Earth and Planetary Science Letters 197 (2002) 35^50
www.elsevier.com/locate/epsl
Relationship between the early Kerguelen plume and
continental £ood basalts of the paleo-Eastern Gondwanan
margins
Stephanie Ingle a; , Dominique Weis a;1 , James S. Scoates a ,
Frederick A. Frey b
a
De¤partement des Sciences de la Terre et de l’Environnement, Universite¤ Libre de Bruxelles, P.O. Box 160/02, Ave. F.D. Roosevelt 50,
B-1050 Brussels, Belgium
b
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Building 54-1226, Cambridge,
MA 02139, USA
Received 4 June 2001; received in revised form 10 December 2001; accepted 10 January 2002
Abstract
Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show
remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern
India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr^Nd^Pb
isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from
5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen
Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal
role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for
the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly
formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments
of continental crust, isolated at shallow levels in the Indian Ocean lithosphere. ß 2002 Elsevier Science B.V. All
rights reserved.
Keywords: Kerguelen Plateau; ODP Site 1137; continental crust; enrichment; mantle; Indian Ocean
1. Introduction
Oceanic plateaus result from voluminous mag-
* Corresponding author.
Tel.: +32-2-650-22.40; Fax: 32-2-650-37.48.
E-mail address: single@ulb.ac.be (S. Ingle).
1
Directeur de Recherches, F.N.R.S.
matic activity over a (typically) short period of
geological time (106 years) and create lithosphere
that is as much as four times the thickness of
normal oceanic lithosphere [1]. The Cretaceous
Kerguelen Plateau in the southern Indian Ocean
is the second largest oceanic plateau [1] and has
been interpreted as the surface manifestation of
Kerguelen mantle plume activity during the opening of the Indian Ocean between India and Australia^Antarctica [2,3]. Ocean Drilling Program
0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 2 - 8 2 1 X ( 0 2 ) 0 0 4 7 3 - 9
36
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
(ODP) Legs 119, 120 and 183 have sampled Kerguelen Plateau crust. Contamination of the recovered basalts by continental lithosphere was proposed to explain extreme isotopic and trace
element features at Site 738 (Leg 119) on the
southernmost edge of the Kerguelen Plateau
(Fig. 1 [4]). Moreover, drilling during Leg 183
recovered rocks of unequivocal continental origin
(i.e., garnet^biotite gneiss) from £uvial units intercalated with basalts at Site 1137 on Elan Bank, a
western salient protruding from between the
Southern and Central Kerguelen Plateau (Fig. 1;
e.g., [5]). Basaltic lavas recovered at Site 1137
erupted during the earlier stages of Kerguelen Plateau formation, at V109 Ma (Fig. 1 [6]), within
the range of volcanism on the Southern Kerguelen
Plateau (119^109 Ma [7,8]) and prior to that of
the Central Kerguelen Plateau (100^85 Ma [7,8]).
Site 1137 basalts are, therefore, critical to deciphering the early magmatic history of the Kerguelen
plume system, its mantle sources and the possible
continental contaminants.
Early Cretaceous volcanism in proximity to the
Kerguelen Plateau also occurred along the paleoEastern Gondwanan margins of Australia and India. The Bunbury basalt of Western Australia records two periods of geochemically distinct continental basalt outpouring at 132 Ma (Casuarina
Group [8,9]) and 123 Ma (Gosselin Group [9]).
The Rajmahal Traps of eastern India erupted at
Fig. 1. (a) Physiographic map depicting major volcanic features of the Indian Ocean and surrounding continents [56]. ODP basement sites are marked by ¢lled circles and dredge locations are marked by ¢lled squares. Rectangles delineate the Elan Bank,
Kerguelen Plateau (location of Site 1137), the Bunbury basalt (southwestern Australia) and the Rajmahal Traps (eastern India).
The four main physiographic provinces of the Kerguelen Plateau are labeled: SKP (Southern Kerguelen Plateau), CKP (Central
Kerguelen Plateau), Elan Bank and the NKP (Northern Kerguelen Plateau). (b) Summary of approximate ages for basalts discussed in the text. Age data sources are as follows: Bunbury [8,9], Rajmahal [10,11], SKP Site 1136 and CKP Site 1138, NKP
and Broken Ridge [7], SKP Sites 738, 749 and 750 [8], Site 1137 [6], Kerguelen Archipelago [57^59]. (c) Site 1137 downcore hardrock log (mbsf is meters below sea £oor) from [17]. The depth, section number, recovery, unit number, general lithology, % vesicularity, £ow type and dominant mineralogy (for phyric rocks) are depicted.
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
117 Ma [10^12] and have also been divided into
two groups on the basis of geochemistry (Groups
I and II [13]). Geochemical similarities between
the Rajmahal Traps and Bunbury basalt and basalts of the Kerguelen Plateau have been noted
previously [9] and the volcanism attributed to impingement of the Kerguelen plume beneath the
lithosphere of Eastern Gondwana. Most workers
have inferred involvement of heat derived from
the plume, rather than substance in their petrogenesis [9,11,14].
Recent wide-angle seismic pro¢ling has shown
that the crustal structure of Elan Bank has geophysical characteristics similar to continental
crust [15,16]. Although it is currently submerged,
basement Units 1^4, 7, 8 and 10 are subaerially
erupted basalt £ows with brecciated upper margins and Units 5, 6 and 9 are £uvial volcaniclastic deposits (Fig. 1 [17]). A surprising result at
Site 1137 was the recovery of a volcaniclastic
polymict conglomerate containing pebble-sized
clasts of variable lithologies, including some of
continental derivation [5,18,19]. In this paper, we
expand upon an initial study of the basalts [20]
with a much more detailed sampling and thorough geochemical investigation to characterize
and quantify the mantle sources and continental
contaminants in early Kerguelen Plateau basalts.
In light of a comparison with basalts of the paleo-Eastern Gondwanan margins, we re-examine
existing models for the timing and emplacement
mechanism of the Kerguelen plume, which range
from post-continental breakup and rapid initial
eruption rates (e.g., [5,21]) to pre-continental
breakup and long-term incubation of the Kerguelen plume beneath Gondwanan lithosphere
(e.g., [22,23]). Our results support the presence
of the Kerguelen plume coincident with early
separation of eastern Gondwana and suggest
that the Kerguelen plume may have played an
active role in the isolation of continental material
at shallow levels within the incipient Indian
Ocean crust.
2. Analytical techniques
The freshest core samples were selected from
37
each basalt £ow unit. To remove sur¢cial contamination by the drill bit and saws, the exterior of
each sample was sanded with silicon carbide paper and rinsed using an ultrasonic bath of ultrapure water. Chips from each coarse-crushed sample were hand-picked under a binocular
microscope to remove visibly altered pieces; remaining chips were powdered in an agate swing
mill. Rock powders were analyzed by X-ray £uorescence (XRF) methods (University of Massachusetts) and instrumental neutron activation
analysis (INAA ; Massachusetts Institute of Technology) for major and trace elements (Table 1).
All XRF data reported for major elements are
averages of duplicate analyses. Details of accuracy
and precision for both methods are reported by
[24].
Representative samples from each £ow unit
were analyzed for radiogenic (Sr, Nd and Pb) isotopic compositions (Table 2). To remove the effects of post-magmatic alteration, between 250
and 300 mg of rock powder was placed in a Teflon vial and leached with 6 N HCl following
‘cold’ leaching methods [25,26]. On average, samples were leached eight times (until the leachate
was clear) and lost V30 wt%. The reproducibility
of the leaching and the e¡ectiveness at removing
secondary alteration minerals can be assessed by
our duplicate analysis of 31R-1-5^10 (Table 2).
Chemical procedures for dissolution follow [27]
and [26]. All radiogenic isotopic compositions
were determined with a VG Elemental Sector 54
multicollector thermal ionization mass spectrometer at the Universite¤ Libre de Bruxelles. Sr and
Nd isotopic compositions were measured on single Ta and triple Re^Ta ¢laments, respectively, in
dynamic mode. Sr isotopic ratios were normalized
to 86 Sr/88 Sr = 0.1184 and Nd isotopic data were
normalized using 146 Nd/144 Nd = 0.7219. The average 87 Sr/86 Sr value for the NBS 987 Sr standard
was 0.710279 þ 7 (2c) on the basis of 65 analyses.
The average Nd standard (Rennes [28]) 143 Nd/
144
Nd value was 0.511964 þ 11 (2c) for 75 runs.
Pb isotopic ratios were measured on single Re
¢laments using the H3 PO4 ^silica gel technique.
All Pb isotopic ratios were corrected by 1.2x
per atomic mass unit to account for mass fractionation on the ¢lament, based on 80 analyses
38
Table 1
Major element oxide (wt%) and trace element concentration (ppm) data for Site 1137 basaltic rocks
Core
Interval
Deptha
Group
25R-1
64^72
229.04
25R-2
46^53
230.3
25R-2
25R-3
119^126 72^79
231.03
231.94
25R-5
52^60
234.38
25R-7
90^98
237.08
26R-1
31^43
238.31
26R-1
26R-2
143^146 17^23
239.43
239.65
26R-3
51^60
241.45
27R-4
57^63
252.49
28R-3
29R-1
123^128 30^37
256.62
257.5
29R-2
31^38
258.77
30R-2
21^26
263.45
31R-3
5^10
269.65
Unit 1
Unit 1
Unit 1
Unit 1
Unit 2
Unit 2
Unit 2
Unit 2
Unit 2
Unit 3
Unit 3
Unit 3
Unit 4
Unit 4
51.45
2.36
15.09
10.79
0.10
5.70
8.97
3.74
1.02
0.34
99.56
4.82
0.51
0.15
25.8
^
97
^
^
^
^
^
^
^
^
^
19.7
4.73
0.79
44.7
25.7
6.27
2.11
0.91
2
0.3
2.09
52.27
2.33
14.69
10.88
0.14
5.86
9.29
3.00
0.78
0.34
99.58
2.85
0.52
31.15
26.2
205
96
22
112
22
6
536
26.9
210
14.9
296
20.5
5.25
0.88
49.9
28.8
6.56
2.15
0.89
2.19
0.31
2.9
52.70
2.33
14.46
11.08
0.15
5.84
9.23
2.97
0.84
0.33
99.93
2.37
0.51
31.26
25.7
202
88
21
110
22
12
530
26.9
208
14.6
310
20.5
5.16
0.86
48.1
26.7
6.26
2.15
1.03
2.22
0.31
2.16
52.10
2.39
14.50
11.20
0.14
5.91
9.26
2.97
0.75
0.33
99.55
2.88
0.51
31.13
26.2
211
82
20
113
22
8.5
537
26.3
212
15
305
19.8
5.29
0.76
48
28.4
6.39
2.13
0.86
2.09
0.3
2.4
Unit 1
50.93
2.47
15.32
10.79
0.12
6.23
9.24
3.35
0.70
0.35
99.50
4.28
0.53
30.37
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
53.87
2.51
14.11
11.76
0.10
5.54
5.72
3.23
2.78
0.37
99.99
4.46
0.48
0.50
24.8
215
51
17
118
22
31.8
405
28
229
16.3
296
22.2
5.52
0.86
53.8
31
6.8
2.24
1.08
2.32
0.32
2.5
52.30
2.45
14.79
10.64
0.11
5.53
8.99
3.11
0.98
0.35
99.24
3.62
0.51
30.84
26.2
216
85
20
113
23
12.2
536
26.5
216
15
322
20.2
5.03
0.81
47.9
27.9
6.41
2.21
1
2.2
0.32
2.27
52.83
2.34
14.40
10.99
0.15
5.79
9.20
3.04
0.96
0.34
100.04
1.80
0.51
31.12
26.3
207
86
21
111
22
12.9
520
27.4
211
14.9
325
21.0
4.77
0.82
51.1
28.2
6.71
2.17
1.21
2.23
0.33
2.22
52.87
2.11
14.66
10.27
0.15
6.11
9.59
3.04
0.92
0.30
100.01
1.53
0.54
31.18
26.7
193
100
21
101
22
12.3
529
24.4
185
12.8
301
18.5
4.36
0.74
43.8
24.6
5.77
2.03
0.81
1.95
0.28
2
Unit 2
53.15
2.54
14.12
11.26
0.16
5.15
8.75
3.18
1.13
0.38
99.82
0.96
0.48
30.93
25.4
212
48
15
115
22
20.2
508
29.4
233
16.3
364
23.7
5.04
0.9
54.1
31
7.03
2.32
0.99
2.37
0.36
2.56
51.89
2.66
14.64
11.52
0.15
5.43
7.35
3.38
2.24
0.40
99.66
3.43
0.48
0.85
25.9
^
50
^
^
^
^
^
^
^
^
^
23.6
5.7
0.94
56.5
31.1
7.18
2.34
1.07
2.59
0.34
2.67
51.51
2.58
15.10
11.48
0.11
6.54
7.94
3.21
1.30
0.38
100.15
6.22
0.53
30.12
26
227
63
24
122
23
17.9
523
29.1
233
16.3
294
22.3
5.53
0.83
52.6
29.4
6.89
2.28
0.95
2.26
0.35
2.36
52.65
2.54
14.41
11.46
0.15
5.62
8.98
3.08
0.87
0.37
100.13
2.73
0.49
31.10
25.9
209
60
21
115
23
16
532
28.8
232
16
333
22.5
5.47
0.85
54.2
29.4
7.07
2.32
0.98
2.32
0.33
2.4
Unit 3
51.15
2.66
15.20
11.02
0.12
5.77
9.08
3.29
0.77
0.39
99.45
5.03
0.51
30.44
27.4
226
63
23
122
24
6.6
566
30.1
241
16.8
340
23.5
5.52
0.94
56.7
31
7.27
2.42
1.09
2.38
0.38
2.54
50.65
2.69
15.28
11.55
0.12
6.59
7.43
3.50
1.69
0.39
99.89
5.05
0.53
0.88
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
52.20
2.04
15.62
9.99
0.08
6.51
7.87
3.22
2.13
0.27
99.93
7.94
0.56
0.47
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
52.20
2.09
16.19
9.83
0.11
6.02
9.31
3.01
1.07
0.28
100.11
4.42
0.55
30.80
23.5
204
146
40
97
22
11
540
23.1
173
12.4
202
16.5
3.96
0.63
39.4
23.4
5.41
1.89
0.74
1.85
0.27
1.64
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O 5
Total
LOIb
Mg#c
AId
Sce
V
Cr
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
La
Hf
Ta
Ce
Nd
Sm
Eu
Tb
Yb
Lu
Th
24R-2
10^17
220.39
Upper
Table 1 (Continued)
Core
Interval
Deptha
Group
a
b
c
d
33R-1
53^60
286.03
37R-3
12^20
324.84
Lower
38R-1
73^78
328.23
38R-3
63^68
329.95
38R-4
71^77
331.39
39R-1
39R-2
101^108 50^54
333.21
334.00
40R-3
41R-1
117^124 3^10
340.62
343.03
41R-1
67^78
343.67
45R-3
57^63
364.84
Unit 4
Unit 4
Unit 4
Unit 7
Unit 7
Unit 7
Unit 7
Unit 7
Unit 8
Unit 8
Unit 10 Unit 10 Unit 10 Unit 10
51.53
2.23
16.39
10.02
0.14
5.79
9.94
3.08
0.62
0.32
100.05
3.66
0.53
30.94
24.8
202
141
37
103
23
11.2
573
25
189
13.6
227
18.0
4.41
0.77
43.7
25.5
6.02
2.02
0.97
2.01
0.29
2.12
Unit 4
50.40
2.19
16.62
10.09
0.18
6.58
8.69
3.24
1.37
0.30
99.66
3.79
0.56
0.39
24.4
^
153
^
^
^
^
^
^
^
^
^
17.5
4.48
0.76
41.1
24.5
5.71
2.03
0.83
1.91
0.3
1.82
53.02
2.06
15.62
10.04
0.11
7.07
4.62
3.13
3.38
0.29
99.34
7.97
0.58
1.32
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
Unit 4
50.84
2.53
15.61
11.34
0.14
5.83
9.20
3.29
0.71
0.36
99.85
4.74
0.50
30.38
25.4
225
106
37
120
23
12.7
545
28
216
15.9
256
20.0
4.79
0.95
49.4
27.6
6.63
2.16
0.88
2.1
0.32
2.08
50.40
2.58
16.06
11.50
0.14
5.78
9.23
3.30
0.56
0.37
99.92
4.96
0.50
30.36
25.5
233
105
39
122
24
8.6
560
27.9
219
15.8
258
20.2
4.88
0.87
48.1
28.2
6.54
2.2
0.98
1.93
0.29
2.52
51.61
2.09
16.25
10.98
0.30
6.99
4.90
3.15
3.09
0.29
99.65
10.94
0.56
1.57
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
^
51.05
2.19
16.73
10.64
0.24
6.16
7.49
3.52
1.70
0.31
100.03
4.57
0.53
0.76
26
^
101
^
^
^
^
^
^
^
^
^
21.3
4.46
0.82
47.6
25.6
5.95
1.94
0.92
2.02
0.32
2.93
51.40
2.12
15.91
10.78
0.14
5.79
9.40
3.10
0.59
0.30
99.53
4.48
0.52
30.90
25.5
^
93
^
^
^
^
^
^
^
^
^
20.4
4.61
0.7
47.8
27.1
6.1
2.04
0.94
2.1
0.34
3.09
52.97
2.04
15.30
10.45
0.17
5.46
9.41
3.05
0.61
0.29
99.75
1.92
0.51
31.51
25.3
200
95
29
111
21
16.5
485
27.3
190
13.2
268
20.2
4.57
0.7
47.7
27.1
6.03
2.03
0.9
2.04
0.33
3.28
52.61
2.06
15.50
10.78
0.17
5.50
9.43
3.09
0.62
0.29
100.05
2.37
0.50
31.33
25.2
198
95
28
111
22
16.7
484
27.8
192
13.3
260
20.6
4.56
0.7
47.9
26.9
6.17
2.02
0.91
2.46
0.3
3.23
Unit 7
50.46
2.21
16.40
11.26
0.17
6.81
7.47
3.49
1.37
0.31
99.95
6.00
0.55
0.62
^
230
^
32
116
22
23.6
477
26.6
199
13.8
450
^
^
^
^
^
^
^
^
^
^
^
53.19
2.02
15.23
10.23
0.13
5.16
8.82
2.67
1.52
0.28
99.25
2.37
0.50
31.06
25.3
202
90
27
107
21
36.1
456
27.7
193
12.9
288
21.0
4.64
0.76
49.8
26.5
6.34
2.00
0.96
2.21
0.35
3.65
Unit 8
51.83
2.13
15.99
11.02
0.21
5.59
9.33
3.11
0.61
0.30
100.12
4.19
0.50
31.03
25.6
208
90
28
114
22
9.3
483
28.8
201
13.6
296
21.0
4.82
0.75
50.5
26.6
6.23
2.04
1.0
2.64
0.31
3.7
49.06
2.47
18.69
12.32
0.28
4.01
7.79
3.28
0.73
0.35
98.98
8.54
0.39
30.29
^
246
^
35
131
26
7.7
562
27.2
231
15.6
266
^
^
^
^
^
^
^
^
^
^
^
49.90
2.60
17.27
10.85
0.17
4.86
8.38
3.07
1.56
0.37
99.03
3.76
0.47
0.6
23.3
^
133
^
^
^
^
^
^
^
^
^
22.0
4.94
0.87
50.8
29.9
6.7
2.21
1.05
2.32
0.36
2.68
46R-1
51^56
367.51
51.41
2.61
15.94
11.02
0.15
4.90
9.68
3.33
0.64
0.35
100.03
3.04
0.47
30.62
24.6
191
144
54
114
23
14.2
494
30.6
204
14.7
304
20.7
4.96
0.81
49.5
28.9
6.72
2.22
1.06
2.44
0.39
2.72
46R-2
7^15
368.54
51.69
2.63
15.45
11.24
0.15
4.45
9.41
3.43
0.78
0.37
99.60
2.15
0.44
30.49
24.4
189
139
53
116
23
19.9
478
31
214
15.2
318
21.3
5.15
0.85
52.1
29.3
6.9
2.27
1.03
2.66
0.34
3.02
46R-3
12^18
369.92
51.85
2.76
14.83
11.86
0.16
4.77
9.27
3.24
0.78
0.38
99.90
2.36
0.44
30.73
25.9
196
148
52
121
23
19.3
455
32.2
221
15.9
325
22.7
5.4
0.85
54.1
29.7
7.32
2.3
1.17
2.71
0.39
2.87
Depth is reported in meters below sea £oor.
LOI is wt% loss-on-ignition at 1000‡C.
Mg# = (wt% MgO/40.311)/[(wt% MgO/40.311)+(wt% FeO/71.846)], assuming all Fe as Feþ2 (FeO = 0.8998UFe2 O3 ).
Alkali index, calculated from [29] as AI = (Na2 O+K2 O)3(SiO2 U0.37^14.43); tholeiitic series rocks have negative values and alkalic series have positive values.
Sc, Hf, Ta, Th and the rare earth elements are by INAA methods; all other elements are by XRF methods.
39
e
31R-7
32R-6
125^130 66^71
276.14
282.88
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O 5
Total
LOIb
Mg#c
AId
Sce
V
Cr
Ni
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
La
Hf
Ta
Ce
Nd
Sm
Eu
Tb
Yb
Lu
Th
31R-3
31R-6
85^90
24^30
270.45 273.63
Upper, cont.
40
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
of NBS 981 Pb standard run at a temperature
between 1090‡C and 1200‡C. Samples were run
with close control of the temperature between
1080‡C and 1170‡C.
1.1; where negative values are tholeiitic and positive values indicate alkalic compositions [29]).
Downcore plots of MgO and TiO2 wt% and AI
show no trends, with the exception of Unit 10, the
lowermost basalt unit, which has slightly lower
MgO with higher TiO2 wt% (Fig. 2).
3. Results
3.2. Alteration
3.1. Major element oxide geochemistry
The ma¢c volcanic lavas at Site 1137 range
from evolved basalts to basaltic andesites
(Mg# = 0.44^0.58; Table 1) and most are tholeiitic to transitional (alkali index (AI) = 31.9 to
Basalts recovered at Site 1137 show a general
positive correlation between weight loss on ignition and total alkali content (wt% Na2 O+K2 O;
not shown). Because of post-magmatic alteration,
the more mobile incompatible elements including
Fig. 2. Geochemical variations downcore of MgO and TiO2 wt%, alkali index, (La/Yb)PM , (La/Nb)PM and (Th/Ta)PM (subscript
‘PM’ indicates normalization to primitive mantle values [30]) for Site 1137 basalts. The basalts are divided into an upper group
(Units 1^4, ¢lled diamonds) and a lower group (Units 7, 8 and 10, open diamonds). The alkali index [29] indicates the relative
displacement of a sample from the tholeiitic^alkalic series dividing line; with tholeiitic series rocks having a negative displacement
(and a negative alkali index) and alkalic series rocks having a positive displacement (the equation is given as
AI = (Na2 O+K2 O)3(0.37USiO2 314.43)).
Table 2
Sr, Nd and Pb isotopic ratios in Site 1137 basaltic rocks
Core, Interval Depth Unit (87 Sr/86 Sr)m Error
Rb/86 Sr
(87 Sr/86 Sr)T (143 Nd/144 Nd)m
Error
147
Sm
144 Nd
(143 Nd/144 Nd)T
ONd (T)
206
Pb
207
Pb
208
Pb
204 Pb
204 Pb
204 Pb
220.39
229.04
230.30
231.03
237.08
238.31
239.43
239.65
252.49
256.62
257.50
269.65
269.65
270.45
282.88
286.03
1
1
1
1
2
2
2
2
3
3
3
4
4
4
4
4
0.705012
0.705005
0.705066
0.704967
0.704987
0.704999
0.705029
0.705092
0.704921
0.704920
0.704825
0.704930
0.704940
0.704932
0.704885
0.704879
7
6
7
6
7
7
6
7
5
6
6
6
8
6
7
6
^
0.0324
0.0655
0.0458
0.0658
0.0718
0.0673
0.115
0.0990
0.0870
0.0337
0.0589
^
0.0565
0.0674
0.0444
^
0.70496
0.70497
0.70490
0.70489
0.70489
0.70493
0.70492
0.70477
0.70479
0.70477
0.70484
^
0.70485
0.70478
0.70481
0.512553
0.512566
0.512549
0.512559
0.512583
0.512576
0.512555
0.512550
0.512565
0.512580
0.512577
0.512563
0.512546
0.512566
0.512600
0.512565
10
9
10
10
15
8
9
9
9
7
9
7
10
12
10
10
0.148
0.138
0.142
0.136
0.139
0.144
0.142
0.137
0.142
0.145
0.142
0.140
^
0.143
0.145
0.140
0.51245
0.51247
0.51245
0.51246
0.51248
0.51247
0.51245
0.51245
0.51246
0.51248
0.51248
0.51246
^
0.51247
0.51250
0.51247
31.0
30.6
31.0
30.7
30.3
30.5
30.9
30.9
30.7
30.4
30.4
30.8
^
30.7
0.0
30.6
18.018
18.009
18.018
17.993
17.989
18.016
17.997
18.002
18.012
18.010
17.995
18.072
18.066
18.036
18.015
18.019
15.620
15.619
15.625
15.616
15.620
15.644
15.617
15.616
15.623
15.608
15.617
15.635
15.625
15.630
15.605
15.608
38.592
38.567
38.625
38.551
38.575
38.663
38.617
38.634
38.681
38.603
38.622
38.847
38.816
38.741
38.631
38.692
329.95
331.39
333.21
337.74
340.62
343.03
364.84
367.51
368.54
369.92
7
7
7
8
8
8
10
10
10
10
0.705625
0.705690
0.705774
0.706098
0.706256
0.706027
0.705650
0.705633
0.705728
0.705680
7
8
7
8
7
7
8
7
7
9
^
0.0984
0.0998
^
0.229
0.0557
^
0.0832
0.120
0.123
^
0.70554
0.70562
^
0.70590
0.70594
^
0.70550
0.70554
0.70549
0.512519
0.512495
0.512488
0.512463
0.512455
0.512461
0.512512
0.512543
0.512546
0.512541
12
11
10
10
10
8
10
13
10
7
0.136
0.135
0.139
^
0.145
0.142
0.136
0.141
0.142
0.149
0.51242
0.51240
0.51239
^
0.51235
0.51236
0.51242
0.51244
0.51245
0.51244
31.5
31.9
32.1
^
32.8
32.7
31.6
31.1
31.0
31.2
17.962
17.974
18.004
17.961
17.999
17.969
18.011
18.006
18.016
18.011
15.639
15.641
15.662
15.667
15.668
15.665
15.654
15.646
15.662
15.658
38.887
38.910
39.033
39.065
39.144
39.059
38.946
38.876
38.931
38.915
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
Upper group
24R-2-10^17
25R-1-64^72
25R-2-46^53
25R-2-119^126
25R-7-90^98
26R-1-31^43
26R-1-143^146
26R-2-17^23
27R-4-57^63
28R-3-123^128
29R-1-30^37
31R-3-5^10
31R-3-5^10*
31R-3-85^90
32R-6-66^71
33R-1-53^60
Lower group
38R-3-63^68
38R-4-71^77
39R-1-101^108
40R-1-14^16
40R-3-117^124
41R-1-3^10
45R-3-57^63
46R-1-51^56
46R-2-7^15
46R-3-12^18
87
Depth is reported in meters below sea £oor. * denotes duplicate analysis. Subscript ‘m’ denotes a measured ratio and ‘T’ denotes the ratio corrected for 109 Myr
of in situ decay [6]. Error reported is the measured mean (2c). Measured error (2c) for 206 Pb/204 Pb is 0.014, 207 Pb/204 Pb is 0.016 and 208 Pb/204 Pb is 0.047. Rb and
Sr concentrations are by XRF, Sm and Nd are by NA. ONd (T) is calculated using 143 Nd/144 Nd value for CHUR (today) of 0.512638 [31].
41
42
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
Rb, K and Ba do not correlate well with immobile
incompatible elements although immobile, incompatible elements (e.g., Zr and Nb) correlate well
with each other (Table 1). For this reason, only
data on immobile trace elements and isotope data
on leached samples are discussed as being relevant
to petrogenetic interpretations.
3.3. Trace element geochemistry
Downcore plots of primitive mantle-normalized
incompatible trace element ratios (designated by
the subscript ‘PM’ [30]) demonstrate limited, yet
important variations between the upper basalt
group, Units 1^4, and the lower basalt group,
Units 7, 8 and 10 (Fig. 2). The variation of (La/
Yb)PM is fairly limited, from 5.5 to 8. All basalts
are enriched in incompatible elements overall
when compared to values for the primitive mantle
[30]. (La/Nb)PM is greater than unity for all samples, but highest for the lower basalt Units 7 and
8; (Th/Ta)PM values show an even stronger distinction between the upper basalt group, which
has values between 1 and 1.5, and the lower basalt
group, which has signi¢cantly higher values, up to
2.5. In Units 3 and 4, (Th/Ta)PM values are the
closest to unity, due mainly to the fact that Th is
less enriched in these two units (20^30Uprimitive
mantle) than in Units 1 and 2 (25^35U primitive
mantle) and the lowermost Units 7, 8 and 10 (30^
45U primitive mantle).
3.4. Sr, Nd and Pb isotopic compositions
Sr, Nd and Pb isotopes demonstrate systematic
trends downcore (Table 2). Initial 87 Sr/86 Sr, at 109
Ma [6], shows limited variation in Units 1^4
((87 Sr/86 Sr)T = 0.70477^0.70497). These values are
distinctly lower than those of the lower basaltic
lava £ows, Units 7, 8 and 10 ((87 Sr/86 Sr)T =
0.70549^0.70595). ONd (T) (calculated from
CHUR values of Goldstein et al. [31]) mirrors
these downcore trends, where the highest values
are found in the upper basaltic £ows (ONd (T) = 31
to 0) and the lower values are found in the lower
basaltic £ows (ONd (T) = 32.8 to 31). Unit 8 has
the highest (87 Sr/86 Sr)T and the lowest ONd (T) of
all basalt £ows. There is no signi¢cant downcore
variation in 206 Pb/204 Pb, but the highest values
occur in Unit 4 (206 Pb/204 Pb = 18.07) and the lowest values in Unit 8 (206 Pb/204 Pb = 17.96). For
207
Pb/204 Pb, the highest overall values are in the
lowermost units; the highest values of 208 Pb/204 Pb
are in Unit 8 (208 Pb/204 Pb = 39.14) and the lowest
in Unit 1 (208 Pb/204 Pb = 38.55).
The substantial range of (87 Sr/86 Sr)T and ONd (T)
in the Bunbury and Rajmahal basalt samples
[9,11] encompasses that of the Site 1137 basalts
(Fig. 3). Rajmahal Group II basalts show two
subgroups, Group IIa and IIb; distinctly more
radiogenic (87 Sr/86 Sr)T and less radiogenic ONd (T)
are found in Group IIb. The small range in (87 Sr/
86
Sr)T and ONd (T) of the Rajmahal Group IIa
lavas is enveloped by the Site 1137 basalts. The
Bunbury basalt of Western Australia also has two
groups that match well with the two major Rajmahal groups [9]. The Bunbury Casuarina group
shows minimal evidence for contamination by
continental crust (similar to Rajmahal Group I),
whereas the Bunbury Gosselin group have Sr and
Nd isotopic compositions near the extreme end of
the Rajmahal Group IIb [9]. Some basalts from
Site 749 on the SKP have Sr and Nd isotopic
compositions similar to those of Rajmahal Group
I and Bunbury Casuarina basalts.
Isotope plots of 207 Pb/204 Pb and 208 Pb/204 Pb vs.
206
Pb/204 Pb reveal very steep (nearly vertical)
trends for Bunbury, Rajmahal, Site 749 and Site
1137 basalts with a 206 Pb/204 Pb value of V18
(Fig. 3). Site 749, Rajmahal Group I and Bunbury
Casuarina basalts have the lowest 207 Pb/204 Pb and
208
Pb/204 Pb values. Rajmahal Group IIa and Site
1137 upper group have values beginning within
this range and extending slightly higher. Bunbury
Gosselin group, Rajmahal Group IIb and Site
1137 lower group have the highest 207 Pb/204 Pb
and 208 Pb/204 Pb values (although most Gosselin
samples have low 208 Pb/204 Pb). Site 738 basalts
have distinctly lower 206 Pb/204 Pb, higher 207 Pb/
204
Pb and similar 208 Pb/204 Pb values. The trend
observed for other basalts erupted early on the
Kerguelen Plateau (Sites 747 and 750) is very different from that of the above mentioned basalts,
forming a shallow slope toward much lower
206
Pb/204 Pb, 207 Pb/204 Pb and 208 Pb/204 Pb values
([32]; Fig. 3).
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
4. Discussion
4.1. Comparison of the petrogenesis of Site 1137
basalts with Cretaceous basalts of Australia
and India
Geochemical similarities exist between Cretaceous continental tholeiitic basalts from the eastern Indian and southwestern Australian margins
and basalts from the Kerguelen Plateau (e.g.,
43
[4,9,11,13]). The basalts at Site 1137 and Site
749 display striking similarities to the Bunbury
basalt and the Rajmahal Traps, speci¢cally in
Pb^Pb isotope plots (Fig. 3). The Rajmahal Traps
and Bunbury basalt have been interpreted as
being variably contaminated by continental lithosphere [4,13], continental crust [11,14] or continental upper crust [9] and this variation forms
the basis of their division into separate groups
[9]. The Site 1137 basalts also show two distinct
groups (upper and lower groups), both with geochemical characteristics more in common with the
Bunbury Gosselin and Rajmahal Group II than
with Bunbury Casuarina or Rajmahal Group I.
All Bunbury Gosselin, Rajmahal Group II and
Site 1137 basalts have elevated (La/Nb)PM , (La/
Ta)PM , (La/Th)PM , 87 Sr/86 Sr and 207 Pb/204 Pb.
These geochemical characteristics in Bunbury
and Rajmahal basalts have been interpreted as
representing contamination by continental material, most likely continental crust [9,11]. We suggest
that assimilation of variable amounts of continental lithosphere also explains these geochemical
characteristics of the Site 1137 basalts.
6
Fig. 3. (a) ONd (T) vs. (87 Sr/86 Sr)T for Indian Ocean, Kerguelen Plateau and Eastern Gondwanan continental basalts. Indian MORB data are from [62], Ninetyeast Ridge (NER)
data are from [26], Site 738 data are from [4] and values for
the Kerguelen plume (labeled ‘plume’) are from [41]. Bunbury and Rajmahal continental tholeiite data from [9,11], respectively and Kerguelen Plateau data from [32]. The thick
gray line represents mixing between a garnet biotite gneiss
also recovered in the Site 1137 drill core [19] and the least
contaminated basalts from Site 1137. The inset shows the entire mixing line between the Site 1137 basalts and the
gneisses (black cross), small white circles are marked with an
estimated percentages of assimilation (italicized numbers).
208
Pb/204 Pb vs. 206 Pb/204 Pb (b) and 207 Pb/204 Pb vs. 206 Pb/204 Pb
(c) for Indian Ocean, Kerguelen and Eastern Gondwanan
continental basalts. 2c error bars are shown for each ¢gure.
The average W (20) and U (4.2) have been calculated for basalts considered to be representative of the Kerguelen plume
[41]; the thickness of the Pb evolution line represents the inherent uncertainties associated with this calculation. Small
white circles are shown for each 50 Myr interval, beginning
at the plume ¢eld with 0 Myr. Data sources and symbols are
the same as in panel a.
44
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
4.2. Evidence for assimilation of continental
lithosphere
4.2.1. Identi¢cation of the continental contaminant
The general characteristics of incompatible
trace elements and trends in the isotope diagrams
presented above are mostly likely created by the
assimilation of continental lithospheric material
during ascent of basalt magmas. Distinguishing
between the assimilation of subcontinental mantle
lithosphere (SCML) and the assimilation of continental crust is a subject of much debate (e.g.,
[33^35]). McDonough [36] argued that most samples (peridotite xenoliths) from the SCML are not
depleted in Nb or Ta; additionally, the SCML
also has much lower concentrations of Sr and
Nd relative to a plume-derived basalt (e.g., [37]).
Thus, contamination by SCML appears unlikely
to produce the geochemical characteristics observed in the Site 1137 lower basalt group. Furthermore, as samples of unequivocal continental
crust crop out within the Site 1137 core [5], these
crust samples are the most conspicuous contaminant for the Site 1137 basalts. The garnet^biotite
gneiss has very high abundances of La, Th, Pb
and Nd, very high 87 Sr/86 Sr values and very low
concentrations of Nb and Ta [19], comparable to
the geochemical characteristics of continental
crust [38,39].
Increasing SiO2 wt% (or decreasing MgO wt%)
in combination with increasing radiogenic isotope
ratios characteristically enriched in the continental crust (e.g., 87 Sr/86 Sr, 207 Pb/204 Pb or 208 Pb/
204
Pb) is indicative of assimilation of continental
crust by basaltic magmas (e.g., [40]). Although
these trends are recognized in the Rajmahal Traps
lavas, Site 1137 upper and lower basalt groups
span a similar range in SiO2 wt% and are only
distinguished by the more radiogenic 87 Sr/86 Sr of
the lower basalt group (Fig. 4). We interpret this
to indicate that the Site 1137 basalts did not assimilate enough continental crust to have a¡ected
their major element concentrations and that the
lower basalt group is more contaminated than the
upper basalt group.
Continental crust is typically highly depleted in
elements such as Ta and Nb [39]. Upper continental crust is enriched in La and Th while lower
Fig. 4. (a) Variation of (87 Sr/86 Sr)T vs. SiO2 wt% for basalts
associated temporally and spatially with the early Kerguelen
plume. (b) (Th/Ta)PM vs. (La/Ta)PM for the same basalts as
in panel a. Arrows indicate typical vectors created by assimilation of upper continental crust (UCC) and lower continental crust (LCC). (c) (87 Sr/86 Sr)T vs. (La/Ta)PM for the same
basalts as in panel a. The arrow points in the direction of
typical continental crust, which is generally enriched in 87 Sr/
86
Sr and La relative to Ta [37,38]. Bunbury Gosselin basalts
fall outside the plot perimeter toward much higher (La/
Ta)PM values. Data sources and symbols are the same as in
Fig. 3.
continental crust is not usually enriched in Th
[38]. The Site 1137 basalts, in addition to those
of Site 749, Rajmahal and Bunbury, follow a
trend toward upper continental crust in a plot
of (Th/Ta)PM vs. (La/Ta)PM (Fig. 4). All samples
de¢ne a line that could represent mixing between
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
mantle melts and upper continental crust, except
for a few Bunbury Casuarina samples. Additionally, contamination by upper continental crust
would readily explain an increase in 87 Sr/86 Sr
coupled with an increase in (La/Ta)PM (Fig. 4).
4.2.2. Quanti¢cation of the continental
contaminant
For the mantle endmember of the calculation,
we use the least contaminated basalt samples from
the Site 1137 core. Unit 4 of the upper basalt
group has the lowest 207 Pb/204 Pb, 208 Pb/204 Pb,
(87 Sr/86 Sr)T , the highest ONd (T) and La/Nb, La/
Ta and La/Th closest to primitive mantle values.
These general characteristics suggest that contamination is minimal in Unit 4 basalts and that their
composition may best approximate the Kerguelen
plume at the time of eruption of the Site 1137
basalts. The evolution of the Kerguelen plume
Pb isotopic composition is estimated based on a
W (238 U/204 Pb) of 20 and U (232 Th/238 U) of 4.2
(Fig. 3; calculated on the basis of average compositions considered to be representative of the
Kerguelen plume [41]). These values are similar
to those of Lassiter et al. [42], calculated on the
basis of average Kerguelen Islands data from Sun
and McDonough [30]. The Kerguelen plume
would intersect with the least contaminated basalts of Site 1137 around 150 Ma; thus, it is a
reasonable assumption that these basalts record
the Kerguelen plume composition. Additionally,
the Kerguelen plume appears to be an appropriate
mantle source for the Bunbury and Rajmahal basalts. However, because the Bunbury Casuarina,
Rajmahal Group I and Site 749 basalts plot below
the Pb evolution curve of the Kerguelen plume;
we thus infer an additional, depleted mantle component in their source.
The composition of the continent-derived gneiss
clasts can be used as the potential contaminant in
the Site 1137 basalts. These gneiss clasts have an
isotopic composition similar to rocks of the Eastern Ghats Belt, a Paleoproterozoic terrane in eastern India [19,43]. For both Sr and Nd isotopes,
using the equations of Vollmer [44] we obtain independently consistent results. A calculation assuming endmembers of the least contaminated
Site 1137 basalt and the gneiss clast requires ap-
45
proximately 9% of the garnet^biotite gneiss to be
assimilated by the basaltic magmas in order to
reach the composition of the most contaminated
basalts (Fig. 3). Pb isotopic systematics are more
di⁄cult to explain because the 206 Pb/204 Pb remains fairly constant from the least contaminated
basalt to the most. Unfortunately no Pb concentration data is available for the basalts, but using
various estimations for Pb concentrations in tholeiitic basalts (from 1 to 5 ppm), both the 207 Pb/
204
Pb and the 208 Pb/204 Pb yield consistent results
for the amount of contaminant required to explain the data (V5%), however the quantity of
contamination required is somewhat less than
that required to explain the Sr^Nd systematics.
The above estimates on the percent assimilate
are based on simple bulk mixing equations. When
we combine the e¡ects of assimilation and fractional crystallization, we can more rigorously constrain the amount of assimilate to original magma. We use the equations of DePaolo [45] and
estimate the rate of assimilation to fractional crystallization using graphical interpolations as demonstrated by Aitcheson and Forrest [46]; this results in a rate of assimilation to rate of fractional
crystallization (r) of 0.1. Using this more realistic
approach than bulk mixing, the fraction of contaminating gneiss is calculated to be between 6.5
and 7% for the most contaminated basalts (Unit
8). This is essentially in the same range as the bulk
mixing prediction, especially if the errors associated with calculating the bulk solid/melt partition
coe⁄cients for Sr and Nd concentrations in basaltic rocks are taken into account. Therefore, we
place the upper limit on the amount of crustal
contamination as V7%, based on the results calculated using Sr and Nd concentrations and isotopes, and the lower limit at 5%, based on the
results of the Pb isotope systematics. Alternatively, if a depleted mantle source is considered
(rather than an enriched, plume-like source), attempts at quanti¢cation of the contaminant yield
highly inconsistent results. For example, the small
shifts in Sr isotopic composition would require
only a few percent assimilate while the huge shifts
in Nd isotopes would require vastly greater quantities of contaminating material, as much as an
order of magnitude more.
46
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
4.3. Tectonic and age constraints for the
involvement of the Kerguelen plume in the
Bunbury, Rajmahal and Kerguelen Plateau
basalts
It would be relatively straightforward to explain the common geochemical characteristics between continental basalts of the eastern Indian
and southwestern Australian margins and those
of the Kerguelen Plateau were the Kerguelen
plume implicated in the breakup of India, Australia and Antarctica (Fig. 5). If the early Cretaceous
Bunbury, Rajmahal and Site 1137 basalts were
derived from the Kerguelen plume, they represent
V23 Myr of volcanic activity. All of these basalts
post-date the oldest known magnetic anomalies in
the Indian Ocean which lie o¡ the northwest coast
of Australia [47], and this makes their geochemical similarities somewhat di⁄cult to reconcile.
Magnetic anomalies as old as M25 (V154 Ma)
occur o¡ the northwestern coast of Australia.
Fig. 5. Generalized plate tectonic reconstruction prior to
complete breakup between West Australia, East Antarctica
and eastern Greater India ca. 130 Ma (after [48,60]). The
major Proterozoic mobile belts are indicated (locations from
[63]). The gneiss clasts recovered from Site 1137 used to
model the crustal contaminant in the basalt £ows may originate from the Eastern Ghats Belt [19]. Elan Bank may be a
microcontinent splintered from the eastern margin of India
through interaction of the Kerguelen plume with the newly
formed margin (e.g., [8,19,61]).
However, magnetic anomalies get younger to the
south along the western coast of Australia and
nearest the location of the Bunbury basalt, the
oldest anomalies are M10N (V132 Ma; magnetic
anomalies reported by Royer and Co⁄n [48] and
ages from the time scale of Gradstein et al. [49]).
This implies that the separation between northern
India and West Australia may have proceeded as
an unzipping from north to south over a time
interval spanning more than 20 Myr. Alternatively, Mu«ller et al. [50] proposed that separation
proceeded by transform motion within Greater
India, the northern half moving away from Australia before the southern half. The separation
between eastern India and East Antarctica is less
well constrained. Mu«ller et al. [50] demonstrated
that breakup must have occurred later than 120
Ma while previous reconstructions have placed it
near 130 Ma [51]. Most recently, Co⁄n et al. [8]
suggest that eastern India was fairly well separated from both southwestern Australia and eastern Antarctica by about 130 Ma. To further explore the tectonic setting of the early Kerguelen
plume, we propose two possibilities :
1. Post-breakup oceanic plume. In this scenario,
the initial magmatism related to the Kerguelen
plume took place well after continental rifting,
and the initial Kerguelen Plateau was constructed in the Indian Ocean basin. Geochemical similarities between the Casuarina group
of Bunbury, Rajmahal Group I and Site 749
and between the Gosselin group of Bunbury,
Rajmahal Group II and Site 1137 would, in
this case, be coincidental. Here, the 130^123
Ma Bunbury basalt is associated with the prolonged stretching phase of continental breakup, i.e., they represent small amounts of volcanism typical of non-volcanic rifted margins
(e.g., [52]). The unusually limited variation in
206
Pb/204 Pb of the Bunbury, Rajmahal and
some Kerguelen basalts would be di⁄cult to
reconcile in this scenario, especially if the higher 207 Pb/204 Pb and 208 Pb/204 Pb result from contamination by continental crust, which tends to
be very heterogeneous. Duncan [7] proposes
that the entire Kerguelen Plateau may have
formed around 118 Ma (adhering to the decompressing plume head model [53]) and that
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
younger volcanism (e.g., Site 1137) represents
small volume magmatism from the Kerguelen
plume tail as the Kerguelen Plateau migrated
south relative to the ¢xed hotspot. These
younger magmas could have assimilated material from the overlying (V118 Ma) plateau
crust, but this is di⁄cult to assess geochemically.
2. Pre-breakup or breakup plume. Storey et al. [22]
proposed that the Kerguelen plume was
present prior to continental rifting and Kent
[23] advocated a long-term, incubating plume
head prior to continental breakup. If so, then
the geochemical similarities between Bunbury,
Rajmahal and Kerguelen basalts would re£ect
a common origin by partial melting of similarly geochemically characterized mantle material contaminated by similarly-aged continental
crust. However, the reportedly small volume of
the Bunbury basalt (V103 km3 [8]) is signi¢cantly lower than that typically associated with
plume-related continental £ood basalt volcanism (V105 ^106 km3 [54]). Co⁄n et al. [8] suggested that the initial Kerguelen plume head
might have broken into ‘droplets’ of variable
sizes within the rapidly convecting upper
mantle, resulting in spatially and temporally
displaced magmatic events of di¡ering volumes, all ultimately related to the Kerguelen
plume.
The second model best explains the geochemical similarities between Bunbury, Rajmahal, Site
749 and Site 1137 basalts. The plume source
would be readily available for these di¡erent basalt groups. The geochemical variations could
then be explained by crust contamination and/or
an additional depleted component (required to
explain the group including the Bunbury Casuarina, Rajmahal Group I and Site 749 basalts).
However, at this time, we cannot explain why
continental £ood basalt volcanism does not appear to pre-date the rifting, as is typical of areas
where plumes are present prior to the onset of
rifting (e.g., [55]). Nevertheless, it appears clear
that the Kerguelen plume did interact with the
continental margins of eastern India and western
Australia and was involved with isolating microcontinents from their margins [8,19,50].
47
5. Conclusions
Geochemical similarities between select early
Cretaceous basalts of the Indian Ocean and Kerguelen Plateau indicate two broad groups, based
on basalt geochemistry : the ¢rst group includes
the Bunbury Casuarina group, the Rajmahal
Group I and Site 749 on the Southern Kerguelen
Plateau and the second group includes the Bunbury Gosselin group, the Rajmahal Group II and
Site 1137. Although these basalts are presently
located several thousand kilometers apart, when
they erupted they were separated by less than
1000 km. Both the geochemical data and plate
tectonic reconstructions are permissive of an origin for both groups from the Kerguelen plume.
However, the ¢rst group requires some input from
depleted asthenospheric mantle, while in the second group, no depleted component is required.
Site 1137 basalts show evidence for contamination
by continental crust as has been previously suggested for the Bunbury basalt and Rajmahal
Traps [9,11]. In Site 1137 basalts, assimilation of
as much as 7% of upper crustal material is required to explain the compositions of the most
contaminated basalts.
Until the timing of rifting between Australia,
India and Antarctica is better constrained, we
cannot ascertain if the Kerguelen plume played
a causal role during the early stages of continental
rifting. It remains possible that the arrival of the
Kerguelen plume in an already rifting environment was fortuitous. Increasingly, however, geochemical and geophysical evidence requires the
presence of a mechanism which aided in the isolation of fragments of continental crust at shallow
levels in the early Indian Ocean basin during
the early rifting stages of Eastern Gondwana.
The Kerguelen plume may have been present during early rifting activity but not necessarily the
driving force behind the opening of the Indian
Ocean.
Acknowledgements
Thanks go to G. Bru«gmann, M. Co⁄n, D.
Damasceno, R. Kent, and N. Mattielli, for dis-
48
S. Ingle et al. / Earth and Planetary Science Letters 197 (2002) 35^50
cussions that improved some of the ideas presented here. A. Saunders and A. le Roex are
greatly thanked for providing critical reviews. C.
Maerschalk is thanked for his help in the laboratory. We thank M. Rhodes and the University of
Massachusetts for X-ray £uorescence analyses.
Dr. P. Ila is thanked for supervision of the
INAA facility at MIT. The ¢rst author is supported by, and the work presented here is funded
by an ARC grant (#03/233) from the Communaute¤ franc°aise de Belgique. The Fonds National
de la Recherche Scienti¢que funds Belgian membership in the Ocean Drilling Program (ODP) and
supported the second author’s participation in
ODP Leg 183, where sampling was carried out
(F.N.R.S. Grant 3.4579.99). The ODP is sponsored by the U.S. National Science Foundation
and participating member countries under the
management of Joint Oceanographic Institutions,
Inc. Additional funding for this research was provided by the U.S. Science Support Program
(USSSP).[BOYLE]
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