Decoupled sources of the 2.3-2.2 Ga Yuanjiacun banded iron formation: Implications for the Nd cycle in Earths early oceans
The recognized worldwide gap in BIF deposition between 2.4 and 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 elementyttrium (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 (<0.5 wt %) and high field strength elements (HFSE) concentrations (<10 ppm) indicate an essentially detritusfree 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.
Yuanjiacun Archean and Paleoproterozoic Seawater Neodymium BIF
Changle Wang Kurt O.Konhauser Lianchang Zhang Mingguo Zhai Wenjun Li
Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Science Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Science
国际会议
中国科学院地质与地球物理研究所2016年度(第16届)学术年会
北京
英文
354-354
2017-01-01（万方平台首次上网日期，不代表论文的发表时间）