Li, H.K. & Niu, Yaoling
(2003). Multi-collector ICP-MS analysis of Pb isotope ratios in actual rocks: High quality data, simple procedure and needed cautions. Acta Geologica Sinica (English Edition) 77
Author(s) from Durham
We measured Pb isotope compositions of seven USGS rock reference standards AGV-1, AGV-2, BHVO-1, BHVO-2, BCR-2, BIR-1/1 and W-2 together with NBS 981 using a Micromass Isoprobe multi-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) at The University of Queensland. We used thallium (with 203 Tl and 205Tl isotopes) as an internal standard to correct for mass-dependant isotopic fractionation. Our results for both NBS 981 and USGS rock standards AGV-1, AGV-2, BHVO-1 and BCR-2 are comparable to or better than double- and triple-spike TIMS data in precision. Our data for BHVO-2 are reproducibly lower for 208Pb/206Pb and 207Pb/206Pb, and higher for 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb than double-spike TIMS data in the literature. We also obtained Pb isotope data for BIR-1/1 and W-2, which may be used as reference values in future studies. Linear correction for mass-dependent Pb isotopic fractionation is adequate with the results identical to those corrected following exponential law or power law. Experiments done on rock solutions with Pb concentration levels of over three orders of magnitude show little matrix effect on Pb isotope ratios. Precise 207Pb/206Pb, 208Pb/206Pb and 208Pb/207Pb ratios can be acquired for sample solutions with Pb 1 ppb. However, Pb isotope ratios involving 204Pb (i.e., 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) are reliable for solutions with Pb 40 ppb Pb. The error for Pb isotope ratio analysis using MC-ICP-MS is dominated by errors in the analysis of 204Pb, which is commonly ascribed to the difficulty and imprecise correction for a 204Hg isobaric interference. While 204Hg interference is a source of error, and short-term peak shift (up to ± 0.2 amu) may also contribute to the error, we found however that the major source of error on 204Pb comes from the tailings of mass 203Tl and mass 205Tl. These mass tailings lead to over-subtraction of the baseline for 204Pb, which is often measured at ± 0.5 amu on both sides of mass-204 (i.e., at amu 203.5 and 204.5 respectively). Such errors are insignificant for Pb-rich samples (i.e., high Pb/Tl ratios in sample solutions to be measured), but can be severe for low-Pb samples when sample solutions are over-spiked with Tl. We suggest that a minimum concentration ratio of Pb/Tl > 5 in Tl-spiked solutions is required to ensure reliable 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotopic ratios. The tailings of 203Tl and 205Tl can also lead to over-subtraction of baselines for 202Hg (at amu 202.5) and 206Pb (at amu 205.5). Therefore, the elegance of using 203 Tl and 205Tl isotopes for mass fractionation correction becomes a severe problem in low-Pb rock solutions - caution is required. Alternative internal standards for mass fractionation correction may be considered. Certainly, significant instrumental refinement in mass resolution (<< 0.5 amu) is in demand.