February 1, 2011

Learning from the gentry: Banded Iron Formations

I've only ever seen one of these, when vacationing around Lake Vermilion in Minnesota in my late teen years. At the time I had no idea the origins of the rock nor the processes that went into making the multibanded swirls. Luckily I found an old photo taken around a shore of the lake during a respite when my family was canoeing.
BIF outcrop on Lake Vermilion, Minnesota

The outcrop I happened to stop at shows a Banded Iron Formation that's quite deformed, with an identifiable fault, folding, and a series of what appears to be aplite cross-cuts that, if I recall correctly, were a few cm's thick. This was before the days I knew to put an object in the frame to define scale.

I have a novice understanding of BIF's, but novice doesn't cut it anymore, especially in the latter half of my degree pursuit. So, armed with more experience and resources, I delved into a deeper understanding of these interesting Precambrian rocks. Hopefully it helps for my Structural Geology course, and takes me from BIF novice - BIF intermediate.

Banded Iron formations are a unique rock strata that is formed of alternating bands of iron-rich layers interbedded with iron poor silica layers of mostly chert. The amorphous silica is typically entrapped by iron oxide laminae to form bands of chert. The bands always alternate, and have minimal comingling of particles/minerals at the boundaries. Studies have found that the boundary between the iron-rich layers and the iron-poor silica layers is distinctly abrupt, even on a micrometer level. Each band can be millimeters thin to meters thick.

The mineral content of the iron-rich layers have hematite and/or magnetite and/or siderite, whilst the iron-poor silica layers can have varying mineral content depending. This diverse mineral makeup of the siliceous bands, coupled with an additionally diverse array of trace & guide fossils, has spurred plenty of research into the origins of individual BIF's....whether the silica content is derived from continental erosion/deposition or oceanic currents or a dynamic mix of the two. Most Banded Iron Formations are dated from 3.8Ga (early Archaean) - 1.8Ga (late Paleoproterozoic). That's plenty of time on Earth, and thus most are of a highly deformed variety (discussed below). There is a widespread connection of deformed BIF's to the major greenstone belts of the world, thus establishing a likely mafic origin of their parent rocks. Because of their age, BIF's are found mostly in continental cratons, and all continents have one:
  • In North America, the aforementioned Vermilion area (47° 49.679'N 92° 14.358'W)
  • In South America, the Guyana shield near Port Kaituma (7° 39.824'N 59° 51.393'W)
  • In Europe, the Voronezh Massif of the Ukranian shield (49° 16.685'N 30° 20.518'E)
  • In Africa, the Liberian shield (6° 48.427'N 10° 18.602'W)
  • In Asia, the state of Karnataka on the Indian craton (15° 3.781'N 76° 35.442'E)
  • In Australia, the Yilgarn craton of Western Australia (26° 35.627'S 118° 29.693'E)
  • In Antarctica, exposures of Mount Rucker on East Antarctic craton (78°11.000′S 162°32.000′E)
The oldest one (3.8 Ga) is found near Isua, Greenland (60° 20.788'N 45° 26.724'W).
Various Precambrian shields of Earth
Most studies conclude that the source of iron for the formations is from hydrothermal vents (black smokers). In a cyclical mechanism, iron was scavenged from the oceanic crust and re-deposited on the ocean floor by hydrothermal fluids. High-temperature hydrothermal alteration of early Archean oceanic crust played an important role in the deposition of the formations. Also playing a role in the construction of the alternating bands was the buildup of siliceous material near the black smokers, which eventually led to submarine landslides that covered the iron-rich layers. Those layers were evened out with the assistance of turbidity and deep ocean density currents, as seen in the following diagram:
Hypothesized steps to achieve alternating bands of iron-rich & iron-poor layers
Favored depositional environments for this style of Banded Iron formation mechanism include island arc basins on flanks, back arc basins of convergent plate boundaries, and rifting zones that developed within Archaean cratons. Thus deposition occurred in a shallow marine environment under transgressing/regressing seas, and even possibly on continental shelves of passive margins. 

Just like every facet of any geological discipline, there is categorization that has emerged for Banded Iron Formations. The two different classes, derived in 1983, are of course named after where they were first studied in some detail. First we have the Superior-type sequences; not hard to figure out where those were discovered and what craton they belong to. They are thought to be deposited in shallow marine environments as largely granular & oolitic iron-formations grading into laminated iron. Deposition was primarily in adjacent basins surrounding the perimeter of uplands that projected above sea level, and seen today the type is relatively unmetamorphosed and undeformed, with widespread uncovered outcrops alongside such members as conglomerate and quartzite rocks. That is indicative of environments that were high energy, but that somewhat buffered the shield BIF's deposited in basins from marginal deformation. Secondly, we have the Algoma-type sequences, named after another part of the Canadian shield not far away from Superior (northern Ontario), and deposited in deep marine environments. Algoma BIF's are highly metamorphosed and deformed, and considered to have parent rocks that were volcanic in origin, thus they reside in extensive greenstone belts where other volcanic entities (ex. pillow lava) are adjacent to them, and their outcrops are more discontinuous and klipped relative to the Superior class.

That was all the nitty-gritty foundational details of Banded Iron Formations, however the genesis of their unique thin bands is apparently one of the more highly debated topics in current paleogeology. Looking through online journal sources, there seems to be repetition of two major hypotheses as to the precipitation of the iron-rich layers. One approach to precipitation of free iron produced by black smokers involves a downward diffusion of CO2 that intercepts the iron upwelling from deep waters, resulting in a chemical reaction between the compounds and thus preventing iron from reaching shallow waters. This posits diffusion of CO2 as an even process in the Archaean/Proterozoic Earth, or at least for the time given for creation of banded iron formations, so that we observe the iron-rich bands as an expression of a downward diffusion of a finite volume of CO2 interacting with the iron; following that is either the quiescence of hydrothermal vent activity or heightened depositional activity from terrigenous sources or a combination of the two.

An emerging hypothesis involves anoxygenic phototrophic organisms living beneath the ocean's windmixed surface layer, and these organisms precipitated the free iron provided by black smokers into iron oxide, an explanation for BIF deposition in a stratified ancient ocean at a constricted depth of a few hundred meters. One of the links in the additional info leads to a paper where geochemical analyses has derived a thickness of the layer where these phototrophs must live to both stay below cyanobacteria above and still get enough sunlight penetration. These phototrophs living below a mixed layer inhabited by cyanobacteria could have been responsible for the absence of iron in shallow waters and for Precambrian BIF deposition in a stratified ancient ocean. Take a look:
diagram of layers involved in precipitation of Fe2O3 by anoxygenic phototrophs
The phototrophs work hard when the hydrothermal vents pump out the free iron, and thus the paper considers the alternating bands evidence of either hydrothermal quiescence or a seasonality mechanism in the early Earth atmosphere. 

The several papers I've read tend to focus highly on the quantitative geochemical elements of Banded Iron Formations, and are lacking in observational geological elements. Essentially the linking between the two facets is limited so far, and such a key connection is hopefully what future studies will focus on. As far as I'm concerned, I simply wish to have more first-hand experience with BIF's, but unfortunately for the time being I reside in a geologically young and active area, which is the polar opposite of the geologically old and inactive areas where you find these beautiful swirling rocks.

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