Millard, A.R. and Hedges, R.M., 1996, Geochim. Cosmochim.
Acta, 60, 21392152.
Pike et al., 2002, Geochimica et Cosmochimica Acta 66, 42734286.
Open University UraniumSeries Laboratory  
Earth and Environmental Sciences, The Open University, Milton Keynes, UK  
The
diffusionadsorption model


Millard & Hedges (1996) developed a U uptake model based on a physicochemical description of U–bonemineral interaction. Put simply, it describes U in the groundwater as uranyl complexes entering the bone porespace by a process of diffusion, where it adsorbs on to the large surface presented by the bone mineral, hydroxyapatite. At the core of their model is the parameter D/R which is a function of the diffusion coefficient and the partition coefficient of U for bone. It is related to the structure of the bone and the hydrology of the burial environment. The model not only predicts the rate of U uptake, but also the spatial distribution of U within a bone. Figure 1 shows the development of U profiles over time according to the model assuming constant conditions. U is taken up rapidly at the edges of the bone to form a Ushaped profile that gradually flattens with time until equilibrium is reached between U in the groundwater and U in the bone. 



Figure 1 Simulated U profiles in bone. The parameter t’ which defines the shape of the profile is related to time and the parameter D/R 

Pike et al.,(2002) deveoped the the diffusionadsorption model to incorporate Useries disequilibrium to dating of bone. The model predicts the development of U and Th isotope ratios across a bone section, shown in Figure 2 as closed system Useries dates. The figure shows the modelled results for a 10ky bone and different values of the parameter D/R. The edges of the bone, that reach equilibrium relatively quickly, give closed system dates close to 10ky, but the centre of the bone may give a gross underestimation of the age. 



Figure 2 Simulated closed system date profiles in a 10ky bone. The parameter D/R is related to the structure of both the bone and the soil. 

Millard, A.R. and Hedges, R.M., 1996, Geochim. Cosmochim.
Acta, 60, 21392152. 

To Top  

© Peter van Calsteren  Last
updated:
December 23, 2011 11:37
