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

Abstract : In-situ measurement of Sm-Nd 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 Sm-Nd 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 143 Nd/ 144 Nd, 145 Nd/ 144 Nd and 147 Sm/ 144 Nd 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 Sm-Nd 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 143 Nd/ 144 Nd and 145 Nd/ 144 Nd isotopic ratios. The value of the 147 Sm/ 144 Nd ratio, however, is correlated to the size of the spot (Fisher et al., 2020). More importantly, we show that 147 Sm/ 144 Nd 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 143 Nd/ 144 Nd 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).
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Régis Doucelance, Emilie Bruand, Simon Matte, Chantal Bosq, Delphine Auclair, et al.. In-situ determination of Nd isotope ratios in apatite by LA-MC-ICPMS: Challenges and limitations. Chemical Geology, Elsevier, 2020, 550, pp.119740. ⟨10.1016/j.chemgeo.2020.119740⟩. ⟨hal-02880485⟩

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