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

The Sm-147-Nd-143 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 Nd-143/Nd-144 measurements with high spatial resolution, but its ability to measure Sm-147-Nd-144 accurately is still poorly constrained. A critical analytical issue is the fractionation effects between Sm-147 and Nd-144 in IA-MC-ICPMS, which must be carefully corrected to determine robust initial Nd-143/Nd-144 particularly for ancient geological samples. Here we show that LA-MC-ICPMS can produce substantial (> 5%) Sm-147-Nd-144 fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of similar to 1% by using an independently defined Sm-147-Nd-143 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 mu m single spots. The LA-MC-ICPMS determinations of the Nd-145-Nd-144 for all samples and Nd-143-Nd-144 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 Nd-143/Nd-144 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 Nd-143/Nd-144 were calculated by taking into account the uncertainty in (i) the Nd isotopic normalization to the synthetic monazite, (ii) the correction factor for Sm-147-Nd-144 fractionation and (iii) the age, which results in 2 sigma 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. (C) 2011 Elsevier B.V. All rights reserved.