Overcoming analytical limitations with 5-line multi-dynamic Nd isotope measurements by thermal ionisation mass spectrometry

Overview & Synopsis

The extinct 146 Sm -142 Nd radio-isotope chronometer is a valuable tool to study the first 500 Myr of Earth's history, although the entire range of variations of the 142 Nd/ 144 Nd ratio in terrestrial materials is on the order of 20 ppm only. The challenge is that measuring small isotope variations needs very high precision measurements (better than 5 ppm), which are difficult to obtain.Here we present high-precision measurements of Nd isotope ratios performed over a period of three years using the latest generation Thermal Ionisation Mass Spectrometer (TIMS) developed by Nu Instruments -Nu TIMS. The Nu TIMS comprises 16 fixed Faraday cups and a zoom optics system that allow 5-line multi-dynamic analyses with the acquisition of three dynamic ratios for all Nd isotopes. This new method, coupled with an enhanced Nd + signal provides precise measurements with internal errors lower than 2 ppm on 142 Nd/ 144 Nd.We assess the performance of the method using multiple measurements of JNdi-1 and AMES Rennes Nd pure standards, together with rock reference materials. For example, typical JNdi-1 measurements performed over a period of 19 months average at 142 Nd/ 144 Nd = 1.1418299 ± 36 (2sd -3.2 ppm), 143 Nd/ 144 Nd = 0.5121007 ± 10 (2.0 ppm), 145 Nd/ 144 Nd = 0.3484046 ± 6 (1.8 ppm), 148 Nd/ 144 Nd = 0.2415790 ± 10 (4.1 ppm) and 150 Nd/ 144 Nd = 0.2364481 ± 27 (11.3 ppm). With such precision and accuracy, subtle variations of 142 Nd/ 144 Nd ratios, as low as 3 ppm, can now be detected in natural samples.Minute differences in 145 Nd/ 144 Nd, 148 Nd/ 144 Nd and 150 Nd/ 144 Nd ratios can now also be detected in natural samples, allowing better identification of nucleosynthetic anomalies in extra-terrestrial samples. For terrestrial samples, it allows detection of traces of samarium, and of nuclear field shift effect due to analytical issues. In particular, we demonstrate that some Sm is present in the JNdi-1 standard and affects both its 148 Nd/ 144 Nd and 150 Nd/ 144 Nd ratios, making a systematic interference correction necessary. By detecting the existence of nuclear field shift effect on Nd isotopes, we can now prevent mis-interpretations of 142 Nd/ 144 Nd deviations that could be wrongly attributed to radiogenic signatures acquired during the Hadean.