Effects of Domain Mixing on High-Precision 142Nd/ 144Nd Measurements Using Thermal-Ionization Mass Spectrometry

Abstract

The short-lived 146Sm-142Nd systematics is a powerful tool for reconstructing the early history of the Solar System and Earth. However, variations in the 142Nd/ 144Nd isotope ratio are extremely small and require high-precision measurements. Current analytical precision typically ranges from 3 to 5 ppm, which is sufficient for the study of very old samples but limits the ability to resolve isotope variations in more recent samples for which 142Nd/ 144Nd differences are of smaller magnitude. To improve both internal and external precision of 142Nd/ 144Nd measurements, we conducted extended Thermal-Ionization Mass Spectrometry runs lasting 16 hours, increasing the Nd load on rhenium filaments from 750 ng to 1000 ng and thus doubling the number of measurement cycles in our 4-line dynamic method from 540 to 1080. Our dataset consists of 58 JNdi-1 standard analyses, including 44 extended 1080cycle runs, 4 extended 1620-cycle runs, and 10 'conventional' 540-cycle runs, acquired over 11 analytical sessions spanning 18 months. Unexpected correlations are observed for static-and dynamiccorrected Nd isotope ratios, resulting in degraded external precision compared to the expected 3-5 ppm.Monte Carlo modeling and Scanning Electron Microscope analysis of Nd loads indicate that domain mixing is the cause of these correlations. This effect arises from the mixing of sub-domains within the load that undergo distinct fractionation trends, leading to an overestimation of the 142Nd/ 144Nd isotope ratio. Corrected for domain mixing, our 1080-cycle dataset yields an overall external precision of 2.4 ppm. This improved precision enables the detection of small isotopic variations that were previously unresolvable.