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The Role of Continental Crust and Lithospheric

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The Role of Continental Crust and. Lithospheric Mantle .... provides an excellent tool for discrimination between assimilation of ...... therefore used handpicked rock chips as raw material for ...... what elevated 187Os/188Os of 0Á1384. However ...
JOURNAL OF PETROLOGY

VOLUME 46

NUMBER 1

PAGES 169–190

2005

doi:10.1093/petrology/egh067

K. RANKENBURG1,2*, J. C. LASSITER1 AND G. BREY2 ¨ R CHEMIE, ABT. GEOCHEMIE, POSTFACH 3060, 55020 MAINZ, GERMANY MAX-PLANCK-INSTITUT FU ¨ INSTITUT FUR MINERALOGIE, SENCKENBERGANLAGE 28, 60054 FRANKFURT/MAIN, GERMANY

1 2

RECEIVED AUGUST 26, 2002; ACCEPTED AUGUST 3, 2004 ADVANCE ACCESS PUBLICATION OCTOBER 1, 2004 We present a combined Sr, Nd, Pb and Os isotope study of lavas and associated genetically related megacrysts from the Biu and Jos Plateaux, northern Cameroon Volcanic Line (CVL). Comparison of lavas and megacrysts allows us to distinguish between two contamination paths of the primary magmas. The first is characterized by both increasing 206Pb/204Pb (1982–2033) and 87Sr/86Sr (070290–070310), and decreasing eNd (70–60), and involves addition of an enriched sub-continental lithospheric mantle-derived melt. The second contamination path is characterized by decreasing 206 Pb/204Pb (1982–1903), but also increasing 87Sr/86Sr (070290–070359), increasing 187Os/188Os ( 0130– 0245) and decreasing eNd (70–46), and involves addition of up to 8% bulk continental crust. Isotopic systematics of some lavas from the oceanic sector of the CVL also imply the involvement of a continental crustal component. Assuming that the line as a whole shares a common source, we propose that the continental signature seen in the oceanic sector of the CVL is caused by shallow contamination, either by continentderived sediments or by rafted crustal blocks that became trapped in the oceanic lithosphere during continental breakup in the Mesozoic.

The Cameroon Volcanic Line (CVL) comprises a genetically related series of Cenozoic intraplate volcanoes that

extend for 1600 km from the island of Annobon (formerly known as Pagalu) in the South Atlantic Ocean to the continental interior of West Africa (Fig. 1). The northern end of the continental part of the CVL is marked by the Cenozoic volcanism of the Biu Plateau, Nigeria. Fitton & Dunlop (1985) showed that basaltic rocks in the oceanic and continental sectors of the CVL are geochemically and isotopically (87Sr/86Sr) similar and suggested that a line or zone of hot asthenospheric mantle is upwelling underneath the region, partial melting of which has generated parental magmas without any substantial involvement of the overlying lithosphere. This simple picture was challenged by combined Nd, Sr, Pb and O isotope studies of Halliday et al. (1988, 1990), in which those workers found a distinctive 206 Pb/204Pb anomaly in CVL lavas focused at the continent–ocean boundary (c.o.b.), which diminishes over a distance of 400 km to either side. Halliday et al. considered this HIMU Pb isotope signature (high m  high 238U/204Pb, leading to time-integrated high 206 Pb/204Pb) to be inherited from relatively recent U/ Pb fractionation at 125 Ma during impregnation of the uppermost mantle by the St. Helena hotspot when the Equatorial Atlantic opened. The observed Pb isotope heterogeneity of the CVL lavas was therefore proposed to be derived from remelting of variably metasomatized lithosphere rather than reflecting primary asthenospheric source heterogeneity. From a study of peridotite xenoliths

*Corresponding author. Telephone: þ1 281 244 1084. Fax: þ1 281 483 1573. E-mail: kai.rankenburg1.jsc.nasa.gov

Journal of Petrology vol. 46 issue 1 # Oxford University Press 2004; all rights reserved

KEY WORDS:

crustal contamination; CVL; megacrysts; ocean floor;

osmium isotopes

INTRODUCTION

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The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

JOURNAL OF PETROLOGY

VOLUME 46

NUMBER 1

JANUARY 2005

Lee et al. (1996) provided evidence that portions of the lithospheric mantle beneath the CVL are isotopically enriched. There is also qualitative evidence for interaction with the continental crust in some evolved lavas of the continental sector based upon large variations in Hf isotopes (Ballentine et al., 1997), 87Sr/86Sr as high as 0705–0714 (Marzoli et al., 1999) and the Sr–Nd isotope systematics of lavas and genetically related megacrysts (Rankenburg et al., 2004). In this study, we examine the respective contributions of crustal contamination and assimilation of subcontinental lithospheric mantle (SCLM) by comparing the isotopic (Sr, Nd, Pb and Os) and trace element variations of Biu and Jos Plateau lavas with the compositions

of genetically related megacrysts that grew at mantle depth. We have analysed Sr, Nd and Pb isotopes in 36 whole rocks and 13 megacrysts collected from the Biu and Jos Plateaux, as well as osmium isotopes of a subset of 17 rock samples. The Re–Os isotope system provides an excellent tool for discrimination between assimilation of continental crust or the SCLM. Unlike Sr, Nd and Pb isotope compositions, which may overlap in both continental crust and the SCLM, there is generally a strong contrast in osmium isotopes between the continental crust and the peridotitic SCLM as a result of the compatible behaviour of Os during mantle melting. Whereas continental crust generally has developed variable but high 187Os/188Os ratios over time

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Fig. 1. Geological map showing the eruption ages of the major volcanic centres of the Cameroon Volcanic Line and the Gulf of Guinea [adapted from Fitton & Dunlop (1985)]. Ages compiled from Fitton & Dunlop (1985), Halliday et al. (1990), Lee et al. (1994) and Ngounouno et al. (1997). The Jos volcanics are located 400 km to the NW of the line axis and are usually not included in CVL magmatism. However, no occurrence of continental Cenozoic volcanism has been recorded west of the Jos Plateau. Sample locations are indicated by grey triangle (Biu Plateau) and black square ( Jos Plateau).

RANKENBURG et al.

CAMEROON VOLCANIC LINE LAVAS

Geological setting: the Benue Trough and Biu and Jos Plateaux The continental sector of the CVL has a Y-shaped form (see Fig. 1). Whereas most previous studies considered the Biu Plateau as the end of the NNW branch of the continental sector of the CVL (e.g. Turner, 1978; Fitton, 1980; Halliday et al., 1988; Poudjom-Djomani et al., 1995; Lee et al., 1996; Ballentine et al., 1997; Barfod et al., 1999; Marzoli et al., 2000), the Jos Plateau (located c. 400 km to the NW of the central CVL axis, see Fig. 1) is usually not assigned to CVL volcanism. However, the timing of the Jos Plateau volcanism is very similar to that of the other CVL volcanic centres (Grant et al., 1972). The Biu and Jos Plateau lavas have similar major and trace element chemistry, and Jos Plateau lavas also span a similar range in isotopic compositions, overlapping the data of the CVL as a whole (Rankenburg et al., 2004). We therefore consider the Jos Plateau to be associated with CVL volcanism in the following discussion. According to Turner (1978), the Biu Plateau was constructed in three stages during two periods of volcanism: (1) an early fissure type eruption; (2) formation of relatively large tephra ring volcanoes and building up of localized thick lava piles (up to 250 m) in the southern part of the plateau. Lavas of this plateau-building stage range in composition from hy-normative basalt to basanite, with K/Ar ages from 535 to 084 Ma (Grant et al., 1972; Fitton & Dunlop, 1985). Extensive weathering and laterite formation suggest a hiatus after this episode. (3) Resumption of igneous activity with the formation of over 80 NNW–SSE-aligned cinder cones with similar chemistry to the earlier basalts. A rough estimate of the age of

the last magmatic period is 25 ka based on pollen dating of maar sediments from the Biu Plateau (Salzmann, 2000). As with the Biu Plateau, volcanic activity on the Jos Plateau occurred in two periods and thus the basalts from this region have been divided into an earlier and a more recent group (McLeod et al., 1971). There are no isotopic age determinations available for the older basalts, but Wright (1976) suggested a Paleocene age, roughly synchronous with Benue Trough folding and uplift. The more recent activity formed a group of 22 cinder cones. Radiometric K–Ar ages (Grant et al., 1972) suggest, unlike on the Biu Plateau, continuous volcanism between 21 and 09 Ma. The younger volcanics of both the Biu and Jos Plateaux are characterized by abundant inclusions of mantle xenoliths and megacrysts. The megacryst suites of the Biu and Jos Plateaux were described in detail by Wright (1970) and Frisch & Wright (1971), and comprise chemically homogeneous crystals of clinopyroxene (cpx), garnet (gnt), plagioclase (plag) and ilmenite (ilm) with diameters of up to several centimetres, whereas crystals of olivine (ol), amphibole (amph), spinel (sp), apatite (apa), zircon (zr) and blue corundum (cor) are extremely rare.

SAMPLING AND ANALYTICAL TECHNIQUES Major and trace element data were obtained for 27 volcanic rocks from the younger Biu Plateau suite, four rocks from the older, plateau-building suite of the Biu Plateau, and five rocks from the younger Jos Plateau suites (Table 1). The lavas were first coarsely crushed in steel mortars. Selected chips free of obvious xenocrysts or alteration were then powdered in an agate ring-disc mill. The powders were analysed by X-ray fluorescence spectroscopy (XRF) with a Philips PW 1404 instrument at the University of Frankfurt using Li-borate glass discs for major elements and at the University of Mainz using pressed powder pellets for trace elements. Rock powders were commercially analysed at the University of Goettingen, Germany (all samples) and at the Memorial University of Newfoundland, Canada (subset of 17 samples) by inductively coupled plasma mass spectrometry (ICP-MS) following HF– HNO3 acid dissolution [analytical details have been given by Jenner et al. (1990)]. A subset of 20 samples was additionally analysed for rare earth element (REE) concentrations by inductively coupled plasma atomic emission spectrometry (ICP-AES) following sinter dissolution at the GeoForschungsZentrum in Potsdam (Zuleger & Erzinger, 1988). Comparison of all the datasets revealed problems of the Goettingen ICP-MS laboratory with respect to accurate determination of high field strength element (HFSE)

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(e.g. Esser & Turekian, 1993; Esperanca et al., 1997), the SCLM generally has complementary unradiogenic 187 Os/188Os ratios (e.g. Walker et al., 1989). Thus, if a melt is contaminated by old crust-derived material, it should have an unusually radiogenic Os isotope signature. In contrast, contaminants derived from the peridotitic SCLM should have unradiogenic Os isotope compositions. Pyroxenite xenoliths derived from the SCLM may also have a radiogenic Os isotope signature (e.g. Reisberg et al., 1991; Roy-Barman et al., 1996; Lassiter et al., 2000; Pearson & Nowell, 2004). Thus melting of pyroxenite layers or veins in the SCLM has been invoked to explain the ubiquity of elevated Os isotope ratios in ocean island basalt (OIB) (e.g. Hauri & Hart, 1993; Schiano et al., 1997; Lassiter et al., 2000; Hauri, 2002; Kogiso et al., 2004). However, contamination with pyroxenite-derived melts may be distinguished from crustal material based upon other geochemical tracers, such as, for example, Pb isotope and trace element signatures.

JOURNAL OF PETROLOGY

VOLUME 46

NUMBER 1

JANUARY 2005

Table 1: Major (wt %) and trace (ppm) element analyses of Biu and Jos Plateau lavas Sample:

ZAGU

JIGU 1

JIGU-M

X

PELA JUNG

KOROKO

PELA ALT

DAM

DAM2

Group:

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

SiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Total Rb Ba Th U

46.42 2.61

49.17 2.17

44.34 3.08

47.82 2.33

49.44 2.19

59.88 0.35

51.05 2.26

44.33 2.99

47.80 2.60

14.39 10.49

14.74 9.71

12.66 11.28

14.43 10.40

14.74 9.71

20.50 3.10

15.25 9.27

12.50 11.48

14.12 10.20

0.18 9.15

0.15 8.64

0.16 8.81

0.15 8.67

0.18 0.37

0.14 7.16

0.20

0.16 9.23

9.26 3.71 1.82

9.18 3.32

0.19 10.78 9.89

9.89 3.16

8.41 3.42

2.02 8.33

7.18 3.60

1.38

3.75 1.77

0.80 98.83

0.47 98.92

0.99 98.74

1.29 0.54 98.84

1.62 0.56 98.92

4.81 0.12 99.65

2.45 0.59 98.97

50.4 786 7.93 1.99

32.4 469 4.36 1.06

Nb

92

51

Ta

5.34 58.8

3.43 30.2

La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sc

110 3.08 11.7 973 48.7 272

59.4 2.21 6.74 610 28.1 187

6.93 9.38

5.27 6.00

3.18 7.72

2.08 5.36

1.19 6.07

0.86 4.48

1.11 27.3

0.84 20.4

2.71 0.36

2.11 0.27

2.03 0.29

1.57 0.23

20.8

21.3

46.2 598 8.38 2.43 104 6.19 67.8 138 3.55 15.3 1009 62.4 352 8.66 12.4 3.90 9.67 1.41 6.96 1.22 29.5 3.10 0.37 2.20 0.31 20.4

36.1

39.0

432

500

5.30 1.28

6.06 1.54

62 n.m. 37.5 72.9 2.34

668

339

40.6 77.5

120

703

33.2

35.0

183

230 5.78 7.02 2.37

n.m. 7.01 2.32 5.95

5.94 0.97

0.93 4.88

4.87 0.87

0.87 23.3

21.8 2.17

2.26 0.29

0.26 1.61

1.74 0.25

0.21 19.4

24.2

36.75 10.04

67 3.96

2.87 8.57

8.10

196 1135

19.25

206 16.90 20 1012 64 808 17.84 11 3.1 8.5 1.2 7.0 1.3 34 3.7 0.48 3.5 0.51 0.75

66.8 830 7.57 1.98 93 n.m. 53.8 101 3.60 10.7 805 42.8 346 n.m. 8.05 2.67 6.44 0.94 4.36 0.76 20.2 1.86 0.23 1.22 0.18 16.0

11.50 10.08 3.28 1.56 0.80 98.72 40.7 736 7.06 1.89 85 n.m. 55.2 113 2.95 12.6 858 51.3 305 n.m. 10.6 3.32 8.30 1.25 6.25 1.11 27.1 2.93 0.36 2.07 0.31 22.0

8.71 3.16 2.15 0.72 98.86 55.8 747 7.86 1.94 89 5.46 52.7 104 3.88 11.2 872 45.4 313 7.28 9.19 2.93 7.28 1.11 5.39 0.95 24.2 2.36 0.30 1.69 0.23 18.6

V

155

164

201

183

166

9

184

192

Cr

231

299

379

287

279

10

214

401

246

Ni

202

191

278

176

210

3 34.3

202

291

222

Zr/Nb

28.9 2.96

19.2 3.67

30.8 3.38

21.6 2.95

25.2 3.43

Ce/Pb

35.7

26.9

38.8

31.2

27.0

La/Yb

K/U

7612

10792

6061

8400

8737

172

2.38 12.2 3974

183

44.1 3.72

26.6 3.59

31.1 3.52

28.2

38.2

26.8

10263

6838

9208

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TiO2

RANKENBURG et al.

CAMEROON VOLCANIC LINE LAVAS

BUGOR

SE BUGOR

HILIA 1

HILIA 2

TAMZA

GUFKA

MIR

GULD-UMBUR

PELA 2

Group:

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

46.97 2.43

46.35 2.45

46.41 2.59

49.26 2.19

46.51 3.06

51.17 1.93

53.24 1.92

45.98 2.59

45.54 2.88

13.95 10.40

14.16 10.42

14.56 10.48

14.69 9.96

14.17 10.62

15.34 9.03

16.19 7.51

12.91 10.65

13.23 10.98

0.17 9.16

0.15 8.59

0.17 9.28

0.13 7.35

0.12 5.91

9.23

9.27 3.17 1.16

8.86

8.90

3.36 2.00

3.49 1.14

SiO2 TiO2 Al2O3 FeO MnO

0.16

0.17

MgO

10.82 8.94

10.15 9.57

3.13 1.46

3.43 1.53

CaO Na2O K2O

3.72 1.77

0.18

0.18

6.80

11.87 9.35

10.18 10.05

3.94 2.93

2.93 1.61

3.35 1.62

P2O5

0.58

0.62

0.74

0.46

0.79

0.52

0.59

0.75

0.75

Total

98.84 37.9

98.84 38.3

98.83 50.2

98.89 28.3

98.82 43.5

98.99 24.3

99.16 73.2

98.81 45.5

98.78 45.1

Rb Ba Th U

441 5.20 1.39

438 5.65 1.51

628 6.49 1.67

Nb

65

73

83

Ta

n.m. 38.1

n.m. 39.5

n.m. 50.0

La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sc

72.9 2.13 8.34 668 33.9 211

76.6 2.16 8.67 714 34.5 206

96.5 2.64 10.2 860 42.1 259

n.m. 7.03

n.m. 7.01

n.m. 8.49

2.37 6.02 0.99

2.43 6.11

2.72 6.87

0.95 4.93

1.05 5.39

5.12 0.93

379

604

4.07 0.65 51

8.69 2.36 103

3.40 30.2 60.1 1.78

6.19 58.2 115 3.67 12.3

6.85 567

1013

28.2 183

49.7 338

5.20 6.17 2.20

10.0 3.20

8.29

5.33 0.86

7.82 1.20 5.75 1.01

440 4.24 1.01 52 2.87 30.6 58.7 1.86 6.67 667 27.4 173 4.10 5.81 2.03 4.95 0.78 3.93

864 10.29 2.67 120 7.48 62.9 120 3.91 12.5 1075 47.2 355

531 7.37 2.12 86 4.51 50.6 97.8 2.76 10.6 804 44.4 263

8.97

6.03

8.7 2.83

8.72 2.87

6.75 0.92

6.78 1.08 5.50 0.98

579 6.39 1.52 82 n.m. 51.0 99.2 2.84 10.9 824 46.5 279 n.m. 9.16 3.01 7.20 1.09 5.64

0.89

0.95

4.49 0.82

0.69

4.03 0.64

23.9 2.39

23.0 2.33

26.7 2.48

20.3 2.04

25.4 2.60

17.8 1.67

16.0 1.55

25.1 2.54

29.3 2.54

0.30 1.74

0.31 1.76

0.32 1.82

0.27 1.53

0.32 1.86

0.22 1.23

0.17 0.99

0.33 1.90

0.33 1.84

0.25 22.1

0.26 21.6

0.26 22.2

0.22

0.27

21.1

18.3

0.17 17.4

0.14 11.7

0.27 21.4

0.99

0.28 24.2

V

184

181

160

171

197

141

122

190

195

Cr

368

290

196

273

224

242

122

387

397

Ni

307

231

183

220

202

196

123

346

243

La/Yb Zr/Nb Ce/Pb K/U

21.9 3.25 34.3 8736

22.4 2.82 35.4 8416

27.5 3.12 36.6 8814

19.8 3.59

31.3 3.28

33.7 14900

31.3 7038

173

24.8 3.33 31.5 9324

63.8 2.96 30.8 9120

26.7 3.06 35.5 6295

27.7 3.40 34.9 8845

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Sample:

JOURNAL OF PETROLOGY

VOLUME 46

NUMBER 1

JANUARY 2005

Table 1: continued

WIGA

GWARAM

ZUMTA

HIZSHI

TUM

ETUM

GUMJA

TILA 1

TILA STR

Group:

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

Biu young

45.66 3.04

50.10 2.38

50.03 2.20

47.77 2.67

46.06 2.40

48.28 2.57

47.49 2.69

46.30 3.09

47.40 2.77

13.55 10.88

15.17 9.28

15.42 8.89

14.25 10.22

13.20 10.58

14.34 10.16

14.46 10.02

13.48 11.03

14.22 11.14

MnO

0.17

0.14 7.78

0.17 8.81

0.18 9.16

0.15 9.43

0.16

10.69 9.71

0.15 7.04

0.16

MgO

0.15 9.33

7.27 4.55 2.33

7.67 3.77 2.45

8.10 4.48 1.59

8.03 3.55 1.88

9.57 3.13 1.37

0.70 98.97 61.0

0.67

0.81 98.86 83.8

0.72 98.87 49.2

0.58

0.59

99.01 66.9

98.88 32.7

98.77 35.8

SiO2 TiO2 Al2O3 FeO

CaO Na2O K2O

2.67 1.59

P2O5

0.85

Total

98.79 33.3

Rb Ba Th U

519 7.36 1.96

858 10.5 2.73

Nb

88

115

Ta La

n.m. 54.9

n.m. 72.0

Ce

106

139

Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sc

2.94 11.5 895 46.7 264 n.m. 9.57

4.46 15.1 1044 59.7 429 n.m. 11.5

805 8.22 2.00 96 6.47 54.5 102 3.39 11.0 918 42.8 282 8.16 8.04

12.89 8.71 2.83 1.41 0.57 98.82 42.1

723

498

8.98 2.39

5.78 1.43

109

73 4.40

n.m. 67.9

40.2 78.1

133 3.75 14.4

2.22 8.55

1017

692

56.7

34.2

386

220

n.m. 11.3

518 7.83 2.08 94 n.m. 54.7 108 3.31 12.1

453 6.57 1.72 72 3.95 42.7 78.6 2.60 8.43

918 48.8

748

367

200

34.5

6.06 6.89

n.m. 9.55

5.15 6.77

3.02 7.49

3.67 8.79

2.63 6.33

3.49 8.68

2.29 5.94

3.09 7.90

2.31 5.80

1.15 5.61

1.30 6.10

0.95 4.33

1.26 6.09

0.89 4.64

1.24 6.70

0.92 4.88

0.94 24.4

1.01 26.2

0.70 18.1

1.03 27.4

0.82 20.9

1.22 28.2

0.86 21.7

2.44 0.31

2.55 0.29

1.78 0.20

2.67 0.30

2.09 0.27

2.94 0.39

2.18 0.29

1.67 0.24

1.64 0.22

1.21 0.16

1.73 0.25

1.61 0.24

2.38 0.33

1.70 0.24

20.0

14.3

15.7

17.3

23.7

17.2

20.4

10.34 9.83 2.55 1.41

384 4.54 1.11 61 n.m. 36.6 76.7 2.19 9.01 671 38.9 252 n.m.

9.21 2.42 1.60 0.53 98.76 40.7 383 4.14 1.10 58 n.m. 31.1 64.2 1.88 7.49 708 31.1 216 n.m.

8.48 2.71

7.04 2.38

6.93 1.09

5.88 0.95

5.51 0.96

4.91 0.85

25.7 2.41

21.3 2.13

0.30 1.76

0.25 1.50

0.25

0.20 16.7

23.8

V

205

145

153

162

195

149

197

210

185

Cr

300

187

217

265

452

333

229

286

283

Ni

269

174

202

233

435

241 23.0

229

230

246 20.7

Zr/Nb

32.8 3.00

43.9 3.73

45.0 2.94

39.3 3.54

24.9 3.01

Ce/Pb

36.2

31.3

30.1

35.3

35.1

La/Yb

K/U

6742

7065

10167

5523

8237

174

3.90 32.6 7476

25.1 2.78

20.8 4.13

30.2

35.1

6638

10499

3.72 34.1 12093

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Sample:

RANKENBURG et al.

CAMEROON VOLCANIC LINE LAVAS

Sample:

DAI

KERANG

AMPANG

PIDONG-M

PIDONG-S

Biu4

Biu5

Biu8

Biu9

melt incl.

Group:

Jos

Jos

Jos

Jos

Jos

Biu old

Biu old

Biu old

Biu old

Biu young (mean of 5)

TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total Rb Ba Th U

46.26 2.44

44.84 2.66

46.69 2.30

45.63 2.43

47.82 2.67

46.18 2.96

45.93 3.15

48.01 2.21

46.63 2.77

53.22 2.43

13.78 10.45

13.68 11.29

13.16 10.48

13.46 10.68

16.30 10.63

13.30 10.89

13.13 11.13

13.95 10.21

13.84 10.54

17.56 6.24

0.17 10.77 9.77

0.19 9.49

0.18

0.16 9.97

0.19 10.14 9.74

2.92 1.69

3.99 1.81

3.33 1.80

3.60 1.79

10.71 2.69

10.70 9.68

0.07 1.49

8.64 3.47 2.32

0.17 10.48 10.31

0.17

10.79 9.54

0.17 6.13

9.91

0.18 11.05 9.12

2.38 1.50

2.41 1.13

2.63 1.62

0.60 98.84

0.89 98.74

0.72 98.83

0.71 98.81

0.66 98.82

0.58 98.76 41.8

0.40

0.73

0.99

98.86 24.6

98.83 44.9

95.56

45.1 554 6.08 1.47

Nb

72

Ta

n.m. 42.5

La Ce Pb Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Ho Y Er Tm Yb Lu Sc

84.1 2.84 9.24 803 37.9 231 n.m. 7.74 2.57 6.44 0.99 4.98 0.89 23.3 2.27 0.28 1.64 0.24 22.8

54.5 734 8.31 2.01 102 5.43 67.5 128 4.25 13.6 1024 53.5 293 6.58 10.3 3.51 8.58 1.24 5.91 0.98 27.7 2.54 0.30 1.67 0.23 20.2

53.0 726 7.46 1.79 86 4.57 56.5 107 4.56 11.3 858 44.2 247

54.5 615 7.21 1.80

56.7 795 7.74 1.95

86

97

n.m. 52.4

n.m. 62.4

101

125

3.81 10.8 841 43.7 257

4.21 13.4 1128 52.2 331

5.55 8.5

n.m. 8.90

2.83 6.81

2.93 6.96

3.15 7.47

1.00 4.92

1.07 5.23

1.08 5.21

0.84 22.0

0.92 24.7

0.91 21.4

2.13 0.26

2.35 0.29

2.33 0.29

1.49 0.22

1.57 0.24

1.62 0.24

18.6

20.7

n.m. 9.6

13.7

1.32 0.61 98.79 29.0 498 4.21 1.12 63 4.16 34.4 70.6 1.87 8.37 712 35.1 240 7.29 7.59 2.53

390 4.66 1.16 63 n.m. 36.8 78.1 2.13 8.97 635 38.3 257 n.m.

351 3.08 0.76 43 n.m. 25.3 51.8 1.90 6.14 497 25.1 164 n.m.

8.11 2.75

5.52 1.84

6.50 1.07

6.96 1.08

4.63 0.74

5.27 0.91

5.20 0.93

3.97 0.72

24.1 2.32

24.0 2.34

18.6 1.78

0.29 1.65

0.28 1.54

0.23 1.44

0.23 21.2

0.22 21.8

0.21 20.8

535 6.49 1.73

4.50 5.93 3.13

n.m. 1179 n.m. n.m.

88

190

n.m. 53.1

n.m. 79.5

105 3.53 11.4 888 47.5 297 n.m. 9.61 2.99 7.61 1.16 5.98 1.06 29.1 2.68 0.35 2.04 0.30 22.9

156 n.m 18.4 1598 66.6 361 8.2 11.8 3.75 7.48 0.78 3.54 0.57 11.9 1.05 b.d. 0.09 b.d. n.m.

V

175

167

156

157

145

206

211

174

179

n.m.

Cr

323

205

417

347

45

355

305

396

358

b.d.

Ni

230

161

345

260

75

244

230

242

226

b.d.

Zr/Nb

25.9 3.21

40.5 2.87

37.8 2.87

33.3 2.99

38.6 3.41

20.9 3.81

24.0 4.08

17.6 3.81

26.1 3.38

Ce/Pb

29.6

30.2

23.5

26.6

29.7

37.8

36.7

27.2

29.8

La/Yb

K/U

9521

7445

8326

8276

9854

n.m., not measured; b.d., below detection limit.

175

9791

10697

12237

7755

880 1.9

Downloaded from http://petrology.oxfordjournals.org/ at Stadt und Universitatsbibiothek / Section Medizinische Hauptbibliothek on November 29, 2011

SiO2

JOURNAL OF PETROLOGY

VOLUME 46

JANUARY 2005

onto Pt filaments with a mixed Na(OH)–Ba(OH)2 emitter. The concentrations and isotopic compositions reported in Table 2 were measured at the Max-PlanckInstitut, Mainz, by thermal ionization mass spectrometry in negative ion mode (N-TIMS) using a Finnigan MAT262 system. The effects of fractionation during Os runs were corrected for by normalizing the Os isotope ratios to 192 Os/188Os ¼ 30827 (Luck & Allegre, 1983). Six procedural blanks for Os ranged from 016 pg to 145 pg with 187Os/188Os between 023 and 039, resulting in corrections on sample 187Os/188Os of