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

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology Pub Date : 2025-02-15 DOI:10.1016/j.chemgeo.2025.122686

C. Israel , C. Chauvel , T.-H. Luu , E.C. Inglis , D. Roberts

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引用次数: 0

Abstract

The extinct ¹⁴⁶Sm¹⁴²Nd 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 ¹⁴²Nd/¹⁴⁴Nd 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 ¹⁴²Nd/¹⁴⁴Nd.

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 ¹⁴²Nd/¹⁴⁴Nd = 1.1418299 ± 36 (2sd – 3.2 ppm), ¹⁴³Nd/¹⁴⁴Nd = 0.5121007 ± 10 (2.0 ppm), ¹⁴⁵Nd/¹⁴⁴Nd = 0.3484046 ± 6 (1.8 ppm), ¹⁴⁸Nd/¹⁴⁴Nd = 0.2415790 ± 10 (4.1 ppm) and ¹⁵⁰Nd/¹⁴⁴Nd = 0.2364481 ± 27 (11.3 ppm). With such precision and accuracy, subtle variations of ¹⁴²Nd/¹⁴⁴Nd ratios, as low as 3 ppm, can now be detected in natural samples.

Minute differences in ¹⁴⁵Nd/¹⁴⁴Nd, ¹⁴⁸Nd/¹⁴⁴Nd and ¹⁵⁰Nd/¹⁴⁴Nd 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 ¹⁴⁸Nd/¹⁴⁴Nd and ¹⁵⁰Nd/¹⁴⁴Nd 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 ¹⁴²Nd/¹⁴⁴Nd deviations that could be wrongly attributed to radiogenic signatures acquired during the Hadean.

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来源期刊

Chemical Geology 地学-地球化学与地球物理

CiteScore

7.20

自引率

10.30%

发文量

374

审稿时长

3.6 months

期刊介绍： Chemical Geology is an international journal that publishes original research papers on isotopic and elemental geochemistry, geochronology and cosmochemistry. The Journal focuses on chemical processes in igneous, metamorphic, and sedimentary petrology, low- and high-temperature aqueous solutions, biogeochemistry, the environment and cosmochemistry. Papers that are field, experimentally, or computationally based are appropriate if they are of broad international interest. The Journal generally does not publish papers that are primarily of regional or local interest, or which are primarily focused on remediation and applied geochemistry. The Journal also welcomes innovative papers dealing with significant analytical advances that are of wide interest in the community and extend significantly beyond the scope of what would be included in the methods section of a standard research paper.