A seawater origin for greenalite in iron formation

Abstract

Iron formations (IF) were widely deposited in the Archean Eon and Palaeoproterozoic Era and hold potential as an archive of marine biogeochemistry. However, reconstructions are challenging due to their fine-grained nature and complex mineralogy. Recent work has identified greenalite, an Fe(II)-silicate mineral, as an abundant, primary phase in IF. Several depositional mechanisms have been proposed for greenalite precipitation, spanning hydrothermal vent systems and shelf environments. We report new in situ rare earth element and yttrium (REY) data from greenalite in the Palaeoproterozoic Kuruman Iron Formation from the Transvaal Supergroup, South Africa. We use a new approach, cutting samples parallel to banding to expose concentrated greenalite and developed a methodology to quantify the proportion of greenalite in each ablation crater. Greenalite consistently shows heavy REE enrichment (YbSN/NdSN 4–21), a small positive Eu anomaly (∼1.41), super-chondritic Y/Ho ratios (32–43), and low ∑REE (1.4–13.1), consistent with formation in a shelf environment. While greenalite may have been an abundant precipitate in vent settings, at least some of the greenalite preserved in the rock record formed directly on the shelf. This necessitates another trigger for its formation, which may have included small amounts of Fe3+, local increases in pH, or warmer oceans. Our findings suggest that greenalite could provide a valuable archive of local seawater chemistry.