Banded Iron Formation (BIF)
Published in Science, 26 september 2008, Vol 321, pages 1828-1831
Collected by the team of the Earth and Planetary Science Department of the McGill University, Montreal, Quebec, Canada, same used in the study !
Specimen is a 1.44g slice
The oldest direct evidence of life on earth !!!!
Although iron formations are generally thought to represent chemical precipitates produced by marine exhalations (Graf, 1978; Uitterdijk Appel, 1983; Gross, 1983; Jacobsen and Pimentel-Klose, 1988; Olivarez and Owen, 1991), the causes of Fe precipitation throughout geologic time are less well understood. The Proterozoic Superior-type banded Fe-formations are believed to be formed by oxidation on shallow continental shelves of Fe2+ rising from deep ocean basins. This scenario cannot, however, explain the Algoma-type Fe-formations that typify earlier Archean greenstone belts, because they would have been precipitated under anoxic conditions (Holland, 1994, 2005). It has recently been discovered that Fe2+ can be directly oxidized by microbial activity under anaerobic conditions (Lovley et al., 1987; Widdel et al., 1993). Indeed, Konhauser et al. (2002) have proposed that Algoma-type banded Fe-formations were precipitated by the activity of anoxic bacteria in the Archean. The origin of life on Earth and its evolution over time has been highly debated in past years (e.g., Mojzsis et al., 1996; Fedo and Whitehouse, 2002a, 2002b; Mojzsis and Harrisson, 2002b; Lepland et al., 2005; Moorbath, 2005), and establishing the chemical sedimentary origin for a rock is a prerequisite to demonstrating potential biological activity and addressing the controversial question of the timing of the origin of life on Earth.
The concave-up depleted LREE profile of the Nuvvuagittuq BIF, combined with its flat HREE profile and positive Eu and Y anomalies with respect to PAAS, are characteristic features of Archean Algoma-type banded Fe-formations of marine exhalite origin (Fryer, 1977; Graf Jr., 1978; Fryer et al., 1979; Jacobsen and Pimentel-Klose, 1988; Fig. 3.4-9). BIF from the Nuvvuagittuq belt has heavier Fe isotopic compositions than the surrounding igneous lithologies (Fig. 3.4-10), a feature that is also consistent with an origin as a chemical precipitate. The Fe isotopic compositions of the BIF from the Nuvvuagittuq belt are similar to those in the Akilia BIF (SW Greenland), which are also enriched in heavy Fe isotopes (0.1 to 0.5‰/amu) relative to their surrounding igneous lithologies (Dauphas et al., 2004b), suggesting a common depositional process for both these example of Eoarchean BIF. Although the Fe isotopic enrichment observed in the amphibolites directly adjacent to the banded Fe-formation suggest some degree of local exchange, the fact that such diverse meta-igneous lithologies, including the fauxamphibolites that are interpreted to have been altered, the amphibolitic sill margins, the gabbro and ultramafic sills, as well as the tonalite and the felsic bands, all share the same Fe isotopic composition suggests that the heavy Fe isotopic enrichment displayed by the Nuvvuagittuq BIF is not a metasomatic or alteration feature. The REE+Y profiles and the Fe isotopic compositions of Nuvvuagittuq’s BIF confirm their origin as marine exhalites and, although the mechanism(s) responsible are not well understood, the Fe isotopic fractionation observed in the BIF of Nuvvuagittuq and Akilia raises the possibility that life was already established on Earth at 3.8 Ga.