Radiation-damage rims in quartz from uranium-bearing sandstones

Using cathodoluminescence (CL) mode of a scanning electron microscope (SEM), radiation damage in quartz is made visible in uranium-vanadium-bearing sandstones located in the Salt Wash Member of the Morrison Formation (Jurassic) from the Colorado Plateau (U.S.A.). Detrital quartz grains surrounded by uranium minerals display radiation-damage rims. Three concentric rims with thicknesses of 37 mu m, 24 mu m and 16 mu m have been observed. The rims are similar to halos surrounding uranium- and thorium-rich accessory minerals observed by Owen (1988). They are due to alpha particles, which create damage when penetrating a medium. The width of the rims agrees well with the Bragg-Kleeman rule and corresponds to the most energetic alpha particles in the ²³⁸ U decay series. Sandstones entirely cemented by vanadium clays also exhibit radioactive rims, which demonstrates that uranium has been removed during diagenesis. These results have important implications for the uranium ore genesis in the Salt Wash sandstones, because they imply an ancient episode of uranium leaching prior to recent weathering. Therefore, cathodoluminescence in sandstones may be useful in recognizing ancient zones or episodes of accumulation and leaching of alpha-emitting nuclides from the uranium series.