Sm-Nd Isotope Systematics by Laser Ablation-Multicollector-Inductively Coupled Plasma Mass Spectrometry: Methods and Potential Natural and Synthetic Reference Materials

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An improved method for precise and accurate in situ determination of Sm–Nd isotopes, with high spatial resolution, and high sample throughput, in light rare earth element enriched accessory minerals by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is discussed. The accuracy of the method is demonstrated by comparison of LA-MC-ICPMS analyses of several natural minerals (Durango apatite, Hondo Canyon titanite, Daibosatsu allanite, Mae Klang monazite, and Trebilcock monazite) to isotope dilution-thermal ionization mass spectrometry (ID-TIMS) analyses of the same minerals. The TIMS analyses demonstrate that each of these minerals, with the possible exception of Durango apatite, is more homogeneous in Nd isotopic composition than can be measured based upon the internal precision of the LA-MC-ICPMS analyses in this study. Thus, these natural minerals may be considered as isotopically homogeneous reference materials for Nd isotopic determinations. In addition two synthetic glasses produced specifically for in situ Sm–Nd isotopic determination and external calibration are presented here.

The three main obstacles in obtaining accurate and precise Sm–Nd isotopic measurements by LA-MC-ICPMS are the isobaric interference correction of 144Sm on 144Nd, determination of the Sm mass bias, and accurately measuring the 147Sm/144Nd which is imperative for producing robust initial Nd isotopic compositions. The 144Sm interference correction was calculated using the measured 149Sm and recently published Sm isotopic abundances. The Sm mass bias was determined using an exponential law and the 147Sm/149Sm measured in the sample. Determination of 147Sm/144Nd was done by calibration to an external glass reference material synthesized specifically for this purpose. This approach produced 147Sm/144Nd that is in agreement with ID-TIMS analysis. The MC-ICPMS instrument configuration used in this study also allows for the determination of Eu and Gd, thereby permitting simultaneous determination of Eu anomalies along with Sm–Nd isotopic composition. The high spatial resolution and sample throughput at a geologically useful level of accuracy and precision possible with LA-MC-ICPMS offers numerous potential geochemical applications including provenance tracing, terrane reconstruction, crustal growth studies, fluid composition and evolution, metamorphic reaction pathways, and silicic magma generation and evolution.