Ridge–hotspot interaction: the Pacific–Antarctic Ridge and the foundation seamounts

Ž .Marine Geology 160 1999 199–223www.elsevier.nlrlocatermargeo

Ridge–hotspot interaction: the Pacific–Antarctic Ridge and thefoundation seamounts

Roger Hekinian a,), Peter Stoffers b,1, Dietrich Ackermand c, Sidonie Revillon d,´Marcia Maia e, Marcel Bohn f

a IFREMER, Centre de Brest, Geosciences Marines, 29280 Plouzane, France´ ´b UniÕersity of Kiel, Geologisch-Palaontologisches Institut, D.24118 Kiel, Germany¨

c UniÕersity of Kiel, Mineralogish-petrographisches Institut, D.24118 Kiel, Germanyd UPR 4661 Geosciences Rennes, UniÕersite de Rennes 1, Campus de Beaulieu, 35042 Rennes, France´ ´e CNRS-UMR 6538 UniÕersite Europeen de la Mer, Place Nicolas Copernic, 29280 Plouzane, France´ ´ ´

f CNRS-UMR 6538, IFREMER, Centre de Brest, 29280 Plouzane, France´

Received 27 August 1998; accepted 12 March 1999

Abstract

Ž .The study of different magmatic provinces between the Resolution fracture zone 338S–1318W and the Pacific–AntarcticŽ . Ž .Ridge PAR axis 378S–1118W suggests that similar processes of interaction between hotspot and spreading axial

magmatism occurred 20–25 Ma and 0–5 Ma ago. There is evidence of this process from the changes in compositionŽ X .observed in the lavas erupted near 400–300 km between the present day PAR axis 378S–111 W and the eastern tip of the

Ž . X wFoundation Seamount FS hotspot near 36820 S–1148W where the last alkali enriched volcanics KrTi)0.30, ZrrY)6Ž . xand CerYb )4 have erupted. This transitional province between the PAR and the FS consists of volcanic cones built onN

Ž . wseveral volcanic ridges -200 km in length which have erupted less enriched volcanics such as E- KrTis0.25–0.33,Ž . x w Ž . xZrrYs5–6 and CerYb s3–4 and T- KrTis0.11–0.25, ZrrYs2–4 and CerYb s1–2 MORBs than thoseN N

Ž .from the FS. It is also noticed that there is a general decrease in the degree of the basalt alkalinity more T-MORBs towardsthe PAR axis. The limit of the FS hotspot influence corresponds to the area where the VR intersect the PAR axis for adistance of about 100 km along its strike between 37810XS and 38820XS. Indeed, the lava erupted further to the north and to

w Ž . xthe south of these latitudes contains N-MORBs KrTis0.05–0.11, ZrrY-3 and CerYb s-2 . Many Old PacificNŽ . Ž .Seamounts OPS, )20 Ma also built on volcanic ridges are identified west )1200 km from the PAR axis of a Failed

Ž .Rift Propagator FRP forming the eastern boundary of the ancient Selkirk microplate. Some of these seamounts made ofŽ .alkali basalts were built during the initiation of the FS hotspot 20–23 Ma ago. The interaction and the influence thermal of

mantle plume magmatism with the ancient spreading ridge of the Farallon–Pacific plates was responsible for the eruptions ofthe T-MORBs and andesitic lavas. This situation is comparable to that presently observed on the PAR axis where siliciclavas are also erupted in association with T-MORBs. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: intraplate volcanism; ridge magmatism; structure; petrology; mineralogy

) Corresponding author. Tel.: q33-98-22-42-52; Fax: q33-04-98-22-45-70; E-mail: [email protected] Fax: q49-0431-880-28-50.

0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0025-3227 99 00027-4

( )R. Hekinian et al.rMarine Geology 160 1999 199–223200

1. Introduction

In order to understand global mantle convectionand upwelling of mantle plumes it is important totrace the timing and petrological consequences ofhotspots and ridge related magmatism. Althoughridge crest and hotspot magmatism have been inves-tigated independently, very few observations exist

Žwhich relate both magmatic systems Poreda et al.,1986; Schilling, 1991; Zhang and Tanimoto, 1992;Small, 1995; Kincald et al., 1995; Ribe et al., 1995;Devey et al., 1996; Hekinian et al., 1997; Phipps

.Morgan, 1997 . Only a few regions on the Earthpresent linear volcanic chains that intersect modern

Ž .spreading ridge axes. Shen et al. 1995 have pro-Ž .posed that South-East Pacific Rise SEPR off-axis

Ž .seamounts 178–188S were produced from the up-welling of ‘small plumes’ transporting lower mantle

Ž .material about 670 km underneath the ridge axis.The leaking of these lower mantle material will giverise to axial and off-axial volcanism during meltmigration. The concept of the presence of ‘plume-like’ components beneath the SEPR axis which wereadmixed with depleted MORB types of mantle was

Ž .suggested by Bach et al. 1994 , Mahoney et al.Ž . Ž .1994 and Niu et al. 1996 . Based on some incom-

Ž .patible element variations i.e., NbrZr, KrTi , Nie-Ž .dermann et al. 1997 have shown that the eruption

of enriched melts is due to the mixing of lowermantle components with the depleted upper mantlematerial. Another volcanic chain located in the South

ŽPacific called ‘the Foundation Line’ by Mammer-. Ž .ickx, 1992 and called Foundation Seamount FS

Žwas recently investigated Devey et al., 1996;.Hekinian et al., 1997; Maia et al., 1999 . The FS is

made up of a series of seamounts and short ridgesegments located within a 180-km-wide and 2000-km-long zone between 338S–1318W near the resolu-tion fracture zone and 378S–1118W near the Pacific

Ž . Ž .Antarctic Ridge PAR Fig. 1 . Satellite altimetricdata have provided a framework for the generaltectonic and structural interpretation of this areaŽMolnar et al., 1974; Haxby and Weissel, 1986;Craig and Sandwell, 1988; Cande and Haxby, 1991;

. ŽMammerickx, 1992; Sandwell and Smith, 1995 Fig..1 . A recent model of plate reconstruction of this

Ž .region by Tebbens and Cande 1997 showed thepresence of a short-lived microplate at about 20 Ma

Žago when the ridge of the Pacific–Farallon PAC–.FAR plates was active. This microplate was called

‘Selkirk’, after the real name of Robinson Crusoe,from the novel by D. Defoe in 1719, based on thestory of a sailor who was abandoned and lived for 4years on the island of Juan Fernandez near the coastof Chile.

Ž .A previous study of the Sonne leg 100 cruise ofŽ1995 carried out along the FS chain about 1300 km

.long had indicated the existence of a mantle plumeŽ . Žin the vicinities of the ridge axis 500 km Devey et

al., 1996; Hekinian et al., 1997; Hemond and Devey,´.1997 , in an area where the topography changes from

more circular features to short elongated ridges.These elongated structures, called Volcanic RidgesŽ .VR , are topped with volcanic cones and are shal-

Ž .lower 1500–1700 m depth with respect to theirŽ .adjacent PAR segments 2300–2600 m depth . The

decrease in alkalinity in samples from the FS provinceŽ .sites 69 and 70 at 410 km from the ridge axis to the

Ž .VR 306–20 km from the ridge axis and the PAR isbelieved to be the result of an interaction of the FSmantle plume with the PAR ridge magmatismŽ . ŽHekinian et al., 1997 . A recent cruise January–

.March, 1996 enabled us to carry out further investi-Žgations using multichannel bathymetry Simrad

.EM12 , gravity and magnetic data with the NrOL’Atalante as a complement to the French–Germanprogram of collaboration. This cruise surveyed in

Ž .more detail the Old Pacific Seamount OPS regionsŽ .20–25 Ma in more detailed as well as the more

Ž .recent PAR -5 Ma region, including the off-axisVR, in order to better constrain the limits of the FShotspot influences.

While previous work was mainly focused on theŽ .study of the FS and the more recent -7–5 Ma

VR, the present study intends to offer a more exten-sive overview on the ridge vs. off-axial magmatism

Ž . Ž .of both recent -7–5 Ma and ancient 20–25 Maintraplate provinces. This study is based on thecomparative petrology in relation to the structuralsettings between the OPS province associated with

Ž .the Failed Rift Propagator FRP of the SelkirkŽ .microplate, and the more recent -5 Ma VR asso-

ciated with the PAR axis. We intend to show thatthere is a compositional as well as structural relation-

Žship between the recent VR–PAR off-axis volca-. Ž .noes and that of OPS 20–23 Ma and Selkirk

()

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Ž . Ž .Fig. 1. Regional altimetry map derived from gravity data of the South East Pacific Sandwell and Smith, 1995 showing the distribution of R.V. Sonne leg 100 and NrOŽ . ŽL’Atalante dredge hauls obtained in 1995 and 1997, respectively. Details on the dredges from the R.V. Sonne empty dots were published elsewhere Devey et al., 1996;

. Ž .Hekinian et al., 1997 . The recent sample locations filled dots were obtained from the R.V. L’Atalante and are reported in Tables 1 and 2.


Ž .microplate Tebbens and Cande, 1997 FRP. A com-parison with the volcanics from the FS indicates thatthe same mantle plume which could have been re-sponsible for the PAR–VR formation could havecontributed to the construction of the OPS 20–25 Maago. Finally, based on the range in compositionalchanges of the volcanics from the PAR axis, we

intend to show the continuous influence of the man-tle plume along the ridge strike.

2. Geological provinces

The various volcanic provinces as previously de-Ž .fined Hekinian et al., 1997 on the basis of

Ž .Fig. 2. The sketched maps show the morphology of the PAR axis a and the distribution of the VR and sample stations located on a recentŽ . Ž . Ž . Ž .0–7 Ma b , and a more ancient 20–25 Ma c lithosphere associated with the PAR and the FRP, respectively. The FRP represent the

Ž .eastern branch of the Selkirk microplate existing at about 20–23 Ma ago Tebbens and Cande, 1997 . The contour lines of the majorŽ . Ž . Ž .positive structures were retraced from satellite altimetric data after Sandwell 1984 , Haxby 1987 and Maia and Diament 1991 . The

Ž . Ž . Ždashed line represent the trace of the regional magnetic anomalies Tebbens and Cande, 1997 at chrone 7 ;25 Ma and chrone 3 ;4. Ž .Ma . SosR.V. Sonne, FHsFoundation Hotline cruise of the R.V. L’Atalante followed by the dredge stations number i.e., 11 . The

Ž . Ždiscontinuous lines on the Delcano ridge indicate en-echelon faulted ? linear features interpreted from multichannel bathymetry Maia et. Ž . Ž . Ž .al., 1999 . The arrows with numbers indicate the general geometry orientations of the various geological structures. The PAR a1 axis

Ž .is oriented to 0158. The VR associated with the PAR and the FRP showing at least two main orientations at 254–2508 a3 and 286–2968

Ž . Ž . Ž .a2 , respectively. These recent VR -7 Ma and the more ancient ones 20–25 Ma associated with the OPS are believed to be originallyŽ . Ž .transformed andror pulled apart linear features topped with volcanic edifices. Alk. B.salkali basalts and normal N- , transitional T- , and

Ž . Ž . Ž . Ž .enriched E- Mid Ocean Basalt MORB are shown a, b and c . The composition of the dredged volcanics plain numbers from the SonneŽ .cruise are published elsewhere Devey et al., 1996; Hekinian et al., 1997 . Samples 71 to 73 are E-MORBs, 74 are N-MORBs and 75 to 93

Ž .are mainly T-MORBs. Dredge FH13 contain one vesicular porphhyric basalt plagioclase, olivine and spinel ; no analyses available.

( )R. Hekinian et al.rMarine Geology 160 1999 199–223 203

Ž .Fig. 2 continued .

morpho-structural and compositional variations areŽ .summarized as follows: 1 the PAR is made up of

several en-echelon segments where both glassyŽ .Mid-Ocean Ridge Basalts MORBs as well as an-

Ž . Ž .desites and dacites have erupted Figs. 1 and 2a , 2the VR located up to 400 km from the PAR axis aretopped with seamounts made up essentially of transi-

Ž . Ž .tional T and enriched E MORBs and alkali basaltsŽ . Ž .Fig. 2b , 3 the OPS built on crust older than 23

ŽMa located west of longitude 1248W )1300 km.from the PAR axis consist of N- and T-MORB and

Ž .alkali basalts comparable to the VR, 4 the FRPwhich was part of the western branch of the Selkirk

Žmicroplate constructed 20–23 Ma ago Tebbens and


. Ž . Ž .Cande, 1997 Fig. 2c , and 5 the FS volcanic linelocated between longitudes 1248W and 1158W at410–1300 km from the PAR axis, which is betweendredge stations So28, FH5, FH6 and FH14, FH15

Ž .and So70, respectively Fig. 2b and c . The FSvolcanics are the most enriched in incompatible ele-

Žments and have erupted alkalic lavas alkali basalt,.trachybasalt and trachyandesite with highly incom-

w Ž . Ž .patible elements K O 0.50–1.1% , Zr )150 ppm2Ž .x Ž .and Ce )48 ppm , high ZrrY 6–10 , KrTi

Ž . Ž . Ž .x2.8–5 and CerYb 4–7 values. These type ofN

lavas are believed to be typical of hotspot origin andmore details on their compositions and geological

Žsetting are found in Devey et al., 1996; Hekinian et.al., 1997 .

2.1. Variability of the Õolcanics along the Pacific–Antarctic Ridge

The PAR axial region at 36830X–398S made up ofŽ .several en-echelon at least 5 segments of about 30

km long, separated by second and third order discon-Ž . Žtinuities Hekinian et al., 1997 Figs. 1 and 2a and

.b . The depths along the PAR axis vary between2230 m and 2650 m and the summit region of the

Žridge crest is less than 1 km wide. Basalts FH10,. Ž-09, -85, -102 and -106 and silicic lavas 86, 91, and

. Ž .92 were dredged along the ridge axis Fig. 2a . Nearsite stations 85 and 86, the axial graben is 300 mwide and 50 m deep between 38815XS and 37830XSŽ . XFig. 2a and b . At 37825 S the ridge becomes widerand there is a circular topographic feature, maybe anaxial seamount, which also has a small graben madeof fresh lobated flow as observed by a deep towed

Ž .bottom camera Fig. 2a . The crossing of the deeptowed camera in a NW direction showed the pres-ence of extinct hydrothermal chimneys associatedwith an older terrain made up of lobated flows andinterstitial sediment near 37827XS–110848.20X W at2200 m depths. Both active and inactive hydrother-

mal vents were seen in the axial graben betweenX X Ž .37840 S and 37836 S Fig. 2a .

Ž .Previously dredged Hekinian et al., 1997 sta-tions along the portion of the ridge crest intersected

X Ž . X Ž .by the VR between 38810 S 92 and 37811 S 105showed a great variety of rock types such as an-

Ž . Ž .desites 105, 92-03 , dacites 86-02, 91-01 , andŽ .T-MORBs 102-1, 85-01 with a K O of 0.17%,2

Na OqK O near 3%, KrTi ratios of 0.11–0.252 2Ž .and Mga of 55–65 Table 2, Fig. 3a and c . How-

ever, the new dredges taken at the extremities of thepreviously sampled PAR segments show the pres-

Žence of N-MORBS whose glassy margins FH10-07,-08, -09, -05 and FH09-01, -03, 04, -07, -09, -13,

. Ž . Ž .-13, -11 have a low K O 0.11% , KrTi -0.122Ž . Ž .and Na OqK O 2.61% values Fig. 3c . The2 2Ž .T-MORBs recovered FH09-02, 05 also contain low

Ž . Ž . ŽK O 0.12% and KrTi 0.12 and total alkali -2. Ž .3.2% contents Tables 2 and 3c . Their bulk trace

Ž .element content show a low ZrrY 2–3.5 ppm ,Ž . Ž . ŽCerYb 1–1.5 and low REE Las3–7 ppm,N

.Sms2–4 ppm which is similar to other SEPRŽ .N-MORBs Table 3; Figs. 3b,d, 4a and 5a,b . The

plagioclase of these N-MORBs is depleted in ortho-Ž .clase an , ab , or content and consists73 – 85 14 – 26 0 – 0.3

Ž .mainly of labradorite Fig. 6 . However, the plagio-Žclase of the silicic lavas consists of andesine ab ,45 – 65

.or and shows a low trend of variability for the0.3 – 1.0

albite–orthoclase contents when compared to otherŽ .enriched lavas Fig. 6 .

2.2. The Õolcanic ridges near 378S–1148W

ŽThe VR as described elsewhere Hekinian et al.,.1997 are linear structures of about 70–200 km in

length and 30–40 km wide with two sets of direc-tions: 2548 at 100–130 km located at dredge sites 71

Ž .to 74 112–1148W and 2868 located at dredge sitesŽ X .75 to 87 111830 –1128W at less than 306 km from

ŽFig. 3. Compositional variations along the PAR crest along axial bathymetry profile constructed from the multichannel Hydrosweep and. Ž . Ž .SIMRAD echosounder data obtained during the RrV Sonne cruise So leg 100 in 1995 and the N.O. L’Atalante in 1997. a The dredge

Ž .stations are indicated along the profile. The sub-horizontal bars show the structural discontinuities of second overlapping spreading centerŽ . Ž . Ž .and third order small fissural offsets discontinuities. b, c and d Rock compositional variations based on the CerYb , KrTi and ZrrYN

ratios.


Ž .Fig. 4. REE normalized to chondrite C1 Sun and MacDonough, 1989 distribution pattern of the bulk rock analyses of volcanics from theŽ .intraplate Old Pacific SeamountsOPS, Foundation seamountsFS and Failed Rift PropagatorsFRP and PAR provinces of the South

East Pacific near 378S.

Ž .the PAR axis Figs. 1 and 2b . These structures arecharacterized by the presence of volcanic cones whichcoalesced together forming single linear structures.

ŽThe set of VR nearer to the PAR axis oriented at. Ž2868 are in generally depths shallower 1600–1700

.m than that of the adjacent average PAR axisŽ . Ž .2200–2400 m Devey et al., 1996 . These VR are

Žessentially made up of E-MORBs KrTis0.12–0.25, Na OqK Os3.6% Zrs120–140 ppm and2 2

. ŽYs34–40 ppm , and T-MORBs KrTis0.20–


0.30, Na OqK Os2.83.6%, 72–120, Ys22–302 2. Ž . Ž .ppm Hekinian et al., 1997 Tables 2 and 3 . The

present data indicates that some of the volcaniccones forming these VR at sites FH14, FH15, So69and So70 are alkali lavas comparable to the FS

Ž .hotspot volcanics Fig. 2b, Tables 2 and 3 . Thesealkali basalts are more enriched in incompatible ele-

Žments KrTi s 0.35–0.8, Na O q K O s 3.2–2 2.4.5%; Zrs190–270 ppm; Ys33–40 ppm and

Ž . ŽLight Rare Earth Element LREE contents Table 3,.Fig. 4b . They also differ from the MORBs by their

Ž .K enrichment of their plagioclase ab , or20 – 40 0.5 – 1.2Ž .Table 1, Fig. 6 . Dredge FH11, taken at about 336km from the PAR axis, consists essentially of glassylobated lavas and pillows and slabs. Many of the

Ž .samples are picritic FH11-02, -3, -10, -12, -16 withmore than 15% olivine phenocrysts. Picritic basalts

Ž .containing spinel 3–10% were also found further tothe East at station So74 and FH11 on a volcanic

Ž .cone along the same VR Table 1 . These leastŽ .evolved picritic lavas MgO in glass )8% from the

Ž .Oblique Ridge are depleted in Zr, ZrrY -2 andŽ .Ni 250–350 ppm , and are believed to have formed

Žduring crystal fractionation Table 3; Fig. 5a,b and. Ž .c . Olivine basalts FH14-15, -16 and -17 with

Ž .olivine 8–10% and spinel were also observed. TheŽ .other dredge hauls FH12, -13, -14, -15 consist

Žmainly of pillow lavas and hyaloclastites samples. Ž .FH12-03, -07 . One dredge haul FH13 containŽ .FeTi basalts TiO s3.5%, and Fes14% which2

are considered as being E-MORB because of theirŽ . Ž .relatively high KrTi 0.26 ratio Table 2 .

2.3. The old Pacific seamounts and the failed riftpropagator

The various structures encountered on the ancientŽ .)23 Ma crust have been regrouped together and

Ž .called the OPS province. They include 1 the FRP.and 2 the volcanoes forming some of the elongated

ridges in the area.

2.3.1. The failed rift propagatorThe FRP is about 190 km in length and 10 km in

Ž .width with a N–S 3578 orientation which are prop-Ž .agated south Figs. 1 and 2c . This FRP was inferred

to be the western boundary of the Selkirk microplate

Ž .Tebbens and Cande, 1997 while the eastern branchcorrespond to the present day PAR axis at 378S. Thecrust at the western side of the FRP is covered by amultitude of seamounts most of which form complexlinear features similar to the so-called pseudo-fault

Ž .structure described by Naar et al. 1991 and Lons-Ž .dale 1994 in the Easter microplate region and the

PAR. The eastern side of the FRP appears to bemore tectonized with fewer constructional edifices

Ž .oriented in a NS 3578 direction. To the north, at the4000 m contour line, this FRP encloses a largeseamount observed at dredge station So28 made up

Žof alkalic lavas Devey et al., 1996; Hekinian et al.,.1997 . This seamount is one of the tallest edifices

Ž .4000 m above the sea floor with a summit at 500 mdepth. It probably belongs to the FS chain whichconstitutes the area of the hotspot believed to beactive 20–25 Ma ago. The only dredge collected

Ž .from the FRP FH7 consists essentially of pillowlavas, some with preserved glassy margins, massivedolerites and breccias made up of subrounded basaltic

Žpebbles. The composition of the rocks based ontheir plagioclase variation, glassy margin and bulk

.rock analyses indicates that they are N-MORBsŽ . Ž .Tables 1–3 . These rocks show a low KrTi -0.1 ,

Ž . Ž . Ž . Ž .Na OqK O -3% , ZrrY -3 , CerYb -32 2 N

low LREE, and have a plagioclase composition com-prised in the low albite–orthoclase variability trendŽ . Ž-ab –or Fig. 4d, Fig. 5a,b, Fig. 6; Tables36 0.3

.1–3 .

2.3.2. The old Pacific seamountsThe OPS are located between 32840XS–1318W

X Žand 33830 S–1268W at dredge stations FH01 and.-13–19 more than 1300 km from the PAR axis, on a

crust older than 23 Ma, and probably formed whenthe Pacific fracture zones and accreting ridge axis

Žhad a ‘Farallon plate’ orientation Tebbens and.Cande, 1997 . The OPS consist of circular volcanoes

associated with elongated structures. The availableŽ38 39 .age dating ArrAr ; O’Connor et al., 1998 at

Žseveral dredge stations So18s16 Ma, FH01s18.Ma, So28s9 Ma and So25s13 Ma indicates a

younger age than the crust on which they were builtŽ .Fig. 2c . This younger age was interpreted as being

Ž .due to the continued influence channeling materialŽ .of the FS mantle plume O’Connor et al., 1998 . The


Fig. 6. Albite–orthoclase distribution of fresh plagioclase from the various types of volcanics recovered from the different geologicalŽ . Ž .settings of intraplate and accreting ridges provinces. OPSsold Pacific seamounts 20–25 Ma which includes a sample FH7-1 from a

Ž .FRP; FSs foundation seamounts consisting essentially of alkalic lavas; VRsvolcanic ridges -7 Ma and PARsPacific–AntarcticŽ .Ridge comprising silicic lavas as well as T-MORBs and N-MORBs. The fields of plagioclase-enriched alkalic lavas and -depleted

Ž .MORBs in K are shown.

Žcontoured depth data 4000, 3000, 2000, and 1000. Žm obtained from altimetric measurements Sandwell

.and Smith, 1995 indicate that these elongated ridgesŽ .100–350 km long, and 30–40 km wide are topped

Ž .Fig. 5. Trace element variations of the volcanics from the various South Pacific accreting ridge segments and intraplate provinces. a and bŽ . Ž . Ž .ZrrY and CerYb ratios of basalts and silicic lavas from the PAR axis, VR, FS and OPS )23 Ma . The subscript N indicatesN

Ž .Normalized to chondrite C1 Sun and MacDonough, 1989 . These compared with other intraplate provinces and South East Pacific RiseŽ . Ž .SEPR segments at 78S–298S Sinton et al., 1991; Bach et al., 1994; Mahoney et al., 1994; Hekinian et al., 1995b excluding the Easter and

Ž .Juan Fernandez microplates. The calculated crystal fractionation LLDsLiquid Line of Descent , mixing and partial melting trends areŽ . Ž .shown. The annotation 10% and 5% near the melting curve indicates the amount of partial melting. c Ni–Zr variation diagram

Ž . Žemphasizing the range of crystal fractionation and the presence of the olivine enriched picritic basalt Ni)200 ppm . A mixing curve filled. Ž . Ž . Ž .line is traced between a moderately depleted So 85-01 and alkali enriched So 28-1 melts. A partial melting curve dashed line falling

along andror near the mixing curve was calculated for a non-modal fractional melting model of a moderately enriched MORB mantleŽ . Ž .source Zrs15 ppm, Ys6.3 ppm and Ces2.49 ppm, Ybs0.62 ppm Hekinian et al., 1995b . The source used consists of 85% spinel

Ž .lherzolite and 15% clinopyroxenite 60% olivine, 17% orthopyroxene, 20% clinopyroxene, 3% spinel . The partition coefficients used are:Ž . Ž . Ž . ŽZr 0.1 olv, 0.02 opx, 0.107 cpx, 0.05 spn , Y 0.1 olv, 0.2 opx, 0.6 cpx, 0.05 spn , Ce 0.01 olv, 0.05 opx, 0.1 cpx, 0.0005 spn , Yb 0.059

. Ž . Ž . Ž .olv, 0.075 opx, 0.5 cpx, 0.0045 spn and Ni 13 olv, 3.8 opx, 4.4 cpx, 5 spn compiled in Hekinian et al., 1995b . N sN-MORB,Ž .T sT-MORB and Alksalkalic trends. The E-MORBs and probably most of the enriched T-MORBs have resulted from magma mixing

Ž . Ž . Ž .between an alkali basalt melt i.e., 28-01 , N- i.e., 74-03 andror T-MORB 85-01 type of melts. The LLD70-01 line with a kink along itsŽ . Ž .trend corresponds to the extensive )10% crystallization of clinopyroxene. The VR consists of rare N-MORB 74-01, -02, 03, -04, 79-06 ,

Ž . Žabundant T-MORB 75-01, 78-01, 82-02, 83-01, 84-01, HF11-1, -3, -12, -16, -10, FH12-08 , common E-MORB 71-01, 71-02, 76-04,. Ž . Ž . Ž87-01, FH12-08, -5, 14-04, 15-03, -06 and -15 and rare alkali basalt FH 14-07, -05, FH15-04, -10 Hekinian et al., 1997 Tables 2 and

. Ž .3 . The samples with prefix FH are from the 1997 cruise of the R.V. L’Atalante; the others with numbers or So prefix are from theŽ .previous R.V. Sonne cruise Devey et al., 1996; Hekinian et al., 1997 .


Table 1ŽLocations of dredge hauls from the South PAR and intraplate seamounts and the foundation intraplate regions at 378S–1128W R.V.

.L’Atalante, 1997

Sample Lat. S Long. W Depth Provinces Rock type PlagioclaseŽ .m an ab or

X XFH1-01 32830.33 127830.09 3200 OPS B. Andes 54.30 45.10 0.60X XFH1-02 32830.33 127830.09 3200 OPS B. Andes – – –

XFH4-01 33846 .69 126843.83 3508 OPS N-MORB 64.44 35.25 0.32XFH5-2 33846 .69 126843.83 3508 OPS Alk. B. – – –

X XFH5-03 34855.11 126813.79 3350 OPS Alk. B. 58.23 40.54 1.22X XFH5-04 34855.11 126813.79 3350 OPS Alk. B. – – –X XFH5-06 34855.11 126813.79 3350 OPS Alk. B. – – –X XFH5-08 34855.11 126813.79 3350 OPS Alk. B. – – –X XFH5-12 34855.11 126813.79 3350 OPS Alk. B. – – –X XFH6-01 34834.00 125816.40 2979 OPS Alk. B. 69.94 29.31 0.75X XFH7-02 35820.85 124845.72 4842 FRP N-MORB 64.61 35.10 0.29X XFH7-03 35820.85 124845.72 4843 FRP N-MORB – – –XFH11-01 36855.48 113804.57 2599 VR T-MORB 81.44 18.32 0.24XFH11-03 36855.48 113804.58 2600 VR T-MORB

FH11-07 36855.48X 113804.56 2598 VR T-MORB 76.10 23.76 0.14XFH11-10 36855.48 113804.58 2600 VR T-MORB – – –XFH11-12 36855.48 113804.59 2601 VR T-MORB – – –XFH11-16 36855.48 113804.58 2600 VR T-MORB 71.81 27.87 0.31XFH12-05 36855.48 113804.59 2601 VR E-MORB – – –X XFH12-08 37855.93 113842.37 2770 VR E-MORB – – –X XFH12-09 37855.93 113842.37 2770 VR T-MORB 65.02 34.47 0.51X XFH14-05 37827.78 113834.77 2598 VR E-MORB 65.73 33.66 0.61X XFH14-06 37827.78 113834.77 2598 VR E-MORB – – –X XFH14-07 37827.78 113834.77 2598 VR Alk. B. 68.91 30.31 0.79X XFH15-03 35844.98 114821.99 2405 VR Alk. B. 62.23 37.12 0.66X XFH15-04 35844.98 114821.99 2406 VR Alk. B. – – –X XFH15-06 35844.98 114821.99 2405 VR E-MORB 75.47 24.11 0.42X XFH15-10 35844.98 114821.99 2406 VR Alk. B. – – –X XFH15-15 35844.98 114821.99 2405 VR E-MORB 76.02 23.63 0.35X XFH9-01 38841.84 111809.02 2273 PAR N-MORB 74.71 25.10 0.19

FH9-02 38841.84X 111809.02X 2273 PAR T-MORB 71.91 27.98 0.12X XFH9-03 38841.84 111809.02 2273 PAR N-MORB – – –X XFH9-04 38841.84 111809.02 2273 PAR N-MORB 74.93 24.96 0.10X XFH9-05 38841.84 111809.02 2273 PAR T-MORB 70.80 29.04 0.16X XFH9-07 38841.84 111809.02 2273 PAR N-MORB 67.52 32.43 0.05X XFH9-09 38841.84 111809.02 2273 PAR N-MORB – – –X XFH9-11 38841.84 111809.02 2273 PAR N-MORB 80.77 19.13 0.10X XFH9-13 38841.84 111809.02 2273 PAR N-MORB 73.09 26.79 0.12X XFH10-05 36830.04 110831.59 2820 PAR N-MORB – – –X XFH10-07 36830.04 110831.59 2820 PAR N-MORB 85.91 14.09 0.00X XFH10-08 36830.04 110831.59 2820 PAR N-MORB 79.21 20.70 0.09X XFH10-09 36830.04 110831.59 2820 PAR N-MORB 78.80 21.13 0.07

OPSsold Pacific seamount, VRsvolcanic ridge, PARsPacific-Antarctic Ridge and FRPs failed rift propagator. Sample FH4-01 isŽ . Ž . Ž . Ž .from a volcanic cone of the Delcano ridge. N normal , T transitional and E enriched MORB mid-ocean ridge basalt are indicated. B.

Ž . ŽAndessbasaltic andesite, Alk. B.salkali basalt, FHsFoundation Hotline cruise 1997 . The plagioclase microprobe analyses average of.)5 were carried out at the center of the crystals. ansanorthite, absalbite and orsorthoclase. FH11-03, -10, 12 and -16 are picritic

basalts.

Ž .with volcanic cones up to 1000–2500 m in heightand are similar to the VR constructions observed

near 378S–1148W between the PAR axis and the FSŽ .Fig. 2b,c . The OPS oblique features show two


Ž . Ž .general sets of orientations, 2508 a3 and 2968 a2Ž .directions with respect to the FRP 3578 which are

Ž .close to that observed on the more recent -7 MaŽ . Ž .VR 2548 and 2868 Fig. 2b,c . In the case of the

Delcano ridge where multichannel bathymetric cov-Ž .erage is available Maia et al., 1999 , it is observed

that it consists of several en-echelon features withsteep slopes probably due to faulting. This structure,oriented in a 2508 direction, is topped with volcanic

Ž .cones. A dredge haul FH04 at the summit of thevolcanic cone at the eastern tip of the ridge and adeep-towed video camera observations carried out at

Ž X X .the western tip of the ridge 34817 S–129825 Wrevealed the presence of lobated and altered pillowlavas, partially coated with a dull, thin Mn-oxidecrust at less than 1900 m depth. The top of theseamount is partially covered by pelagic sedimentand a few patches of hydrothermal Fe-oxyhydroxideproducts were observed. Another structure with ageneral orientation in a 2968 direction is also toppedby at least three volcanic cones. A dredge haulŽ .FH05 was collected at 3350 m depth near theeastern tip of this oblique structure near 358S–1268WŽ . Ž .Fig. 2c . The same dredge haul FH5 containsaltered pillow lava fragments, some of which haveglassy chilled margins and a rusty colored, chertylooking material which has the appearence of an

Ž .indurated Fe-rich sediment Table 1 . The dredgesFH01 and So13–19 are also from tall seamountsŽ .500–1500 m depth forming the volcanic cones

Ž .topping the other oblique ridges Fig. 2c .The bulk rock, glassy margin and mineral analy-Ž .ses plagioclase were used to classify the various

Ž .rock types encountered Table 1, Fig. 6 . AlkaliŽ .lavas FH6-01, FH5-03, FH4-01 , and basaltic an-Ž .desites FH1-01, -02 have erupted on the volcanic

cones topping the volcanic ridges. The alkali basaltsŽ .are enriched in KrTi 0.35–0.40 and total alkali

Ž .Na OqK O)4% content. Their trace elements2 2Ž . Ž .expressed in terms of ZrrY 7–8 and CerYb N

Ž .4–5 variation with respect to their correspondingŽ . Ž . Ž .Zr 260–400 ppm and Ce 90–100 , concentra-N

Ž .tion, fall in the field of the FS volcanics Fig. 5a,b .ŽSample FH4-01 was too altered abundant Fe-oxide.matrix and iddingsitized olivine to be used it for a

petrogenesis discussion. The microprobe data ob-tained on fresh plagioclase laths indicates that they

Žare depleted in their orthoclase content an , ab ,64.4 35.7

.or and the plagioclase is similar to that observed0.3Ž .in the PAR axis N- and T-MORBs Fig. 6 .

2.4. Magmatic processes

Ž .The algorithms of Nielsen 1988, 1990 , WeaverŽ . Ž .and Langmuir 1990 and Nielsen and Delong 1992

have been used to model the crystal fractionationpaths of the various types of glassy MORBs, alkalibasalts and silicic lavas. When modelling crystalfractionation, it was found that most PAR, VR, andOPS volcanics plot close to the liquid lines of de-scent obtained from the fractionation of T-MORBŽ . Ž .i.e., sample 85-01 , E-MORB 70-1 and Alkali

Ž . Ž .basalt 28-1 trends Fig. 5a and b . This is shown byŽ . Ž .the large range in ZrrY 2–12 and CerYb N

Ž .1–8 with respect to corresponding variation in ZrŽ . Ž .and Ce contents Fig. 5a and b . The most de-N

Žpleted N-MORB i.e., picritic basalt 74-01, from the.VR , enriched in olivine phenocrysts, shows the

lowest crystal fractionation trend among the vol-canics collected from the various provinces which

Ž .extend up to the level of the SEPR samples Fig. 5 .The highest extent of crystal–liquid fractionationŽ .about 64% solids, Hekinian et al., 1997 whichfollows the T-MORB liquid line of descent is ob-

Žserved for the silicic lavas erupted on the PAR Fig.. Ž5a,b . Some samples essentially from the VR FH11,

. Ž .-01, -03, -12 and the OPS FH1-1 and 18-1 have aŽ . Ž .ZrrY 3–4 , and Zr of 80–122 ppm, CerYb ofN

Ž . Ž .1–2.3 and Ce of 25–30 Table 3 . These samplesN

fall in a field of crystal fractionation defined by thatŽ . Ž .of the T-MORB i.e., 85-01 Fig. 5a,b . Other E-

Ž .MORBs 87-01, 76-04, 73-04, FH12-05 departingfrom the T-MORB trend of variability are more

Ž .enriched in total alkalis Na OqK Os3–4% , and2 2Ž . Žin their KrTi 0.21–0.35 , ZrrY sample 87-01s

. Ž . Ž4.2, FH12-05 s 5.74 and CerYb samplesN. Ž .FH12-05, 73-04, 87-01s2.3–3 ratios Fig. 5a,b .

Ž .Alkali basalts from the VR FH14-05, FH15-04, -10Ž .and OPS FH5, FH6-1 fall in the field of other

Ž .enriched lavas from the FS Fig. 5a,b . The basalticŽ .andesites from the OPS FH1-01, -02 showing a

Ž . Ž . Ž . Ž .ZrrY 4.8 CerYb 2 , Zrs410–420 ppm andNŽ . Ž .Ce 99 ppm fall along a trend of crystal–liquidN

fractionation which is part of the field defined by theŽ . Ž .T-MORB 85-1 from the PAR axis Fig. 5a,b . The


Table 2Average micropobe analyses of glassy volcanics from the PAR axis near 378S

ample a FH9-01 FH9-02 FH9-03 FH9-04 FH9-05 FH9-07 FH9-09 FH9-13Types N-MORB T-MORB N-MORB N-MORB T-MORB N-MORB N-MORB N-MORBAverage 4 9 7 9 9 9 9 13

SiO 50.53 50.54 50.37 50.58 50.61 50.24 50.23 50.242

TiO 1.67 1.67 1.50 1.50 1.68 1.81 1.72 1.542

Al O 14.02 13.93 13.90 13.97 13.88 13.65 13.86 13.892 3

FeO 10.61 10.35 10.45 10.56 10.57 10.94 10.77 10.44MnO 0.24 0.23 0.17 0.16 0.24 0.19 0.21 0.15MgO 7.37 7.23 7.49 7.44 7.23 7.09 7.19 7.52CaO 11.90 11.90 12.28 12.05 11.88 11.52 11.91 12.11Na O 2.76 2.67 2.69 2.70 2.80 2.81 2.78 2.692

K O 0.13 0.15 0.09 0.11 0.14 0.14 0.13 0.112

P O 0.09 0.16 0.08 0.11 0.19 0.14 0.16 0.072 5

Total 99.35 98.88 99.06 99.20 99.25 98.57 98.99 98.80KrTi 0.11 0.12 0.08 0.10 0.12 0.11 0.10 0.10Mga 57.92 58.06 58.68 58.26 57.51 56.19 56.93 58.79Na OqK O 2.89 2.82 2.78 2.81 2.94 2.95 2.90 2.802 2

Microprobe analyses of the volcanics from off-axis and intraplate seamounts from the south Pacific

Ž .OPS FRP Volcanic ridges VR

Sample a FH1-01 FH5-03 FH06-01 FH7-03 FH11-10 FH11-16 FH12-08 FH12-09Types B. Andes Alk. B. Alk. B. N-MORB T-MORB T-MORB E-MORB T-MORBAverage 3 5 7 6 9 4 4 7

SiO 52.19 49.20 47.36 49.71 47.00 47.05 49.21 49.072

TiO 3.44 4.29 3.20 2.67 1.50 1.54 2.25 2.352

Al O 12.31 13.32 15.65 13.38 16.44 16.80 14.47 14.422 3

FeO 13.83 13.01 10.63 13.17 10.20 10.06 11.01 11.18MnO 0.34 0.25 0.16 0.23 0.16 0.10 0.24 0.17MgO 3.73 4.44 6.11 6.84 8.38 8.16 6.46 6.49CaO 7.90 9.24 9.87 10.57 11.15 11.17 10.55 10.57Na O 2.35 2.57 3.56 2.04 2.85 2.89 3.51 3.372

K O 0.72 1.07 0.83 0.14 0.18 0.22 0.46 0.432

P O 0.62 0.71 0.55 0.25 0.17 0.18 0.31 0.292 5

Total 97.45 98.13 97.97 99.01 98.07 98.27 98.48 98.35KrTi 0.29 0.35 0.36 0.07 0.16 0.20 0.28 0.25Mga 34.82 40.34 53.25 50.71 61.95 61.62 53.74 53.49Na OqK O 3.07 3.64 4.39 2.18 3.03 3.11 3.97 3.802 2

Ž .Distance km 1430.5 1399.9 1325.7 1250.3 218.0 218.0 229.0 229.0

Ž .The analyses were done with the SX 50 Camebax IFREMER, Brest .Ž .The analytical precisions and standard deviations are given in Hekinian et al., 1997 .

MgasMg2qrMg2qqFe2q with Fe2qrFe2qqFe3qs0.9.Ž . Ž .OPSsold Pacific seamounts )20 Ma are associated with volcanic ridges topped with volcanic cones. The distances km are calculated

from the PAR axis.Ž .FRPs failed rift propagator part of the Skelkirk microplate. Dredge FH11 contain mainly picritic basalts olivine)10% .

least depleted samples of N-MORBs from the VRŽ . Ž .i.e., 74 , from the PAR axis FH9-2, FH10-5, 106-1

Ž .and from the FRP FH7-3, -5 fall in a field which isŽ . Ž . Ž .lower in ZrrY -3.5 and CerYb -1.5 andN

closer to the N-MORB trend of fractionation which

Ž .is more in the field of the SEPR samples Fig. 5a,b .The new data obtained on the samples collected fromthe VR confirms the range of variabilities previouslyobserved when modelling a mixing curve between

Ž . Ž .the most depleted N- i.e., 74 and T- 85-01 MORB


FH9-11 FH10-05 FH10-07 FH10-08 FH10-09 FH10-09N-MORB N-MORB N-MORB N-MORB N-MORB N-MORB8 3 3 2 3 3

50.69 50.25 50.90 50.78 50.06 51.101.52 1.71 1.30 1.32 1.75 1.28

13.95 14.13 14.09 14.11 13.99 13.9810.55 10.89 10.20 10.55 11.12 10.100.22 0.10 0.24 0.12 0.16 0.177.50 7.19 7.68 7.69 7.24 7.70

12.06 11.74 12.36 12.37 11.64 12.532.69 2.75 2.33 2.27 2.77 2.330.12 0.13 0.08 0.05 0.11 0.080.11 0.07 0.07 0.17 0.13 0.06

99.49 99.00 99.26 99.47 99.06 99.360.11 0.11 0.09 0.05 0.09 0.09

58.46 56.66 59.87 59.08 56.33 60.152.82 2.88 2.41 2.32 2.88 2.41

FH13-01 FH14-05 FH14-06 FH14-07 FH-15-03 FH15-06 FH15-06 FH15-15E-MORB E-MORB E-MORB Alk. B. E-MORB E-MORB E-MORB E-MORB5 10 9 4 3 4 7 7

50.01 46.15 46.96 47.76 49.74 48.87 48.66 49.353.49 2.90 2.62 3.04 3.28 2.36 2.35 2.76

13.39 14.13 15.52 16.11 13.71 13.99 13.84 13.7814.44 11.87 11.57 10.34 13.01 11.21 11.38 11.710.24 0.19 0.20 0.22 0.20 0.18 0.20 0.164.38 9.35 6.35 5.94 4.78 5.84 5.92 5.708.83 8.65 10.10 9.26 9.16 11.00 10.95 10.232.76 3.49 3.69 3.85 3.16 3.23 3.22 3.440.65 0.70 0.54 0.95 0.76 0.59 0.56 0.640.51 0.51 0.39 0.70 0.51 0.31 0.35 0.36

98.82 97.94 97.93 98.16 98.32 97.58 97.43 98.140.26 0.33 0.29 0.43 0.32 0.34 0.33 0.32

37.51 60.95 52.09 53.24 42.13 50.79 50.75 49.083.41 4.19 4.23 4.80 3.92 3.82 3.78 4.08

334.0 357.6 357.6 357.6 370.0 370.0 370.0 370.0

Ž .melts and the most enriched FS lavas So28Ž .Hekinian et al., 1997 . Most of the least evolved

Ž .melts 85-01, 70-02 and 28-01 encountered in ourstudy fall near a mixing line which also correlateswith the calculated partial melting trend obtained

Ž .from a heterogeneous mantle source Fig. 5a,b . Anexample of a heterogeneous mantle consisting of a

Ž .mix of 85% lherzolite Frey et al., 1985 and 15%Ž . Žclinopyroxenite Irving, 1980 was proposed Bideau

.and Hekinian, 1995; Hekinian et al., 1995b . This

Ž .type of mantle with moderate ZrY 2.38 andŽ . Ž .CerYb 1.12 ratios rather than a garnet lherzo-N

w Ž .lite i.e., 66SAL-1, ZrrYs3.7, CerYb s1.54,NxFrey, 1980 was chosen because it gives a better fit

in calculating a potential source for our samples. TheŽ .melting curves calculated for the Ce vs. CerYbN N

and Zr vs. ZrrY fall close to the calculated mixingŽlines which enclose the T- and E-MORBs Fig.

.5a,b . The N-MORBs are likely to have been derivedfrom a more depleted lherzolite source with lower


Table 3Bulk rock analyses of samples from the PAR axis and adjacent intraplate provinces collected during the 1997 Cruise of the N.O. L’Atalante

Ž .Old Pacific seamounts OPS

Volcanic ridges near 1268W FRP

Sample 01-01 01-02 05-02 05-03 05-04 05-06 05-08 05-12 06-01 07-03 07-05Type B. Andes B. Andes Alk. B. Alk. B. Alk. B. Alk. B. Alk. B. Alk. B. Alk. B. N-MORB N-MORB

Ž .Sc ppm 42.12 46.38 53.15 41.20 38.69 39.01 43.06 40.79 33.91 nd ndCr 42.10 38.50 151.40 103.79 102.17 102.25 109.20 107.98 231.59 226.00 228.00Co 43.00 45.90 49.37 49.08 43.64 47.69 40.67 47.81 53.93 46.00 50.00Ni 21.09 21.64 38.36 37.80 36.46 39.16 29.75 37.68 141.48 132.00 115.00Cu 38.79 38.30 54.08 54.55 52.56 54.14 51.40 53.49 46.53 nd ndZn 187.69 193.13 172.85 174.63 166.54 174.65 173.10 174.60 126.08 156.00 124.00Rb 7.72 7.91 10.22 10.35 9.96 10.03 17.28 10.12 12.54 8.15 13.00Sr 258.85 269.21 453.11 467.49 456.16 460.67 448.91 469.64 461.12 106.00 97.20Y 84.57 88.73 48.03 49.61 45.94 49.02 46.91 49.14 37.56 61.00 58.80Zr 411.79 428.30 354.52 370.10 344.89 361.48 358.70 361.25 262.83 188.00 179.00Nb 22.91 24.14 28.29 29.17 27.34 28.41 28.54 28.58 32.86 6.74 6.20Ba 97.53 98.47 152.75 157.15 150.75 154.99 138.44 156.64 156.16 15.10 27.20Pb 1.53 1.44 1.47 1.51 1.46 1.51 1.56 1.49 1.28 0.76 0.65

REELa 23.52 23.76 25.14 26.00 24.35 25.62 24.95 25.58 24.95 7.22 6.63Ce 60.68 60.95 61.55 64.29 60.63 62.20 60.67 62.75 54.55 20.60 19.40Pr 8.95 9.02 8.72 9.04 8.46 8.86 8.75 8.88 7.15 3.51 3.31Nd 41.77 41.86 39.30 40.70 38.34 39.68 39.44 39.95 30.24 18.30 17.20Sm 12.16 12.25 10.08 10.39 9.90 10.28 10.28 10.39 7.35 6.44 6.16Eu 3.59 3.61 3.15 3.25 3.06 3.19 3.20 3.22 2.40 2.03 1.94Gd 14.23 14.32 10.40 10.78 10.09 10.57 10.52 10.60 7.75 8.58 8.17Tb 2.37 2.40 1.59 1.62 1.53 1.60 1.59 1.59 1.19 1.53 1.47Dy 15.30 15.25 9.27 9.54 8.92 9.33 9.29 9.42 7.05 10.20 9.88Ho 3.07 3.08 1.74 1.77 1.67 1.76 1.73 1.77 1.35 2.13 2.05Er 8.97 8.97 4.79 4.90 4.57 4.85 4.70 4.81 3.74 6.07 5.82Tm 1.26 1.28 0.63 0.66 0.61 0.65 0.63 0.64 0.50 0.92 0.88Yb 8.40 8.34 4.03 4.18 3.87 4.11 4.06 4.12 3.24 6.03 5.83Lu 1.22 1.23 0.58 0.60 0.56 0.58 0.57 0.58 0.46 0.88 0.86

Ž .LarSm 1.25 1.25 1.61 1.61 1.59 1.61 1.57 1.59 2.19 0.8704 0.8364N

Yb 49.43 49.06 23.71 24.62 22.79 24.17 23.87 24.21 19.04 35.47 34.29N

Ce 99.16 99.60 100.56 105.05 99.07 101.64 99.14 102.53 89.14 33.66 31.70NŽ .CerYb 2.01 2.03 4.24 4.27 4.35 4.20 4.15 4.23 4.68 0.95 0.92N

ZrrY 4.87 4.83 7.38 7.46 7.51 7.37 7.65 7.35 7.00 3.08 3.04Distance 1430.5 1430.5 1399.9 1399.9 1399.9 1399.9 1399.9 1399.9 1325.7 1250.3 1250.3Ž .km

Ž .Volcanic ridges VR near 1138–1148W PAR

Sample DR 11 DR 11-01 DR 11-03 DR 11-12 DR 12-05 DR 14-05 DR 15-04 DR 15-10 09-02 10-05 BHVO-Type T-MORB T-MORB T-MORB T-MORB E-MORB E-MORB Alk. B. Alk. B. T-MORB N-MORB standard

Ž .Sc ppm 48.27 49.20 49.22 48.06 45.85 32.73 42.67 41.00 54.15 47.45 27.89Cr 252.34 269.10 551.20 439.16 85.65 226.58 21.61 21.14 287.23 347.17 267.24Co 57.06 61.27 69.54 66.32 55.69 53.59 48.05 53.50 57.34 57.31 42.45Ni 165.59 181.73 285.17 258.22 48.96 167.11 23.11 30.44 74.48 103.07 114.32Co 93.64 93.89 101.66 96.89 68.29 52.58 50.63 48.97 86.10 99.34 132.25Zn 82.82 85.35 79.37 82.08 124.57 130.55 127.12 129.46 107.32 90.30 103.13Rb 1.25 1.23 0.80 0.93 5.37 9.16 14.16 13.36 2.22 1.00 8.73


Ž .Table 3 continued

Ž .Volcanic ridges VR near 1138–1148W PAR

Sample DR 11 DR 11-01 DR 11-03 DR 11-12 DR 12-05 DR 14-05 DR 15-04 DR 15-10 09-02 10-05 BHVO-Type T-MORB T-MORB T-MORB T-MORB E-MORB E-MORB Alk. B. Alk. B. T-MORB N-MORB standard

Sr 226.64 228.86 215.62 199.98 386.67 411.06 380.01 370.81 171.29 98.00 378.04Y 30.07 30.60 26.08 27.38 33.20 40.15 33.10 33.19 34.63 25.49 21.95Zr 119.85 122.31 95.09 94.97 190.64 266.46 223.15 219.41 118.89 69.97 155.00Nb 5.05 5.25 2.58 3.47 14.83 26.13 35.58 34.83 6.54 3.39 16.30Ba 15.41 16.43 8.43 10.68 71.03 118.16 145.34 143.88 26.92 14.89 123.55Pb 0.53 0.69 0.44 0.41 0.88 1.10 1.29 1.30 0.48 0.34 1.86

La 5.89 6.08 3.71 4.21 13.28 21.16 25.62 25.17 6.21 3.17 15.13Ce 15.54 15.71 10.66 11.35 31.42 47.80 54.96 53.30 15.95 8.81 35.18Pr 2.35 2.38 1.71 1.78 4.39 6.48 6.89 6.76 2.49 1.43 5.00Nd 10.94 11.07 8.42 8.51 19.74 28.41 28.29 27.76 12.14 7.48 21.91Sm 3.35 3.40 2.70 2.75 5.49 7.45 6.63 6.54 3.98 2.66 5.54Eu 1.22 1.23 1.03 1.03 1.88 2.42 2.14 2.10 1.38 0.97 1.83Gd 4.32 4.34 3.49 3.64 6.18 8.11 6.85 6.80 5.06 3.66 5.56Tb 0.76 0.76 0.63 0.65 0.98 1.26 1.04 1.03 0.88 0.66 0.85Dy 5.11 5.15 4.33 4.47 6.06 7.50 6.29 6.21 5.82 4.45 4.77Ho 1.08 1.09 0.91 0.95 1.17 1.44 1.20 1.18 1.20 0.91 0.86Er 3.24 3.27 2.73 2.90 3.34 3.99 3.34 3.27 3.54 2.75 2.27Tm 0.48 0.47 0.40 0.43 0.45 0.54 0.46 0.45 0.50 0.38 0.31Yb 3.24 3.18 2.71 2.90 2.90 3.53 2.97 2.87 3.28 2.58 1.84Lu 0.48 0.48 0.40 0.44 0.42 0.51 0.42 0.41 0.49 0.37 0.25

ŽLar 1.14 1.16 0.89 0.99 1.56 1.83 2.49 2.49 1.01 0.77.Sm N

Yb 19.05 18.73 15.93 17.08 17.03 20.75 17.50 16.87 19.29 15.20N

Ce 25.39 25.67 17.42 18.54 51.35 78.11 89.80 87.09 26.07 14.40NŽCer 1.33 1.37 1.09 1.09 3.01 3.76 5.13 5.16 1.35 0.95

.Yb N

ZrrY 3.99 4.00 3.65 3.47 5.74 6.64 6.74 6.61 3.43 2.75Distance 218.0 218.0 218.0 218.0 229.0 357.6 370.0 370.0 axis axisŽ .km

Ž . ŽThe analyses were performed by Inductively Coupled Plasma Spectrometry ICP-MS at the University of Kiel Geologisch-Palaontolo-¨.gisches Institut , on hand picked glasses.

Ž .OPSsold Pacific seamount constructed on ancient )20 Ma lithosphere.Ž .FRPs failed rift propagator. The distances km are obtained from the PAR axis.

Dredge FH11 contains picritic basalts.

Ž . Ž . Ž .ZrrY -2 and CerYb -1 than that usedNŽ .here source: 85r15 .

3. Discussions: interaction between hotspot andridge magmatism

An interaction between the mantle plume of theintraplate region and the migrating ridge system has

Ž .been suggested by Schilling 1991 , Zhang and Tani-Ž . Ž . Ž .moto 1992 , Ito and Lin 1995 , Kerr et al. 1995

Ž .and others. Ito and Lin 1995 suggested that excessmantle temperature anomalies of 50–2508C couldinfluence hotspot–ridge interaction, up to a maxi-mum distance of 500 km. The present study indicatesthat the same hotspot giving rise to the FS has

Ž . Žinteracted with ancient 20–23 Ma and recent -5.Ma ridge type magmatism to produce similar types

of linear structures and volcanic cones. In both cases,the hotspot was located less than 400 km away from

Žthe corresponding nearby ridge axis precursor of the. Ž .FRP and PAR Fig. 2b,c . The age constraints on


ŽŽ . .Fig. 7. Regional compositional KrTi, ZrrY, CerYb and T8C of eruption variation of the glassy basalts erupted along the PAR, theNŽ . Ž .Oblique Ridges 0–306 km , the FS between 306 and 1300 km from the PAR axis , the OPS and the FRP constructed on the Farallon crust

Ž . Ž . Ž . Ž)1300 km from the PAR axis a, b, c and d . The best fit regression curve is constructed to guide the readers’ eyes. b and c The ZrrY. Ž .and CerYb ratios variation trends are from bulk rock analyses. d The temperature of eruption were calculated from the glassy basaltsN

Ž .using the experimental data on glassy basalt obtained by Bender et al. 1984 from elsewhere.

the initiation of the hot spot may be extrapolatedfrom the reconstruction of the microplate starting 20

Ž Ž .Ma ago Tebbens and Cande 1997 . Probably bothFRP and the east Ridge precursor of the PAR were

Fig. 8. Sketched model of intraplate-ridge interaction between FS and PAR magmatism in the South Pacific near 378S–1118W, assumingŽ .that the FS hotspot is fixed. Depleted N-MORB type of melts are produced at shallower depth underneath the PAR axis, while enriched

Ž . Ž .magmas alkali basalt are formed deeper in the mantle underneath the FS dredge stations FH14, FH15, 69 and 70 prior to 5 Ma. SinceŽ .then, that is from 5.8 Ma to about 2.7 Ma ago magma mixing between depleted and enriched alkali basalt melts have produced T- and

Ž . Ž . Ž .E-MORBs essentially. a The formation of the first set of VR oriented 0708 started at this period about 5 Ma with the extrusion ofŽ . Ž . Ž . Ž .mainly alkali basalts dredge sites FHF14, FH15 and MORBs FH13, FH12, 93 . b The formation about -4 Ma ago of the other set

Ž . Žoriented 2868 of VR occur from shallower partial melting of a more ridge centered heterogeneous mantle plume FS plume plus PAR.mantle material with diminuishing supply of deeper enriched melts and a continued eruption of essentially T-MORBs which lasted until

less than 1 Ma. The lack of enriched alkali basalts in this period might be due to the larger extent of melting of more depleted meltproduction andror decrease in deep mantle plume supply. Another alternative hypothesis is that the FS mantle plume is still underneath the

Ž Ž ..present location of sample sites FH14, FH15, 69 and 70 as about 5 Ma ago, see a and the gradual compositional changes observed withinŽ .the VR to the PAR axis volcanics is the result of mixing between the two mantle sources FS less vigorous than the PAR . At the present

time, the FS mantle plume might still be effective in maintaining high thermal conditions within the lithosphere at the level of the PARX X Ž . Ž .segments between 37810 S and 38820 S. spr indicates the relative spreading rates between the Pacific about 50 mmryear and Antarctic

Ž .6 mmryear plates.


active at 20 Ma and correspond to the constructionof the seamount at dredge station So28 along the tip

Žof the FRP. The linear structures having 2968 and.2508 directions associated with the accreting ridge


segments may represent large ridge axial discontinu-Ž . Žities transform faults andror faulted graben pull-

.apart formed during the plate reorientation and read-Ž .justment Fig. 2b,c . These types of linear ridges


were previously described from another intraplateregion located between the Crough seamount and the

ŽEaster microplate Searle et al., 1995; Hekinian et.al., 1995a . The compositional variabilities from al-

kali basalts to less enriched MORBs observed for theŽvolcanics erupted on the various structures off-axis

.volcanoes and ridge axes suggests that there wassome extent of mixing between two types of mantle:Ž . Ž .1 an enriched mantle source with high ZrrY )6

. Ž .and CerYb )4 giving rise to the alkali basaltsNŽ .and 2 a more depleted source with lower inŽ . . Ž .ZrrY -5 and CerYb -3 giving rise toN

Ž .MORBs Table 3 . The regional pattern of basaltsŽ .N-, T- and E-MORBs and alkali basalts variabilityis more pronounced among the volcanics from theVR–PAR and OPS–FRP provinces than those from

Ž . Ž .the FS alkali basalts Fig. 7a,c,d . The highestŽ .temperature of eruptions 1200–12208C are encoun-

tered on the VR–PAR and the lowest for the FSŽ . Ž .1170–12008C volcanics Fig. 7b . This lower tem-perature anomaly for the FS plume volcanics isprobably related to the higher volatile content in thealkali melts than the MORBs. It is also observed thatthe FS samples have generally higher degrees of

Ž .rock vesicularity )10–15 vol% vesicles whenŽcompared to the other N- and T-MORBs Hekinian

.et al., 1997 .On the basis of a reconstruction of the Pacific–

Ž .Antarctic plate, Hekinian et al. 1997 showed thatthe mantle plume 5 Ma ago in the region of the VRŽ X X . Ž .36820 S–114830 W was nearer 110 km to the

Ž X .PAR 37830 S–1118W axis and was able to channelenriched mantle components towards the ridge axis.

ŽAlso, because the Pacific plate is moving faster 50.mryear; Mayes et al., 1990; Searle et al., 1993 than

Ž .the Antarctic plate 6 mmryr , the PAR axis isŽcloser today to the FS hotspot Hekinian et al.,

.1997 . This suggests that the off-axis mantle plumeis today nearer andror underneath the PAR axis andis less effective in transporting deep mantle plume

Ž .material Shen et al., 1995; Niedermann et al., 1997to shallower levels undernath the lithosphere wheremore depleted mantle components are produced. In

Ž .this scenario, the ascent of melts from deep plumemantle melting zones decreases as the PAR axis

Ž .approaches the FS hotspot Fig. 8b . The variableŽdegrees of mixing of mantle plume material en-

.riched in incompatible elements with that givingŽ .rise to ridge magmatism depleted N-MORB was

responsible for the production of the E- and T-MORBs encountered on the VR located between the

Ž .FS sites FH14, -15, -69 and -70 and the PAR axisŽ .at about 5–2 Ma Fig. 8 . In addition, the area where

the VR intersects the ridge axis is marked by theeruption of T-MORBs as well as silicic lavas alongthe strike of the PAR axis for about 110 km between

X X Ž .37810 S and 38820 S Fig. 2a,b . This is believed tobe caused by the influence of the FS hotspot whichincreased the regional temperature of the lithosphereand enhanced the existence of a large magmaticreservoir which would allow extensive crystal–liquidfractionation and the extrusion of viscous lavas.

Another alternative hypothesis is that the mantleplume is located underneath the present day FS

Žhotspot position whose eastern boundary mantle.front is delineated by the eruption of enriched lavas

Ž .alkali basalts at dredge sites FH14, FH15, 69 and70, which are at about 360–400 km from PAR axisŽ .Fig. 2b and comparable to Fig. 8a . This is in

Ž .agreement with the model of Schilling 1991 regard-

Ž .Fig. 9. Sketched reconstruction of the major volcanic structures based on the tectonic evolutionary model of Tebbens and Cande 1997Ž .from early Oligocene 20–25 Ma . This time span was chosen in order to include the formation of the two sets of the VR on which were

Ž .built the OPS during the construction of the Selkirk microplate. a The spreading ridge axis and the Delcano transform of theŽ . Ž .Farallon–Pacific plates are shown at 25 Ma. Depleted melt is produced on a ridge axis and transform stations FH04 . b and c The off-axis

Ž . Ž .volcanic ridges Fig. 2c might represent ancient discontinuities transform zones filled by volcanic cones during lithospheric readjustment.Ž . Ž .This could have resulted from extentional and shearing motions of the Pacific PAC –Farallon FAR plates during the construction of the

Ž . Ž .Selkirk microplate 23–20 Ma , while enriched magmas alkali basalt are formed in an area of about 150 km in diameter including dredgestations FH05, 06 and So28 at 20–23 Ma ago. Station FH01 containing andesitic lava was thermally influenced by the hotspot along theFRP. This scenario is somewhat similar to that suggested for the origin of the VR in the vicinities of the PAR -7 Ma ago. In both cases,the interaction of the Foundation hotspot with accreting ridge magmatism was prominent for the formations of these off-axial volcanic

Ž . Ž . Ž .structures. c After the construction of the microplate, hotspot volcanism FS continued sites 28 and 22–25 , the western ridge propagatorŽ . Ž . Ž .failed FRP and the eastern ridge precursor of the modern PAR jumped eastwards -20 Ma; Tebbens and Cande, 1997 .


ing mantle sourcermigrating ridge sink, where theplume source could feed and dynamically affect amagmatic flow at the base of the lithosphere. In thiscase, the formation of the tall volcanic edifices which

Ž .are shallower 1500–1700 m depths than the PARŽ .axis 2200 m is the result of intermittent off-axial

volcanic activities, which might explain the absenceof fresh lava flows on the VR sites. However, withthis model, one could expect to find melts erupted onthe PAR axis which are as enriched as those found atthe FS sites; this is not the case from our presentobservation. Instead we observe a diminishing inputof FS plume components at the PAR axis.

Ž .The similarities between the recent 0–7 MaŽ .VR–PAR structures and the older 20–23 Ma OPS

structures is seen in their respective geological set-ting and in their range of compositional variabilitiesof the volcanics. Alkalic lavas, and MORBs areassociated with the volcanic cones topping linear

Ž .ridges and the FRP Fig. 2b . The first occurrencesŽ .of undepleted alkali-basalt lavas found in the re-

Žgion of dredge sites So28, FH5 and FH6 near.348S–1258W covers an area of about 150 km in

diameter. This is believed to coincide with the initia-tion of the FS volcanic chain as well as the begin-

Žning of the Selkirk microplate 20–23 Ma ago Fig..9a,b . The creation of the Selkirk microplate with its

western boundary represented by the FRP took placeduring the reorientation of the Pacific and Farallonplates which was responsible for the stretching andthinning of the lithosphere in this region. Such dif-ferential motions of plate reorganization gave rise to

Ž .local faulting pull-apart and facilitated localizedmagmatic upwelling. The formation of the Delcanoridge might correspond to this period of plate read-

Ž . Žjustment giving rise to leaky transforms FH04 Fig.. Ž9b . The production of enriched lavas alkalic and

.E-MORBs caused the construction of the volcanicŽ .cones at the sites of FH5 and FH6 Fig. 9a and b .

When the formation of the Selkirk microplate beganŽ .20–23 Ma , the ridge type of magmatic activityshifted to the eastern spreading branch while thewestern branch continued to propagate southward

Žgiving rise to the N-MORB type of volcanism sta-. Ž .tion FH07 , N-MORB dredge FH04 , and T-MORBs

Ž .dredges 13–19, Hekinian et al., 1997 . The an-Ž .desitic basalts dredge FH1 found on the FRP and

on the seamounts near 32830XS–127830X W, are be-

lieved to have been formed from a spreading ridgetype of magmatism taking place on the old Farallon

Ž .plate, precursor of the FRP Fig. 9a,b . The broadarea covered by the FS hotspot magmatism at dredgesites So28, FH05 and FH06 at the beginning of theSelkirk microplate, could have enhanced the produc-tion of the T-MORB and the andesitic lavas, in thesame way as for the PAR silicic lavas. Hotspot

Ž .volcanism site 28 continued when the western ridgeŽ .of the microplate became a failed rift FRP and the

Ž .eastern ridge precursor of the PAR jumped furtherŽeast about 20–16 Ma ago Tebbens and Cande,

. Ž .1997 Fig. 9c .The mantle plume activity giving rise to the FS,

and its interaction with the ridge type of magmatismin the OPS–FRP and VR–PAR provinces, has not

Žchanged in composition Devey et al., 1996; Hekinian. Žet al., 1997 nor decreased in intensity size of the

.edifices from at least the last 20–23 Ma to about 5Ma ago. This observation agrees with that made by

Ž .Phipps Morgan 1997 concerning the independenceof hotspot melting from the overriding lithosphericages.

4. Conclusions

The interaction between a mantle plume givingrise to the FS and spreading ridge magmatism hasbeen responsable for the formation of off-axialseamounts and volcanic ridges in the area locatedbetween the resolution fracture near 32856X–130845X W and the PAR near 37830X–110850X W.

ŽThese ridges -2000 m high, -350 km long and.-50 km wide are topped with volcanic conesŽ .forming the OPS 20–25 Ma near 348S–1278W and

Ž .the younger -7 Ma VR near 378S–1138W. TheFS between 34815X S–122814X W and 37822X S–

X Ž114806 W about 1000 km in length and 200 km.wide , were constructed from the upwelling of a

Žmantle plume located about 1300 km near 348S–.1258W at dredge stations So28, FH06 and FH05

from the present day PAR axis. The last eruptedenriched melts of hotspot origin are identified in anarea comprised between 35845–1148W and 37835XS–

X Ž114820 W located at about 306–410 km dredge.sites So29, So70, FH14 and FH15 from the present

day PAR axis, on crust about 5 Ma old.


Ž . ŽThe FS mantle plume started 20–23 Ma Teb-.bens and Cande, 1997 during andror slightly before

the formation of the Selkirk microplate and gave riseŽmainly to alkalic lavas sample sites So28, FH6 and

. Ž . Ž .FH5 with high KrTi )0.33 , ZrrY )6 and. Ž .CerYb )4 . The Selkirk microplate is boundedN

to the west by the FRP and to the east by theprecursor of the PAR spreading axis. The presence

Ž .of basaltic andesites samples FH1-01, -02 withŽ .relatively high SiO 52–53% is believed to be due2

to axial or near off-axial volcanism related to anancient spreading center associated with thePacific–Farallon plates. The basaltic andesites and

Ž .the N-MORBs sites FH7 and FH4 sampled fromŽ .the OPS, the FRP and the Delcano FH4 ridge,

respectively, are comparable to the present day vol-canics encountered on the PAR axis. These latervolcanics were probably erupted on the ancientspreading ridge of the Pacific–Farallon plates duringandror prior to the initiation of the FS hotspot andof the Selkirk microplate.

The most detailed survey conducted from theŽ X .western limit 37822 S–1148W of the VR coincid-

ing with the FS hotspot up to the PAR axis shows achange in the volcanics composition going from

walkali basalts KrTi ) 0.30, ZrrY ) 6 andŽ . x wCerYb )4 to E- KrTis0.25–0.33, ZrrYsN

Ž . x w5–6 and CerYb s3–4 and T-MORBs KrTisNŽ . x0.11–0.25, ZrrYs2–4 and CerYb s1–2 , es-N

sentially. This compositional variability from thehotspot to more of a ridge type magmatism is also

Ždocumented by the mineralogical plagioclase com-.position and petrological changes involving differ-

ent parental magmatic sources. It is believed that ahotspot magma produced by the FS mantle plumewas mixed with the PAR type of magmatism toproduce the T- and E-MORBs observed along the

Ž .VR -5 Ma . However, in order to explain thedecrease in melt enrichment in most VR samples it islikely that there is a diminishing supply of FS mantleplume material towards the PAR axis.

The PAR axis at the level of its intersection withthe VR near 37830XS–110850X W, is made up essen-

Ž .tially of transitional T MORBs, Fe–Ti basaltsŽ . Ž .sample 106-02 , andesites SiO s55–59% , and2

Ž .dacites SiO s60–65% . They are comparable to2

the T-MORBs and the andesitic lavas of the OPSformed on the ancient spreading ridge in the

Pacific–Farallon plate. The presence of silicic lavason the PAR and the OPS suggests the existence ofthermal anomalies related to a mantle plume affect-ing the lithosphere during the eruption of these vis-cous lavas.

Acknowledgements

We are grateful to Capts. J.C. Gourmelon and H.Andresen, the officers and the crew of the R.V.

Ž .L’Atalante and Sonne leg 100 for their help andpatience during the ‘Foundation’ and the ‘Hotline’cruise, respectively. We are also grateful to thescientific colleagues participating on these cruises.These cruises were initiated within the framework ofthe French–German collaboration on the study ofIntraplate Volcanism. We are grateful to Ronan Ap-prioual for his invaluable help in the handling of thesamples and providing the thin sections on board.The microprobe analyses were performed on aCamebax SX50 at IFREMER. The bulk chemicalanalyses were done at the University of KielŽ . Ž .Germany . We are indebted to BMFT Germany ,IFREMER, INSU, and the University of BretagneOccidentale for their support. We are indebted toDrs. H. Chamley, R. Batiza, and an anonymousreviewer who helped us a great deal in amelioratingthe manuscript.

References

Bach, Hegner, W.E., Erzinger, J., Satir, M., 1994. Chemical andisotopic variations along the superfast spreading East PacificRise from 6 to 308S. Contrib. Miner. Petrol. 116, 365–380.

Bender, J.F., Langmuir, C.H., Hanson, G.N., 1984. Basalt glassesfrom the Tamayo region, East Pacific Rise. J. Petrol. 25,213–254.

Bideau, D., Hekinian, R., 1995. A dynamic model for generatingsmall-scale heterogeneities in ocean floor basalts. J. Geophys.Res. 100, 10141–10162.

Cande, S., Haxby, W.F., 1991. Eocene propagating rifts in theSouthwest Pacific and their conjugated features on the NazcaPlate. J. Geophys. Res. 96, 19609–19622.

Craig, C.H., Sandwell, D.T., 1988. Global distribution ofseamounts from Seasat profiles. J. Geophys. Res. 100, 17931–17946.

Devey, Hekinian, C.R., Stoffers, P., Ackermand, D., Binard, N.,


Drusch, M., Francke, B., Hemond, C., Kapsimalis, V., Lorenc,S., Maia, M., Connor, O., Perrot, K., Pracht, J., Ramm, D.,Rogers, T., Statteger, K., Victor, P., 1996. A first survey andsampling of the Foundation Seamount Chain, S.E. Pacific.Marine Geol. 137, 191–200.

Frey, F.A., 1980. The origin of pyroxenites and garnet pyroxenitesfrom Salt Lake Crater, Ohau, Hawaii: trace element evidence.Am. J. Sci. 280A, 427–449.

Frey, F.A., Suen, C.J., Stockman, H.W., 1985. The Ronda hightemperature peridotite: geochemistry and petrogenesis.Geochim. Cosmochim. Acta 49, 2469–2491.

Haxby, W.F., 1987. Gravity field of the world’s oceans, U.S.Office Naval Res.

Haxby, W.F., Weissel, J.K., 1986. Evidence for small-scale con-vection from Seasat altimeter data. J. Geophys. Res. 91,3507–3520.

Hemond, C., Devey, C., 1997. The foundation seamount chain:´first isotopic evidence of a newly discovered hotspot track,Ž .abstract presented at goldsmith conference. Heilderberg 1,255.

Hekinian, R., Stoffers, P., Ackermand, D., Binard, N., Francheteau,J., Devey, C., Garbe-Shonberg, D., 1995a. Magmatic evolu-

Žtion of the easter microplate-crough seamount region South.East Pacific . Marine Geophys. Res. 17, 375–397.

Hekinian, R., Bideau, D., Hebert, R., Niu, Y., 1995b. Magmatism´Ž X .in the Garrett transform fault East Pacific rise near 13827 S .

J. Geophys. Res. 100, 10163–10185.Hekinian, R., Stoffers, P., Devey, C., Ackermand, D., Hemond,´

C., O’Connor, J., Binard, N., Maia, M., 1997. Intraplate versusridge volcanism on the Pacific, Antarctic Ridge near 378S–1118W. J. Geophys. Res. 102, 12265–12286.

Irving, A.J., 1980. Petrology and geochemistry of compositeultramafic xenoliths in alkalic basalts and implications formagmatic processes within the mantle. Am. J. Sci. 280, 389–426.

Ito, G., Lin, J., 1995. Oceanic spreading center–hotspot interac-tions: Constraints from along isochron bathymetric and gravityanomalies. Geology 23, 657–660.

Kerr, A.C., Sounders, A.D., Tarney, J., Berry, V.H., Hards, V.L.,1995. Depleted mantle-plume geochemical signatures: noparadox for plume theories. Geology 23, 843–846.

Kincald, C., Ito, G., Gable, C., 1995. Laboratory investigation ofthe interaction of off-axis mantle plumes and spreading cen-tres. Nature 376, 758.

Lonsdale, P., 1994. Geomorphology and structural segmentationŽ .of the crest of the southern Pacific–Antarctic East pacific

Rise. J. Geophys. Res. 99, 4683–4702.Mahoney, J.J., Sinton, J.M., Kurz, M.D., Macdougall, J.D.,

Spencer, K.J., Langmuir, C.W., 1994. Isotope and trace ele-ments characteristics of a super-fast spreading ridge: EastPacific Rise, 13–238S. Earth Planet. Sci. Lett. 121, 173–193.

Maia, M., Diament, M., 1991. An analysis of the altimetric geoidin various wave bonds in the central Pacific Ocean: constraintson the origin of intraplate features. Tectonophysics 190, 133–153.

Maia, M., Hekinian, R., Ackermand, D., Delghani, G.A., Gente,P., Naar, D., O’Connor, J., Perrot, K., Phips Morgan, J.,

Ramillien, G., Revillon, S., Sabetian, A., Sandwell, D., Stof-´fers, P., 1999, The Foundation Seamounts: A Ridge–hotspotinteraction in the South Pacific, Geology, submitted.

Mammerickx, J., 1992. The Foundation Seamounts: tectonic of anewly discovered seamount chain in the South Pacific. EarthPlanet. Sci. Lett. 113, 293–306.

Mayes, C.L., Lawver, L.A., Sandwell, D.T., 1990. Tectonic his-tory and new isochron chart of the South Pacific. J. Geophys.Res. 95, 8543–8567.

Molnar, P., Atwater, T., Mammerickx, J., Smith, S.M., 1974.Magnetic anomalies, bathymetry and the tectonic evolution ofthe south Pacific since the Late Cretaceous. Geophys. J.R.Astron. Soc. 40, 383–420.

Naar, D.F., Martinez, F., Hey, R.N., Reed, T.B. IV, Stein, S.,1991. Pito Rift: How large-offset rift propagates. MarineGeophys. Res. 13, 287–309.

Niedermann, S., Bach, W., Erzinger, J., 1997. Noble gas evidencefor a lower mantle component in MORBs from the southernEast Pacific Rise: decoupling of helium and neon isotopesystematics. Geochim. Cosmoschim. Acta 61, 2697–2715.

Nielsen, R.L., 1988. A model for the simulation of combinedmajor and trace element liquid lines of descent. Geochim.Cosmochim. Acta 52, 27–38.

Nielsen, R.L., 1990. Theory and application of a model of openmagmatic system processes. In: Nicholls, J., Russell, J.K.Ž .Eds. , Modern Methods of Igneous Petrology. Reviews inMineralogy, vol. 24, Mineralogical Society of America, Wash-ington DC, pp. 65–106.

Nielsen, R.L., Delong, S.E., 1992. A numerical approach toboundary layer fractionation: application to differentiation innatural magma systems. Contrib. Mineral. Petrol. 110, 355–369.

Niu, Y., Waggoner, D.G., Sinton, J.M., Mahoney, J.J., 1996.Mantle source heterogeneity and melting processes beneathseafloor spreading centers: the East Pacific Rise, 18–198S. J.Geophys. Res. 101, 27711–27733.

O’Connor, J., Stoffers, P., Wijbrans, J.R., 1998. Migration rate ofvolcanism along the Foundation Chain, SE Pacific. EarthPlanet. Sci. Lett. 164, 41–59.

Phipps Morgan, J., 1997. The generation of a compositionallithosphere by mid-ocean ridge melting and its effect onsubsequent off-axis hotspot upwelling and melting. EarthPlanet. Sci. Lett. 146, 213–232.

Poreda, R.J., Schilling, J.G., Craig, H., 1986. Helium and hydro-gen isotopes in ocean-ridge basalts north and south of Iceland.Earth Planet. Sci. Lett. 78, 1–17.

Ribe, N.M., Christensen, U.R., Theibing, J., 1995. The dynamicsof plume–ridge interaction: 1. Ridge-centered plumes. EarthPlanet. Sci. Lett. 134, 155–168.

Sandwell, D.T., 1984. A detailed view of the South Pacific geoidfrom satellite altimetry. J. Geophys. Res. 89, 1089–1104.

Sandwell, D.T., Smith, W.H.F., 1995. Marine Gravity Anomalyfrom Satellite Altimetry, Geological Data Center Scripps Insti-tution of Oceanography, La Jolla, CA.

Schilling, J.-G., 1991. Fluxes and excess temperatures of mantleplumes inferred from their interaction with migrating mid-oc-ean ridges. Nature 352, 397–403.


Searle, R.C., Bird, R.T., Rusby, R.I., Naar, D.F., 1993. Thedevelopment of two oceanic microplates: easter and JuanFernandez microplates, East Pacific Rise. J. Geological Soc.London 150, 965–976.

Searle, R.C., Francheteau, J., Corniglia, B., 1995. New observa-tions on mid-plate volcanism and the tectonic history of thePacific Plate. Earth Planet. Sci. Lett. 131, 395–421.

Sinton, J.M., Smaglik, S.M., Mahoney, J.J., Macdonald, K.C.,1991. Magmatic processes at superfast spreading oceanicridges: glass variations along the East Pacific Rise, 138S–238S.J. Geophys. Res. 96, 6133–6155.

Shen, Y., Scheirer, D.S., Forsyth, D.W., Macdonald, K., 1995.Trade-off in production between adjacent seamount chainsnear the East Pacific Rise. Nature 373, 140–143.

Small, C., 1995. Observations of ridge–hotspot interactions in theSouthern Ocean. J. Geophys. Res. 100, 17931–17946.

Sun, S.-s, MacDonough, 1989. Chemical and isotopic systematicsof oceanic basalts: implications for mantle compositions and

Ž .processes. In: Saunders, A.D., Norry, M.C. Eds. , Magmatismin the Ocean Basins. Geol. Soc. Spec. Publication, 42, pp.313–345.

Tebbens, S.F., Cande, S.C., 1997. Southeast Pacific tectonicevolutionfrom from early Oligocene to present. J. Geophys.Res. 148, 12063–12084.

Weaver, J.S., Langmuir, C.H., 1990. Calculation of phase equilib-rium in mineral-melt systems. Comput. Geosci. 16, 1–19.

Zhang, Tanimoto, T., 1992. Ridges, hotspots and their interactionas observed in seismic velocity maps. Nature 355, 45–49.

Ridge–hotspot interaction: the Pacific–Antarctic Ridge and the foundation seamounts

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