Seismic detection of sublithospheric plume head residuebeneath the Pitcairn hot-spot chain

Sebastian Rost �, Quentin WilliamsDepartment of Earth Sciences, Center for the Study of Imaging and Dynamics of the Earth, University of California Santa Cruz,

1156 High Street, Santa Cruz, CA 95064, USA

Received 15 September 2002; received in revised form 29 January 2003; accepted 5 February 2003

Abstract

We study array seismograms of French nuclear explosions from two islands in French Polynesia and use these toconstrain the structure in the upper mantle beneath the islands. Seismograms of nuclear explosions on the hot-spot-related Fangataufa atoll show discrete, large-amplitude P-coda phases which are not observed in recordings ofexplosions on Mururoa, an atoll V40 km to the north. The source for these P-coda phases is located beneath theFangataufa atoll, indicating that structural heterogeneities are present in the oceanic upper mantle in this region onvery small scale lengths. Synthetic seismograms for models of the upper mantle beneath Fangataufa require a layerwith P-wave velocity elevated by V10% between depths of 51 and 85 km with a sharp termination in the north,possibly at the Austral Fracture Zone, to match these P-coda phases. Few mineralogic scenarios exist that can explainthis structure, and the properties of this layer imply that extensive enrichment in garnet occurs in this depth range.The garnet-enriched layer is likely of similar origin to the well-known xenolithic ‘garnet megacryst’ suite found inkimberlitic regions. We propose a model for the formation of the Fangataufa and Mururoa atolls involving garnet-enriched zones being generated at depth through magmatic processes at the Pitcairn plume head. Thus, the initiationof hot-spots could produce complex geochemical and structural heterogeneities at depth in the suboceanic mantle.4 2003 Elsevier Science B.V. All rights reserved.

Keywords: oceanic lithosphere; hot-spots; mantle plumes; high-velocity zones; seismology

1. Introduction

From 1966 to 1996, France tested nuclearbombs on two islands in French Polynesia. TheMururoa (MUR) and Fangataufa (FAN) atollsare located in the vicinity of the South Paci¢c

superswell [1] and are part of the Pitcairn volcanicchain, a young (V10 Ma) volcanic island chainproduced by the Pitcairn hot-spot [2] now located90^100 km southeast of Pitcairn island [3] andV1000 km southeast from MUR and FAN(Fig. 1). The ages of MUR and FAN are closeto the maximum proposed age of the Pitcairn hot-spot, so they likely were generated at the initiationof this hot-spot chain [2]. MUR formed10.42 A 0.1 to 11.06A 0.11 Ma ago [2,4,5] and islocated on the Austral Fracture zone (AFZ)(Fig. 1), which divides V41 Ma old oceanic lith-

0012-821X / 03 / $ ^ see front matter 4 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0012-821X(03)00075-X

* Correponding author. Tel.: +1-831-459-1437;Fax: +1-831-459-3074.E-mail addresses: srost@es.ucsc.edu (S. Rost),

qwilliams@earthsci.ucsc.edu (Q. Williams).

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osphere to the north from V34 Ma old litho-sphere to the south [6]. Unlike most other oceanicislands MUR is elongated along a N80‡ azimuth,probably due to channeling of volcanism alongthe AFZ [7,8]. FAN was formed 10.12 A 0.10 to12.44A 0.24 Ma ago [2,9] and is located V40 kmsouth of MUR and AFZ on V34 Ma old litho-sphere, and shows a roughly hexagonal form ap-proximately half the size of MUR. Both atolls arelocated on a small oceanic swell V200 km by 150km striking S80‡W that is approximately 1000 mshallower than the surrounding ocean £oor [10].French nuclear devices were detonated on bothislands in 0.5^1.1 km deep bore-holes in the ba-saltic bodies of the volcanos beneath the coral rimand lagoon of both islands, with MUR as themain test site [11]. The exact locations of the testswere not published by the French Commissariat a'l’Energie Atomique, but the atolls are quite small,so that an assignment of the explosions to theislands can be uniquely made (Table 1). There-fore, these explosions provide an unusual oppor-tunity to interrogate the deep structure of oceanicatolls.

Although both islands appear to lie on thesame hot-spot track, their lavas and associatedxenoliths are geochemically and petrologically dis-

tinct [12]. Both islands have been sampled exten-sively in bore-holes up to 1100 m deep [9,12^14].MUR’s basalts are highly alkaline, and hydrousphases, such as micas and amphiboles, are presentin xenoliths [7,13]. In contrast, FAN’s basalts arequite conventional for oceanic hot-spot magma-tism: initially tholeiitic basalts, followed by a ve-neer of alkali basalts. The anomalous petrology ofMUR rocks has led the association of this islandwith the Pitcairn hot-spot to be questioned, and agenetic connection with the AFZ has instead beenproposed [13,14].

The ¢rst tens of seconds of the seismograms ofexplosions from the two islands are very di¡erent,with FAN events showing two dominant P-codaphases. We can model the P-coda phases by in-troducing a layer with a high (vvpV+10%) P-ve-locity beneath FAN that is absent beneath MUR.Other sources for the coda phases from FAN,such as the impact of the overburden after itsballistic £ight or a reverberation in the water col-umn, can be ruled out due to both travel time andamplitude discrepancies.

Our mineralogic modeling shows that a highpercentage of garnet is the only possible meansof generating such a large velocity jump in theoceanic upper mantle. We develop a model of

Table 1French nuclear explosions at MUR and FAN

Event # Origin Lat. Lon. mb

[‡] [‡]

1a 1996 January 27 21:29:57.8 322.240 3138.820 5.32 1995 December 27 21:29:58.0 321.880 3138.970 5.13 1995 November 21 21:29:58.1 321.990 3139.030 4.84 1995 October 27 21:59:58.2 321.890 3138.980 5.45a 1995 October 01 23:29:58.0 322.250 3138.740 5.46 1995 September 05 21:29:58.4 321.850 3138.840 4.87a 1988 November 30 17:55:00.0 322.233 3138.740 5.58 1987 November 19 16:31:00.2 321.845 3138.941 5.79 1986 May 30 17:25:00.1 321.862 3138.948 5.510 1980 July 19 23:47:00.0 321.861 3138.934 5.711a 1990 November 14 18:11:58.0 322.258 3138.805 5.512a 1990 June 26 17:59:58.2 322.215 3138.841 5.513a 1989 November 27 17:00:00.0 322.250 3138.722 5.514a 1991 May 29 18:59:58.2 322.256 3138.794 5.5

Origin time and location of explosions are taken from the Preliminary International Data Center (PIDC) Database of the Com-prehensive Nuclear Test-Ban Treaty. mb, body wave magnitude.a Fangataufa events.

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the hot-spot-related volcanism at these two is-lands that explains the geochemical, mineral-phys-ical and seismological data.

These observations demonstrate that the ocean-ic upper mantle can be heterogeneous on rela-tively small scale lengths, and that hot-spots areable to produce geochemical heterogeneities that,when entrained into Earth’s convective mantle,

could produce geochemical signatures that resem-ble subduction-generated heterogeneities.

2. Seismological data

We study recordings of the small-aperture,short-period seismological arrays in Yellowknife

Fig. 1. (a) Source^receiver combinations. The stations used are marked by triangles (BDFB: Brasilia, Brasil ; LPAZ: La Paz, Bo-livia; CPUP: Villa Florida, Paraguay; PLCA: Paso Flores, Argentina; TXAR: Texas Array, USA). Mainly data of the small-aperture, short-period, vertical arrays of Yellowknife (YKA, Canada) and Warramunga (WRA, Australia) are used. The insertshows the form and location of MUR and FAN. Both islands are atolls with a coral rim and lagoon. The Austral Fracture zone(AFZ) strikes N80‡E. MUR and FAN are V40 km apart. (b) Bathymetric map of the surroundings of MUR/FAN in FrenchPolynesia. The 500 m isolines are given as solid lines. The dashed lines give lithospheric ages [6]. The Austral and MarquesasFracture zones are clearly visible in lithospheric age. Presumed hot-spots of the region are marked by gray circles (SOC: Society;MAC: MacDonald; MAR: Marquesas; PIT: Pitcairn). MUR and FAN are located in the center of the ¢gure.

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and Warramunga (YKA and WRA, respectively),located in northern Canada and Australia (Fig.1). Epicentral distances from MUR and FAN tothe two arrays are similar with vYKA = 86.4‡ andvWRA =79.8‡. Both arrays have apertures of V20km and 18 and 20 vertical, short-period stations,respectively. WRA is built in an L-shaped con¢g-uration, whereas YKA is cross-shaped. Both ar-rays were built to monitor nuclear explosionsglobally and have operated since the early 1960s.They are especially sensitive to high-frequencyP-waves from teleseismic distances and are partof the International Monitoring System to deter-mine compliance with the Comprehensive NuclearTest Ban Treaty.

Fig. 2 shows recordings from YKA for explo-sions at MUR and FAN. For event location weuse the database of the Prototype InternationalData Center (PIDC). The di¡erences of the seis-

mograms recorded at the same stations from thetwo closely spaced atolls are striking. The YKArecordings from explosions on MUR show a verysimple P-coda, whereas two large-amplitudeP-coda phases at V16 and V24 s are visible forFAN explosions. The events are recorded at thesame stations and the di¡erence in arrival angle isnegligible. Therefore, a receiver side structure asthe source for these phases can be completelyruled out. The paths of P from MURCYKAand FANCYKA are very similar. The P-waveturns V300 km above the CMB, so that anystructural di¡erence along the path in the deepmantle is very unlikely and highly heterogeneousregions of the mantle like DQ are not sampled bythe P-waves. The source for the large coda phasesis thus most likely located in the direct vicinity ofthe atolls. Due to the explosive origin of the seis-mic energy, the source mechanism is simple and

Fig. 2. YKA recordings of explosions in MUR and FAN (events 2, 3, 1, 5) of the 1995/1996 explosion series. Shown are band-pass-¢ltered seismograms (fourth-order bandpass with cut-o¡ frequencies of 0.5 and 1.4 Hz). All seismograms are normalized tothe ¢rst arrival. The two large onsets (marked by arrows) in the P-coda at V16 and V24 s are clearly visible for the FAN ex-plosions. In contrast, the coda of MUR events is very simple. The polarity of the 16 s phase is opposite to the P-onset, whereasthe 24 s phase has the same polarity.

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any systematic di¡erence of source mechanismcan be ruled out. The almost circular form andsmall size of FAN rules out the possibility of anenergetic reverberation within the volcanic edi¢ce.Moreover, surface-wave to P-wave conversions inthe atoll would arrive earlier in the seismogram[8], as would the impact of the overburden abovethe shot-point after its ballistic £ight. Syntheticseismograms show that phases like PmP (Mohore£ection) and PmwP (Moho re£ection and anadditional leg in the water column) would arriveearlier in the coda. PmwP arrives V11.3 s after Pfor a 15 km deep Moho and a water depth of 4 kmand with much smaller amplitude than observedin the data. Shallower Moho depths would in-crease the time discrepancy. We also expect thatthe ballistic £ight time would be the same forexplosions on both islands, since no di¡erence in

hypocentral depth is observed. Reverberations inthe water column should also be identical betweenthe two islands, since water depth around bothatolls is very similar. We constrained the originof the additional phases using frequency^wave-number analysis [15], and no azimuthal deviationfrom the great-circle path was resolvable. Bothphases show P-wave slownesses, indicating thatthe coda phases originate close to the atolls.

We calculated synthetic seismograms using there£ectivity method [16] for a source^receiver com-bination analogous to FANCYKA and an arrayin YKA con¢guration. Fig. 3 shows a comparisonbetween data (traces 1^6) and some synthetic seis-mograms (traces 7^15). The synthetic seismo-grams show that the FAN explosions are bestmodeled by models including a high-velocity

C

Fig. 3. (a) Data and synthetics beam traces for Fangataufaevents recorded at YKA. Traces 1^6 are array beam tracesfor events 5, 11, 12, 1, 13, 14, respectively. All these eventsare FAN explosions. Traces 7^15 are synthetic beam tracescalculated using the re£ectivity method [16] for theYKACFAN source^receiver combination. The array used inthe calculation of the synthetic seismograms has the samecon¢guration as YKA. We use an altered IASP91 modelthat lacks the strong Moho discontinuity, and therefore lacksthe strong arrival from this discontinuity. The synthetic seis-mograms calculated using the original IASP91 and the modi-¢ed IAPS91 models are shown as traces 13 and 14, respec-tively. The asterisk in trace 13 marks the strong arrival fromthe IASP91 Moho, an arrival we do not observe in the dataand which is lacking in the modi¢ed model. The cross marksthe arrival of the modeled Moho phase in our modi¢ed mod-el. The data are best modeled by models containing a high-velocity layer (HVZ) at depth. Trace 7 shows the resultingseismogram for a HVZ between 51 and 89 km with a +20%and 310% impedance contrast, respectively. The model fortrace 8 shows a +20% impedance change at 50 km and a315% change at 85 km and the model for trace 9, A 20%changes at 45 and 89 km. Traces 9^12 show seismograms forHVZ between 51 and 89 km with +10% and 310% impe-dance changes (trace 10); +15% and 310% (trace 11) and+20% and 310%. We do not have control on the velocitystructure in the HVZ, since the internal velocity structurehas a minor e¡ect on waveforms and travel times. Trace 15shows a calculation for a 10% low-velocity layer between 50and 88 km. This seismogram lacks the prominent onset at16 s. (b) Best ¢tting S- and P-wave velocities and densitymodel for HVZ.

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zone (HVZ) beneath the atoll. The observed codaarrivals at 16 and 24 s are identi¢ed as reverber-ations between the upper boundary or lowerboundary of the HVZ and the surface (Fig. 5),i.e. P51pP and P85pP, for the best-¢tting model.The one-dimensional reference Earth modelIASP91 [17] (trace 13 in Fig. 3) is not able toproduce the large coda phases, but predicts alarge arrival V10 s after P, which is not observedin the data. This predicted phase is due to thestrong P-wave discontinuity at 35 km depth inIASP91. For the modeling, we use a modi¢edIASP91 model, which lacks this boundary buthas a 12% velocity jump at 20 km and the HVZat greater depths. The precise depth of this simu-lated Moho only a¡ects the calculated waveformsnear the direct P arrivals and has no resolvablee¡ect at time greater than V8 s. The syntheticwaveforms of the altered velocity model are verysimilar to the data, although IASP91 is a mainlycontinental model. If additional phases observablein the FAN explosions are due to the standard

discontinuities present in oceanic lithosphere, wewould expect similar phases with larger di¡eren-tial time to P also in the MUR explosions, whichare not observed. The larger di¡erential traveltime would come from a thickening of the litho-sphere due to the advanced cooling of the olderoceanic lithosphere sampled by the MUR explo-sions. The model without a HVZ is unable toproduce the prominent coda arrivals at 16 and24 s (trace 14). Traces 7 and 8 extremely accu-rately reproduce the observed 16 and 24 s arrivals.These models incorporate a layer with an approx-imately 10% elevation in P-wave velocity and den-sity initiating around 50 km depth, and taperingto a 10^15% impedance contrast between 85 and90 km depth. The depth of the lower discontinuityis less well-determined than that of the upper dis-continuity, because the travel times of the 24 sphase vary by V2 s between di¡erent explosions.Additionally, the amplitudes of the 24 s phasevary between the di¡erent explosions. This couldbe the result of either a fuzzy lower boundary of

Fig. 4. (a) WRA beam traces of events at FAN (event 7) and MUR (events 8^10). All events show the 16 s phase (dashed line)and some evidence for a 24 s phase. All events are normalized to the P amplitude. The FAN event shows only very smallP-coda amplitudes, which might be related to the small P amplitudes for MUR events recorded at WRA [19]. (b^d) Recordingsfor three di¡erent events at FAN (b,d) and MUR (c). The 16 s phase is marked by the dashed lines. Some recordings show aphase with the correct travel time and amplitude, but the single-station data quality is in general worse than for the array. Forarrays (WRA, ASAR, TXAR) array beam traces are shown. ASAR is the Alice Springs array in central Australia.

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the HVZ, or small-scale topography on the lowerboundary.

Synthetic tests show that the upper boundary ofthe HVZ must be very sharp, with the velocitychange occurring over less than 2 km, and theupper boundary seems to be locally £at, as indi-cated by the consistent travel times and ampli-tudes of the 16 s phase. The 24 s phase (P85pP)shows more time variation and lower amplitudes,making both the depth and amplitude of the im-pedance jump at the lower boundary less preciselydetermined.

The uppermost trace of Fig. 3 shows the resultfor a low-velocity zone between 50 and 88 km.Such reduced velocity models are clearly notable to produce the large 16 s phase. Low-velocityzones are common features in the uppermostmantle [18] at depths of 60^200 km and arethought to be produced by the transition fromthe rheologically de¢ned lithosphere to the asthe-nosphere. The feature observed beneath FAN isclearly not in agreement with a low-velocity zone.Our models are only slightly sensitive to the ve-locity gradient within the HVZ and all modelsshown here have no velocity and density increaseor decrease in the HVZ. A normal velocity gra-dient within the HVZ would alter the inferreddepth of the lower boundary slightly. Fig. 4 showsadditional data from stations in Australia, Southand North America that are used to determine thelateral extent of the high-velocity body that gen-erates the additional P-coda phases. Fig. 4a showsWRA beam traces of explosions on FAN (toptrace, event #7) and MUR (events #8 to #10).These recordings show evidence for the 16 and24 s phase both in MUR and FAN explosions.The identi¢cation of the coda phases in WRAdata is di⁄cult, because the incoherent coda am-plitude for WRA recordings from explosions atMUR and FAN relative to the coherent P-arrivalsis about four times higher than in YKA record-ings [19,20]. The high relative coda amplitudes aredue to an amplitude reduction of the ¢rst arrivalat WRA rather than an increase in the coda am-plitude [19]. Therefore, the WRA amplitude pat-tern produces more complex seismograms. None-theless, since Fig. 4a shows array beam traces, thecoda phases stack coherently across the array and

the 16 s phase can be easily identi¢ed, provingthat the high-velocity body extends to the westof FAN and MUR, making a termination of thelayer at the AFZ likely. Fig. 4b,d shows record-ings from stations in North and South Americaand Australia of explosions on FAN, while Fig.4c shows recordings of an explosion on MUR.

Fig. 5. (a) Sketch of ray paths for phases P, P51pP andP85pP. The phases P51pP and P85pP are re£ections from theupper and lower boundary of the high-velocity body detectedbeneath FAN. They arrive 16 and 24 s after the main P ar-rival at YKA. (b) Location of the re£ection points of P51pP(circles) and P85pP (squares) towards YKA and WRA forthe nuclear explosions (stars). The solid lines give the litho-spheric age in Myr [6]. The path from MUR to YKA sam-ples older lithosphere north of AFZ, whereas MURCWRA,FANCWRA and FANCYKA sample young lithosphere orthe fracture zone.

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The South American stations (BDFB, LPAZ,PLCA) are single stations, which prevents arrayprocessing for these stations. Therefore, the iden-ti¢cation of the 16 and 24 s phases is more di⁄-cult. Phases arriving in this time window might bereverberations or conversions from discontinuitiesbeneath the stations. The stations ASAR (Austra-lia) and TXAR (USA) are mini-arrays that enablearray processing, but have very similar azimuthsas the paths from MUR/FAN to WRA andYKA, and therefore do not yield much additionalinformation. In spite of the low quality of thedata from the South American stations we ¢ndevidence for the 16 s phase in these recordings.Many of these stations seem to have a coherentonset with the appropriate travel time. Therefore,we speculate that the high-velocity body also ex-tends to the east of the two atolls, and thus alateral extent of V50 by V50 km is likely.

The HVZ detected below FAN disappears tothe north on a scale length of 9 30 km, sincethe P-coda phases P51pP and P85pP cannot bedetected beneath MUR on paths to YKA. It islikely that the northern boundary of the HVZ isthe AFZ: The detection of the coda phases fromMUR and FAN to WRA and to some of theSouth American stations indicates an extensionof the HVZ to the east and the west, approxi-mately paralleling the fracture zone, while thepath from MUR to YKA de¢nitely samples nor-mal oceanic mantle. Any extension of the HVZ tothe south cannot be determined since no record-ings from Antarctica of the explosions exist. Wesearched the ISC catalog for identi¢cations of ad-ditional phases in the P-coda for these explosions,with no results in the relevant P-wave distancenecessary to determine the extent of this velocityanomaly.

Regional and global gravity data [21,22] andgeoid [22] for the MUR^FAN region data wereexamined for any anomaly in the region, withoutsuccess. The high-velocity body might be isostati-cally compensated, or small enough relative to itsdepth extent that it shows no resolvable signal inthe gravity and geoid ¢eld. The presence of aheat-£ow anomaly could also be indicative of avolcanism-related source of the HVZ. Unfortu-nately, no heat-£ow measurements in close prox-

imity to MUR and FAN exist that might indicatethe presence or absence of a heat-£ow anomaly[23,24]. Indeed, the young age of the lithosphereand low sediment load render such measurementsdi⁄cult in this region.

3. Mineralogic modeling

The mineralogic options for generating thehigh-velocity layer required by the seismic dataare restrictive, and thus provide key constraintson the make-up and genesis of this layer. Amongplausible rock assemblages, eclogites with mid-ocean ridge basalt chemistry have been calculatedto have P-wave velocities elevated by only V2%(with no change in shear velocity) relative to bothpyrolite and harzburgite in the 90 km depth range[25]. Eclogites with chemistries that generate high-er garnet contents and corresponding loweramounts of pyroxene have progressively largerP-wave velocity elevations [26]. Yet, even compa-ratively garnet-rich eclogite compositions do notapproach the V10% P-velocity contrast requiredto produce the observed postcursors. Indeed,among the major upper mantle minerals, olivine,ortho- and clinopyroxene and garnet, only a dra-matic increase in the modal concentration of gar-net, signi¢cantly in excess of that present withineclogites, can generate changes in P-wave velocityof near 10%. This is a simple consequence of therelative incompressibility of garnets relative toother upper mantle phases. For mantle garnets,typical values of the ambient temperature bulkmodulus are 170 ( A 3) GPa, while those of oliv-ines are V130 GPa and pyroxenes lie in the 104^116 GPa range [27]. For comparison, the respec-tive shear moduli of the three phases are 92 ( A 3),82 and 74^78 GPa, with pyropic garnet beingabout 10% denser than olivine and pyroxene. Ac-cordingly, a 10% increase in P-wave velocity im-plies a dramatic increase in the garnet concentra-tion (and, relative to eclogites, virtual eliminationof the lower-velocity pyroxene concentration).

We model the 10% compressional velocity con-trast that we observe at the top of this layer usingelastic parameters from Du¡y and Anderson [27],evaluated at a depth of 50 km and for an adiabat

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initiating at 1400‡C. We assume that the garnethas an Mg/Mg+Fe number of 0.8, with a grossu-lar component of 10 mol% (our results are onlyslightly a¡ected by the Ca content of the garnet),and that olivine and pyroxene each have Mg/Mg+Fe numbers of 0.9. A transition between ma-terial of approximately harzburgitic chemistry

(90% olivine and 10% orthopyroxene) and a layerof between 96 and 100% garnet (with the balancebeing either olivine or pyroxene) generates a 10%compressional velocity contrast. We model onlythe velocity contrast at the top of the garnet-rich body, since the re£ections from the top arenot sensitive to the absolute value of the seismic

Fig. 6. (a) Topographical map [22] of the MUR/FAN region. The dashed line shows the vertical pro¢le sketched in panel b. Thechanneling of material along AFZ (dot-dashed line) is clearly visible. (b) Sketch of formation of megacryst suite beneath FAN.Plume material is deposited at V50 km depth at a mechanical boundary (MBL). The material is either a pulse of material risingalong the plume conduit or, more likely, the plume head connected with the start of the Pitcairn volcanic chain. The hot, likelyvolatile-rich material is trapped at this mechanical boundary. The boundary could be associated with the spinel-to-garnet transi-tion [45], the lower boundary of the lithosphere or a change in viscosity and water content of the mantle material [46]. The initialpulse of plume material is tapped along the weak zone of the AFZ, and generates the volatile-rich minerals and the topographyalong AFZ associated with MUR (panel a). The trapped, partially molten material undergoes in situ fractional crystallization,building a garnet-rich megacryst layer at depth, while FAN is built up from less volatile-rich melts originating from nearer thecenter of the plume head.

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velocity but rather provide a robust constraint onthe velocity contrast. The amount of garnet canbe mildly reduced if the amount of pyroxene inthe overlying material is increased: nevertheless,our results indicate that the transition we observeis entirely compatible with a transition from de-pleted mantle to a lens of nearly pure garnet.Such a pure garnet layer could likely only be gen-erated through fractional crystallization in a mag-matic system. In this context, it is notable thatslowly ascending picritic magmas, or those ar-rested at depth, will produce garnet as a majorphase during fractional crystallization [28]. Thisobservation led to the proposal that many ‘blindconduits may exist at great depths that are ¢lledwith alkalic picrites or their fractionation prod-ucts’ [28]. The feature that we document to existbeneath FAN can be explained as a large sill ofgarnet, generated by fractional crystallization oflarge quantities of (likely ponded) silicate magmathat were present during the genesis of the Pit-cairn hot-spot. Additionally, the inferred mineral-ogy of this layer is fully consistent with the min-eralogy of the ‘garnet megacryst’ suite [29^31].The garnet megacryst suite is a set of xenolithscomposed predominantly of monomineralic gar-net, with crystal sizes varying from the cm to mlength scale. This suite has been interpreted as thefractional crystallization product of volatile-richmantle melts, possibly including kimberlites andalno«ites [32], an explanation compatible with theobservation of hydrous xenoliths on MUR.

4. Discussion

We have presented seismic evidence for a high-velocity body beneath the Fangataufa atoll. Ex-plosions from the Mururoa atoll V40 km furthernorth prove that the anomaly likely ends at theAustral Fracture zone. Synthetic seismogramsshow that the velocity in the body is V10% high-er than its overlying layer. Such a velocity in-crease can only be obtained by a dramatic in-crease of garnet content within the layer.

In the 1970s and early 1980s, several long-rangeseismic refraction studies focused on the structureof oceanic lithosphere using ocean-bottom seis-

mometers (OBSs) [33^36] with line lengths of800^1500 km. Some of these studies found spo-radic evidence for a high-velocity layer at depthsof 45^75 km [33^36], probably with velocity in-creases of up to 10%. The study areas were oce-anic basins (Lesser Antilles, east Mariana basinand west Philippine Sea) containing old oceaniclithosphere and it has been speculated that thishigh-velocity layer might be an inherent featureof old oceanic lithosphere [34]. The high-velocitylayer was interpreted as being either a layer ofanisotropic olivine or a layer with substantial en-richment in garnet [34,37] similar to our model-ing, although no source for the garnet increasewas previously proposed. It is di⁄cult to obtaina localized 10% P-velocity jump by means of ani-sotropy alone. Nonetheless, anisotropy mightcontribute to the observed velocity jump. How-ever, the contribution would be small due to thesmall inherent anisotropy of garnet.

The early OBS studies have poor signal-to-noise ratio and the data are di⁄cult to interpret.Since the tectonic setting of these studies is di¡er-ent from the one in our study area, we do notnecessarily think that the former studies andours sample the same feature. Nevertheless, thewestern Paci¢c and Caribbean regions sampledby the OBS studies are zones that have experi-enced extensive non-ridge-associated volcanic ac-tivity. For comparison, we develop a model of thegeneration of the HVZ beneath FAN based onthe likely origin of MUR/FAN from hot-spot vol-canism and the hypothesis that the HVZ is due toa massive enrichment of garnet formed via frac-tional crystallization.

Based on its geographic position at the start ofthe Pitcairn chain, the structural heterogeneity be-neath FAN is likely a residual feature associatedwith the genesis of the Pitcairn hot-spot. We pro-pose the following sequential model for the for-mation of MUR, FAN and the HVZ beneathFAN. The plume head associated with the initia-tion of the Pitcairn chain (possibly associated witha ‘mantle wet-spot’ [38]) may have been largelytrapped beneath the young lithosphere. Such trap-ping is in accordance with recent geodynamicmodeling of the interaction between ascendingplumes and the overlying lithosphere [39,40]. We

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assume that the Pitcairn chain arose from the as-cent of a relatively small plume, and we do notdraw conclusions on the depth of origin of theplume. Nevertheless, the plume ascended into rea-sonably young lithosphere. Therefore, its thermalbuoyancy may have been small relative to theoverlying plate. The ascending plume head waslikely trapped. Thus, extensive melting and asso-ciated fractional crystallization was generated atsub-lithospheric depth. Extensive sub-lithosphericlateral transport of melt (perhaps for many hun-dreds of km) may have occurred, as has beenproposed in other locales [41,42], with early tap-ping of volatile-rich material at MUR along theAFZ, and movement of large quantities of meltaway from the zone of extensive melting near thecenter of the plume head. Thus, the amount ofmelt produced at the plume head could havebeen far greater than that represented by the pres-ently observed volcanic edi¢ces. The seismologicaldata indicate that the HVZ has a sharp lateraltermination to the north. This might be due tothe thicker (older) lithosphere north of MUR hin-dering melt transport in this direction. Fractionalcrystallization of the plume head beneath MURwas likely minimal, since the magma could betapped through the AFZ and thus transportedeasily to the surface. Therefore, large quantitiesof residual fractional crystallization productscould progressively accumulate within the trappedhead beneath FAN: the estimated volume of thegarnet-rich layer that we observe is 40U50U50km3

W105 km3, likely implying that three to tentimes this volume [29] of magma was processed atdepth through this region. Indeed, the amount ofapparent magmatism in this region is far greaterthan that represented solely by MUR and FAN.MUR is part of a submarine volcanic range fol-lowing a N80‡ trend [7] and a topographic highstretches along the AFZ for nearly 200 km (Fig.6a), so that the AFZ might have served as a ‘leakyfracture zone’ for the plume head. The Pitcairnplume head thus appears to have produced dis-tributed regional volcanism, with the primarypresent-day record of the head being the largemagmatically generated feature documented atdepth. This sub-surface trapping of liquid andlarge-scale lateral transport provides a notable

contrast with the canonical association of plumeheads with £ood basalts. In this instance, meltappears to have been emplaced at depth and es-caped primarily along zones of weakness. Indeed,if hemorrhaging (as opposed to £ooding) of largeamounts of magma occurs in other plume heads,then the total mass and heat £ux from hot-spotswould be underestimated: large-scale sub-litho-spheric deposition of hot-spot-related materialhas not been taken into account in such £ux esti-mations. Therefore, the processes that have oc-curred beneath Pitcairn may be more representa-tive of those that occur when small plumesimpinge on young lithosphere, in which the pri-mary long-term manifestation of the plume headis con¢ned to comparatively small regions of thesub-surface.

Finally, geochemical constraints have indicatedthat plume-head material may contaminate thesource regions of the Pitcairn Island basalts them-selves [43]. Regardless of whether this plume-de-rived material is associated with the genesis of thePitcairn chain or represents residua from older,recycled heads, our results show that plume headslikely produce large-amplitude heterogeneities atdepth in the oceanic mantle on the tens of kmlength scale: a length scale of importance for geo-chemical studies of the Earth’s mantle [44]. There-fore, plumes appear able to generate geochemi-cally anomalous features at depth that, whenentrained into Earth’s convecting mantle, can pro-duce geochemical signatures that resemble sub-duction-generated heterogeneities. Finally, wespeculate that such garnet-enriched features couldbe common beneath the start of hot-spot tracksthat lack associated £ood basalts.

Acknowledgements

We thank Elise Knittle, Justin Revenaugh,Richard Von Herzen and Thorne Lay for help-ful comments and discussions. We thank J. Gaher-ty and N. Sleep for their constructive reviews.Funding for this study was provided byNSFgrant EAR-9905733. This is CSIDE (for-merly Institute of Tectonics) contribution number459.[SK]

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