flood basalt from mont tourmente in the central kerguelen archipelago: the change from transitional...

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 7 PAGES 1367–1387 2002

Flood Basalt from Mont Tourmente in theCentral Kerguelen Archipelago: the Changefrom Transitional to Alkalic Basalt at>25 Ma

F. A. FREY1∗, K. NICOLAYSEN1†, B. K. KUBIT2, D. WEIS3 ANDA. GIRET4

1DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF

TECHNOLOGY, CAMBRIDGE, MA 02139, USA2DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF MASSACHUSETTS, AMHERST, MA 01003, USA3DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, CP 160/02, UNIVERSITE LIBRE DE BRUXELLES,

AV. F. D. ROOSEVELT, 50, B-1050, BRUSSELS, BELGIUM4LABORATOIRE DE GEOLOGIE, UNIVERSITE JEAN MONNET, CNRS–UMR 6524, 23 RUE DU DOCTEUR PAUL

MICHELON, 42023, SAINT-ETIENNE, CEDEX 2, FRANCE

RECEIVED MAY 7, 2001; REVISED TYPESCRIPT ACCEPTED FEBRUARY 14, 2002

the eastern sections, which have been proposed to be characteristicThe surface of the Cenozoic Kerguelen Archipelago, constructed onof the Kerguelen plume. Consequently, the Mont Tourmente isotopicthe Kerguelen Plateau in the southern Indian Ocean, is dominantlydata may reflect heterogeneity within the plume or a constantflood basalt. With the objective of understanding the Cenozoicproportion of a depleted component mixed with the high 87Sr/86Srhistory of the Kerguelen mantle plume, the age and geochemicaland low 143Nd/144Nd plume. In contrast to many of the Cretaceouscharacteristics of this flood basalt province are being determined byKerguelen Plateau lavas, there is no evidence in trace elementstudying stratigraphic sections of basalt flows at several locations.abundances or Sr and Nd isotopic ratios that the Cenozoic KerguelenSections from the NW, north–central, east and SE parts of theArchipelago lavas were influenced by continental lithosphere.archipelago have been studied. Here we report results for a 597 m

succession of lavas from Mont Tourmente from the Plateau Central,a region of the archipelago that has not been studied in detail. MontTourmente lavas range from >26 Ma dominantly transitional

KEY WORDS: Kerguelen Archipelago; Kerguelen plume; flood basalt; igneousbasalts in the lower 80% of the section to>25·3 Ma dominantlygeochemistryalkalic basalts in the upper part of the section. The timing of this

change from transitional to alkalic volcanism within the MontTourmente section is consistent with that defined by the older>28–29 Ma transitional basalts in the north and the >25 Ma

INTRODUCTIONalkalic lavas erupted in the east. This change in basalt compositionmay be related to migration of the archipelago away from the plume To understand the Cenozoic volcanism attributed to theor to increasing lithosphere thickness over the >5 Myr of flood Kerguelen plume we are studying stratigraphic sectionsbasalt volcanism. The alkalic and transitional Tourmente lavas are of the flood basalt lavas that cover most of the Kerguelennearly homogeneous in isotopic ratios of Sr, Nd and Pb. They have Archipelago (Fig. 1). Our objective is to define and

understand the temporal and spatial variations in thelower 87Sr/86Sr and higher 143Nd/144Nd compared with lavas from

∗Corresponding author. E-mail: [email protected]†Present address: Geology Department, Kansas State University,Manhattan, KS 66506, USA. Oxford University Press 2002

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 7 JULY 2002

Fig. 1. Map of the Kerguelen Archipelago showing geographical regions, such as the Plateau Central, the major geological units and thelocation of studied stratigraphic sections of the flood basalts: Ravins du Charbon and Jaune in the SE Province (Frey et al., 2000), Mont Crozierin the NE (Damasceno et al., 2002), Mont Bureau and Mont Rabouillere in the north (Yang et al., 1998), Mont des Ruches and Mont Fontainein the NW (Doucet et al., 2002) and Mont Tourmente in the Plateau Central (this study). Mont Ross is the youngest edifice in the archipelago(Weis et al., 1998). Inset is a map of the eastern Indian Ocean showing volcanic structures attributed to the Kerguelen plume; i.e. the largeigneous province formed by the now separated Cretaceous Kerguelen Plateau and Broken Ridge; the hotspot track formed by the Cretaceousto Cenozoic Ninetyeast Ridge and the Cenozoic Kerguelen Archipelago and Heard Islands (×s) located on the northern Kerguelen Plateau.Χ, drilling and dredging sites on the Ninetyeast Ridge, Broken Ridge, Naturaliste Plateau and Kerguelen Plateau.

geochemical characteristics of these lavas. Studies of In this paper we focus on a section from Mont Tour-sections from the SE (Frey et al., 2000), NE (Damasceno mente, which is located in the central part of the ar-et al., 2002), north–central (Yang et al., 1998) and NW chipelago in a region known as Plateau Central (Fig. 1).(Doucet et al., 2002) show that there are important In a survey study of the flood basalts Gautier et al. (1990)regional geochemical differences in the flood basalts. found that the Sr and Nd isotopic ratios of the transitionalThese differences include: (1) a temporal change from basalts from this region (four samples) are intermediateolder tholeiitic–transitional to younger alkalic lavas and between those of the alkalic basalts in the SE and NE(2) isotopic (Sr, Nd, Pb) heterogeneity in the oldest, and the tholeiitic to transitional basalts in the NW. A>28–29 Ma basalts (Yang et al., 1998; Doucet et al., goal in this paper is to determine the generality of the2002), which contrasts with the isotopic homogeneity of survey results by Gautier et al. (1990) and to answeryounger, 25 Ma, sections (Weis et al., 1998; Frey et al., the question—are lavas erupted in the Plateau Central

geochemically distinct from basalts erupted in other parts2000).

1368

FREY et al. FLOOD BASALT FROM MONT TOURMENTE

Fig. 2. Location of studied samples (black horizons with sample numbers) in the Mont Tourmente section. Ages ( 40Ar/39Ar) indicated forsamples 93-414 and 83-354 are from Nicolaysen et al. (2000). The white levels are those without outcrop or very weathered.

of the archipelago? Previously, Frey et al. (2000) showed This region is characterized by basaltic flows that extendfor very long distances and are only cut locally bythat the trend to more alkalic lavas with decreasing age

in the Kerguelen Archipelago reflects (1) a decreasing small streams; glaciation, however, has exposed verticalsections, up to 800 m high. The total volume of mag-supply of magma from the plume, (2) a temporal decrease

in extent of melting and (3) a temporal increase in depth matism represented by the Plateau Central is a significantportion of the flood basalts. Like the other volcanoes inof melt segregation. If radiogenic isotopic ratios of Plateau

Central lavas differ from those of younger alkalic flood the area, Mont Tourmente has a general oval shape withbasalts, these data bear on important issues such as the a clear east–west orientation.heterogeneity of the Kerguelen plume and changes inmixing ratios between source components derived fromplume, asthenosphere and oceanic lithosphere.

SAMPLE DESCRIPTIONSixty-four samples (93-351 to 93-414) were collected from

GEOLOGY a 597 m section (Fig. 2). On the basis of 40Ar/39Ar dating,Nicolaysen et al. (2000) reported eruption ages of 26·0Mont Tourmente is located in the northern part of the

Plateau Central in the Kerguelen Archipelago (Fig. 1). ± 1·0 Ma for sample 93-414 at the bottom of the section

1369


Table 1: Major (wt % oxides) and trace element abundance (ppm) in lavas from Mont Tourmente

Sample: 93-351 93-352 93-353 93-354 93-355 93-356 93-357 93-358 93-359 93-360 93-361

Height (m): 597 577 563 555 545 537 533 529 519 512 504

XRF

SiO2 46·62 47·85 48·32 47·57 46·44 46·97 46·61 46·89 47·85 48·07 47·05

TiO2 3·96 3·84 3·82 4·26 4·24 4·22 3·86 3·83 3·41 3·30 3·59

Al2O3 13·68 13·10 13·12 12·74 13·58 13·33 13·29 13·14 13·79 13·81 13·64

Fe2O3 16·15 15·80 15·88 16·71 16·16 16·40 16·37 16·47 15·27 14·91 15·94

MnO 0·36 0·24 0·26 0·26 0·33 0·24 0·27 0·25 0·22 0·23 0·23

MgO 5·38 4·98 5·09 4·80 4·97 4·81 5·48 5·29 5·19 5·42 5·42

CaO 9·85 9·88 9·57 9·18 9·98 9·53 10·02 10·04 10·70 10·64 9·98

Na2O 2·79 2·76 3·01 3·12 2·97 3·03 2·85 2·91 2·76 2·80 2·72

K2O 0·62 0·66 0·68 0·96 0·62 1·11 0·68 0·60 0·39 0·39 0·48

P2O3 0·46 0·45 0·45 0·52 0·50 0·48 0·42 0·42 0·34 0·35 0·44

Total 99·87 99·56 100·20 100·12 99·79 100·10 99·83 99·84 99·90 99·91 99·49

V 382 352 349 397 392 377 379 383 342 331 345

Cr 70 65 63 38 62 62 57 74 50 67 93

Ni 35 33 30 27 35 36 40 41 35 39 37

Zn 169 154 161 175 169 169 168 163 137 134 160

Ga 26 24 24 25 25 28 26 24 23 23 25

Rb 5·9 6·9 10·7 17·4 3·3 25·8 6·5 3·4 1·9 1·3 4·7

Sr 338 328 330 317 347 323 317 335 338 348 326

Ba 193 209 228 222 222 215 181 195 159 173 199

Y 40·8 37·2 37·5 41·0 40·7 39·2 36·7 36·3 30·6 31·1 38·2

Zr 269 255 280 286 282 282 244 243 205 204 254

Nb 30·1 28·3 31·3 32·1 31·4 31·7 27·0 26·8 21·7 22·1 27·5

Ce 52 51 47 58 54 55 44 44 38 41 53

Sample: 93-362 93-363 93-364 93-365 93-366 93-367 93-368 93-369 93-370 93-371 93-372

Height (m) 496 485 475 466 462 456 445 436 431 424 418

XRF

SiO2 46·62 46·72 49·18 47·83 49·40 49·67 49·66 49·41 49·49 49·28 50·38

TiO2 3·71 3·93 3·47 3·36 3·20 3·36 3·14 3·11 3·13 3·65 3·56

Al2O3 13·53 13·42 13·01 14·03 13·32 13·19 13·49 13·45 13·47 12·76 13·15

Fe2O3∗ 16·54 16·56 15·51 15·13 14·63 15·04 14·67 14·61 14·59 15·92 14·82

MnO 0·24 0·24 0·24 0·24 0·22 0·24 0·24 0·24 0·23 0·26 0·24

MgO 5·41 5·35 5·05 5·54 5·31 4·83 5·45 5·49 5·40 4·75 4·22

CaO 10·01 10·40 9·59 10·37 9·91 9·61 9·99 10·05 10·06 9·27 8·83

Na2O 2·77 2·66 2·99 2·68 3·15 2·88 2·91 2·85 2·83 2·90 2·95

K2O 0·36 0·38 0·57 0·40 0·58 0·47 0·53 0·48 0·47 0·48 1·22

P2O3 0·45 0·45 0·39 0·41 0·37 0·42 0·33 0·35 0·34 0·43 0·41

Total 99·64 100·10 100·00 99·99 100·09 99·70 100·41 100·05 100·01 99·69 99·78

V 363 356 345 340 317 313 327 310 320 349 354

Cr 64 58 58 79 65 47 67 69 69 26 28

Ni 39 41 41 50 46 39 47 49 51 33 28

Zn 168 166 152 151 139 142 141 138 141 158 160

Ga 24 24 23 23 23 23 22 22 22 24 24

Rb 1·5 2·1 9·7 1·3 10·9 13·7 9·4 7·2 9·0 14·9 42·5

Sr 335 340 324 333 326 320 323 329 325 320 322

Ba 181 203 200 168 192 183 171 173 173 184 197

Y 39·9 38·3 34·1 35·0 32·2 34·8 30·7 31·3 31·4 37·1 35·3

Zr 261 270 248 244 233 250 212 213 215 260 251

Nb 28·1 28·7 26·6 26·1 24·8 26·6 23·0 22·8 23·3 28·0 27·1

Ce 50 53 45 48 43 50 38 43 42 52 46

1370


Sample: 93-373 93-374 93-375 93-376 93-377 93-378 93-379 93-380 93-381 93-382 93-383

Height (m): 412 406 396 392 363 345 345 341 340 329 324

XRF

SiO2 49·86 50·24 49·28 49·29 49·22 49·04 49·30 47·75 48·87 49·43 50·03

TiO2 3·45 3·41 3·63 3·06 3·73 3·03 2·92 3·70 2·88 2·88 3·18

Al2O3 13·09 12·96 13·20 14·29 13·21 14·93 14·49 13·91 14·29 13·88 13·47

Fe2O3∗ 15·05 14·72 15·41 13·49 15·42 13·24 13·50 15·02 13·57 13·61 14·62

MnO 0·23 0·22 0·24 0·20 0·23 0·22 0·21 0·27 0·20 0·20 0·23

MgO 4·89 4·72 4·72 5·45 4·83 5·47 5·32 5·57 5·99 5·65 4·64

CaO 9·37 9·24 8·84 10·29 9·03 10·38 10·34 10·22 10·50 10·74 9·23

Na2O 2·95 3·01 2·88 2·82 3·08 2·82 2·73 2·76 2·80 2·76 3·26

K2O 0·79 0·68 0·85 0·47 0·76 0·75 0·58 0·29 0·43 0·53 0·59

P2O3 0·40 0·38 0·42 0·38 0·46 0·37 0·35 0·39 0·29 0·31 0·43

Total 100·08 99·58 99·46 99·74 99·96 100·25 99·73 99·87 100·28 99·99 99·68

V 347 342 362 333 353 299 291 372 313 292 308

Cr 46 61 40 107 40 109 107 61 137 126 37

Ni 37 38 28 43 26 51 48 41 60 58 27

Zn 156 148 161 142 153 136 131 166 124 122 147

Ga 23 23 24 23 24 24 23 25 22 23 24

Rb 9·6 18·4 10·3 14·3 8·2 6·1 4·5 0·6 2·2 5·0 13·4

Sr 336 322 335 342 340 353 346 358 343 326 333

Ba 205 198 219 192 229 187 192 143 153 160 224

Y 36·0 34·1 37·7 32·8 38·0 33·9 31·3 36·1 26·9 27·1 36·0

Zr 242 250 275 232 273 227 218 257 180 181 261

Nb 26·0 27·1 29·4 25·6 29·3 24·5 23·6 28·6 19·2 19·5 28·5

Ce 51 46 50 46 55 47 45 49 29 35 54

Sample: 93-384 93-385 93-386 93-387 93-388 93-389 93-390 93-391 93-392 93-393 93-394

Height (m): 316 308 296 287 281 272 256 248 232 222 216

XRF

SiO2 50·67 49·09 50·57 49·05 49·36 48·52 50·85 50·49 49·78 49·57 48·95

TiO2 3·21 3·63 3·54 3·05 3·19 3·67 4·08 3·88 3·24 3·25 3·76

Al2O3 13·49 13·15 13·33 14·28 13·46 13·41 12·78 13·26 14·02 13·31 13·90

Fe2O3 14·33 15·31 14·51 13·47 14·41 14·72 14·76 14·28 13·74 14·34 14·17

MnO 0·22 0·23 0·22 0·20 0·22 0·24 0·21 0·23 0·21 0·22 0·24

MgO 4·52 4·89 4·39 5·48 5·46 5·39 4·50 4·80 5·52 5·28 5·02

CaO 8·79 9·59 8·61 10·30 9·77 9·87 8·67 8·84 9·80 10·18 9·09

Na2O 3·06 3·02 3·23 2·88 2·91 2·87 3·27 3·10 2·88 2·88 3·03

K2O 1·15 0·45 0·90 0·47 0·65 0·60 0·63 0·65 0·50 0·59 1·38

P2O3 0·43 0·41 0·45 0·37 0·42 0·42 0·55 0·49 0·43 0·38 0·45

Total 99·86 99·77 99·74 99·54 99·86 99·71 100·21 100·02 100·11 100·00 99·99

V 313 348 339 304 320 366 366 337 305 315 374

Cr 33 54 50 126 133 72 34 90 160 82 105

Ni 26 36 32 56 56 38 32 57 59 52 50

Zn 152 147 147 131 144 143 156 151 137 141 146

Ga 24 24 25 23 23 23 25 25 23 23 26

Rb 30·3 13·1 13·3 3·2 9·6 7·4 24·6 17·8 8·5 9·1 31·5

Sr 330 327 352 363 328 347 325 342 333 328 320

Ba 227 184 257 192 201 191 233 237 199 182 197

Y 36·1 35·1 38·7 31·8 34·3 34·2 41·2 40·3 36·3 32·8 37·8

Zr 263 247 294 225 243 235 314 311 259 234 265

Nb 28·4 27·2 31·4 24·5 26·9 26·4 34·2 33·8 27·3 25·1 29·0

Ce 54 50 58 49 50 47 63 60 51 48 50

1371


Table 1: continued

Sample: 93-395 93-396 93-397 93-398 93-399 93-400 93-401 93-402 93-403 93-404 93-405

Height (m): 211 207 197 187 183 177 170 144 137 130 125

XRF

SiO2 49·38 49·86 49·93 50·52 50·22 49·39 50·11 48·94 48·48 48·36 50·47

TiO2 3·71 3·63 3·65 3·67 3·46 3·51 3·18 2·79 2·84 2·89 3·41

Al2O3 13·74 13·43 13·50 13·47 12·98 13·30 13·93 13·99 14·20 14·36 13·25

Fe2O3∗ 14·07 13·98 13·96 13·95 15·39 15·66 13·27 13·12 13·41 13·42 14·59

MnO 0·21 0·22 0·22 0·21 0·23 0·23 0·22 0·20 0·21 0·21 0·22

MgO 5·09 4·94 5·01 4·34 4·49 4·61 5·09 6·01 6·20 6·38 4·64

CaO 9·47 9·20 9·27 8·35 8·80 8·85 10·10 11·09 11·13 11·04 8·89

Na2O 3·06 3·11 3·07 3·28 3·24 2·97 2·82 2·67 2·66 2·62 3·12

K2O 0·67 0·77 0·75 1·25 0·57 0·93 0·78 0·44 0·41 0·25 0·64

P2O3 0·44 0·44 0·44 0·56 0·44 0·45 0·36 0·30 0·30 0·31 0·43

Total 99·84 99·58 99·79 99·60 99·81 99·89 99·85 99·55 99·84 99·84 99·63

V 364 362 354 355 330 342 298 291 291 294 315

Cr 101 101 100 68 24 24 152 166 163 229 25

Ni 51 50 51 39 20 20 60 70 69 86 24

Zn 140 141 139 153 152 155 127 119 119 124 143

Ga 24 24 24 26 24 24 24 21 22 23 24

Rb 5·3 10·4 11·3 26·7 14·5 13·1 15·7 3·2 2·2 1·1 13·7

Sr 350 336 339 335 335 342 338 350 356 348 337

Ba 210 209 207 242 217 212 173 153 153 131 234

Y 37·1 36·5 36·5 43·3 55·7 37·1 32·2 26·4 27·3 28·6 35·9

Zr 259 257 255 315 268 271 222 183 185 192 263

Nb 28·1 28·1 27·8 34·0 28·8 29·0 23·5 19·7 20·0 20·5 28·2

Ce 52 51 53 65 54 55 46 39 37 39 53

Sample: 93-406 93-407 93-408 93-409 93-410 93-411 93-412 93-413 93-414

Height (m): 112 104 101 92 81 56 45 33 16

XRF

SiO2 50·42 51·15 51·20 49·60 49·91 49·79 49·95 50·47 50·25

TiO2 3·51 3·33 3·39 2·98 3·07 3·33 3·31 3·27 3·30

Al2O3 13·43 13·96 13·48 13·85 13·48 13·46 13·42 13·20 13·57

Fe2O3∗ 15·07 14·06 14·28 13·78 13·99 14·97 15·04 14·85 14·57

MnO 0·24 0·22 0·22 0·22 0·22 0·22 0·23 0·22 0·21

MgO 4·20 4·05 4·40 5·57 5·43 4·88 4·71 4·68 4·63

CaO 8·10 7·87 8·45 10·16 9·70 9·20 8·84 8·96 8·86

Na2O 3·15 3·23 3·23 2·77 2·95 3·30 3·22 3·28 3·12

K2O 0·96 1·31 0·89 0·38 0·56 0·46 0·75 0·78 0·88

P2O3 0·51 0·41 0·48 0·35 0·35 0·45 0·45 0·45 0·45

Total 99·58 99·59 100·02 99·65 99·66 100·06 99·91 100·15 99·83

V 286 310 293 297 312 288 310 292 321

Cr 12 6 22 69 45 17 16 14 20

Ni 15 18 17 39 32 24 22 22 24

Zn 166 159 144 125 133 142 143 135 147

Ga 25 25 25 22 22 23 23 24 24

Rb 10·1 16·0 10·1 7·4 8·9 12·7 13·5 11·1 15·9

Sr 330 317 337 337 340 348 332 326 338

Ba 259 282 254 162 186 215 221 225 228

Y 55·6 43·7 36·5 28·2 29·8 37·2 35·5 34·9 36·2

Zr 311 331 276 199 208 251 251 249 254

Nb 32·5 34·5 28·9 21·3 22·5 27·1 26·9 26·7 27·4

Ce 73 64 55 39 43 56 51 53 54

1372


Tab

le1

:co

ntin

ued

Sam

ple

:351

352

356

358

360

365

376

380

381

390

392

394

402

403

404

406

407

409

414

Hei

gh

t

(m):

597

577

537

529

512

466

392

341

340

256

232

216

144

137

130

112

104

9216

INA

A

Sc

33·0

31·5

31·7

33·9

33·4

33·6

31·0

34·4

33·6±

628

·830

·7±

0·3

30·4

32·3

33·0±

0·1

32·3

28·0

25·4

31·0

30·3

Cr

6664

5968

6777

102

5913

5±4

3915

8±1

102

163

166±

124

210

1068

20

La24

·824

·025

·421

·818

·423

·621

·924

·216

·5±

0·1

28·5

24·2±

0·2

25·7

17·1

17·3±

0·1

18·3

39·3

31·3

18·8

25·8

Ce

59·5

56·6

61·2

52·9

45·9

54·6

50·7

60·1

40·1±

0·1

66·7

57·0±

1·3

60·8

41·4

42·6±

0·2

44·5

84·8

71·6

45·5

61·4

Nd

36·5

32·9

34·3

31·7

27·3

32·2

31·2

34·7

25·3±

0·6

39·0

33·4±

0·6

33·9

23·7

25·0±

0·6

24·9

44·3

39·8

27·7

34·9

Sm

8·50

7·96

8·37

7·84

6·63

7·72

7·33

7·96

5·74±

0·13

9·23

8·02±

0·05

8·41

6·00

6·08±

0·02

6·22

10·5

9·13

6·44

8·21

Eu

2·70

2·46

2·64

2·54

2·20

2·43

2·38

2·61

1·95±

0·3

2·71

2·47±

0·07

2·63

1·95

2·02±

0·01

2·06

3·15

2·55

2·10

2·56

Tb

1·32

1·16

1·31

1·15

1·04

1·18

1·19

1·25

0·96±

0·06

1·92

1·19±

0·05

1·31

0·95

1·01±

0·03

1·03

1·69

1·43

0·99

1·27

Yb

3·11

3·19

3·45

2·79

2·35

2·90

2·71

3·15

2·22±

0·04

3·45

3·03±

0·02

3·11

2·19

2·13±

0·04

2·24

4·09

3·56

2·23

2·89

Lu0·

490·

480·

460·

450·

360·

420·

390·

450·

32±

0·04

0·46

0·45±

0·01

0·46

0·31

0·33±

0·02

0·32

0·56

0·53

0·33

0·45

Hf

6·16

5·81

6·07

5·51

4·73

5·61

5·43

6·05

4·21±

0·01

7·01

6·02±

0·12

6·09

4·22

4·49±

0·11

4·57

7·05

7·37

4·74

5·84

Th

2·63

2·42

2·51

2·16

1·82

2·50

2·24

2·40

1·52±

0·02

3·2

2·6±

0·2

2·45

1·59

1·67±

0·02

1·98

3·41

4·25

2·19

2·97

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Fig. 3. Na2O + K2O vs SiO2 classification plot showing that the Mont Tourmente lavas (Χ and ×) straddle the alkalic–tholeiitic dividing lineof Macdonald & Katsura (1964).×, 13 samples from the upper 112 m of the section. Major element data were adjusted to a FeO/Fe2O3 molarratio of 0·85. In general, these >26–25 Ma Mont Tourmente lavas are not as alkalic as the slightly younger >25 Ma flood basalts erupted inthe SE Province (Frey et al., 2000) or at Mont Crozier (Damasceno et al., 2002). Most Mont Tourmente lavas overlap with the older, 28–29 Ma,lavas erupted at Mont Bureau and Mont Rabouillere in the north–central part of the archipelago (Yang et al., 1998).

Fig. 4. Alkalinity index and SiO2 abundance (wt %) vs stratigraphic height (meters) in the Mont Tourmente section. Alkalinity index [Na2O+ K2O − 0·37(SiO2 − 39)] is a measure of the deviation in wt % Na2O + K2O from the Macdonald & Katsura (1964) line in Fig. 3 withpositive values (Α ) indicating samples within the alkalic field. It should be noted that nine of the uppermost 13 lavas are alkalic and all 13samples have relatively low SiO2 contents. Sample numbers are indicated for the two alkalic lavas lower in the section and transitional sample380, which has a low SiO2 content.

and 25·3 ± 0·7 Ma for sample 93-354 from near the plagioclase and 3–4% clinopyroxene phenocrysts andmicrophenocrysts. The scarcity of phenocrysts in mosttop of the section (Fig. 2); therefore this section formed

within 1 Myr. Except for two samples, all of the samples of the Mont Tourmente lavas contrasts with the abundantphenocrysts, especially plagioclase, in the younger,contain <5% phenocrysts and microphenocrysts, dom-

inantly plagioclase with lesser amounts of clinopyroxene. 24–25 Ma, alkalic flood basalts erupted to the east atMont Crozier (Damsaceno et al., 2002).In contrast, samples 93-401 and 93-404 contain 8–10%

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Fig. 5. TiO2, P2O5, CaO, Al2O3, SiO2 and Na2O vs MgO content (all in wt %). Χ and Α, transitional and alkalic samples, respectively. Mostof the alkalic lavas have lower SiO2 and higher TiO2 and P2O5 contents than the tholeiitic lavas, especially the nine alkalic lavas from the upper112 m, labelled as Upper Alkalic Group. Also labelled are the four transitional samples with >6% MgO.

SE (Ravin Jaune and du Charbon sections), and theyANALYTICAL TECHNIQUESoverlap with the transitional flood basalts forming the

Abundances of major and several trace elements (Rb, Mont Bureau and Mont Rabouillere sections in theSr, Ba, V, Cr, Ni, Zn, Ga, Y, Zr, Nb and Ce) in 64

north–central part of the Kerguelen Archipelago (Fig.samples were determined by X-ray fluorescence (XRF),3). Within the Mont Tourmente section there is, however,and in a subset of 19 samples abundances of Sc, REE,a temporal change from transitional to alkalic basalt.Hf and Th were determined by instrumental neutronThe uppermost 13 lavas, 93-351 to 93-363 from 597 toactivation analysis (INAA) (Table 1). Sixteen samples485 m, are characterized by relatively low SiO2 contents,were analyzed for Sr and Nd isotopes and 13 for Pband 11 of the 13 are alkalic or very close to the tholeiitic–isotopes (Table 2).alkalic boundary (Figs 3 and 4). We conclude that these11 lavas form a distinctive geochemical group, which wedesignate as the ‘Upper Alkalic Group’. Two other alkalic

RESULTS lavas (93-394, 93-398) occur lower in the section; theydiffer from the uppermost samples in having relativelyMajor elementshigh K2O (>1·3–1·4%, Table 1) and SiO2 (>49%)In a total alkalis vs SiO2 classification plot, Mont Tour-contents (Fig. 4b).mente lavas straddle the alkalic–tholeiitic boundary (Fig.

The 64 Mont Tourmente lavas have low MgO contents3). Most of the Mont Tourmente lavas are less alkalic(Table 1), ranging from 4·05% (sample 93-407) to 6·38%than the slightly younger (>25 Ma) alkalic lavas forming

the flood basalts in the NE (Mont Crozier section) and (sample 93-404). At a given MgO content, relative to the

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Trace elementsLike TiO2 and P2O5, the abundances of incompatibletrace elements, such as Nb, define a broad inverse trendwith MgO content with the upper alkalic lavas generallyhaving the highest abundances of incompatible elementsat a given MgO content (Figs 5 and 6). Consistent withtheir relatively low MgO contents, the Mont Tourmentelavas have low Ni contents (<100 ppm). The positivecorrelations of Ni and Cr with MgO are consistentwith fractionation of mafic phases, such as olivine andpyroxene (Fig. 6); however, olivine is not present in theselavas. Abundance of Sc ranges from 30 to 34 ppm forMgO >5·5%, but MgO and Sc contents are positivelycorrelated at lower MgO contents (Fig. 6). Like CaO,Sc is compatible in clinopyroxene; therefore both theCaO–MgO and Sc–MgO trends are consistent withclinopyroxene as a major fractionating phase for magmaswith <5·5% MgO.

Plagioclase is the major phenocryst phase in these lavas.Abundance of Sr, an element compatible in plagioclase, isnot positively correlated with abundances of an in-compatible element, such as Nb; in fact, there is a poorlydefined negative trend (Fig. 7). An important role forplagioclase fractionation is indicated by Sr/Ce ratios,which are close to primitive mantle ratios in the highestMgO lavas but decrease to (Sr/Ce)PM >0·4 with de-creasing MgO (Fig. 8). This relative depletion of Sr isapparent in primitive mantle (PM) normalized plotswhere Sr abundances are nearly constant at 15–17 timesthe PM abundance (Fig. 9); the mean Sr content is 335± 11 ppm for 64 lavas from 597 m of section. Thelimited range in Sr and Sc contents (>13% and 30%,respectively) relative to the range of Nb abundances, afactor of>1·8, implies that the bulk solid–melt partitioncoefficients for Sr and Sc were near unity. This inferencerequires a fractionating assemblage dominated by theobserved phenocryst phases, qualitatively>50% feldsparand >30% clinopyroxene (assuming that the mineral–melt partition coefficient for Sr is>2 for plagioclase and

Fig. 6. Abundances of Ni, Cr, Sc and Nb (in ppm) vs MgO content >3 for clinopyroxene); these proportions are typical(wt %). Symbols as in Fig. 5. As for TiO2 and P2O5 in Fig. 5, the

for low-pressure evolution of basaltic magma (Toplis &Upper Alkalic Group is enriched in Nb at a given MgO; two transitionalCarroll, 1995).lavas 93-390 and 93-391 also have high Nb content.

Other important features in the primitive mantle-normalized plots are the negative slopes from Nb to Ybtransitional lavas the upper alkalic lavas have higher(Fig. 9); that is, unlike some basalts from the Cretaceousabundances of P2O5 and TiO2 and lower SiO2 contentsKerguelen Plateau (e.g. Mahoney et al., 1995; Frey et al.,(Fig. 5). In contrast, all samples define a similar negative2002), these Cenozoic Kerguelen Archipelago lavas aretrend for Na2O–MgO and a positive trend for CaO–MgOnot relatively depleted in Nb (or Ta). In fact, none ofthat levels off at>11% CaO when MgO exceeds>5·5%the flood basalt sections in the Kerguelen Archipelago(Fig. 5). The latter trend is consistent with control ofinclude lavas that are relatively depleted in Nb (e.g. Yanglava compositions by the onset of clinopyroxene frac-et al., 1998; Frey et al., 2000; Doucet et al., 2002). Theretionation as MgO decreased below 5·5%. Abundance ofare strong positive correlations between abundances ofAl2O3 also decreases with decreasing MgO content, butincompatible elements, such as Nb, Ce, Zr and Y, whichthere is considerable scatter with no distinction between

alkalic and transitional lavas (Fig. 5). are relatively immobile during post-magmatic alteration;

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Fig. 7. Abundances of Ce, Rb, Ba, Sr, Zr and Y vs Nb content (all in ppm). Symbols as in Fig. 5. The highest Nb abundances are in foursamples (93-390, -391, -398, -407) with relatively low MgO (Table 1a). Sample 93-406 has anomalously high Ce and Y. In contrast to the otherelements, neither Rb nor Sr contents vary systematically with Nb content. Rb abundances are highly variable, whereas Sr abundances vary by<15%.

as with TiO2 and P2O5 abundances, the Upper Alkalic southeastern Australia (Price et al., 1991). The enrichmentprocess can occur during the earliest stages of weatheringGroup samples tend to have the highest abundances of

these incompatible elements (Fig. 7). and involves localized mobilization of Y and REE withtheir deposition in groundmass phosphate. High rainfall,The aphyric sample 93-406 is anomalously enriched

in Y and Ce (Fig. 7) and several other incompatible which is characteristic of the windward sides of Hawaiianshields and the Kerguelen Archipelago, may promoteelements (Fig. 9). Part of this enrichment reflects its

relatively low MgO content (4·2%) and the effects of this process.Another major feature of the primitive mantle-nor-fractional crystallization [low Sc and (Sr/Ce)PM in Figs 6

and 8, respectively]. However, the exceptionally high La malized plots are the strong depletions in Rb and veryhigh ratios of Ba/Rb (Fig. 9), a ratio that is usually fairlyand Ce contents (Fig. 9) and deviation from the trend

defined by other Mont Tourmente lavas (Nb–Ce panel constant in unaltered oceanic basalts (Hofmann & White,1983). Also, abundances of Rb and Nb are not correlatedin Fig. 7) may reflect post-magmatic alteration. In both

hand specimen and thin section this sample is highly (Fig. 7), and the range in Rb content, from 0·6 to42·5 ppm, is much greater than that for the immobileweathered. Anomalous enrichments in Y and rare earth

elements (REE) have been observed in many localities, incompatible element Nb (factor of>1·8). We infer thatRb was mobile during post-magmatic alteration. Theincluding Hawaiian tholeiitic shields (Fodor et al., 1992;

Frey et al., 1994), alkalic lavas in French Polynesia ( Joron wide range in K/Rb, 210–4000, is also consistent withthis interpretation. In particular, the trend to highet al., 1991; Cotten et al., 1995) and tholeiitic lavas in

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Fig. 8. (Sr/Ce)PM vs MgO content (wt %) showing a decreasing (Sr/Ce)PM with decreasing MgO. Subscript ‘PM’ designates normalized toprimitive mantle estimate of 11·9 (Sun & McDonough, 1989).

K/Rb at low K2O/P2O5 (Fig. 10) is the same alteration of the Kerguelen Archipelago flood basalts; i.e. the MontBureau and Mont Rabouillere (>29 Ma) sections andtrend as shown by Hawaiian shield lavas (e.g. Feigenson

et al., 1983; Roden et al., 1994); i.e. Rb is more mobile the Mont Fontaine and Mont des Ruches (>28 Ma)sections (Fig. 11a). Mont Tourmente lavas have 87Sr/than K2O. Despite the obvious differences in climate, in

these areas of high rainfall the alkali metals, K and 86Sr and 143Nd/144Nd intermediate to the extremes definedby lavas from these older sections.especially Rb, are leached from the lavas, leading to

anomalously high K/Rb and low K2O/P2O5. A similar Relative to the >25 Ma alkalic basalts erupted in theeastern part of the archipelago (Mont Crozier and Ravinresult was found for Kerguelen Archipelago flood basalts

from Monts Bureau and Rabouillere (see Yang et al., du Charbon in Fig. 1) the Tourmente lavas have lower87Sr/86Sr and higher 143Nd/144Nd (Fig. 11a). Despite these1998, fig. 4b). For Mont Tourmente lavas loss of K could

affect the tholeiitic–alkalic classification plot by converting differences in Sr and Nd isotopic ratios, Pb isotopicratios of Mont Tourmente lavas largely overlap with thealkalic lavas to tholeiitic lavas, but in general both types

of lavas range widely in K2O/P2O5 (Fig. 10). younger alkalic flood basalts from Mont Crozier andRavin du Charbon; however, some Mont Tourmentelavas extend to lower Pb isotopic ratios and overlap withthe fields for the older 28–29 Ma basaltic sections from

Isotopes (Sr, Nd, Pb) Mont Fontaine and Mont Bureau (Fig. 11b).The Sr and Nd isotopic ratios of 16 Mont Tourmentelavas define an inverse 143Nd/144Nd–87Sr/86Sr trendcentered at 0·5127 and 0·7047, respectively (Fig. 11a).Compared with the wide range of Sr and Nd isotopic DISCUSSIONratios found in lavas from the Kerguelen Archipelago,

Temporal geochemical evolution of theMont Tourmente lavas are relatively homogeneous inKerguelen Archipelago flood basaltsisotopic ratios of Sr, Nd and Pb with no systematicImportant geochemical characteristics of the flood basaltsisotopic differences between the alkalic and transitionalforming the Kerguelen Archipelago vary systematicallylavas (Fig. 11, Table 2). In contrast to the relative isotopicwith location and eruption age. Transitional basalts arehomogeneity of lavas at Mont Tourmente (25–26 Ma),

isotopic heterogeneity is typical in slightly older sections exposed in north–central sections [the >29 Ma Mont

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bottom of the section to 25·3 ± 0·7 Ma for an alkalicbasalt near the top of the section (Fig. 2), is consistentwith the change from transitional basalt in the northernsections (28–29 Ma) to alkalic basalt in the eastern sections(>25 Ma). At a given MgO content, the alkalic lavashave higher abundances of incompatible elements thanmost of the transitional lavas (e.g. P, Ti and Nb in Figs5 and 6). The inferred decrease in extent of melting withdecreasing eruption age at Mont Tourmente is consistentwith the temporal trend inferred from studies of basaltsections in the eastern part of the archipelago (Frey etal., 2000).

Like the flood basalts in the eastern sections, the alkalicbasalts in the Mont Tourmente section are distinguishedby their relatively low SiO2 content (Fig. 12a), a resultconsistent with melt segregation from a peridotite atrelatively high pressure (e.g. Gaetani & Grove, 1998,table 8). For alkalic lavas from the SE Province andMont Crozier, pressures of melt segregation within thegarnet stability field are inferred on the basis of relativelylow heavy rare earth element (HREE) and Y contentsat a given abundance of a highly incompatible element,such as Nb or Th (Fig. 12b; Frey et al., 2000, fig. 6).There is, however, no evidence for residual garnet duringthe petrogenesis of the transitional or alkalic Mont Tour-mente lavas, which define a Y–Nb trend that is distinctfrom that of basalts in the SE Province and Mont Croziersections. The Y–Nb trend of Mont Tourmente lavasoverlaps with the data for the transitional flood basaltsfrom Mont Bureau and Mont Rabouillere (Fig. 12). At

Fig. 9. Incompatible element abundances in selected alkalic and trans- a given MgO content, there are also important differencesitional Tourmente lavas normalized to the primitive mantle estimates between flood basalt sections in Ce/Y, La/Yb and Nb/of Sun & McDonough (1989). Important features are the negative

Zr (Fig. 13). Such ratios increase as extent of meltingslopes from Nb to Yb with a pronounced relative depletion in Sr andlarge variation in Ba/Rb generally caused by Rb depletion. For the decreases, especially when garnet is a residual phase.transitional lavas (a) there is a general increase in abundances of These ratios in Mont Tourmente lavas, both transitionalincompatible elements with decreasing MgO content (indicated and alkalic, from the Plateau Central are lower than inin wt %). Except for Rb contents, the patterns for the alkalic lavas are

the alkalic flood basalts from the eastern sections (Croziersimilar to those of the transitional lavas. The shaded field in (b) indicatesthe range for 14 transitional samples; the upper limit exceeds values and SE Province), and they overlap with the P-typefor the alkalic lavas because some transitional lavas (such as 93-407) transitional lavas in the north–central Mont Bureau andare more evolved with low MgO content (4·05%) and high abundances

Mont Rabouillere sections (Fig. 13). Apparently all ofof incompatible elements (see Nb–MgO panel in Fig. 6).the Mont Tourmente magmas segregated within thestability field of spinel peridotite, but on the basis of their

Bureau and Mont Rabouillere sections (Yang et al., 1998)] relatively low SiO2 content (Fig. 12a) the uppermostand in NW sections [the >28 Ma Mont Fontaine and alkalic magmas segregated at pressures higher than theMont des Ruches sections (Doucet et al., 2002)], whereas transitional lavas.>25 Ma alkalic basalts are exposed in eastern sections In summary, studies of several basalt sections through[Ravin du Charbon and Ravin Jaune sections (Frey et the flood basalt forming the Kerguelen Archipelago showal., 2000) and Mont Crozier (Damasceno et al., 2002)]. that at >25 Ma the extent of melting decreased andIn the Mont Tourmente section from the Plateau Central melt segregation occurred at higher pressures, reaching(Fig. 1) a change from transitional to alkalic lavas occurs the garnet stability field beneath the eastern part of thewithin the succession of lavas with primarily transitional archipelago. At>25 Ma there were, however, importantbasalt in the lower 80% of the section and dominantly differences between the magma sources. The Sr and Ndalkalic basalt in the upper 112 m of this 597 m section. isotopic ratios of Mont Tourmente lavas are distinct fromThe timing of this change in lava composition at Mont those of the alkalic lavas erupted in the east (Fig. 11a).

In fact, these isotopic ratios in Mont Tourmente lavasTourmente, 26·0± 1 Ma for a transitional basalt at the

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Fig. 10. Alkalinity index (see Fig. 4 caption) and K/Rb vs K2O/P2O5 in Mont Tourmente lavas. Symbols as in Fig. 5. The trend to low K2O/P2O5 and high K/Rb reflects relative loss in K and especially Rb during low-temperature subaerial alteration. Both the alkalic and transitionallavas range in extent of alteration, as reflected by variable K2O/P2O5.

are intermediate between the isotopic extremes of the in the Kerguelen Archipelago (Mont Crozier and Ravinolder transitional basalt erupted in the northern sections du Charbon in Fig. 11) representing the low meltingat Mt. Bureau and Mt. des Ruches (Fig. 11a). component in the plume. This component, with relatively

high 87Sr/86Sr and low 143Nd/144Nd, also dominates inthe P-type transitional flood basalts from Mont Bureauand Mont Rabouillere (Fig. 11). Other transitional floodHeterogeneous Kerguelen plume or varyingbasalts from the northern sections (Mont Fontaine, Montmixing ratios between plume,des Ruches, D-type from Mont Bureau and Mont Ra-asthenosphere and lithosphere?bouillere) range to lower 87Sr/86Sr and higher 143Nd/Isotopic data for ocean-island basalt suites attributed to 144Nd (Fig. 11). These transitional lavas may contain amantle plumes, such as Hawaii, Iceland and Galapagos,relatively depleted component intrinsic to the Kerguelenindicate considerable isotopic heterogeneity within eachplume that was sampled only at higher extents of melting.suite. Does this heterogeneity dominantly reflect intrinsic

Within the Kerguelen Archipelago the isotopic char-differences within the plume or variable proportions ofacteristics of Mont Tourmente lavas are intermediatecomponents derived from the plume, asthenosphere andbetween the enriched and depleted extremes. Althoughlithosphere? This question has generated considerablethis intermediate character may represent mixing, andebate (for Hawaii: Lassiter & Hauri, 1998; Keller et al.important result is that the alkalic and transitional lavas2000; Regelous et al., 2002; for Iceland: Hanan et al.,from Mont Tourmente have similar isotopic ratios (Table2000; Kempton et al., 2000; for Galapagos: White et al.,2). Two-component melt mixing will result in a cor-1993; Blichert-Toft & White, 2001).relation between isotopic ratios and magma composition;Similarly, the wide range of radiogenic isotopic ratiostherefore it is unlikely that Mont Tourmente lavas reflectin lavas from the Kerguelen Archipelago (Fig. 11) maymixing between enriched alkalic and depleted transitionalbe explained by two endmember interpretations.to tholeiitic magmas. Mont Tourmente lavas may rep-(1) Intrinsic geochemical heterogeneities embedded

within the Kerguelen plume with the alkalic flood basalts resent magmas derived from variable extents of melting

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Table 2: Sr, Nd and Pb isotopic ratios and parent/daughter abundance ratios (calculated from data in Table

1) of lavas from Mont Tourmente

Sample (87Sr/86Sr) 2� ( 87Rb/86Sr) (87Sr/86Sr) (143Nd/144Nd) 2� 147Sm/144Nd 143Nd/144Nd 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

number meas. at 25·7 Ma meas. at 25·7 Ma

351 (a) 0·704705 9 0·0505 0·70469 0·512715 23 0·1408 0·51270

352 (t) 0·704703 6 0·0609 0·70468 0·512730 9 0·1463 0·51270 18·449 15·543 38·860

356 (a) 0·704753 15 0·2311 0·70467 0·512706 8 0·1475 0·51268 18·434 15·576 38·920

358 (a) 0·704755 9 0·0294 0·70471 0·512721 6 0·1495 0·51270 18·384 15·520 38·775

360 (t) 0·704718 8 0·0108 0·70471 0·512732 11 0·1468 0·51271 18·394 15·526 38·770

365 (t) 0·704747 12 0·0113 0·70474 0·512727 7 0·1449 0·51270 18·384 15·520 38·708

376 (t) 0·704810 7 0·1210 0·70477 0·512700 10 0·1420 0·51268 18·445 15·560 38·889

380 (t) 0·704799 7 0·0048 0·70480 0·512712 10 0·1387 0·51269

381 (t) 0·704795 8 0·0186 0·70479 0·512720 8 0·1372 0·51270 18·324 15·517 38·703

0·704785 6 0·512716 11

392 (t) 0·704755 6 0·0739 0·70473 0·512704 13 0·1452 0·51268 18·496 15·568 38·978

394 (a) 0·704813 7 0·2849 0·70471 0·512726 12 0·1500 0·51270 18·419 15·516 38·824

402 (t) 0·704789 12 0·0265 0·70478 0·512706 7 0·1531 0·51268 18·406 15·563 38·885

403 (t) 0·704799 11 0·0179 0·70479 0·512696 11 0·1470 0·51267

404 (t) 0·704678 6 0·0091 0·70467 0·512741 9 0·1510 0·51272 18·407 15·508 38·707

409 (t) 0·704693 10 0·0635 0·70467 0·512717 12 0·1406 0·51269 18·436 15·523 38·825

414 (t) 0·704780 7 0·1361 0·70473 0·512712 8 0·1422 0·51269 18·423 15·527 38·883

All samples were acid-leached before analysis for isotopic ratios. All analyses were carried out at Universite Libre deBruxelles following procedures described by Weis & Frey (1991). Normalization procedures, blanks and data for standardsare as given by Frey et al. (2002). a or t within parentheses following sample number indicates alkalic or transitional lava,respectively (see Fig. 4). 2� indicates uncertainty in the last significant digits of the isotopic ratios. Sample 381 was analyzedin duplicate for Sr and Nd isotopic ratios.

from a nearly homogeneous part of the plume with 86Sr and high 143Nd/144Nd (Group D in Fig. 11) coupledwith the trace element characteristics of plagioclase-richintermediate isotopic ratios.

(2) Alternatively, the isotopic heterogeneity of Ker- cumulates; Yang et al. (1998) concluded that these lavasrepresent plume-derived magmas that assimilated plagio-guelen Archipelago lavas may reflect varying mixing

proportions between components derived from the clase-rich cumulate rocks in the lower oceanic crust.Lavas with these geochemical characteristics do not occurplume, asthenosphere and lithosphere, either oceanic or

continental (Doucet et al., 2002). in the Mont Tourmente section.A role for a mid-ocean ridge basalt (MORB)-relatedIn regard to continental lithosphere, in several areas

sampled by basement drilling Cretaceous plume-derived asthenosphere component in the source of KerguelenArchipelago lavas was inferred by Storey et al. (1988) andlavas forming the uppermost part of the Kerguelen

Plateau were apparently contaminated by continental Gautier et al. (1990), who argued that as the KerguelenArchipelago evolved from an early ridge-centered stagelithosphere (Mahoney et al., 1995; Weis et al., 2001; Frey

et al., 2002; Ingle et al., 2002). Also, mantle xenoliths with at>40 Ma to its present intraplate setting, the proportionof depleted asthenosphere in the source decreased. Thisa continental affinity have been found in Kerguelen

Archipelago lavas (Hassler & Shimizu, 1998; Mattielli et interpretation was based on an inferred increase in 87Sr/86Sr and decrease in 143Nd/144Nd with eruption age inal., 1999). There is, however, no compositional evidence

that Kerguelen Archipelago basalts, including lavas from the Kerguelen Archipelago. Today this model is lesscompelling because Yang et al. (1998) found that mostMont Tourmente, contain a continental component (e.g.

Fig. 9; Yang et al., 1998; Doucet et al., 2002; Frey et al., of the lavas in the oldest studied sections of the floodbasalt, the transitional basalt from Mont Bureau and2002, fig. 10).

In regard to the role of oceanic lithosphere,>15% of Mont Rabouillere, have Sr and Nd isotopic ratios thatoverlap with those of the younger alkalic flood basalts atthe lava flows in the 29–30 Ma sections from Mont

Bureau and Mont Rabouillere have relatively low 87Sr/ Mont Crozier and Ravin du Charbon (Fig. 11). Although

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Fig. 11.

there is no simple trend of Sr and Nd isotopic ratios which is a hotspot track related to the Kerguelen plume(Fig. 1 inset; Frey & Weis, 1995).varying with age, the archipelago lavas with relatively

low 87Sr/86Sr and high 143Nd/144Nd occur only withinthe oldest, 28–30 Ma, sections of the flood basalt (Yanget al., 1998; Doucet et al., 2002). A larger proportion of

SUMMARYa depleted component is inferred for some of these oldestlavas, but there is no evidence that the uppermost parts Studies of flood basalt sections from the Kerguelen Ar-of the flood basalt formed at a time when the archipelago chipelago show several important trends. With decreasingwas near the Southeast Indian Ridge (SEIR); i.e. the eruption age from >29 Ma in the north to >25 Ma inoldest flood basalts in the archipelago erupted at>30 Ma the east, the flood basalts of the Kerguelen Archipelago(Nicolaysen et al., 2000; Doucet et al., 2002) much younger changed from transitional to alkalic basalt. Consistentthan the >40 Ma juxtaposition of the archipelago and with this result, the 26·0–25·3 Ma, 597 m lava section atSEIR. There are, however, intercalated >34 Ma Mont Tourmente in the Plateau Central region includesMORB- and plume-related lavas at Ocean Drilling Pro- an abrupt change from transitional to alkalic flood basalts.gram (ODP) Leg 183 Site 1140 on the northernmost By analogy with the growth stages of Hawaiian volcanoesKerguelen Plateau (Weis & Frey, 2002). (Clague & Dalrymple 1987), this temporal trend implies

In summary, the near isotopic homogeneity of Mont a decrease in extent of melting and may indicate thatTourmente lavas (Fig. 11) contrasts with the isotopic the archipelago was not centered above the plume atdiversity of intercalated lava flows at ODP Site 1140 north >25 Ma. Alternatively, magma flux from the mantleof the Kerguelen Archipelago and at Monts Fontaine, des decreased at >25 Ma as the lithospheric thickness be-Ruches, Bureau and Rabouillere in the archipelago. In neath the archipelago increased. Consistent with thisaddition, the isotopic ratios of Mont Tourmente lavas inference, the role of residual garnet during partial melt-differ from those proposed for the Kerguelen plume ing was important for the eastern alkalic basalt sections(Weis et al., 1998). If much of the Plateau Central region (Frey et al., 2000), but not for the northern transitionalof the Kerguelen Archipelago proves to be isotopically basalt sections. This change in residual mineralogy is notsimilar to the Mont Tourmente lavas, this may be evi- observed in the Mont Tourmente Section from the

Plateau Central. Another possibility is that at >25 Madence for intrinsic isotopic heterogeneity of the Kerguelenplume that complements the evidence for plume hetero- the intrinsic plume flux was diminishing, possibly be-

coming less focused (Frey et al., 2000). Evidence in favorgeneity provided by basalts from the Ninetyeast Ridge,

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Fig. 11. (a) 143Nd/144Nd vs 87Sr/86Sr and (b) 207Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb, showing data and fields for various sections of theKerguelen Archipelago flood basalts. Data for Sr and Nd, but not Pb, are age corrected (Table 2). Data sources: Mont Tourmente, this study;Mont Crozier, D. Weis, unpublished data (2000); SE Province, Weis et al. (1993) and Frey et al. (2000); Monts Bureau and Rabouillere, Yang etal. (1998); and Monts Fontaine and des Ruches, Doucet et al. (2002). In the 143Nd/144Nd vs 87Sr/86Sr plot the two fields for Bureau and Rabouillererepresent the D (depleted) and P (plume) groups of Yang et al. (1998). Shown for comparison is a field for Southeast Indian Ridge N- and E-MORB (Mahoney et al., 2002). Insets with expanded scales show that Mont Tourmente lavas range slightly beyond ±2� precision indicatedby error bars, but there is no systematic difference between alkalic and transitional lavas (Α and Χ, respectively). Gautier et al. (1990) analyzedfour samples from the Plateau Central. The Sr and Nd data are not shown because the samples were not acid-leached and age correctionscannot be made for all samples. +, lead data for two Plateau Central samples from Gautier et al. (1990); they overlap with Mont Tourmentedata.

of the latter alternative is that over the last 20 Myr alkalic located between the Kerguelen Archipelago and HeardIsland (Weis et al., 2002).lavas have erupted as post-flood basalts in the Kerguelen

Archipelago (Weis et al., 1993), and have formed Heard Samples of flood basalt from the oldest sections of theKerguelen Archipelago are isotopically heterogeneous,and McDonald Islands, >400 km SE of the Kerguelen

Archipelago (Barling et al., 1994), and several seamounts whereas those from the younger sections are relatively

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Fig. 12. Comparisons of different sections of the Kerguelen Archipelago flood basalt (Bureau and Rabouillere data from Yang et al., 1998; SEProvince data from Frey et al., 2000; Crozier data from F. A. Frey, unpublished data, 2000; Tourmente data, this study). (a) SiO2 vs MgO(wt %). Although there is some overlap, lavas from the Mont Bureau and Mont Rabouillere sections in the north–central region generally havehigher SiO2 contents at a given MgO content than lavas from Mont Crozier and the SE Province. Data points are indicated for Mont Crozierand the SE Province sections to show that most, but not all, of these lavas are offset to lower SiO2. Most of the Mont Tourmente lavas arewithin the field defined by Mont Bureau and Mont Rabouillere lavas, but the Upper Alkalic group in the Mont Tourmente section has relativelylow SiO2 like most of the alkalic lavas from the Mont Crozier and SE Province sections. (b) The Y vs Nb (in ppm) panel shows that Y and Nbdefine a similar coherent trend in lavas from Monts Tourmente, Bureau and Rabouillere. In contrast, alkalic lavas from the SE and MontCrozier sections define a more scattered trend with a shallower slope that extends to higher Nb contents; that is, at a given Nb content, MontCrozier and SE lavas have lower Y contents. As Y is compatible in garnet but not in pyroxenes and spinel and Nb is incompatible in all ofthese phases, a plausible interpretation is that the SE and Crozier magmas formed by a lower extent of melting from a garnet-bearing source.

homogeneous with relatively high 87Sr/86Sr, and low archipelago lavas. The depleted component, relativelylow 87Sr/86Sr and high 143Nd/144Nd, may be intrinsic143Nd/144Nd and radiogenic Pb isotope ratios. This en-

riched component is also present in the oldest >29 Ma, within the plume or related to the oceanic lithosphere

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Fig. 13. Ce/Y, La/Yb and Nb/Zr vs MgO (wt %). In each case the Mont Tourmente lavas overlap with the Group P Mont Bureau and MontRabouillere lavas, and at a given MgO content they have lower ratios than lavas from the SE and Mont Crozier sections. These differences areconsistent with the SE and Mont Crozier magmas segregating at lower extent of melting within the garnet stability field, whereas the petrogenesisof Mont Tourmente lavas was most similar to that of lavas from Monts Bureau and Rabouillere.

(Yang et al., 1998) or asthenosphere (Doucet et al., 2002). ponents derived from continental lithosphere. It is pos-sible that Mont Tourmente lavas dominantly reflect theThe lavas in the Mont Tourmente section are nearlyKerguelen plume.isotopically homogeneous, with Sr, Nd and Pb isotopic

ratios intermediate between the enriched and depletedcomponents. Also, there is no isotopic distinction between

ACKNOWLEDGEMENTStransitional and alkalic lavas. In contrast to CretaceousKerguelen Plateau basalts (e.g. Frey et al., 2002), there is This research was supported by US NSF EAR Grant

9814313 (F.A.F.) and ARC GRANT 98/03-233 (D.W.),no evidence that Mont Tourmente lavas contain com-

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Indian Ocean): evolution of the mantle sources from ridge to anand FNRS Grant 1.5.186.98 (D.W.). We thank Dr J.intraplate position. Earth and Planetary Science Letters 100, 59–76.Scoates for discussions regarding the geology of the

Hanan, B. B., Blichert-Toft, J., Kingsley, R. & Schilling, J.-G. (2000).Plateau Central, Dr P. Ila for supervision of the MITDepleted Iceland mantle plume geochemical signature: artifact of

Neutron Activation Analysis Facility, C. Maerschalk for multicomponent mixing? Geochemistry, Geophysics, Geosystemschemical processing of samples before isotopic analyses, 1999GC000009.and G. Xu for assistance in preparing figures. Finally, Hassler, D. & Shimizu, N. (1998). Osmium isotopic evidence for ancient

subcontinental mantle beneath the Kerguelen Islands, Southernwe thank J. Barling, G. Fitton and R. Kent for theirIndian Ocean. Science 280, 418–421.constructive reviews.

Hofmann, A. W. & White, W. M. (1983). Ba, Rb and Cs in the Earth’smantle. Zeitschrift fur Naturforschung 38A, 258–266.

Ila, P. & Frey, F. A. (1984). Utilization of neutron activation analysisin the study of geologic materials. In: Harling, O. K., Clark, L. &

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