A joint interpretation of electromagnetic and seismic tomography models suggests the mantle...

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A joint interpretation of electromagnetic and seismic tomography models suggeststhe mantle transition zone below Europe is dry

Hisashi Utada a,⁎, Takao Koyama a, Masayuki Obayashi b, Yoshio Fukao b

a Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japanb Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Natsushima 2-15, Yokosuka 237-0061, Japan

a b s t r a c ta r t i c l e i n f o

Article history:Received 9 July 2008Received in revised form 16 February 2009Accepted 17 February 2009Available online 28 March 2009

Editor: R.D. van der Hilst

Keywords:electrical conductivityseismic tomographymantle transition zonestagnant slabwater

The three-dimensional (3D) electrical conductivity distribution in the mantle transition zone (MTZ) belowEurope is resolved by directly inverting geomagnetic response functions from 12 selected Europeanobservatories. Comparison of the conductivity image with the results of global seismic tomography reveals agood correlation between low-conductivity and high-velocity anomalies at depths of 400–700 km beneaththe Mediterranean subduction zone in central Europe. Interpretation of the 3D conductivity model combinedwith seismic tomography indicates that the European MTZ is drier than the MTZ beneath the western Pacificsubduction zone, even though the subducted slab material is stagnating similarly in both regions.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The electrical conductivity of Earthmaterials is a property sensitiveto temperature and chemical composition, and is also stronglydependent on the amount of water present. Water, a source ofhydrogen, is of particular importance with respect to the evolution ofthe Earth (Karato 1991, 2006). In recent studies on themineral physicsof deep mantle materials (Huang et al., 2005; Hae et al., 2006), it hasbeen found that the distribution of electrical conductivity in themantle transition zone (MTZ; ca. 400–660 kmdepth) suggests that theMTZ might act as a water reservoir in the solid Earth (Ohtani et al.,2004). Although geomagnetic observations are capable of probing thedistribution of electrical conductivity at such depths (Banks, 1969),there remain few observation-derived results related to water contentin the MTZ due to the difficulty of performing three-dimensional (3D)inversions of the observational data.

The conductivity distribution in the MTZ or at deeper levels can bederived by global induction based on electromagnetic (EM) fieldvariations on the scale of 1 day or longer. There have been a number ofstudies on the global mantle structure assuming a one-dimensional(1D) Earth (Banks, 1969; Honkura and Matsushima, 1998), with thefirst studies performed in the 1950s. However, in a simple 1D model,

two 3D induction effect are ignored: that caused by the distribution ofland and sea, and that due to the heterogeneous mantle. The former,known as the ocean induction effect, was originally thought to benegligible when considering EM variations with such long periods(e.g., longer than 1 day). However, recent 3D modeling (Kuvshinovet al., 2002) has revealed that the effect can be significant, even forvariations with periods of up to 20–30 days. This finding indicates thatrigorous 3D treatment is necessary, at least for the surface, even if the1D conductivity distribution in the mantle is only to be estimated to afirst-order approximation.

A 1D model of mantle conductivity has been constructed (Utadaet al., 2003) for the North Pacific based on data acquired by a newlydeployed network of geophysical observatories, the Ocean Hemi-sphere Network (Shimizu and Utada, 1999). This model is the firstestimated by a global approach considering the ocean induction effect.A global 1D model was more recently estimated using satellite dataand rigorous consideration of the ocean induction effects (Kuvshinovand Olsen, 2006). One-dimensional models estimated by suchapproaches can be adopted as reference models for 3D inversion asa next step in the investigation of the MTZ, where departures of theobserved responses from those predicted by the model can beregarded as anomalies due to 3D heterogeneities in the mantle.

A 3D semi-global inversion has been reported for the North Pacific(Koyama, 2001; Fukao et al., 2004). This approach, a form of EMtomography analogous to seismic tomography, affords a 3D map ofmantle conductivity. Comparison of the map constructed in the North

Earth and Planetary Science Letters 281 (2009) 249–257

⁎ Corresponding author. Tel.: +81 35841 5722; fax: +81 3812 9417.E-mail addresses: [email protected] (H. Utada), [email protected]

(T. Koyama), [email protected] (M. Obayashi), [email protected] (Y. Fukao).

0012-821X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2009.02.027

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Pacific study, having a lateral resolution of 15°×15° anddepth resolutionof 100 km, with global seismic tomography (e.g., Fukao et al., 2003)suggests the possible presence of water in the MTZ below the westernPacific subduction zone (Koyama et al., 2006). TheMTZ can be probed inthis way only in regions where many geomagnetic observatories havebeen deployed. Apart from the western Pacific, Europe is therefore theonly other region where such an investigation is possible at present.Geomagnetic observatories have been in operation in Europe for manydecades (Olsen, 1999), and maps of the lateral heterogeneity ofconductivity beneath Europe have been reported (e.g., Semenov andJozwiak, 2006). Previous maps, however, were obtained by simplycompiling the conductivity profiles obtained by individual 1D inversionfor each observatory, where it was assumed that the frequencydependence of the response functions at a given site is simply due tothe radial variation in conductivity. Therefore, if lateral heterogeneityeffects, including ocean induction effects, are not negligible, assuggested above, previous studies may have yielded erroneous results(Shimizu et al., in press). In the present study, EM tomography is appliedto the geomagnetic variationdata acquiredbyEuropeanobservatories inorder to obtain a map of 3D conductivity in the MTZ that accounts forlateral heterogeneity effects.

2. Data and inversion

The 12 European geomagnetic observatories at which data wereacquired for the present study are shown in Fig. 1. Hourly mean valuesof the three components of the geomagnetic field recorded over aperiod of longer than 7 years are available for all of the selectedobservatories. Observatories at high latitudewere excluded in order toavoid the influence of the local ionospheric current system in theauroral oval (Fujii and Schultz, 2002). The geomagnetic deep sounding

(GDS) responses (Banks, 1969) were obtained for 11 discrete periodsof 5–50 days for each observatory. The GDS response is calculated by

RobsGDS ωi; rj

� �=

Bz ωi; rj� �

Bx ωi; rj� � ð1Þ

where ωi is frequency, rj is the observation location, and Bz and Bx arethe vertical and northward components of the geomagnetic field,respectively. The GDS response is thus a ratio of the vertical (Bz(ωi;rj))to the northward (Bx(ωi;rj)) components of geomagnetic fieldvariations and is a complex quantity containing information on theconductivity structure of the Earth. The period range considered in thepresent study allows the electrical conductivity to be probed at thedepth of the MTZ and the shallow lower mantle.

Fig. 1. Locations of selected geomagnetic observatories in Europe. Major plate boundaries are indicated by dotted lines.

Fig. 2. Decrease in the misfit between observed and calculated GDS responses given byEq. (2) with increasing iterations in 3D electromagnetic inversion of European data.

250 H. Utada et al. / Earth and Planetary Science Letters 281 (2009) 249–257


The set of GDS responses is then inverted to obtain the 3D electricalconductivity distribution by minimizing the misfit between observedand calculated responses. This procedure can be expressed formally as

ψ =Xi;j

j 1eij RobsGDS ωi; rj

� �−Rcal

GDS ωi; rj;σ3D r; θ;/ð Þ� �n o j 2Ymin: ð2Þ

where ψ is the misfit, RGDScal (ωi;rj;σ3D(r,θ,ϕ)) is a calculated response,and ɛij is the estimation error for RGDSobs (ωi;rj).

In the calculation of model responses, the 3D conductivity distribu-tion given by is separated into three parts: a radially symmetric part (1Dreference model) given by, a 3 km-thick heterogeneous layer at thesurface (r=r0) representing the distribution of land and sea and given

by Δσ(r0,θ,ϕ), and the lateral perturbation from the reference model inthe mantle as given by δσ(r,θ,ϕ). The conductivity distribution is thusexpressed by

σ3D r; θ;/ð Þ = Δσ r0; θ;/ð Þσ1D rð Þ + δσ r; θ;/ð Þ

for r = r0for r b r0

�ð3Þ

The 1Dmodel for the North Pacific region (Utada et al., 2003) is usedas the referencemodel (σ1D(r)). A European 1D referencemodel was notdetermined in the present study because the European observatories donot cover a sufficiently wide area for estimating a spatially averagedmodel structure. The surfaceheterogeneous layer (Δσ(r0,θ,ϕ)) ismodeledin reference to the bathymetry of the ETOPO-5 dataset (NOAA), and the

Fig. 3. Comparison of observed response functions (dots with error bars) with those calculated for the initial (light-blue dashed lines) and final (blue dotted lines) models forobservatories (a) WNG, CLF, HAD, FUR, NGK, and AQU, and (b) SPT, BDV, EBR, BEL DOU, and HLP.

251H. Utada et al. / Earth and Planetary Science Letters 281 (2009) 249–257


conductivity of seawater is set uniformly at 3 S/m. The heterogeneousstructure is considered only below the area shown in Fig. 1, where theunknown conductivity perturbation δσ(r,θ,ϕ) corresponds to the hetero-geneity in themiddlemantle (400–800 km). The conductivity in regionsoutside the target region, for which the present dataset is not sensitive(Fukao et al., 2004), is assumed to be laterally homogeneous and is thusexpressed only by.

The target region is composed of a number of blocks with uniformconductivity perturbation (δσ(r,θ,ϕ)). The size of each block is 5°×5°laterally and 100 km radially, affording a total ofM=9×9×5 unknownparameters. The discretized model parameter is defined as

mk = log 1 +δσ rk; θk;/kð Þ

σ1D rkð Þ� �

ð4Þ

where (rk,θk,ϕk) is the location of the center of the kth block. The gridspacing for numerical integration is 1°×1° laterally and 50 km radially.

The modified iterative dissipative method (Singer, 1995) is employedas a forward solver, inwhichMaxwell equations are solved in the form ofan integral equation tocalculate the theoretical responseof the conductingEarth under external excitation ofmagnetic field variations. A ring currentis assumed for the external source current configuration (Banks, 1969;Koyama, 2001). The optimum set of parameters {mk} minimizing themisfit between the observed and calculated GDS responses is obtained bya search using theQuasi-Newtonmethodwith the BFGSupdate algorithm(Koyama, 2001). The initial condition assumes the absence of perturba-tions in each block (i.e., mk=0 for k=1,…,M). The set of unknownparameters was corrected iteratively until convergence in 40 iterations,reducing the misfit by approximately 75% from 5.50 to 1.39 (Fig. 2). The

Fig. 3 (continued).



responses calculated from the finalmodel are in good agreementwith theobserved responses within the range of observational error (Fig. 3(a,b)).The misfit between the 1D models with and without the land–seadistribution is approximately 1.3, whereas that between the initial andfinal models, additionally involving 3D mantle heterogeneity is approxi-mately 4.1. This result indicates that the effect due to 3D mantleheterogeneity is substantially stronger than that due to ocean inductionin the present case.

3. Result

The 3D conductivity anomaly maps determined in the presentstudy are shown in Fig. 4 for the MTZ at depths of 400, 500, and600 km. A low-conductivity anomaly is conspicuous in these maps. Alow-conductance anomaly in central Europe has been reported bySemenov and Jozwiak (2006), who interpreted the anomaly as aninduction effect attributable to the Mediterranean Sea. However, the

Fig. 4. Comparison of P- and S-wave tomography and present EM tomography results at depths of 400, 500, and 600 km.



present full 3D inversion result indicates that the anomaly is due notto an induction effect related to near-surface heterogeneity but toinduction related to a deep heterogeneity.

The obtained conductivity anomaly maps are compared withglobal P- and S-wave seismic tomography (Obayashi et al., 2006;Megnin and Romanowicz, 2000) in Fig. 4. Due mainly to differences inspatial resolution, the regional P-wave tomography of Piromallo andMorelli (2003) affords locally stronger P-wave velocity (VP) anoma-lies, and the regional P- and S-wave joint tomography of Schmid et al.(2006) yields stronger S-wave (VS) anomalies than the global P- andS-wave tomographic models employed here. The global models wereadopted in the present study because the long-wavelength featurescomparable in spatial resolution to the present conductivity anomalymodel are of most interest.

The sensitivity of the conductivity of the kth block is calculated by

S mkð Þ =Xi; j

j 1eij ARcalGDS ωi; rj;mk

� �Amk j 2 ð5Þ

The 1D reference model is employed to calculate the partialderivative of the GDS response, since the calculation of derivatives in afull 3Dmodel is very timeconsuming. The 1D sensitivity canbe regardedas representative of that for the 3Dmodel becausemore than 90% of thevariance reduction is obtained in the first iteration (Fig. 2), with whichthe 1D sensitivity is involved.

The resulting sensitivity maps are shown in Fig. 5 for depths shownin Fig. 4. The sensitivity seems to be primarily related to the distributionof observation sites, and tends to increasewith depth due to the increasein background 1D conductivity. The sensitivity of the blocks in theimmediate vicinity of observatories are more than 10 times higher thanthose in areas more distal from the observatories.

Themost notable feature in Fig. 4 is the correlation between the low-conductivity and high-velocity anomalies beneath the central part ofEurope, where slab material subducted from the Tyrrhenian trenchstagnates in the MTZ (Faccenna et al., 2003). The inverted values of thislow-conductivityanomalyare regardedas themost reliable, consideringthe high sensitivity values of the corresponding blocks. This correlationimplies that the low-conductivity and high-velocity characteristics ofthe stagnant slab (Fukao et al., 2001) have a common origin, most likelytemperature. This result is contrary to that obtained for the northwestPacific region (Koyama et al., 2006), where a conduction mechanismadditional to the temperature effect (e.g., hydrogen conduction) isrequired to account for a particular conductivity anomaly (Fig. 6).Qualitatively, these results suggest that the European MTZ is relativelydry. This inference will be verified in the next section by performingfurther analyses of the three independent anomaly sets.

4. Discussion

The present results from Europe are compared with previous resultsfor the Pacific region (Fukao et al., 2004) by normalization assuming asimple temperature dependence for electrical conductivity, whichallows us to separate temperature and other effects on conductivity.Using the Arrhenius relation, the inverted 3D conductivities can thus beexpressed as

σ3D r; θ;/ð Þ = σ0exp−H

k T0 rð Þ + δT r; θ;/ð Þf g� �

ð6Þ

where σ0 is a pre-exponential term,H is the activation enthalpy, k is theBoltzmann constant, T0 is the standard 1D temperature profile in theMTZ (Ito and Katsura, 1989), and δT is the temperature anomaly.

The activation enthalpy is a constant representing the conductionmechanism, while the pre-exponential term represents the totalamount of charge responsible for conduction. Under dry mantleconditions, where the effect of hydrogen conduction is negligible,small polarons (lower temperature) or magnesium vacancies (highertemperature) act as the charge carriers responsible for conduction overthe pressure and temperature range of interest (Constable, 2006).Hydrogen conduction can be distinguished from intrinsic conduction ofsmall polarons ormagnesiumvacancies by applying a smaller value ofH(Huang et al., 2005; Yoshino et al., 2008).

The temperature dependence of 3D conductivity anomalies canbe examined by converting the velocity anomalies of the global seismicP- and S-wave tomography (Obayashi et al., 2006; Megnin andRomanowicz, 2000) into the temperature anomalies δTP(r,θ,ϕ) andvelocity–temperature conversion is performed using a simple relation(Karato, 1993) that was applied in the previous study (Fukao et al.,2004). Values of δTP or δTS are substituted for the temperature anomalyδT in Eq. (6) after decimating the resolution of corresponding velocityanomalies to a block size of 5°×5°×100 km. The 3D conductivity is thenplotted against T0+δTP and T0+δTS thus obtained for each block asshown in Fig. 7. Although it is possible in theory to estimate the values ofσ0 andHbya least-squaresfit to eachof these conductivity–temperaturerelations, it is not attempted in this study because of the largeuncertainty in the amplitude estimates of anomalies. In fact, theconductivity–temperature relation for T0+δTS apparently shows thesteeper gradient than that for T0+δTP as can be seen in Fig. 7. Thisdifference is attributable to the smaller variance of δTS due probably tothe poorer spatial resolution of the global VS tomography in this region.

The conductivity–temperature relations in the region correspondingto the high-conductivity anomaly beneath the Philippine Sea (Fig. 6) arealso plotted in Fig. 7 for comparison. The Pacific data are significantlyshifted to higher conductivity side compared to the European data. This

Fig. 5. Sensitivity of conductivity for each block at depths of 400, 500, and 600 km below Europe.



discrepancy in the conductivity–temperature relations between Europeand the western Pacific might also be ascribed to the different spatialresolutions of different tomography techniques employed. Thereforewereduced the resolution of European data to 15°×15° horizontally

(similar to that of the western Pacific case), selected blocks with highsensitivity and compared them with the western Pacific results toexamine the resolution effect (Fig. 7). As shown in Fig. 7, the generaltendencies do not depend appreciably on the spatial resolution.

Fig. 7. Arrhenius relation between conductivity and converted temperature at depths of 400 (red), 500 (green), and 600 km (blue). Conductivity values are determined by EMtomography, and temperatures are converted from global P-wave (left) and S-wave (right) tomography. Plus and square symbols denote European results obtained with a spatialresolution of 5°×5° and 15°×15°, respectively. Solid circles denote the corresponding relations for the high-conductivity anomaly beneath the Philippine Sea (Fig. 6). Red dotted (β)and blue dashed (γ) lines are those predicted from experimental data for dry wadsleyite and ringwoodite, while red and blue solid lines are those for water containing minerals(Yoshino et al., 2008), respectively.

Fig. 6. Comparison of P-wave, S-wave, and EM tomography results for the Pacific region at depths of 400, 500, and 600 km (modified after Fukao et al., 2004). Block size for EMtomography is 15°×15°×100 km. Intense high-conductivity anomaly in the MTZ beneath the Philippine Sea is indicated by a box (no accompanying velocity anomaly is apparent).



Note that the temperatures shown in Fig. 7 are not very accurate, asthey are estimated by using global tomographic models with reducedspatial resolution. Amplitudes of actual temperature anomalies could belarger if we refer to regional tomographic models (e.g. Schmid et al.,2006). Even if more accurate temperature estimates are used that areshifted to other temperatures, however, the main difference inconductivity between the Pacific and Europe will still remain. Experi-mental results indicate that extremely large difference in temperaturestructure between the two regions would be required to reasonablyaccount for the difference in conductivity, if we tried to explain thedifference only by temperature effects. Given what we know about thetwo regions, however, such large temperature differences are hardlyplausible.

Seismic velocities can also be reduced by the presence of water (e.g.van der Lee et al., 2008; Inoue et al.,1998). If significant amount ofwateris present in the high velocity anomaly region, amplitudes of the highvelocity anomaly are expected to be more reduced thanwhat would bedue solely to low temperatures. If the Pacific result is more affected bythe presence of water than the European result, it would lead velocityestimates biased to the lower values and therefore temperatureestimates biased to the higher values in the simple temperature–velocity conversion applied in the present study. However, propercorrection of the effect will result in more enhanced difference inconductivity between the two regions.

Thus, the difference in the conductivity–temperature relations isregarded as a robust feature in Fig. 7. The presence of considerablymore water in the western Pacific than in the European MTZ (Koyamaet al., 2006) can be a likely interpretation, assuming similar mineraland chemical compositions. Suetsugu et al. (2006) estimated watercontent of approximately 1 wt.% in the MTZ by comparing a receiverfunction analysis with the tomography results in the western Pacificregion, which supports our interpretation above. In Fig. 7, theEuropean results with reduced resolution are more or less consistentwith the experimental data for dry MTZ minerals. On the other hand,the Pacific data distribute around lines expected by laboratoryexperiments on wet MTZ minerals. It is also noticeable that thePacific data for lower temperatures (or shallower depths) tend todeviate more from the overall trend of the European results, whichmay indicate a smaller value of H in the Pacific than that in theEuropean MTZ. This tendency may also imply the different watercontent at different depths in the Pacific MTZ; structure at 400 kmmainly reflects mantle wedge conditions containing water trans-ported by down-going flow, while structure at 600 km should ratherbe dominated by flat lying slab that has already dehydrated. As aresult, the difference between Pacific and European data in Fig. 7 tendsto be smaller for greater depths. However, observations of higherdensity are required especially in the Pacific subduction zone forfurther discussion along this direction, as present dataset can onlyprovide limited spatial resolution.

The present result indicates that hydrogen conduction is not adominant mechanism in bulk electrical conduction in the MTZbeneath central Europe, confirming the inference that little waterwas transported into the MTZ by the Mediterranean subduction. Thisstate can be explained by the expulsion of water into the uppermantle(Maruyama and Okamoto, 2007) due to the slow subduction of ayoung, hot plate (Faccenna et al., 2003). Such an explanation is alsoconsistent with the identification of a high-conductivity anomaly inthe deeper upper mantle beneath the French Alps, which suggests thepossible presence of water (Tarits et al., 2004). A receiver functionanalysis for the underlying region (van der Maijde et al., 2003) hasrevealed a thickening of the 410 km seismic discontinuity, whichsuggests that approximately 0.1 wt.% water is transported into theshallowest part of the MTZ. This result is not inconsistent with ourresult, because the conductivity of MTZ minerals with water contentof 0.1 wt.% does not show a significant difference from that of dryminerals (Yoshino et al., 2008).

In active subduction zones of old and cold oceanic crust, such as inthewestern Pacific, water entrained in the downwardmantle flow canbe transported deep into the mantle (Kawakatsu and Watada, 2007),readily to the MTZ (Maruyama and Okamoto, 2007). Such a process isconsidered to play an important role in the global water cycle(Bercovici and Karato, 2003; Bolfan-Casanova, 2007), suggesting thatthe MTZ may act as a water reservoir (Ohtani et al., 2004). In thewestern Pacific subduction zone, a high-conductivity anomaly in theMTZ was identified beneath the Philippine Sea (Fig. 6), below whichthe subducted Pacific slab is stagnating. From this observationalevidence, we suggest that the western Pacific MTZ is wetter thanEuropean MTZ. Considering the absence of an accompanying low-velocity anomaly (Fukao et al., 2004), this high-conductivity anomalyis less relevant to the presence of a melt layer (Bercovici and Karato,2003; Toffelmier and Tyburczy, 2007), although further detailed studyis needed to clarify this point.

In the discussion above, the differences in present day slab age andsubduction rate are inferred to cause the different degree of hydration ofthe MTZ in the two regions. If we consider the large horizontal scale ofhydration of more than 1000 km, however, the difference in the timelength of slab subduction could be another possible interpretation.Subduction under thewestern Pacific is a muchmore long-lived (largerscale) process than that under the central-western Mediterranean. Thedry European MTZ may be a consequence of the total subduction timelength that is insufficient to hydrate the MTZ over such large scales.

5. Conclusions

A 3D model of electrical conductivity in the MTZ below Europe wasderived by inversion of geomagnetic observation data. The modelresolves a low-conductivity feature that correlates well with a high-velocity anomaly identified by VP and VS tomography which has beeninferred to correspond to a stagnant slab in the MTZ. Comparing thepresent results with previous results for the western Pacific subductionzone, it was implied that the conductivity–temperature relation for theEuropean results can be explained only by considering the temperatureeffect, whereas the western Pacific results need a contribution ofadditional conduction mechanism such as hydrogen conduction. If thedifference in conductivities is interpreted in terms of water content, theEuropean MTZ is suggested to be drier. Either smaller amount of watertransported to the MTZ or shorter subduction time for hydration of theMTZ is suggested to be a probable cause for the dryMTZ condition in theMediterranean subduction zone.

Acknowledgements

The European observatories andWDC Kyoto are gratefully acknowl-edged for providing geomagnetic data. Critical comments by twoanonymous reviewers and the editor (Dr. R.D. van der Hilst) werehelpful in improving themanuscript. This studywas partially supportedby the Stagnant Slab Project (#16075202 and #16075204).

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A joint interpretation of electromagnetic and seismic tomography models suggests the mantle...

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