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
EARTH AND PLANETARY SCIENCE LETTERS(2023)
摘要
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.
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关键词
Re-Os,ironstones,mid-Proterozoic,paleo-environment,atmospheric oxygen level
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