| The fractionation of sulfur isotopes during geological processes is distinct,and therefore sulfur isotopes are widely used as tracers in geology.Conventional bulk analytical techniques for sulfur isotope ratios include the gas-source mass spectrometry(GS-MS)and the solution nebulization multi-collector inductively coupled plasma mass spectrometer(MC-ICPMS).But with the development of in-situ analysis technique,a growing number of in-situ geochemistry information give new insights into geological process,thus sulfur isotope analysis in micro-area is important.The method of MC-ICPMS has widely detection limit,good accuracy and precision in isotope analysis.Therefore,LA-MC-ICPMS is a promising method to analyze sulfur isotopes in micro-area.Previous study demonstrate that many factors will affect accuracy and precision of results,such as laser induced fractionation,matrix effects,doubly charged ions and polyatomic ion interference,ionization condition in ICP.So the experiments about these factors should be designed to improve the method.The measurements use standard-sample bracketing to correct mass bias,with sulfur isotope compositions presented as values relative to a reference;thus,the standard is a key factor in sulfur measurement.A series of sulfur-bearing materials were prepared in this article,including natural sulfides(pyrite,chalcopyrite,sphalerite,galena),pressed sulfides powder tablets(pyrite,chalcopyrite,sphalerite,galena,Ag2S and synthetic sulfide)and the sulfides glasses(pyrite,chalcopyrite)by high temperature in inert gas environment.The sulfur isotope homogeneities of these materials were tested by long-time tests using LA-MC-ICPMS.Solution MC-ICPMS and GS-MS were used to determine the sulfur isotope composition of these materials.A femtosecond laser system and an ArF excimer laser system were connected with a high resolution MC-ICPMS to analysis sulfur isotopes and the laser induced fractionation was studied.Experiments show that the femtosecond laser has advantages in sulfur analysis.It lacks of thermal effects and generates sample aerosol particles with minor and uniform size,which make a high sensitivity and less fractionation.But the matrix effects still exist because the transfer and ionization efficiency of aerosol particles is insufficient.On the one hand,although the particle size is smaller and more homogeneous than ns-laser,fs-laser generates the large spherical particles(hundreds of nanometer)and nanoparticle agglomerates produced by femtosecond laser may result in a differentiated transfer,vaporization and ionization efficiency.On the other hand,high mass load and the position dependent fractionation in ablation cell also lead to matrix effects.The pressed powder tablets and natural sulfides have different response to laser parameters,which aggravate matrix effects.In addition,the crater ablated by fs-laser is irregular because its energy distribution is asymmetric.As the hole getting deeper,the transfer efficiency become worse and the signal drop faster,this limits the development of single spot ablation mode by fs-laser.According to the results,matrix effects will decrease at a lower fluence and smaller spot size.And an ablation cell design with high aerosol transfer efficiency is important for in-situ sulfur isotope analysis by fs-laser MC-ICPMS.The thermal effects are common and the sizes of aerosol particles are diverse when ablated sulfide using ArF excimer laser.The large and small particles have different transfer and ionization efficiency which result in matrix effects.Especially for chalcopyrite and galena,the serious thermal effects will cause the obvious down hole fractionation.The results will keep the lowest matrix effects at the laser fulence of 3.5 J/cm2-4 J/cm2 and low frequency(3 or 4 Hz).The heavy matrix mass will aggravate the DHF and matrix effects.Ablation with line scan mode is a way to reduce DHF and matrix effects,but it also limit space resolution.Reducing the makeup gas(Ar)can minish the matrix effects of some sulfides,but it is not effective for Ag2S and galena.Adding a small amount of N2 in makeup gas can markedly control fractionation and increase sensitivity in low Ar condition.The matrix effects can be removed when the N2 flow rate is 4 mL/min and Ar flow rate was adjusted to obtain the highest signal intensity.However,the matrix effects are instability because the matrix effects are very sensitive to the gas flow rate,especially to N2 flow rate.The long time measurements for matrix effects should be executed to monitor the range of fluctuation of matrix effects.Based on the research of the method,the genesis of a controversial Zn-Pb deposit(BFS-ELH)was studied by in-situ S isotope analysis,in order to distinguish the different ore-forming stages and identify their material source.The results showed that the δ34S values for coarse sphalerite of the main mineralizing stages were concentrated in a small range,with a heavy S isotope composition(about 12‰).The sulfides(sphalerite,galena and pyrite)with of a typical alteration structure and have a relatively light S isotope composition(about 7‰).These compositions are similar to those of the sulfides in late sulfide-rich quartz veins crossing the major orebody(about 4‰)and in granodiorite rocks(about 6‰).The heavy sulfur of sphalerite and galena in the major orebody may come from the Thermochemical Sulfate Reduction(TSR)of Devonian seawater,and the light sulfur of the late stage sulfides could result from a metamorphic and magmatic hydrotherm during the Indosinian orogenesis. |
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