Direct lead isotope analysis in Hg-rich sulfides by LA-MC-ICP-MS with a gas exchange device and matrix-matched calibration

摘要

In situ Pb isotope data of sulfide samples measured by LA-MC-ICP-MS provide valuable geochemical information for studies of the origin and evolution of ore deposits. However, the severe isobaric interference of Hg-204 on Pb-204 and the lack of matrix-matched sulfide reference materials limit the precision of Pb isotopic analyses for Hg-rich sulfides. In this study, we observe that Hg forms vapor and can be completely removed from sample aerosol particles produced by laser ablation using a gas exchange device. Additionally, this device does not influence the signal intensities of Pb isotopes. The within-run precision, the external reproducibility and the analytical accuracy are significantly improved for the Hg-rich sulfide samples using this mercury-vapor-removing device. Matrix effects are observed when using silicate glass reference materials as the external standards to assess the relationship of mass fractionation factors between Tl and Pb in sulfide samples, resulting in a maximum deviation of similar to 0.20% for Pb-20x/Pb-204. Matrix-matched reference materials are therefore required for the highly precise and accurate Pb isotope analyses of sulfide samples. We investigated two sulfide samples, MASS-1 (the Unites States Geological Survey reference materials) and Sph-HYLM (a natural sphalerite), as potential candidates. Repeated analyses of the two proposed sulfide reference materials by LA-MC-ICP-MS yield good external reproducibility of <0.04% (RSD, k = 2) for Pb-20x/206Pb and < 0.06% (RSD, k = 2) for Pb-20x/Pb-204 with the exception of Pb-20x/Pb-204 in MASS-1, which provided an external reproducibility of 0.24% (RSD, k = 2). Because the concentration of Pb in MASS-1 (76 mu g g(-1)) is similar to 5.2 times lower than that in Sph-HYLM (394 +/- 264 mu g g(-1)). The in situ analytical results of MASS-1 and Sph-HYLM are consistent with the values obtained by solution MC-ICP-MS, demonstrating the reliability and robustness of our analytical protocol. (C) 2016 Elsevier B.V. All rights reserved.