Examination of Banded Iron Formation Through Petrographic, Geochemical and Iron Isotope Analyses

Banded iron formation (BIF) has the potential to preserve geochemical signatures critical to interpretation of early Earth because: (1) it is found within the Precambrian when fundamental changes in Earth’s physical, biological and chemical evolution occurred, and (2) it may preserve a record of the depositional environment. Rare earth elements (REE) are useful for investigating BIF because they have characteristic features. One goal of this study was to evaluate the calculation of certain REE ratios to determine if the calculation affected the interpretation. It was concluded that the method for calculating certain ratios may affect the interpretation of redox conditions for a given sample. Another goal was to determine paleo-oceanic and atmospheric redox conditions. REE concentrations in BIF samples from the >3.7 Ga Isua Greenstone Belt, the ~3.0 Ga Buhwa Greenstone Belt, and the ~1.9 Ga Gunflint Iron Formation were examined and it was concluded that the ~1.9 Ga ocean was not fully ventilated, but that oxidative continental weather was occurring. A third goal was to evaluate the degree of heterogeneity in BIF mineral REE chemistry. Substantial microscale heterogeneity in mineral spot chemistry was observed and is most likely a result of inorganic, post- depositional alteration processes. Lastly, this study examined a lithology whose age and origin has been highly debated, with some arguing for a BIF protolith and others arguing for an ultramafic protolith. To investigate the unit’s protolith, I analyzed mafic mineral compositions and found that they possess trace-element signatures indicative of an igneous origin, rendering a BIF protolith unnecessary. To investigate the reported age, I measured the orientation of foliations, mineral lineations and fold axes on either side of the contact used to date the contentious lithology and concluded that the contact is tectonic and thus cannot be used to constrain the depositional age of the debated unit.