Precise and accurate determination of ¹⁴⁷Sm/¹⁴⁴Nd and ¹⁴³Nd/¹⁴⁴Nd in monazite using laser ablation-MC-ICPMS

The ¹⁴⁷Sm-¹⁴³Nd radioactive decay system has been widely used in the Earth sciences as a geochronometer and isotopic tracer. Recent studies have shown that laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) is capable of precise and rapid ¹⁴³Nd/¹⁴⁴Nd measurements with high spatial resolution, but its ability to measure ¹⁴⁷Sm/¹⁴⁴Nd accurately is still poorly constrained. A critical analytical issue is the fractionation effects between ¹⁴⁷Sm and ¹⁴⁴Nd in LA-MC-ICPMS, which must be carefully corrected to determine robust initial ¹⁴³Nd/¹⁴⁴Nd particularly for ancient geological samples. Here we show that LA-MC-ICPMS can produce substantial (>5%) ¹⁴⁷Sm/¹⁴⁴Nd fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of ~1% by using an independently defined ¹⁴⁷Sm-¹⁴³Nd isochron for an Archean monazite standard. In addition, we describe a method for improving the precision and accuracy of LA-MC-ICPMS isotopic measurements by calibration of Faraday amplifier response and isotopic ratio normalization using a synthetic standard. This method has been applied to one Neogene and two Archean monazites, which were also analyzed by isotope dilution-thermal ionization mass spectrometry (ID-TIMS). The LA-MC-ICPMS data were acquired from 16-37μm single spots. The LA-MC-ICPMS determinations of the ¹⁴⁵Nd/¹⁴⁴Nd for all samples and ¹⁴³Nd/¹⁴⁴Nd for the Neogene monazite compare well with the ID-TIMS results, indicating the robustness of our method to measure Nd isotopic ratios. For the Archean monazites, the method yields initial ¹⁴³Nd/¹⁴⁴Nd values identical to the ID-TIMS values with reproducibilities (2s.d.) of 39 and 58ppm. Analytical precisions of individual spot analyses for initial ¹⁴³Nd/¹⁴⁴Nd were calculated by taking into account the uncertainty in (i) the Nd isotopic normalization to the synthetic monazite, (ii) the correction factor for ¹⁴⁷Sm/¹⁴⁴Nd fractionation and (iii) the age, which results in 2σ of 45-87ppm. These results clearly demonstrate the capabilities of the LA-MC-ICPMS technique applied to monazite Sm-Nd isotope systematics for early crustal evolution studies.