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goal of the research is to discern if the process by which IFs formed might possibly be constrained. Though not the direct focus of this thesis, elucidating the manner in which IFs precipitated in ancient seawater is attractive, as these sediments remain a geochemical mystery even after more than 50 years of study. The general approach followed is to measure the isotopic ratios of the rare earth elements Sm and Nd, assuming that these measured ratios reflect the ambient seawater that precipitated the studied IFs. The inferred Sm-Nd isotopic signature of seawater would reflect different sources of Sm and Nd to ancient oceans, and would constrain the relative importance of these sources on seawater chemistry. As ancient oceanic and continental crust possess distinctly different Sm-Nd isotopic signatures, it should be possible to determine if weathering of oceanic or continental crust was more important in controlling seawater chemistry. 1.2. Structure of thesis This thesis is structured as a compilation of independent manuscripts, with the exception of this, the introductory first chapter, and the final chapter that contains concluding remarks. Each individual manuscript contains an abstract, background information, the discussion of results, conclusions, and references. The thesis in total consists of six chapters that are outlined in the Table of Contents. This, the first chapter, introduces the research goals and provides background information relevant to the thesis which is not covered extensively in the individual manuscripts. The manuscripts that follow are generally written to facilitate publication in international academic research journals. One exception is Chapter 2, as it describes the relevant analytical methods employed within the Jacobs University Bremen (Jacobs) Geochemistry Lab, and which has been published as Jacobs Technical Report No. 18. 2
As a result of this approach, slight differences will exist from chapter to chapter regarding organization or format. If already published or submitted for publication in an academic journal, the manuscripts reflect the individual formatting and length requirements of the respective journal, and where appropriate, the manuscript/chapter is the printed version of the journal article. 1.3. Study area The geologic samples used in this thesis are exclusively from South Africa, and Figure 1 lists the major relevant geologic features of southern Africa. Since the research goal revolves around the chemistry of ancient seawater, this part of the world is an ideal area for study. The Kaapvaal craton is one of the oldest and most stable cratons known in the world (Tankard, 1982), and the oldest rock yet dated on the Kaapvaal craton, and probably the oldest on the African continent (Poujol et al., 2002), is a 3644 ±4 Ma tonalitic gneiss from northwest Swaziland (Compston and Kröner, 1988). The Kaapvaal craton hosts several Archean sequences containing iron-formation deposited in different tectonic settings, which is advantageous as these represent seawater precipitates from different marine environments, e.g., island-arc or stable continental shelf. Examples include IF within the >3.2 Ga Barberton greenstone belt, considered to have formed in an oceanic plateau/island-arc setting (Lowe, 1994), as well as IF deposited on the stable Kaapvaal craton during the formation of the ~2.5 Ga Transvaal Supergroup (Beukes, 1983) The IFs discussed in this thesis were selected as they represent seawater precipitates from different tectonic settings, yet are of a similar age (ca. 2.9-3.0 Ga). Samples were obtained from the 2.9 Ga Mozaan Group of the Pongola Supergroup, which was deposited within a relatively shallow, near-shore continental shelf 3
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
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Page 18 and 19: 100 10 MORB 1 0.1 raw data (mg/kg)
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Page 52 and 53: Abstract The benefits of inductivel
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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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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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School of Engineering and Science - Jacobs University

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