ABSTRACT

A rapid and precise routine Pb isotope analytical technique for rock samples was developed using multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The mass discrimination of the Pb isotope ratios was corrected with the ²⁰⁵Tl/²⁰³Tl ratio of a Tl isotope reference material (NIST SRM 997) added to samples. The optimization of the analytical parameters and the minimization of the memory and matrix effects enabled to acquire the Pb isotope data rapidly (three samples per hour) with a reproducibility of 0.01% by consumption of 150 ng Pb per analysis. Repeated analyses of a Pb isotope reference material, NIST SRM 981, gave the average values of ²⁰⁶Pb/²⁰⁴Pb = 16.9308 ± 0.0010, ²⁰⁷Pb/²⁰⁴Pb = 15.4839 ± 0.0011, and ²⁰⁸Pb/²⁰⁴b = 36.6743 ± 0.0030 (2σ).
Pb isotope ratios of eleven Geological Survey of Japan rock reference samples were determined using this technique. In anion-exchange chemical separation, complete removal of Hg and Tl from the Pb fraction was confirmed. It was revealed that the Tl-normalization minimized the matrix effects caused by remaining impurities and medium, and the reproducibility of seven independent analyses of JB-2 was comparable to that of NIST SRM 981. Pb isotope ratios obtained for the rock reference samples showed a good agreement with those given previously.