Na, K, Ca, Mg, and U-series in fossil bone and the proposal of a radial diffusion-adsorption model of uranium uptake.

Fossil bones are often the only materials available for chronological reconstruction of important archeo- logicalsites.However,sinceboneisanopen system foruranium,itcannotbe dateddirectlyand thereforeit is necessary to develop models for the U uptake. Hence, a radial diffusioneadsorption (RDA) model is described. Unlike the classic diffusioneadsorption (DeA) model, RDA uses a cylindrical geometry to describe the U uptake in fossil bones. The model was applied across a transverse section of a tibia of an extinct megamammal Macrauchenia patachonica from the La Paz Local Fauna, Montevideo State, Uruguay. Measurements of spatial distribution of Na, K, Ca, and Mg were also performed by neutron activation analysis (NAA). Gamma-ray spectrometric U-series dating was applied to determine the age of the bone sample. From U concentration profile, it was possible to observe the occurrence of a relatively slow and continuous uranium uptake under constant conditions that had not yet reached equilibrium, since the uranium distribution is a ∪-shaped closed-system. Predictions of the RDA model were obtained for a specific geochemical scenario, indicating that the effective diffusion coefficient D/R in this fossil bone is (2.4±0.6)10 � 12 cm 2 s � 1 .Mean values of Na,K,Ca, and Mgcontents along the radialline of the fossil tibia are consistent with the expected behavior for spatial distributions of these mineral elements across a modern bone section. This result indicates that the fossil tibia may have its mineral structure preserved.