The Development Of The Analytical Method Of Sulfur Isotopes By MC-ICP-MS And Its Geological Applications

Multi-collector inductively coupled plasma mass spectrometry(MC-ICP-MS)has been widely applied to the analysis of sulfur isotopic compositions and has benefited the geochemical research on sulfur isotopes a lot.In this thesis,the spectral inter-ferences,instrumental mass bias and matrix effects during sulfur isotopic analysis by MC-ICP-MS have been investigated in detail and a solid method has been established which is suitable for both bulk and in situ sulfur isotopic determination.The spectral interferences on sulfur isotopes mainly come from O2+ polyatomic interferences,which can be separated from S+ efficiently in medium mass resolution mode to achieve accurate ?34S results.The measurement of ?33S must be performed in high mass resolution mode to distinguish 32S1H from 33S.In the MC-ICP-MS sulfur isotopic measurement,the instrumental mass bias is corrected by the standard-sample bracketing(SSB)method,so the concentration effects and matrix effects need to be paid attention to.The concentration effects during sulfur isotopic measurement are very obvious while can be corrected quantitatively.The concentration effect correction curve can be applied to the samples varying in sulfur concentrations to simplify the analytical procedure.The investigation of matrix effects deepens our understanding of matrix effects during MC-ICP-MS measurement.A few important conclusions have been obtained based on the exploration of the matrix effects of Ca:(1)it is the absolute concentration of matrix element rather than the matrix/analyte ratio that dominates the matrix effects;(2)the presence of matrix element will influence(increase or suppress)the signal sensitivity of the analyte and the influence is also mainly dependent on the absolute concentration of matrix element;(3)compared with wet plasma,the matrix effects under dry plasma condition are more significant.The studies of matrix effects also simplify the analytical procedure of pore water and gypsum samples,which are beneficial to the related geochemical research.In this thesis,the established analytical method has been used to study the sulfur isotopic compositions of the sediment pore water samples of South China Sea(by bulk analysis)and the pyrite of Xinyu banded iron formation(BIF)(by in situ analysis).The results of pore water samples indicate that obvious dissimilatory sulfate reduction pro-cesses occur at Site QDN-1,and at this site,the anaerobic oxidation of methane(AOM)reaction plays an important role in the sulfate reduction processes.The calculation re-sults based on the Rayleigh distillation law shows that the fractionation factor between sulfate and sulfide at Site QND-1 is 13.18‰.This relatively low fractionation factor is probably the result of the relatively fast rate of sulfate reduction.The results of the sulfur isotopic compositions of the pyrite samples in Xinyu BIF show that the pyrite samples exhibit a very large range(?47‰)of ?b34S values and the lowest ?34S value is-40.2‰.The lowest ?34S value probably reflects the low ?34Ssulfate value(probably less than 10‰)of the ocean when the pyrite formed.The low ?34Ssulfate value of the ocean probably results from the increased sulfur flux by sulfide weathering from the rivers to the sea.The large variability in ?34Spyrite in the Neoproterozoic Xinyu BIF can be explained as a consequence of low sulfate concentrations and rapidly fluctuating?34Ssulfate in seawater.