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

Abstract

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 multicollector inductively-coupled plasma mass spectrometer (MC-ICP-MS) at the University of Queensland. 203Tl-205 Tl 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 206 205 Tl. 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/Tl ratios), but can be severe for low-Pb sample solutions when over-"spiked" with Tl. Experiments in this study suggest that a minimum concentration ratio of Pb/Tl>5 in Tl-"spiked" solutions be required to ensure reliable 206 Tl 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 Tl and 205 Tl 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.