two mantle plumes beneath the east african rift system: sr, nd and pb isotope evidence from kenya...
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Two mantle plumes beneath the East African rift system:Sr, Nd and Pb isotope evidence from Kenya Rift basalts
Nick Rogers a;*, Ray Macdonald b, J. Godfrey Fitton c, Rhiannon George a,Martin Smith d, Barbara Barreiro e;1
a Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UKb Department of Environmental Sciences, Lancaster University, Lancaster LA1 4YQ, UK
c Department of Geology and Geophysics, University of Edinburgh, West Mains Rd., Edinburgh EH9 3JW, UKd British Geological Survey, Murchison House, West Mains Rd., Edinburgh EH9 3LA, UK
e NERC Isotope Geoscience Laboratory, Keyworth, Nottingham NG12 5GG, UK
Received 9 July 1999; received in revised form 6 December 1999; accepted 12 January 2000
Abstract
Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recentbasalts (MgOs 4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend tonephelinite. A broadly negative correlation between Zr/Nb (6 2^7) and Ce/Y (1^8) indicates derivation from a garnet-bearing mantle source region as a result of 6 3% melting. Isotope ratios in basalts from the axial regions of the KenyaRift have 143Nd/144Nd = 0.51300 to 0.51255, 87Sr/86Sr = 0.7030 to 0.7055 and 206Pb/204Pb6 18 to s 20, broadly similarto values from OIB. The Kenya Rift cuts through basement of different ages and aspects of the composition of maficmagmas reflect the anisotropy of the underlying lithosphere. Specifically, those basalts from that part of the riftunderlain by the Tanzanian craton (TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through thePanafrican Mozambique belt (MB) implying an origin either at greater depth or from a more trace element-enrichedsource region. Samples erupted through the zone of reactivated craton margin (RCM) share the characteristics of maficlavas from both the craton and the mobile belt. MB samples have 143Nd/144Nd = 0.5130^0.5127, 87Sr/86Sr = 0.7030^0.7035 and 206Pb/204Pb = 18.3^19.8, defining a steep negative trend on the Nd^Sr diagram and plotting close to theNHRL on conventional Pb isotope diagrams. By contrast TC and RCM samples have 143Nd/144Nd = 0.5124^0.51275,87Sr/86Sr = 0.7035^0.7056 and 206Pb/204Pb = 17.6^21.2, defining flat-lying arrays on Nd^Sr plots and a much greaterscatter and spread on Pb isotope diagrams, with many analyses plotting above the NHRL. Both groups of analysestrend towards a common end member on a plot of 143Nd/144Nd against 87Sr/86Sr, at 143Nd/144NdV0.51275 and 87Sr/86SrV0.7035. These values are suggested to reflect the isotopic characteristics of the sub-lithospheric Kenyan mantle,inferred to be the Kenya mantle plume. Comparison with data from Afar suggest that the Kenya plume is distinct fromthe Afar plume, implying that the east African Rift is underlain by at least two distinct mantle plumes. Eocene andOligocene basalts from southern Ethiopia bear a closer resemblance to the Kenyan basalts than to those from Afar and
0012-821X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 0 ) 0 0 0 1 2 - 1
* Corresponding author. E-mail: [email protected]
1 Present address: 153 Pray Hill Road, Ossipee, New Hampshire 03864, USA.
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Earth and Planetary Science Letters 176 (2000) 387^400
www.elsevier.com/locate/epsl
the Ethiopian plateau, suggesting that the Kenya plume has been active for at least 45 Ma. Migration of magmatismfrom southern Ethiopia southwards is consistent with the northeastward migration of the African plate over the Kenyaplume over the past 50 My. ß 2000 Elsevier Science B.V. All rights reserved.
Keywords: Kenya Rift valley; basalts; lithosphere; mantle plumes
1. Introduction
The interaction between mantle plumes and thecontinental lithosphere, and the lateral in£uenceof plumes beneath continental plates, are both thesubjects of topical debate. Does the lithosphereremain inert, acting as a refractory lid restrictingmelting in the upwelling plume to sub-lithosphericdepths [1] or does the lithosphere contribute tomagmatism because it contains hydrous phases[2^4] or because it is enriched in easily fusibleveins [5]? Similarly, is the lateral in£uence ofmantle plumes limited to those uplifted regionswith large regional gravity anomalies or do theyspread out laterally beneath large areas of conti-nental plates channelled by the inverse topogra-phy of the base of the lithosphere [6]?
One region that is central to these debates is theeast African rift system (EARS). Basalt-domi-nated magmatism occurs throughout the totallength of the EARS, which cuts across two pla-teaus, dynamically supported by upwelling mantlefrom sub-lithospheric depths [7]. Until recentlythese plateaus were assumed to relate to two dis-tinct mantle upwellings or plumes but this ortho-doxy was questioned by Ebinger and Sleep [6].They suggested that magmatism throughoutmuch of continental Africa could be explainedby just one deep mantle plume, the ¢rst manifes-tations of which were 45 Ma old basalts eruptedin southern Ethiopia.
There is a general consensus, from geophysical,petrological and geochemical evidence, that theAfar mantle plume underlies the Ethiopian pla-teau [7,8]. By contrast there is less agreement onthe presence or otherwise of a distinct mantleplume beneath the east African plateau [6,9].The present-day topography of the east Africanplateau and its associated gravity anomaly suggestdynamic support from sub-lithospheric depths [7],and the volume and trace element composition of
rift-related basalts from Kenya have been inter-preted to require a signi¢cant contribution froma mantle plume [10]. A further complexity is therelationship between the east African plateau anda much broader region of dynamic topographycovering the whole of southern and eastern Afri-ca. The support for this larger region is attributedto a major upwelling in the deep mantle, althoughthe anomalous topography of east Africa is theresult of that part of the density anomaly locatedin the upper 500 km of the mantle [11]. What isless clear from this latter study is whether thedynamic topography of the Ethiopian plateau isrelated to the seismically imaged anomaly beneathsouthern and eastern Africa.
In this paper we address these questions usingradiogenic isotope analyses of Neogene to Recentbasalts erupted within the central graben andalong the £anks of the Kenya Rift to map outchanges in mantle source regions along the lengthof the EARS. The results are integrated with pub-lished analyses of basalts from the Kenya Riftand reveal the importance of the mantle litho-sphere in controlling the isotope geochemistry ofKenya Rift basalts. The data also suggest that theKenya and Afar mantle plumes are composition-ally distinct and that there are at least two mantleplumes beneath the EARS. In addition, variationsin basalt compositions throughout the Kenya/Ethiopia system are used to investigate the lon-gevity of these two mantle upwellings.
2. Geological and geophysical background
The magmatism of the Kenya and Ethiopiarifts, the associated £ood basalts of north Ethio-pia and the volcanism of the Afar depression rep-resent one of the largest continental igneous prov-inces presently active on Earth (Fig. 1). The tworifts have a total length in excess of 2000 km and
EPSL 5372 1-3-00
N. Rogers et al. / Earth and Planetary Science Letters 176 (2000) 387^400388
are most volcanically active where they cut acrossthe Ethiopian and east African plateaus. L factorsare generally low (6 1.1) [12] and so the presenceof voluminous magmatism in both rifts impliesthe presence of either elevated asthenospheric po-tential temperatures or easily fusible mantle litho-sphere or both.
The basement to the Kenya Rift is complex andcan be divided into three zones: the ArchaeanTanzanian craton (TC), the late Proterozoic Pan-african Mozambique belt (MB) and a zone ofcraton margin reactivated during the Panafricanorogeny (RCM) (Fig. 1) [12]. On a broad scale thevolumes and compositions of erupted magmas re-£ect the anisotropy of the subjacent lithosphere.Where the rift follows the late Proterozoic Pan-african mobile belt, as is the case in the northernpart of the Kenya Rift, volcanism is characterisedby voluminous alkali and transitional basalts andtheir evolved derivatives. By contrast, in those fewlocalities where the rift cuts into the craton, e.g.western Kenya, eastern Uganda and northernTanzania, volcanism is generally of more limitedvolume, highly alkaline and dominated by neph-elinites, basanites and carbonatites. Where the riftcuts the reworked craton margin, volcanism hascharacteristics that are a mixture of craton andmobile belt regimes, and strongly alkaline mag-mas and basalts are accompanied by locally largevolumes of trachyte, pantellerite and phonolite[13].
3. Analytical techniques
Major and trace elements were determined bystandard XRF techniques at the University ofEdinburgh [14]. Sr, Nd and Pb analysed at theOpen University were separated by standardion-exchange techniques and analysed on Finni-gan0 MAT261 (Sr and Pb) and MAT262 (Nd)multi-collector mass spectrometers. Repeat analy-ses of NBS987 gave a mean 87Sr/86Sr ratio of0.71028 þ 0.000030 (2c) and our in-house John-son^Matthey Nd standard a mean 143Nd/144Ndratio of 0.511865 þ 0.000020 (2c), both on tenanalyses. Blank levels were 6 1.5 ng for Sr and6 0.4 ng for Nd. Pb was analysed in temperature-
controlled runs at 1100³C and ratios were cor-rected for 1x AMU31 relative to the recom-mended values for NBS981. Replicate analysesindicate an external precision of þ 0.2x for allthree Pb isotope ratios. Pb blanks were less than 1ng. Analytical procedures for the BGS laboratoryare similar and described in [15] and analysesfrom the two laboratories have been normalisedto a common value of 0.71025 for 87Sr/86Sr inNBS987 and 0.51186 for 143Nd/144Nd in John-son^Matthey Nd.
4. Results
4.1. Major and trace elements
Major and trace element analyses for KenyaRift basalts from the mobile belt and remobilised
Fig. 1. Sketch map of the Kenya and Ethiopia rifts, showingthe distribution of Tertiary^Recent volcanism, cratonic andmobile belt and the reworked craton margin as mapped outin Kenya [13]. The curved solid lines denote the extent ofthe Ethiopian and east African plateaus and the faint linesmark the strike of the major border faults of the main riftvalleys.
EPSL 5372 1-3-00
N. Rogers et al. / Earth and Planetary Science Letters 176 (2000) 387^400 389
Tab
le1
Maj
oran
dtr
ace
elem
ent
and
Sr,
Nd
and
Pb
isot
ope
anal
yses
ofba
salt
sfr
omth
ere
acti
vate
dcr
aton
mar
gin
and
mob
ilebe
ltzo
nes
ofth
eK
enya
rift
Rif
ted
crat
onm
argi
nM
obile
belt
43/3
7W
203
KL
R10
220
9-1
KL
R36
KL
R67
TH
EL
2K
B18
3K
B24
1K
B16
2K
B15
6K
B19
7K
B21
4K
B24
KB
42K
B63
KB
65K
B66
KB
71K
B10
6K
B8
5/35
KB
260
KB
267
KB
265
KB
167
KB
203
KB
226
KB
259
KB
45
Age
1.9
7.5
2.57
12.5
1.54
1.54
0.25
0.00
50.
005
1.55
0.00
53.
20.
005
0.00
20.
0005
0.11
0.06
40.
110.
126.
92.
753.
33.
20.
250.
253.
21.
870.
12
Roc
kty
peA
OB
AO
BA
OB
AO
BA
OB
HN
BA
OB
BSN
BSN
BSN
BSN
BSN
BSN
AO
BA
OB
AO
BA
OB
AO
BA
OB
AO
BA
OB
AO
BA
OB
AO
BA
OB
AO
BA
OB
HN
BH
NB
HN
B
SiO
242
.14
47.5
644
.74
44.8
447
.13
47.3
846
.56
46.8
447
.83
45.5
44.8
845
.61
45.6
349
.05
47.2
645
.86
46.1
746
.34
46.3
846
.23
47.8
443
.37
47.4
47.3
949
.57
48.4
347
.66
48.3
649
.39
47.2
6
TiO
24.
479
1.81
4.66
62.
556
1.85
72.
568
2.28
2.08
2.82
1.96
21.
912.
821
2.87
12.
511.
933.
163.
352.
953.
412.
761.
902
2.89
43.
104
3.51
13.
333
2.42
21.
452
2.70
32.
963
2.70
2
Al 2
O3
14.4
414
.99
11.5
18.
2215
.19
14.5
914
.69
14.9
315
.48
13.5
514
.46
14.9
615
.714
.99
15.7
413
.95
13.8
613
.68
13.0
113
.81
16.7
515
.48
14.1
14.8
314
.49
15.2
519
.27
14.5
716
.27
15.6
5
Fe 2
O3
15.3
812
.24
15.1
511
.88
12.4
13.6
112
.39
11.7
812
.09
11.2
110
.79
12.4
512
.33
12.5
311
.41
15.4
15.7
215
.45
16.3
715
.07
11.2
714
.36
16.0
414
.42
13.5
613
.02
8.8
14.7
713
.04
13.6
7
MnO
0.21
40.
185
0.19
20.
162
0.19
70.
204
0.17
10.
180.
243
0.16
90.
171
0.2
0.19
80.
210.
170.
220.
210.
210.
210.
210.
183
0.20
30.
242
0.22
20.
227
0.20
70.
142
0.23
20.
215
0.20
9
MgO
6.4
6.73
6.77
11.0
46.
66.
047.
517.
575.
2510
.78
10.2
6.87
5.72
5.47
7.64
6.21
5.59
5.98
5.08
6.19
6.98
6.96
4.95
4.89
4.35
5.46
6.23
4.54
4.09
6.29
CaO
9.28
11.9
810
.25
15.4
812
.38
10.9
410
.67
9.93
8.92
12.4
813
.14
10.4
810
.75
9.86
11.5
810
.76
10.0
911
10.2
711
.22
11.0
210
.69.
969.
728.
6110
.23
12.8
89.
367.
539.
9
Na 2
O3.
152.
642.
912.
212.
832.
822.
953.
634.
482.
72.
633.
583.
723.
963.
173.
13.
453.
163.
43.
043.
072.
523.
53.
294.
13.
922.
763.
554.
083.
13
K2O
1.07
60.
706
1.72
21.
172
0.68
41.
038
1.23
91.
11.
698
1.03
80.
981.
432
1.35
41.
150.
50.
851.
040.
760.
980.
670.
695
0.94
0.91
91.
422
1.57
10.
826
0.43
41.
221.
697
0.89
8
P2O
50.
689
0.23
90.
813
0.37
60.
281
0.46
30.
863
0.31
1.11
40.
419
0.41
60.
875
0.88
40.
460.
230.
590.
630.
460.
680.
420.
306
0.79
40.
415
0.61
60.
798
0.42
50.
232
0.46
0.55
10.
432
Tot
al97
.26
99.0
798
.73
97.9
499
.54
99.6
699
.32
98.3
599
.92
99.8
299
.57
99.2
999
.17
100.
1999
.63
100.
110
0.11
99.9
999
.79
99.6
210
0.02
98.1
310
0.63
100.
3110
0.6
100.
1899
.85
99.7
899
.82
100.
14
Nb
9162
9367
2643
6524
7146
5459
5626
1824
3625
3220
2437
3970
6542
2155
8732
Zr
381
256
488
192
112
163
129
124
198
137
150
164
158
201
121
150
185
145
190
135
143
9121
826
230
517
888
210
254
178
Y40
.627
.843
.119
.727
.836
.224
.823
38.1
23.3
22.3
3130
.940
2536
4135
4434
30.5
21.4
42.9
38.8
55.2
40.3
21.8
40.6
37.6
35.7
Sr92
556
288
171
144
945
415
2638
167
951
268
369
774
434
441
844
142
947
945
247
943
912
1742
660
443
735
838
739
662
751
6
Rb
24.2
31.9
57.2
29.7
13.9
20.9
39.4
1638
.725
.522
.630
.430
.218
1016
2114
2214
15.9
22.9
20.2
32.5
38.6
21.6
1028
.347
.316
.3
Th
6.5
5.4
9.2
4.7
2.1
3.2
9.5
6.3
3.9
5.8
6.3
5.5
27.
16
6.8
6.7
4.8
1.7
7.8
104.
4
Pb
4.4
6.2
4.1
1.9
3.8
53.
72.
13.
83.
21.
31.
61.
52.
61.
54.
22.
41.
73.
93.
51.
5
La
5641
7049
2031
9116
4226
2936
3630
2343
3441
3417
2724
4139
2011
2961
23
Ce
133
9817
110
446
6916
033
100
5868
8181
116
4494
8561
8170
4162
5893
9452
2969
110
54
Nd
6846
8445
2536
5652
2933
4244
2233
3447
5131
1735
5031
Zn
132
107
139
8683
9884
7010
176
7489
8081
6710
510
887
108
7970
7611
196
106
9956
106
9210
3
Cu
4112
145
127
131
164
6190
4811
111
065
6267
6068
6965
5789
5556
7584
2773
7298
2141
Ni
1613
880
139
7863
142
7047
212
176
9057
2758
3731
2520
2954
6918
1416
4082
235
32
Cr
1537
316
247
484
7331
411
497
491
341
180
9877
7282
6961
4162
133
9625
1031
122
205
345
58
V32
127
934
127
326
329
222
917
925
227
125
122
321
424
936
429
324
230
221
336
619
529
5
Ba
590
366
575
481
375
518
1744
198
996
346
365
815
825
395
210
490
490
352
435
354
287
680
295
627
373
268
152
385
862
475
Sc20
.428
.820
.843
.238
.629
.324
.316
.434
.333
.823
.318
.129
.215
.331
.418
.920
.731
2724
15.6
26
Lab
OU
OU
OU
OU
OU
OU
OU
BG
SB
GS
BG
SB
GS
BG
SO
UB
GS
BG
SB
GS
BG
SB
GS
BG
SB
GS
BG
SO
UB
GS
BG
SB
GS
BG
SO
UB
GS
BG
SB
GS
87Sr
/86Sr
0.70
377
0.70
369
0.70
408
0.70
382
0.70
417
0.70
419
0.70
472
0.70
317
0.70
326
0.70
323
0.70
336
0.70
330
0.70
326
0.70
303
0.70
339
0.70
321
0.70
327
0.70
335
0.70
335
0.70
329
0.70
325
0.70
344
0.70
319
0.70
313
0.70
320
0.70
309
0.70
319
0.70
316
0.70
339
0.70
351
143N
d/14
4N
d0.
5127
30.
5126
90.
5125
90.
5127
20.
5126
90.
5126
20.
5129
80.
5129
00.
5129
10.
5129
00.
5127
60.
5129
30.
5129
80.
5128
40.
5128
30.
5128
50.
5128
30.
5128
50.
5128
90.
5127
50.
5129
50.
5129
50.
5129
30.
5130
00.
5128
00.
5129
60.
5128
40.
5127
9
206P
b/20
4P
b19
.575
18.3
8219
.500
19.9
6019
.800
19.9
5820
.336
19.0
5719
.297
19.3
4319
.419
19.4
1918
.407
18.9
4818
.779
19.0
0619
.150
19.1
0418
.789
19.0
3619
.519
18.9
4519
.196
19.7
5419
.083
18.7
05
207P
b/20
4P
b15
.600
15.5
8115
.716
15.6
2315
.674
15.7
0815
.696
15.5
7715
.595
15.6
0415
.617
15.5
9715
.533
15.6
1015
.549
15.5
7415
.630
15.5
7115
.554
15.5
8015
.605
15.6
1715
.572
15.6
3215
.630
15.5
33
208P
b/20
4P
b39
.320
38.2
2039
.741
39.7
3239
.444
39.7
2742
.983
38.8
9839
.124
39.1
7639
.303
39.2
2438
.358
38.8
1638
.591
38.8
1539
.107
38.9
0838
.819
38.8
3339
.264
38.8
5238
.965
39.5
3639
.009
38.7
13
Ana
lyse
sla
bele
dB
GS
wer
ede
term
ined
atth
eB
riti
shG
eolo
gica
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rvey
usin
gte
chni
ques
asde
scri
bed
in[1
5]an
dth
ose
labe
led
OU
wer
ede
term
ined
atth
eO
pen
Uni
vers
ity.
Ana
lyse
sfr
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etw
ola
bora
tori
esw
ere
norm
alis
edto
aco
mm
onva
lue
of0.
7102
5fo
r87
Sr/86
Sr
inN
BS9
87an
d0.
5118
6fo
r14
3N
d/14
4N
din
John
son^
Mat
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craton margin sectors are listed in Table 1 andcritical aspects of the compositional variations il-lustrated in Figs. 2 and 3. Data plotted in both¢gures are extracted from a recently compiled da-tabase of Kenya Rift basalts [16] and the follow-ing description of the data represents a summaryof that larger dataset which includes the analysesin Table 1. Fig. 2 illustrates the variation of totalalkali contents with silica in samples from thelength of the Kenya Rift. Virtually all are alkalinewith a relatively small number of hypersthene-normative basalts. Alkali basalts, basanites andnephelinites dominate the array, together withlesser numbers of hawaiites, and tephrites andone phono-tephrite. While there is no clear rela-tionship between lava composition and the ageand nature of the underlying lithosphere, lavaserupted through the mobile belt (open circles)are dominated by alkali basalts, basanites andhawaiites. There are also proportionately morenephelinites in regions underlain by cratonic base-ment than in those underlain by mobile belt [12].
Aspects of the incompatible trace element var-iation in these samples are illustrated in Fig. 3.The plot of Ce/Y and Zr/Nb (Fig. 3upper panel)shows a broad negative array. As with the majorelements, there is considerable overlap betweenthe three groups of samples, although there is a
tendency for the MB samples to plot at low Ce/Yand higher Zr/Nb ratios than the TC samples, thelatter being characterised by Ce/Y ratios s 3.Similarly the RCM samples fall into two groups,one with Ce/Y6 3 and a second with Ce/Ys 3,although both groups have comparable ranges ofZr/Nb ratio. Superimposed on Fig. 3, upper panelis a grid showing the compositions of melts de-rived from a fertile lherzolite source region with2U chondritic abundances of Ce, Y, Zr and Nb.The data are consistent with 0.3^3% melting ofsuch a source region with variable modal garnet
Fig. 2. Total alkali^silica diagram for analyses of ma¢c lavasfrom the Kenya Rift. Samples from the Tanzanian craton(TC) are denoted with black circles, from the reactivated cra-ton margin (RCM) by grey circles and those from the mobilebelt (MB) by open circles.
Fig. 3. Variation of Ce/Y ratio with (upper panel) Zr/Nband (lower panel) Ce abundance in ma¢c lavas from theKenya Rift. Key as in Fig. 2. Curved, ticked lines denotecalculated fractional melts from a mantle source region with2U chondritic abundances of the elements shown. D-valuesfrom [46]. Tick marks denote melt fraction and italicisednumbers at the end of each curve denote the modal abun-dance of garnet in the source. Note that virtually all of theanalyses can be generated by between 0.2 and 5% melting ofsuch a source.
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in the source. The RCM lavas with low Ce/Yratios and most of the MB lavas require 0^3%garnet in their source regions, whereas those sam-ples with high Ce/Y from the RCM group and thecratonic samples require 3^8% garnet. Melt frac-tions also vary from 2^3% at low Ce/Y and highZr/Nb to 0.3% in the most enriched cratonic sam-ples, which represent smaller melt fractions de-rived from a deeper, more garnet-rich mantlesource.
The variation of the Ce/Y ratio with Ce con-centration is shown in Fig. 3, lower panel. Themodelled lines illustrate the e¡ects of 0^5% melt-ing of two lherzolite source regions with 2Uchondritic abundances of both Ce and Y, onewith 0% and the other with 6% modal garnet.Most of the data lie between the two lines,although a signi¢cant number in the RCM andTC groups have Ce abundances greater than thelimiting concentration for a zero melt fraction.The samples plotted in this ¢gure all haveMgOs 5 wt% and so the e¡ects of fractionationon the concentrations of the incompatible ele-ments are not dominant. Thus Ce concentrationsin many of the TC and RCM lavas, and even asmall number of MB lavas are higher than can beaccounted for in one melting episode of a lherzo-lite with 2U chondritic abundances of incompat-ible elements. This observation suggests that man-tle source regions beneath the Kenya Rift havebeen enriched in incompatible trace elementsand that melt fractions of up to 3% indicated inFig. 3 are minimum values. Such values arebroadly consistent with estimates of melt fractionspresent in the mantle beneath the rift axis fromseismic velocity changes [29].
4.2. Radiogenic isotopes
The Sr, Nd and Pb isotope results are presentedin Table 1 and illustrated in Figs. 4 and 5. Thesenew data have been combined with existing anal-yses from elsewhere in the Kenya sector of therift, in particular from northern Tanzania [17],and the following description and discussion focuson the contrasts within this larger data set.
In general, the isotope ratios lie within theranges de¢ned by ocean island basalts, with
143Nd/144Nd ranging from 0.51300 to 0.51255,87Sr/86Sr from 0.7030 to 0.7055 and 206Pb/204Pbfrom 6 18 to s 20. However within these rangesthere is clear evidence of the strong control ex-erted by the structure of the underlying litho-sphere and in particular the boundary betweenthe mobile belt and the reworked craton marginis most signi¢cant. Those basalts erupted throughthe craton or its reworked margins have 143Nd/144Nd ratios 6 0.51275 and 87Sr/86Srs 0.7035,while those erupted through the mobile belthave 143Nd/144Nd ratios s 0.51275 and 87Sr/86Sr6 0.7035, de¢ning a steep negative array.Within the craton and craton margin group, thedata de¢ne two sub-parallel, £at-lying trends inwhich 87Sr/86Sr increases with little variation in143Nd/144Nd.
In conventional Pb isotope diagrams (Fig. 5)the distinction between basalts erupted throughthe craton and mobile belt is less clear. The206Pb/204Pb ratio of the basalts from the cratoniczone ranges from 17.5 to 21.5 compared with18.5^20.0 for the basalts erupted through the mo-bile belt. Within the craton group, 206Pb/204Pbratios are highly variable and when plottedagainst 207Pb/204Pb, de¢ne a broadly linear array,
Fig. 4. Conventional Nd^Sr isotope diagram of basalts fromthe Kenya Rift valley. Basalts erupted where the rift cutsthrough the craton (open diamonds) or reworked cratonmargin (open squares) are distinct from those erupted wherethe rift cuts across the Panafrican (Mozambique) mobile belt(¢lled circles). Data from the cratonic zone, where the KenyaRift intersects the Tanzanian craton from [16]; other datafrom [14,15].
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which is steeper than and crosses the NHRL(northern hemisphere reference line [18]). By con-trast, the majority of the basalts from the mobilebelt zone plot close to the NHRL at a 206Pb/204Pbratio of V19.5, while a small number trend tolow 206Pb/204Pb along the NHRL and only fouranalyses are displaced signi¢cantly above it.
While the basalts erupted through the cratonmargin have a similar range of Nd and Sr isotoperatios to the basalts from the craton zone, theirPb isotope ratios are more scattered. They plot onor above the NHRL, particularly on the 207Pb/204Pb vs. 206Pb/204Pb diagram, and locally, an in-crease in 207Pb/204Pb is associated with a decreasein 206Pb/204Pb ratios and an increase in 87Sr/86Sr.These variations are related to crustal contamina-tion [19].
5. Discussion
5.1. Crustal contamination
Whereas crustal contamination has been recog-nised as a controlling in£uence on the radiogenicisotope composition of basalts from the Naivashaarea [19], it is unlikely that the whole range in Srand Nd isotopes in this study can be related tocrustal interaction. In Fig. 6, all of the available87Sr/86Sr data are plotted against the inverse oftheir Sr concentrations (expressed as 1000/Sr)
Fig. 6. Plot of 87Sr/86Sr versus (upper panel) the inverse ofSr concentration and (lower panel) MgO content for theKenyan basalts, (key as in Fig. 2). Note the tendency overallfor samples with high Sr contents to have the highest andmost variable 87Sr/86Sr ratios, whereas low Sr contents areassociated with low and much less variable 87Sr/86Sr ratios.Such variations are unlikely to be controlled by additions ofcontinental crust, which tends to have Sr contents 6 300ppm. Similarly there is as greater range in 87Sr/86Sr ratios athigh MgO contents as there is in lavas with low MgO.
Fig. 5. Plot of Pb isotope ratios for the Kenyan basaltsshowing general relationship to the NHRL for both 207Pb/204Pb and 208Pb/204Pb. The analyses of basalts from the cra-tonic zone of the rift are plotted separately together with thebest ¢t regression line which is equivalent to an age ofV2200 Ma.
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and their MgO contents, used as an index of frac-tionation. The Naivasha basalts are highlightedon Fig. 6 and de¢ne a steep trend to high 87Sr/86Sr with only a slight increase in Sr content, con-sistent with assimilation and fractional crystallisa-tion (AFC) in a crustal-level magma chamber [19].However, similar trends are not seen in the re-maining samples and there is a marked tendencyamongst those samples from the craton zone tohave both high 87Sr/86Sr and high Sr concentra-tions. The continental crust and crustal melts arecharacterised by low Sr concentrations and mostof the silicic magmas erupted in the Kenya Rifthave Sr concentrations below 300 ppm [15,19,20].Hence the tendency for high 87Sr/86Sr to be asso-ciated with high Sr concentrations cannot easilybe reconciled with mixing with crustal melts. Sim-ilarly, although the basalts erupted through themobile belt plot at lower 87Sr/86Sr, they containa similar range of Sr concentrations and reveal alimited correlation. As with the basalts from thecratonic zone the highest 87Sr/86Sr ratios tend tobe associated with the highest Sr contents, imply-ing that elevated 87Sr/86Sr is associated with acomponent with high Sr and low Rb/Sr, consis-tent with a mantle rather than a crustal origin.
In Fig. 6, lower panel, there is no sense of co-variation between 87Sr/86Sr ratios and MgO, therange of 87Sr/86Sr ratios in lavas with 10% MgObeing similar to that in lavas with 6% MgO. Thereis a tendency within the Naivasha group for 87Sr/86Sr ratios to increase as MgO decreases, sugges-tive of some crustal interaction concomitant withfractionation of these samples and this is consis-tent with variations in Pb isotopes. However, suchvariations are the exception and studies of ma¢cvolcanics from both the rift £anks in northernKenya [21] and the rift axis in northern Tanzania[17] conclude that contamination is not a control-ling in£uence on isotope geochemistry, whichlargely re£ects processes operating at mantledepths.
Thus signi¢cant crustal contamination appearsto be geographically restricted to the basaltserupted through the reworked craton margin.This is the region of the rift in which large vol-umes of evolved magmas are found and it may bethat contamination is only signi¢cant here be-
cause this region is characterised by high crustalheat £ow and crustal melting. To the north, in themobile belt zone, Mesozoic and Tertiary exten-sion reduced crustal thickness [22,23] and volumesof evolved magmas are lower. The cratonic crust,by contrast, is of normal thickness [23] but thevolatile-rich and highly alkaline ma¢c magmascharacteristic of this part of the rift tend to pass
Fig. 7. a: Comparison of the range of Nd and Sr isotope ra-tios of the Kenya Rift basalts with analyses of basalts fromAfar [8] and the Red Sea and Gulf of Aden [28,29]. Notethe restricted overlap in isotope ratios and the generally low-er 143Nd/144Nd ratios in the Kenyan basalts compared withthose from Afar at a given value of 87Sr/86Sr. b: Comparisonof Nd and Sr isotope ratios from basalts from southernEthiopia with data ¢elds from both Kenya and Afar. Notethe close similarity between basalts from southern Ethiopiawith those from the Kenya Rift. Southern Ethiopian datafrom [31,41], HT2 data from [43] refer to analyses of theplume-related HT2 £ood basalts with R/Ra ratios up to 19.6[34].
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through the crust very rapidly and have little timefor interaction. Moreover, they are typically moreenriched in incompatible elements and so theirisotopic compositions are less susceptible to thee¡ects of crustal interaction. It is only in the re-worked craton margin that a particular combina-tion of crustal thickness and magma compositionlocally conspires to generate the observed contam-ination^fractionation trends.
5.2. Mantle source regions
The trace element variations described aboveindicate that Kenyan basalts are derived from agarnet-bearing mantle source region and that meltfractions are smaller and from greater depthwhere the lithosphere is older and thicker. Super-¢cially, this observation is consistent with meltingin a mantle plume, in which average melt depth iscontrolled by the thickness of the lithospheric lid[1]. However the radiogenic isotope variations re-veal that the origin of Kenya Rift basalts is morecomplex than simply the result of melting of ahomogeneous mantle source. Despite the lack ofcrustal interaction, the lithosphere exerts as stronga control on the isotope composition of the ba-salts as it does on the tectonic fabric of the rift[12]. Thus either the magma source region is lo-cated within the lithosphere or magmas from sub-lithospheric depths interact with the mantle litho-sphere en route to the surface. It is likely that agreater proportion of Kenyan basalts were de-rived from the mantle lithosphere than suggestedfrom trace element studies alone [10]. For exam-ple, Paslick et al. [17] noted that while the north-ern Tanzanian volcanics (the TC samples of thisstudy) have trace element characteristics broadlysimilar to those of OIB, isotopically there are sig-ni¢cant di¡erences between them and OIB. Theyconcluded that the northern Tanzanian magmaswere derived from lithospheric source regions thathad been enriched by small melt fractions subse-quent to stabilisation. Similar conclusions weredrawn concerning the origins of carbonatitesfrom Uganda [24], and ma¢c potassic volcanicsfrom the western rift [25,26].
A possible argument against a lithospheric ori-gin for the TC samples concerns the major ele-
ment composition of Archaean mantle litho-sphere, which many studies have shown isdepleted in `basaltic' elements, particularly CaO,Al2O3 and FeO [3,27,29]. These characteristicsshould be re£ected in magmas derived fromsuch ancient source regions. While xenolithsfrom Tanzanian volcanics do share these depletedcharacteristics, those with the deepest origins,which are also the only garnet-bearing types, arerelatively rich in FeO, CaO and Al2O3. Since thesource region of the TC lavas is garnet-bearing, itis most likely that these deep and relatively fertilesamples are more likely to be representative of thecratonic mantle source region than the shallowermore depleted peridotites. Furthermore, we notethat the deviation of 143Nd/144Nd ratios of the TCsamples from depleted mantle implies maximumsource ages in the region of 0.5^1 Ga and thatsimilar ages have been calculated for potassiclavas from the western rift [25]. In the lattercase it is now recognised that the isotopic signa-ture of the mantle source re£ects the approximateage of the last enrichment or metasomatic eventto have a¡ected the mantle lithosphere [26]. It isargued that this is also the case in the TC sourceregion which is located in the deeper parts of themantle lithosphere that were thermally accretedlater than the Archaean.
The distinctive isotopic characteristics of theTC lavas are less apparent in the samples fromthe MB sector of the Kenya Rift although thevariations in 143Nd/144Nd ratios still imply a de-gree of source heterogeneity. However, there areno correlations between isotope ratios and traceelement indices of melt fraction or depths. Thesenon-systematic variations contrast with resultsfrom the rift £ank [21] in which magmas withdeep trace element signatures (e.g. high Ce/Y)have distinctive isotopic characteristics. Geophys-ical studies of the Kenya Rift [29] have shownthat mantle with asthenospheric properties as-cends almost to the Moho beneath the rift axiswhereas the mantle lithosphere extends to a depthof 90 km beneath the rift £anks. This contrast inboth the thermal structure of the mantle beneaththe axis and £anks of the rift and in the degree ofisotopic heterogeneity of the ma¢c volcanics mayre£ect the more extensive interaction between
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plume and lithosphere components in the mantlebeneath the rift axis. The lack of correlation be-tween trace element and isotope ratios is inferredto re£ect the generation and mixing of plume- andlithosphere-derived melts over a range of depths.
Thus the balance between lithospheric anddeeper source regions is probably variable alongthe length of the rift. Lithospheric source regionsdominate volcanic rocks from the cratonic zone ofthe rift, whereas those from the MB zone prob-ably include contributions from both the litho-sphere and the underlying mantle plume. Assum-ing that the same convecting mantle sourcecontributes to basalts from both the cratonicand mobile belt regions, its composition will becommon to both groups of analyses on Fig. 4.The common end member of these two groupshas a 143Nd/144Nd ratio of V0.51275 and an87Sr/86Sr ratio of V0.7035. Therefore if the sub-lithospheric mantle beneath the Kenya Rift hasbeen sampled then this is its probable composi-tion. It lies within the range of radiogenic isotoperatios in the global OIB database and it is sug-gested that it represents the composition of theKenya mantle plume.
The overlap and large range of Pb isotope anal-yses from the volcanic rocks from the di¡erentzones of the rift do not allow the simple de¢nitionfor the possible Pb isotopic composition for thiscommon component in Kenya. Most of the MBzone samples have 206Pb/204Pb analyses between19 and 19.5 but the limited data from the RCMzone appear to be anchored to the NHRL at val-ues as high as 20.5. Further detailed studies of thedi¡erent zones of the rift are required to resolvethis problem.
5.3. Comparison with the Afar mantle plume
The Sr and Nd isotope analyses of the Kenyanbasalts di¡er markedly from those from Afar, i.e.those most likely to have been derived from theAfar mantle plume (Fig. 7a). Signi¢cantly, theAfar data do not lie on the same trends as theKenyan analyses, having generally higher 143Nd/144Nd ratios for a given value of 87Sr/86Sr. Themajority plot close to a 143Nd/144Nd ratio ofV0.5129 and an 87Sr/86Sr ratio of V0.7035 and
this is now widely accepted as the isotopic com-position of the Afar mantle plume, which also hasa 206Pb/204Pb ratio of V19.0. The remainder ofthe analyses radiate out towards MORB or moreenriched isotope ratios, commonly associated withthe depleted upper mantle and continental litho-sphere respectively [30^33].
The focal point for the Kenyan basalts, as dis-cussed above, is at a signi¢cantly lower 143Nd/144Nd ratio than that of the Afar plume, although87Sr/86Sr ratios are similar. It is therefore inferredthat the Kenya and Afar mantle plumes are di¡er-ent entities with contrasting 143Nd/144Nd ratios.This inference is supported by the available 3He/4He analyses of basalts from both the Ethiopianand Kenya Rifts. The Afar plume is amongstthose characterised by elevated 3He/4He signa-tures (up to 19.6 R/Ra) [34,35] and basalts fromthis region with high R/Ra also have 143Nd/144Ndratios of 0.5129, 87Sr/86Sr ratios of 0.703^0.704and 206Pb/204Pb ratios of around 18.8^19.0 [32].High 3He/4He are also present in the ground-waters of the Ethiopian rift and extend as farsouth as V7³N [35], the approximate southernlimit of the Ethiopian plateau. Although thereare no He analyses of basalts from the KenyaRift, the maximum R/Ra in groundwaters is 8and this appears to be the best indication of theHe isotope ratio of the Kenya mantle [36]. Thus itwould appear that Afar plume material is notpresent beneath the Kenya Rift and probablydoes not extend further south than 7³N. Theeast African plateau is therefore inferred to besupported by a mantle plume that is physicallyand compositionally distinct from the Afar plume,implying that two distinct sub-lithospheric uppermantle source regions contribute to the basalts ofthe EARS within a length scale of 2000 km.
The composition of the Afar mantle plume isvery close to estimates of the common componentidenti¢ed from oceanic basalts, thought to be de-rived from the lower and/or undegassed mantle(e.g. C [37] FOZO [38] and PHEM [39]). Thiscontrasts with the absence of such material fromthe plume beneath the Kenya Rift. Although thesource of the Kenyan basalts may be dominatedby the mantle lithosphere, there is also no geo-chemical evidence for a contribution from a
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deep mantle component. This compositional dif-ference can be tentatively related to tomographicstudies of the mantle beneath south and east Afri-ca, which reveal a broad upwelling in the lowermantle, supporting the dynamic topography ofsouthern Africa [11]. Uplift of the east Africanplateau is related to a smaller anomaly, princi-pally located within the upper mantle. The lackof a C/FOZO component in the Kenyan basalts isconsistent with this interpretation and suggeststhere is little transfer of material between theselower and upper mantle upwellings. Whether theC/FOZO-dominated Afar plume is linked to ananomaly in the deep mantle awaits high-resolu-tion tomographic images of this region.
5.4. Age of the plumes
The longevity of the Kenya and Afar mantleplumes can be investigated by tracing the geo-chemical characteristics of basalt source regionsback through time. The oldest Tertiary basalticmagmatism in the EARS occurs in southernEthiopia and previous studies have suggestedthat this may be the ¢rst manifestation of mag-matism associated with the Afar plume [6,40]. TheSr and Nd analyses of basalts from southernEthiopia are plotted in Fig. 7b, and comparedwith ¢elds de¢ned by similar data from northernEthiopia, Afar and the Red Sea/Gulf of Aden.Quite clearly the Eocene and Oligocene basaltsfrom southern Ethiopia are distinct from thosefrom Afar, having 143Nd/144Nd ratios lying be-tween 0.51285 and 0.51255, at lower values thanthose typical of the Afar plume. However, thesevalues are similar to those seen in the cratonic andcraton margin zones of the Kenya Rift, and arealso similar to the proposed isotope compositionof the Kenya mantle plume.
Previous conclusions [40] that the southernEthiopian basalts were the ¢rst manifestations ofthe Afar plume were largely a consequence of thelocation of southern Ethiopia within the present-day in£uence of the Afar plume. This is now con-sidered an unlikely interpretation, in part becausethe African plate has migrated steadily northeast-wards over the past 50 Ma, relative to the hotspotreference frame. Studies of the ages of seamounts
along the tracks of the St Helena and Tristanplumes reveal motion of 20 mm Yr31 since 19^30 Ma and 30 mm Yr31 prior to that date [41].Moving the African plate back along this track tothe time of the eruption of the earliest southernEthiopian basalts (35^45 Ma) places southernEthiopia at the same latitude as present-dayLake Victoria; in other words well within the in-£uence of the Kenya plume [9]. The similaritybetween the Nd and Sr isotope analyses of theseplume-related tholeiites [42] with the analyses ofthe Kenyan basalts (Fig. 7) strongly support thiscontention.
By contrast, the earliest basaltic magmas with aradiogenic isotope composition similar to thepresent-day Afar plume are to be found in theEthiopian £ood basalts. These are dated at 29^30 Ma and were related to the initial rifting ofthe Red Sea and the break-up of the Afro-Ara-bian plate [43]. Thus it is inferred that the ¢rstmagmatic activity associated with the Afar plumewas the eruption of the northern Ethiopian £oodbasalts at 30 Ma [8], since which time the plumehas spread out laterally beneath the lithosphere,but probably only to the extent of the present-dayEthiopian plateau. The Kenya plume is an olderfeature and is responsible for magmatism extend-ing from southern Ethiopia, southwards throughKenya to northern Tanzania. However the tem-poral migration of magmatism from north tosouth is probably more easily reconciled withthe slow northward drift of the African plateover the Kenya plume and not the lateral migra-tion of Afar mantle plume material [9]. Within theEthiopian plateau and Afar there is no sense ofmigration of the onset of magmatism as there is inthe Kenya Rift. This is in part because during thepast 30 My, the African plate has been moving ata slower rate than prior to 19^30 Ma [41]. Inaddition the location of magmatism in the Afar/Red Sea/Gulf of Aden system is even more con-trolled by extension than it is in the Kenya Rift.
Finally, the isotopic similarity of the basaltsfrom southern Ethiopia, which is underlain al-most exclusively by Panafrican mobile belt, withthose from the craton and craton margin zones ofthe Kenya Rift, implies that the relationship be-tween basalt isotope geochemistry and the age of
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the underlying lithosphere is not as simple as thatsuggested from the Kenya Rift alone. Rather theSr and Nd isotopes may re£ect the age of themost recent tectonothermal process to have af-fected the mantle lithosphere. Hence, those ba-salts from the northern mobile belt zone of theKenya Rift were also erupted through that partof the Kenyan basement a¡ected by the exten-sional episode that produced the Mesozoic Anzagraben [44]. Mantle lithosphere attenuated duringthis event would be gradually replaced by the ac-cretion of cooled asthenosphere and so the deeperparts of the lithosphere beneath this part of themobile belt may only be 100 Ma old. Hence theywould be isotopically similar to an MORB sourceonly recently isolated from the convecting mantle.Areas away from the Anza graben, by contrastwould re£ect Panafrican ages and this appearsto be the case certainly for Nd and Sr isotopesin the southern Ethiopian basalts [45].
6. Conclusions
The radiogenic isotopes of Neogene to Recentbasalts and other ma¢c magmas from the axis ofthe Kenya Rift re£ect marked changes in the ageof the subjacent continental lithosphere. This sug-gests that a signi¢cant and probably dominantproportion of the ma¢c magmas from the KenyaRift are derived from the mantle lithospherewhich has OIB-like trace element characteristicsbut variable radiogenic isotope ratios that re£ectlithosphere age. The systematics of Sr and Nd inthe whole suite suggest that the isotope composi-tion of the Kenya mantle plume is distinct fromthe Afar plume. In particular it has a lower 143Nd/144Nd ratio (0.51275) and 3He/4He of 9 R/Ra.Hence there must be at least two distinct mantleplumes beneath the EARS, providing separate dy-namic support for the Ethiopian and east Africanplateaus respectively. The isotopic characteristicsof the Afar plume suggest a deep origin in anundegassed part of the mantle whereas the Kenyaplume may have a shallow origin, as inferred fromseismic tomography. However, the Kenya plumeis the longest-lived feature as Eocene basalts insouthern Ethiopia have radiogenic isotope charac-
teristics very similar to those of present-day Ken-yan basalts. The ¢rst manifestations of the Afarmantle plume are the Oligocene £ood basalts ofnorthern Ethiopia. Channelling of the Afar plumebeneath the African plate is therefore limitedlargely to the area of the present Ethiopian pla-teau.
Acknowledgements
We would like to thank Raphael Pik, GarethDavies and Roberta Rudnick for their construc-tive reviews. We are also grateful to Simon Turn-er, Ian Parkinson, Chris Hawkesworth and DavidGraham for comments on an earlier version ofthis paper. Radiogenic isotope research at theOpen University is partly supported by theNERC. Paper published with the permission ofthe Director, British Geological Survey (NERC).NERC Isotope Geoscience Lab contribution no.386.[AH]
Appendix. Locations of analysed samples
(BSN, basanite; AOB, alkali olivine basalt ; HNB,hypersthene-normative basalt)Samples from the remobilised craton margin(RCM)THEL2: AOB, Theloi basalt formation, EldamaRavine43/37: AOB, Turasha basalt, GilgilW203: AOB, lower Simbara basalt, AberdareRangeKLR102: AOB, Group 2, Olokisalie volcano209-1: AOB, Lisudwa volcanics, MagadiKLR36: AOB, Ol Tepesi volcanics, southern rift,KenyaKLR67: HNB, Ol Tepesi basalts, southern rift,KenyaSamples from the mobile belt (MB)KB183: BSN, northern £ank, Namarunu centreKB241: BSN, Recent £ow, Teliki's volcano, Bar-rier complexKB162: BSN, Recent £ow, Andrews, Barriercomplex
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KB156: BSN, Logipi basalt, Barrier complexKB197: BSN, older £ow, southern £anks, BarriercomplexKB214: BSN, Latarr basalt, Barrier complexKB24: AOB, northern £anks, Paka volcanoKB42: AOB, northeastern £anks, Paka volcanoKB63: AOB, Lokoyamana basalt, Silali volcanoKB65: AOB, young £ow, northern periphery, Si-lali volcanoKB66: AOB, Flank Fissure basalts, northern£anks, Silali volcanoKB71: AOB, Katenmening basalt, Silali volcanoKB106: AOB, Flank Fissure basalts, northern£anks, Silali volcanoKB8: AOB, south £ank, Korosi volcano5/35: AOB, Tirioko basaltKB260: AOB, Pliocene basalt, Tirr Tirr plateauKB267: AOB, Pliocene basalt, east rift marginnear the BarrierKB265: AOB, Lorikipi basalt, northwest Namar-unuKB167: AOB, young £ows, axis of inner trough,NamarunuKB203: AOB, young £ows, axis of inner trough,NamarunuKB226: HNB, Lorikipi basalt, southwest Namar-unuKB259: HNB, Plio-Pleistocene basalt, western riftmarginKB45: HNB, western £anks, Korosi volcano
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