Using BIF magnetite of the Badampahar greenstone belt, Iron Ore Group, East Indian Shield to reconstruct the water chemistry of a 3.3–3.1 Ga sea during iron oxyhydroxides precipitation

Abstract

The rare earth elements and Y (REY) geochemistry is considered to be an important tool to interpret the origin of banded iron formations (BIF) and paleo-ocean chemistry. The interpretations are based on the assumption that there was minimal fractionation of REY during precipitation of iron oxyhydroxide phases in the Precambrian oceans. However, the study of iron precipitation in laboratory and modern marine setting shows that precipitating iron oxyhydroxide phases significantly fractionate REY from solution. The purpose of this work is to inspect the validity of using BIF as a direct proxy for ancient seawater and establish the potentials of REY fractionation into BIF. The BIF magnetite of the 3.3–3.1Ga Badampahar greenstone belt (BGB) is studied by LA-ICP-MS for trace and REY compositions. Considering the REY fractionation in BIF, the corresponding seawater (3.3–3.1Ga) REY compositions are reconstructed from the BIF magnetite data using experimental REY partitioning coefficients and compared with modern seawater compositions. The calculated seawater REY patterns (shale normalized) show more pronounced W-type lanthanide tetrad effect relative to modern seawater implying high rate of iron precipitation. The calculated seawater data also show true negative Ce anomalies which indicate that iron was oxidized in presence of free oxygen derived from microbial photosynthesis.