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available, it should be possible to distinguish if ca. 3.0 Ga seawater near the Kaapvaal craton was similar to modern seawater in possessing negative Є Nd (t). 1.5.4. Iron-formations as seawater archives Reconstructing Nd isotopic ratios in paleoseawater is possible as isotopic fractionation during incorporation of Nd into pure marine chemical sediments has not been observed (Frank, 2002). Even marine precipitates possessing strongly fractionated REY patterns relative to seawater, such as hydrogenetic Fe-Mn crusts (Fig. 8), accurately record the Nd isotopic composition of ambient seawater (Goldstein and Hemming, 2003, and references therein). Therefore, sufficiently pure marine chemical precipitates from the Archean should represent suitable archives for reconstructing primary seawater Nd isotopic ratios. Potential seawater archives for the Archean would be carbonate rocks such as limestones or dolomites; however, the carbonate record prior to ~2.7 Ga is poor, consisting primarily of thin, discontinuous, and poorly preserved occurrences (Grotzinger, 1989). Prior to 2.7 Ga the best candidates for archives of seawater 143 Nd/ 144 Nd ratios are iron-formations, as they are ubiquitous in both time and space for this period of Earth’s history. While the exact process by which they formed remains unknown, the frequent association of IFs with marine carbonates and sea level rise (Klein and Beukes, 1989; Simonson and Hassler, 1996) supports the general consensus that IFs are chemical sediments precipitated from seawater (cf. Simonson, 2003). The geochemical evidence used in this thesis to screen IF samples for suitability as seawater archives primarily rests upon measured concentrations of elements such as Al, Ti, and Th. These refractory elements are good indicators of clastic detritus that may contaminate pure marine precipitates. Data presented in the following chapters suggests 24
10 0 10 -1 Isua IF (3.8 Ga) Kuruman IF (2.5 Ga) 10 -4 Campbellrand dolomite (2.5 Ga) Holocene carbonate reef La Ce Pr Nd PmSm Eu Gd Tb Dy Y Ho Er Tm Yb Lu sample/PAAS 10 -2 10 -3 seawater x10 4 Figure 12. Distributions of the REY in average world seawater (same as Fig. 4), and various chemical sediments that are free from contamination with Al-rich clastic detritus. Data representing the Isua IF (Greenland) are for the IF-G reference standard analyzed at Jacobs University Bremen as discussed in Chapter 5. Kuruman (South Africa) IF data represent the average of two analyses as reported in Bau et al. (1997a). Campbellrand dolomite represents unpublished data from GKP-01 drillcore (Agouron- Griqualand Paleoproterozoic Drilling Project, administered by University of Johannesburg). Holocene (≤10,000 years old) carbonate reef data from Webb and Kamber (2000). The REY patterns of IFs and both ancient and modern carbonates are very similar to each other and modern seawater. These observations suggest that Archean IFs possessing seawater-like REY distributions are likely to be excellent archives for the Nd isotopic ratios present in contemporaneous seawater. that, as a general rule, IF samples containing more than 0.5-0.7% Al 2 O 3 are unsuitable for reconstructing seawater Nd isotopic ratios. Additionally, the distribution of the REY is very useful at screening potential seawater archives, as Archean marine precipitates frequently display REY patterns that are very similar to those observed in modern seawater. Figure 12 shows REY patterns observed in Archean IFs and both modern and ancient marine carbonates. The distribution of the REY is generally indistinguishable between modern seawater and carbonate reefs, as well as Archean dolomite and IFs. The general enrichment of the HREE over the LREE and positive La and Y anomalies found in seawater are common to all marine precipitates shown in Fig. 12 regardless of age. The primary differences are for Ce, which reflects the different oxidation states of 25
Page 1: Iron sedimentation and the neodymiu
Page 4 and 5: The process(es) by which IFs were d
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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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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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