• Number 300  |
• November 23, 2009

Why bands of iron formed billions of years ago

No one knows why massive bands of iron—some hundreds of kilometers long—mysteriously began precipitating on Earth’s surface about 3.5 billion years ago. Or why, almost 2 billion years later, the precipitation stopped.

Earlier attempts to explain their existence could not satisfactorily explain all the observations made by geologists, particularly the existence of alternating structural bands of silica-rich layers with iron-rich layers in these deposits. However, in an October issue of Nature Geoscience, researchers from DOE's Sandia National Laboratories and elsewhere have proposed a new approach. A key component of the process, found using computer simulations, may have been the absence of aluminum in early oceanic rocks, which chemically favored the banded iron formations. The continual enrichment of oceanic crust by aluminum as Earth evolved ultimately ended the era of iron band formation.

The researchers suggest that iron- and silicon-rich fluids were generated by hydrothermal action on the seafloor. Their calculations show that the formation of bands was generated by internal interactions of the chemical system, rather than from external forcing by unexplained changes such as ocean surface temperature variations.  “This concept of the self-organizational origin of banded iron formations is very important,” says Sandia principal investigator Yifeng Wang. “It allows us to explain a lot of things about them, like their occurrence and band thickness.” Wang coauthored the paper with Enrique Merino, Indiana University, and Huifang Xu and Hironomi Konishi, of the University of Wisconsin, Madison.

[Julie Hall, 505.284.7761,
jchall@sandia.gov]