Fe and Mg isotope fractionations during primary precipitation and early diagenesis in a 3.2 Ga BIF

Abstract

We investigated variations in the bulk Fe and Mg isotopic ratios on ferruginous sedimentary rocks that were deposited in a shallow ocean in ~3.2 Ga Moodies Group in the Barberton Greenstone Belt, South Africa. The bulk Fe isotope ratios of the samples tend to decrease with decreasing the Fe/Al ratios of the bulk chemical compositions as well as clay/sand ratio of the detrital components, suggesting that they preserve isotope fractionation during primary precipitation of dissolved ferrous iron from seawater. On the other hand, the bulk Mg isotope ratios are explained by the mixing of the two endmembers: carbonate and silicate minerals. The uniform isotope ratio of the endmember carbonate Mg implies that the carbonate minerals were formed during very early diagenesis.