Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak metamorphic temperatures of <200, <340, ∼500, <550, and <740°C respectively. δ56Fe of bulk layers ranges from −0.8 to +0.8‰; in some samples the layers may differ by >1‰ on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which δ56Fe of magnetite > silicate ≥ carbonate. The inter-mineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature-dependent fractionation factors. Textural evidence reveals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions therefore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are characteristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observation confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record.