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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
earth and planetary sciences
Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis
Geochimica et Cosmochimica Acta, Volume 72, No. 1, Year 2008
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Description
The voluminous 2.5 Ga banded iron formations (BIFs) from the Hamersley Basin (Australia) and Transvaal Craton (South Africa) record an extensive period of Fe redox cycling. The major Fe-bearing minerals in the Hamersley-Transvaal BIFs, magnetite and siderite, did not form in Fe isotope equilibrium, but instead reflect distinct formation pathways. The near-zero average δ56Fe values for magnetite record a strong inheritance from Fe3+ oxide/hydroxide precursors that formed in the upper water column through complete or near-complete oxidation. Transformation of the Fe3+ oxide/hydroxide precursors to magnetite occurred through several diagenetic processes that produced a range of δ56Fe values: (1) addition of marine hydrothermal Fe2 +aq, (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with excess Fe2 +aq that had low δ56Fe values and was produced by DIR. Most siderite has slightly negative δ56Fe values of ∼ -0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative δ56Fe values may record DIR. Support for an important role of DIR in siderite formation in BIFs comes from previously published C isotope data on siderite, which may be explained as a mixture of C from bacterial and seawater sources. Several factors likely contributed to the important role that DIR played in BIF formation, including high rates of ferric oxide/hydroxide formation in the upper water column, delivery of organic carbon produced by photosynthesis, and low clastic input. We infer that DIR-driven Fe redox cycling was much more important at this time than in modern marine systems. The low pyrite contents of magnetite- and siderite-facies BIFs suggests that bacterial sulfate reduction was minor, at least in the environments of BIF formation, and the absence of sulfide was important in preserving magnetite and siderite in the BIFs, minerals that are poorly preserved in the modern marine record. The paucity of negative δ56Fe values in older (Early Archean) and younger (Early Proterozoic) BIFs suggests that the extensive 2.5 Ga Hamersley-Transvaal BIFs may record a period of maximum expansion of DIR in Earth's history. © 2007 Elsevier Ltd. All rights reserved.
Authors & Co-Authors
Johnson, Clark M.
United States, Madison
University of Wisconsin-madison
Beard, Brian L.
United States, Madison
University of Wisconsin-madison
Klein, Cornelis
United States, Albuquerque
The University of new Mexico
Beukes, Nicholas Johannes
South Africa, Johannesburg
University of Johannesburg
Roden, Eric E.
United States, Madison
University of Wisconsin-madison
Statistics
Citations: 289
Authors: 5
Affiliations: 3
Identifiers
Doi:
10.1016/j.gca.2007.10.013
ISSN:
00167037
Research Areas
Environmental
Study Locations
South Africa