Analysis of In situ Pb Isotope in Sulfides by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry

Pb isotope is an important geochemical tracer in the study of ore-forming material source, mineralization process and deposit formation mechanism. In this study, the matrix effect on Pb isotope determination by 193 nm excimer laser and multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was investigated, and the results showed that it was of importance to select an appropriate matrix matching standard material for determination of Pb isotope composition in sulfide. By the simulation, it was found that the accurate determination of Tl and Hg fractionation factor could effectively correct the interference of Hg-204-Pb-204 in the range of Hg-204/Pb-204 < 2. It was also found that the spot size (24-160 mu m) and the pulse frequency (2 - 20 Hz) did not affect the Pb isotope analysis, so changing the laser ablation parameters could solve the problem of narrow detection range for MC-ICP-MS. To solve the lack of sulfide solid standard materials, the preparation of sulfide standard materials by pressing and rapid melting method was attempted. The pellet samples showed a good homogeneity of Pb isotope composition. For the rapid melting method, there was Pb isotope fractionation in different batches. But the homogeneous Pb isotopic composition was found in the single-batch molten samples. The results showed that these two methods were promising candidates for the preparation of sulfide standard samples, though the rapid melting method needed to be further modified. The Pb isotopic compositions in natural sulfide samples (pyrite and sphalerite) and synthetic sulfides were determined by the established method. The measured values were consistent with the solution values in the error range.