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Author's personal copy B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 153 Fig. 8. Nd isotopic evolution lines for 2.9–3.0 Ga South African IFs. Mozaan IF data does not include the two samples shown in Fig. 7 with Є Nd (2.9 Ga) lower than −8.0. The left vertical axis represents the minimum age for the Mozaan Group of 2837±5 Ma (Gutzmer et al., 1999); the best estimate of the maximum age for the Mozaan IFs is 2977±5 Ma (Nhleko, 2003). The lower age limit for the Pietersburg IFs is indicated as 2958±2 Ma (De Wit et al., 1993). The geologic evidence and the limited radiometric age data do not preclude the possibility that deposition of the Pietersburg and Mozaan IFs overlapped in time. The isotopic evolution lines are generally subparallel for all IF samples, reflecting the similar 147 Sm/ 144 Nd ratios in these samples. The Pietersburg IFs are consistently more radiogenic than the Mozaan IFs, and are interpreted to represent Archean seawater masses with higher 143 Nd/ 144 Nd than the seawater that precipitated the Mozaan IFs. environment similar to a modern mafic back-arc setting, whereas the Mozaan IF were clearly deposited in a marine shelf setting along a stable cratonic margin (Beukes and Cairncross, 1991). The negative Є Nd (2.9 Ga) values observed in the Mozaan IF are not surprising considering the occurrence of ca. 2.9 Ga igneous rocks possessing negative Є Nd (t) near the South Africa–Swaziland border (Hegner et al., 1984, 1994). These felsic and mafic rocks (e.g., rhyolite, andesite, basalt, and gabbro) possess Є Nd (t) very similar to the Mozaan IFs (Fig. 7), and are consistent with published Mozaan Group shale Є Nd (t) values(Stevenson and Patchett, 1990; Alexander et al., 2008). 6.4. Implications of Nd isotopic variations within the Archean ocean The fact that many Archean IFs with seawater-like REY distributions possess positive Є Nd (t) implies a significant hydrothermal flux from mantle-derived source rocks to seawater until at least 2.5 Ga. Certainly more studies of Mesoarchean IF (3.0–3.5 Ga) are necessary, but existing Nd isotopic data are consistent with the increased mantle-derived hydrothermal flux implied from Sr isotopic studies of Archean carbonates (Veizer and Mackenzie, 2004). With the exception of the Mozaan IF, the first clear evidence of seawater displaying negative Є Nd (t) is found in N. American IFs possessing Є Nd (2.65 Ga) of −1.5 (Frei et al., 2007), which is considerably more radiogenic than the older Mozaan IF. However, it is possible that negative (and highly variable) Є Nd (t) values were more prevalent in ca. 2.9 Ga seawater than is suggested by Fig. 7, as it does not include data from Frei et al. (2007) for oxide-facies IF from the N. American Nemo iron-formation. This is because the Nemo IF is poorly constrained in age (2.56–2.9 Ga), and therefore could display Є Nd (2.9 Ga) between −2.76 and +3.71, or Є Nd (2.56 Ga) between −5.24 and 0.78. The clearly distinct Є Nd (t) values between the Pietersburg and Mozaan IF are considered to arise from their different depositional environments, as the Mozaan IF are the oldest IF deposited on a stable cratonic margin, whereas older IF like the Pietersburg are associated with greenstone belts. If ca. 3.0 Ga seawater was homogenous with respect to 143 Nd/ 144 Nd, then world oceans would have become dramatically less radiogenic in the time span between deposition of the Pietersburg and Mozaan IF. We consider this unlikely, as it would further require a reversion of world seawater Є Nd (t) back to chondritic or positive values for the remainder of the Archean. We therefore interpret the data as indicating that Archean seawater could be inhomogeneous with respect to 143 Nd/ 144 Nd ratios, primarily dependent upon the nature of local crust exposed for weathering, a situation that is similar to that observed in modern oceans. The degree of Nd isotopic inhomogeneity in Archean seawater is unclear; it may have been that only small parts of the world ocean deviated significantly from an ‘average’ 143 Nd/ 144 Nd ratio. Excluding the Mozaan IF, seawater appears to have displayed relatively constant Є Nd (t) between 0 and +2.5 from 2.7 to 3.8 Ga. This is remarkable, and though detailed discussions of mantle evolution and the growth of continental crust are beyond the scope of this study, crustal sources of Nd were certainly evolving isotopically during this time period (Fig. 7). However, regardless of the model employed, there is little evidence that isotopic evolution within Nd source(s) to world oceans significantly altered the 143 Nd/ 144 Nd ratio of seawater throughout most of the Archean. This implies that prior to 2.7 Ga, the great bulk of seawater at any time t was relatively homogeneous with respect to 143 Nd/ 144 Nd, and water masses that deviated from a bulk seawater average Є Nd (t) of +1 to +2 (i.e., those that formed the Mozaan IF), were minor components of the world ocean. An isotopically homogenous world ocean is best explained by widespread anoxic conditions in Archean seawater (as suggested by the presence of IF themselves) that would increase the residence time of Nd in seawater. Evidence for deep water anoxia is provided by U and Th behavior in altered Archean basalts (Nakamura and Kato, 2007), and this environment would prohibit the formation of important REY sinks, increasing the residence time of Nd in Archean bottom seawater. Shallow (coastal) seawater that could precipitate oxide-facies IF was most likely characterized by the same short marine residence time of Nd as is observed in modern seawater, and was dominated by less radiogenic ‘local’ continental Nd. However, the relatively constant positive Є Nd (t) values observed in world IFs between 2.7 and 3.8 Ga in age that possess seawater-like REY distributions suggests hydrothermal alteration of oceanic crust was dominating the Nd (and hence Fe?) flux to bulk seawater throughout most of the Archean. 7. Conclusions Prior to 2.7 Ga, iron-formations possessing seawater-like REY distributions are the best archives for reconstructing paleoseawater 143 Nd/ 144 Nd ratios. Ambient seawater at the time of deposition of the Pietersburg IF possessed positive Є Nd (2.95 Ga) of at least +1, which is slightly lower than the best estimate for seawater Є Nd (3.8 Ga) as suggested by Isua IF. Local clastic aluminosilicate material, presumably derived from an Archean equivalent of a modern mafic back-arc setting, possessed lower Є Nd (t) of approximately −0.5, and relatively small contributions of this material (≥0.5% Al 2 O 3 ) were sufficient to dominate the Nd budget in the Pietersburg IF. Crustal sources of Nd to seawater near the Kaapvaal craton 2.9–3.0 Ga possessed distinctly different Є Nd (t), and could locally influence seawater 143 Nd/ 144 Nd ratios, as shown by the Mozaan IF, for example. The relatively consistent Nd isotopic ratios observed throughout most of the Archean for IFs with seawater-like REY distributions suggests that, on average, bulk Archean seawater displayed positive Є Nd (t) between +1 and +2 until at least 2.7 Ga. However, the range in Є Nd (t) values observed between the similarly aged Pietersburg and Mozaan IFs implies that world oceans were not entirely well-mixed with respect to 143 Nd/ 144 Nd ratios, but that this variability was restricted to local water masses, and was likely the consequence of
Author's personal copy 154 B.W. Alexander et al. / Earth and Planetary Science Letters 283 (2009) 144–155 different depositional environments. In conjunction with the ubiquitous positive Eu anomalies observed in IFs, the Nd isotopic data indicate high-temperature hydrothermal alteration of mantle-derived oceanic crust was a dominant control on bulk seawater composition for much of the Archean. Acknowledgements This research was funded by Exchange Grant 1327 within the European Science Foundation ArchEnviron Research Networking Programme. Mark Le Grange is thanked for his assistance with sampling. The authors would like to gratefully acknowledge the contributions of M. Fischerström and H. Schöberg regarding the Sm– Nd isotopic analyses performed at LIG. This manuscript significantly benefited from the comments of M. Gutjahr and one anonymous reviewer, as well as suggestions by M.L. Delaney. References Abbey, S., McLeod, C.R., Liang-Guo, W., 1983. 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Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling. Mar. Chem. 56, 123–131. Beukes, N.J., 1983. Palaeoenvironmental setting of iron-formations in the depositional basin of the Transvaal Supergroup, South Africa, in: Trendall, A.F., Morris, R.C. (Eds.), Iron-formation Facts and Problems, 131–198, Vol. 6 Developments in Precambrian Geology (Ed. B.F. Windley), Elsevier, Amsterdam. Beukes, N.J., Cairncross, B., 1991. A lithostratigraphic sedimentological reference profile for the late Archaean Mozaan Group, Pongola Sequence: application to sequence stratigraphy and correlation with the Witwatersrand Supergroup. S. Afr. J. Geol. 94, 44–69. Bolhar, R., Kamber, B.S., Moorbath, S., Fedo, C.M., Whitehouse, M.J., 2004. Characterisation of early Archaean chemical sediments by trace element signatures. Earth Planet. Sci. Lett. 222, 43–60. Broecker, W.S., Peng, T.H., 1982. Tracers in the Sea. Eldigio Press, New York. Byrne, R.H., Kim, K., 1990. Rare earth element scavenging in seawater. Geochim. Cosmochim. Acta 54, 2645–2656. Byron, C.L., Barton, J.M., 1990. The setting of mineralization in a portion of the Eersteling goldfield, Pietersburg granite–greenstone terrane, South Africa. S. Afr. J. Geol. 93, 463–472. Cantrell, K.J., Byrne, R.H., 1987. Rare earth element complexation by carbonate and oxalate ions. Geochim. Cosmochim. Acta 51, 597–605. Condie, K.C., 1993. Chemical composition and evolution of the upper continental crust: contrasting results from surface samples and shales. Chem. Geol. 104, 1–37. Danielson, A., Möller, P., Dulski, P., 1992. The europium anomalies in banded iron formations and the thermal history of the oceanic crust. Chem. Geol. 97, 89–100. DePaolo, D.J., Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249–252. Derry, L.A., Jacobsen, S.B., 1990. The chemical evolution of Precambrian seawater: Evidence from rare earth elements in banded iron formations. 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Holland, H.H., 1984. The Chemical Evolution of the Atmosphere and the Oceans. Princeton Univ. Press. Jacobsen, S.B., Pimentel-Klose, M.R., 1988a. A Nd isotopic study of the Hamersley and Michipicoten banded iron formations: the source of REE and Fe in Archean oceans. Earth Planet. Sci. Lett. 87, 29–44. Jacobsen, S.B., Pimentel-Klose, M.R., 1988b. Nd isotopic variations in Precambrian banded iron formations. Geophys. Res. Lett. 15, 393–396. Jeandel, C., Bishop, K., Zindler, A., 1995. Exchange of neodymium and its isotopes between seawater and small and large particles in the Sargasso Sea. Geochim. Cosmochim. Acta 59, 535–547. Jochum, K.P., Nohl, U., Herwig, K., Lammel, E., Stoll, B., Hofmann, A.W., 2005. GeoReM: A new geochemical database for reference materials and isotopic standards. Geostand. Geoanal. Res. 29, 333–338. Jones, M.G., 1990. The geology of the Mt Mare area, Pietersburg greenstone belt, South Africa. Ph.D. thesis, Imperial College, Univ. London. 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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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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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