Gondwana Research 16 (2009) 482–490

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Seismological evidence for compositional variations at the base of the mantletransition zone under Japan Islands

Akira Yamada a,⁎, Dapeng Zhao b, Toru Inoue a, Daisuke Suetsugu c, Masayuki Obayashi c

a Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japanb Department of Geophysics, Tohoku University, Sendai 980-8578, Japanc Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan

⁎ Corresponding author.E-mail address: yamada@sci.ehime-u.ac.jp (A. Yama

1342-937X/$ – see front matter © 2009 International Adoi:10.1016/j.gr.2009.04.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 15 November 2008Received in revised form 14 April 2009Accepted 15 April 2009Available online 22 April 2009

Keywords:Mantle transition zoneSeismic observationsTemperature anomalyWater contentSubducting slab

Temperature and water content anomalies just above the 660-km discontinuity under the Japan Islands areestimated from seismological observations. Two sets of seismological observations of P-wave velocityperturbations and depth variations of the 660-km discontinuity are used, which are (1) long-wavelength(~500 km) variations from seismic tomography based on a grid parameterization and waveform analysis ofScS reverberations and (2) moderate-wavelength (~150 km) variations from seismic tomography with ablock parameterization and receiver-function analysis. To estimate temperature and water contentanomalies, partial derivatives of velocity and depth variations with respect to temperature and watercontent determined by mineral physics studies are used. Under Southwest Japan, low temperature and highwater-content anomalies are obtained from both sets of seismological observations, which have already beenfound by a previous study. Under Northeast Japan, however, there are discrepancies between the resultsestimated from the two data sets, possibly due to the different resolution scales. This discrepancy isdismissed when examining distributions of the temperature and water content anomalies. These anomaliescan be grouped into two geographical locations, one group under Southwest Japan, the other underNortheast Japan. The two groups are clearly separated and exhibit common features in both sets of theseismological observations. The grouping is interpreted by a compositional difference between thesubducting Pacific slab under Southwest Japan and a normal mantle under Northeast Japan.

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

1. Introduction

Dehydration processes associated with the subducting slabs playan important role in mantle dynamics, volcanism and seismicity insubduction zones (Iwamori et al., 2007; Hasegawa et al., 2009-thisissue; Zhao and Ohtani, 2009-this issue). Low seismic velocityanomalies in the mantle wedge under the Japan Islands are clearlyimaged by seismic tomography (e.g., Nakajima et al., 2001; Zhao,2009; see also Igarashi, 2009-this issue;Mishina, 2009-this issue). Thelow velocity anomalies are interpreted as upwelling flows caused bythe slab dehydration and corner flow in the mantle wedge. Seismicanisotropy in themantlewedge, induced by the flows, was revealed byshear-wave splitting observations (e.g., Nakajima and Hasegawa,2004) and by inversion of P-wave arrival times for both velocityheterogeneity and anisotropy (Ishise and Oda, 2005; Wang and Zhao,2008, 2009). Combining these seismological observations withmineral physics and petrological studies, we can deduce that theupwelling flows in the mantle wedge are activated by the subductingslab, which is one of the key sources for the seismic and volcanicactivity as well as other tectonic movements in the arc regions.

da).

ssociation for Gondwana Research.

Recent seismological analyses revealed dehydration and watertransportation from the subducting slab. Kawakatsu and Watada(2007) showed receiver-function images of the subducting Pacific slabunder Northeast Japan. They interpreted a change in polarity ofdiscontinuity at the upper surface of the subducting Pacific slab downto about 150 km depth in terms of a dehydration process associatedwith subduction of the oceanic basalt layer. Deeper regions of themantle wedge and the subducting Pacific slab are imaged also byreceiver function analyses (Tonegawa et al., 2008). They found a clearinclined discontinuity from a depth of about 330 km to the 410-kmdiscontinuity in the mantle wedge just above the subducting slab.They argued that the discontinuity represents the upper boundary ofhydrous peridotite which is produced by dehydration of the sub-ducting slab. They also suggested that the hydrous peridotite can becarried down to the mantle transition zone. Thus these previousstudies indicate that the subducting Pacific slab can carry water intonot only the upper mantle but also the mantle transition zone.

Water solubility in the mantle minerals is determined by mineralphysics studies. The solubility into olivine and its high-pressurephases is, for example, up to 0.12–0.7 wt.% in α phase (Kohlstedt et al.,1996; Chen et al., 2002), 3.1 wt.% in β (wadsleyite) (Inoue et al., 1995),and 2.2–2.6 wt.% in γ (ringwoodite) (Inoue et al., 1998; Ohtani et al.,2000). Therefore, the mantle transition zone can be a large water

Published by Elsevier B.V. All rights reserved.

Fig. 1. Tectonic setting of the study region. Solid lines show plate boundaries betweenPacific, Philippine Sea, North American and Eurasian plates (Bird, 2003). Broken linesshow depth contours of the subducting Pacific and Philippine Sea slabs (Gudmundssonand Sambridge, 1998).

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reservoir in the Earth. Seismological observations for several regionsindicate a low velocity layer just above the 410-km discontinuity(Revenaugh and Sipkin, 1994; Song et al., 2004). Bercovici and Karato(2003) proposed a water filter model which hypothesizes a thin meltlayer just above the 410-km discontinuity. Note that a larger density ofthe melt than the surrounding mantle is necessary for stable layeringof the melts, and whether the melt layer actually exists or not is still acontroversial issue (Matsukage et al., 2005; Sakamaki et al., 2006).

Deep dehydration and the upwelling induced from the transitionzone are also suggested by seismic observations. A low-velocityanomaly was revealed extending from the surface beneath theChangbai intraplate volcano down to the mantle transition zone justabove the stagnant Pacific slab under Northeast Asia (Lei and Zhao,2005; Zhao et al., 2009). A hot upwelling through the 410-kmdiscontinuity in the mantle wedge up to the Lau back-arc spreadingcenter in the Tonga–Fiji region is visible in the tomographic image byZhao et al. (1997). This upwelling was also suggested by Helffrich(2005) based on the study of the depth variation of the 410-kmdiscontinuity under the region (Tibi and Wiens, 2005). A recenttomography also showed that a hydrated zone exists down to a depthof at least 400 km under the Lau back-arc basin (Conder and Wiens,2006).

Although many workers have suggested the possible existence ofwater in the transition zone, the actual evidence is still not sufficientand the amount of water is still unknown. Attempts to estimate actualwater content in the mantle transition zone are made by Blum andShen (2004) beneath the Kaapvaal Craton and by Suetsugu et al.(2006) beneath Southwest Japan. Both studies suggested existence ofwater by a few wt.%, which is consistent with mineral physics studies.Huang et al. (2005) and Yoshino et al. (2008) estimated water contentin the transition zone by measuring the electrical conductivity of thehydrous and anhydrous wadsleyite and ringwoodite. Huang et al.(2005) suggested water content of about 0.1–0.2 wt.% and Yoshinoet al. (2008) claimed no or less than 0.1 wt.% water, though theseestimated water contents strongly depend on the geophysicallyobserved values which they adopted.

In this study, we estimate the temperature and water contentanomalies in a depth range just above the 660-km discontinuity, thatis, the lowermost part of the mantle transition zone. The estimation ismade for the entire Japan Islands (Fig. 1), which is an extension of theprevious work by Suetsugu et al. (2006) who studied only SouthwestJapan. Evidence for deep dehydration, hydrous mantle transition zoneand their implications for mantle dynamics in the western Pacificregion are actively discussed in the recent issues of GondwanaResearch. For example, low-velocity anomalies in and around thetransition zone are clearly imaged by seismic tomography studies,which are interpreted as upwelling flows associated with thedehydration processes of the hydrous stagnant slab (e.g., Zhao andOhtani, 2009-this issue). Hydrousmantle is petrochemically predictedby Komiya and Maruyama (2007). Findings of deep dehydration andhydrous mantle are synthesized by Maruyama et al. (2007) forunderstanding the mantle dynamics from the past to the future in thewestern Pacific region. In this study we present our new findingswhich establish that the relationship between the temperature andwater content anomalies is different in the subducting slab and thesurrounding mantle, regardless of the difference in seismic resolutionscale. The difference may be caused by compositional differences,which improve our understanding of the mantle dynamics in thewestern Pacific region.

2. Estimation of temperature and water content anomalies

Temperature and water content anomalies are converted from theP-velocity perturbations and the depth variation of the 660-kmdiscontinuity by using the same method as in Suetsugu et al. (2006).

Here we only make a brief description of the seismological data andmethod used. For details, see Suetsugu et al. (2006).

Variations in Earth's physical property revealed by seismicobservations are often interpreted as temperature anomaly alone.Recent mineral physics studies, however, reveal that seismic velocitiesare dependent not only on temperature but also on water content inminerals in the mantle transition zone (Inoue et al., 1998, 2004).Another seismic observation is depth variations of the mantlediscontinuities caused by phase transitions of mantle minerals. Recentstudies of mineral physics also showed that the phase transition depthis changed not only by temperature (Clapeyron slope) but also bywater content (Higo et al., 2001; Chen et al., 2002). Therefore a jointanalysis of both the velocity perturbations and the discontinuity depthvariations can be made to determine two parameters, temperatureand water content.

In this study we assume the following linear relations between thetwo kinds of seismic observations and the two physical parameters:

ΔVp = AVp= ATð ÞΔT + AVp = Awð ÞΔw ð1Þ

Δd = Ad = ATð ÞΔT + Ad= Awð ÞΔw ð2Þ

Four coefficients needed for solving the linear equations are partialderivatives of velocity (Vp) and the 660-km discontinuity depth (d)with respect to temperature (T) and water content (w) (∂Vp/∂T, ∂d/∂T, ∂Vp/∂w and ∂d/∂w). We use the same values of the partialderivatives as those used by Suetsugu et al. (2006); ∂Vp/∂T=−5.5×10−4 [km/s/K], ∂d/∂T=−0.06 [km/K], ∂Vp/∂w=−0.2 [km/s/wt.%] and ∂d/∂w=2.7 [km/wt.%]. The temperature and watercontent anomalies can be estimated simultaneously by solving the

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equations. Note that this simple assumption may be valid only for theregion just above the 660-km discontinuity. Unlike phase transitionsfrom α to β (the 410-km discontinuity) and β to γ (the 520-kmdiscontinuity), phase transition of γ to perovskite and periclase at the660-km discontinuity does not form a significant phase loop in aphase diagram by changing Fe content (for a review, see Agee, 1998).Therefore we can ignore the effect of Fe content on the depthvariations of the 660-km discontinuity, while it is not the case for the410- and 520-km discontinuities.

Hereafter the P-velocity perturbations and the depth variations ofthe 660-km discontinuity are called seismological observations. Similarto Suetsugu et al. (2006), we used two sets of seismologicalobservations in this study. One is the velocity perturbations by Zhao(2004) and the 660-km depth variation by Yamada and Zhao (2007).The former is the result of global tomography by using a grid-parameterization with a grid interval of 5°. The latter is estimated byusing the waveform modeling of sScS precursor (a reflection from the

Fig. 2. (a, b) Long-wavelength seismic observations under Japan Islands. (a) Depth variatio(Zhao, 2004) just above the discontinuity. (c, d) The same as (a, b) but for moderate-wavel2005). (d) P-velocity perturbations (Fukao et al., 2001). Note that stars in (a) and (b) show t

660-km discontinuity) by using low-pass (50 s) filtered waveformswhich have a size of Fresnel zone of about 500 km at the reflectiondepth of 660 km. Thus this set of seismic data can be regarded as long-wavelength observations. The other set of seismic data is the velocityperturbations by Fukao et al. (2001) and the 660-km depth variationby Niu et al. (2005). The former is the result of seismic tomography byusing a block-inversion scheme with a block size of about 1.4°, whilethe latter is from a receiver-function analysis using waveforms with apredominant period of 3–4 s which has a Fresnel zone of about 150 kmat the conversion depth of 660 km. The second set of data can beregarded as moderate-wavelength observations. Fig. 2 shows the twosets of seismological observations (the same as Plates 2 and 3 inSuetsugu et al. (2006)). In the previous study of Suetsugu et al.(2006), only the variations under Southwest Japan were discussedbecause there are unexplainable discrepancies between the long- andmoderate-wavelength observations except for the region underSouthwest Japan. For detailed explanations of the uncertainties and

n of the 660-km discontinuity (Yamada and Zhao, 2007). (b) P-velocity perturbationsength seismic observations. (c) Depth variation of the 660-km discontinuity (Niu et al.,he values at source-side bounce points of sScS precursors (see Yamada and Zhao, 2007).

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errors, caused by the experimentally determined partial derivatives inEqs. (1) and (2), as well as the seismological observations of P-velocityperturbations and depth variations of the 660-km discontinuities, seeSuetsugu et al. (2006).

Fig. 3 shows the temperature and water content anomalies underthe whole Japanese Islands obtained in this study. The variations inFig. 3 indicate not absolute but relative values with respect to averagevalues in this study region, since all the values in Eqs. (1) and (2) arerelative values. Thus the apparently unreasonable negative values ofwater content shown in Fig. 3b and d (orange to red colors) should betaken as lesser water content than average water content. If aminimum value of “relative” water content anomalies in Fig. 3b and d(ca. −0.5 wt.%) is regarded as a situation of absolutely null water(dry), water content may be possible to be 0.5 wt.% on average and upto 1–2 wt.% in this region. These values are inconsistent with Yoshinoet al. (2008) who suggested no water and Huang et al. (2005) whosuggested 0.1–0.2 wt.% water content. Both of the mineral-physics

Fig. 3. Temperature and water content anomalies estimated from the two sets of the seismolong-wavelength seismic data. (c, d) The same as (a, b) but from the moderate-wavelength

studies compared their experimental results with observations ofelectrical conductivity for a semi-global averaging (one quarter of theEarth). In this study we investigate a much narrower region of theJapan subduction zone. Therefore the differences between our presentresult and those of the two mineral-physics studies may be due to thescale difference in the study areas. Note that the observations ofelectrical conductivity are still progressing. Shimizu et al. (2009)recently revised their conductivity model. Combining their revisedversionwith the result of Yoshino et al. (2008), up to 0.5 wt.% water ispossible just above the 660-km discontinuity.

In the following we describe the obtained results of this work(Fig. 3). In Southwest Japan (westward from 135°E), higher watercontent and lower temperature is shown in both datasets, possiblysuggesting the cold and wet subducting Pacific slab. This may be anevidence for water transportation into the mantle transition zone bywhole portion of the subducting Pacific slab, however, it is uncertainhow and why water can be incorporated into the whole slab. The

logical observations. (a, b) Temperature (a) and water content (b) anomalies from theseismic data.

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seismological observations by Kawakatsu and Watada (2007) andTonegawa et al. (2008) argue dehydration processes of upper layers ofthe subducting Pacific slab, that is, the sediment layer and oceaniccrust. Experimental studies indicate that diffusion length of hydrogenis small for minerals in the transition zone, 5.3 km in 109 years forwadsleyite (Hae et al., 2006) and 9.5 km in 108 years for ringwoodite(Kudo et al. (2006). It may be difficult to diffuse water from surfaceportion of the slab into deeper andwhole portion of the slab. Althougha cold lithospheric mantle hydrated by sea water through pathwayscaused by normal faulting just before the subduction has beensuggested (Ranero et al., 2003), it is not clear whether the whole slabof about 100 km thick can be hydrated or not. The process of watertransportation into the mantle transition zone is one of the mostimportant issues to be investigated in detail. Although the presentresult suggests the existence of water at the bottom of the transitionzone, clarifying the transportation process is beyond the scope of thisstudy.

The discrepancies between the seismological observations (Fig. 2)lead directly to the differences in estimations of temperature andwater content anomalies in Fig. 3. In Northeast Japan (especiallyaround 37 and 42°N), differences both in water and temperature arevisible (compare Fig. 3a and c, and Fig. 3b and d, respectively).Especially, an opposite topography of the 660 kmdiscontinuity around

Fig. 4. Relations between the seismological observations and geographical distribution of thedata. (a) Distribution of the P-velocity perturbations and the depth variations of the 660-kmsame as (b) but for showing two sequences (groups). Blue and red points are in the range ofSlopes of the blue and red lines are −350 and −380 [K/wt.%], respectively. (d) GeographicFig. 6. (For interpretation of the references to colour in this figure legend, the reader is refe

42°N (Fig. 2a and c) can cause different estimation of water andtemperature anomalies. This may be caused by poor data coverage inthis area; compared with other area, fewer seismic stations used inYamada and Zhao (2007) and lesser conversion points from P to S atthe 660 km discontinuity in Niu et al. (2005). The differences arevisible not only in pattern but also in amplitude. Temperatureanomalies range from −300 K to 300 K in Fig. 3a and from −800 Kto 200 K in Fig. 3c, water content anomalies from −0.5 to 0.8 wt.% inFig. 3b and from −0.5 wt.% to 1.8 wt.% in Fig. 3d.

The amplitude difference can be attributed to the difference in thehorizontal resolution scale of the seismic observations. Although thepattern difference is difficult to interpret, further examination of theresults shown in Fig. 3 has revealed one common geographical patternin temperature and water content anomalies.

Fig. 4 shows relations between the seismic observations and theestimated parameters. The variation of the long-wavelength seismicobservations (Fig. 4a) is scattered, indicating no clear correlationbetween them. However, the converted values into temperature andwater content anomalies (Fig. 4b) show that two negatively inclinedlines can be delineated to group them (Fig. 4c). The same groupingseems also possible in Fig. 4a by a close inspection. The geographicaldistribution of the colored points along the two lines is shown inFig. 4d. The points along the lower line in Fig. 4c are located under

temperature and water content anomalies estimated from the long-wavelength seismicdiscontinuity. (b) Distribution of the temperature and water content anomalies. (c) The30 K (10% of whole temperature range) with respect to blue and red lines, respectively.al distribution of colored points in (c). Green line shows the location of cross section inrred to the web version of this article.)

Fig. 5. The same as Fig. 4 but for the moderate-wavelength seismic data. Colored points are in the range of 50 K around the colored lines (c). Slopes of the blue and red lines in (c) are−380 and −360 [K/wt.%], respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

487A. Yamada et al. / Gondwana Research 16 (2009) 482–490

Southwest Japan, while those along the upper line are located underNortheast Japan. Fig. 5 shows the same plots as Fig. 4 but for themoderate-wavelength seismic observations. The variations of the Pvelocity and the 660-km discontinuity depth are also scattered(Fig. 5a). Note that our procedure to smooth the variations in theanalyses might lead to some artificial sequences of points in Fig. 5a.The distribution of temperature and water content anomalies (Fig. 5band c) also shows a gathering of points along two lines as seen inFig. 4b and c. The same geographical grouping of points as in Fig. 4d isclearly visible in Fig. 5d.

Note that, to determine lines (Figs. 4c and 5c) in a quantitativemanner, there is room for further investigation. In this study wedetermined two lines in Figs. 4c and 5c in the following way. A line isassumed and points along the line are geographically plotted. Wechange slopes of lines in a range from −200 to −500 [K/wt.%] andinspect geographical distributions of the points along the lines.Among many trials by changing the slopes, the red and blue lines inFigs. 4c and 5c are those which generate most apparent and clearseparation of two regions in geographical plotting for both of the twodatasets. Although the slopes of the lines in Figs. 4c and 5c are found tobe−350 to−380 [K/wt.%] in this study, it is better to regard them asa preliminary estimation because of large uncertainties in the currentseismological observations. In addition, other lines might exist inFigs. 4b and 5b. In future studies we may adopt a more objective andquantitative procedure to determine the lines (groupings) and valuesof slopes when we have more precise seismological data.

3. Discussion

Despite the different wavelengths of the seismic observations(Fig. 2) leading to different pattern and amplitude of the temperatureand water content anomalies (Fig. 3), the same geographical groupingwas obtained, which are two regions of Southwest Japan andNortheast Japan (Figs. 4d and 5d). This feature becomes obviousafter the seismological observations are converted into another set ofphysical parameters. In seismological analyses, variations or patternsof observations obtained from different procedures, wavelength scaleand frequency of waveforms are often very much different from eachother. The present study may be a good example that a commonfeature can be extracted from multiple sets of observations if theoriginal seismic observations can be converted into another set ofparameters, especially into physical and/or compositional parametersby using values frommineral physics. Although the secondary derivedvalues may be hard to explain in a quantitative manner due to theuncertainties and errors involved in the original observations, suchconversion may lead to new insight into the Earth's structure anddynamics.

3.1. Effects of uncertainties in the seismic data

Before discussing the implications of the obtained results, weexamine the effects of uncertainties in the seismological observationson the two lines in Figs. 4c and 5c. Suetsugu et al. (2006) investigated

Fig. 6. Schematic diagram showing a vertical cross section along a line in Figs. 4d and 5d.Horizontal axis indicates distance in degree from point A in Figs. 4d and 5d. Uppersurface of the subducting Pacific slab is indicated in solid line by interpolating slabcontours by Gudmundsson and Sambridge (1998). Lower surface (broken line) isdelineated by assuming a slab thickness of 100 km. Horizontal dotted lines show the 410and 660 km discontinuities. A and A′ correspond to endpoints of the line in Figs. 4dand 5d. B and B′ correspond to boundaries between red- and blue-colored regions inFigs. 4d and 5d, respectively. Red and blue arrows indicate two regions in Figs. 4d and5d. (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

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the influences of errors in P-velocity perturbations and depthvariations of the discontinuity on the estimations of temperatureand water content anomalies. They showed that as P-velocityincreases for a constant 660-km depth, the estimated amount ofwater content decreases. Since the P-velocity anomalies in atomographic model tend to be underestimated by a regularizationprocess, absolute values of actual P-velocity anomalies could begreater than shown in Fig. 2b and d. Southwest Japan, where exhibitsfast anomalies, may have even faster P-velocities than in Fig. 2.Therefore, the water content anomalies in Southwest Japan estimatedfrom the true P-velocity anomalies could be smaller than in Fig. 3b andd, which should shift the “blue” lines in Figs. 4c and 5c to the left. InNortheast Japan, where slow anomalies are indicated, the true(negative) P-velocity anomalies should shift the “red” lines to theright. Thus the true offset could be larger than in Figs. 4c and 5c.

Second, if we use a 1-D velocity reference model to determine the660-km discontinuity depths, the case for Fig. 2a obtained by Yamadaand Zhao (2007), errors in the assumed velocities could influence theobtained discontinuity depth. If an assumed velocity is lower than atrue velocity, the discontinuity depth should be underestimated. ForFig. 2c obtained by Niu et al. (2005), a 3-D velocity model is used tomap the discontinuity depth. However, since the S-velocity anomaliesare calculated from the P-velocity anomalies by using Vp/Vs ratios ofthe iasp91 velocity model (Kennett and Engdahl, 1991), inaccuracy indetermining the depth variation of the discontinuity may still remain.Under Southwest Japan, actual depths of the discontinuity may bedeeper than those shown in Fig. 2a and c, because high velocityanomalies of the stagnant Pacific slab artificially lead to shallowerestimation of the discontinuity. Suetsugu et al. (2006) examined theeffects of depth shift of the discontinuity and found that deepened(elevated) discontinuity by 10 km could cause a shift of temperatureestimation to colder (warmer) by 100–200 K and a shift of watercontent estimation to greater (lesser) by 0.5 wt.%, respectively. Thisindicates that for the case of Southwest Japanwhere a deeper 660-kmdiscontinuity is expected, every blue points in Figs. 4c and 5c movetoward right (greater amount of water) andmove downward (colder).Therefore errors in the 660-km depth determination shift each pointqualitatively along the line but do not significantly shift the line itself.Thus the offset between the lines in Figs. 4c and 5c should not be dueto the errors of the 660-km depth determination and velocity. Whilemore precise determinations of the velocity perturbations and the660-km depth variations (these two may be linked to each other) aredesirable in future quantitative works, it is qualitatively robust thatvariations in temperature and water content anomalies can begrouped onto the two lines in the study region.

3.2. Temperature dependence of water solubility

The two lines (Figs. 4c and 5c) have a negative slope of about−300 to −400 (K/wt.%) (or −2.5 to −3.3 wt.%/1000 K). If theminerals are assumed to be saturated with water, the latter value mayindicate a temperature derivative of solubility of water in mantleminerals at the depth of 660 km. The slope obtained here is stronglydependent on the values of the partial derivatives in the Eqs. (1) and(2). Eq. (1) can be rewritten as

Δw = ΔVp= AVp= Awð Þ− AVp= ATð Þ= AVp = Awð Þf gΔT ð3Þ

By substituting the derivative values used in this study derivedfromdata onmineral physics, the ratio in the second term is calculatedto be −2.8 (wt.%/1000 K), which is consistent with the value of theslope in Figs. 4c and 5c. This suggests that the value of the slope in thisstudy should not be regarded as a new estimation.

However, we can infer a consistency between different mineralphysics studies on water solubility and elastic velocity and temper-ature dependence of them. The maximum solubility of water into

ringwoodite is determined to be 2.6 wt.% at 1573 K by Inoue et al.(1998) and 1.5–2.6 wt.% at 1573–1673 K and about 0.3 wt.% at 1973–2073 K by Ohtani et al. (2000). A rough estimate of temperaturedependence of water content in saturated ringwoodite from thesestudies is −2.2 to −5.1 wt.%/1000 K, which is consistent with theestimation in the present study. The consistency means that themineral physics results used here are robust.

3.3. Compositional difference

The two regions with different ratios of temperature and watercontent in Figs. 4d and 5d may indicate a difference in mineralogicalcomposition at the bottom of the mantle transition zone. If thesubducting Pacific slab is assumed to be 100 km thick and has a dipangle of about 30°under the Japan Islands (e.g., Zhao, 2004, 2009), thewestern region (the blue area in Figs. 4d and 5d) corresponds to thesubducting Pacific slab, while the eastern region (the red area inFigs. 4d and 5d) corresponds to the normal mantle under the Pacificslab (Fig. 6). Points B and B′ in Fig. 6 show the “red–blue” boundariesfor the two datasets, respectively (Figs. 4d and 5d). The boundaryshould coincide with each other if it indicates a boundary of themineralogical composition. An offset between B and B′ in Fig. 6 mayshow differences in uncertainties and/or errors in the two datasetswhich are hard to be accurately evaluated. For example, by a carefulinspection in geometry of the depth contours of the Pacific slab, thepoints B and B′ are located at positions where the contours becomeconvex. Such a complicated slab geometry may lead to a differenteffect on perturbations of seismic ray paths in the two dataset, that is,P to SV conversion in the receiver functions and SH to SH reflections inthe ScS reverberations. Although an accurate position of boundaryseparating the two portions may be still uncertain, a general feature,“red” in Southwest Japan and “blue” in Northeast Japan, is the same forthe two dataset.

The offset between the two lines in Figs. 4c and 5c, may suggest adifferent water solubility of the slab material and the surroundingmantle due to a compositional difference. A large dependence of watersolubility on temperature has been reported for ringwoodite byOhtani et al. (2000) and Bolfan-Casanova et al. (2006), showing thatthe lower temperature ringwoodite has the higher solubility. Therelative enrichment in water in Southwest Japan with respect to theNortheast Japan may be caused partially by higher solubility due tolower temperatures in Southwest Japan (Fig. 3). A water transportassociated with the subducted slab may also contribute to the

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enrichment in Southwest Japan. However, the offset indicates that, fora given temperature, a water content anomaly in Southwest Japan(slab material) is lower than that in Northeast Japan (surroundingmantle), which cannot be attributed to the temperature dependenceof the solubility. A possibility of the compositional factor is adifference inMg/Fe in the slab and surroundingmantle. A lithosphericportion of subducted slabs is considered to be Mg-rich (Fe-poor) ascompared with the surrounding mantle. It may cause a differentnature of hydration and solubility in the slab and surrounding mantle,while the compositional dependence (e.g., Mg/Fe and/or Al content)on water solubility into ringwoodite remains to be studied.

It is necessary to take into account the temperature dependence on∂Vp/∂T due to the effect of anelasticity for such a case of largetemperature difference between the subducting slab and thesurrounding mantle, because the same value of ∂Vp/∂T is assumedfor all the regions in this study. As discussed in Suetsugu et al. (2006),the absolute value of ∂Vp/∂T becomes larger as Q value decreases. Weuse the ∂Vp/∂T value for infinite Q in this study, which may beappropriate for the cold subducting slab. The surroundingmantle mayexhibit a large ∂Vp/∂T due to its low-Q value compared with that ofthe subducting slab. A large ∂Vp/∂T leads to an estimation of largeamount of water content (Suetsugu et al., 2006). If we apply a large∂Vp/∂T to the surrounding mantle region, water content there wouldbecome larger, that is, the red lines in Figs. 4c and 5c will move to theright, while the blue lines will stay there. This will result in a largeoffset between the two lines in Figs. 4c and 5c. Therefore the groupingof the anomalies obtained in this study is not due to the anelasticeffect but reflect a real feature caused by the compositional difference.Yamazaki et al. (2005) showed that softening of a subducting cold slabmight occur due to a slow rate of grain growth of ringwoodite after thephase transition. Although there is a great difference in time scalebetween rheological deformation and vibration by seismic waves, thesoftening effect might lead to low-Q slab at the base of the transitionzone. To clarify this issue, the attenuation (Q) structure of the mantletransition zone should be investigated by both seismology andmineral physics in the near future.

4. Conclusions

Seismological observations of P-velocity perturbations and depthvariations of the 660-km discontinuity are used to estimate thetemperature and water content distribution at the bottom of themantle transition zone under the entire Japan Islands. The obtainedtemperature and water content anomalies show discrepanciesbetween different seismic data sets with long and moderatewavelengths. However, variations in temperature and water contentanomalies show two distinct sequences (groups), which correspondto a lower temperature and high water-content anomaly in SouthwestJapan and a higher temperature and low water-content anomaly inNortheast Japan. A remarkable new finding is the coincidence of thegeographical locations of the anomalies estimated from severaldifferent seismological studies with different data sets and methods.The two distinct anomalies appeared in the distribution of temper-ature and water content anomalies indicate the spatial variations incomposition at the bottom of the mantle transition zone, onecorresponding to the lithospheric portion of the subducting Pacificslab under Southwest Japan, the other corresponding to the normalmantle under Northeast Japan.

Acknowledgements

We thank F. Niu for providing us with his data of the topographyof the 660 km discontinuity. This study was supported by theGlobal-COE Programs for Deep Earth Mineralogy at Ehime Uni-versity and for Earth and Planetary Dynamics at Tohoku University.Figures were made by using GMT (Wessel and Smith, 1998). We

appreciate the editorial effort by Prof. M. Santosh and Prof. S.Maruyama and helpful review comments and suggestions from twoanonymous reviewers.

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