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In-situ determination of Nd isotope ratios in apatite by LA-MC-ICPMS: Challenges and limitations
Chemical Geology ( IF 3.6 ) Pub Date : 2020-09-01 , DOI: 10.1016/j.chemgeo.2020.119740
Régis Doucelance , Emilie Bruand , Simon Matte , Chantal Bosq , Delphine Auclair , Abdel-Mouhcine Gannoun

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 eNd-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 eNd-units).

中文翻译:

LA-MC-ICPMS 原位测定磷灰石中 Nd 同位素比率:挑战和局限性

近年来,含稀土元素矿物中 Sm-Nd 同位素的原位测量已成功用于了解旧地球材料的历史(例如 Hammerli 等人,2019 年;Fisher 等人,2020 年)。然而,用于 Sm-Nd 测量的激光烧蚀-多收集器-电感耦合等离子体质谱 (LA-MC-ICPMS) 的分析协议在很大程度上取决于仪器、分析参数和应用的校正。在本文中,我们打算总结设置此技术的一些困难。我们通过评估激光参数(光斑尺寸、能量密度、频率和 He/N2 气流)对 143 Nd/ 144 Nd、145 Nd/ 144 Nd 和 147 Sm/ 144 Nd 比率的影响来展示新结果。我们还报告了锥体几何和干涉和质量歧视校正测试结果,对 Nd 同位素分析的准确性和精确度进行了测试。我们报告了由 TIMS 和 LA-MC-ICPMS 对杜兰戈参考磷灰石(墨西哥)和来自 2 种碳酸盐岩(Fogo,佛得角和 Phalaborwa,南非)的磷灰石晶体进行的新 Sm-Nd 测量。我们从这些测量中得出结论,激光参数对 143 Nd/ 144 Nd 和 145 Nd/ 144 Nd 同位素比没有影响。然而,147 Sm/144 Nd 比率的值与光斑大小相关(Fisher 等人,2020 年)。更重要的是,我们表明,当 He/N2 气流在分析过程中甚至准时地改变(例如样品交换)时,147 Sm/144 Nd 比率测量值可能会发生变化,并且在白天系统性地减少,我们将其与系统的仪器漂移有关。我们还得出结论,我们的 Durango 晶体的 Nd 同位素比率是同质的,而 Sm/Nd 的水平在 ±1-1.5% 的水平上略有异质性,因此在用作外标时可以进行准确测量。我们检索了近期 Fogo(4 Ma 旧)和古代 Phalaborwa(2060 Ma 旧)样品的预期同位素比率,我们实现了 125-150 ppm(1.3-1.5 eNd 单位)的 143 Nd/144 Nd 的精度对于 40 µm 的激光光斑,这可以被认为是大多数地质样品中磷灰石晶体的合理尺寸。最后,我们提出了光栅测量,允许在 70 ppm(0.7 eNd 单位)下将精度提高约 2 倍。并且 Sm/Nd 在 ±1-1.5% 的水平上略有异质性,当用作外部标准时允许准确测量。我们检索了近期 Fogo(4 Ma 旧)和古代 Phalaborwa(2060 Ma 旧)样品的预期同位素比率,我们实现了 125-150 ppm(1.3-1.5 eNd 单位)的 143 Nd/144 Nd 的精度对于 40 µm 的激光光斑,这可以被认为是大多数地质样品中磷灰石晶体的合理尺寸。最后,我们提出了光栅测量,允许在 70 ppm(0.7 eNd 单位)下将精度提高约 2 倍。并且 Sm/Nd 在 ±1-1.5% 的水平上略有异质性,当用作外部标准时允许准确测量。我们检索了近期 Fogo(4 Ma 旧)和古代 Phalaborwa(2060 Ma 旧)样品的预期同位素比率,我们实现了 125-150 ppm(1.3-1.5 eNd 单位)的 143 Nd/144 Nd 的精度对于 40 µm 的激光光斑,这可以被认为是大多数地质样品中磷灰石晶体的合理尺寸。最后,我们提出了光栅测量,允许在 70 ppm(0.7 eNd 单位)下将精度提高约 2 倍。5 eNd 单位)对于 40 µm 激光光斑的 143 Nd/144 Nd 比率,这可以被认为是大多数地质样品中磷灰石晶体的合理尺寸。最后,我们提出了光栅测量,允许在 70 ppm(0.7 eNd 单位)下将精度提高约 2 倍。5 eNd 单位)对于 40 µm 激光光斑的 143 Nd/144 Nd 比率,这可以被认为是大多数地质样品中磷灰石晶体的合理尺寸。最后,我们提出了光栅测量,允许在 70 ppm(0.7 eNd 单位)下将精度提高约 2 倍。
更新日期:2020-09-01
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