Understanding Redox Processes in Surface Environments from Iron Oxide Transformations and Multiple Sulfur Isotope Fractionations

Open Access
Author:
Otake, Tsubasa
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 04, 2008
Committee Members:
  • Hiroshi Ohmoto, Committee Chair
  • Hubert Lloyd Barnes, Committee Member
  • Susan Louise Brantley, Committee Member
  • Peter J Heaney, Committee Member
  • Kwadwo Osseo Asare, Committee Member
  • David J Wesolowski, Committee Member
Keywords:
  • mass-independent fractionation
  • sulfur isotope
  • banded iron formation
  • transformation
  • iron oxide
  • ab initio calculation
Abstract:
The ultimate goal of this thesis was to increase our understanding of the evolution of the atmosphere, hydrosphere, biosphere, and thermal structure of early Earth. I approached this goal by evaluating the origins of magnetite and hematite in Banded Iron Formations (BIFs), associated iron ores, and mass-independently-fractionated sulfur isotopes (MIF-S) in Precambrian sedimentary rocks. The major deposition of BIFs before 1.9 Ga has been thought to reflect a period of globally anoxic oceans, at least the deep ocean. This theory springs from the conventional model which requires the transport of Fe2+ from the deep ocean to the continental shelf. Researchers have proposed that magnetite, the most common Fe-bearing mineral in typical oxide-type BIFs (e.g., Algoma-type), was transformed from hematite by a redox reaction with organic carbon or siderite during metamorphism. However, another process that can transform hematite to magnetite may be a non-redox reaction with Fe2+-rich hydrothermal fluids. I have experimentally investigated the transformation mechanisms of iron oxides under hydrothermal conditions. Magnetite or hematite was reacted with an acid under H2-rich hydrothermal conditions (100-250ºC). Non-redox transformations between magnetite and hematite: Fe3O4(mt) + 2H+(aq) &#8596; Fe2O3(hm) + Fe2+(aq) + H2O (1), occurred rapidly in both direactions, and much more efficiently than redox transformations under low temperature hydrothermal conditions (e.g., 150ºC). Non-redox transformation of hematite to magnetite may have been responsible for the transformation of primary hematite to magnetite in BIFs that were poor in reductants (e.g., organic matter). Because non-redox transformation of hematite to magnetite, which increases molar volume, is most likely to occur during early diagenesis, the Fe source for the BIFs would have been proximal to the depositional sites. The transformation further suggests that formation of BIFs was a local phenomenon, rather than the long distance transport of Fe from the deep ocean to continental shelves through a global anoxic ocean. This experimental study also indicates that the redox transformation of hematite to magnetite by hydrogen can be important even at 150ºC under certain conditions (e.g., pH, presence of catalyst). Both of the transformation mechanisms may have been important in many oxide-type BIFs that are associated with volcanic rocks, are not subjected to high-grade metamorphism, and have very low organic carbon content (<0.01%). The world’s major iron ores are high-grade hematite iron ores developed from low-grade BIFs through transformation of magnetite to hematite. In all previous models, molecular oxygen from the atmosphere has been considered to be the oxidant. Therefore, such ore formation has been linked to the oxygenation of the atmosphere. However, I have experimentally demonstrated that magnetite is transformed to hematite by a reaction with an acidic fluid under hydrothermal conditions. Because this transformation process facilitates water-rock interactions by increasing porosity of rocks, it is more efficient than oxidation of magnetite by O2-bearing fluids. The results suggest that the occurrence of high-grade hematite iron ores is not evidence for free oxygen, and that the formation of hematite-rich iron ores developed from BIFs may have occurred throughout geologic time. Since the discovery of MIF-S in many sedimentary rocks older than 2.45 Ga, the MIF-S record of such rocks has been cited by many researchers as unequivocal evidence for a dramatic change from an anoxic to oxic atmosphere around 2.4 Ga. However, multiple sulfur isotope data of these natural samples have been interpreted under the following two premises. The first premise is that the equilibrium fractionation of sulfur isotopes in nature always conforms to the Bigeleisen-Mayer approximations. The second premise is that UV photolysis of SO2 in an O2-poor atmosphere is the only cause for MIF-S in nature. I have theoretically evaluated the two premises using ab initio methods. The results demonstrate that mass-dependent relationships (i.e., fractionation factor ratios) during equilibrium reactions vary for S-species couples such as SO42-(aq) - H2S (e.g., 0.505 – 0517 for value), and that MIF-S signatures may be produced during adsorption, particularly at high temperatures (e.g., 200ºC). These results suggest that MIF-S signatures in Archean sedimentary rocks may have been created by heterogeneous reactions between organic matter and S-bearing aqueous solution under hydrothermal conditions, rather than by UV photolysis of volcanic SO2 in an O2-poor atmosphere. Studies in this thesis indicate that conventional models for the deposition of BIFs, formation of hematite-rich iron ores, and production of MIF-S may not be correct. Thus, their temporal distributions are not unequivocal evidence for the oxygenation of atmosphere. Both of the studies on iron oxide transformations and multiple S isotope fractionations illuminate the significance of hydrothermal activity in the genesis of BIFs and MIF-S signatures in sedimentary rocks. On a broader scale, it outlines the influence such activity had on global geochemical cycles and redox-sensitive elements on early Earth. The abundance of magnetite-rich BIFs in the Archean and early Proterozoic sequences may have been the result of higher heat flux from the interior to the surface of the early Earth. MIF-S signatures found in some Archean sedimentary rocks may have been the result of large-scale hydrothermal activity in sedimentary basins where abundant organic matter accumulated.