磷灰石Sr-Nd同位素的激光剥蚀-多接收器电感耦合等离子体质谱微区分析

In situ Sr-Nd Isotopic Measurement of Apatite Using Laser Ablation Multi-collector Inductively Coupled Plasma-Mass Spectrometry

磷灰石是常见的副矿物,具有较高的Sr-Nd含量和较低的Rb含量,对其微区Sr-Nd同位素组成的准确测定可以为精细地质作用过程的探讨提供重要的地球化学信息。激光剥蚀-多接收器电感耦合等离子体质谱(LA-MC-ICPMS)具有分析速度快、分析精度高和空间分辨率高的特点,特别适合大量细颗粒磷灰石样品的Sr-Nd同位素分析,而同位素干扰的精确扣除和仪器质量歧视校正是原位微区分析准确获得Sr-Nd同位素比值的关键。本文利用LA-MC-ICPMS技术,综合最新发表的Kr、Rb、稀土二价离子及钙聚合物对Sr同位素的干扰扣除方法和Sm对Nd同位素的干扰扣除方法,对仪器的质量歧视进行了校正,建立了磷灰石原位Sr-Nd同位素分析方法。用此方法对一个磷灰石国际标准样品Durango和两个实验室标准Apatite 1和PE进行了详细的Sr-Nd同位素测定,结果表明,对Sr-Nd含量足够高的磷灰石样品可以准确地获得其Sr-Nd同位素组成,测试结果与文献报道值或热电离质谱(TIMS)测试值在误差范围内一致,Sr同位素的测试精度<0.015%(2SD),Nd同位素的测试精度<0.005%(2SD),达到了国际同类实验室水平;且三个磷灰石标准样品同位素组成较为均一,都是理想的原位Sr-Nd同位素分析参考物质。

Apatite is a common accessory mineral, having a high Sr-Nd content and a low Rb content. The precise determination of micro-area Sr-Nd isotopic composition for apatite will provide important geochemical information for discussing the details of the geological process. Laser Ablation Multi-collector Inductively Coupled Plasma-Mass Spectrometry （LA-MC-ICPMS） technique has the characteristics of rapid analysis, high analytical precision and high spatial resolution, and is particularly suitable for Sr-Nd isotopic analysis of a large quantity of fine-grained apatite samples. The precise deduction of isotopic interference and calibration of instrument mass bias are crucial for in-situ analysis to obtain Sr-Nd isotopic ratio. In this article a method is described, which adopts LA-MC-ICPMS and combines with the newly published method for deduction of interferences, like Kr, Rb, divalent ions of REEs and calcium-containing polymer for Sr isotopes, Sm for Nd isotopes. The calibration of instrument mass bias is carried out, and the in-situ Sr-Nd isotopic analysis approach for apatite is established. The test results of Durango apatite standard, Apatite 1 and PE show that Sr-Nd isotopic composition of apatite sample with high Sr-Nd content can be accurately determined. The specific results are consistent with literature or the measured values by Thermal Ionization Mass Spectrometry （TIMS） within error. The measurement precision of Sr isotopes is 〈0.015% （2SD）, and that of Nd isotopes is 〈0.005% （2SD）. The established method is applied for Sr-Nd isotopic analysis of Durango apatite standard, Apatite 1 and PE （laboratory standard）, respectively. The analysis results indicate that the Sr-Nd isotopic compositions of the three apatite samples are basically consistent, and can serve as ideal reference samples for in-situ Sr-Nd isotopic analysis.