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Author's personal copy B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 149 Fig. 3. REY distributions in IF samples normalized to Al-rich IF7 (5.3% Al 2 O 3 ). Inset displays REY distribution in Al-rich IF7 normalized to chondritic meteorite (Anders and Grevesse, 1989) and continental crust as represented by Post Archean Australian Shale (PAAS, McLennan, 1989). PAAS possesses a negative Eu anomaly compared to bulk Earth, and PAAS-normalized data will correspondingly display positive Eu anomalies when no Eu fractionation exists. This is the case for Al-rich IF7, for which no anomalous behavior of Eu is observed. The remaining samples all display positive Eu anomalies relative to IF7 that significantly vary in magnitude, with greater Eu enrichments in samples of lower total REY abundance. Positive La, Gd, and Y anomalies are present in many of the samples and, similar to Eu, the magnitude of these anomalies increases with decreasing REY concentration. The effect of detrital contamination is observed in samples IF3 and IF4, which have the second highest Al 2 O 3 contents (1.5%) after IF7, and relatively flat REY patterns. However, IF4 also possesses greater total REY concentrations that IF7, which coupled with the positive Eu anomaly in IF4 indicates that a significant fraction of its REY budget was supplied by the chemical precipitate which produced the IF. Sample IF7a illustrates the large range of REY concentrations (and REY fractionation) observed on cm-scales within the drill core, as it was obtained from a homogenous band adjacent to sample IF7. The range of concentrations and REY distributions observed throughout the core argues against the widespread mobility of REY at any point following deposition. nickel (Ni), and scandium (Sc)), including relatively mobile rubidium (Rb) (Fig. 2). These correlations remain even if the single sample with the highest Ti and Al content (IF7) is not considered (Fig. 2). The iron-formations possess REE and yttrium (REY) concentrations that vary by an order of magnitude between the samples, and the distribution patterns of these elements vary significantly as well. This may be illustrated by normalizing REY data to IF7, which possesses the highest Al 2 O 3 content (5.3%), a procedure that effectively removes the influence of any detrital material that may be present (Fig. 3). The Alrich IF7 itself is light rare earth element (LREE) enriched when normalized to chondrite (subscript CN , Anders and Grevesse, 1989) and shows gadolinium/ytterbium ratios ((Gd/Yb) CN ) of ~1 and no REY anomalies (Fig. 3). The two samples with the next highest Al 2 O 3 concentrations (1.5%, IF3 and IF4) have higher REY abundances than the remaining samples, and REY concentrations change significantly on small scales (1–2 cm) throughout the core as a function of Al content. This is illustrated by samples IF5 and IF5a, which represent individual ≤1 cm thick bands sampled within 2–3 cm of each other, and that contain Al, Ti, and REY contents that vary by a factor of ~3. However, a significant fraction of the REY in most of the samples must be derived from the chemical precipitate which produced the IF. Sample IF4 has one-third the Al of sample IF7, yet IF4 has higher REY concentrations (Fig. 3). In the remaining samples four of the REY display anomalous behavior to varying degrees, with strong positive europium (Eu) and Y anomalies and smaller positive lanthanum (La) and Gd anomalies. The IFs also possess slight positive lutetium (Lu) anomalies, and the magnitude of all anomalies generally increases as Sm–Nd analytical quality was monitored using the basalt reference standard BCR-2 (United States Geologic Survey), which yielded 143 Nd/ 144 Nd=0.512628±0.000002. Additionally, the Isua iron-formation standard IF-G, issued by IWG-GIT (International Working Group – Groupe International de Travail) was analyzed to provide reference data for subsequent Sm–Nd isotopic studies of iron-formations, and these data match well with the single previously published Sm–Nd isotopic analysis of IF-G (Stecher et al., 1986). 5. Results Analytical data are presented in Tables 1 and 2. Excluding samples IF2a and IF7, all samples are more than 91% silicon- and iron-oxides (SiO 2 and Fe 2 O 3 ), with SiO 2 between 29 and 62%, and Fe 2 O 3 between 37 and 69%. Sample IF2a is enriched in calcium (Ca) and magnesium (Mg) compared to most samples, and IF7 possesses significantly more aluminum (Al) and Mg than any other sample. While Al contents are typically very low (b0.8%), consistent with very low amounts of detrital aluminosilicates, IF3 and IF4 each contain 1.5% Al 2 O 3 , and IF7 contains 5.3% Al 2 O 3 . Trace elements that are considered immobile, such as zirconium (Zr), hafnium (Hf), and thorium (Th), correlate well with Al and titanium (Ti), as do most trace metals (e.g., cobalt (Co), Fig. 4. Nd concentrations and Є Nd (t) of Pietersburg IF samples as a function of Al content. Top figure (a): Nd concentrations generally decrease with decreasing Al concentrations. However, significant Nd must be derived from the chemical precipitate that formed the IFs, as IF4 contains less than one-third the Al of IF7, but higher Nd concentrations. Bottom figure (b): Iron-formation samples with Al 2 O 3 N0.5% possess very consistent negative Є Nd (t), whereas positive Є Nd (t) values are only observed in samples with the lowest Al concentrations, indicating that contamination with minor detrital aluminosilicates drives Є Nd (t) to more negative values. This suggests that preexisting crust that provided aluminosilicates to the IF depositional environment possessed Є Nd (t) of approximately −0.5, and ambient Fe-rich seawater possessed Є Nd (t) equal to or greater than +1. Error bars for Є Nd (t) calculated by assuming average errors in 147 Sm/ 144 Nd ratios of 0.5%.
Author's personal copy 150 B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 REY abundances decrease. However, when comparing samples of similar REY content the relative size of the anomalies varies. For example, IF5 possesses a REY distribution and positive La and Y anomalies almost identical to samples of similar concentration, yet IF5 displays a significantly larger Eu anomaly. The possibility of significant alteration of the primary REY distributions is considered unlikely. Mobility of the REY has not been observed in Archean IFs (Bau, 1993), and concentrations of REY may vary considerably between adjacent Fe- and Si-rich bands in Archean– Paleoproterozoic IFs (Bau and Dulski, 1992). This behavior is also observed in samples IF5 and IF5a, which were obtained from adjacent layers in a ~4 cm section of drill core and yet possess distinct REY patterns and very different REY concentrations (Fig. 3). We emphasize that the lack of significant diagenetic or metamorphic alteration is further supported by the excellent correlation of relatively mobile Rb compared to highly immobile elements such as Ti and Th (Fig. 2). Similarly to immobile trace elements, Є Nd (2.95 Ga) of the IF samples varies as a function of Al content (Fig. 4), with samples containing more than 0.5% Al 2 O 3 displaying relatively uniform negative Є Nd (2.95 Ga) between −0.35 and −0.66. Samples containing less than 0.5% Al 2 O 3 are significantly more radiogenic, displaying positive Є Nd (2.95 Ga) between +0.29 and +0.99. The difference between the two groups of Є Nd (2.95 Ga) values exceeds the analytical error of the measurements. The correlation between increasing immobile element contents (e.g., Al, Th, Zr, Rb, and Hf) and decreasing Є Nd (t) suggests that crustal sources contributing detritus to the Pietersburg IF depositional basin possessed Є Nd (2.95 Ga) of approximately −0.5, and seawater responsible for precipitating the IF was more radiogenic with Є Nd (2.95 Ga) similar to or greater than +1. 6. Discussion 6.1. Nature of the detrital aluminosilicate source The higher Al 2 O 3 content (≥1.5%) of some IF samples indicates contamination with detrital aluminosilicates, and candidates for a detrital source(s) to the iron-formation include lithologies in the simatic basement. Published data from the Eersteling area are primarily for major elements and transition metals, and 80 analyses have characterized metagabbros, metabasalts, amphibolites, serpentinites, as well as quartz porphyrys (Saager and Meyer, 1982; Jones, 1990; Byron and Barton, 1990). Aluminum averaged 9.7% in these samples and only four contained more than 15% Al 2 O 3 (maximum 16.4%), suggesting that if the aluminosilicate fraction in the iron-formation was derived from pre-existing simatic basement then 15% Al 2 O 3 is the likely upper limit for this material, which agrees well with estimates for Archean mafic rocks (Condie, 1993). The strong correlations between immobile trace elements and Al allows reasonable estimates to be made regarding the trace metal distribution in the detrital source, and by comparing this estimated distribution with trace element data from the literature it may be possible to restrict the provenance of the detrital component. Estimated trace metal data (Fig. 5) for the most Alrich sample (IF7) at higher Al 2 O 3 contents of 10–15% match reasonably well with a mixture of Archean felsic volcanics and komatiites possessing compositions as proposed by Condie (1993), and komatiites in the PGB have been described previously (Saager and Mayer, 1982). However, few trace element data exist for PGB mafic rocks, making it difficult to characterize the nature of this aluminosilicate source, except to state that physical weathering of serpentinized material is an unlikely candidate for any detritus present in the Pietersburg IF. The relatively high Al 2 O 3 content in some samples (e.g., IF3, IF4, and IF7) renders them unsuitable as archives for the marine fluid that precipitated the Pietersburg IF, but these samples do provide Nd isotopic information regarding the clastic detrital source during deposition of the IF. Samples with Al 2 O 3 greater than ~0.5% possess very consistent Є Nd (t) close to −0.5 (Fig. 4), which is considered to reflect the Nd isotopic composition of the detrital aluminosilicate source. Close examination of the Al–Є Nd (t) relationships (Fig. 4) suggeststhattwocomponent conservative mixing between a detrital aluminosilicate source and the pure chemical precipitate that formed the IFs might adequately model the Nd isotopic data. However, two-component mixing models that attempt to account for detrital contamination Fig. 5. Trace element discrimination diagrams for simatic basement samples from the southwest region of the PGB. The grey area in both diagrams represents the range of predicted element concentrations in sample IF7, and was calculated from linear regressions of the metal contents as a function of Al. The lower border of the grey area represents 10% Al 2 O 3 and the upper border represents 15% Al 2 O 3 , which is a reasonable range for the detrital source (see Section 6). Diagram a contains literature data for possible source lithologies (Jones, 1990; Byron and Barton, 1990), of which pillow lavas and amphibolites are broadly consistent with the predicted distribution pattern. Diagram b shows the distribution patterns for model Archean komatiites and felsic volcanics of Condie (1993), a mixture of which could reasonably produce the pattern observed in the detrital fraction of the iron-formation samples, and komatiites are present within the oldest sections of the PGB (Saager and Meyer, 1982).
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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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determinations in carbonate rocks r
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1.50 JMn-1 (n=4) 1.25 JUB / referen
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