Effect of submarine hydrothermal activity and emerged landmasses on Paleoarchean Ocean chemistry

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 (REYs

_N) patterns with positive La

_SN, Eu

_SN, Gd

_SN anomalies, super-chondritic Y/Ho ratios, absence of negative Ce

_SN anomalies, and an enrichment of heavy over light REY

_SN. 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' εHf

_i 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.