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

Abstract

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 16–37 μm 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 (2 s.d.) of 39 and 58 ppm. 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 45–87 ppm. These results clearly demonstrate the capabilities of the LA-MC-ICPMS technique applied to monazite Sm–Nd isotope systematics for early crustal evolution studies.