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appropriate to briefly discuss: 1) basic rare earth element geochemistry, 2) rare earth element behavior in seawater, 3) Sm-Nd isotopic signatures in the Earth’s crust and seawater, and 4) the use of IFs as archives of ancient seawater chemistry. 1.5.1. Rare earth element geochemistry The rare earth elements include Sc, Y, and the fifteen lanthanide elements which have atomic masses between 57 and 71 (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). Hereafter in this thesis, the lanthanides will be exclusively referred to as the rare earth elements (REE), excluding Sc and Y. However, as described below, yttrium is useful when grouped with the lanthanides for studying geochemical processes, and where yttrium data are available discussions of the REE and Y will be indicated by the acronym REY. Among the lanthanides promethium (Pm) is a special case, as it has three radioactive isotopes ( 145 Pm, 146 Pm, 147 Pm), of which 145 Pm has the longest half-life of ~17.7 years. As such Pm is considered an extinct element in natural samples and is not included in discussions of REY behavior in geological studies. The remaining REY display similar geochemical behavior, primarily as a result of their +3 valence state, though Ce and Eu may also exist in the +4 and +2 valence states, respectively. This geochemically coherent behavior among the REY is also a consequence of very similar ionic radii between 0.977 Å (Lu) and 1.16 Å (La). The similar ionic radii results from a progressive filling of the 4f electron orbital that produces a monotonic decrease in the ionic radii with increasing atomic mass (Figure 2). As a result, lanthanide behavior tends to vary smoothly and predictably as a function of ionic radii (i.e., atomic mass) in many geochemical systems. 6
4 Ce valency 3 Sc Lu Y La Eu 2 0.8 0.9 1.0 1.1 1.2 1.3 ionic radius (Å) Figure 2. Valence state and ionic radius (coordination number 8) for rare earth elements. Yttrium has an ionic radius that falls between Dy and Ho. Cerium and Eu are the only rare earth elements that exist at valence states other than +3 (ionic radii data from Shannon, 1976). Rare earth element data are typically normalized for plotting, as REY concentrations may vary by an order of magnitude (or more) between adjacent elements in the series, due to the greater abundance of elements with even atomic numbers relative to elements with odd atomic numbers (Oddo-Harkins rule). Figure 3 shows data for mid-ocean ridge basalt (MORB) that has been normalized to C1 chondritic meteorite (Anders and Grevesse, 1989), as well as non-normalized data for comparison, and where Y has been inserted between Dy and Ho according to its ionic radius (see Figure 2). If none of the REY exhibit anomalous behavior then the normalized REY pattern will vary smoothly across the series as seen in Figure 3. It should be noted that the C1 chondritic meteorite is often used for normalizing REY data, as it is considered to possess a REY distribution equivalent to bulk silicate Earth. However, it is sometimes useful to normalize REY data to other geochemical reservoirs, and another commonly used REY dataset for normalization purposes (and one used extensively in this thesis) is upper continental crust, as represented by the Post Archean Average Shale (PAAS) of McLennan (1989). 7
Page 1: Iron sedimentation and the neodymiu
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Page 52 and 53: Abstract The benefits of inductivel
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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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