S And Pb Isotope Analyses Using Solution Nebulization And Femtosecond Laser Ablation-Multiple Collector Inductively Coupled Plasma Mass Spectrometry And The Application In Sulfides

In this paper, the sulfur and lead isotope compositions in sulfides analyzed by solution and femtosecond laser ablation-mulitiple collector-inductively coupled plasma mass spectrometry (SN/fsLA-MC-ICPMS). A sulfide sample digestion method was setup followed by S isotope purification using AG 50-X8 resin chromatography. Both SN desolvating solution nebulization (DSN) sampling techniques for sulfur isotope composition measured with both standard MC-ICPMS (NP-II) and large dimesion MC-ICPMS (NP-1700) were comparied. An in situ microanalysis of Pb isotope compositions in sulfides analyzed by 266nm fsLA-MC-ICP-MS was set up.1. A sulfide digestion method using concentrated nitric acid (HNO3) and hydrochloric acid (HC1) was established. Sulfur was oxidized to the highest valence (SO42-).The S isotope separation and purification procedures was set up using, the AG 50-X8 cation resin chromatography. The recovery of S was higher than 97%.2. S isotope analysis was carried out by N P-II MC-ICPMS using standard-sample bracketing method to correct instrumental mass bias. The signals was acquiered using time-resolved-analysis (TRA) mode and the blank was substracted by On-Peak-Zero (OPZ) method. Ultra pure water (MQ-H2O) shows superior to 2% HN03 in terms of the wash out efficiency of background and S concentration matching range between standard and samples. Since the isobaric interference of (like O2+) could be removed in DSN mode, the mass resolutions of DSN and SN sampling modes were medium resolution and high resolution, respectively. Mass bias of S isotopes during MC-ICPMS measurement corrected by Cl was evaluated, and the results show that the Cl is not suitable for S mass bias correction due to the different fractionation behavior during MC-ICPMS analyses. Several reference materials from the International Atomic Energy Agency (IAEA) and Aesar Alfa (Alfa-S) were measured, and the results agreeed with the literature values within 2s uncertainties.3. A sulfur isotopes using Nu Plasma 1700 MC-ICPMS in high mass resolution mode (resolving power>12000) was set up. S isotopic compositions of some interational reference materials (including IAEA-S-2, S-3, NBS-123, Alfa-S and Spex-S) analyzed in this work were consistent with the previous studies within 2s uncertainties.4. An in situ microanalysis of Pb isotopes in sulfides using 266 nm fsLA-MC-ICP-MS was carried out. Fractionation and mass discrimination effects existing in the ICP-MS analytical processes were corrected using an internal reference Tl in conjunction with an external reference NIST SRM 610. The proposed method was used to analyze the Pb isotopic compositions of chalcopyrite, pyrite, and sphalerite from the Dulong Sn-Zn-In polymetallic ore district. The results showed that in this ore district, the sulfide minerals and different grains of the same sulfide mineral show a large variation in Pb content up to 1000-fold. The studied pyrites show relatively higher Pb contents and homogeneous Pb isotopic compositions, whereas the sphalerites have low Pb contents but most variable Pb isotopic compositions. It is suggested that the large variation of Pb isotopic composition may reflect a late hydrothermal superimposition on the primary sulfide formation. In addition, radiogenic Pb accumulated by radioactive decay of trace amounts of U over time in the host minerals may also be one of the causes for the large variation range of Pb content and Pb isotopic composition of those low-Pb sphalerites. Chalcopyrite and sphalerite grains with Pb content greater than 10 μg/g presented a consistent Pb isotopic distribution, while all the sulfide grains with Pb content greater than 100 μg/g had consistent Pb isotopic composition within 2s measurement uncertainties. The in-situ analysis of Pb isotopic composition agreed well with the results obtained by conventional chemical methods within 2s measurement uncertainties, indicating that the data obtained by fsLA-MC-ICP-MS are reliable. Additionally, this study indicates that the Pb isotopic composition could truthfully record the source of ore-forming minerals only for sulfide minerals with high Pb content. On the contrary, the Pb isotopic composition of low-Pb sulfide minerals may be affected by trace amounts of U in the host minerals that may lead to a highly radiogenic Pb isotope ratio. Alternatively, it is also possible that late fluid metasomatic overprinting may alter the Pb isotopic compositions.