Decoupled sources of the 2.3-2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earth’s early oceans
Wang et al
The recognized worldwide gap in BIF deposition between 2.4-2.0 billion years ago has long been considered as an obstacle to fully determining the geochemical composition of seawater at that time. However, the recently dated 2.3-2.2 Ga Yuanjiacun banded iron formation (BIF) in the North China Craton (NCC) offers a possibility to redress these uncertainties. Shale-normalized rare earth element-yttrium (REE+Y) patterns of the BIF and interlayered meta-chert samples show features characteristic of other Archean and Paleoproterozoic BIFs, with HREE enrichment relative to LREE, positive La and Eu anomalies, and superchondritic Y/Ho ratios comparable to modern seawater. Very low Al2O3 (less than 0.5 wt%) and high field strength elements (HFSE) concentrations (less than 10 ppm) indicate an essentially detritus-free depositional setting, while positive Eu anomalies are attributed to an imprint of high-temperature hydrothermal fluids. Sm-Nd isotopic features further point to two periodically interacting water masses controlling the deposition of the BIF. The first is seafloor-vented hydrothermal fluids (εNd(t)∼+3.5) derived from interaction with a depleted mantle source and associated with high Fe fluxes. The second is ambient surface seawater (εNd(t)∼-2.4), which obtained its signature through weathering of the nearby landmasses and associated with high Si fluxes. Our findings suggest that the REE budget of the oceans prior to 2.3 Ga was generally dominated by hydrothermal circulation of seawater through depleted mantle-derived source rocks. However, where evolved local continental crust and/or an enriched mantle source were present, this positive mantle Nd signal became discernible in the BIFs. By comparing the Nd isotopic features of similar aged BIFs and marine carbonates, it is concluded that similar to modern oceans, the early Precambrian ocean was not well-mixed with respect to its Nd isotopic composition.