Banded Iron Formations as archives for ca. 3.5 Ga old marine environments: Insights from REE and Hf-Nd isotope signatures

Banded Iron Formations (BIFs) are authigenic marine sedimentary rocks that record the composition of Precambrian seawater and provide key insights into early marine environments. The Paleoarchean Algoma-type Tomka BIF from the Daitari Greenstone Belt (India) is considered to be ~3.37–3.50 Ga old and to have experienced only greenschist-facies metamorphism, in contrast to many Eo- to Paleoarchean BIFs that were metamorphosed under much higher amphibolite-facies conditions. Despite this relatively low metamorphic overprint, the potential of the Tomka BIF as a reliable archive of ancient seawater chemistry has not yet been evaluated. Still, this location may be crucial to better understand the evolution of Palaeoarchean marine habitats and their interactions with early landmasses and the atmosphere.

To better constrain both the depositional age and the paleoenvironmental conditions of the Tomka BIF, we analysed major and trace element abundances together with radiogenic Hf–Nd isotope compositions of individual Fe- and Si-rich BIF layers, as well as an associated shale. Tomka BIF samples lacking detrital contamination and post-depositional alteration display typical Archean, shale-normalised seawater-like rare earth and yttrium (REY_SN​) patterns. These include positive La_SN, Eu_SN​, and Gd_SN​ anomalies, superchondritic Y/Ho ratios, the absence of negative Ce_SN​ anomalies, and enrichment of heavy relative to light REY_SN​. Collectively, these signatures indicate deposition in an anoxic marine environment influenced by high-temperature submarine hydrothermal activity.

BIF samples preserving pristine Hf–Nd isotope compositions define coherent trends along the ¹⁷⁶Lu–¹⁷⁶Hf and ¹⁴⁷Sm–¹⁴³Nd reference isochrons corresponding to the inferred depositional age of 3.37–3.50 Ga. Initial εNd values (+0.1 to +5.3) indicate a juvenile source contribution to Tomka seawater, while the associated shale (εNd = -0.3 to +1.1) reflects a similarly juvenile provenance for the detrital component. In contrast, initial εHf​ values of the BIFs (-4.8 to +145) are strongly decoupled from the Nd isotope system and from the so-called terrestrial array, which reflects the coupled behaviour of Hf-Nd in magmatic systems. A Hf-Nd isotope decoupling in low-temperature systems, however, is related to incongruent Hf weathering, as described by the so-called zircon effect. Applied to the Daitari BIFs, this decoupling likely reflects the emergence and weathering of a zircon-bearing crust in the proto-Singhbhum Craton, which influenced Archean seawater chemistry by at least 3.37 Ga.