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deviation of the 143 Nd/ 144 Nd ratio measured in a sample relative to the 143 Nd/ 144 Nd ratio in a chondritic uniform reservoir (CHUR) in parts per 10 4 : Є Nd (t) = ⎡( ⎢ ⎢⎣ 143 Nd/ I 144 t CHUR Nd) i ⎤ − 1⎥ ⎥⎦ x 10 4 where ( 143 Nd/ 144 Nd) i is the initial ratio in the sample (i.e., at the time it formed, t), I t CHUR is the CHUR 143 Nd/ 144 Nd ratio at the time the sample formed, and CHUR is considered to represent bulk silicate Earth. Since continental crust is characterized by low Sm/Nd ratios that produce relatively low amounts of radiogenic Nd, it displays negative Є Nd (t) values. Conversely, mafic oceanic crust derived from a depleted upper mantle is characterized by positive Є Nd (t) values, as MORB has relatively high Sm/Nd ratios and excess radiogenic Nd. As the REY in modern seawater are primarily sourced from the weathering of continental crust, modern seawater possesses negative Є Nd (0) values between -1 and -20, though extreme values in this range are restricted to shallow waters (Goldstein and Hemming, 2003). Figure 11 shows average Є Nd (0) values for different world oceans, as well as simple models for the Nd isotopic evolution in continental crust and a depleted mantle over the past 4.0 Ga. The significant Nd isotopic variations between ocean basins indicates that the marine residence time of Nd (τ Nd ) is less than the mixing time of the oceans (~1000 years, e.g., Broecker and Peng, 1982), and estimates of τ Nd in oxic seawater typically range from 300-1500 years (Jeandel et al., 1995; Tachikawa et al., 2003). This is consistent with the particle reactive nature of the REE and the efficient Nd scavenging processes operating in modern oceans, as REE-rich hydrothermal fluid with highly positive Є Nd (0) has a negligible effect on 143 Nd/ 144 Nd ratios in world seawater (Fig. 11). 22
15 10 TAG hydrothermal fluid 5 depleted mantle ε Nd (t) 0 -5 Pacific Ocean Indian Ocean -10 Atlantic Ocean -15 mafic igneous -20 felsic igneous 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Ga continental crust Figure 11. The range of Nd isotopic ratios (grey bar) as observed in modern seawater (Goldstein and Hemming, 2003), as well as Nd isotopic evolution in a depleted mantle and average continental crust. Average ocean basin seawater data from Piepgras and Wasserburg, (1980), and Trans-Atlantic Geotraverse (TAG) mid-ocean ridge hydrothermal fluid data reported by Mills et al. (2001). Isotopic evolution lines represent simple linear models where Є Nd (4.56 Ga) = 0, depleted mantle Є Nd (0) = +9 (Salters and Stracke, 2004), and average continental crust Є Nd (0) = -17 (Goldstein et al., 1984). Also shown are data for mafic and felsic igneous rocks from the Kaapvaal craton, illustrating that ca. 3.0 Ga crust in the study area displayed a wide range of Є Nd (t) values. Igneous rock data from Hegner et al. (1984), Wilson and Carlson (1989), Nelson et al. (1992), Kröner and Tegtmeyer (1994), Kröner et al. (1996), and Chavagnac (2004). For the early Earth, the isotopic evolution of Nd in continental crust and a depleted mantle reservoir is debated (cf. Nägler and Kramers, 1998; Patchett and Samson, 2003), and the isotopic evolution lines in Fig. 11 represent simple linear models. Regardless of the model favored, however, it appears that by ~3.8 Ga Є Nd values in the upper mantle were at least +2 (Bennet, 2003), implying that significant crustal differentiation was occurring in the early Archean. For southern Africa, the range of observed Є Nd (t) values observed for >2.7 Ga felsic and igneous rocks is large (≥10 Є Nd - units, Fig. 11), indicating that crustal sources of Nd to Archean seawater in the vicinity of the Kaapvaal craton were isotopically diverse. Due to the large range in Є Nd (t) values for Nd sources, and assuming suitable archives for seawater 143 Nd/ 144 Nd ratios are 23
Page 1: Iron sedimentation and the neodymiu
Page 4 and 5: The process(es) by which IFs were d
Page 7: This thesis represents original and
Page 10 and 11: viii
Page 12 and 13: goal of the research is to discern
Page 14 and 15: Figure 1. Map of southern Africa sh
Page 16 and 17: appropriate to briefly discuss: 1)
Page 18 and 19: 100 10 MORB 1 0.1 raw data (mg/kg)
Page 20 and 21: 10 -2 hydrothermal fluid 10 -3 10 -
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Page 24 and 25: 100 MCO + 3 + M(CO 3 )- 2 rare eart
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Page 30 and 31: sound geochemical basis for anomalo
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Page 38 and 39: Bau M., Möller P., and Dulski P. (
Page 40 and 41: Elderfield H. and Greaves M.J. (198
Page 42 and 43: Haley B.A., Klinkhammer G.P. (2003)
Page 44 and 45: Lee J.H. and Byrne R.H. (1993) Comp
Page 46 and 47: Piepgras D.J. and Wasserburg G.J. (
Page 48 and 49: Tosiani T., Loubet M., Viers J., Va
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Page 52 and 53: Abstract The benefits of inductivel
Page 54 and 55: List of Figures Figure 1. Sample de
Page 56 and 57: 1. Introduction The advent of induc
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For the more recent, full 32 elemen
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isotopic data for geologic CRMs, av
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Table 4. Interferences observed for
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value uncertainty (as %RSD) is repr
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1.50 FeR-4 (n=2) 1.25 JUB / referen
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1 IF-G/shale 0.1 0.01 La Ce Pr Nd P
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consistent (App. 1), and similar to
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1.50 SGR-1b (n=3) 1.25 JUB / refere
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1.50 JCh-1 (n=3) 1.25 JUB / referen
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1.50 JDo-1 (n=5) 1.25 JUB / referen
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determinations in carbonate rocks r
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1.50 JMn-1 (n=4) 1.25 JUB / referen
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eference value. The result is that
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eference value and the measured JUB
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the 32 analyzed elements, with Ti b
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Govindaraju K. (1995) 1995 working
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Appendix 1. Analytical data Appendi
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Appendix 1. Literature reference va
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Appendix 1 continued. Data in mg/kg
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the concentration of the major elem
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Potential interferences on monoisot
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1500 0.5 M HCl 4.0 interference on
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4000 0.40 0.20 3500 0.17 mg/kg 0.18
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interference on 95 Mo in sample sol
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Nd isotopes in 2.9 Ga Archean surfa
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Table 2 Trace element concentration
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Author's personal copy B.W. Alexand
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and preservation of oxide-facies IF
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TOP relative stratigraphic position
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The mechanism by which the alkali m
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6. Widdel, F. et al. Ferrous iron o
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82 PRESERVATION OF PRIMARY REE PATT
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ulk, open ocean seawater tended to
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