Evaluating the removal of non-detrital matter from soils and sediment using uranium isotopes

The time elapsed since detrital minerals were reduced to < 63 μm by weathering can be constrained by applying the comminution dating method, which is based on the (234U/238U) activity ratio and surface area properties of the detrital minerals. In order to constrain an accurate age, the detrital minerals should be isolated and non-detrital matter present must be completely removed. Here we evaluate current sample pre-treatment procedures for removing non-detrital matter using uranium isotopes. The (234U/238U) activity ratio of the solid residue decreased stepwise throughout sequential extraction procedures, which is attributed to the removal of non-detrital matter. Despite the heterogeneity observed in the untreated samples, the final (234U/238U) activity ratio of solid residues from replicate experiments were within analytical error. This shows that the (234U/238U) activity ratio of the detrital minerals is consistent following removal of non-detrital matter. The addition of a complexing agent (sodium citrate) to extraction reagents decreased the readsorption of uranium, but did not affect the final (234U/238U) activity ratio. Mild HF/HCl etching experiments showed that the (234U/238U) activity ratio can be further decreased following sequential extraction. Particle-size distribution measurements revealed that the decrease in the (234U/238U) activity ratio is likely due to the dissolution of clay minerals. Mild HF/HCl etching of a rock standard also revealed a small amount of preferential leaching of 234U (< 1%). The inferred comminution ages are generally beyond the limit of the technique (1000 ka). By assuming an initial activity ratio of 0.95 to account for preferential leaching effects, the ages of samples following sequential extraction were within analytical error. Mild HF/HCl etching following sequential extraction results in older ages, which is attributed to the further removal of clay minerals. We recommend sequential extraction followed by mild HF/HCl etching as sample pre-treatment for comminution dating studies.