Sequential Pb-Sr-LREE separation from silicates for isotopic analysis

Abstract

An optimised and improved process is presented for the sequential separation of Pb, Sr, and light rare earth elements (LREEs) from a single aliquot of digested silicate sample. Cation exchange resin, Sr resin, and Ln resin columns are used to achieve 80-90% recovery yield of each element with high purity. Pb and Sr are recovered using a miniaturised Sr column after the separation of the Pb-Sr fraction from the digested silicate sample using cation exchange resin (AG50W-X8). The rare earth elements (REE) fraction recovered from the same step is used to separate Nd using Ln resin. We have used dilute HNO₃ to elute a Ce and Sm free Nd fraction (Ce/Nd < 10^-5). The REE fraction was oxidised with > 5 mM NaBrO₃ solution prior to loading on the Ln resin. The required concentration of NaBrO₃ was tested using reference rock JB-2 and 5 mM was found to be sufficient for REE from 20 mg rock powder. The separation process also yields highly pure Ba, La, Ce, and Sm as by-products which can be used for isotopic analyses. The activity of the NaBrO₃ solution degrades rapidly, and therefore should be used within 2-3 days of preparation. Pb-Sr-Nd Isotopic ratios of nine reference rocks and sediments from GSJ and USGS were measured using TIMS and are found to be accurate and reproducible. We also report the first Pb-Sr-Nd isotopic ratios for JSL-1 (slate from GSJ) and Pb isotopic ratio of SDO-1 (shale from USGS).