Uranium Redox and Deposition Transitions Embedded in Deep‐Time Geochemical Models and Mineral Chemistry Networks

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 25; no. 2
• Main Authors: Moore, E. K., Li, J., Zhang, A., Hao, J., Morrison, S. M., Hummer, D. R., Yee, N.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.02.2024; Wiley
• Subjects: Anoxia; Anoxic conditions; Anoxic sediments; Basalt; Chemical elements; Chemistry; Coefficient of variation; deep‐time; Distribution; Earth; Earth history; Energy resources; Energy sources; Environmental changes; Environmental conditions; Fossil fuels; geochemical model; Geochemistry; Greenhouse gases; Groundwater; History; mineral; Minerals; Modelling; network; Nuclear energy; Nuclear power plants; Organic matter; Oxic conditions; Oxidation; Oxidoreductions; Oxygenation; Phanerozoic; Power plants; redox; Relative abundance; Trace elements; Uranium; Weathering
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011267

Uranium (U) is an important global energy resource and a redox sensitive trace element that reflects changing environmental conditions and geochemical cycling. The redox evolution of U mineral chemistry can be interrogated to understand the formation and distribution of U deposits and the redox processes involved in U geochemistry throughout Earth history. In this study, geochemical modeling using thermodynamic data, and mineral chemistry network analysis are used to investigate U geochemistry and deposition through time. The number of U6+ mineral localities surpasses the number of U4+ mineral localities in the Paleoproterozoic. Moreover, the number of sedimentary U6+ mineral localities increases earlier in the Phanerozoic than the number of U4+ sedimentary mineral localities, likely due to the necessity of sufficient sedimentary organic matter to reduce U6+–U4+. Indeed, modeling calculations indicate that increased oxidative weathering due to surface oxygenation limited U4+ uraninite (UO2) formation from weathered granite and basalt. Louvain network community detection shows that U6+ forms minerals with many more shared elements and redox states than U4+. The range of weighted Mineral Element Electronegativity Coefficient of Variation (wMEECV) values of U6+ minerals increases through time, particularly during the Phanerozoic. Conversely, the range of wMEECV values of U4+ minerals is consistent through time due to the relative abundance of uraninite, coffinite, and brannerite. The late oxidation and formation of U6+ minerals compared to S6+ minerals illustrates the importance of the development of land plants, organic matter deposition, and redox‐controlled U deposition from ground water in continental sediments during this time‐period. Plain Language Summary Uranium (U) is the most widely used fuel in nuclear fission power plants, and nuclear power results in lower greenhouse gas emissions than fossil fuel energy. The different types of U deposits and minerals have evolved throughout the Earth history with changing environmental conditions and formation processes. In this study, we use modeling calculations and network analysis to understand how the transforming Earth system impacted the global U cycle, evolving U mineral chemistry, and deposit formation through time. Modeling calculations show that the abundant U mineral uraninite (UO2) formed in much greater quantities in the anoxic conditions of the Archean eon than present day oxic conditions. We also find that U minerals are increasingly oxidized through time, in agreement with modeling calculations, resulting in new minerals with diversifying chemical element associations and expanding distribution of U in the environment. The earlier increase in the number of oxidized U minerals in sedimentary localities than unoxidized U minerals in sedimentary localities 350–250 million years ago reflects the importance of land plants and organic matter in the formation of unoxidized U minerals in sedimentary settings. Key Points Model calculations indicate Earth surface oxidation limited U4+ uraninite formation, in agreement with increased observed U6+ minerals U oxidation increases the diversity of mineral chemical element associations and distribution in the environment The later formation of U6+ minerals compared to S6+ minerals represents differential Earth surface oxidation