Kenji Ohta et al- Phase transitions in pyrolite and MORB at lowermost mantle conditions:...

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Phase transitions in pyrolite and MORB at lowermost mantle conditions:Implications for a MORB-rich pile above the coremantle boundary

Kenji Ohta a, Kei Hirose a,b,, Thorne Lay c, Nagayoshi Sata b, Yasuo Ohishi d

aDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japanb Institute for Research on Earth Evolution, Japan Agency for MarineEarth Science and Technology, Yokosuka, Kanagawa 237-0061, Japan

c Department of Earth and Planetary Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064, USAd Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan

Received 31 January 2007; received in revised form 17 October 2007; accepted 19 November 2007

Editor: R.D. van der Hilst

Abstract

Subduction of mid-oceanic ridge basalt (MORB) gives rise to strong chemical heterogeneities in the Earth's mantle, possibly extending downto the coremantle boundary. Phase relations in both pyrolite and MORB compositions are precisely determined at high pressures andtemperatures corresponding to lowermost mantle conditions. The results demonstrate that the post-perovskite phase transition occurs in pyrolitebetween 116 and 121 GPa at 2500 K, while post-perovskite and SiO2 phase transitions occur in MORB at4 GPa lower pressure at the sametemperature. Theory predicts that these phase changes in pyrolite and MORB cause shear wave velocity increase and decrease, respectively. Nearthe northern margin of the large low shear velocity province in the lowermost mantle beneath the Pacific, reflections from a negative shear velocity

jump near 2520-km depth are followed by reflections from a positive velocity jump 135 to 155-km deeper. These negative and positive velocitychanges are consistent with the expected phase transitions in a dense pile containing a mixture of MORB and pyrolitic material. This may be adirect demonstration of the presence of accumulations of subducted MORB crust in the deep mantle. 2007 Elsevier B.V. All rights reserved.

Keywords: D"; post-perovskite; core-mantle boundary; perovskite; superplume

1. Introduction

The 6-km thick basaltic oceanic crust (MORB) is injected

into the mantle in significant quantities as oceanic lithospheresinks at subduction zones. The average oceanic crust productionrate during Mesozoic to present time is estimated to be about25 km3 per year (Reymer and Schubert, 1984). Assuming this

production (=subduction) rate has been constant during the last4 billion years (a much larger production rate has been sug-gested for the Archean (Komiya, 2004)), the total amount ofoceanic crust that has subducted corresponds to at least 11% of

the Earth's mantle in volume. It is thus probable that there arelarge quantities of MORB residing in the mantle.

Subducting MORB crust undergoes phase transitions as

pressure and temperature increase with depth, and MORB'sdistinct chemistry from the balance of the slab and surroundingmantle produces chemical and density heterogeneities over thedepth range of subduction, possibly extending down to the coremantle boundary (CMB). The chemicalcomposition of subductedMORB crust is not significantly modified in the Earth's deepinterior because the solid-state diffusion rate is very slowespecially in the lower mantle (Holzapfel et al., 2005), althoughthe upper part of MORB crust is hydrated before subduction andloses minor amounts of SiO2 andAl2O3 upondehydration (Kesselet al., 2005). Previous density measurements (Ono et al., 2005;Hirose et al., 2005) and geodynamical simulations (Christensenand Hofmann, 1994; Tackley, 1998; McNamara and Zhong,

2004; Nakagawa and Tackley, 2005) suggest that dense MORB

Available online at www.sciencedirect.com

Earth and Planetary Science Letters 267 (2008) 107 117www.elsevier.com/locate/epsl

Corresponding author. Department of Earth and Planetary Sciences, TokyoInstitute of Technology, Meguro, Tokyo, 152-8551, Japan. Tel.: +81 3 5734 2618;fax: +81 3 5734 3538.

E-mail address: [email protected] (K. Hirose).

0012-821X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2007.11.037


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crust may plausibly have accumulated in chemically distinct pilesintheD region at the bottom of the mantle. While strong seismicvelocity heterogeneities in the lowermost mantle have been de-tected (see Lay and Garnero, 2004 for a review), direct seis-mological evidence for the presence of MORB piles has not been

presented, in part because the properties of MORB are not wellcharacterized for lowermost mantle conditions and in partbecausethere are alternate interpretations of seismic heterogeneities in thelower mantle. Nishihara (2003) calculated seismic velocities of

pyrolite and MORB, showing that they are very similar to each

other at lower mantle conditions except around 1600-km depthwhere the velocities significantly decrease in MORB materialsdue to ferroelastic-type phase transition in SiO2 phase; however,absolute seismic velocities do not uniquely resolve specific min-eralogical structure in the deep mantle because there are strongtrade-offs with temperature. The non-uniqueness of interpretationof seismic structure can be reduced if phase transitions in themedium are detected and can be compared to the expectations fora given mineralogy.

MgSiO3-rich perovskite is expected to be the most abundantmineral in both pyrolite and MORB materials under lower mantleconditions down to great depths (e.g., Kesson et al., 1994, 1998).However, the perovskite to post-perovskite phase transition re-cently discovered in MgSiO3 (e.g., Murakami et al., 2004;Tsuchiya et al., 2004; Oganov and Ono, 2004) is expected for

pressuretemperature (PT) conditions a few hundred kilometers

above the CMB. Subsequent high-pressure experimental studieshave shown that the post-perovskite phase transition also occursin pyrolite and MORB compositions for lowermost mantle con-ditions (Ono et al., 2005; Hirose et al., 2005; Murakami et al.,

2005; Ono and Oganov, 2005). However, the exact locations ofthe phase transition boundary and its width have not yet beendetermined for pyrolite and MORB. In addition, a phase transitionin SiO2 takes place in MORB at similar high PT conditions(Murakami et al., 2003; Hirose et al., 2005). If the effects of these

phase changes in pyrolite and MORB can be related to observeddeep mantle seismic velocity discontinuities, there may be seis-mological indicators of the presenceof any MORBaccumulationsnear the base of the mantle.

In this study, we examine the deep mantle phase relations inboth pyrolite and MORB compositions based on X-ray dif-fraction measurements in-situ at high PT conditions appro-

priate for the D

region. We also analyze new seismic data tofurther resolve the seismic velocity discontinuity structure in thelowermost mantle near the northern margin of the Pacific largelow shear velocity province (LLSVP) (Fig. 1a) (Avants et al.,2006; Lay et al., 2006). The LLSVPs under the Pacific andAfrica appear to be chemically distinct from the surroundingdeep mantle, and it has been proposed that they may involvedense debris from ancient oceanic slabs (Fig. 1b) (see Tackley,1998; McNamara and Zhong, 2004; Lay and Garnero, 2004;

Nakagawa and Tackley, 2005). The possible accumulation ofsubducted MORB crust in a pile in the D region is discussed,

based on joint consideration of our mineral physics and seis-mological findings.

2. Methods

2.1. High-pressure experiments

HighPTconditions were generated in a laser-heated diamond-anvil cell (LHDAC). Starting materials were prepared as amor-

phous gels with chemical compositions of natural KLB-1peridotite (Takahashi, 1986), which is similar in composition topyrolite, and normal MORB (Hirose et al., 1999) (Table 1). Thestarting materials, sample configurations, and high-pressure ex-

perimental techniques are the same as those used in our previous

studies (Hirose et al., 2005; Murakami et al., 2005). The sampleswere mixed with fine gold powder and loaded into a hole in arhenium gasket together with thermal insulation layers of NaCl or

Fig. 1. (a) S-wave velocity variations in thelowermost 200-kmof themantle in theseismic tomography model of Grand (2002). The large low shear velocity

provinces (LLSVP) regions appear to be dense and chemically distinct from sur-rounding mantle (Lay and Garnero, 2004). (b) Schematic cross-section through anLLSVP, suggesting that a mix of dense, separated MORB and pyrolitic mantlecomprises the pile. Phase transitions within the pile give rise to the reflectivity(Fig. 8) shown for the northern edge of the Pacific LLSVP (see small box in (a)

corresponds to Fig. 2c).

Table 1Compositions of starting materials

KLB-1 MORB

SiO2 44.80 49.80TiO2 0.16 1.65Al2O3 3.62 14.93FeO a 8.16 11.47MgO 39.50 8.54CaO 3.46 10.58

Na2O 0.30 2.91K2O 0.12Total 100.00 100.00

a Total Fe as FeO.

108 K. Ohta et al. / Earth and Planetary Science Letters 267 (2008) 107117


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pure samples unmixed with gold. They were compressed withbeveled diamond anvils with 200-m culet. Heating was done

using a multi-mode Nd:YAG laser or TEM01-mode Nd:YLFlaser using a double-sided heating technique that minimizes axialtemperature gradients within the sample (Shen et al., 1996).Temperature was measured by the spectroradiometric method(Watanuki et al., 2001). Angle-dispersive X-ray diffractionspectra of the samples were collected on a CCD detector or animaging plate at BL10XU of SPring-8. A monochromatic in-cident X-ray beam with a wavelength of about 0.41 was col-limated to 20-m in diameter. The diffraction data were obtainedin-situ at high PT during heating and after quenching tem-perature to 300 K. Two-dimensional X-ray diffraction imageswere integrated as a function of 2-theta angle in order to give aconventional one-dimensional diffraction profile using the fit-2D

program (Hammersley, 1996).Recently, there has been an extensive debate on the accuracy of

pressure scales for high-pressure experiments. The experimen-

tally-determined pressure estimates for the post-perovskite phasetransition in MgSiO3 differ by as much as 15 GPa, depending on

the internal pressure standard and its PVT equation of state(EOS) used in each study (see a review by Hirose, 2006). TheMgO pressure scale may currently be the most practical because ithas been extensively studied and is least controversial. Indeed, the

post-spinel phase transition boundary pressure in Mg2SiO4 basedon the PVT EOS of MgO proposed by Speziale et al. (2001)matches the associated 660-km seismic discontinuity pressure(Fei et al., 2004). Moreover, when Speziale's MgO pressure scaleis used, the MgSiO3 post-perovskite phase transition isdetermined to occur at 119 GPa for a temperature of 2500 K,which is consistent with thetypical observed depthof the D shearvelocity discontinuity commonly attributed to this phase change(Hirose et al., 2006). In the present study, however, MgO couldnot be used as a direct pressure indicator because it reacts readilywith the samples. Therefore, we measured the unit-cell volume ofAu mixed with the sample from (111) and (200) diffraction lines,

Table 2Experimental conditions and results

Run no. Pressure a Error1 b Error2 c Volume (Au) Error Temperature d Assemblage e

(GPa) (3) (K)

KLB-1

#1 108.4 1.3 2.1 51.907 0.106 1780 MgPv + Mw + CaPv110.3 0.9 2.5 51.929 0.073 1960 MgPv + Mw + CaPv113.5 1.5 3.4 52.263 0.121 2540 MgPv + Mw + CaPv

#2 111.1 1.0 2.2 51.702 0.083 1800 MgPv + Mw + CaPv + PPv(trace)112.4 1.1 2.5 51.747 0.088 1950 MgPv + Mw + CaPv + PPv(trace)

#3 116.3 0.7 2.5 51.426 0.055 1940 PPv + MgPv + Mw+ CaPv#4 97.6 0.3 2.9 53.433 0.025 2300 MgPv + Mw + CaPv#5 106.9 1.9 2.6 52.334 0.165 2070 MgPv + Mw + CaPv#6 122.1 0.2 3.5 51.561 0.014 2550 PPv + Mw+ CaPv#7 128.5 1.9 3.1 50.790 0.137 2250 PPv + Mw+ CaPv#8 131.9 2.8 3.4 50.724 0.202 2450 PPv + Mw+ CaPv

MORB

#9 109.5 0.5 2.7 52.142 0.045 2100 MgPv + CaCl2-type SiO2+CaPv+CF+PPv(trace)#10 112.0 0.6 2.6 51.882 0.049 2050 MgPv + CaCl2-type SiO2+CaPv+CF+PPv#11 114.1 1.2 2.9 51.883 0.097 2220 PPv + MgPv +-PbO2-type SiO2+CaPv+CF#12 117.5 0.7 3.2 51.779 0.059 2390 PPv + MgPv +-PbO2-type SiO2+CaPv+CF#13 88.4 0.0 2.4 53.968 0.002 1980 MgPv + CaCl2-type SiO2+CaPv+CF

103.8 1.2 2.6 52.598 0.105 2060 MgPv + CaCl2-type SiO2+CaPv+CF#14 117.4 1.3 3.0 51.630 0.100 2240 PPv +-PbO2-type SiO2+CaPv+CF#15 129.2 0.1 3.0 50.664 0.005 2170 PPv +-PbO2-type SiO2+CaPv+CF

132.0 1.2 3.2 50.564 0.085 2280 PPv +-PbO2-type SiO2+CaPv+CF

Runs #48 and #1315 were reported by Murakami et al. (2005) and Hirose et al. (2005), respectively.a Pressures were calculated from PVT EOS of Au (Hirose et al., submitted for publication) that is consistent with MgO pressure scale ( Speziale et al., 2001)

This new pressure scale is represented by a third-order BirchMurnaghan EOS:

P 3

2KT;0

VT;0

V 73

VT;0

V 53

" # 1 3

44 KV

VT;0

V 23

1" #( )where isothermal bulk modulus K300,0=167 GPa at 1 bar and 300 K, and its pressure derivative K'=5.58 and temperature dependence dK/dT=0.028 GPa/K. Thevolume at ambient condition V300,0=67.85

3, and those at high temperatures

VT;0 V300;0 exp

ZT300

aT;0dT; where aT;0 3:179 105 1:477 108 T:

b Pressure error derived from uncertainty in the unit-cell volume of Au.c Pressure difference corresponding to temperature variation in the sample.d Temperature variations are less than 10%.e MgPv, MgSiO3-rich perovskite; Mw, (Mg,Fe)O magnesiowstite; CaPv, CaSiO3 perovskite; PPv, post-perovskite; CF, Ca-ferrite-type Al-phase.

109K. Ohta et al. / Earth and Planetary Science Letters 267 (2008) 107117


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and estimated the pressure using a new PVT EOS of Au(Hirose et al., submitted for publication) that is consistent with theSpeziale's MgO pressure scale (Table 2). This new EOS of Auwas obtained by the simultaneous volume measurements of Au

and MgO at 0140 GPa and 3002330 K, using Speziale's MgOscale as a reference pressure scale. It predicts higher pressure than

previously reported EOS of Au at high temperatures above1500 K (e.g., Shim et al., 2002; Tsuchiya, 2003).

The errors in our pressure measurements using PVTEOSare derived from errors in volume and temperature, with theabsolute uncertainty due to the uncertainty in EOS itself. Theerrors in the unit-cell volume of Au based on X-ray diffractionmeasurements were relatively small, resulting in pressureuncertainty of typically about 1 GPa (Table 2). On the otherhand, temperature variations are generally large in laser-heatedsamples; up to 10% variations within a 20-m area from which

X-ray diffractions were collected (Hirose et al., 2006). Suchtemperature gradient causes more than 2 GPa pressure gradientdue to the effect of thermal pressure; high temperature increasingthe pressure and low temperature reducing the pressure (e.g.,Dewaele et al., 1998).

2.2. Seismological observations

If MORB is present in the deep mantle, it could either belocated in recently subducted material, localized in a thin layer ofa potentially very strongly contorted slab, or it could haveaccumulated as large quantities of dense dregs, separated overtime from ancient subducted slabs and swept by deep mantle

flow into piles away from current downwellings. While therehave been some seismological interpretations of thin lamellae ofanomalous material in the deep mantle that could correspond to aformer crustal layer (Weber, 1994; Thomas et al., 1998), it isvery difficult to seek such structures or to interpret them reliably.

We instead consider the possibility that large amounts of MORBmaterial has accumulated into dense piles in the lowermostmantle, such that volumetrically the material has distinctive ob-servable seismic properties; in particular, phase changes give

rise to seismic velocity contrasts that reflect elastic waves. Wefocus on the shear velocity structure near the northern margin ofthe Pacific LLSVP (Fig. 2), as this is one of the few regionswhereit is possible to determine the detailedvelocity structure ofa chemically distinct region of D.

We substantially augment previously analyzed seismic datasampling the D region beneath a localized domain under thecentral Pacific (Avants et al., 2006; Lay et al., 2006), to refine theregional shear velocity structure. Ourdata are from earthquakes inthe TongaFiji region recorded at broadband stations in western

North America (Fig. 2a). We focus on shear wave observations inthe distance range 7080, for which there is enough time sepa-

ration between the CMB reflection, ScS, and the direct waveturning in the mid-mantle, S(Fig. 2b), such that we can seek anyreflectivity within the lowermost 400-km of the LLSVP. Weconsider S waves rather than P waves due to the greater timeseparation of relevant phases and the observed tendency forS-wave reflections from the deep mantle to be stronger than

P wave reflections, as expected for computed post-perovskitephase transition reflectivity (Stackhouse et al.,2005; Wentzcovitchet al., 2006).

The detailed data processing is identical to that described byLay et al. (2006); we use transverse component ground dis-

placements corrected for receiver anisotropy and deconvolvedby the stacked ScS wavelet to remove source time function

complexity. The deconvolved traces from all events are used in adouble-array stacking procedure that determines the 1D reflec-tivity in the signals as a function of depth relative to the CMB.Identical analysis of corresponding synthetic seismograms forlocalized 1D velocity models is used to match the observed

Fig. 2. (a) Globe map showing the source locations (S) and receiver locations (R) for our seismic data, the 444 great-circle raypaths, and D velocities for thetomographic shear velocity model ofGrand (2002). The coremantle boundary (CMB) reflection points forScS phases are shown by black circles. (b) Cross section

along the great-circle arc marked by white dashes (a), showing representative raypaths of Sand ScSphases and the tomographic shear velocity variations in that great-circle plane. (c) Map showing the ScS reflection points (black circles) and the tomographic variations in the lowermost 200-km of the mantle. Note that the pathssample the northern margin of the LLSVP under the Pacific shown in Fig. 1. This region has high density, strong lateral margins, and 3 to 7% low shear velocities (Layand Garnero, 2004).



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stacks, perturbing the absolute velocities and any velocity dis-continuities in the structure. Our final data set involves 444 highquality SH signals sampling a very localized region of about500750 km in horizontal extent (Fig. 2c) near the northern

boundary of the Pacific LLSVP, corresponding to one of the 3subregions studied in (Lay et al., 2006).

3. Deep mantle phase relations

3.1. Pyrolitic mantle

Combining this study and that ofMurakami et al. (2005), eightseparate sets of experiments were conducted on pyrolitic mantle(KLB-1 peridotite) composition in the pressure range between 98and 132 GPa (Table 2). In order to avoid kinetic hindering of

phase transformation especially in such a multi-component sys-

tem, each run was conducted on amorphous starting material ata single PTcondition of interest, except that we changed temperature during heating in run #1. The diffraction peaks ofMgSiO3-rich perovskite (Mg-perovskite), (Mg,Fe)O magnesio-wstite, and CaSiO3-rich perovskite (Ca-perovskite) were ob-

served below 114 GPa and 2540 K (Fig. 3b). At higher pressures,the peaks from post-perovskite were observed together with thesethree phases (Fig. 3a). Mg-perovskite was not present above122 GPa at 2550 K. These results demonstrate that at a tem-

perature of 2500 K the post-perovskite phase transition occursbetween 116 and 122 GPa, corresponding to 2550 to 2640-kmdepth in the mantle (Fig. 4). This is similar to the transition

pressure in pure MgSiO3 (119 GPa at 2500 K for MgSiO3 (Hiroseet al., 2006)). The dP/dT slope of post-perovskite-in and Mg-

perovskite-out curves is not tightly constrained but is best esti-mated to be +8(4) MPa/K, consistent with a previous report(Ono and Oganov, 2005).

The post-perovskite transition occurs within a 5 GPa pres-sure range in pyrolitic material, corresponding to a lowermostmantle depth range of 90-km. Such a width is much narrowerthan for post-perovskite phase transitions in MgSiO3Al2O3

(Akber-Knutson et al., 2005) and MgSiO3

FeSiO3 (Mao et al.,2004; Tateno et al., 2007). This is similar to the majorite to perovskite phase transition; the two-phase coexisting field inMgSiO3Al2O3 (Irifune et al., 1996) is much wider than that innatural MORB composition (Hirose et al., 1999). However, a

Fig. 3. X-ray diffraction patterns for pyrolite at (a) 116 GPa, 1940 K and (b) 110 GPa, 1960 K, and for MORB obtained after quenching temperature from (c) 109 GPa,2100 K and (d) 117 GPa, 2240 K. (a)(c) are from this study and (d) is from Hirose et al. (2005). MP, MgSiO3-rich perovskite; PPv, post-perovskite; Mw,magnesiowstite; CaPv, CaSiO3 perovskite; SC, CaCl2-type SiO2 phase; SA, -PbO2-type SiO2 phase; CF, Ca-ferrite-type Al-phase; Au, gold; NaCl, pressuremedium.



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5 GPa pressure interval for post-perovskite phase transition inpyrolite is still larger than estimates of the sharpness of the D

seismic discontinuity. Seismological studies have inferred adepth extent of velocity increase up to 50 to 75-km (Revenaughand Jordan, 1991; Weber et al., 1996), or less than 30-km (Layand Young, 1989). This discrepancy with experimental resultsmay be due to the errors in pressure determinations in theexperiments. On the other hand, there has been extensive debateon a similar discrepancy for the 410-km seismic discontinuity.The effective width of the olivine to -spinel transition may beless than half of two-phase coexisting region (Stixrude, 1997),

possibly this is also true for the post-perovskite phase transition.Further seismological studies are needed to better constrain thesharpness of the D discontinuity.

The perovskite to post-perovskite phase transition in pureMgSiO3 is theoretically predicted to cause a 1.5% shear velocityincrease (e.g., Stackhouse et al., 2005; Wentzcovitch et al., 2006),although the incorporation of Fe and Al significantly reducesthe velocity contrast (Caracas and Cohen, 2005). Perovskite in a

pyrolitic mantle contains relatively little Fe and Al (Murakamiet al., 2005), and the phase transition to post-perovskite producesabout 1% S-wave velocity increase (Tsuchiya and Tsuchiya,2006).

3.2. MORB crust

We conducted seven separate sets of experiments on normalMORB composition at 88 to 132 GPa in this study and Hiroseet al. (2005) (Table 2). Similarly to the pyrolite experiments,heating was made at a single PTcondition for each run except

run #13. We observed a four-phase assemblage of Mg-perovskite,Ca-perovskite, Ca-ferrite-type Al-phase, and CaCl2-type SiO2

phase below 104 GPa and 2060 K. Detailed chemical analyses arediscussed in Hirose et al. (2005) and Sinmyo et al. (2006). The

most intense (022) line of post-perovskite was found as a veryweak peak at 110 GPa and 2100 K (Fig. 3c), indicating that the

post-perovskite-in curve should be very close to this PT con-dition. SiO2 phase also transformed from CaCl2-type to -PbO2-type structure between 112 and 114 GPa at 20502220 K(Fig. 3d). Mg-perovskite disappeared above 117 GPa at 2240 K.These results show that post-perovskite phase transition takes

place in MORB material between 112 and 118 GPa (2480 and2580-km depth) at 2500 K (Fig.5), a range shifted by about 4 GPalower pressure than that for a pyrolitic composition. This isa small but, we believe, sensible difference, even allowing forthe pressure and temperature uncertainties in our measurements

(Figs.4and5). This pressure shift is supported by theory (Caracasand Cohen, 2005; Stackhouse et al., 2006; Zhang and Oganov,2006). Perovskite and post-perovskite in MORB are enriched in

bothAlandFe(Hirose et al., 2005). Theoretical calculations havedemonstrated that small increase in the transition pressure by Al isnegligible compared to the large reduction by Fe. Indeed, thecalculations suggest much lower transition pressure in MORBthan the present experimental results, due to the effect of Fe.

The SiO2phase transition in MORB occurs at an indistinguish-able pressure from the post-perovskite transition. This is con-sistent with previous experimental results indicating that the post-

perovskite phase transition boundary in MgSiO3 end-membercomposition is very close to the CaCl2-type to-PbO2-type phase

transition boundary in pure SiO2 (Murakami et al., 2003, 2004).Theory predicts that both MORB phase transitions involve

decreases in shear velocity. The calculations have shown thatthe incorporation of Al and Fe remarkably reduces the velocity

Fig. 4. Phase relations in a pyrolitic lowermost mantle. Open and solid symbolsindicate the stabilities of Mg-perovskite and post-perovskite, respectively. Half-filled symbols show the coexistence of perovskite and post-perovskite.Magnesiowstite and Ca-perovskite were observed in all the experiments. Thehorizontal error bars indicate pressure uncertainty due to the error in unit-cellvolume of pressure standard. The oblique bars represent a pressure gradientcorresponding to a temperature gradient (10%) in the sample. The uncertaintiesin the locations of post-perovskite (PPv)-in and perovskite (Pv)-out curves areshown by gray. A general depth range of the D discontinuity (2600 to 2700-kmdepth) is indicated by a bar (Wysession et al., 1998).

Fig. 5. Phase relationsin a subductedMORB crustat lowermostmantle conditions.The stabilities of Mg-perovskiteand post-perovskite wereshown by openand solidsymbols, respectively. Half-filled symbols indicate the coexistence of perovskite

and post-perovskite. The boundaries are assumed to have the same pressure/temperature slope as that for pyrolite, and the gray indicates the uncertainties intheir locations. The dashed line indicates the phase transition boundary betweenCaCl2-type (circles) and -PbO2-type (squares) SiO2 phases.



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contrast between perovskite and post-perovskite (Caracas andCohen, 2005) and results in a negative S-wave discontinuity atthe phase transition in MORB (Tsuchiya and Tsuchiya, 2006).The (Al, Fe)-enriched perovskite in MORB contains 14 wt.%

Al2O3 and 20 wt.% FeO (Hirose et al., 2005), and its trans-formation to post-perovskite produces a 1.5% shear velocitydiscontinuity, assuming simple linear effects of Al and Fe.The S-wave velocity change for the SiO2 phase transition fromCaCl2-type to -PbO2-type structure has been also calculatedas 1 to 2% (Karki et al., 1997). Note that velocity contrast forthe SiO2 phase transition and the effects of Al and Fe impuritieson post-perovskite phase change have been calculated only atT=0 K. We therefore assume here that the magnitude of ve-locity jump is the same at high temperatures. Since both (Fe,Al)-enriched post-perovskite and SiO2 phase are importantconstituent minerals in MORB (our MORB composition in-

cludes 38% post-perovskite and 23% SiO2 phase at lowermostmantle conditions (Hirose et al., 2005)), a negative shear wavevelocity gradient and/or jump of about1% (0.6% by post-

perovskite phase change and another0.5% by SiO2 phasetransition) is expected around 2500 to 2550-km depth, for anysubducted MORB component present.

Given the experimental evidence for a possible depth shiftbetween MORB-related phase changes that cause a velocity de-crease and pyrolitic phase change that causes a velocity increase,we explore seismic data for any corresponding structures at aboutthe right pressure conditions.

4. Seismological analysis

Our seismic data display waveform complexities that indicatethe presence of complex structure in the deep mantle under thecentral Pacific. Representative examples of the deconvolvedwaveforms are shown in Figs.6and7. Fig.6 shows representativewaveforms from four events, aligned on the ScS arrivals. Thedominant periods for all of our data are from 3 to 5 s. There areseveral arrivals between the Sand ScSpeaks, the largest being a

positive amplitude arrival labeled B, which is preceded by anegative peak labeled A. Side-lobes are present on the major

peaks in the traces due to the source wavelet deconvolution, butthis doesnot account for the A or B features. The A and B arrivals

are observed to shift in time relative to ScS systematically withdistance, as shown in the two event profiles shown in Fig. 7,indicating that these arrivals are associated with deep mantlestructure, not source or receiver reverberations. Move-out relativeto S is weak over the narrow distance range spanned by ourlocalized sampling, since the reflected waves are not turning farapart, but there are clear shifts in timing relative to S from eventto event, as expected given variable source depths and distanceranges for each event. The phases have stable relative behavior,indicating that they are not scattered from out of plane, and tofirst-order at least, the data can be characterized by a 1D struc-ture. Clearly, however, the arrivals are small (the weak arrivalscannot be tracked in raw displacement data) and noise anddeconvolution artifacts are present in the signals, so a waveformstacking procedure is needed to quantify the stability andcharacteristics of the A and B arrivals, including their move-out

relative to Sand ScSfor events at different distances and sourcedepths.

Given the very localized sampling of our 444 signals, we

assume that the regional structure can be represented by a 1Dvelocity structure, justifying the double-array stacking methodthat is employed (Lay et al., 2006). Earlier studies of a moredistributed data set haveestablished that there are lateral variationsin the wave field over scale lengths of 1000-km in this region(Avants et al., 2006; Lay et al., 2006), but we restrict our specificdata set to a subregion with lateral scale of less than 750-km,allowing for the lateral averaging effects of the grazing raypaths inthe deep mantle. A single 1D stack is thus obtained, having about40% greater sampling in our localized area than in previous workdue to the inclusion of more recent earthquakes, giving a total of26 events. The large increase in data was achieved by using

several larger events than in earlier work (Lay et al., 2006), whichhave clear signal windows for detecting reflectivity associatedwith the A and B arrivals in double-array stacks.

The double-array stack that is obtained is shown in Fig. 8a,with the stack amplitude plotted as a function of vertical distancefrom the CMB. The reference velocity model used in the stackingis the same regional model used in Lay et al. (2006), and thevertical distances are apparent depths due to dependence onthis reference model. Note that the data stack has very tightuncertainty estimates, with strong peaks associated with ScSattheCMB, and clear positive reflection B and negative reflection A. Inthis case, the traces were aligned and normalized on ScS, so allamplitudes are relative to the stack amplitude at the CMB.Additional negative arrivals just above the CMB, labeled Cand D, are inferred from the analysis by Lay et al. (2006). Theadditional data from this study tend to obscure features very close

Fig. 6. Examples of SH ground displacement waveforms for different stationsand events, indicating systematic negative (A) and positive (B) peaks between Sand ScSarrivals. These signalshave beendeconvolved by the averageScS sourcewavelet foreachevent (Avantset al.,2006), andlowpassfiltered toretain periodslonger than 3 s. Variations in the time differences between S and ScS, and

between ScS and A and B are consequences of variable source depth (d) andepicentral distance ().



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(Fig. 8b). This model predicts the data well whether ScSorSareused as reference phases, and provides good fit to the ScSS

differential times (Supplemental Fig. S1). The velocity changescould be distributed over transition zones extending as much as30-km in depth, rather than being the sharp discontinuities foundin model SPAC3, and still produce synthetics that match the datawell. This is an intrinsic limitation of using wide-angle reflectionswith 35 s periods. The actual velocity jumps may beunderestimated somewhat due to modeling of extensively stackedsignals and the assumption of a 1D structure. The depth interval

between A and B is estimated to have about 20-km uncertainty,from 135 to 155-km based on the fitting of stacks aligned on Sversus ScS, and assuming simple intervening velocity structure.The predicted reflections are small (see Supplemental Fig. S2),

accounting for the difficulty of readily seeing them in individualtraces, and therefore stacking of many signals is required to haveconfident detection of the weak phases.

5. Implications for a MORB-rich pile above the CMB

Thermally-equilibrated subducted MORB crust is denser thanthe average mantle at all depths except between 660 and 720-km(Irifune andRingwood, 1993; Hirose et al., 1999; Ono et al., 2005;Hirose et al., 2005). The density contrast is about 3% at the base ofthe mantle (Hirose et al., 2005), so if subducted slabs penetrate tothe D region, MORB material could be stable there. Geodyna-mical simulations have demonstrated that this magnitude ofdensity contrast can induce separation and accumulation of denseMORB crust at the base of the mantle (Christensen and Hofmann,1994; Tackley, 1998; McNamara and Zhong, 2004; Nakagawa

and Tackley, 2005). It has been further suggested that denseMORB-enriched materials are swept into piles concentrated

underneath lower mantle upwellings (Christensen and Hofmann,1994; Tackley, 1998; McNamara and Zhong, 2004; Nakagawaand Tackley, 2005). MORB material may therefore contribute toseismic heterogeneities in the D region, either within recentlysubducted slab material or in piles of separated MORB fromancient subduction, which possibly represents the LLSVP.

Our experimental results indicate that Mg-perovskite begins totransform to post-perovskite in MORB about 70-km shallowerthan the post-perovskite phase transition in pyrolite at the sametemperature (Figs. 4 and 5). In addition, SiO2 phase in MORBundergoes a phase transformation from CaCl2-type to -PbO2-type structureat the same depth as thepost-perovskite transition in

MORB.If MORB has accumulated into substantial piles of mixed, but

not homogenized MORB and pyrolitic material, within whichphase boundaries could exist laterally over large extent, seis-mology may be able to detect the small velocity effects of the

phase changes in MORB. The LLSVP in the lowermost mantleunder the southern Pacific may involve such a large accumulationof dense MORB-enriched materials (Christensen and Hofmann,1994; Tackley, 1998; McNamara and Zhong, 2004; Nakagawaand Tackley, 2005). If this is the case, post-perovskite and SiO2

phase transitions in the MORB component of the mixed chemicalpile could account for discontinuity A, with post-perovskite tran-sition in the pyrolitic component accounting for discontinuity B.

Comparing the magnitudes of observed velocity jumps(0.5% at discontinuity A and +0.6% at discontinuity B) withthose predicted from mineral physics (1% for MORBand+ 1%

Fig. 8. Seismogram stack and velocity profile for the northern margin of the Pacific LLSVP. (a) Double-array stack of 444 transverse component (SH) recordings ofTongaFiji earthquakes made at broadband stations in California (see example waveforms in Figs. 6 and 7). The traces were aligned and normalized on the coremantle boundary (CMB) reflected phase ScS, and stacked to determine reflectivity of energy as a function of depth relative to the CMB (solid black line). Bootstrapuncertainties on the stack are shown by dashed lines (Avants et al., 2006). The stacked data for depths more than 450-km above the core are contaminated by coda fromdirectSarrivals that turn at shallower depths. The blue dashed curve shows the results of stacking of similarly processed synthetics for model PREM (Dziewonski andAnderson, 1981). The red curve shows the results of stacking synthetics for model SPAC3. (b) The velocity profiles for PREM and SPAC3. SPAC3 had four velocitydiscontinuities (A, B, C, D) that produce reflectivity associated with the corresponding labeled peaks on the left. For the interpretation of this region as a MORB-

pyrolite mixed pile, discontinuity A results from post-perovskite and SiO2 phase transitions in the MORB component, discontinuity B from post-perovskite phasetransition in the pyrolitic component, discontinuity C from back transformation of the post-perovskite to perovskite in the pyrolitic material due to rapid temperatureincrease (Lay et al., 2006), and discontinuity D from onset of partial melting just above the CMB.



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for pyrolite), a 50/50 mix of MORB and pyrolite can be re-conciled with the seismic data, generally accounting for thedepth and strength of the A and B discontinuities, althoughcollective uncertainties in all of the measurements provide only

loose bounds on the bulk composition. The chemical reaction between the MORB and pyrolite should be negligible unlesspartial melting is involved because the solid-state diffusion isvery slow; only 1 m in 109 to 1010 yr in perovskite lithology(Holzapfel et al., 2005), so the mixed material can be very old.

The depths and signs of the seismic discontinuities in modelSPAC3 are generally consistent with the MORB-enriched pilescenario. The 2520-km depth for discontinuity A matches the

pressure of post-perovskite and SiO2 phase transitions in MORBat about 2500 K (Fig. 5). The temperature at discontinuity Bwould have to be about 400500 K higher than that at dis-continuity A to reconcile the depth of discontinuity B with post-

perovskite phase transition in pyrolite (Fig. 4), although this valuestrongly depends on the dP/dT slope of the phase transitionboundary and may be an overestimate. This temperature increasecould be attributed to a strong temperature gradient withina densechemical pile as predicted in geodynamic models (Nakagawa andTackley, 2005).

Interpreting deep mantle seismic structure involves significantnon-uniqueness, so other possible interpretations can be ad-vanced. Lay et al. (2006) interpreted discontinuity A as the effectof a change in composition at the top of the LLSVP, possiblyinvolving iron enrichment that causes an abrupt shear velocitydecrease, with discontinuity B being a post-perovskite transfor-mation. They further suggest that discontinuity C is the result of

post-perovskite converting back to perovskite in a steep thermalboundary layer, with discontinuity D being the upper boundary ofan ultra-low velocity zone, perhaps involving partial melt. Thekey difference relative to the current paper involves the expla-nation for discontinuity A. Given the new mineral physics results,the MORB-pile interpretation appears at least equally viable, andin terms of accounting for a large-scale heterogeneous composi-tion, the MORB-pile notion is much more straightforward thanaccounting for a large iron-enriched domain.

Comparable detailed seismic analysis of reflectivity in otherregions of thePacific andAfrican LLSVPs is needed (see a reviewof recentresults in Lay and Garnero, 2007), but the seismic model

for the region under the central Pacific in Fig. 8 is generallyconsistent with expected phase transitions in a chemically distinct

pile of mixed MORB and pyrolitic material. This inference isclearly right at the frontier limits of both mineral physics ex-

periments and seismological imaging of the deep interior, but thelarge data sets used forboth arenasat least establish the viability ofthis interpretation. If further mapping of the structure sustains theinterpretation in terms of phase transitions in a MORB-enriched

pile, geophysicists may finally have a clear indication of the fateof large quantities of subducted oceanic crust.

Acknowledgments

We thank M. Murakami for technical advice, and T. Nakagawaand J. Hernlund, along with three anonymous reviewers and theEditor for thoughtful comments. The synchrotron X-ray ex-

periments were conducted at SPring-8 (proposal no. 2005B6892-PUl-np, 2005B0010-LD2-np, and 2006A0099). Seismic data wereobtained from the IRIS, Berkeley, and Caltech/USGS TRInet datacenters. This work was supported by grants from the Japan Society

for the Promotion of Science and the U.S. National ScienceFoundation (EAR-0125595, EAR-0453884, and EAR-0635570).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, atdoi:10.1016/j.epsl.2007.11.037.

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