Pb isotopic analysis of standards using a 207Pb-204Pb double spike and thallium to correct for mass bias with a double focusing MC-ICP-MS

• Published in: Chemical Geology
• Main Authors: BAKER J. A, PEATE D, WAIGHT T, MEYZEN, CHRISTINE MARIE
• Other Authors: Baker, J. A., Peate, D, Waight, T, Meyzen, CHRISTINE MARIE
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV:PO Box 211, 1000 AE Amsterdam Netherlands:011 31 20 4853757, 011 31 20 4853642, 011 31 20 4853641, EMAIL: [e-mail], INTERNET: http://www.elsevier.nl, Fax: [phone] 2004
• Subjects: Iceland
• Online Access: http://hdl.handle.net/11577/154423; https://doi.org/10.1016/j.chemgeo.2004.06.030; http://www.sciencedirect.com/science/article/pii/S0009254104002529

A Pb isotope analytical method using a Pb-207-Pb-204 double spike (DS) and a double-focusing MC-ICP-MS routinely yields standard and sample data with an external reproducibility <100 ppm for all Pb isotope ratios. With modest increases in analysis times (<10 min), external reproducibility of :! 50 ppm is attainable, e.g., SRM 981-Pb-206/Pb-204=16.9418 +/- 6; Pb-207/Pb-204=15.5000 +/- 6; Pb-208/Pb-204=36.7265 +/- 19 (n=26; 2 S.D.). This is achievable even when unspiked and spiked runs are analysed sequentially, demonstrating that memory due to use of the DS can be easily overcome by short and careful washout routines coupled with on-peak zeroes measurements. Replicate analyses of three mixtures of SRM 981 and SRM 982, in variable proportions, lie within 42 ppm of a mixing line between SRM 981 and SRM 982 Pb isotope ratios. Coupled use of thallium to correct for mass bias during the same analyses of standards and samples does not produce Pb isotopic data of comparable quality to the DS-corrected analyses. While the approach of Woodhead (A simple method for obtaining highly accurate Pb isotope data by MC-ICP-MS, J. Anal. At. Spectrom., 17 (2002) 1381-1385) allows us to produce accurate thallium-corrected Ph isotopic data, the DS data are at least five times more precise and accurate than those obtained when using thallium to correct for mass bias. Relatively rapid analytical throughput (20-25 samples/day), elimination of differential loading blanks between unspiked and spiked runs, easy optimal spiking and the lack of anomalous 207 Ph behaviour as observed sometimes during TIMS analysis make DS analysis by MC-ICP-MS an attractive procedure. New Ph DS data on lavas from the Torfajokull volcanic centre (Iceland) define near-perfect binary mixing within our errors, even over a very small range in Pb-207/Pb-204 (<300 ppm), comparable to or smaller than the reproducibility of thallium-corrected Pb isotopic data. We also show that recently published Pb isotopic data for Theistareykir picrites (Stracke, et al., ...