Bands of Iron Ore and Chert
The strikingly banded rocks scattered across the upper Midwest and elsewhere throughout the
world are actually ambassadors from the past, offering clues to the environment of the
early Earth more than 2 billion years ago.
Called banded iron formations or BIFs, these ancient rocks formed between 3.8 and 1.7 billion
years ago at what was then the bottom of the ocean. The stripes represent alternating layers
of silica-rich chert and iron-rich minerals like hematite and magnetite.
A Source of Iron and Environmental Information
First mined as a major iron source for modern industrialization, BIFs are also a rich source
of information about the geochemical conditions that existed on Earth when the rocks were
made. However, interpreting their clues requires understanding how the bands formed, a topic
that has been controversial for decades, says Huifang Xu, a geology professor at UW-Madison.
A study in Nature Geoscience offers a new picture of how these colorful bands developed and
what they reveal about the composition of the early ocean floor, seawater, and atmosphere
during the evolution of the Earth.
Previous Formation Hypotheses
Previous hypotheses about band formation involved seasonal fluctuations, temperature shifts,
or periodic blooms of microorganisms, all of which left many open questions about how BIFs
dominated the global marine landscape for two billion years and why they abruptly disappeared
1.7 billion years ago.
A New Model for BIF Deposition
With Yifeng Wang of Sandia National Laboratories, Enrique Merino of Indiana University and
UW-Madison postdoc Hiromi Konishi, Xu developed a BIF formation model that offers a more
complete picture of the environment at the time, including interactions between rocks, water,
"They are all connected," Xu explains. "The lithosphere affects the hydrosphere, the hydrosphere
affects the atmosphere, and all those eventually affect the biosphere on the early Earth."
Their model shows how BIFs could have formed when hydrothermal fluids, from interactions between
seawater and hot oceanic crust from deep in the Earth's mantle, mixed with surface seawater.
This mixing triggered the oscillating production of iron- and silica-rich minerals, which were
deposited in layers on the seafloor.
They used a series of thermodynamic calculations to determine that the source material for BIFs
must have come from oceanic rocks with a very low aluminum content, unlike modern oceanic
basalts that contain high levels of aluminum.
"The modern-day ocean floor is basalt, common black basalt like the Hawaiian islands. But
during that time, there was also a strange kind of rock called komatiites," says Xu. "When
ocean water reacts with that kind of rock, it can produce about equal amounts of iron and
silica" - a composition ideally suited to making BIFs.
Such a mixture can create distinct alternating layers - which range in thickness from 10
micrometers to about 1 centimeter - due to a constantly shifting state that, like a
competition between two well-matched players, resists resolving to a single outcome and
instead see-saws between two extremes.
The Age of Iron Deposition
BIFs dominated the global oceans 3.8 to 1.7 billion years ago, a time period known to
geologists as the Archaean-Early Proterozoic, then abruptly disappeared from the geologic
record. Their absence in more recent rocks indicates that the geochemical conditions
changed around 1.7 billion years ago, Xu says.
This change likely had wide-ranging effects on the physical and biological composition
of the Earth. For example, the end of BIF deposition would have starved iron-dependent
bacteria and shifted in favor of microbes with sulfur-based metabolisms. In addition,
chemical and pH changes in the ocean and rising atmospheric oxygen may have allowed the
emergence and spread of oxygen-dependent organisms.
The new study was partly funded by the NASA Astrobiology Institute, and Xu hopes to look for
biosignatures trapped in the rock bands for additional clues to the changes that occurred
1.7 billion years ago and what may have triggered them.
Additional support was provided by the National Science Foundation and the U.S. Department of Energy.
|Banded Iron Formation: Close-up of a banded iron formation. In this specimen bands of hematite (silver) alternate with bands of jasper (red). This photo spans an area of rock about eight inches wide. Photo taken by André Karwath, GNU Free Documentation License.
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