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Author's personal copy B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 145 However, the exact process of IF deposition is unclear, as it implies contradictory mechanisms in which seawater is reducing enough to transport large amounts of soluble Fe(II), yet oxidizing enough to episodically precipitate sediments that may contain N90% Fe (measured as Fe 2 O 3 ). Therefore, at least with respect to the marine Fe cycle, Archean seawater during IF deposition was clearly distinct from modern seawater, and several studies have attempted to quantify the importance of an Fe-rich hydrothermal flux by examining samarium– neodymium (Sm–Nd) isotopes in IFs (e.g., Miller and O'Nions, 1985; Jacobsen and Pimentel-Klose; 1988a, b; Derry and Jacobsen, 1990; Alibert and McCulloch; 1993; Bau et al., 1997a; Alexander et al., 2008). This application of Sm–Nd isotopes is possible because fractionation during crustal differentiation produces different Sm/Nd ratios in oceanic and continental crust, resulting in unique 143 Nd/ 144 Nd ratios in these reservoirs due to the subsequent decay of 147 Sm to 143 Nd. Measured 143 Nd/ 144 Nd ratios are typically expressed using the Є Nd (t) notation (DePaolo and Wasserburg, 1976), which describes the deviation of 143 Nd/ 144 Nd in parts per 10 4 from CHUR (chondritic uniform reservoir, i.e., bulk silicate Earth) at time t. Continental crust, with a low Sm/Nd ratio and little radiogenic 143 Nd, displays negative Є Nd (t), whereas oceanic crust derived from a depleted mantle has a higher Sm/Nd ratio, more radiogenic 143 Nd, and positive Є Nd (t) values. Therefore, Sm–Nd isotope ratios in a sample of known age can distinguish whether Nd has been primarily derived from continental or oceanic crust. The purpose of this study is to screen IFs for seawater provenance on the basis of REE patterns, and then re-evaluate the isotopic evolution of Nd in Archean seawater. Though data between 3.0 and 3.5 Ga are few, it seems that seawater possessed relatively constant Є Nd (t)of approximately +1 to +2 from 3.8 Ga until ~2.6 Ga. New Nd isotopic results for 2.95 Ga IF from the Pietersburg greenstone belt (PGB) in South Africa support this conclusion, and the PGB samples show strong evidence for mixing between a detrital sediment source with slightly negative Є Nd (t) and ambient seawater with positive Є Nd (t) equal to or greater than +1. It is concluded that, unlike modern seawater, the great majority of Nd in bulk seawater prior to 2.7 Ga originated from mantle-derived mafic source rocks. However, comparison of the Pietersburg IF to similarly aged IF from South Africa indicates that where evolved local continental crust was present this mantle Nd signal was not discernible in shallow Archean seawater (Alexander et al., 2008), implying that like modern oceans, the Archean ocean was not well-mixed with respect to its Nd isotopic composition. 2. Nd in seawater and previous IF studies Seawater in modern oceans possesses negative Є Nd (0) between −1 and −20, though extreme values in this range are restricted to shallow waters (Frank, 2002; Goldstein and Hemming, 2003). There is no evidence that radiogenic Nd derived from hydrothermal alteration of mid-ocean ridge basalt contributes significantly to modern seawater, and in fact, efficient scavenging of REE by precipitating metal oxides near hydrothermal vent sites is considered to provide an ultimate sink for REE in seawater (German et al., 1990). As a result, Nd derived from continental crust dominates world seawater (e.g., Piepgras and Wasserburg, 1980; Andersson et al., 2008, and references therein). The persistence of a ~20 Є Nd -unit variation between ocean basins indicates that the marine residence time of Nd (τ Nd ) is less than the mixing time of the oceans (~1500 yr, e.g., Broecker and Peng, 1982), and estimates of τ Nd in modern oxic seawater based on isotopic studies typically range from ~500 yr (Tachikawa et al., 2003) to 1000– 2000 yr (Jeandel et al., 1995;). Temporal variability in seawater Є Nd (t) has also been noted, and Recent (b20 Ka) seawater Nd ratios appear to have fluctuated by 2–3 Є Nd -units on 1000–5000 yr time scales (e.g., Piotrowski et al., 2005; Gutjahr et al., 2008). Early Nd isotopic studies of IFs attempted to constrain the source of Fe to coeval seawater, based upon the premise that initial 143 Nd/ 144 Nd ratios in IFs would indicate if Fe was derived from hydrothermal fluids originating in ancient oceanic crust, or if the Fe was sourced from weathering of contemporaneous continental crust. However, this approach is problematic as recently discussed by Alexander et al. (2008), as the different geochemical behaviors of Fe and Nd do not allow definitive statements to be made regarding the Fe source to IF based upon Nd isotopes. Regardless of the Fe source to IFs, the use of Sm–Nd isotopes for discerning the relative impact of oceanic crust-derived hydrothermal fluxes to Archean seawater should be possible. Neodymium isotopic data for IFs have varied, with Miller and O'Nions (1985) concluding that continental crust supplied much of the Nd to Archean oceans. However, later studies by Jacobsen and Pimentel-Klose (1988a, b), Alibert and McCulloch (1993), and Bau et al. (1997a) concluded that contemporaneous seawater Nd was primarily derived from hydrothermal alteration of mafic oceanic crust. A recent study of 2.9 Ga IF by Alexander et al. (2008) indicates that shallow Archean seawater along cratonic margins was dominated by continentally-derived Nd, similar to coastal seawater in modern oceans. Whereas early studies have extrapolated data from relatively small numbers of samples and locations to be representative of the entire Archean ocean (e.g., Miller and O'Nions, 1985; Jacobsen and Pimentel-Klose, 1988b), the conflicting interpretations could reflect spatial and temporal heterogeneity with respect to 143 Nd/ 144 Nd ratios in Earth's early oceans. This study addresses the possibility that Nd isotopic heterogeneity existed in the Archean ocean, similar to what is observed in modern oceans. It is necessary to examine the possibility that primary Nd isotopic signals recorded in IFs may be reset by post-depositional processes; however, the Sm–Nd isotopic system is resistant to metamorphism and consistent Nd isotopic results are obtained for IFs that have experienced a wide range of metamorphic overprint. Bau et al. (1997a) reported Sm–Nd data for the stratigraphically equivalent Kuruman and Penge IFs of the Transvaal Supergroup in South Africa (Beukes, 1983). Whereas the Kuruman IFs were weakly metamorphosed (b200 °C) and the Penge IFs suffered intense metamorphic overprint (N500 °C), both IFs yield very similar Sm–Nd isotopic ratios, and REE patterns are indistinguishable between the two. Moreover, 2.5–3.7 Ga IFs from a number of locations and with very different metamorphic histories display remarkably similar, seawater-like REE distributions (e.g., Alibert and McCulloch, 1993; Bolhar et al., 2004), and in a detailed study of worldwide IFs Bau (1993) concluded that postdepositional REE mobility in IFs is negligible. 3. Geologic setting The Pietersburg greenstone belt outcrops over a 150 km long by 70 km wide area located in northeast South Africa (Fig. 1). The northeastern portion of the PGB consists of schists, is bounded by the Limpopo metamorphic province, and experienced the highest degrees of deformation and lower to upper amphibolite grade metamorphism (De Wit et al., 1992). The southwestern portion of the belt is surrounded by igneous intrusives and younger cover of the Traansvaal Supergroup, and experienced less tectonic deformation and lower degrees of metamorphism (De Wit et al., 1992). The PGB contains two major tectonic units separated by a distinct unconformity, above which lies the sedimentary Uitkyk Formation (De Wit et al., 1992). Below the unconformity, the South African Committee for Stratigraphy (SACS, 1980) recognized five formations, from oldest to youngest, the Mothiba, Ysterberg, Eersteling, Zandrivierspoort, and the Vrischgewaagd. Only the lowermost Mothiba and Ysterberg Formations contain IF. However, De Wit (1991) observed significant structural repetition in the southwestern portion of the PGB, where the IF samples for this study originated, and concluded that a strict SACS stratigraphy was not applicable. Therefore, we adopt the stratigraphy of De Wit (1991) for this area, which labels all units below the unconformity as simatic basement, due to the silica-magnesia
Author's personal copy 146 B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 Fig. 1. Map showing the location of 2.9–3.5 Ga greenstone belts associated with the eastern margin of the Kaapvaal craton, including the Pietersburg greenstone belt (PGB), its distribution, and a geologic map of the southwestern region of the PGB (modified after De Wit et al., 1992). The oldest epicratonic sediments in South Africa are located in the ~2.90 Ga Pongola Supergroup, which consists of the older Nsuze Group and the younger, IF-bearing Mozaan Group. Mozaan IF discussed in the text was sampled from the Wit Mfolozi inlier. Also shown on the detailed map are the type localities for the Pietersburg geologic formations. lithologies that dominate. The simatic basement primarily consists of mafic volcanic rocks including metagabbros, serpentinized peridotites, and massive pillowed metatholeiites, with accessory interlayered IF, Fe-rich shales, and minor cherts and carbonates (De Wit, 1991). Few radiometric age data exist for the PGB. The maximum age of the Uitkyk Formation is interpreted as ~2.90 Ga, as determined by three detrital zircon uranium–lead (U–Pb) ages that range from 2901±2 Ma to 2957±8 Ma (De Wit et al., 1993). A minimum age of 2687 ±2 Ma for the PGB is provided by a U–Pb single zircon analysis of the Uitloop granite (Fig. 1), which cross-cuts all units of the PGB (De Wit et al., 1993). For the simatic basement below the unconformity a Pb–Pb date for mafic metavolcanics from the Eersteling area of 3455±128 Ma was reported by Byron and Barton (1990), but this age was interpreted as reflecting a secondary metamorphic overprint. The only other data for the simatic basement consists of two zircon analyses for a metaquartz porphyry associated with IF in the Ysterberg area. These analyses yielded a Pb–Pb evaporation age of 2939±26 Ma, and a precise U–Pb zircon age of 2949±0.2 Ma (Kröner et al., 2000). A granitoid located 15 km west of the Ysterberg area that intrudes into, and is deformed with, the lower simatic basement yielded a U–Pb zircon age of 2958±2 Ma (De Wit et al., 1993). These data indicate that the oldest sections of the PGB are greater than 2.96 Ga in age, and the fact that iron-formation is described only in the oldest portions of the simatic basement (Mothiba and Ysterberg Formations) suggests that IF deposition occurred no later than 2.95 Ga. Therefore, considering the precision of the U–Pb date (2949±0.2 Ma) reported by Kröner et al. (2000) for the IF-bearing sequence in the Ysterberg Formation, we assign the IF in the southwest portion of the PGB a minimum age of 2.95 Ga for the purposes of interpreting Nd isotopic data. For comparison, the Pietersburg samples are discussed in relation to IF from the ~2.9 Ga Mozaan Group, which is part of the Pongola Supergroup located southeast of the PGB (Fig. 1). The Mozaan IF were part of the first sediments unconformably deposited on the older Nsuze Group, and are considered ideal archives for shallow Archean seawater (Alexander et al., 2008). Few age constraints exist for the Mozaan Group. A rhyolite occurring in the upper third of the older Nsuze Group yielded a SHRIMP U–Pb date of 2977±5 Ma for single zircons (Nhleko, 2003), and is the best constraint for the upper age limit of the Mozaan Group (see also Hegner et al., 1994). The youngest depositional age for the Mozaan Group has been interpreted as 2837±5 Ma from a SHRIMP U–Pb zircon age obtained from a quartz-porphyry sill (Gutzmer et al., 1999). Based on the limited data, it appears that the oldest parts of the Mozaan Group (the IF) may be as old as ~2.95 Ga, and these sediments are unlikely to be younger than ~2.84 Ga. 4. Sampling and analytical methods The Pietersburg IF was sampled from an approximately 50 cm long continuous section of drill core that was subdivided into seven pieces
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Iron sedimentation and the neodymiu
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The process(es) by which IFs were d
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This thesis represents original and
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viii
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goal of the research is to discern
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Figure 1. Map of southern Africa sh
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appropriate to briefly discuss: 1)
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100 10 MORB 1 0.1 raw data (mg/kg)
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10 -2 hydrothermal fluid 10 -3 10 -
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seawater and returned to the sedime
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100 MCO + 3 + M(CO 3 )- 2 rare eart
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net effect of carbonate complexatio
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feature was not recognized in early
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sound geochemical basis for anomalo
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deviation of the 143 Nd/ 144 Nd rat
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available, it should be possible to
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modern and Archean oceans, as well
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Bau M., Möller P., and Dulski P. (
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Elderfield H. and Greaves M.J. (198
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Haley B.A., Klinkhammer G.P. (2003)
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Lee J.H. and Byrne R.H. (1993) Comp
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Piepgras D.J. and Wasserburg G.J. (
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Tosiani T., Loubet M., Viers J., Va
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Abstract The benefits of inductivel
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List of Figures Figure 1. Sample de
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1. Introduction The advent of induc
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Figure 1. Flow-chart diagram of the
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during decomposition (SiF 4 and CO
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Fractionation between the three iso
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cps, then 3000 cps must be subtract
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variety of ways. These range from r
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Table 3. Change in HFSE concentrati
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0.90, corresponding to an anomalous
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10 2 JDo-1 reference values 10 1 IF
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Figure 5. Diagram depicting various
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20 18 basalt BHVO-2 (n=5) run preci
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The method precision for elements p
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Two approaches may be utilized to d
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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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