Chemical Separation Method Research Of Lu-Hf Isotopes For The Low Hf Samples

The Lu-Hf isotope system is stable and robust comparing to conventional dating systems such as Rb-Sr and Sm-Nd. The half life of Lu lies between short half life systems (U-Th-Pb) and long half life systems (Rb-Sr and Sm-Nd). Therefore, the Lu-Hf isotopic system can solve not only the geologic chronology problems such as the terrain formation time, the crust-mantle differentiation and the evolution of crustal growth of ancient geologic terrain, but also the geological chronology of young orogenic movements. The development of instrument analytical technology of hafnium isotope restrains the application of the Lu-Hf isotopic system due to the chemical separation difficulty and high ionization energy of Hf. Currently, the Lu-Hf isotope data can be obtained by two methods:laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) and solution nebulization multiple collector inductively coupled plasma mass spectrometry (SN-MC-ICPMS). The LA-MC-ICPMS method can obtain in situ hafnium isotopic composition from high hafnium content samples (zircon, baddeleyite, etc.) with high spatial resolution, but it is inefficient for those samples with low Hf concentration (mantle basalt rocks, eclogite). Alternatively, the SN-MC-ICPMS method can get hafnium isotopic composition of low hafnium samples. The second method is comprised of following chemical steps:completely dissolve samples; separate concentrated pure lutetium and hafnium cuts by the chemical separation using ion chromatography; then analyze lutetium and hafnium isotopic composition using MC-ICPMS. This method is effective for both high and low hafnium content samples. The available hafnium isotope chemical separation method still need to be improved in terms of hafnium recovery, especially the Lu-Hf isotope chemical separation method. It is important to establish a stable and efficient Lu-Hf isotope chemical separation technology for samples with low Hf content.The present hafnium isotope chemical separation method in the State Key Laboratory of Continental Dynamics is fast but low hafnium recovery (< 60%). The main aim of this work is improving the Lu-Hf isotope separation method, and increasing the lutetium and hafnium recovery for low Hf content samples. Based on Connelly's method (Connelly et al.,2006),Ⅰbuilt a new chemical separation method for low hafnium content samples using international basalt standards (BCR-2, BHVO-2 from USGS). The method developed here including:1) completely dissolve samples with acids on the hotplate,2) add boric acid to the sample solution to remove residual hydrofluoric acid, and destroy the fluoride precipitations,3) separate Lu-Hf cuts using Cation-Exchange and extraction chromatography. After fourteen months experiments, a fast two-stage Hf separation method is developed here achieving the hafnium and lutetium recovery of> 90% and 30～60%, respectively. The life of the resin is prolonged due to the replacement of H2O2 with dilute nitric acid in order to elute titanium. However, further experiments are needed to obtain stable Lu separation method with high recovery. Using the method developed in this work, we separate hafnium from USGS BCR-2 and BHVO-2, and the results agree well with references in 2σ. This work is significant for hafnium isotopic composition measurement of the low hafnium samples and Lu-Hf chemical separation researches.