Ancient geochemical cycling in the Earth as inferred from Fe isotope studies of banded iron formations from the Transvaal Craton

Variations in the isotopic composition of Fe in Late Archean to Early Proterozoic Banded Iron Formations (BIFs) from the Transvaal Supergroup, South Africa, span nearly the entire range yet measured on Earth, from -2.5 to + 1.0parts per thousand in Fe-56/Fe-54 ratios relative to the bulk Earth. With a current state-of-the-art precision of +/-0.05parts per thousand, for the Fe-56/Fe-54 ratio, this range is 70 times analytical error, demonstrating that significant Fe isotope variations can be preserved in ancient rocks. Significant variation in Fe isotope compositions of rocks and minerals appears to be restricted to chemically precipitated sediments, and the range measured for BlFs stands in marked contrast to the isotopic homogeneity of igneous rocks, which have delta(56)Fe= 0.00 +/- 0.05%., as well as the majority of modern loess, aerosols, riverine loads, marine sediments, and Proterozoic shales. The Fe isotope compositions of hematite, magnetite, Fe carbonate, and pyrite measured in BlFs appears to reflect a combination of (1) mineral-specific equilibrium isotope fractionation, (2) variations in the isotope compositions of the fluids from which they were precipitated, and (3) the effects of metabolic processing of Fe by bacteria. For minerals that may have been in isotopic equilibrium during initial precipitation or early diagenesis, the relative order of delta(56)Fe values appears to decrease in the order magnetite > siderite > ankerite, similar to that estimated from spectroscopic data, although the measured isotopic differences are much smaller than those predicted at low temperature. In combination with on-going experimental determinations of equilibrium Fe isotope fractionation factors, the data for BIF minerals place additional constraints on the equilibrium Fe isotope fractionation factors for the system Fe(III)-Fe(II)-hematite-magnetite-Fe carbonate. delta(56)Fe values for pyrite are the lowest yet measured for natural minerals, and stand in marked contrast to the high delta(56)Fe values that are predicted from spectroscopic data. Some samples contain hematite and magnetite and have positive delta(56)Fe values; these seem best explained through production of high Fe-56/Fe-54 reservoirs by photosynthetic Fe oxidation. It is not yet clear if the low delta(56)Fe values measured for some oxides, as well as Fe carbonates, reflect biologic processes, or inorganic precipitation from low-delta(56) Fe ferrous-Fe-rich fluids. However, the present results demonstrate the great potential for Fe isotopes in tracing the geochemical cycling of Fe, and highlight the need for an extensive experimental program for determining equilibrium Fe isotope fractionation factors for minerals and fluids that are pertinent to sedimentary environments.