Re-Os, Sr-Nd isotopic and PGE elemental constraints for the formation of mid-Proterozoic ironstones in North China Craton: Implications for the atmospheric oxygen level

• Published in: Earth and planetary science letters Vol. 621; p. 118367
• Main Authors: Chu, Zhuyin, Qiu, Yifan, Zhou, Xiqiang, Yang, Xuli, Peng, Peng, Zhao, Taiping, Xu, Jifeng
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.11.2023
• Subjects: atmospheric oxygen level; ironstones; mid-Proterozoic; paleo-environment; Re-Os; paleo-environment; ironstones; atmospheric oxygen level; Re-Os; mid-Proterozoic
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2023.118367

The origin (hydrothermal vs. continental Fe source) and environmental implications in particular for the atmospheric oxygen level of mid-Proterozoic ironstones have been widely studied, but remain highly debated. Here, we present Re-Os, Sr-Nd isotopic and PGE elemental data for ironstones from the ∼1.7 Ga Yunmengshan (YMS) Formation in south margin of the North China Craton to address the controversy. The initial 87Sr/86Sr ratios of the YMS ironstones are ∼0.7063, which are roughly comparable with seawater values around that period, suggesting a seawater signal with minor/minimal direct contributions from submarine hydrothermal fluids. The Nd isotopic compositions (εNd(t): −6.6 to −6.0) indicate a dominantly continental crustal signal for the formation of the ironstones. The PGE patterns of the YMS ironstones are distinctly similar to those of hydrogenous Fe-Mn crusts/nodules in modern ocean, also suggesting deposition from normal seawater conditions. Importantly, the YMS ironstones give initial 187Os/188Os ratios of ∼0.6 at ∼1.7 Ga, providing critical constraints for contemporaneous seawater values. This seawater 187Os/188Os value of ∼0.6 strongly suggests a result of oxidative weathering and transportation of Os in continental crust into the ocean, implying a relatively high atmospheric oxygen level at ∼1.7 Ga. •87Sr/86Sr and PGE-Re patterns of the YMS ironstones suggest a normal seawater signal.•Nd isotopes suggest a primarily crustal source for the YMS ironstones.•YMS ironstones record elevated seawater 187Os/188Os values to ∼0.6 at ∼1.7 Ga.•Os isotopes in YMS ironstones imply a pulsed rise of atmospheric O2 level at ∼1.7 Ga.•Ironstone is good candidate for Os isotope chemo-stratigraphic studies.