Multi‐collector ICP‐MS Analysis of Pb Isotope Ratios in Rocks: Data, Procedure and Caution

The authors measured Pb isotope compositions of seven USGS rock reference standards, i.e. 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. 203 T1‐ 205 T1 isotopes were used as an internal standard to correct for mass‐dependant isotopic fractionation. The results for both NBS 981 and USGS rock standards AGV‐1 and BHVO‐1 are comparable to or better than double‐ and triple‐spike TIMS (thermal ionization mass spectrometry) data in precision. The data for BHVO‐2 and, to a lesser extent, AGV‐2 and BCR‐2 are reproducibly higher for 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb than double‐spike TIMS data in the literature. The authors also obtained the Pb isotope data for BIR‐1/1 and W‐2, which may be used as reference values in future studies. It is found that linear correction for Pb isotopic fractionation is adequate with the results identical to those corrected following an exponential law or a power law. Precise 207 Pb/ 206 Pb, 208 Pb/ 206 Pb and 208 Pb/ 207 Pb ratios can be acquired for sample solutions with Pb≥1 ppb. However, Pb isotope ratios involving 204 Pb (i.e., 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb) are reliable for solutions with Pb≥40 ppb. The errors for Pb isotope ratio analysis using the MC‐ICP‐MS are dominated by errors in the analysis of 204 Pb, which is commonly ascribed to the difficulty and imprecise correction for a 204 Hg isobaric interference. It is found however that the major errors on 204 Pb come from the tailings of mass 203 T1 and mass 205 T1. These mass tailings lead to over‐subtraction of the baseline for 204 Pb, which is 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 sample solutions (i.e., high Pb/T1 ratios), but can be severe for low‐Pb sample solutions when over‐“spiked” with T1. Experiments in this study suggest that a minimum concentration ratio of Pb/T1>5 in T1‐“spiked” solutions be required to ensure reliable 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb isotopic ratios. The tailings of 203 T1 and 205 T1 can also lead to over‐subtraction of baselines for 202 Hg (at amu 202.5) and 206 Pb (at amu 205.5). Therefore, the elegance of using 203 T1 and 205 T1 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. Of course, significant instrumental refinement in abundance sensitivity is in demand.