Mechanisms of U enrichment and helium generation potential in marine black shales following U isotope-constrained Neoproterozoic Oxidation Event.

Abstract

Following the NOE, the early Cambrian witnessed the global deposition of marine black shales with high U concentrations. This study analyzes the Lower Cambrian Yuertusi Formation in the Tarim Basin, China, focusing on U isotopes to elucidate U enrichment mechanisms in black shales and their potential for helium generation. In wells 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 continental weathering, 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 and nutrient elements, which were then transported to the ocean by rivers, laying the foundation for U enrichment in black shales and the accumulation of organic matter. The δ²³⁸U values of the Yuertusi Formation range from -0.44 ‰ to 0.37 ‰, showing two phases of first positive and then negative drift in δ²³⁸U values, reflecting a process where the area of oceanic oxidation experienced an expansion followed by contraction. During the expansion of the oceanic oxidation area, the paleoproductivity and U concentration in the oceanic oxidation layer increased, allowing soluble U elements to accumulate in black shales through reduction and organic matter adsorption in deep water anoxic environments. Conversely, during the contraction of the oceanic oxidation area, the U concentration in the oceanic oxidation layer decreased, resulting in significantly lower U concentration in the deposited dolostones or limestones compared to black shales. The early Cambrian black shales enriched with U can serve as effective helium source rocks, with an estimated cumulative release of approximately 1382 × 10⁸ m³ of helium gas. The insights gained from this study are significant for understanding the redox state of the ocean following the NOE and for guiding the exploration of ultra-deep helium gas.