Directly determining ~(143)Nd/~(144)Nd isotope ratios using thermal ionization mass spectrometry for geological samples without separation of Sm-Nd

摘要

Sm-Nd isotopic system is a powerful tracer and dating tool in geochemistry and cosmochemistry. Thermal ionization mass spectrometry (TIMS) is the benchmark method for obtaining high precision Nd isotope ratios. Traditionally, a two-step separation was employed in order to obtain high purity Nd fraction, which makes the sample preparation time-consuming. In this study, we present a new method to directly obtain precise and accurate Nd isotope ratios of geological samples without Sm-Nd separation. The key point is to well correct isobaric interferences of ~(144)Sm on ~(144)Nd for this method. The accurate intensity of ~(144)Sm can be obtained using ~(147)Sm/~(144)Sm ratio of 4.87090 and mass discrimination factor of Sm (β_(sm)); which is achieved using ~(147)Sm/~(149)Sm ratio of 1.08583 with Russell equation. Consequently, the real intensity of ~(144)Nd was achieved, then also raw ~(146)Nd/~(144)Nd, ~(143)Nd/~(144)Nd ratios. Finally, ~(143)Nd/~(144)Nd ratio is normalized using ~(146)Nd/~(144)Nd ratio of 0.7219 following exponential law. The accuracy of this method is validated by replicating TIMS measurements for thirteen international reference materials of silicate rock, spanning a wide range of Sm/Nd from 0.13 to 0.46 and bulk compositions. The measured ~(143)Nd/~(144)Nd ratios of the rock standards are in good agreement with the reported values of within error of ±0.004%. Our new method shortens the analytical procedure and significantly improves sample throughput which greatly contributes to those studies requiring a large dataset and quick analysis.