Kerguelen basic and ultrabasic xenoliths: Evidence for long-lived Kerguelen hotspot activity

LITHOS 0

ELSEVIER Lithos 37 ( 19%) 26 l-280

Kerguelen basic and ultrabasic xenoliths: Evidence for long-lived Kerguelen hotspot activity

N. Mattielli a, D. Weis a, M. Gr6goire b, J.P. Mennessier a, J.Y. Cottin b, A. Giret b a Universitt Libre de Bruxelles, Unite’ de Pgtrologie et Giodynamique chimique CP160/02, Au. F.-D. Roosevelt 50, 1050 Brussels,

Belgium b Universiti Jean Monnet, Laboratoire de Gblogie-CNRS UAIO, Rue du Docteur Paul Michelon 23,42023 Saint-Etienne Ckdex 02,

Frunce

Received 15 October 1994; accepted 15 September 1995

Abstract

The xenoliths from the Southeast Province of the Kerguelen Archipelago derived from the lower crust or the upper mantle, can contribute to define the characteristics of the mantle sources below Kerguelen and improve the constraints on the formation of the Kerguelen Islands and plateau. Our petrographic, geochemical and isotopic (Sr, Nd and Pb) study focuses on peridotites (Type la: harzburgite/clinopyroxene-poor lherzolite and Type Ip: dunite), 2-pyroxenes-spine1 bearing ultrabasic and basic xenoliths [Type Ila: clinopyroxene-rich lherzolite, wehrlite, ( f olivine f plagioclase) websterite, (+ garnet f sapphirine) metagabbro and anorthosite] and ibnenite metagabbros (Type UC).

The large ranges of isotopic ratios for the xenoliths reflect different degrees of interaction between a depleted MORB-type component, quite abundant in the Type II xenoliths, and the Kerguelen plume, distinctly predominant in the Type I xenoliths. Type I peridotites are residues of a previous partial melting event of the Kerguelen plume; residues that subsequently interacted with a percolating alkaline melt.

2-pyroxenes-spine1 bearing ultrabasic and basic xenoliths (Type IIa) and ilmenite metagabbroic xenoliths (Type 11~) are deep cumulates crystallized from tholeiitic magmas.

The isotopic results for the xenoliths strengthen the hypothesis of an oceanic origin for the Kerguelen Islands and refute the existence of pieces of old continental crust beneath the Islands and the northern part of the Kerguelen Plateau. They also confirm the importance of plume-spreading ridge interactions throughout the history of the Kerguelen plume.

The isotopic and geochemical characteristics of the Type IIa and IIc xenoliths are consistent with the hypothesis of an Iceland-type setting for the northern part of the Kerguelen Plateau. The results for the Type I xenoliths on the other hand suggest a similarity between the Hawaii-type midplate volcanic structure and that of Kerguelen Islands.

The isotopic data suggest that the Kerguelen xenoliths were formed recently ( 5 45 Ma), and thus support the hypothesis of the formation of the Plateau by the arrival of the plume at the base of the lithosphere (- 115 Ma ago). The Plateau would have grown through several pulses of plume activity ( - 115, _ 80, _ 40 Ma), while the geotectonic environment changed with time (from a ridge-centered position to the present intraplate position). The occurrence of deep Type IIa and IIc xenoliths can explain the crustal thickening and provides evidence for the growth of oceanic plateaus by vertical accretion.

00244937/%/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0024-4937(95)00040-2

262 N. Mattirlli et ul./Litho.s 37 (19961261-280

1. Introduction

The Kerguelen Islands (Fig. 1) are located in the Southern Indian Ocean (48”30’-5O”S, 68”30’-71 “El.

They lie on the northern part of the second largest

oceanic plateau: the Kerguelen Plateau/Broken

Ridge LIP (Large Igneous Province; Coffin and Eld-

holm, 1994). The formation of this LIP has been attributed to the activity of the Kerguelen plume, a long-lived source of magmatism (- 115 m.y.>. The

emplacement of the LIP started with the formation of the - 115-l 10 Ma southern part of the Kerguelen

Plateau, contemporary with the building of the 116

Ma Rajmahal Traps (Mahoney et al., 1983; Baksi,

QUATERNARY DEPOSITS

PLUTONIC COMPLEXES

SUBACTUAL VOLCANOES

SAMFLED XENOLITHS LOCALITIES

PLATEAU

CENTRAL

Fig. I. Geological sketch map of Kerguelen Archipelago (modified from Gautier et al., 1990) showing the different major formations and

the locations of samples (solid diamonds) discussed in the text. Inset shows the major tectonic features of the eastern Indian Ocean (adapted

from Mahoney et al., 199.5). Ocean Drilling Program (ODP) and Deep Sea Drilling Project (DSDP) drill sites are indicated as dots.

N. Mattielli et al. / Lithos 37 (1996) 261-280 263

1994). It continued with the formation of the central Kerguelen Plateau and Broken Ridge (- 85 Ma), and finally that of the northernmost part of the Kerguelen Plateau and Kerguelen, Heard and Mc- Donald Islands (- 45-O Ma) (Barling and Gold- stein, 1990; Duncan, 1991; Royer and Coffin, 1992; Weis et al., 1992; Munschy et al., 1994). The formation of the Ninetyeast Ridge seems to have captured the plume for 82-39 Ma (Weis and Frey, 1991). The tectonic setting of the Plateau has changed with time, as for instance reflected by the evolution of the archipelago from a ridge-centered position, above or close to the Southeast Indian Ridge (SEIR) 45 Ma ago, to its present intraplate position.

Geological activity on the Kerguelen Islands has focused on three specific themes. Firstly, initial re- search focused on the numerous alkaline volcano- plutonic complexes that are quite unusual in an oceanic environment. This brought up the question of a continental origin for the archipelago (Watkins et al., 1974; Giret, 1990; Weis and Giret, 1994).

Secondly, studies focused on the basaltic series. Large variations in the isotopic compositions (con- tinuum between EM1 and EM21 of basalts, and plutonic complexes (Doss0 and Murthy, 1980; Storey et al., 1992; Weis et al., 1992; Weis and Giret, 1994) were interpreted (Gautier et al., 1990) as reflecting variable degrees of mixing between a depleted MORB mantle component, quite abundant in the older tholeiitic basalts, and a Kerguelen plume component (carrier of the DUPAL isotopic anomaly; Hart, 1984; Dupre and AlEgre, 19831, distinctly predominant in the younger highly alkaline rocks. The isotopic ratios of Kerguelen Islands basalts do not correspond to a simple systematic temporal evolution. They reflect variations of the mixing proportions between plume and MORB components on a very short time scale, as the spreading ridge axis migrated away from the archipelago; they also illus- trate the complexity of plume-ridge interaction processes (Frey et al., 1994).

Thirdly, more recent studies, and the focus of this paper, deal with the basic and ultrabasic xenoliths of the Kerguelen Islands in order to provide informa- tion about the deep structure of the Kerguelen Plateau and about the processes which lead to the formation of the major Kerguelen Plateau/Broken Ridge LIP and the role of the Kerguelen plume. By extension

these studies are necessary to understand the mecha- nisms that control the formation of oceanic plateaus and their thickening.

This paper presents the first isotopic (Sr, Nd and Pb) and geochemical data, together with some petrological observations, on Kerguelen ultrabasic and basic xenoliths. These xenoliths were sampled in highly alkaline basaltic rocks from the Southeast Province of the Kerguelen Archipelago. We compare their geochemical and isotopic features with those of the basaltic series of the Archipelago and Plateau and with other features related to the activity of the Kerguelen plume. The results from this study help in answering/constraining some unresolved questions, such as: do the xenoliths confirm the occurrence of the DUPAL anomaly in the upper mantle below Kerguelen?; what is the evidence for continental material under the northern part of the Kerguelen Plateau?; how do the xenoliths constrain the temporal evolution of the Kerguelen plume and related to that, is it pure coincidence that the Kerguelen Islands are the oldest islands in the youngest ocean?

2. Geological setting

High resolution satellite altimetry data, seismic reflection studies and age constrains lead to the division of the Kerguelen Plateau into four distinct tectonic domains: the northern and southern parts; the Labuan Basin and the Elan Bank (KGnnecke and Coffin, 1994). Gravity and seismic data together with the study of sedimentary sections, lead Coffin and Eldholm (1994) to infer a breakup between the Mesozoic and Cenozoic parts of the Plateau just south of the Kerguelen Archipelago.

The northern Kerguelen Plateau (46”-54%) is formed by basaltic crust and is considered to be dominantly volcanic. The southern part (54”-64%) is characterized by the absence of volcanic morphol- ogy, because of subaerial erosion preceding subsidence (Schaming and Rotstein, 1990). It is tectoni- cally much more complex, containing uplifts, block faulting, horst and graben structures and various types of evidence for subsidence (Royer and Coffin, 1992).

Seismic refraction experiments (Charvis and Op- erto, 19921, supported by gravity modelling (Houtz

264 N. Mottirlli et ul. / Lrthm 37 (1996) 261-280

et al., 19771, indicate crustal thicknesses for the Cretaceous Kerguelen Plateau of between 20 to 25 km. The crustal structure below the Archipelago differs significantly from that of the rest of the Plateau. The upper crust is indeed thicker, - 10 km, and the lower crust thinner, - 4-10 km (Recq et al., 1990; Charvis et al., 1993).

The dominant lithology of Kerguelen Islands is basaltic lava flows, which have been intruded by rare volcanic necks and numerous dikes bearing abundant xenoliths. The Southeast Province is characterized by several extrusions of differentiated lavas, which form plugs, cupolas, sugar-loaf structures, dome needles and lava flows. There are two distinct volcanic series in the SE Province: a lower Miocene (20-22 Ma) basalt-trachyte series, including the Diime Rouge intrusion, and an upper Miocene (6.6-10.2 Ma) series (basanites, tephriphonolites and phonolites) (Nougier et al., 1983; Leyrit, 1992; Weis et al., 1993).

Xenoliths of the SE Province were sampled in the upper Miocene highly-alkaline basaltic series at Mt. Tizard (10.2 _t 0.3 Ma pipe; Nougier et al., 1983), D6me Rouge (basanitic dike) and Pointe Suzanne (limburgitic flow) on the Jeanne d’Arc and Prince de Galles peninsulas, respectively. Mt. Tizard and Dame Rouge are the dominant exposures of xenoliths in the SE Province (Fig. 1) (L.eyrit, 1992; Grkgoire, 1994).

3. Petrological description

The sampling in the Kerguelen SE Province has provided abundant xenoliths, characterized by a wide variety of petrographic types, derived from the lower crust or upper mantle. They can be classified into seven compositional groups, according to the Frey and Prinz scheme (1978), on the basis of structural and mineralogical characteristics (Grkgoire, 1994): Type I - peridotites; Type IIa - 2-pyroxenes-spine1

DBme Rouge 41%

Mont Tizard 34%

Pointe Suzanne 7%

Other Outcrops 18%

Type la

Type IP

Type I/a

Type I/b

Type Ill

,‘,<,* q ,L,\,\ \\\ Composites

Fig. 2. Relative proportions of the 7 compositional groups of xenoliths and percentage of the sampled xenoliths from the different

provenances in the Southeast Province. The total percentage of the different xenoliths arc: 20% of Type la (harzburgite/clinopyroxene-poor

Iherzolite); 15% of Type ID (dunite); 45% of Type lla (2-pyroxenes-spine1 bearing ultrabasic and basic xenoliths); 10% of Type Ilb

(clinopyroxene-ilmenite-spine1 ultrabasic and basic xenoliths); 7% of Type llc (ilmenite metagabbros); 2% of Type Ill (Illa = homblenditic,

lllb = biotitic series); 1% of composites (comprising Type I p associated with Type lla or Type Ill, and Type lla associated with Type lib)

(CZgoire, 1994).

N. Mattielli et al./ Lithos 37 (1996) 261-280 265

bearing ultrabasic-basic xenoliths; Type IIb - clinopyroxene-ilmenite-spine1 bearing ultrabasic- basic xenoliths; Type IIc - ilmenite metagabbros; Type IIIa - hornblenditic series; Type IIIb - biotitic series. The relative proportions of the xenoliths and the groups from the different localities of the SE Province are illustrated in Fig. 2. We have selected representative samples of the four most abundant groups of xenoliths:

3.1. Type ICX : Harzburgites, clinopyroxene-poor lherzolites - Type Ifi: Dunites

Coarse grained peridotites contain olivine (Fo,,_,, in Type I (Y ; Fo,,_~, in Type I p 1, orthopyroxene (En,,_,,), clinopyroxene (Cr-diopside or Cr-Mg augite) and spine1 (Mg-Al chromite).

In Type Ia, clinopyroxene occurs as discrete grains and/or intergrown with spine1 and orthopyroxene. Sometimes clinopyroxene is also poikilitic (including orthopyroxene/olivine/ spine]), enriched in Na, Cr (Na,O: 0.69-2.15 wt.%; Cr,O,: 1.57-2.17 wt.%) with low magnesium to iron ratios with mg * < 92.2, where mg * = [Mg/(Mg + Fe>] * 100. Con- sequently, we distinguish Type Ia- and Type ICY-~, the latter corresponding to peridotites with poikilitic clinopyroxene (Gregoire, 1994).

3.2. Type Ma: 2-pyroxenes-spine1 bearing ultrabasic-basic xenoliths

Type IIa xenoliths grade from clinopyroxene-rich lherzolites, wehrlites, ( f olivine + plagioclase) web- sterites, (&garnet + sapphirine) metagabbros to anorthosites. The main minerals are Al-diopside (mg * > 73.5), enstatite (mg * > 70.51, Al-spine1 (mg * > 54), plagioclase (An > 50) and olivine (Fo > 79). They have heterogranular granoblastic-mosaic texture with numerous relicts of magmatic textures (layering and cumulate textures). These xenoliths have undergone an extensive subsolidus history as indicated by the presence of exsolution lamellae, coronas and intergrowths, including sapphirine. The mineral assemblage of the Type IIa metagabbroic xenoliths (Al-diopside + orthopyroxene + plagioclase + Al-spine1 + garnet f sapphirine) is typical of granulite facies, corresponding to P-T conditions of 0.6 GPa, 750°C to 1.6 GPa, 1000°C.

The presence of sapphirine in these xenoliths constitutes the first reported occurrence of this mineral in an oceanic environment (Gregoire et al., 1994).

3.3. Type llc: Ilmenite metagabbros

Type IIc xenoliths contain clinopyroxene (Al-Na diopside or augite), plagioclase (An,, _ 32 ), ilmenite, garnet, orthopyroxene (mg * = 69-631, and less commonly rutile and Al-spinel. They are characterized by heterogranular granoblastic textures. They also contain relicts of cumulate textures, corona structures and exsolution lamellae (ilmenite, plagioclase, garnet and orthopyroxene) in clinopyroxene.

4. Major and trace element composition

Only general petrological and geochemical features pertinent to the isotopic studies will be pre- sented here (see Gregoire, 1994, for detailed descrip- tions).

4.1. Major elements

The Kerguelen xenoliths as a whole have large variations in the chemical compositions. Type Ia peridotite whole rock compositions are, however, very homogeneous. Their mg’ (90-92) value and low CaO (< 1.10 wt.%), Na,O (<0.18 wt.%), Al,O, (< 1.15 wt.%), TiO, (< 0.13 wt.%) contents indicate a refractory nature. The major element compositions of the Type I/3 dunites are more variable than those of the Type ICY harzburgites/clinopyroxene-poor Iherzolites, as reflected by a larger mg * range (86-91), and Al,O, (0.15-1.91 wt.%) and CaO (0.34-1.6 wt.%) contents. These variations are related to the proportions of modal clinopyroxene- spinel.

The 2-pyroxenes-spine1 bearing ultrabasic-basic xenoliths (Type IIa) have the highest mg * (91-76) value of all Type II inclusions. The variation from ultrabasic (clinopyroxene-rich lherzolite) to basic (metagabbro) rocks correlates with a simultaneous decrease in MgO and an increase in Al,O, in a constant Fe0 content. This reflects the variations of the proportions of modal olivine, pyroxenes and plagioclase. Type IIa xenoliths are characterized by

266 N. Muttirlli rt ul. /Lithm 37 (1996) 261-280

low contents of TiO, (< 0.38 wt.%), K,O (< 0.6

wt.%) and P,O, (< 0.05 wt.%). Ilmenite metagabbroic xenoliths (Type 11~) have

uniform compositions, with low mg * (50-641 values

and K,O (< 0.5 wt.%) and P,O, (< 0.06 wt.%) contents.

4.2. Trace elements

The whole rock rare earth element (REE) contents

of Kerguelen xenoliths were determined on a VG

plasmaquad PQ2 + ICP-MS at the “Mu&e Royal de

1’Afrique Centrale” (for a detailed description of the

ICP-MS analytical method, see Andre and Ashchep- kov, submitted). The samples were prepared under

ultraclean conditions and five international standards

(PCC 1, DTS 1, W 1, ANG, BHVOl 1 were calibrated externally. The analytical accuracy was checked by analyzing of other international standards (Nim-N,

Nim-D, UB-N) (L. Andre, pers. commun.1. How- ever, because of the poor knowledge of the exact low REE contents in the ultramafic international stan-

dards, we preferred to check the quality of the analyses using duplicate samples (prepared with al-

kaline and acid dissolutions) (Table lc). For REE

abundances > 1 ppm, 1-O. 1 ppm, < 100 ppb and < 10 ppb, the reproducibility of measurements cor- responds respectively to 5%, lo-15%, 50-100% and > 100% of the measured value.

Type I xenoliths are enriched in LREE relative to chondrites (Table 1 a) (Fig. 3a). Type I o-2 harzburgites/clinopyroxene-poor lherzolites have the most LREE-enriched patterns with a slight negative slope (Ce(,,/Yb,,, = 1 1.8-23.2). The Type I a-1 harzburgites and dunites have convex-down profiles (a slight

U-shape) (Ce,,,/Yb,,, = 1 .O- 12.1). Ion microprobe

analyses (obtained on a Cameca IMS-3f ion microprobe at Woods Hole Oceanographic Institution, following the techniques reported by Shimizu and Hart (1982) (the uncertainties are + 7- 15% for LREE and f5-7% for intermediate and heavy REE) (Shimizu and Weis, 1991; Mattielli et al., 1992 and N. Shimizu, pers. commun.) of clinopyroxenes from several harzburgites and a clinopyroxene-poor lherzolite are also reported in Fig. 3a. The chondrite-normalized REE profiles of the clinopyroxenes are similar to those of the corresponding whole rocks. However,

the absolute REE abundances and LREE contents are

much higher in the clinopyroxenes (Ce,,,/Yb,,, = 3.0-31.7) than in the whole rocks. Schiano et al.

( 1994) reported significant HFSE negative anomalies

with Ti/Zr = 30 in clinopyroxenes from the same peridotites (MG-91-8 and LVLK 133Al- 1). Hassler

et al. ( 1994) observed similar LREE enrichment and

HFSE negative anomalies in clinopyroxenes from

the Lac SupCrieur locality, northeastern Kerguelen Islands.

The REE profiles of the clinopyroxenes of the Kerguelen peridotites are comparable to those of the

clinopyroxenes from Hawaii (Kaau, Pali and Kalihi;

Salt Lake Crater) spine1 lherzolites, i.e. they are LREE-enriched with “spoon-shaped” patterns (Sen

et al., 1993). The REE of Type IIa xenoliths show uniformly

low values and nearly flat chondrite-normalized pat-

terns (Table lb) (Fig. 3b). Positive Eu anomalies

occur in all the ultrabasic and basic samples. GrCgoire et al. (1994) reported the same characteristics for REE in clinopyroxene, garnet, sapphirine and plagio-

clase from a sapphirine-bearing metagabbro. The REE levels and the positive Eu anomalies of Type IIa xenoliths reflect the troctolitic protolith composition, which was inferred by Gregoire et al. (19941 from the Type Ila normative compositions (trocto-

lites or olivine gabbros). The REE concentrations of Type IIc xenoliths are

greater (5 to 20 times chondritic values) than those of Type IIa (Table 1 b) (Fig. 3b). Chondrite-normalized REE patterns display a slight enrichment in the intermediate REE (La#m,,, = 0.3-0.7) providing

a concave-up profile, without an Eu anomaly.

5. Isotopic geochemistry

5.1. Analytical procedures

Because Frey et al. (1991) demonstrated that grinding in agate produced no obvious contamination effects, especially for Pb isotopes, we crushed about 200 g of each sample in an agate mortar. We reserved grain sizes of loo-250 pm for bulk rock analyses, and coarse grains for mineral separation.

Minerals were concentrated with a Franz mag- netic separator and hand-picked (with further crush-

N. Mattielli et al./Lithns 37 (1996) 261-280 267

ing to finer grain sizes) to avoid grains with cracks and/or cloudy surfaces.

Whole rock samples (- 250 mg) were leached

100

.z

j lo

q

“a

8 l v)

0.1

0.01

100

e, lo 2

8

B “a E

d

1

0.1

I I I I I I I I I

La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb

I I 1 1 I I I I I 1 I La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb

following the procedure described by Weis and Frey (1991). We also performed a 4-step [methanol, HCl (6 NJ, HF (5%), HCl (2.5 N)] leaching on separated minerals (- 60 mg) according to an original method (adapted from those of Shimizu and Hart, 1973; Jagoutz et al., 1980 and PolvC, 1983).

Samples were dissolved in a mixture HF + HNO,( +HClO,) (6: 1: 1) (perchloric acid was only used for samples especially resistant to dissolution) in a Teflon vessel. Insoluble residues, if present, were transferred to a Teflon bomb with a solution of HF + HNO,.

The chemical separation techniques were similar to those reported by Weis et al. (1987) and Weis and Frey (1991). Total Pb blank values for the entire chemical procedure, including sample dissolution, were less than 0.7 ng. The blank values for the entire Sr and Nd chemical separation methods were less than 1.5 ng and 0.3 ng, respectively.

Elemental concentrations (Rb, Sr, Sm, Nd, U and Pb) were measured, by the isotope dilution tech- nique, on a Finnigan Mat 260 mass spectrometer. Sr, Nd and Pb isotopic compositions were measured with a VG Sector 54 multicollector mass spectrometer. For each run, the 146-145-144-143 and 88- 87-86-84 isotopes were normalized to ‘46Nd/ 144Nd = 0.7219 and 86Sr/ **Sr = 0.1194, respectively. Repeated measurements of standards yielded ‘43Nd/ 144Nd = 0.511732 f 10 and ‘45Nd/ 144Nd = 0.348409 + 4 (2~,, on 32 measurements) for the Merck Nd standard and *6Sr/ ‘*Sr = 0.710274 f 6 (2~,, on 32 measurements) for the NBS987 standard. The Pb isotopic ratios were cor- rected for mass fractionation by repeated analyses of the NBS981 standard IO.12 k 0.006% per a.m.u. (2am, n = 18>]. Between-run precision was better

Fig. 3. Chondrite-normahsed rare earth element abundances of the

Kerguelen xenoliths: (a) Type I peridotites: Type 1~1 harzbur-

gites - Type I a-2 harzburgites and clinopyroxene-poor lherzolites

- Type lb dunites. REE contents of clinopyroxenes arc also

reported (open squares for Type I a- 1 clinopyroxenes; open trian-

gles for Type la-2 chnopyroxenes); (b) Type II xenoliths: Type Ha pyroxenes f sapphirine f garnet metagabbros (data of Type IIa

websterite (open triangles) and wehrlite (open diamonds) are also

reported); Type IIb = chnopyroxene-ilmenite-spine1 ultrabasic

and basic xenohths; Type Ilc = ilmenite metagabbros. The nor-

malizing values used are those of Sun and McDonough (1989).

Tab

le

I

(a)

Rar

e ea

rth

elem

ent

abun

danc

es

(ppm

) of

Typ

e I

xeno

lith

who

le

rock

s an

d cl

inop

yrox

enes

. (b

) R

are

eart

h el

emen

t ab

unda

nces

(p

pm)

of T

ype

Ha

and

11~

xeno

lith

who

le

WA

sam

ples

. (c

) R

are

eart

h el

emen

t ab

unda

nces

(p

pm)

of d

uplic

ate

sam

ples

(p

repa

red

by

alka

line

and

acid

di

ssol

utio

n)

(a)

Typ

e I

a-l

Typ

e I

a-2

Typ

eIP

3

harz

burg

ite

harz

burg

ite

cpx

harz

burg

ite

cpx

cpx-

poor

cp

x ha

rzbu

rgite

ha

rzbu

rgite

cp

x du

nite

du

nite

du

nite

Z

M

G-9

1-8

MG

-91-

38

MC

-91

-42

lher

zolit

e G

M-9

2-50

2 G

M-9

2-50

9 M

G-9

1-40

M

C-9

1 I

I4

GM

-92-

286

2.

: w

.r.

w.r.

w

.r.

LV

LK

w

.r.

w.r.

w

.r.

w.r.

w

.r.

2 l3

3Al-

I w

.r.

c i..

La

0.42

0.

10

1.80

0.

39

4.55

2.

82

78.4

2.

78

I .25

16

.4

0.12

0.

96

I.34

c

Ce

0.77

0.

16

2.10

0.

69

4.7

I 7.

52

189

4.56

3.

06

47.6

0.

26

I.41

I .

74

s %

Pr

0.08

0.

02

_ 0.

08

_ I .

03

_ 0.

49

0.42

0.

04

0.13

0.

16

it;

Nd

0.29

0.

07

0.45

0.

28

0.89

3.

96

1.76

I.

81

25.3

0.

20

0.35

0.

49

;=_

Sm

0.05

0.

01

0.15

0.

05

0.43

0.

74

_ 0.

3 I

0.34

5.

33

0.06

0.

04

0.07

%

E

u 0.

02

0.00

4 0.

03

0.02

0.

14

0.20

3.

55

0.10

0.

11

1.65

0.

03

0.0

I 0.

03

5

Gd

0.04

0.

0 I

_ 0.

06

_ 0.

7 I

_ 0.

35

0.32

_

0.09

0.

04

0.07

3

DY

0.

04

0.0

I 0.

1 I

0.05

0.

57

0.42

6.

37

0.19

0.

18

2.49

0.

1 I

0.04

0.

07

I H

o 0.

01

0.00

3 0.

01

0.06

_

0.03

0.

03

_ _

_ 0.

02

0.0

I 0.

01

Z

Er

0.02

0.

01

0.18

0.

03

0.41

0.

15

3.32

0.

08

0.08

0.

95

0.07

0.

03

0.04

Y

b 0.

03

0.02

0.

19

0.04

0.

50

0.09

I.

61

0.07

0.

07

0.98

0.

07

0.05

0.

04

Nor

mul

ized

La

I .77

Ce

1.26

Pr

0.84

Nd

0.62

Sm

0.33

Eu

0.34

Gd

0.19

DY

0.

16

Ho

0.18

Er

0.12

Yb

0.18

0.42

7.

59

1.65

0.

26

3.44

I.

12

0.2

I _

0.80

0.15

0.

97

0.57

0.07

0.

97

0.33

0.07

0.

52

0.29

0.05

_

0.29

0.04

0.

43

0.18

0.05

_

0.18

0.06

1.

06

0.18

0.12

I.

13

0.22

19.2

I I

.9

7.70

12

.3

_ 10

.8

I.90

8.

48

2.80

4.

84

2.36

3.

45

3.45

2.23

I .

65

_ 1.

06

2.50

0.

91

2.95

0.

53

331

I I.

7 5.

27

309

7.45

5.

00

_ 5.

21

4.44

_ 3.

78

3.88

_ 2.

05

2.20

61.2

1.

69

I .83

_ 1.

69

I .56

25.1

0.

76

0.70

0.60

0.

57

20.1

0.

5 I

0.49

9.

47

0.41

0.

42

69.3

0.

51

4.05

5.

65

77.8

0.

42

2.30

2.

84

_ 0.

42

I .37

I .

68

54.2

0.

43

0.75

I .

05

34.8

0.

39

0.26

0.

46

28.4

0.

52

0.17

0.

52

_ 0.

44

0.19

0.

34

9.80

0.

43

0.16

0.

28

_ 0.

35

0.18

0.

18

5.71

0.

42

0.18

0.

24

5.76

0.

41

0.29

0.

24

- .-

-

. .

. -

- -

_ _

. _

,,,

.,

(b)

Typ

e II

a T

ype

IIc

weh

rlite

sa

+ g

t m

etag

abbr

o w

ebst

exite

w

ebst

erite

gt

met

agab

bro

sa m

etag

abbr

o ilm

met

agab

bro

ilm m

etag

abbr

o

MG

-91-

105

MC

-91-

I 16

G

M-9

2-23

4 G

M-9

2-25

2 G

M-9

2406

A

RC

83

5 w

.r.

MG

-91-

106

w.r.

G

M-9

2-41

9

w.r.

w.r.

w

.r.

w.r.

w

.r.

w.r.

La

0.18

0.

34

0.29

1.

01

1.01

0.

50

1.81

2.

08

Ce

0.40

0.

79

Pr

0.09

0.

10

Nd

0.47

0.

34

Sm

0.17

0.

07

EU

0.

07

0.08

Gd

0.27

0.

08

DY

0.

33

0.07

Ho

0.07

0.

02

Er

0.19

0.

04

Yb

0.21

0.

04

Norm

alize

d

La

0.76

Ce

0.65

Pr

0.89

Nd

1.01

Sm

1.13

Eu

1.26

Gd

I .29

DY

1.

30

Ho

1.24

Er

1.15

Yb

1.24

I .43

1.

22

4.26

4.

26

2.11

7.

64

8.78

1.29

0.

72

3.99

2.

81

1.72

9.

35

9.18

I .05

0.

84

4.00

3.

05

1.68

12

.8

11.1

0.73

0.

84

4.11

3.

00

1.54

15

.3

11.8

0.46

0.

85

3.92

3.

27

I .50

17

.6

13.1

1.38

I .

03

4.66

5.

52

4.3

1 20

.9

13.6

0.39

0.

97

3.89

3.

45

1.46

16

.6

13.8

0.28

0.

94

3.39

3.

50

1.42

17

.8

13.1

0.35

0.

88

3.18

3.

36

1.41

15

.0

12.2

0.24

0.

79

3.02

3.

20

1.21

16

.1

11.1

0.24

0.

76

2.76

3.

00

1.12

12

.3

10.2

0.39

0.13

0.06

0.20

0.24

0.05

0.13

0.13

2.44

1.

72

1.05

5.

72

5.62

0.38

0.

29

0.16

1.

22

I .05

1.92

1.

40

0.72

7.

15

5.50

0.60

0.

50

0.23

2.

69

2.01

0.27

0.

32

0.25

1.

21

0.79

0.80

0.

71

0.30

3.

41

2.84

0.86

0.

89

0.36

4.

53

3.34

0.18

0.

19

0.08

0.

85

0.69

0.50

0.

53

0.20

2.

67

1.84

0.47

0.

5 1

0.19

2.

09

1.74

(c)

harz

burg

ite

MG

-91-

38

acid

di

s.

MG

-91-

38

alka

line

dis.

weh

rlite

ilm

met

agab

bro

MG

-91-

105

MG

-91-

105

MG

-91-

106

MG

-91-

106

acid

di

s.

alka

line

dig.

ac

id

dis.

ac

id

dis.

La

0.10

0.

10

0.19

0.

18

1.81

1.

87

Ce

0.16

0.

17

0.39

0.

40

5.72

5.

74

Pr

0.02

0.

02

0.09

0.

09

1.22

1.

19

Nd

0.06

0.

07

0.45

0.

47

7.15

6.

%

Sm

0.01

0.

01

0.18

0.

17

2.69

2.

59

Eu

0.00

4 0.

004

0.08

0.

07

1.21

1.

19

Gd

0.01

0.

01

0.26

0.

27

3.41

3.

25

DY

0.

01

0.01

0.

30

0.33

4.

53

4.54

Ho

0.00

3 0.

003

0.07

0.

07

0.85

0.

86

Er

0.00

8 0.

01

0.18

0.

19

2.67

2.

73

Yb

0.02

0.

02

0.19

0.

2 1

2.09

2.

20

RE

E

conc

entr

atio

ns

are

norm

aliz

ed

to t

he

Cl

chon

drite

va

lues

of

Sun

an

d M

cDon

ough

(1

989)

.

270 N. Matfielli et al./Lithos 37 (1996) 261-280

Lower Miocene

Fig. 4. Present-day s’Sr/ mSr vs. *06Pb/ *04Pb ratios of Southeast Province xenoliths and Kerguelen Islands basaltic rocks. Kerguelen

Archipelago volcanic rocks include: Foch Island tholeiitic basalts (White and Hofman, 1982); Basaltic series (Gautier et al., 1990) and

Southeast Province upper and lower Miocene series (Weis et al., 1993). Data for Heard Island lavas (solid squares) (Barling and Goldstein,

1990) are also reported.

than 0.1% for 206 Pb/ *04Pb and *“Pb/ ‘04Pb and

better than 0.15% for 208Pb/ 204Pb. To test the leaching procedure we compared the

isotopic compositions of a leached sample (GM-92- 286) and its leachate solution (Table 2). They are clearly different. The reliability and the efficiency of the leaching method for separated minerals was also demonstrated by the very similar isotopic ratios (with

an even lower Sr isotopic value) obtained on an altered clinopyroxene (LVLK I33,l -I”> and a clinopyroxene without any distinct alteration (MG-

91-8”) (Fig. 4).

In order to test the reproducibility of the measurements and chemical procedures, duplicate leaching, dissolution and analyses were performed for 5 sam-

ples (MG-9 l-38, MG-9 1 - 116, GM-92-286, GM-92- 502 and ARC8351 and yielded entirely reproducible isotopic ratios (Table 2).

5.2. Results

Table 2 and Figs. 4 and 5a-c present Sr, Nd and Pb isotopic data for leached whole rocks and clinopyroxenes of ultrabasic and basic xenoliths from

Fig. 5. (a) Present-day ‘G Nd/ ‘44Nd vs. “Sr/ 86Sr diagrams for Southeast Province xenoliths. For comparison, the data of basalts from the

Kerguelen Archipelago (data sources as in Fig. 4) and Plateau (Weis et al., 1989; Salters et al., 1991), and from the Indian Ridges (Indian

Ocean Triple junction, Rodriguez, Southwest Indian Ridge, Southeast Indian Ridge, East Indian Ridge: references in Weis et al., 1993) and

Ninetyeast Ridge (ODP Leg 121, Sites 756, 757, 758: Weis and Frey, 1991) are also plotted. (Mantle components from Zindler and Hart,

1986). Inset shows isotopic data of the xenoliths and the host lavas (Mont Tizard: stippled squares; Dome Rouge: solid squares). (b)

Present-day “‘Pb/ *04Pb vs. *06Pb/ *04Pb diagrams. Data sources and symbols as in Fig. 5a. Data from Broken Ridge (open squares),

Naturaliste Plateau (solid squares) (Mahoney et al., 1995) and Bunbury (circles) lavas (Frey et al., submitted) are also reported. (c)

Present-day *‘sPb/ *04Pb vs. *06Pb/ *04Pb diagrams. Data sources and symbols as in Fig. 5a.

N. Manielli et al./Lithou 37 (19%) 261-280 271

(4 0.5134

0.512f

0.51%

0.512:

r I “3Nd/“‘Nd

0.5130 - Type I/a I

143Nd/144Nd

0.5126 I / 0.51B - Kergoelen Islands 1

Basalfs

A?? A

0.5124 - Type la+p ~‘w*%r

0.704 0 705

Lower Miocene Series

0.706

upper mocene benes

A

l drilled o dredged

I C)_EMI Type la+/3

0.702

09 15.70

0.703 0.704 0.705 0.706 0.707 0.708

2 0 7 P b/z 0 4 P b

15.65

15.60

15.55

15.50

15.45

1.5.4c

15.35

15.3c

Indian Ridges

17.2 17.4 17.6

Nin,etyeast Ridge ]

17.3 18.0 '8.2 15.45 ryperrc Type U 206Pb/204Pb

206Pb12%‘b 18.0 182 184 18.6 18.8

Tab

le

2

Sr.

Nd

and

Pb

isot

opic

da

ta

for

leac

hed

who

le

rock

s an

d cl

inop

yrox

rnrs

of

ultr

abas

ic

and

basi

c xe

nolit

hs

from

th

e So

uthe

ast

Prov

mce

of

K

ergu

elen

A

rchl

prla

go.

Ele

men

tal

conc

entr

atio

ns

of

Sr.

R

b,

Nd.

Sm

. P

b an

d P

b (b

y Is

otop

e di

lutio

n)

Sam

ple

Nat

ure

Loc

ality

“S

r/

2un,

, “‘

Rb/

Sr

R

b C

Nd

‘47S

mj

Nd

Sm

20hP

b/’

“‘P

b;’

zOxP

b/

21xP

b,

Pb

(1 p

pm

=sr

‘“?P

b/

2 un

, X

b Sr

ppm

pp

m

ld4N

d la

4Nd

ppm

pp

m

‘04P

b L

WPb

“‘

“Pb

LO

4 Pb

ppm

Typ

e la

-l

MG

-91-

8 a

MG

-91-

8

MC

-91

-38

MG

-91-

42

Typ

e la

-2

LV

LK

I3

3 A

l-l

a

LV

LK

133

Al-l

GM

-92-

502

harz

burg

lte

DB

me

Rou

ge

0.70

551

I 0.

0023

52

.9

harz

burg

ite

D&

ne

Rou

ge

0.70

552

2 1.

68

harz

burg

ite

Dam

e R

ouge

0.

7050

6 1

0.69

harz

burg

ite

DB

me

Rou

ge

0.70

530

1 0.

0827

2.

90

0.04

0.

5124

4 2

0.51

248

2

0.05

0.08

0.

5126

0 5

~ 3.

84

0.01

6

-3.0

6 0.

189

0.08

89

- 0.

80

0.13

8

-0.1

0 O

.lll

- 0.

66

0. I

I6

- 2.

3 I

0.20

4

-3.8

8 0.

130

25.7

0.

69

18.4

05

15.5

46

38.8

46

21.7

I.

1 0.

38

0.16

0.

05

18.4

74

15.5

73

39.0

74

25.1

0.

04

0.01

0.07

0.

01

18.3

90

15.5

63

38.8

83

6.75

0.

04

0.00

5

0.12

0.

03

18.4

46

39.0

24

15.5

85

22.2

0.

06

0.02

0.05

0.

0012

I1

4

4.37

0.07

35

3.07

0.51

263

4

0.51

260

2

IO5

19.2

4.00

0.

77

18.4

56

15.5

93

39.0

00

I I .6

0.

54

0.10

0.08

0.

004

0.08

0.

512.

52

3 0.

37

0.13

tn

.602

15

.666

0.

04

0.00

6

0.01

95

10.7

0.

07

0.51

244

I I.

48

0.32

0.

05

0.00

6

0.01

30

Il.8

0.

05

0.51

268

2 0.

78

0.21

8 0.

25

0.09

18

.254

15

.523

38

.597

8.

93

0.04

0.

006

0.00

82

IO.3

0.

03

0.51

244

II -

3.86

0.

104

0.34

0.

06

18.2

70

15.5

30

38.9

60

0.07

DB

me

Rou

ge

Dam

e R

ouge

Poin

te

SWL

aIlll

e

0.70

509

I cp

x-po

or

lher

zolit

e

cpx-

poor

lher

zolit

e

harz

burg

ite

0.70

512

1

0.70

576

I

0.70

592

2

0.70

577

I G

M-9

2-50

9 ha

rzbu

rgite

Po

inte

Suz

anne

Typ

e l/

3

MG

-91-

40

GM

-92-

286

duni

te

DB

me

Rou

ge

duni

te

Mon

t T

izar

d 0.

7049

7 I

0.70

536

I 0.

7054

6 I

0.70

623

I 18

.294

IS

.602

39

.002

le

acha

te

Mon

t T

izar

d

Typ

e IIu

M

G-9

1-10

5 M

G-9

1-11

6 w

ehrl

ite

sa *

gt

met

agab

bro

Mon

t T

izar

d 0.

7045

8 I

0.00

56

36.7

0.

07

1.21

2.

45

Mon

t T

izar

d 0.

7043

2 I

0.00

24

I12

0.09

0.

16

0.03

18.1

34

15.5

37

38.3

28

2.41

0.

07

0.00

3

0.03

0.

006

Mon

t T

izar

d

Mon

t T

izar

d

Mon

t T

izar

d

Mon

t T

izar

d

0.70

422

I 0.

0020

0.

17

0.05

0.12

0.37

0.

13

1.94

1.

72

18.1

74

15.5

70

38.1

81

6.75

0.70

441

I 0.

7039

8 I

0.00

24

41.2

0.

03

43.4

0.51

284

IO

3.90

0.

179

0.51

286

3 4.

40

0.20

6

0.51

288

4 4.

62

0.53

6

IS.3

23

15.5

84

38.6

85

1.76

0.

07

18.3

39

15.4

94

38.4

70

13.0

0.

09

0.00

2

0.02

Mon

t T

izar

d 0.

7044

5 I

0.00

73

106

0.27

0.

5127

7 3

2.59

0.

264

1.06

0.

45

IS.1

02

15.5

24

38.5

92

1.24

0.

14

0.00

3

Mon

t T

izar

d 0.

7043

4 I

0.00

90

7.11

0.

02

0.51

284

2 3.

92

0.24

4 2.

12

0.85

0.

2 I

0.02

Mon

t T

izar

d 0.

7042

7 3

0.00

62

129

0.28

0.

72

0.23

IS

.159

15

.548

38

.67

I 2.

73

0.23

Mon

t T

izar

d 0.

7043

0 I

0.00

52

129

0.23

0.

5128

0 2

3.24

0.

198

0.75

18

.134

15

.501

38

.551

0.

23

Mon

t T

izar

d 0.

7046

7 I

209

0.51

278

I 2.

84

0.22

I

6.70

2.

45

IS.1

25

15.5

28

38.6

26

Mon

t T

izar

d 0.

7042

5 I

0.01

21

89.8

0.

38

0.51

288

2 4.

78

0.22

9 6.

42

2.43

18

.184

15

.532

38

.464

GM

-92-

234

GM

-92-

252

GM

-92-

406

AR

C

83.5

a

AR

C

835

01 w

ebst

erite

01 +

pl

web

ster

ite

gt m

eta-

gabb

ro

sa m

eta-

gabb

ro

sa m

eta-

gabb

ro

3 $ 2.

0.00

6 s

Typ

e U

C

MG

-91-

106

GM

-92-

419

ilm

met

a-

gabb

ro

ilm

met

a-

gabb

ro

0.15

1.88

0.

20

P

a S

epar

ated

cl

inop

yrox

ene.

l N

d ca

lcul

ated

ac

cord

ing

to

{[(‘

4’N

d/

‘“N

d),

- (1

4’N

d/

‘“N

d),,)

/(‘4

’Nd/

lti

Nd)

,,)

* 10

,000

; w

here

B

E =

0.

5 12

638.

e c s B

%

2 %

8 %

i-

z

274 N. Murrirlli er al. / Lithos 37 (1996) 261-280

the SE Province of the Kerguelen Archipelago. We have also shown the fields for the different basaltic series from the Islands and the Plateau (Storey et al.,

1988; Weis et al., 1989, Weis et al., 1993; Gamier et

al., 1990; Salters et al., 1991). Fig. 5a-c includes the isotope data for the host rocks of the xenoliths (Weis

et al., 1993). The xenoliths have a large range of Sr, Nd and Pb

isotopic ratios: *‘Sr

= 4.78 to - 3.88, ~~~~;~~~~2:,“d~7~~~~~~

. 7 *“Pb/ *04Pb = 15.501 to 15.593, and *‘sPb/ *04Pb

= 38.136 to 39.024. The only other published isotope data for Kergue-

len xenoliths are the Sr and Nd isotopic ratios ob-

tained by Vance et al. (1989) (on a gabbro and two

peridotites) and Hassler et al. (1994) (on a suite of peridotite and granulite xenoliths from the Lac

Sup&ieur locality, northeastern Kerguelen Islands). They are in very good agreement with our isotopic data, except for the extreme values obtained on two

spine1 + phlogopite lherzolites (up to “Sr/ @Sr =

0.70645 and l Nd = - 4.0) and one phlogopite- bearing clinopyroxene megacryst (*‘Sr/ 86Sr =

0.70869 and eNd = - 12.5) by Hassler et al. (19941. Elemental concentrations have been used to calcu-

late the correction for in-situ Rb, Sm and U decay according to the ages of important magmatic events of the Kerguelen Plateau and Archipelago (115, 80, 45 Ma; Leclaire et al., 1987; Whitechurch et al.,

1992; Pringle et al., 1994; Gautier et al., 1990; Weis et al., 1989, 1993). Even if the age corrections can modify the isotopic ratios by up to 1.2~10~~ for 87Sr/86Sr, 1.6~10~~ for 14’Nd/ ‘44Nd. 1.8x10-’

for ‘06Pb/ , ‘04Pb these differences are negligible when compared to the large variations in the isotopic corn ositions between different samples (1.6~10~~

8: for Sr/*‘Sr, 4.4~10~~ for ‘43Nd/‘44Nd, 5x10-’ for ‘06 Pb/ 204 Pb).

The isotopic compositions of the xenoliths as a whole fall within the range of those of the volcanic series on the Archipelago (Storey et al., 1988; Gau- tier et al., 1990; Weis et al., 1993) (Figs. 4 and 5a-c). The isotopic ratios of the xenoliths, however, differ from those of the Kerguelen Plateau (Salters et al., 1991; Weis et al., 1989); they are particularly lower in “‘Pb/ 204Pb.

The xenoliths are isotopically distinct from their host rocks, while the isotopic ratios of the clinopy-

roxenes and the whole rocks for the same sample are very similar.

The isotopic compositions of the xenoliths vary according to their petrographic characteristics and

not according to their provenance. Type I xenoliths

for instance come from Mt. Tizard, Dame Rouge and

Pointe Suzanne and all show comparable isotopic

ratios.

The isotope data of the xenoliths plot into two

distinct groups (Figs. 4 and 5a-c). The first group

includes the large range of isotopic signatures of

Types I (Y and I/3 that shows high *‘Sr/ 86Sr (0.70506-0.70577) and 206Pb/ *04Pb (18.390-

18.456) and low ‘43Nd/ ‘44Nd (0.51244-0.51263)

ratios. The second group comprises the much more uniform isotopic ratios of the Type IIa and IIc

xenoliths (*‘Sr/ 86Sr = 0.70422-0.70447,

‘“‘Nd/ ‘44Nd = 0.51264-0.51288, *06Pb/ *04Pb = 18.102- 18.184).

6. Discussion

6.1. Type I xenoliths

The petrographic and mineralogical characteristics, major element compositions and inferred P-T

conditions (Gregoire, 1994) of the Type I xenoliths

are comparable to those of oceanic lithosphere peridotites. Type I a harzburgites/clinopyroxene-poor lherzolites are similar to San Carlos Group I inclu-

sions (Frey and Prinz, 1978) and Oahu (Hawaii) spine1 lherzolites @en et al., 1993). Type I& xenoliths have a refractory nature, as indicated by the composition of their constituent minerals (high Mg content in olivine, spine1 and pyroxenes; high Cr content in spinel) and their whole rock major element abundances (high mg * ; low CaO, Na,O, Al,O, and TiO, contents). Type I@ dunites are less refractory than Type I cx harzburgites/clinopyroxene-poor lherzolites (lower Mg content in silicates and lower Cr content in spinel).

Type I xenoliths have isotopic compositions that are rare for oceanic peridotites: *‘Sr/ 86Sr up to 0.70577 and eNd down to - 3.9 (or *‘Sr/ 86Sr up to 0.70869 and eNd down to - 12.5, if we take into account the results of Hassler et al., 1994).

The peridotite xenoliths (Type Ia + p) have Pb,

N. Mattielli et al./Lithos 37 (1996) 261-280 275

Nd and Sr isotopic compositions similar to those of the alkaline basalts (Figs. 4 and 5a-c). These lavas include the upper Miocene basaltic series of the Southeast Province (i.e., the basanites, tephriphonolites and phonolites), the enriched isotopic compositions of which have been interpreted as representative of the “pure” composition of the Kerguelen plume (Weis et al., 1993). The similarity between the isotopic compositions of the Type I xenoliths and those of the alkaline basaltic series supports a common mantle source. Assuming this hypothesis to be correct this implies that these xenoliths formed relatively recently, in an age range comparable to that of the alkaline series on the Archipelago, i.e. < 4.5 Ma.

Given the mineralogical characteristics [mg,&e: 90.17-92.37 and cr&,: 37.72-61.63 (Gregoire, 1994)] of the Type Ia xenoliths, very similar to those of peridot&es from Lanzarotte (Siena et al., 1991) and Loihi (Clague, 19881, and their isotopic signatures, Type I xenoliths could be a priori considered to represent residues of a previous partial melting process of the Kerguelen plume.

However, the petrographic and geochemical features of the Type I xenoliths are not consistent with a simple process of partial melting.

The clinopyroxenes of the Type I o-2 harzburgites and clinopyroxene-poor lherzolites have poikilitic textures (including opx/sp/ol) and display significant Na- and Cr-enrichment. Moreover, Type I xenoliths are characterized by LREE enriched or LREE enriched convex-down shapes [Ce/Yb(,, = 3.0-3 1.7, in separated clinopyroxene]. Previous studies reported such mineralogical and geochemical characteristics (especially for the spine1 lherzolites from Salt Lake Crater and Kaau, Pali, Kalihi; Sen et al., 1993) and inferred that these features resulted from metasomatic enrichment of the lithophere caused by reaction with magmas that formed the Honolulu Volcanics.

Schiano et al. (1994) observed three types of secondary cogenetic inclusions trapped in olivine and pyroxenes from Type I peridotites: silicate melt inclusions, carbonate melt inclusions and CO, fluid inclusions. The melt inclusions were significantly enriched in strongly incompatible elements and the silicate-carbonate melt was interpreted as an important “migrating metasomatic melt phase”. These authors described Type I peridotites as fragments of

the upper mantle, metasomatised by a silicate- carbonate melt.

The petrographic and geochemical characteristics, together with the occurrence of the secondary inclusions, of Type I clinopyroxenes indicate that these peridotites have suffered one or several events of melt percolation and reaction.

Furthermore, neon isotopic data of Kerguelen harzburgites strengthen the evidence for the occurrence of a metasomatic event. Valbracht et al. (1996) interpreted the neon isotopic compositions as reflecting a “primitive” mantle source (i.e., one with a low ratio of time-integrated (II + Th) to primordial solar noble gases relative to MORB). The authors suggested that the primitive neon of the Kerguelen peridotites can be introduced by a metasomatic phase, such as the silicate-carbonate melt observed in mineral inclusions, since noble gases are preferentially fractionated into a CO,-rich siliceous melt.

All the characteristics of the Type I xenoliths indicate that they are the residues of a previous partial melting event in the Kerguelen plume; residues that subsequently interacted with a percolating alkaline melt.

4.2. Type II xenoliths

The 2-pyroxenes-spine1 bearing ultrabasic-basic xenoliths (Type IIa) and ilmenite metagabbroic xenoliths (Type 11~) have very uniform isotopic compositions (Fig. 4). This strongly supports a cogenetic origin, which is also indicated by their petrographic and geochemical relationships.

We suggest that Type IIa and IIc xenoliths are deep cumulates from tholeiitic basalts. This inference is justified by (1) the mineralogical assemblages typical of granulite facies, corresponding to P-T conditions of 0.5 GPa, 750°C to 1.6 GPa, 1000°C (Grkgoire, 1994); (2) the occurrence of numerous relict cumulate textures; (3) the major element compositions of the constituent minerals (high Mg content in olivine, spine1 and pyroxenes; high Ca content in plagioclase) and the whole rocks (low K,O, TiO, and P,O, contents), and the geochemical characteristics (uniformly low REE abundance levels, with a positive Eu anomaly) indicating that Type IIa and IIc xenoliths are poorly differentiated rocks (Gregoire, 1994); (4) the similarity between the REE patterns

216 N. Mattielli et ul. /Lithos 37 (19961 261-280

and the major element compositions (Gregoire, 1994)

of Type II xenoliths, and those of the tholeiitic-transitional basalts (Watkins et al., 1974; Gamier et al.,

1990).

Type IIa and IIc xenoliths have high l Nd (up to +4.8) and low Sr isotopic ratios (down to 0.70422)

that are intermediate between those of the Foch

tholeiitic basalts and those of transitional basaltic series (Fig. 5a). But these xenoliths have lower

207Pb/ 204Pb (down to 15.501) for a given

206Pb/ 204Pb (down to 18.102) than any Pb isotopic

ratio of a basalt from either the Archipelago or the Plateau. The Pb isotopic compositions of the xeno-

liths partly overlap those of the Southeast Province

upper Miocene basaltic series, interpreted as the

“pure” composition of the Kerguelen plume, and

considerably overlap those of the Indian Ridges (Fig.

5b, c). The isotopic ratios of Type IIa and IIc xenoliths

are thus distinct from those of the Kerguelen Plateau. but overlap those of basalts from the Archipelago and the Indian Ridges. The similarity between the isotopic compositions of Type IIa and IIc xenoliths and those of the Kerguelen Islands and the Indian Ridges, supports a common mantle source. Assum- ing this inference to be correct, this implies that the formation of these xenoliths is cogenetic or associ-

ated with relatively young magmatic or tectonic events ( N 40-45 Ma), concurrent with the formation of the islands.

We suggest that the isotopic characteristics of Type IIa and IIc xenoliths reflect a mixing of the Kerguelen plume with a large proportion of a MORB-type mantle component. The depleted as- thenosphere component was supplied by the nearby ridge, when the archipelago was close or above the SEIR at its birth - 40 Ma ago.

Mahoney et al. (1995) and Storey et al. (1992) argued that the Kerguelen plume had interacted with the continental lithosphere just before the breakup of Gondwana. The isotopic data of the Kerguelen xenoliths are not consistent with a contribution from the continental lithosphere. It is useful to compare the isotopic characteristics of the Kerguelen xenoliths with those of Naturaliste Plateau, Broken Ridge (Mahoney et al., 1995) and Bunbury lavas (Frey et al., submitted) (Fig. 5b). These latter reflect a strong contamination by a continental crust component, they

have very high 207Pb/ 204Pb relative to 206Pb/ 204Pb

ratios, which are never seen in either the xenoliths or the basalts from the Kerguelen Archipelago.

Our isotopic results consequently refute the exis-

tence of pieces of continental lithosphere beneath the

Archipelago and the northern part of the Kerguelen

Plateau.

6.3. Perspectives for the role of plumes and the growth process of oceanic plateaux

The Kerguelen xenoliths provide important infor-

mation on the evolution of the activity of the Ker-

guelen plume and on the growth process of the Kerguelen Plateau:

(1) there is a current consensus to consider mantle

plumes as the best mechanism for the formation of

LIP (Coffin and Eldholm, 1994). There is, however, considerable debate whether the plume plays an “ac-

tive” role (Griffiths and Campbell, 1990, 1991) or a “passive” role (Kent, 1991; Saunders et al., 1992) in the process. In the first model, the Kerguelen

Plateau would have been formed by the arrival of a new mantle plume at the base of the lithosphere. In the “passive” plume model, the Kerguelen Plateau

would have formed in response to the lithospheric extension of eastern Gondwana above an old pre-ex- isting thermal anomaly. These different models raise

the question of whether plate separation leads to the formation of a mantle plume or whether the occurrence of a plume at the base of the lithosphere causes the plates to split? Our isotopic data suggest a recent formation for the Kerguelen xenoliths (< 45 Ma)

and demonstrate the absence of a continental lithosphere. This is an additional argument in favour of the Kerguelen plume having a lifetime no longer than 115 m.y. It is more consistent with the “active” plume model;

(2) the available geochronological and isotopic

data from the Kerguelen Plateau are consistent with the Plateau having formed in a series of episodes with episodic activity of the Kerguelen plume. Al- though this needs to be documented further by additional drilling, the duration of each period of activity can be relatively long ( _ 45 m.y.). This is evidenced by the last 45 m.y., during which the activity on the archipelago has been more or less continuous with the formation of the xenoliths and the basaltic series,


i.e. deep and subaerial magmatic manifestations of the Kerguelen Plateau, the thickening of the crust the Kerguelen plume, but also by the hotspot track and the occurrence of deep-seated sapphirine-bearing that constitutes the Ninetyeast Ridge. A LIP can thus xenoliths can be the first signs of a continental represent voluminous fluxes of magma emplaced lithosphere nucleation process (Gregoire et al., in over relatively short time periods, through several press). The LIP can represent the future building pulses of plume magmatic activity; blocks of a continental crust.

(3) the primary and secondary mineral assemblages of Type IIa (including sapphirine) and Type IIc xenoliths are typical of granulite facies, corresponding to P-T conditions of 0.5 GPa, 750°C to 1.6 GPa, 1000°C (Gregoire, 1994). The different petrographic facies of Type IIa and IIc xenoliths are the results of (relequilibration at different depths (from the lower crust to the upper mantle). The depth can be as much as 45 km. Grbgoire et al. (1994) speculated that the mineralogical characteristics of these granulitic xenoliths are consistent with the observed compressional wave velocity (VP = 7.2-7.5 km/s) of the transition zone at the crust-mantle boundary below the Kerguelen Archipelago. The mineralogical reequilibrations could have been produced by isobaric cooling, as indicated by the P-T conditions for the formation of spinel-Zpyroxenes intergrowths in pyroxenes-bearing metagabbros (Grhgoire et al., 1994). However, Grbgoire (1994) reported that Type IIa and IIc xenoliths could have experienced a significant increase of depth since their bulk compositions indicate that they are normative troctolites or olivine gabbros. Such a burial process could occur during crustal accretion, or abduction of the gabbros or troctolites. The abduction hypothesis has been proposed by Menke and Levin (1994) for gabbros from the Icelandic crust. Those authors suggest that the gabbros crystallize at the base of magma chambers, at shallow depths, “before being abducted away from the ridge axis and to deeper depths by the ‘seafloor’ spreading process”.

7. Conclusions

The peridotite xenoliths (Type Ia + p) have Sr, Nd and Pb isotopic compositions similar to those of the alkaline basaltic series. The major-element compositions and the isotopic data of Type I xenoliths are consistent with their formation as residues from the partial melting of the Kerguelen plume. How- ever, petrographic, mineralogical and geochemical characteristics require another process to have oc- curred, such as the reaction between the residual Type I peridotites and percolating alkaline melts.

The 2-pyroxenes-spine1 bearing ultrabasic-basic xenoliths (Type IIa> and ilmenite metagabbroic xenoliths (Type 11~) are deep cumulates from tholeiitic magmas. The presence of different mineralogical paragenesis reflect (re)equilibrations at different depths, from the lower crust to the upper mantle. The isotopic compositions of Type IIa and IIc xenoliths are very uniform. They are intermediate between the “depleted” signatures of the Indian Ridges and tholeiitic-transitional basalts from the archipelago, and the “enriched” signatures of the upper Miocene basaltic series of the Southeast Province. These isotopic compositions reflect the mixing of the Kergue- len plume with a large proportion of a depleted MORB-type component.

The deep Type IIa and IIc xenoliths provide another argument in favour of the hypothesis of plateau growth by vertical rather than lateral accretion. Gregoire et al. (1994) suggested that these xenoliths correspond to magmatic underplating con- tributing to the formation of the plateau. However, Coffin and Eldholm (1994) noted that the underplating process should only occur beneath a continental crust where density contrasts between the crust and the underplated material are important;

The isotopic compositions of the Kerguelen xenoliths differ from those of the Kerguelen Plateau, especially for Pb, but they fall within the range of those of the volcanic series of the Archipelago. This later similarity has important implications:

(1) the Kerguelen xenoliths, and especially the Type I xenoliths, are characterized by extreme isotopic compositions similar to those of the DUPAL isotopic anomaly;

(4) the combination of the significant volume of

(2) similarly to the basalts of the Archipelago, this large range of isotope data reflects different degrees of interaction between a depleted MORB-type source (SEIR), quite abundant in the Type II xenoliths, and the Kerguelen plume, distinctly predominant in the

278 N. Mottielli et (11. / Lithos 37 (19961261-280

Type I xenoliths. It confirms the importance of plume-spreading ridge interactions throughout the history of the Kerguelen plume; these interactions were also important in the genesis of the Kerguelen Plateau (115-85 Ma) and the Ninetyeast Ridge (85 38 Ma);

(3) the formation of Kerguelen xenoliths is recent, i.e. associated with relatively young magmatic events (- 40-45 Ma), contemporary with the formation of the Archipelago and the northern part of the Plateau.

The isotopic differences between the Kerguelen xenoliths and the lavas of Naturaliste Plateau, Bro- ken Ridge and Bunbury strengthen the hypothesis of an oceanic origin for the Kerguelen Islands. They also refute the existence of pieces of old continental crust beneath the Islands and the northern part of Kerguelen Plateau, as also evidenced by recent geophysical studies (Munschy et al., 1994).

Geophysical data (Recq et al., 1990, 1995) indicate a crustal structure below the northern Kerguelen Plateau comparable to that of present-day Iceland. The isotopic study of the ultrabasic and basic xenoliths supports this comparison. Indeed, the isotopic compositions of Type IIa and IIc xenoliths, which reflect a major contribution from a MORB-type component (SEIR) in the mixing with the Kerguelen plume, favour the hypothesis of an Iceland-type setting (i.e. where a plume and an active spreading-ridge participate in oceanic magmatism). Nevertheless, the crust below the Kerguelen Islands differs strikingly from the crustal structure of the Plateau as a whole (Recq et al., 1990, 1995). The Archipelago behaves like a midplate volcanic structure such as Hawaii. Data from Type I xenoliths support this comparison. The Type Icr peridotites have characteristics similar to those of Hawaii (Oahu) xenoliths. Furthermore, the extreme isotopic compositions of the Type I xenoliths reflect the contribution of the plume in a midplate setting.

This study of Kerguelen xenoliths places some constraints on the evolution of plume activity and the formation of the Plateau with time. It supports the hypothesis of the Plateau having been formed by the arrival of the plume at the base of the lithosphere after the breakup of Gondwana ( - 115 Ma ago). The plateau would have grown through several pulses of plume magmatic activity (- 115 Ma, - 80 Ma, - 40 Ma) in a geotectonic environment that changed

with time (from a ridge-centered position to the present intraplate position). The occurrence of deep Type IIa and IIc xenoliths reflects a crustal thickening and provides evidence for plateau growth by vertical accretion. Furthermore, these xenoliths and the large volume of magma of the Plateau can be the early signs of future continental lithosphere nucleation (Gregoire et al., in press).

The combination of different tectonic environ- ments (ridge centered to intraplate setting), the longevity of the Kerguelen plume activity and the importance of plume-ridge interactions are probably responsible for the huge volume of magma generated on the major LIP, Kerguelen Plateau/Broken Ridge, for the different features of the four tectonic sectors of the Plateau and for the large variety of petrographic, geochemical and isotopic characteristics of Kerguelen magmatism. It certainly implies that the Kerguelen Plateau should be considered as an unique magmatic construction.

Acknowledgements

We thank the “Mission de Recherche” of the I.F.R.T.P. (Institut FranGais pour la Recherche et

Technologie Polaires), for the essential support in the field, and the Belgian FNRS for travel funds.

We would like to thank J. Michot for his help and much advice, and L. Andre for the REE analyzes on the ICP-MS at the Mu&e Royal de I’Afrique Cen- trafe. N.M. would like to thank J. Barling for long discussions and help in the clean lab, F. Frey for his constructive comments on a first draft of the manuscript and N. Shimizu for the analyses on the ion microprobe (Woods Hole Oceanographic Institu- tion) and interesting discussions. The authors are grateful to the reviewers, J. Ludden and A. Saunders; they helped significantly in improving the paper. N. Cromps is thanked for her help with the drawings.

References

And& L. and Ashchepkov, N.Y., submitted. The high pressure

evolution of alkaline mantle melt: evidence from the VITIM

augite clinopyroxenitic suite. Earth Planet. Sci. Lett.

Baksi, A.K., 1994. Geochronological studies on whole-rock

basalts, Deccan Traps, India: evaluation of the timing of

volcanism relative to the K-T boundary. Earth Planet. Sci.

Lett., 121: 45-56.


Barling. J. and Goldstein, S.L., 1990. Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature, 348(62%): 59-62.

Charvis, P. and Operto, S., 1992. Variations in the crustal structure beneath the Kerguelen Plateau. Eos Trans. Am. Geophys. Union, 73(43): 532.

Charvis, P., Operto, S., Konnecke, L.K., Recq, M., Hello, Y., Houdry, F., Lebellegani, P., Louat, R. and Sage, F., 1993. Structure profonde du domaine nord du plateau de Kerguelen (G&n Indien austral): r6sultat.s pn?liminaires de la campagne MD66/Ke OBS. C.R. Acad. Sci. Paris, 316 (Ser. II): 341-347.

Clague, D.A., 1988. Petrology of ultramatic xenoliths from Loihi Seamount, Hawaii. J. Petrol., 29: 1161-l 186.

Coffm, M.F. and Eldholm, O., 1994. Large igneous provinces: CrUSti st~cture, dimensions and external consequences. Rev. Geophys., 32: l-36.

Giret, A., in press. The Kerguelen archipelago: an hypothetic continental mafic protolith. Terra Antarct.

GrifIIths, R.W. and Campbell, I.H., 1990. Stirring and structure in mantle starting plumes. Earth Planet. Sci. Len., 99: 273-2%.

Griftiths, R.W. and Campbell, I.H., 1991. Interaction of mantle heads with the Earth’s surface and onset of small-scale con- vection. J. Geophys. Res., 96(18): 295-310.

Hart, S.R., 1984. A large scale isotope anomaly in the Southern Hemisphere mantle. Nature, 309: 753-757.

Hassler, D., Shimizu, N., Frey, F., Weis, D., Cottin, J.-Y. and Giret, A., 1994. Geochemistry of petidotite and granulite xenoliths from Courbet Peninsula, northeastern Kerguelen Is- lands. Eos Trans. Am. Geophys. Union, 74 632.

Houtz, R.E.. Hates, D.E. and Mark], R.G., 1977. Kerguelen Plateau bathymetry. sediment distribution and crustal structure. Mar. Geol., 25: 95-130.

Dosso, L. and Murthy, V.R., 1980. A Nd isotopic study of the Kerguelen Islands: inferences on enriched oceanic mantle sources. Earth Planet. Sci. Lett., 48: 268-276.

Duncan, R.A., 1991. Age distribution of volcanism along aseismic ridges in the eastern Indian Ocean. Proc. ODP, Sci. Results, 121: 507-517.

Jagoutz, E.. Carlson, R.W. and Lugmair, G.W., 1980. Equilibrated Nd-unequilibrated Sr isotopes in mantle xenoliths. Nature, 286: 708-710.

Kent, R.W., 1991. Lithospheric uplift in eatem Gonswana: evidence for a long-lived mantle plume system? Geology, 20: 891-894.

Dupre, B. and Allegre, C.J., 1983. Pb-Sr isotope variation in Indian Ocean and mixing phenomena. Nature, 303: 142- 146.

Frey, F.A. and Prinz, M., 1978. Ultramatic inclusions from San Carlos, Arizona. Petrologic and geochemical data bearing on their petrogenesis. Earth Planet. Sci. Lett., 38: 139-176.

Frey, F.A., Jones, W.B., Davies, H. and Weis, D., 1991. Geo- chemical and petrologic data for basalts from Sites 756, 757 and 758: implications for the origin and evolution of Nine- tyeast Ridge. In: J. Weissel, J. Peirce, E. Taylor, J. Alt et al. (Editors), Proc. ODP, Sci. Results, 121: 61 l-659.

Frey, F.A., Yang, H.-J., Weis, D. and Gitet, A., 1994. Early evolution of the Kerguelen Archipelago: Transition of a plume volcano from a ridge-centered to intraplate setting. EOS, Trans. Am. Geophys. Union, 75: 708.

Frey, F.A., McNaughton, N.J., Nelson, D.R., delaeter, J.R. and Duncan, R.A., submitted. Geochemical characteristics of the Bunbury basalts, Western Autralia: Interaction between the Kerguelen plume and Gondwana lithosphere. Earth Planet. Sci. Lett.

Kiinnecke, L. and Coffm, M.F., 1994. Tectonics of the Kerguelen Plateau, Southern Indian Ocean. EOS, Trans. Am. Geophys. Union, 75: 174.

Leclaire, L. et al., 1987. Lower Cretaceous basalts and sediments from the Kerguelen Plateau. Geo-Mar. Lett., 7: 169- 176.

Leyrit, H., 1992. Kerguelen: Cartographic et Magmatologie des Presqu’iles Jeanne d’Arc et Ronarc’h. Place des laves diff&e- nciees. Ph.D. Thesis, Univ. Paris-Sud.

Mahoney, J.J.. MacDougall, J.D., Lugmair, G.W., 1983. Kergue- len hotspot source for Rajmahal Traps and Ninetyeast Ridge? Nature, 303: 385-389.

Gautier, I., Weis, D., Metmessier, J.-P., Vidal, P., Giret, A. and Loubet, M., 1990. Petrology and geochemistry of Kerguelen basalts (South Indian Ocean): evolution of the mantle sources from ridge to an intraplate position. Earth Planet. Sci. Lett., 100: 59-76.

Mahoney, J.J., Jones, W.B., Frey, F.A., Salters, V.J.M., Pyle, D.G. and Davies, H.L., 1995. Geocbemical characteristics of lavas from Broken Ridge, the Naturaliste Plateau and south- ernmost Kerguelen Plateau: Early volcanism of the Kerguelen hotspot. Chem. Geol., 120: 315-345.

Mattielli, N., Weis, D. and Shimizu, N., 1992. Trace element and isotope study of Kerguelen xenoliths: chemical heterogeneity of the Dupal mantle. XXIXth Int. Geol. Gong., Kyoto, Japan, Abstr. Vol., l/3: 177.

Giret, A., 19%. Typology, evolution and origin of the Kerguelen plutonic series, Indian Ocean: a review. Geol. J., 25: 239-247.

Gn?goire, M., 1994. Wtrologie des Enclaves Ultrabasiques et Basiques des Iles Kerguelen. Les Contraintes Mineralogiques et Thermobarom&riques et leurs Implications Gtodynamiques. Ph.D. Thesis, Univ. Jean Monnet.

Menke, W. and Levin, V., 1994. Cold crust in a hot spot. Geophys. Res. Lett., 21(18): 1%7- 1970.

Munschy, M., Fritsch, B., Schlich, R. and Rotstein, Y., 1994. Tectonique extensive sur le plateau de Kerguelen. Mbm. Sot. GBol. Fr., 166: 99-108.

Nougier, J., Pawlowski, D. and Cantagml, J.M., 1983. Chrono- spatial evolution of the volcanic activity in southeastern Ker- guelen (TAAFJ. In: R.L. Oliver, P.R. James and J.B. Jago (Editors), Antarctic Earth Science. Cambridge Univ. Press, pp. 640-645.

Gtt?goire, M., Mattielli, N., Nicollet, C., Cottin, J.-Y., Leyrit, H., PolvC, M., 1983. Les Isotopes du Nd et du Sr dans les Lherzolites Weis, D.. Shimizu, N. and Giret, A., 1994. Granulite facies Grogeniques: Contribution a la Wtetmination de la Structure xenoliths from the Kerguelen Archipelago, Southern Indian et de la Dynamique du Manteau Sup&ieur. Ph.D. Thesis, Ocean. Nature, 367: 360-363. Univ. Paris VII.

Gr6goire. M., Cottin, J.Y., Mattielli, N., Nicollet, C., Weis, D. and Pringle, M.S., Storey, M. and Wijbrans, J., 1994. 4oAr-39Ar

280 N. Mcrtticlli et al. / Litho.\ 37 i 19961261-280

Geochronology of mid-Cretaceous Indian ocean basalts: Con-

straints on the origin of large flood basalt provinces. EOS,

Trans. Am. Geophys. Union, 75: 728.

Recq, M., Brefort, D., Malod, J. and Veinante, J.L., 1990. The

Kerguelen Isles (southern Indian Ocean): New results on deep

structure from refraction protils. Tectonophysics, 182: 227-

248.

Recq, M., Chat+, P., Operto, S., Bredfort, D.. 1995. Deep

structure of the northern Kerguelen Plateau and hot spot-rr-

lated activity. In: 8th EUG Meeting, Strasbourg, France. Terra

Nova, 7: 64.

Royer, J.-Y. and Coffin, M.F., 1992. Jurassic to Eocene plate

tectonic reconstructions in the Kerguelen Plateau region. In:

S.W. Wise, R. Schlich et al. (Editors), Proc. ODP. Sci.

Results, 120: 917-928.

Salters, V.J.M., Storey, M., Sevigny. J.H. and Whitechurch. H..

1991. Trace element and isotopic characteristics of Kergue-

len-Heard plateau basalts. In: S.W. Wise, R. Schlich et al.

(Editors), Proc. ODP, Sci. Results, 120: 55-62.

Saunders, A, Storey, M., Kent, R.W. and Norry. M.J., 1992.

Consequences of plume-lithosphere interactions. In: B.C. Al-

abaster and R.J. Pankhurst (Editors), Magmatism and the

Causes of Continental Break-up. Geol. Sot. Spec. Pub]., 68:

41-60.

Schaming, M. and Rotstein, Y., 1990. Basement reflectors in the

Kerguelen Plateau, south Indian Ocean: Implications for the

structure and early history of the plateau. Geol. Sot. Am.

Bull., 102: 580-592.

Schiano, P., Clocchiatti, R., Shimizu, N., Weis. D. and Mattielli,

N., 1994. Cogenetic silica-rich and carbonate-rich melts

trapped in mantle minerals in Kerguelen ultramafic xenoliths:

Implications for metasomatized upper mantle. Earth Planet.

Sci. Lett.. I I I: 69-82.

Sen, G., Frey, A.F., Shimizu, N. and Leeman, W.P., 1993.

Evolution of the lithosphere beneath Oahu, Hawai: Rare Earth

element abundances in mantle xenoliths. Earth Planet. Sci.

Lett., 119: 53-69.

Shimizu, N. and Hart, S.R., 1973. Differential Dissolution Tech-

nique (DDT): Chemical Separation of Crystals from glass.

Carnegie Inst. Washington Yearb., 72: 269-272.

Shimizu, N. and Hart, S.R., 1982. Applications of the ion micro-

probe to geochemistry and cosmochemistry. Annu. Rev. Earth

Planet. Sci., IO: 483-526.

Shimizu, N. and Weis, D., 1991. Chemical heterogeneity of the

Dupal mantle: a trace element study of the Kerguelen xenolith

suite. In: 6th EUG Meet., Strassbourg. France. Terre Nova, 3:

458.

Siena, F., Beccaluva, L., Coltorti, M., Marchesi, S. and Morra, V., 1991. Ridge to hotspot evolution of the atlantic lithosphere

mantle: evidence from the Lanzarote peridotitc xenoliths

(Canary Islands). In: Orogenic Lherzolites and Mantle Pro-

cesses. J. Petrol., pp. 255-270.

Storey, M., Saunders, A.D., Tamey, J., Leat, P.. Thirlwall, M.F.. Thompson, R.N., Menzies, M.A. and Marriner, G.F., 1988.

Geochemical evidence for plume-mantle interactions beneath

Kerguelen and Heard Islands, Indian Ocean. Nature 336: 371-374.

Storey, M., Kent, R., Saunders, A.D., Salters, V.J., Hergt, J.,

Whitechurch, H., Sevigny, J.H., Thirwall, M.F., Leat, P.,

Chose, N.C. and Gifford, M., 1992. Lower Cretaceous vol-

canic rocks along continental margins and their relationship to

the Kerguelen Plateau. In: SW. Wise, R. Schlich et al.

(Editors), Proc. ODP, Sci. Results, 120: 33-54.

Sun, S. and McDonough, W.F., 1989. Chemical and isotopic

systematics of oceanic basalts: implications for mantle compo-

sition and processes. In: A.D. Saunders and M.J. Norry (Edi-

tors), Magmatism in the Ocean Basins. Oxford, pp. 313-345.

Valbracht, P., Honda, M., Matsumoto, T., Mattielli, N., Mc-

Dougall, I., Ragettli, R. and Weis, D., 1996. Helium, neon and

argon isotope systematics in Kerguelen ultramatic xenoliths:

implications for mantle source signatures. Earth Planet. Sci.

Lett., 138: 29-38.

Vance, D., Stone, J.O.H. and O’Nions, R.K., 1989. He, Sr and Nd

isotopes in xenoliths from Hawaii and other oceanic islands.

Earth Planet. Sci. Lett., 96: 147-160.

Watkins, N.D., Gunn, B.M., Nougier, J. and Baksi, A.K., 1974.

Kerguelen: continental fragment or oceanic island. Geol. Sot.

Am. Bull., 85: 201-212.

Weis, D. and Frey, F.A., 1991. Isotope geochemistry of Nine-

tyeast Ridge basal& Sr, Nd and Pb evidence for the involve-

ment of the Kerguelen hot spot. In: J. Weissel, J. Peirce, E.

Taylor, J. Alt et al. @ditors), Proc. ODP, Sci. Results, 121:

591-610.

Weis, D. and Giret, A., 1994. Kerguelen plutonic complexes: Sr,

Nd. Pb isotopic study and inferences about their sources, ages

and geodynamic setting. Mtm. Sot. Gtol. Fr., N.S., 166:

47-59

Weis, D., Demaiffe, D., Cauet, S. and Javoy, M., 1987. Sr, Nd, 0

and H isotopic ratios in Ascension Island lavas and plutonic

inclusions: cogenetic origin. Earth Planet. Sci. Lett., 82: 255-

268.

Weis, D., Bassias, Y., Gautier, I. and Mennessier, J.-P., 1989.

Dupal anomaly in existence 1 I5 Ma ago: Evidence from

isotopic study of the Kerguelen Plateau (South Indian Ocean).

Geochim. Cosmochim. Acta, 53: 2125-2131.

Weis, D., White, W.M.. Frey, F.A., Duncan, R.A., Dehn, J., Fisk,

M.. Ludden, J., Saunders, A. and Storey, M., 1992. The

influence of mantle plumes in generation of Indian oceanic

crust. In: R.A. Duncan et al. (Editors), Synthesis of Results

from Scientific Drilling in the Indian Ocean. Am. Geophys.

Union, Geophys. Monogr., 70: 57-89.

Weis, D., Frey, F.A., Leyrit, H. and Gautier, I., 1993. Kerguelen Archipelago revisited: geochemical and isotopic study of the

SE Province lavas. Earth Planet. Sci. Lett., 118: IO1 - I 19.

White, W.M. and Hofman, A.W., 1982. Sr and Nd isotope

geochemistry of oceanic basalts and mantle evolution. Nature, 296: 821-825.

Whitechurch, H.R., Montigny, J., Storey, M. and Salters, V.,

1992. K-Ar and @Ar-j9Ar ages of central Kerguelen Plateau basal& In: S.W. Wise, R. Schlich et al. (Editors), Proc. ODP,

Sci. Results, 120: 71-77.

Zindler, A. and Hart, S.R., 1986. Chemical geodynamics. Annu.

Rev. Earth Planet. Sci., 14: 493-571.

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Kerguelen basic and ultrabasic xenoliths: Evidence for long-lived Kerguelen hotspot activity