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

• Published in: Chemical geology 2011-03, Vol.282 (1-2), p.45-57
• Main Authors: Iizuka, Tsuyoshi, Eggins, Stephen M, McCulloch, Malcolm T, Kinsley, Leslie PJ, Mortimer, Graham E
• Format: Article
• Language: English
• Subjects: Ablation; Amplifiers; Fractionation; Lasers; Mathematical analysis; Monazite; Neodymium; Spots
• ISSN: 0009-2541
• DOI: 10.1016/j.chemgeo.2011.01.008

The super(147)Sm- super(143)Nd 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 super(143)Nd/ super(144)Nd measurements with high spatial resolution, but its ability to measure super(147)Sm/ super(144)Nd accurately is still poorly constrained. A critical analytical issue is the fractionation effects between super(147)Sm and super(144)Nd in LA-MC-ICPMS, which must be carefully corrected to determine robust initial super(143)Nd/ super(144)Nd particularly for ancient geological samples. Here we show that LA-MC-ICPMS can produce substantial (>5%) super(147)Sm/ super(144)Nd fractionation during analysis of monazite and that the fractionation can be corrected with uncertainty of ~1% by using an independently defined super(147)Sm-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 mu m single spots. The LA-MC-ICPMS determinations of the super(145)Nd/ super(144)Nd for all samples and super(143)Nd/ super(144)Nd 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 super(143)Nd/ super(144)Nd values identical to the ID-TIMS values with reproducibilities (2s.d.) of 39 and 58ppm. Analytical precisions of individual spot analyses for initial super(143)Nd/ super(144)Nd were calculated by taking into account the uncertainty in (i) the Nd isotopic normalization to the synthetic monazite, (ii) the correction factor for super(147)Sm/ super(144)Nd fractionation and (iii) the age, which results in 2 sigma 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.