Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 151–169 www.elsevier.com/locate/gca Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis Clark M. Johnson a,*, Brian L. Beard a, Cornelis Klein b, Nic J. Beukes c, Eric E. Roden a a Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, USA b Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA c Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa Received 29 May 2007; accepted in revised form 18 October 2007; available online 26 October 2007 Abstract 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 d56Fe 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 pre- cursors to magnetite occurred through several diagenetic processes that produced a range of d56Fe values: (1) addition of mar- 2þ ine hydrothermal Fe aq, (2) complete reduction by bacterial dissimilatory iron reduction (DIR), and (3) interaction with 2þ 56 56 excess Fe aq that had low d Fe values and was produced by DIR. Most siderite has slightly negative d Fe values of À0.5‰ that indicate equilibrium with Late Archean seawater, although some very negative d56Fe 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 clas- tic 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 d56Fe 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. Ó 2007 Elsevier Ltd. All rights reserved. 1. INTRODUCTION than those of the modern iron cycle (e.g., Trendall, 2002; Klein, 2005). The large inventory of Fe3+-bearing oxides Banded iron formations (BIFs) have played a prominent (magnetite, hematite) requires an oxidant during BIF gene- role in discussions on the surface environments of the an- sis, given the fact that all proposed iron sources (riverine, cient Earth because their origin requires redox conditions marine hydrothermal) were Fe2+. Contrary to common be- and iron transport pathways that were markedly different lief, however, hematite or goethite are not the major Fe-bearing phases in fresh BIFs that are absent the effects of weathering, supergene alteration, mineralization, or 2+ * Corresponding author. Fax: +1 608 262 0693. metamorphism (Simonson, 2003). Instead, Fe -bearing E-mail address: [email protected] (C.M. Johnson). minerals such as magnetite and siderite are the major 0016-7037/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.10.013 152 C.M. Johnson et al. / Geochimica et Cosmochimica Acta 72 (2008) 151–169 repositories of Fe in BIFs, reflecting the fact that the that were involved (Fig. 1). Iron isotope compositions are average oxidation state of iron in BIFs is Fe2.4+ (e.g., expressed in terms of 56Fe/54Fe ratios in units of per mil Klein and Beukes, 1992). The sources and pathways for (‰), relative to the average of igneous rocks: Fe2+, therefore, are as important to models of BIF genesis 2+ 56 56 54 56 54 3 as the oxidative pathways. The source of aqueous Fe d Fe = ( Fe/ FeSample/ Fe/ FeIg=Rxs À 1) 10 ð2Þ has been commonly argued to be marine hydrothermal (Beard et al., 2003a). On this scale, the d56Fe value of the systems, largely based on rare earth element (REE) data IRMM-014 standard is À0.09‰ (Beard et al., 2003a). Iso- (e.g., Bau et al., 1997). REE data, however, do not directly topic fractionations between two components, A and B, constrain the Fe sources, nor the iron pathways involved are described by: in BIFs genesis. The role of bacteria in BIF genesis has been extensively 56 56 56 3 D FeA B ¼ d FeA À d FeB 10 ln aA B ð3Þ discussed because the period in which the major BIFs were À À deposited (2.8 to 1.8 Ga) overlaps major changes that are where aAÀB is the isotopic fractionation factor, following thought to have occurred in the biosphere and atmosphere standard practice. (Knoll, 2003; Canfield, 2005). Indirect roles for bacteria The very large quantities of Fe that are required for BIF have been proposed, where Fe2+ oxidation occurred deposition are generally thought to have been originally sup- 2þ through elevated atmospheric O2 produced by oxygenic plied by marine hydrothermal sources as Fe aq (pathway 1, 2þ 3þ photosynthesis (Cloud, 1968). A more direct role for bacte- Fig. 1). During oxidation of Fe aq to Fe aq, followed by 2+ 3þ ria might have involved Fe oxidation that was metaboli- precipitation of Fe aq to ferric oxide/hydroxide, the net iso- cally coupled to reduction of CO2 (Konhauser et al., 2002; topic fractionation between ferric oxide/hydroxide and ini- 2þ Kappler et al., 2005): tial Fe aq is +1.5‰ at room temperature (pathway 2, Fig. 1), reflecting a +2.9‰ Fe3þ –Fe2þ equilibrium frac- 4Fe2þ +7HO+CO CH O + 4FeOOH + 8Hþ 1 aq aq 2 2 ! 2 ð Þ tionation and a À1.4‰ kinetic fractionation between fer- 3þ The ubiquitous magnetite and siderite in Late Archean ric oxide/hydroxide and Fe aq upon precipitation; the to Early Proterozoic BIFs that have not been subjected to later fractionation may vary on the order of 1‰, depend- ore-forming processes or significant metamorphism are ing upon precipitation kinetics (Beard and Johnson, 2004). 2þ common end products of dissimilatory iron reduction The overall ferric oxide/hydroxide–Fe aq fractionation ap- (DIR) of ferric oxide/hydroxides by bacteria (Lovley pears to be similar regardless of the oxidative pathway in- et al., 1987), and a reductive role for bacteria in BIF genesis volved, including abiologic oxidation by O2 (Bullen et al., 2+ has been proposed (Nealson and Myers, 1990; Konhauser 2001), anaerobic photosynthetic Fe oxidation (Croal et al., 2005), which may be represented as the reverse of et al., 2004), oxidation by acidophilic iron-oxidizing bacteria Eq. (1). Despite great speculation in the literature regarding (Balci et al., 2006), or UV-photo oxidation (Staton et al., the potential role of bacteria in BIF formation, clear evi- 2006), where most of these fractionations measured in exper- dence for bacterial processes in BIF genesis is essentially iments vary between +1.0‰ and +2.0‰. Ferric oxide/ non-existent (Klein, 2005). hydroxides that are formed by only a few % oxidation and Here, we report new iron isotope data for the 2.50– precipitation record the maximum fractionation, and hence 56 2.45 Ga Brockman Iron Formation of the Hamersley Ba- will have d Fe values that are 1.5‰ higher than those of 2þ sin, Western Australia, one of the best preserved and most the initial Fe aq. In contrast, complete oxidation and pre- intensively studied BIFs in the world (Trendall and Block- cipitation will produce ferric oxide/hydroxides that have 56 2þ ley, 1970; Trendall et al., 2004). We compare these new data d Fe values equal to those of the initial Fe aq, despite a sig- 2þ 2þ to our previous work on the Kuruman and Griquatown nificant FeðOHÞ3–Fe aq or Fe2O3–Fe aq fractionation fac- 56 iron formations of the Transvaal Craton, South Africa tor. The d Fe values of the flux of ferric oxide/hydroxides (Johnson et al., 2003), which were deposited synchronously from the upper ocean layer to the seafloor (pathway 3, with the Brockman Iron Formation (Pickard, 2003). These Fig. 1) will therefore be determined by the extent and rate results significantly expand the Fe isotope database on of oxidation and precipitation that occurs in the upper part BIFs, which includes additional studies (Dauphas et al., of the water column. 2004, 2007; Rouxel et al., 2005; Frost et al., 2006; Valaas- Transport of the ferric oxide/hydroxide ‘‘rain’’ into 2þ Hyslop et al., in press; Whitehouse and Fedo, 2007). Cou- Fe aq-rich fluids in the deep marine anoxic layer produces pled with a growing database of experimentally determined opportunities for reactions that may produce important Fe isotope fractionation factors, the results presented here BIF minerals, including magnetite, via reactions such as: provide new constraints on the Fe pathways that were in- 2þ þ volved in BIF formation, including evaluation of the role 2FeðOHÞ3 þ Feaq ! Fe3O4 þ 2H2O þ 2H ð4Þ of bacteria in BIF genesis. (pathway 4, Fig. 1). The Fe isotope composition of magne- tite in the above reaction may, for example, reflect simple 2.