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The process(es) by which IFs were deposited in Archean seawater is also investigated through a study of Fe-rich shales formed in close association with the Pongola IFs. Major and trace element concentration data for these Fe-rich shales indicate post-depositional mobility and extreme loss of alkali metals and barium (Ba). Mobility of these elements in modern clastic sediments has been observed in the presence of pore water NH + + 4 , and a significant presence of NH 4 in Fe-rich Archean + sediments would strongly suggest a biologic origin for IFs. However, a conclusive NH 4 signature is not observed in the Pongola Fe-rich shales, and the behavior of the alkali metals and Ba is best explained by the presence of significant concentrations of dissolved Fe 2+ in sediment pore waters and/or bottom seawater. This suggests that precipitation of Fe(III) mineral phases, either in the water column or at the sediment water interface, was an unlikely mechanism for the deposition of the primary Fe-rich sediment. While studies of seawater prior to ~2.7 Ga are generally constrained to investigations of IFs, cherts, and rare fluid inclusions, the occurrence of carbonate rocks is extensive after 2.7 Ga. Many Archean and Paleoproterozoic carbonates are dolomitic and/or highly silicified, and the retention of primary seawater trace metal distributions in these rocks has not been conclusively demonstrated. A study of limestones and silicified dolomites from the ~2.25 Ga Mooidrai carbonates indicates that silicified dolomites retain primary trace metal signatures consistent with seawater and indistinguishable from coeval limestones. Therefore, silicified dolomites free of clastic detritus are considered suitable archives for proxies of ancient seawater. The accuracy and reproducibility of the trace metal concentration data for 32 elements are evaluated, and the inductively coupled plasma mass spectrometry (ICPMS) analytical methods used are described. The ICPMS data for Sm and Nd in IF samples are in excellent agreement with thermal ionization mass spectrometry (TIMS) measurements, supporting the accuracy of the reported REE data. The data quality is discussed in the context of repeated analyses of certified reference materials (CRMs) commonly used in geochemical research. Measured concentrations for IF and carbonate CRMs match reference data well; however, some elements suffer from major element interferences in particular rock types. This includes Co in carbonate rocks, and Nb in Fe-rich rock types. The concentrations of many trace metals in some CRMs are poorly constrained, and early published datasets appear to frequently overestimate the abundances of these elements. ii
ACKNOWLEDGEMENTS A special thank you to my family and friends for their encouragement and support, and particularly to Claudia Nitzschmann, as three years became four, and four years became five. Thanks as well to my advisor, Michael Bau, for introducing me to the numerous, endlessly fascinating topics regarding the geochemical evolution of the early Earth. I must confess a compelling attraction to a subject where it is so difficult to prove my assertions wrong… A debt of gratitude also to the many people that I came to know and appreciate within the geochemistry working group at Jacobs, including, but not limited to, faculty (A. Koschinsky), students (K. Schmidt, S. Kulaksiz), and staff (D. Meißner, J. Mawick, A. Moje). The various excursions, meetings, and long hours in the lab were much more enjoyable because of their presence. from Holland (1972) iii
Page 1: Iron sedimentation and the neodymiu
Page 7: This thesis represents original and
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Page 12 and 13: goal of the research is to discern
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Page 52 and 53: 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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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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