Constraining Rates of Chemical and Physical Erosion Using U-Series Radionuclides

International audienceThis chapter relates recent developments concerning the use of several U-series nuclides, in particular 234 U-238 U and 230 Th-238 U disequilibria, for constraining physical and chemical erosion rates and sediment age. Indeed, the ability to measure these disequilibria with an extremely high precision, even in samples with low concentrations such as natural waters, has opened new avenues for investigating erosional processes. This chapter is articulated in three main parts: a brief introduction and presentation of modern technical methods is followed by a description of how 234 U-238 U and 230 Th-238 U disequilibria measured in dissolved (water) and solid (sediment) river phases can be used to provide quantitative constraints on physical and chemical erosion rates at the basin scale. In parallel, recoil effects occurring during sediment formation can now be modelled and used to estimate the residence time of a sediment within a basin. Finally, the last part of this chapter presents the latest findings concerning the study of weathering profiles and the modelling of U and Th migration within an aquifer system

Structural and biological control of the Cenozoic epithermal uranium concentrations from the Sierra Pena Blanca, Mexico

Epithermal uranium deposits of the Sierra Peña Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45-55°C for the uraninite from the ore body and ~20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ 34S values (~-24. 5‰) in pyrites and the presence of low δ 13C (~-24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments