A Continuing Enigma of Geology
Jelte P. Harnmeijer, University of Washington
Banded Iron Formations (‘BIFs’) are highly controversial chemical precipitates characterized by the presence of alternating layers of iron-rich and amorphous silica-rich layers. This dichotomous compositional layering is usually expressed on several scales at anygiven outcrop, from fine sub-millimetre-scale varve-like laminae to metre-scale bands. Even on a microscopic scale, the boundary between the ferruginous and siliceous layers is distinctly abrupt.
This paper sets out to explore the key aspects of BIFs pertinent to their role in the study of early earth evolution, and outline the major controversies surrounding their interpretation. The focusrests on BIF classification, temporal- and spatial-distribution, behavior in solution, depositional environment, and origin.
An understanding of the origin of BIFs provides useful insight into conditions existing on, and governing, the early lithosphere, hydrosphere and atmosphere. BIFs have proved particularly useful to studies of the early earth due to their intimate coupling with oxygenconcentration. Researchers have benefited tremendously from the narrow temporal-, depositional-, and tectonic- niches inhabited by BIFs in our geological record.
In the context of early earth evolution, it is the possible link to an evolving biosphere that makes BIFs particularly interesting. Iron is used as a metabolic agent by numerous microorganisms. Some of these, including specific species ofoxygenic- and anoxygenic- photoautotrophs and chemoferrotrophs, lend credit to the theory of BIF-deposition being, at least in part, a microbially mediated process. Direct evidence for a microbial role in Archaean BIF deposition remains elusive.
The dominant enigma surrounding BIFs is the spectacular alternating BIF banding. A distal hydrothermal source for iron is favored, while recent workutilizing Ge/Si ratios (Hamade et al., 2003) suggests a continental component in silica flux. Various mechanisms for primary Fe2+ oxidation are explored, including diffusive, photochemical and biological processes.
The favorable characteristics of hydrothermal settings as sites for the emergence of life are well recognized (eg., Russell and Hall, 1997). Likewise, the catalytic role that surfaces ofclay minerals may have played has much support (Cairns-Smith, 1982). Given their presence at the very beginning of the rock record, association with hydrothermal activity, diverse mineralogy, hydrous clay mineral content and probable biogenicity, I propose BIFs as a candidate-site for the emergence of life on Earth.
Table of Contents
1. Observations of BIFs in Australia, southern Africa 4
2.1. Literature Classification and Definition 5
2. BIF Distribution 7
1. Temporal Distribution 7
2. Spatial Distribution 9
3. BIF Chemistry and Metamorphism 10
4. The Behaviour of BIF Components in Solution12
4.1. Fe2+ and Fe3+ in Solution 12
2. Silica in Solution 12
5. Depositional Environment 13
6. Origin of Precambrian BIFs 15
1. Sources of Iron and Silica 15
2. Primary Oxidation 16
1. Near-surface Oxidation 16
2. Photochemical Processes 17
3. Biological Processes 174. Tectonic Origin 21
7. Conclusions 22
iron formation a chemical sedimentary rock, typically thin-bedded and/or finely laminated, containing at least 15% iron of sedimentary origin, and commonly but not necessarily containing layers of chert
- American Geological Institute Glossary of Geology (after...