Mechanisms of U enrichment and helium generation potential in marine black shales following U isotope-constrained Neoproterozoic Oxidation Event
Journal:
Science of The Total Environment
Key Words:
U isotope，Neoproterozoic Oxidation Event，Blackshale，U enrichment，Helium gas，Yuertusi formation
Abstract:
Following the NOE, the early Gambrian witnessed the global deposition of marine black shales with high Uconcentrations. This study analyzes the Lower Cambrian Yuertusi Formation in the Tarim Basin, China, focusingon U isotopes to elucidate U enrichment mechanisms in black shales and their potential for helium generation. Inwells XK-1, LT-1, and LT-3, the average U concentrations in the Yuertusi Formation black shale are 41.7 ppm,29.21 ppm, and 275.28 ppm, respectively. U enrichment in black shales is jointly controlled by continentalweathering, paleoproductivity, oceanic oxidation, and organic matter, A synchronous increase in global atmospheric oxygen levels and weathering processes, leading to the continuous weathering of land rocks rich in U andnutrient elements, which were then transported to the ocean by rivers, laying the foundation for U enrichment inblack shales and the accumulation of organic matter. The 8?38y values of the Yuertusi Formation range from-0.44 %o to 0.37 %o, showing two phases of first positive and then negative drift in S?38u values, reflecting a process where the area of oceanic oxidation experienced an expansion followed by contraction. During theexpansion of the oceanic oxidation area, the paleoproductivity and U concentration in the oceanic oxidatiorlayer increased, allowing soluble U elements to accumulate in black shales through reduction and organic matteiadsorption in deep water anoxic environments. Conversely, during the contraction of the oceanic oxidation areathe U concentration in the oceanic oxidation layer decreased, resulting in significantly lower U concentration irthe deposited dolostones or limestones compared to black shales, The early Cambrian black shales enriched witlU can serve as effective helium source rocks, with an estimated cumulative release of approximately 1382 x 10'm" of helium gas, The insights gained from this study are significant for understanding the redox state of theocean following the NOE and for guiding the exploration of ultra-deep helium gas.
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