A rare earth element and Nd isotopic investigation into the provenance and deposition of the Dahongliutan banded iron formation and associated carbonates, NW China: Implications on Neoproterozoic seawater compositions

摘要

Rare earth element and Nd isotopic data for the recently discovered Neoproterozoic Dahongliutan banded iron formation (BIF) and associated carbonates are employed to infer aspects of the provenance and depositional environment, including ambient seawater compositions. The geochemical evidence is suggestive of deposition of the Dahongliutan marble in an epicontinental shallow-marine environment with riverine influx and low-T hydrothermal input, with restricted exchange to the open ocean. In contrast, the Dahongliutan BIF formed in a more open setting, with high-T hydrothermal input and old continental materials addition. The clearly distinct but negative epsilon(Nd) (0.593 Ga) values indicate that Neoproterozoic seawater at Dahongliutan could be spatially inhomogeneous with respect to Nd-143/Nd-144 ratios, owing to inefficient vertical and horizontal mixing of seawater closely related to the riverine Nd input derived from the weathering of different types and ages of continental crusts of the Western Kunlun orogenic belt and the Tarim Block. The Nd isotopic mass balance calculations of the Dahongliutan BIF indicate that most (> 68%) of the Nd were derived from continental source materials transported by river water. In conjunction with previously published geochemical data of Neoproterozoic BIFs worldwide, we evaluate the chemical compositions of seawater where they formed, represented by a seawater reservoir strongly influenced by continentally derived freshwaters (> 10%) mixed with open ocean and minor (< 5%) high-T hydrothermal fluids as indicated by Sm/Yb and Eu/Sm ratios. By comparing the Nd isotopic features of Neoproterozoic BIFs, it is concluded that similar to modern oceans, the Neoproterozoic ocean was not well-mixed with respect to its Nd isotopic composition and continental source materials transported by river water dominated the great majority of Nd in Neoproterozoic seawater.