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and preservation of oxide-facies IF was common throughout the first 2 billion years of Earth’s geologic history. Oxide-facies IF in Archean marine basins is frequently associated with Fe-rich shales, with the latter commonly assumed to represent mixtures between detrital clay minerals and Fe(III) oxide minerals. The Archean (2.85-2.95 Ga) Pongola Supergroup in southern Africa consists of the predominantly volcanic Nsuze Group, which is unconformably overlain by the predominantly sedimentary Mozaan Group. Previous studies have concluded that the Pongola Supergroup was deposited in near-shore shallow marine waters on a stable cratonic margin 10,11 , and subsequently experienced lower greenschist facies regional metamorphism 10 . This study discusses samples from the Sinqeni Formation of the Mozaan Group, that were obtained from the White Mfolozi inlier where a shallow-water sequence of BIF and ferruginous shale is bounded top and bottom by non-ferruginous shales 12 . The sample set includes the non-ferruginous shales deposited immediately below and above the BIF and the ferruginous shales within the BIF. Contacts between the ferruginous shales and the pure oxide-facies BIF layers can be sharp, but some BIF beds are very impure chemical sediments due to the co-deposition of detrital aluminosilicates. Major element, carbon, nitrogen, and trace element data for the samples are provided in Table 1. Variation in major elements such as Al, Fe, and K is related to the relative position of the samples in the stratigraphic profile. The non-ferruginous shales located below and above the BIF display a major element composition typical for Archean shales (Fig. 1). They are indistinguishable from fine-grained clastic rocks described in a comprehensive study of the Pongola Supergroup conducted by Wronkiewicz and Condie 13 (see Supplementary Table), and represent sediment weathered from the Kaapvaal craton ~2.85-3.0 Ga ago. In contrast, Fe in the ferruginous shales is inversely correlated with Al content (Fig. 1), a relationship that reflects mixing between Al-rich clastic sediment and the Fe-rich chemical precipitate that produced the BIF. On average, the ferruginous shales display half the Al 2 O 3 content and a five-fold increase in Fe 2 O 3 compared to the non-ferruginous shales. Similar to Al and Fe, the alkali metals (with the exception of Na) display variable concentrations and a distinct bimodal distribution with strong depletions for K, Rb, and Cs in the ferruginous shales (Fig. 1). Potassium and Rb average 6.3% and 205 ppm, respectively, in the nonferruginous shales, whereas the ferruginous shales contain less than 0.05% K 2 O and typically less than 1 ppm Rb. Barium concentrations, unlike the other alkaline earth elements, are also distinct between the non-ferruginous and ferruginous shales, averaging 567 ppm and less than 22 ppm, respectively (Fig. 1). 156
Table 1. Major and trace element concentration data for Pongola Supergroup shales. wt.% 1 bottom _________________________________________________ sample position in studied stratigraphic section ______________________________________________________ WM1 WM-A WM-B WM-C WM2 WM3 WM8 WM9 WM10 WM11 WM12 WM14 WM15 WM16 WM171 WM172 WM18 WM19 WM20 Fe-poor shale Fe-poor shale Fe-poor shale Fe-poor shale Fe-poor shale Fe-shale Fe-shale Fe-shale Fe-shale Fe-shale Fe-shale Fe-shale Fe-shale Fe-shale impure BIF impure BIF Fe-poor shale Fe-poor shale top Fe-poor shale AVG (n = 9) Fe-shale AVG (n = 8) AVG (n = 50) Fe-poor shale pelite 4 SiO 2 56.6 56.1 47.4 65.3 51.6 52.4 47.9 54.2 44.9 49.7 49.2 44.5 46.9 36.3 38.0 32.1 49.1 48.6 71.8 47.3 55.8 59.5 TiO 2 0.94 1.11 0.95 0.73 0.90 0.76 0.72 0.82 0.81 0.58 0.47 0.59 0.48 0.83 0.16 0.08 1.35 1.21 0.60 0.67 0.97 0.90 Al 2 O 3 29.2 29.8 30.1 22.2 23.3 14.2 11.9 11.7 14.9 13.3 10.4 11.5 11.0 17.1 4.46 1.86 24.7 26.8 16.7 12.9 25.4 21.1 Fe 2 O 3 0.65 0.88 7.13 3.87 10.5 21.0 29.0 24.2 31.0 29.8 29.0 28.3 27.4 32.2 53.8 63.5 9.16 6.55 2.52 28.0 5.15 6.07 MnO
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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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Page 118 and 119: Appendix 1 continued. Data in mg/kg
Page 120 and 121: the concentration of the major elem
Page 122 and 123: Potential interferences on monoisot
Page 124 and 125: 1500 0.5 M HCl 4.0 interference on
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Page 128 and 129: interference on 95 Mo in sample sol
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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
Page 168 and 169: TOP relative stratigraphic position
Page 170 and 171: The mechanism by which the alkali m
Page 172 and 173: 6. Widdel, F. et al. Ferrous iron o
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