Reconstructing Paleoarchean marine environments: insights from REE and combined Hf-Nd isotopes in banded iron formations from the Daitari Greenstone Belt, India

Banded Iron Formations (BIFs) are Precambrian marine chemical sediments that can be used as prime geochemical archives to reconstruct Earth's early ocean chemistry. The ~3.4-billion-year-old BIF in the Daitari Greenstone Belt, India, is one of the oldest so far known BIF that has only experienced greenschist-facies metamorphism [1] and provides a great opportunity to reconstruct Paleoarchean marine environments.

This study presents new major and trace element data, along with Nd and Hf isotopic compositions from BIF of individual Fe- and Si-rich layers. Exceptionally low concentrations of immobile elements indicate that the BIF are remarkably pure chemical precipitates without significant detrital contamination. Most samples exhibit shale-normalized rare earth element and yttrium (REY_SN) patterns characteristic of Archean seawater, including positive La_SN, Eu_SN, and Gd_SN anomalies, super-chondritic Y/Ho ratios, an absence of negative Ce_SN anomalies, and enrichment of heavy REY_SN over light REY_SN. These features point to an anoxic, open-ocean depositional setting influenced by high-temperature submarine hydrothermal activity. Most BIF samples plot on ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd reference lines between 3.3 and 3.5 Ga, consistent with geological evidence [1]. Initial εNd values range from +1.7 at 3.5 Ga to +3.0 at 3.3 Ga and suggest that mantle-derived sources influenced ancient Daitari seawater. Extremely radiogenic εHf values (from +33.9 to +11.4) reflect either isotopic disturbance or incongruent weathering of emerged continental crust similar to what is observed from the Neoarchean onwards [2].

[1] Hofmann et al., 2022, Earth Sci. Rev. 228.

[2] Viehmann et al., 2014, Geology 42.