Experimental constraints on Fe isotope fractionation during magnetite and Fe carbonate formation coupled to dissimilatory hydrous ferric oxide reduction

Iron isotope fractionation between aqueous Fe(II) and biogenic magnetite and Fe carbonates produced during reduction of hydrous ferric oxide (HFO) by Shewanella putrefaciens, Shewanella algae, and Geobacter sulfurreducens in laboratory experiments is a function of Fe(III) reduction rates and pathways by which biogenic minerals are formed. High Fe(III) reduction rates produced Fe-56/Fe-54 ratios for Fe(II)(aq) that are 2-3parts per thousand lower than the HFO substrate, reflecting a kinetic isotope fractionation that was associated with rapid sorption of Fe(II) to HFO. In long-term experiments at low Fe(III) reduction rates, the Fe(II)(aq)-magnetite fractionation is - 1.3parts per thousand, and this is interpreted to be the equilibrium fractionation factor at 22degreesC in the biologic reduction systems studied here. In experiments where Fe carbonate was the major ferrous product of HFO reduction, the estimated equilibrium Fe(II)(aq)-Fe carbonate fractionations were ca. 0.0parts per thousand for siderite (FeCO3) and ca. +0.9parts per thousand for Ca-substituted siderite (Ca0.15Fe0.85CO3) at 22degreesC. Formation of precursor phases such as amorphous nonmagnetic, noncarbonate Fe(II) solids are important in the pathways to formation of biogenic magnetite or siderite, particularly at high Fe(III) reduction rates, and these solids may have Fe-56/Fe-54 ratios that are up to 1parts per thousand lower than Fe(II)(aq). Under low Fe(III) reduction rates, where equilibrium is likely to be attained, it appears that both sorbed Fe(II) and amorphous Fe(II)(s) components have isotopic compositions that are similar to those of Fe(II)(aq). The relative order of delta(56)Fe values for these biogenic minerals and aqueous Fe(II) is: magnetite > siderite approximate toFe(II)(aq) > Ca-bearing Fe carbonate, and this is similar to that observed for minerals from natural samples sui,h as Banded Iron Formations (BIFs). Where magnetite from BIFs has delta(56)Fe >0parts per thousand, the calculated delta(56)Fe value for aqueous Fe(II) suggests a source from midocean ridge (MOR) hydrothermal fluids. In contrast, magnetite from BIFs that has delta(56)Fe less than or equal to0parts per thousand apparently requires formation from aqueous Fe(II) that had very low delta(56)Fe values. Based on this experimental study, formation of low-delta(56)Fe Fe(II)(aq) in nonsulfidic systems seems most likely to have been produced by dissimilatory reduction of ferric oxides by Fe(III)-reducing bacteria. Copyright (C) 2005 Elsevier Ltd