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In-situ determination of Nd isotope ratios in apatite by LA-MC-ICPMS: Challenges and limitations

2020; Elsevier BV; Volume: 550; Linguagem: Inglês

10.1016/j.chemgeo.2020.119740

1872-6836

Régis Doucelance, Emilie Bruand, Simon Matte, Chantal Bosq, Delphine Auclair, Abdelmouhcine Gannoun,

Geochemistry and Elemental Analysis

In-situ measurement of SmNd isotopes in rare-earth-element bearing minerals has been successfully used in the recent years to access the history of old Earth materials (e.g. Hammerli et al., 2019; Fisher et al., 2020). However, the analytical protocol of Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) for SmNd measurements strongly depends on the instruments, the analytical parameters and the applied corrections. In this paper, we intend to summarize some of the difficulties to set up this technique. We present new results by evaluating the influence of laser parameters (spot size, fluence, frequency and He/N2 gas flows) on 143Nd/144Nd, 145Nd/144Nd and 147Sm/144Nd ratios. We also report results on tests of cone geometry and interference and mass discrimination corrections on both the accuracy and precision of Nd isotopic analyzes. We report new SmNd measurements performed by TIMS and LA-MC-ICPMS on Durango reference apatite (Mexico) and apatite crystals from 2 carbonatites (Fogo, Cape Verde and Phalaborwa, South Africa). We conclude from these measurements that the laser parameters have no influence on the 143Nd/144Nd and 145Nd/144Nd isotopic ratios. The value of the 147Sm/144Nd ratio, however, is correlated to the size of the spot (Fisher et al., 2020). More importantly, we show that 147Sm/144Nd ratio measurements can vary when the He/N2 gas flows are changed even punctually (e.g. sample exchange) during an analytical session and decrease systematically during the day, which we relate to a systematic instrumental drift. We also conclude that our Durango crystal is homogeneous for Nd isotopic ratios, and slightly heterogeneous for Sm/Nd at the level of ±1–1.5% allowing accurate measurements when used as external standard. We retrieve expected isotopic ratios for both recent Fogo (4 Ma old) and ancient Phalaborwa (2060 Ma old) samples and we achieve precisions in the order of 125–150 ppm (1.3–1.5 εNd-units) for the 143Nd/144Nd ratio for a laser spot of 40 μm, which can be considered as a reasonable size for apatite crystals in most geological samples. Finally, we present raster measurements allowing to improve the precision by a factor of ~2 with 70 ppm (0.7 εNd-units).