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

Abstract

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.