Decoupled neodymium and hafnium isotopes in seawater as insights into the Late Neoarchean weathering process

Abstract

We here present trace element and coupled Sm-Nd and Lu-Hf isotope data for both pure and detritus-rich BIF samples collected from the ∼2.51 Ga Zhalanzhangzi BIF, North China Craton to investigate sources of BIF-forming materials to the ambient seawater, and to evaluate factors controlling the decoupling of Hf and Nd isotope systems in Late Neoarchean seawater. Both pure and detritus-rich BIFs exhibit positive Eu anomalies, positive correlation between εNd(2.51Ga) values and Fe2O3 contents, and moderate negative to positive εNd(2.51Ga) values, revealing a combined submarine hydrothermal and terrigenous sources for REY and iron to ambient seawater. Instead of interpreting the correlation between Ge/Si ratio and iron (or silica) content of BIFs as the mixing of hydrothermal and terrigenous sources of silica, we preferred a terrigenous source for silica to Neoarchean seawater. Moreover, by compiling published Ge/Si data, we argue that although hydrothermal fluids might be the dominant source of silica to early Archean oceans, a continent-derived riverine flux was expected to be the main source of silica to ambient seawater, following the substantial increase in land mass during Neoarchean. The εNd(2.51Ga) and εHf(2.51Ga) values of pure BIFs reveal decoupling of Nd and Hf isotopes in ambient seawater, and suggest that incongruent terrestrial weathering had significant effects on oceanic geochemistry at the end of Archean eon. In addition, based on the observation that Neoarchean seawater was characterized by more radiogenic εHf values when compared to its modern counterpart, we posit a weak to moderate chemical weathering conditions for Neoarchean continents.