic crust in the subducting slab is aninevitable consequence of subduction of hydrated oceanic lithosphere. Aqueous fluids released from thesubducting slab also cause partial melting widely in mantle wedge from just above the subducting slab to justbelow overlying crust at volcanic front. Hydrous minerals in the mantle wedge are stable only in shallow(b120 km) areas, and are absent in the layer that is dragged into deep mantle by the subducting slab. Theposition of volcanic front is not restricted by dehydration reactions in the subducting slab but is controlled bydynamics of mantle wedge flow, which governs the thermal structure and partial melting regime in themantle wedge.

© 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Subduction zone is a location where significant chemical differ-entiation processes occur, such as the generation of continental crustand transfer of chemical heterogeneity into the deep mantle, both ofwhich play fundamental roles in the chemical evolution of the Earth.Chemical differentiation in subduction zones is promoted mainly byliberation of aqueous fluids from subducting oceanic lithosphere(slab) and resultant magma generation. Therefore, quantitativemodeling of fluid liberation in the subducting slab and its relevanceto magma genesis in subduction zones is critical for better under-standing of the Earth's chemical evolution. Generation of aqueousfluids and silicate magmas in subduction zones has also been one ofthe most important issues in the area of igneous petrology, and manyresearchers have proposed a variety of petrologic models for the fluidand magma genesis in subduction zones (e.g., Nicholls and Ringwood,1972; Wyllie and Sekine, 1982; Kushiro, 1983, Tatsumi, 1989, Schmidtand Poli, 1998; Kimura and Yoshida, 2006). However, there is littleconsensus on what are keys to explain the important features ofsubduction zone magmatism, such as the existence of volcanic frontand across-arc variations of magma chemistry and volume.

Recent progress in seismological studies of Northeast Japan arc(e.g., Nakajima and Hasegawa 2007; Kita et al., 2006) provided uswith highly precise distribution of intraslab earthquakes and detailed

so).

ssociation for Gondwana Research.

seismic velocity structures in the mantle wedge beneath NE Japan(Hasegawa et al., 2009-this issue). In addition, new experimental andtheoretical data on hydrous phase relations in peridotite and basaltsystems (e.g., Omori et al., 2002, 2004; Komabayashi et al., 2004;Grove et al., 2006) allowed us to know precise pressure–temperatureconditions of fluid and magma generation in subduction zones.Moreover, it is widely accepted that embrittlement caused byliberation of fluids from hydrous minerals in subducting slab is apotential cause of intraslab earthquakes (Raleigh, 1967; Kirby et al.,1996; Seno and Yamanaka, 1996, Jung et al., 2004). In the context ofthis “dehydration embrittlement” hypothesis, the distribution ofintraslab seismicity can be linked with stability limits of hydrousminerals in subducting slab, which in turn allows estimating detailedthermal structure of subducting slab (e.g., Omori et al., 2002; Hackeret al., 2003; Omori et al., 2004, 2009-this issue). The thermal structureof subducting slab is an essential parameter for quantitative modelingof magma genesis in subduction zones. In this paper, we combine thelatest results of seismic studies of NE Japan with hydrous phaserelations in peridotite–basalt systems, and propose a new petrologicmodel for subduction zone magmatism on the basis of the dehydra-tion embrittlement hypothesis.

2. Subduction zone magmatism: key features and previous models

There are several important features that characterize subductionzone magmatism (Fig. 1). Here we review key features of subductionzone magmatism, and illuminate some problems that have not been

Published by Elsevier B.V. All rights reserved.

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http://dx.doi.org/10.1016/j.gr.2009.05.006

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Fig. 1. Schematic illustration of the key features of subduction zone magmatism.

447T. Kogiso et al. / Gondwana Research 16 (2009) 446–457

well explained by previous petrologic models for subduction zonemagmatism.

2.1. Occurrence of magmatism

The most important feature of subduction zone magmatism is theoccurrence of magmatism itself, that is, the fact that hot magmas aregenerated at settings in which the mantle is being cooled bysubduction of cold oceanic lithosphere. There is a consensus thataqueous fluids released from hydrous minerals brought by thesubducting slab play critical roles in generatingmagmas at subductionzones, because H2O considerably lowers solidus temperatures ofmagma source materials (e.g., Kushiro, 1969). However, detailedmechanisms of magma genesis in subduction zones have beendisputed.

One possible mechanism is hydrous partial melting of oceanicbasaltic crust in the subducting slab (e.g., Nicholls and Ringwood,1972; Kay, 1980; Wyllie and Sekine, 1982). This mechanism had beenproposed as the main cause for basaltic–andesitic magmas thatdominate subduction zones. However, later melting experiments onhydrous basaltic systems (e.g., Beard and Lofgren, 1991; Rushmer,1991; Wolf and Wyllie, 1994; Rapp and Watson, 1995) demonstratedthat hydrous melting of basaltic crust dominantly produces silicicmagmas with particular chemical characteristics like tonalite oradakite, which are limited in volcanic arcs beneath which relativelyyoung (=hot) oceanic lithosphere is subducting. In addition,analytical and numerical models on the thermal structure ofsubduction zones suggested that subducting oceanic crust olderthan 20 Myr are unlikely to partially melt beneath volcanic arcs (e.g.,Drummond and Defant, 1990; Molnar and England, 1995; Bourdonet al., 2002). On the other hand, high geotherms of some arcspetrologically estimated from metamorphic rocks suggest thatsubducting oceanic crust as old as 80 Myr can partially melt beneathvolcanic arcs (Kelemen et al., 2003). Partial melting of subductingoceanic sediments has also been proposed from basalt geochemistryfor old subduction zones such as Izu-Mariana (e.g., Ishizuka et al.,2003).

The other possible mechanism is hydrous partial melting in mantlewedge caused by addition of aqueous fluids from dehydration reactionin subducting slab (e.g., McBirney, 1969; Kushiro, 1983; Tatsumi et al.,1983). This has been thought to be more plausible because subductingslab in any subduction zones has been hydrated by interaction withseawater before subduction, which necessarily cause dehydrationreactions during subduction.

2.2. Formation of volcanic front

Another key feature is the formation of volcanic front (Sugimura,1960), in particular its correspondence to the depth of subducting slab

surface around 100–130 km (Gill, 1981; Tatsumi, 1986) (Fig. 1). Thisfeature has led some researchers to propose that the locus of magmagenesis is controlled by depth-dependent dehydration reactionswithin and/or above subducting slab (e.g., Tatsumi, 1986, 1989).Tatsumi (1989) attributed the constancy of the slab surface depthbeneath volcanic front to dehydration reaction of amphibole inperidotite, which he thought occurs at ∼110 km depth just above thesubducting slab and forms a hydrous column to cause flux melting inthe mantle wedge. However, recent compilations of high-pressurehydrous phase relations (e.g., Schmidt and Poli, 1998; Iwamori, 1998;Omori et al., 2002) revealed that amphibole in the peridotitic systembreaks down at pressures less than 3.0 GPa, which corresponds tob∼90 km depth. Moreover, it has been shown that serpentine andchlorite have the largest contribution to the H2O budget in peridotiteand that amphibole breakdown plays only minor role (Schmidt andPoli, 1998; Omori et al., 2002). Because dehydration reactions ofserpentine and chlorite are highly temperature-dependent, there are nomajor depth-dependent dehydration reactions that can produce con-siderable amounts of aqueous fluids inmantlewedge at∼110 kmdepth.

Another explanation for the constancy of the slab surface depth isthat flow and thermal regime in mantle wedge controls the location ofvolcanic front (e.g., Kushiro, 1983; Spiegelman and McKenzie, 1987;Iwamori, 1998). Kushiro (1983) suggested that the location of volcanicfront should be controlled by the position of wet solidus of peridotitein mantle wedge because magmas are generated by continuousaddition of aqueous fluids from subducting slab. Spiegelman andMcKenzie (1987) numerically demonstrated that streamlines of meltin mantle wedge can converge to one point beneath overlying crustowing to corner flow of solid mantle drugged by subducting slab.Iwamori (1998) numerically simulated migration and chemicalreactions of aqueous fluids in solid peridotite flow in mantle wedge,and suggested that extensive melting in mantle wedge occur at fixeddepths because similar flux of fluids and flow pattern of solid can beattained regardless of age of subducting slab.

2.3. Across-arc variations of magma chemistry and volume

Across-arc variations of chemistry and volume of erupted magmasare also important. Kuno (1959, 1960) reported alkali contents ofmagmas increase toward the backarc side in the NE Japan arc. AfterKuno's pioneer works, many researchers reported such increasingalkali contents toward backarc side in many volcanic arcs in the world(e.g., Miyashiro, 1974; Dickinson, 1975; Tatsumi and Eggins, 1995;Kimura and Yoshida, 2006). The across-arc chemical variations inmagmas are observed not only in major elements but also inincompatible trace element concentrations (e.g., Gill, 1981; Sakuyamaand Nesbitt, 1986) and radiogenic and stable isotopic ratios (e.g.,Nohda andWasserburg, 1981; Hawkesworth et al., 1993; Ishikawa andNakamura, 1994). The volume variation was first pointed out bySugimura et al. (1963) who showed that the volume of eruptedmagmas in NE Japan is largest in the volcanic front and sharplydecreases toward the Japan sea. The magma volume decrease towardbackarc side has been reported in many volcanic arcs since then(Marsh, 1979; Tatsumi and Eggins, 1995). Some researchers suggestedthat two volcanic chains occur in many single arcs because themagmavolume does not seem to decrease monotonously to the backarc side(Miyashiro, 1974; Marsh, 1979; Tatsumi and Eggins, 1995), although itis not accepted as a general feature of subduction zone magmatism(Ishikawa and Nakamura, 1994; Kondo et al., 1998; K. Kaneko, pers.comm. 2009).

These across-arc variations in magma chemistry and volume havebeen attributed to physical conditions of magma genesis that tend tobe affected by subduction of oceanic lithosphere. Before the concept ofplate tectonics was established, Kuno (1959) already proposed thatarc magmas are generated along the deep seismic zone (Wadati–Benioff zone) and attributed the chemical variations of arc magmas to

Fig. 2. Summary of notable seismic structures recently discovered in NE Japan arc.

448 T. Kogiso et al. / Gondwana Research 16 (2009) 446–457

the different depth of magma generation. Later high-pressure meltingexperiments on basalt systems (e.g., Green and Ringwood, 1967;Kushiro, 1968) revealed that the depth of the Wadati–Benioff zone(i.e., subducting slab) is too deep to explain chemical characteristics ofmagmas. Nevertheless, it is widely accepted that the depth of magmageneration and/or segregation plays a key role in producing across-arcchemical variations, because further high-pressure experiments(Kushiro, 1983; Tatsumi et al., 1983) has demonstrated that alkalibasalts that dominate in backarc side volcanism can segregate fromperidotite at deeper part in mantle wedge than tholeiitic basalts in thevolcanic front. Chemistry of aqueous fluids released from subductingslab is also an important parameter that affects the across-arcchemical variations, especially in trace element concentrations (Gill,1981; Tatsumi and Kogiso,1997; Hochstaedter et al., 2001; Kimura andYoshida, 2006; Nakamura et al., 2008).

The cause of the volume decrease toward backarc side iscontroversial. One explanation is that flux of slab-derived fluids,which promote partial melting in mantle wedge, is largest beneathvolcanic front and decreases toward backarc side (Gill, 1981; Tatsumi,1989; Cagnioncle et al., 2007). However, as described above, there areno dehydration reactions that can supply large amounts of fluids justbelow volcanic front. Another explanation is the existence of partialmelting zone inclined toward backarc side, in which the largest extentof melting sufficient for melt segregation can be achieved at the upperend below volcanic front (Marsh, 1979; Kushiro 1983; Schmidt andPoli, 1998). Such inclined partial melting zone in mantle wedge can beformed because strong focusing of flow in thewedge corner caused byNewtonian temperature-dependent or non-Newtonian rheologyproduces high temperature zone slanted toward backarc side(Furukawa, 1993; van Keken et al., 2002; Kelemen et al., 2003). Thisthermal structure in the wedge corner can also explain the across-arcdifference in magma segregation depth.

3. New view of subduction zone magmatism

Themost remarkable progress in recent studies of subduction zoneis the increasing acceptance of the dehydration embrittlementhypothesis as the cause of intraslab earthquakes (e.g., Kirby et al.,1996; Jung et al., 2004; Hasegawa et al., 2009-this issue). As Hasegawaet al. (2009-this issue) summarized, an assumption that intraslabearthquakes occur where hydrous minerals dehydrate in subductingslab canwell explainmany important phenomena in subduction zone:the existence of double seismic zone in the slab, uneven distribution ofearthquakes within it, seismic velocity structure of the slab and so on(Omori et al., 2002, 2004; Kita et al., 2006; Tsuji et al., 2008; Omoriet al., 2009-this issue; Nakajima et al., 2009). The dehydrationembrittlement hypothesis has great significance for modeling ofsubduction zone magmatism because it allows us to link intraslabearthquakes to the thermal structure of subducting slab and mantlewedge by comparing loci of earthquakes with pressure–temperaturelimits of hydrous mineral stabilities (Hacker et al., 2003; Omori et al.,2002; Yamasaki and Seno, 2003; Omori et al., 2004; Nakajima et al.,2009). The thermal structure of subducting zone, which is critical tounderstand subduction zone magma genesis, has been estimated onlyby numerical simulations (e.g., Davies and Stevenson,1992; Furukawa,1993; Iwamori, 1998; Kelemen et al., 2003), but now it can bedetermined by seismic observations and hydrous phase relations.Here we summarize recent progress in studies on seismology of NEJapanwith regard to the relevance tomodelingmagma genesis (Fig. 2)as well as recent studies on phase relations of hydrous minerals andmelts in basalt and peridotite systems.

3.1. Seismic observations

A notable feature of the seismic activities in NE Japan is thepresence of a double-planed seismic zone in the subducting pacific

slab (Umino and Hasegawa, 1975; Hasegawa et al., 1978). The twoplanes of the double seismic zone converge to one plane at around180 km depth (Hasegawa et al., 1978), beneath which the earthquakeactivity decreases in the slab. If dehydration embrittlement inducesintraslab earthquakes, the location of the double seismic zone shouldbe loci of dehydration reactions in the subducting slab (Omori et al.,2002; Hacker et al., 2003), which imposes strong constraints on thethermal structure of the subducting slab. Recently, Kita et al. (2006)precisely relocated intermediate-depth (50–150 km) earthquakesbeneath NE Japan by applying the double-difference hypocenterlocation method (Waldhauser and Ellsworth, 2000), and revealeddense concentration of seismicity at depths of 70–90 km in the upperplane of the double seismic zone (Fig. 2). This “upper-plane seismicbelt” is located near the upper boundary of the subducting Pacific slab,probably in the basaltic crust in the slab (Kita et al., 2006). Theexistence of the upper-plane seismic belt implies that majordehydration reactions occur in the basaltic slab at these depths(Hacker et al., 2003; Omori et al., 2004; Omori et al., 2009-this issue;see discussion below). Kita et al. (2006) also show that thicknesses ofboth the upper and lower planes of the double seismic zone are∼15 km or thinner, suggesting that dehydration reactions in the slaboccur just within restricted zones.

Tsuji et al. (2008) determined detailed seismic velocity structure ofthe subducting Pacific slab by double-difference tomography method(Zhang and Thurber, 2003). The obtained tomography image showsthe existence of a thin (b10 km) low-velocity layer at the top of theslab (Fig. 2). The low-velocity layer gradually disappears at depths of70–90 km, which corresponds to the depth of the upper-plane seismicbelt found by Kita et al. (2006). Tsuji et al. (2008) also show that alow-velocity layer exists in the mantle wedge just above the Pacificslab at depths greater than 70–90 km (Fig. 2). This low-velocity layercorresponds to that already found by previous studies (Matsuzawaet al., 1986; Kawakatsu andWatada, 2007), which continues to deeperthan 200 km (Tonegawa et al., 2008). These observations suggest thatdehydration reactions take place in subducting hydrated basaltic slabat depths around 70–90 km where expelled aqueous fluids move tothe mantle wedge and form a hydrous layer just above the slab (Tsujiet al., 2008; Nakajima et al., 2009) and that the hydrous layer in themantle wedge is dragged into depths greater than 200 km by thesubducting slab (Kawakatsu and Watada, 2007; Tonegawa et al.,2008). It is also shown that another thin low-velocity layer existsaround the lower plane of the double seismic zone within thesubducting slab (Tsuji et al., 2008; Nakajima et al., 2009), suggestingsignificant amounts of aqueous fluids released also along the lowerseismic plane.

Intensive seismic tomography studies beneath NE Japan (e.g.,Hasegawa et al., 1991; Zhao et al., 1992; Tsumura et al., 2000; Nakajima


et al., 2001; Hasegawa and Nakajima, 2004) have revealed theexistence of a prominent low-velocity zone in the mantle wedge thatis inclined toward backarc side subparallel to the subducting slab(Fig. 2). This low-velocity zone is observed in the whole region of NEJapan, that is, the low-velocity zone has sheet-like shape. The low-velocity zone also has high Vp/Vs ratios and high seismic attenuation(Zhao et al., 1992; Nakajima et al., 2001; Tsumura et al., 2000), andextends from depths around 150 km to the Moho just beneath thevolcanic front (Zhao and Hasegawa, 1993; Zhao et al., 1994; Nakajimaet al., 2001; Hasegawa and Nakajima, 2004). The existence of thesheet-like inclined low-velocity high-attenuation zone is thought tocorrespond to the locus of upwelling flow in the mantle wedge, inwhich peridotite is partially molten (e.g., Iwamori and Zhao, 2000;Hasegawa and Nakajima, 2004). The presence of small amounts ofpartial melts in the inclined low-velocity zone is supported by detailedinvestigations of its seismic attenuation structure (Nakajima andHasegawa, 2003; Nakajima et al., 2005). Detailed tomographic imagesof the inclined low-velocity zone obtained by Hasegawa and Nakajima(2004) show that there are several very low-velocity areas in the low-

Fig. 3. (a) and (b). Calculated P–T diagrams of stableminerals andH2O contents for peridotitethick lines are boundary of the main dehydration field (see text). Stable phases and boundarolivine (forsterite), opx= orthopyroxene, phA= phase A, srp= serpentine (antigorite), tlc =in theNCFMASHsystem(Omori et al., 2009-this issue). Thick broken lines showapproximate pthe estimated P–T path of the slab surface (Omori et al., 2009-this issue). Thin line is the H2O-for H2O contents is 15 wt.% for lherzolite and harzburgite, and 5 wt.% for MORB. AbbreviationEpAm = epidote amphibolite, EpEc = epidote eclogite, GS = greenschist, LwBS = lawsonitet al., 2006) and that of MORB (Liu et al., 1996; Schmidt and Poli, 1998). Also shown is crindistinguishable (Mibe et al., 2007).

velocity zone, and that their positions well correspond to the locationsof volcanic clusters and loci of low-frequency microearthquakesobserved (Okada and Hasegawa, 2000). On the basis of theseobservations, Hasegawa and Nakajima (2004) proposed that partialmelts segregated from the very low-velocity areas in the low-velocityzone rise vertically to the overlying crust and cause volcanisms andlow-frequency microearthquakes.

3.2. Subsolidus phase equilibria in hydrous systems

Subsolidus phase relations of hydrous minerals in peridotite andbasalt systems have been determined in detail by high-pressureexperiments and thermodynamic calculations in the last decades (e.g.,Schmidt and Poli, 1998; Iwamori, 1998; Omori et al., 2002; Iwamori,2004;Omori et al., 2004;Okamoto andMaruyama, 2004; and referencestherein). Omori et al. (2009-this issue) newly constructed P–T phasediagrams in the model system MgO–Al2O3–SiO2–H2O (MASH) forperidotite compositions and Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O(NCFMASH) for basalt composition by thermodynamic calculations

(a: lherzolite and b: harzburgite) in theMASH system (Omori et al., 2009-this issue). Theies of mineral assembly are shown. Abbreviations are: br = brucite, chl = chlorite, ol =talc. (c) Calculated P–T diagram of stable minerals and H2O contents for averageMORB

osition of the boundaries betweenmajormetamorphic facies. Thick grayarrow indicatessaturated solidus of MORB (see panel d). Note that the maximumvalue of the color scales are: Am = amphibolite, AmEc = amphibole eclogite, BS = blueschist, EC = eclogite,

e blueschist, LwEc = lowsonite eclogite. (d). H2O-saturated solidus of lherzolite (Groveitical curve in peridotite–H2O system above which aqueous fluid and silicate melt are


and semi-quantitative analysis of high-pressure experiments. Details ofcalculation, chemical compositions and thermodynamic data of phasesencountered in themodel aredescribed inOmori et al. (2009-this issue).The phase diagrams determined for model lherzolite, harzburgite, andoceanic crust (mid-oceanic ridge basalt: MORB) compositions areshown in Fig. 3. Principal hydrous phases that can release large amountsof fluids in peridotite systems are serpentine, brucite, talc and chlorite(Fig. 3a–b). Although amphibole is also a major hydrous mineral innatural peridotite systems including Na2O (e.g., Schmidt and Poli, 1998;Grove et al., 2006), its contribution to H2O budget in subducting slabperidotite is much smaller than above three minerals (Iwamori, 1998,2007). So the phase diagram in the MASH system is a goodapproximation of hydrous phase equilibria in peridotite of subductingslab and mantle wedge. Thus the major dehydration reactions insubducting slab occur under P–T conditions where serpentine, brucite,talc and chlorite decompose (between thick lines in Fig. 3a–b). Thehigh-pressure end of this “main dehydration field” is around 6–7 GPa(∼200 km depth) (Omori et al., 2002, 2009-this issue), which is close tothe depth at which the two planes of the double seismic zone converge(Fig. 2). It should be noted that decomposition reactions of theseminerals are highly temperature-dependent, that is, they occur withinnarrow temperature interval (∼200 °C: Fig. 3a–b), suggesting thesereactions are useful to estimate temperature distribution in subductingslab in the context of the dehydration embrittlement hypothesis (e.g.,Omori et al., 2002).

In the hydrous basalt system (Fig. 3c), major dehydration reactionsare decomposition of Ca-amphibole (b2 GPa), epidote (1–2.5 GPa),lawsonite and Na-amphibole (N2 GPa). The Ca-amphibole decom-position reaction is pressure-dependent, that is, its decompositionoccurs at nearly constant pressure over a range of temperature(Fig. 3c). In contrast, decomposition reactions of lawsonite and Na-amphibole are rather temperature-dependent than pressure-depen-dent. Thus, loci of dehydration in basaltic slab is sensitive totemperature at depths greater than ∼70 km. This means that theuneven distribution of earthquakes within the basaltic slab (Kita et al.,2006) is also useful to estimate temperature distribution in subduct-ing slab (Omori et al., 2009-this issue).

Fig. 4. Seismic data in the cross section beneath Kurikoma–Chokai line of NE Japan arc.The position of the cross section is shown as a line in the inset. Red triangles in the insetare Quaternary volcanoes, among which Sengan area (see text) is circled. (a). S-wavevelocity perturbation determined by Nakajima et al. (2001). Gray and red circles aremicroearthquakes and low-frequency microearthquakes, respectively (Okada andHasegawa, 2000). (b). Detailed S-wave velocity perturbation determined withdouble-difference tomography method by Nakajima et al. (2009). The image (a) wasobtained so that the longwave-length (∼20 km) structure around the central part of themantle wedge is clearly determined (Nakajima et al., 2001). On the other hand, theimage (b) was obtained so that the shorter wave-length (∼5 km) structure near the slabsurface is clearly determined (Nakajima et al., 2009). Thus, the difference between (a)and (b) is due to the differences not only in tomographic inversion method but also inregion that is well resolved by each method. (c). Locations of intraslab earthquakesdetermined with double-difference hypocenter location method by Kita et al. (2006,2008). The inclined line is the position of the slab surface (Kita et al., 2008). Colorsindicate distance from the slab surface: red b10 km, green=10–23 km, blue N23 km,gray=above slab surface.

3.3. Melting phase relations of hydrous systems

Grove et al. (2006) precisely determined melting phase equilibriaof hydrous lherzolite at near-solidus conditions by careful observa-tions of high-pressure experiment products, and found that thesolidus temperature of peridotite is as low as 800 °C at 3.2 GPa(Fig. 3d). A surprising result of their experiments is that chloriteappears as a stable phase above the solidus at pressures higher than2.8 GPa. This means that hydrous peridotite will melt even below thestability limit of chlorite. Melting phase equilibria of hydrous basalticsystem determined for a typical MORB composition (Liu et al., 1996;Schmidt and Poli, 1998) showed that the solidus of hydrous MORBbasalt locates around 700 °C at 1–2 GPa, which is also well below thestability limit of chlorite in hydrous peridotite (Fig. 3d). These dataimply that basalt and peridotite necessarily melt at depths greaterthan ∼80–100 km if their temperatures are higher than the chloritestability limit.

The other important feature of hydrous melting of peridotite iscomplete miscibility between aqueous fluid and silicate melt at highpressures (Mibe et al., 2007). In situ observation of high-pressure andhigh-temperature experimental runs by synchrotron radiation X-rayradiography (Mibe et al., 2007) revealed that aqueous fluid andhydrous silicate melt in a peridotite system become indistinguishableabove 3.8 GPa and ∼1000 °C (Fig. 3d). Therefore, at depths greaterthan ∼110 km, there can exist one supercritical liquid at highertemperatures, which changes its composition with increasing tem-perature gradually from H2O-rich to silicate-rich at around 1000 °C.

4. Modeling for magma genesis

On the basis of the recent seismic and petrologic studiessummarized above, we propose a new petrologic model for magma


genesis in NE Japan arc. The seismic data we employed for modelingare the vertical cross sections of seismic tomography in the mantlewedge (Nakajima et al., 2001), high-resolution tomography near thetop of the subducting slab (Nakajima et al., 2009), and distribution ofearthquakes in the subducting slab (Kita et al., 2006, 2008) along theline connecting Kurikomayama and Chokaisan volcanoes (Fig. 4).

4.1. Thermal structure

For estimating temperature distribution in subduction zone, weemploy several major assumptions in the context of the dehydrationembrittlement hypothesis as follows.

(1) The location of the double seismic zone in the subducting Pacificslab corresponds to the main dehydration field in the P–Tdiagram of the hydrous peridotite (Omori et al., 2002) (Fig. 3a).The inner edge of the double seismic zone is where brucite inharzburgite or talc in lherzolite decomposition starts, and theouter edge is where chlorite decomposition ends.

(2) Thickness of basaltic crust in the subducting slab is 7 km.The earthquakes within 7 km from the upper boundary of thesubducting slab are caused by dehydration reactions in thebasaltic system.

(3) The bulk composition of peridotite in the mantle wedge and inthe subducting slab is depleted lherzolite, except that theuppermost portion of the peridotite slab consists of harzburgite(i.e., the upper plane of the double seismic zone is within theharzburgite portion). The bulk of subducting oceanic crust isaverage MORB (Omori et al., 2002, 2009-this issue).

When employing the assumption (1), we didn't take into accountthe earthquakes between the upper and lower plane of the doubleseismic zone at ∼60–90 km depths. The wide distribution of these“interplane” earthquakes at these depths can be explained bycontinuous decomposition of brucite if we include Fe in the bulksystem (Omori et al., 2002; Omori and Komabayashi, 2007). Otherassumptions we employed to determine the thermal structure aredescribed and discussed below. We used here constraints obtainedfrom petrologic studies only in order to clarify the difference from thethermal structure determined by seismic studies and numerical

Fig. 5. Estimated temperature distributionwithin the subducting slab and the mantle wedge(°C) estimatedwith the assumption that intraslab earthquakes are caused by dehydration in tassumed for estimating temperatures. Light blue ovals are approximate positions of dry solgeneration of primitive arc magmas (Ueki and Iwamori, 2007). Red contours are smoothed is

modeling (e.g., Iwamori, 1998; Kelemen et al., 2003; Nakajima andHasegawa, 2003; Nakajima et al., 2005).

The temperature distribution in the mantle wedge is difficult toestimate because few petrologic constraints are available. Ueki andIwamori (2007) investigated thermal and chemical conditions ofmagma genesis beneath the volcanic front of NE Japan by densesampling and petrologic analyses of volcanic rocks in Sengan area(Fig. 4), and revealed by thermodynamic calculations with pMELTS(Ghiorso et al., 2002) that parental magmas were produced at 1250–1300 °C and 1.0–1.5 GPa with 0.3–0.5 wt.% H2O. Thus we assume thatthe temperature at the Moho just below the volcanic front is around1300 °C.

To estimate temperatures around the inclined low-velocity zone inthe mantle wedge, we focused our attention to the change of velocityanomaly values near the center of the low-velocity zone. The S-wavevelocity perturbation value abruptly changes around +1% to −3% onboth side of the low-velocity zone (Fig. 4a). We interpret that thisabrupt change corresponds to a sudden increase of melt fraction,because velocity reduction rates are correlated with melt fraction inaddition to aspect ratio of melt pore and temperature (Nakajima et al.,2005). Melting experiments on hydrous depleted peridotite (Kubo,2003) demonstrated that melt fraction of depleted peridotite underhydrous condition remains very low (b5%) between the hydroussolidus and the dry solidus and steeply increases around dry solidustemperature. Therefore, we assume that the position of the −1 to−3% contours of the S-wave velocity perturbation roughly matchesthe solidus of dry depleted peridotite (Robinson et al., 1998;Wasylenki et al., 2003; Laporte et al., 2004). The temperaturedistribution in the subducting slab and mantle wedge determinedwith these assumptions is shown in Fig. 5.

4.2. Distribution of hydrous minerals and aqueous fluids

From the temperature distribution determined above and hydrousphase relations (Figs. 3 and 5), we estimated the stability fields ofhydrous minerals, aqueous fluids and silicate melts in the subductingslab and mantle wedge (Fig. 6). We assumed that aqueous fluidsliberated from the double seismic zone migrate vertically much fasterthan slab subduction andmantle wedge flow just for simplicity. As the

beneath Chokai–Kurikoma line of NE Japan. Blue digits with color dots are temperatureshe subducting slab (Omori et al., 2009-this issue). Dot colors indicate bulk compositionsidus of depleted peridotite. Dark blue oval indicates the temperature and depth of theotherms. Gray crosses are the location of intraslab earthquakes (Kita et al., 2006, 2008).

Fig. 6. (a) Stability fields of hydrous minerals and partial melt estimated from the temperature distribution (Fig. 5). Black and dark red arrows schematically show movement ofaqueous fluids and silicate melts, respectively. A thick broken line shows the boundary of critical curve below which aqueous fluids and silicate melts exists as supercritical liquid.Abbreviations are as in Fig. 3. (b) Estimated possible distribution of free aqueous fluid, silicate melt, and supercritical liquid.


slab continues subducting, aqueous fluids are continuously supplied,and the liberated fluids hydrate the overlying portion of the slab andthe mantle wedge along migration paths. Thus the slab above thedouble seismic zone and the mantle wedgewould be locally saturatedwith H2O along fluid migration paths. Therefore, we drew the stabilityfields of hydrous minerals and silicate melts, and possible distributionof free aqueous fluids for H2O-saturated conditions (Fig. 6), but we donot mean that these phases are uniformly distributed within therespective stability fields.

Our estimation shows that hydrous minerals (mainly serpentine)can be present in the wedge corner and in the subducting slabbetween the double seismic zone if aqueous fluids are supplied there(Fig. 6a). This is consistent with high Poisson's ratios observed withinthe wedge corner and subducting slab beneath Kanto area of Japan(Kamiya and Kobayashi, 2000; Omori et al., 2002). It should be notedthat the stability fields of hydrous minerals in the mantle wedge arelimited to the wedge corner shallower than 120 km (Fig. 6a). Thisresults from the temperature distribution determined by extrapola-

tion of the outer boundary of the double seismic zone (=stabilitylimits of hydrous phases in peridotite) into the mantle wedge, whichcrosses the slab surface at around 120 km depth. This means that nohydrous minerals exist at the bottom layer of the mantle wedgedeeper than 120 km, that is, the bottom layer dragged by thesubducting slab into deep mantle is free from hydrous minerals eventhough aqueous fluids are being supplied from the subducting slab. Inthe basaltic slab, hydrous minerals are stable up to ∼120 km depth,but major dehydration reactions occur around 60–80 km depths(Fig. 3c). This corresponds to the depth of the lower end of the low-velocity layer at the top of the slab (Fig. 4b; Nakajima et al., 2009),suggesting that the low-velocity layer in the slab is consistent with thepresence of hydrated basaltic slab.

Major dehydration reactions occur between talc/brucite andchlorite stability limits in the peridotite slab (the main dehydrationfield of Fig. 3a–b) as well as at shallower (b120 km) part of thebasaltic slab (Fig. 3c). Therefore, free aqueous fluids can be presentwithin the double seismic zone and basaltic slab because aqueous


fluids are continuously liberated there as the slab sinks (Fig. 6b). In thewedge corner, aqueous fluids are continuously supplied from thesubducting slab, but the mantle flow is thought to be quite slow (e.g.,Kelemen et al., 2003). Therefore, the supplied fluids can hydrateperidotite in the corner until it become oversaturated with H2O alongfluidmigration paths. Thus free fluids can also be present in thewedgecorner (Fig. 6b). On the other hand, in the slab between the upper andlower planes of the double seismic zone, it is unclear whether freefluids can be present. Because the supply of liberated fluids from thelower seismic plane will stop when all hydrous minerals completelydecompose, the liberated fluids will be completely consumed byhydration of surrounding peridotite if their flux is small. The fluid fluxfrom the lower seismic plane depends on the amounts of hydrousminerals there, which is to be quantitatively investigated in futurestudies.

4.3. Locus of partial melting

Since aqueous fluids are supplied to the basaltic slab from thedouble seismic zone below, the basaltic slab can be saturatedwith H2Oalong fluid migration paths even above the stability limits of hydrousminerals in the basalt system (Fig. 3c). This causes partial meltingwithin the basaltic slab at depths around 120 km and deeper (Fig. 6)where the temperature of the basaltic slab exceeds the hydroussolidus of basalt (Fig. 3d). In other words, slab melting necessarilyoccurs beneath NE Japan if we accept the dehydration embrittlementhypothesis.

Partialmelting also occur in awide area of themantlewedge (Fig. 6),because the temperatures inmost parts of themantlewedge exceed thehydrous solidus of peridotite (Figs. 3 and 5). It should be noted that thepartial melting area extends from just above the subducting slab. Meltfraction increases toward the center of the inclined low-velocity zoneand become highest just below the volcanic front, if we assume that theS-wave velocity perturbation correlates with melt fraction (Nakajimaet al., 2005). Thus the position of the volcanic front corresponds to theareaofmaximummelt fraction in themantlewedge, and is not related tohydrous phase stabilities (see discussion below).

5. Discussion

5.1. Slab melting

An important feature of the petrologic model we proposed here isthat partial melting occurs in the subducting slab. In previous studieson subduction zone magmatism, it has widely been thought that slabmelting occurs only where relatively young oceanic lithosphere issubducting (e.g., Drummond and Defant, 1990; Peacock, 2003).Therefore, in the NE Japan, where a very old (∼130 Ma) oceanicplate is subducting, slab melting has never been considered to occur(e.g., Tatsumi, 1989; Iwamori, 2007). However, in the context of thedehydration embrittlement hypothesis, the subducting Pacific oceaniccrust inevitably undergoes partial melting because of the meltingphase relations of hydrous basalt (Fig. 3). This means that slabmeltingcommonly occur in subduction zones worldwide. This is consistentwith the numerical model incorporating temperature-dependentviscosity (Kelemen et al., 2003), which demonstrated that slab surfacetemperature can exceedwet solidus of basalt even if subducting slab isolder than 80 Myr.

Slab melting can be one of the main factors to produce across-arcvariations in arc basalt chemistry. The across-arc geochemicalvariations of arc basalts were well documented by thoroughpetrological and geochemical studies (e.g., Sakuyama and Nesbitt,1986; Shibata and Nakamura,1997; Hochstaedter et al., 2001; Ishizukaet al., 2003; Kimura and Yoshida 2006). These studies attributed theacross-arc variations to many different processes, such as differentdegrees of partial melting in the mantle wedge, difference in

chemistry of slab-derived fluids, differences in amounts of slab-derived components and so on (see Kimura and Yoshida (2006) fordetails). Some of the previous studies suggested that partial melting ofsubducting sediments contribute to produce the across-arc variationseven in old subduction zones like Izu-Mariana (e.g., Johnson andPlank,1999; Ishizuka et al., 2003). Because the solidus temperatures ofsediment and basalt are similar under hydrous conditions (Schmidtand Poli, 1998; Johnson and Plank, 1999), partial melting of basalticslab can occur if sediment melting occurs. In our model, subductingslab releases aqueous fluids into overlying mantle wedge at shallowerdepths (b∼120 km), whereas it supplies partial melts (or supercriticalliquid) from the basaltic slab at deeper depths (N∼120 km). Thusliquid phases released from the subducting slab change their chemicalcompositions with increasing depth, and such changes in chemistry ofslab-derived fluid/melt can be another important factor tomake across-arc chemical variations of basalt chemistry. Detailed experimentalstudies for chemistry of fluids, melts and supercritical liquids releasedfrom hydrous basalt and peridotite compositions will advance ourunderstanding of the role of slab melting to arc basalt chemistry.

5.2. Low-velocity layer at the bottom of the mantle wedge

Another remarkable aspect of our model is the absence of hydrousminerals in themantlewedge just above the subducting slab at depthsdeeper than 110 km. Many previous models have proposed thathydrous minerals are stable in the bottom layer of the mantle wedge ifsubducting slab is as cold as the Pacific slab beneath NE Japan and thatthe layer is dragged into deep mantle by subducting slab (Tatsumi,1989; Schmidt and Poli, 1998; Iwamori, 2004). These previous modelssuggested that hydrous minerals in the dragged layer are stablebeneath volcanic front and deeper. In our model, however, hydrousminerals are stable only at shallow (b120 km) depths in the mantlewedge corner. Therefore, the thin low-velocity layer just above theslab around 70–90 km depths found by the double-differencetomography (Tsuji et al., 2008; Nakajima et al., 2009) corresponds tothe hydrousmineral stability area of ourmodel (Figs. 2 and 6). However,the low-velocity layer found at the deeper portion (Kawakatsu andWatada, 2007; Tonegawa et al., 2008) does not indicate the presence ofhydrous minerals there.

The deep low-velocity layer, which was found by the receiverfunction technique (Kawakatsu and Watada, 2007; Tonegawa et al.,2008), is not necessarily a thin “layer”, because the thickness of thelow-velocity “layer” cannot be constrained solely by the receiverfunctions. Instead, the receiver function technique is sensitive to ansharp velocity change within a short length scale. Such a sharpvelocity change can be attained in our model. Our model predicts thatpartial melting in the mantle wedge occurs just above the subductingslab and in the slab at deeper than ∼120 km (Fig. 6). Below this partialmelting zone, hydrous minerals and/or free aqueous fluids arepresent, and the boundary of these zones are nearly parallel to theslab surface (Fig. 6b). Although amounts of aqueous fluids andhydrous minerals in the slab are difficult to constrain, it is clear thatthe fraction of “liquid” (silicate melt or aqueous fluid) in a certainvolume of peridotite/basalt abruptly increases around the solidus withincreasing temperature in a hydrous peridotite and basalt systems(Fig. 7). Therefore, the fraction of partial melts in the partial meltingzone is much higher than the fraction of aqueous fluids in the slab, atleast along fluid migration paths. Thus, the bottom of the partialmelting zone can be a sharp velocity-jump boundary (Fig. 7), whichcan be detected by the receiver function technique.

5.3. Position of volcanic front

Since the major dehydration reactions in the slab are essentiallytemperature-dependent (Fig. 3), there is no rationale for the idea thatthe position of volcanic front is controlled chiefly by pressure-

Fig. 7. Schematic phase diagrams at below (a) and above (b) the second critical end point in a simplified silicate–H2O system.When the bulk H2O content is fixed, the fraction of fluidis f/g at below-solidus temperature Y. At above-solidus temperature Z, the fraction of melt is f/h, which is much higher than f/g. Thus, the fraction of “liquid” (fluid or melt) abruptlyincreases across the solidus temperature. Even under conditions above the second critical end point, similar abrupt increase in liquid fraction occurs as shown in (b). Therefore, thebottom boundary of the partial melting zone (thick broken line in panel c) can be a sharp velocity-jump boundary.


dependent dehydration reactions in themantlewedge (Tatsumi,1986,1989). Therefore, such “single phase dehydration model” is no longervalid for subduction zone magmatism, as already pointed out bySchmidt and Poli (1998). Our model predicts that chlorite andserpentine decompose beneath the volcanic front in the mantlewedge (Fig. 6), which seems to indicate that these decompositionreactions in the mantle wedge control the position of volcanic front.However, the position of these decomposition reactions is chieflygoverned by the temperature distribution in the mantle wedge. Inaddition, partial melting widely occurs in the mantle wedge, so theposition of chlorite and serpentine decomposition does not havedirect relevance to the position of volcanic front. Thus, the thermalstructure in the mantle wedge is the key to the formation of volcanicfront and the constancy of the depth to the slab surface beneathvolcanic front.

Many researchers proposed that the inclined low-velocity zonecorresponds to the locus of partial melting (Kushiro, 1987; Iwamoriand Zhao, 2000; Hasegawa and Nakajima, 2004; Nakajima et al.,2005). Hasegawa and Nakajima (2004) suggested that the inclinedlow-velocity zone indicates the position of a sheet-like upwelling flowin themantlewedgewhere significant partial melting occurs. Iwamori(1998, 2007) numerically demonstrated that flow pattern in themantle wedge and fluid flux from the slab are not changedsignificantly even if angle and age of subducting slab vary, suggestingthat a similar thermal structure can be achieved in various subductionzones. These studies imply that the position of volcanic front iscontrolled by inclined upwelling flow, which can be formed at a

similar position in various subduction zones. However, it is nottheoretically clarified why the flow pattern in the mantle wedge canbe similar with varying age and angle of subducting slab. Since thereare no petrologic constraints to rationalize the formation of volcanicfront so far, more theoretical investigations on the dynamics of mantlewedge flow is needed.

5.4. Temperature distribution in the mantle wedge

In our model, we assumed that the temperature in the mantlewedge exceed the dry solidus of depleted peridotite near the centralportion of the inclined low-velocity zone (∼1300 °C just below thevolcanic front: Fig. 5) based on recent petrologic studies on arc basalts(Ueki and Iwamori, 2007). This estimation is too high as comparedwith the mantle wedge temperature estimated by numerical model-ing (e.g., Drummond and Defant, 1990; Iwamori, 1998; Peacock,2003). This discrepancy mainly derives from uncertainties in therheological properties of mantle wedge incorporated in numericalcalculations, as demonstrated by van Keken et al. (2002) and Kelemenet al. (2003). Our understanding of the rheological and other physicalproperties of mantle materials is not yet adequate to rigidly constrainthe temperature in the mantle wedge.

We did not draw high temperature anomaly below the backarc-side volcanism (i.e., Chokai volcanic zone) in our model (Fig. 6),because the seismic velocity structure (Fig. 4a) doesn't showsignificant anomalies beneath the backarc-side volcanoes. This canbe explained by localization of melt migration. The occurrence of low-


frequency microearthquakes just below the Chokai volcano (Fig. 4) isthought to be caused by migration of liquid phases (Hasegawa et al.,1991). This suggests that partial melts are migrating upward there(Hasegawa and Nakajima, 2004). Therefore we interpret that magmasof the backarc-side volcanoes are segregated from the inclined partialmelting zone and migrate upward in small length scales that cannotbe detected by seismic tomography.

5.5. Implication of slab melting

If slab melting is a general feature of the subduction zonemagmatism, it has an important implication for deepmantle recyclingof crustal material. As partial melting of subducting oceanic crustproceeds, the composition of its residue approaches bimineraliceclogite (comprised solely of garnet+clinopyroxene) owing to theloss of silica-rich components as partial melts (Kogiso et al., 2004).Since bimineralic eclogite is on the thermal divide in the peridotite–basalt melting regime (O'Hara, 1968; Kogiso et al., 2004), the residuecomposition remains bimineralic until partial melting ceases. There-fore, bimineralic eclogite can be the representative lithology of thecrustal material subducting into deep mantle. If such bimineraliceclogite is recycled in the deep mantle and involved in upwellingmantle plumes, it can generate nepheline-normative melts similar toalkalic ocean island basalts (OIB) that are dominant product in manyhotspot volcanoes (Kogiso et al., 2003; Kogiso and Hirschmann, 2006).Thus, the idea that slab melting is a general phenomenon insubduction zone is consistent with OIB genesis that requires commonoccurrence of bimineralic eclogite in their source regions as recycledcrustal component.

6. Summary

Recent remarkable progress in seismic studies of NE Japan andpetrologic studies of hydrous phase relations in the mantle enabled usto newly build a petrologic model for subduction zone magmatismfrom the viewpoint of the dehydration embrittlement hypothesis. Ourmodel demonstrates that subducting slab inevitably undergoes partialmelting in the basaltic portion. Hydrous minerals in mantle wedge arestable only in shallow (b120 km) part of the wedge corner, and nodehydration reactions occur at deeper areas in the mantle wedge.Aqueous fluids are supplied into the mantle wedge chiefly bytemperature-dependent decomposition reactions of serpentine, talc,brucite and chlorite in subducting slab. Partial melting occurs in awide area of mantle wedge, from just above subducting slab to justbelow volcanic front. The formation of volcanic front is controlled bydistribution of partial melts in mantle wedge, which is governed bydynamics of mantle flow andmelt migration. Petrologic constraints donot provide a rationale for the constancy of slab depth beneathvolcanic front.

Acknowledgments

We are extremely grateful to Junichi Nakajima and Saeko Kita forproviding us with their up-to-date seismic data and related figures.We deeply thank Prof. Hasegawa for his outstanding seismic studies,which stimulated us to review subduction zone magmatism. Themodel proposed here has been developed through extensive discus-sions in the subduction zone meetings held by Prof. Hasegawa's andMaruyama's groups. We thank Profs. Hasegawa, Matsuzawa, Umino,Zhao, and other members who attended the meetings and forconstructive and helpful discussion. Discussions with Katsuya KanekoandMamoru Katowere helpful to improve ourmodel. Themanuscripthas been improved with constructive comments by two anonymousreviewers.

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