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The mechanism by which the alkali metals and Ba were mobilized from the clastic sediments is rather a consequence of high dissolved Fe 2+ concentrations. Ferrous iron may readily exchange with interlayer Na + in smectite 23 , for example, and such Fe 2+ -alkali metal exchange is also observed for montmorillonite at high ionic strength and high chloride activity in experiments intended to model seawater-clay particle interactions 24 . An Fe 2+ -alkali metal exchange is further supported by the fact that extreme K, Rb, Cs, Ba and Na depletion is confined to the Fe-rich clastic sediments. The ferruginous shales are closely associated with oxide-facies BIF. Although the contacts between ferruginous shale beds and BIF beds are often sharp, there also exist impure BIF samples that are transitional between ferruginous shales and pure BIF. Samples WM171 and WM172 have Al 2 O 3 concentrations of 4.46% and 1.86%, respectively, and elevated concentrations of TiO 2 , Zr, Hf, and Th (Supplementary Table) compared to associated pure BIF. Ratios between these immobile elements are similar to those of the ferruginous shales, indicating that the deposition of ferruginous shale continued during the deposition of oxidefacies BIF. We emphasize that the detrital aluminosilicate component in WM171 and WM172 is characterized by the same lack of K, Rb, Cs and Ba (and Na) as the ferruginous shales, demonstrating the abundance of dissolved ferrous iron in an environment which formed chemical sediments that are now oxide-facies BIF. This suggests that Fe(III) oxides precipitated and were preserved in a macro-environment that was reducing with respect to the Fe 2+ /Fe 3+ redox couple, i.e., in a macro-environment that was anoxic. This is supported by the close similarity of the Th/U ratios of the ferruginous and the non-ferruginous shales, as oxidation of U(IV) in the clay minerals during the exchange reaction would have resulted in the preferential mobilization of U(VI) over Th(IV) 25 , which is not observed. The oxidation of part of the available dissolved Fe 2+ and the production of ferric-ironrich sediment could result from a number of processes. These mechanisms include indirect or direct biological processes, i.e., oxidation by reaction with photosynthetically produced molecular oxygen 1,2 , or by anoxygenic photoautotrophs 7,8 , respectively. Abiotic photochemical oxidation of Fe 2+ has also been suggested 3,4 , but is unlikely in the light of recent experimental results 26 . The ferruginous shales associated with the BIF formed when fine-grained clastic sedimentary material was deposited in ferrous iron rich seawater. Ferrous iron was incorporated into the clay minerals at the expense of K, Rb, Cs, Ba and Na which escaped into the water column. The contemporaneous precipitation and preservation of oxide-facies BIF demonstrates that Fe(II) oxidation occurred in an anoxic macro-environment, which is 160
supported by the lack of Th-U fractionation. This renders it unlikely that molecular oxygen was responsible, as either molecular oxygen would have been abundant enough to quantitatively oxidize the available dissolved ferrous iron, preventing the formation of alkali element depleted ferruginous shales, or the Fe(III) oxides would have been reductively dissolved (by Fe 2+ , for example) as soon as all the molecular oxygen was consumed. Only the stabilization and preservation of Fe(III) oxides by photoautotrophs would have allowed for the simultaneous deposition of oxide-facies BIF and ferruginous shales depleted in K, Rb, Cs, Ba and Na. Thus, oxide-facies IF could form via photoferrotrophy throughout the Archean prior to the onset of oxygenic photosynthesis. The formation of Fe(III) (hydr)oxides, i.e., oxide-facies IF, would have been common in a ferrous-iron-rich Archean ocean capable of sustaining postulated anaerobic ecosystems dominated by anoxygenic photoautotrophs 27 , in full agreement with the ubiquitous presence of oxide-facies BIF, jasper, jaspilite and ferruginous chert in the Archean geological record. Acknowledgements This research was supported by the European Science Foundation research program Archean Environmental Studies: the Habitat of Early Life. References 1. Cloud, P. Significance of the Gunflint (Precambrian) microflora. Science 148, 27-35 (1965). 2. Cloud, P. Paleoecological significance of the banded iron-formation. Econ. Geol. 68, 1135-1143 (1973). 3. Cairns-Smith, A.G. Precambrian solution photochemistry, inverse segregation, and banded iron formations. Nature 276, 807-808 (1978). 4. Braterman, P.S., Cairns-Smith, A.G., & Sloper, R.W. Photo-oxidation of hydrated Fe2+ - significance for banded iron formations. Nature 303, 163-164 (1983). 5. Garrels, R.M., Perry Jr., E.A., & Mackenzie, F.T. Genesis of Precambrian ironformations and the development of atmospheric oxygen. Econ. Geol. 68, 1173-1179 (1973). 161
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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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THIS PAGE INTENTIONALLY LEFT BLANK
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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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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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Page 132 and 133: Nd isotopes in 2.9 Ga Archean surfa
Page 136 and 137: Table 2 Trace element concentration
Page 152 and 153: Author's personal copy B.W. Alexand
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