School of Engineering and Science - Jacobs University

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Geochimica et Cosmochimica Acta 72 (2008) 378–394
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Continentally-derived solutes in shallow Archean seawater:
Rare earth element and Nd isotope evidence in iron
formation from the 2.9 Ga Pongola Supergroup, South Africa
Brian W. Alexander a, *, Michael Bau a , Per Andersson b , Peter Dulski c
a Earth and Space Sciences, Campus Ring 1, Research III 102, Jacobs University Bremen, D-28759 Bremen, Germany
b Laboratory for Isotope Geology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
c GeoForschungsZentrum, D-14473 Potsdam, Germany
Received 27 November 2006; accepted in revised form 25 October 2007; available online 17 November 2007
Abstract
The chemical composition of surface water in the photic zone of the Precambrian ocean is almost exclusively known from
studies of stromatolitic carbonates, while banded iron formations (IFs) have provided information on the composition of deeper
waters. Here we discuss the trace element and Nd isotope geochemistry of very shallow-water IF from the Pongola Supergroup,
South Africa, to gain a better understanding of solute sources to Mesoarchean shallow coastal seawater. The Pongola
Supergroup formed on the stable margin of the Kaapvaal craton 2.9 Ga ago and contains banded iron formations (IFs) that
represent the oldest documented Superior-type iron formations. The IFs are near-shore, pure chemical sediments, and shalenormalized
rare earth and yttrium distributions (REY SN ) exhibit positive La SN ,Gd SN , and Y SN anomalies, which are typical
features of marine waters throughout the Archean and Proterozoic. The marine origin of these samples is further supported
by super-chondritic Y/Ho ratios (average Y/Ho = 42). Relative to older Isua IFs (3.7 Ga) from Greenland, and younger
Kuruman IFs (2.5 Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and
appear to record variations in solute fluxes related to sea level rise and fall. Sm–Nd isotopes were used to identify potential
sediment and solute sources within pongola shales and IFs. The Nd (t) for Pongola shales ranges from 2.7 to 4.2, and Nd (t)
values for the coeval iron-formation samples (range 1.9 to 4.3) are generally indistinguishable from those of the shales,
although two IF samples display Nd (t) as low as 8.1 and 10.9. The similarity in Nd isotope signatures between the shale
and iron-formation suggests that mantle-derived REY were not a significant Nd source within the Pongola depositional environment,
though the presence of positive Eu anomalies in the IF samples indicates that high-T hydrothermal input did contribute
to their REY signature. Isotopic mass balance calculations indicate that most (P72%) of the Nd in these seawater
precipitates was derived from continental sources. If previous models of Fe–Nd distributions in Archean IFs are applied, then
the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally
considered.
Ó 2007 Elsevier Ltd. All rights reserved.
1. INTRODUCTION
* Corresponding author.
E-mail address: b.alexander@jacobs-university.de (B.W.
Alexander).
Studies to determine the characteristics of seawater in
the geologic past are dependent upon proxies which reliably
reflect seawater composition. Such studies have
examined sedimentary rocks and minerals which form directly
from seawater solutions, including inorganic precipitates
(e.g., evaporites) or biologically mediated
precipitates (e.g., CaCO 3 ). Attempts to identify reliable
proxies for paleo-seawater rely on chemical tracers, particularly
elements with known and predictable behavior
in modern seawater and modern seawater precipitates.
0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.gca.2007.10.028