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

Abstract

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 ¹⁴⁴Sm on ¹⁴⁴Nd for this method. The accurate intensity of ¹⁴⁴Sm can be obtained using ¹⁴⁷Sm/¹⁴⁴Sm ratio of 4.87090 and mass discrimination factor of Sm (β_Sm), which is achieved using ¹⁴⁷Sm/¹⁴⁹Sm ratio of 1.08583 with Russell equation. Consequently, the real intensity of ¹⁴⁴Nd was achieved, then also raw ¹⁴⁶Nd/¹⁴⁴Nd, ¹⁴³Nd/¹⁴⁴Nd ratios. Finally, ¹⁴³Nd/¹⁴⁴Nd ratio is normalized using ¹⁴⁶Nd/¹⁴⁴Nd 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 ¹⁴³Nd/¹⁴⁴Nd 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.