James R. Budahn Richard I. Grauch J. B. Paces K. R. Simmons Robert A. Zielinski 2004 <p><span>Variably weathered outcrop samples of the Meade Peak Phosphatic Shale Member of the Phosphoria Formation have 5-10% of the contained uranium (U) in a form readily extractable by 0.1 M sodium bicarbonate. Fission track radiography of outcrop samples and other less-weathered channel and core samples indicate that this mobile fraction of U is likely hosted by organic matter, secondary iron oxides and clay minerals, trace uraninite, and very fine-grained apatite cement. During weathering, this extractable U fraction is especially susceptible to redistribution, which produces small but measurable departures (1-15%) from radioactive (secular) equilibrium in the&nbsp;</span>²³⁸<span>U decay-series. The most weath- ered samples show the strongest isotopic evidence for redistribution of U during the last 350 ka, but sequestration of U by alteration products limits open-system losses of U at the whole-rock scale. In less-weathered samples, isotopic evidence for minor U loss (or gain) over longer time periods (1 Ma) is consistent with relatively non-aggressive attack of phosphatic rock during weathering. Comparative extractability of selenium (Se) suggests that a larger fraction of Se (19%) is readily available for mobilization during the earliest stages of weathering.</span></p> application/pdf 10.1016/S1874-2734(04)80011-1 en Elsevier Weathering of the meade peak phosphatic shale member, phosphoria formation: Observations based on uranium and its decay products chapter