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
170
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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
