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Geology; August 2007; v. 35; no. 8; p. 719-722; DOI: 10.1130/G23582A.1
© 2007 Geological Society of America
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Microscale heterogeneity of Fe isotopes in >3.71 Ga banded iron formation from the Isua Greenstone Belt, southwest Greenland

Martin J. Whitehouse*,1 and Christopher M. Fedo*,2

1 Laboratory for Isotope Geology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
2 Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA

We present Fe isotope compositions for magnetite crystals from >3.7 Ga banded iron formation (BIF) from the Isua Greenstone Belt and younger, spatially related pyrite obtained in situ by secondary ion mass spectrometry (SIMS). Across a distance of several millimeters, individual magnetite crystals of BIF show {delta}56Fe values up to 2{per thousand}, and within single magnetite-rich layers, there is a >2{per thousand} range in {delta}56Fe. The highest positive {delta}56Fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric Fe in aqueous solution at a likely Early Archean ocean temperature of ~70 °C, and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal. These high values also imply that the supply of oxidant was limited. Heterogeneity in {delta}56Fe values is attributed to diagenetic reactions occurring in limited-volume pore waters, isolated from the bulk ocean ({delta}56Fe = 0{per thousand}), with development of isotope reservoir effects. Secondary pyrite in BIF and a conglomerate also show a distinct enrichment in heavy Fe isotopes, interpreted as redistribution and/or fractionation during postformational events.

Key Words: Isua Greenstone Belt • BIF • Fe isotopes • secondary ion mass spectrometry • Archean • paleo-oceanography






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