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

Abstract

A Pb isotope analytical method using a 207Pb– 204Pb 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— 206Pb/ 204Pb=16.9418±6; 207Pb/ 204Pb=15.5000±6; 208Pb/ 204Pb=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 Pb 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 207Pb behaviour as observed sometimes during TIMS analysis make DS analysis by MC-ICP-MS an attractive procedure. New Pb DS data on lavas from the Torfajökull volcanic centre (Iceland) define near-perfect binary mixing within our errors, even over a very small range in 207Pb/ 204Pb (<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., Theistareykir revisited, Geochem. Geophys. Geosyst., (2003) Paper no. 2001GC000201), corrected for mass bias using thallium, are up to 3300 ppm inaccurate compared to our DS data and also that of Thirlwall (Interlaboratory and other errors in Pb isotope analyses investigated using a 207Pb– 204Pb double spike, Chem. Geol., 163 (2000) 299–322) and Thirlwall et al. (Mantle components in Iceland and adjacent ridges investigated using double-spike Pb isotope ratios, Geochim. Cosmochim. Acta., 68, (2004) 361–386). These Icelandic data highlight both the potential and the dangers of Pb isotopic analysis by MC-ICP-MS and demonstrate the need for all MC-ICP-MS Pb isotopic studies to include analyses of well-characterized and isotopically homogeneous rock standards in addition to SRM 981. To this end, we present ca. 100 analyses of international standard reference materials, including the NIST 610–612–614 glasses that are used for in situ Pb isotope analysis by laser ablation ICP-MS and SIMS. While the Pb isotope compositions of some standards show poor reproducibility due to variable contamination during initial preparation, four of the reference materials (JB-2, NOD-A-1, AGV-1, BCR-1) and the NIST 610 and 612 glasses reproduce to <200 ppm. We also note that Pb isotopes are fractionated during normal anion exchange chemical separation of Pb, as collection of Pb in HCl significantly fractionates Pb isotope ratios, with the lighter isotopes preferentially eluted first. Provided a sufficiently large volume of ≥6 M HCl is collected (>10× the resin volume), there is no measurable (<50 ppm) fractionation of the Pb isotopes as the light-isotope-depleted tail is fully stripped from the resin.