Abstract
Banded Iron Formations (BIFs), authigenic marine sedimentary rocks, preserve insights into the composition of Precambrian seawater and early Earth marine environments. The Paleoarchean (∼3.37–3.50 Ga) Algoma-type Tomka BIF from the Daitari Greenstone Belt, India, experienced only greenschist-facies metamorphism, in contrast to Eo- to Paleoarchean amphibolite-facies BIFs. Its potential as a seawater archive for palaeo-environmental reconstructions is explored herein.
To better constrain the age and the paleo-environmental conditions during deposition of the Tomka BIF, we analysed major- and trace element concentrations together with radiogenic Hf-Nd isotope ratios of individual Fe- and Si-rich BIF layers and an associated shale. Tomka BIF samples, devoid of detrital contamination and post-depositional alteration, show typical Archean shale-normalised seawater-like rare earth and yttrium (REYsN) patterns with positive LaSN, EuSN, GdSN anomalies, super-chondritic Y/Ho ratios, absence of negative CeSN anomalies, and an enrichment of heavy over light REYSN. These signatures imply deposition in an anoxic marine setting influenced by submarine high-temperature hydrothermal systems.
Samples with pristine Hf-Nd isotope compositions align along 176Lu-176Hf and 147Sm-143Nd age reference lines with the depositional age range (3.37 to 3.50 Ga). Initial εNd values (+0.1 to +5.3) suggest a juvenile source affecting Tomka seawater; the shale (−0.3 to 1.1) reflects a similarly juvenile source for the detrital component. The BIFs' εHfi values (−4.8 to +145) are decoupled from the Nd isotope system and from the terrestrial array. This decoupling likely indicates the emergence and weathering of zircon-bearing felsic crust of the proto-Singhbhum Craton, affecting Archean seawater composition at 3.37 billion years or earlier.
•First major-trace element & Hf-Nd isotope data of Tomka BIF, Daitari Belt, India.•Majority of paleoarchean Tomka BIF samples are devoid of detrital contamination.•Deposition in a marine anoxic setting influenced by high-T hydrothermal systems.•Lu-Hf and Sm-Nd isotope reference lines confirm 3.50–3.37 Ga BIF depositional age.•Hf-Nd isotope decoupling indicates subaerial weathering of evolved landmasses.