In Situ Analysis Of Copper Isotopes In Copper-bearing Minerals By Femtosecond Laser Ablation MC-ICP-MS

In recent years,copper(Cu)isotopes have been widely applied in the study of metallogenesis because they are ubiquitous in major metal deposits.The multiple collector inductively coupled plasma mass spectrometry(MC-ICP-MS)provides a powerful analytical technique for high-precision analysis and fine research of Cu isotopes.On the basis of the successful development of the bulk analysis technology of whole rock Cu isotopes,more and more attention has been paid to the technology of Cu isotope analysis in micro-regions in order to further distinguish Cu isotope variations in single mineral samples with ring structure.In situ analysis technology can be used to determine the Cu isotope composition in individual mineral grain or sub grain scale in copper bearing minerals,which will provide geochemical constraints for the study of complex mineralization mechanism.There is no doubt that the wide variations of Cu isotope composition in minerals made it a potential powerful tracer for metal migration,metallogenic mechanism,and indicate the location of mineralization enrichment.However,the in-situ analysis of copper isotope composition is still bothered by varies problems and more studies are needed.In this study,we provided a detailed study on the determination of Cu isotopes in Cu bearing minerals using microdrill and MC-ICP-MS,then we investigated the non-matrix-matched determination of Cu isotopes in copper bearing minerals by femtosecond laser ablation(fsLA)-MC-ICP-MS and prepared one chalcopyrite reference material as external standard for the in situ determination of Cu isotopic compositions in Cu-dominated sulfide minerals.1.Accurate determination of Cu isotope compositions in Cu bearing minerals using microdrilling and SN-MC-ICP-MSTo obtain the Cu isotope composition within sub-grain,we provide a microsampling method which is a combination of microdrilling with the solution nebulization(SN)-MC-ICP-MS.Eleven different nature copper bearing minerals were sampled with microdrill and analyzed using MC-ICP-MS after digest for half an hour on a 140? hot plate without chromatography purification procedure.The Cu isotope results were calibrated using standard sample bracketing(SSB)method combine with NIST 994 Ga as internal standard.Cu isotope composition of same samples that processed after chromatography purification was also determined to verify the accuracy of this method.Results showed that ?65Cu of all sample solutions with or without chromatography were overlapped and the standard deviation between them is less than 0.04‰,which indicated that this method was applicable to most natural copper bearing minerals.Since the micro drilled copper mineral samples were direct digested without chemical purification,potential matrix elements including metal cations and some anions were still existed in solutions.Matrix effect of metal cations and anions was systematically investigated.Results have shown that the ?65Cu deviations are less than 0.05‰ within the uncertainty when the concentration of the matrix elements Zn,Fe,Ni,Al,Sb,Co/Cu+S was less than 2,Ti/(Cu+S)?0.8,and Mn/(Cu+S)?0.5.The ?65Cu of all sample solutions with or without chromatography are overlapped and the deviations between them are less than 0.04‰,indicating this method is applicable to most natural copper-bearing minerals.This method has the merits of saving time and reagents,low background,good accuracy,and reliability as well as improved spatial resolution.It is easier to achieve in most isotope laboratories compared to other microsampling methods such as laser ablation method.2.A candidate chalcopyrite reference material for LA-MC-ICP-MS copper isotope composition measurement was developed.Standard samples are the cornerstone of in situ analysis technology.A natural chalcopyrite reference material TC1725 has been prepared for in situ Cu isotope analysis by fsLA-MC-ICP-MS.Detailed textural examination and major element determination indicated that the TC1725 chalcopyrite grains have no growth rim or zoning structure.The chalcopyrite grains are homogeneous with respect to Cu isotope compositions based on a large number of measurements by bulk solution nebulizer(SN)-MC-ICP-MS,single grain SN-MC-ICP-MS,and fsLA-MC-ICP-MS.Traditional bulk analyses by SN-MC-ICP-MS yielded a mean ?65Cu value of-0.06±0.03‰(2SD,n=132).Randomly selected single chalcopyrite grain of TC1725 was further determined by SN-MC-ICP-MS without column chromatography,which gave a mean ?65Cu value of-0.06±0.04‰(2SD,n=188).The ?65Cu value determined by fsLA-MC-ICP-MS analyses on individual chalcopyrite particles and bulk strips was-0.06±0.03‰(2SD,n=393),which is consistent with the bulk isotope composition within uncertainty.The results show that the Cu isotope composition of chalcopyrite TC1725 is homogeneous,which is suitable as the external standard for in situ micro-region analysis technology,and the recommended Cu isotope composition is-0.06±0.03‰(2SD,n=525).3.Accurate analysis of Cu isotopes by fsLA-MC-ICP-MS using non-matrix-matched calibration method.The copper isotope compositions of copper-rich minerals have been determined using 206 nm deep ultraviolet femtosecond laser ablation multi collector inductively coupled plasma mass spectrometry.A pure copper wire NWU-Cu-B and a natural chalcopyrite TC1725 were used as bracketing standards for calibration.Reliable ?65Cu values can be obtained using matrix-matched standards under dry plasma condition and calibrated by sample standard bracketing method(SSB).However,the ?65Cu values calibrated by non-matrix-matched standards using 206 nm fsLA-MC-ICP-MS are seriously affected by matrix effect,with a deviation of up to 1.42‰.Therefore,matrix-matched standards are acquired for reliable Cu isotope ratios measurement using 206 nm UV fsLA-MC-ICP-MS and precise(<0.07‰,2SD)?65Cu values can be obtained using matrix-matched standards under dry plasma condition.In addition,the laser frequency and energy density can also affect the reliability of Cu isotope determination.According to the results,the laser fluence of less than 1 J cm-2 and frequency between 6-15Hz were applied to ensure that the laser-induced isotope fractionation can be ignored during the in situ Cu isotope analysis.Although the analytical accuracy(2SD)is slightly improved,the use of Ga as an internal standard does not reduce the systematic deviation caused by the matrix effect.The matrix effect can be significantly suppressed by adding 8.6 ?L min-1 water vapor and 2 mL min-1 nitrogen into the carrier gas.The matrix induced ?65Cu deviation of TC1725(chalcopyrite)calibrated against pure copper NWU-Cu-B is reduced from 0.99‰ in dry plasma condition to 0.03‰ in wet plasma condition.The ?65Cu of chalcopyrite TC1725 is-0.07±0.10‰(2SD,n=100)calibrated by NWU-Cu-B using SSB combine with Ga over six-month period analysis,the result is consistent with the reference value(-0.06±0.03‰,2SD)within the uncertainty.4.Accurate determination of Cu isotopic composition of copper bearing minerals without matrix-matched calibrationThe matrix effect of non-matrix-matched Cu isotope calibration is obviously reduced by adding water vapor and nitrogen into plasma.The in situ determination of Cu isotopes in 12 natural copper rich minerals(including native copper,sulfides,carbonates,oxides,and copper chloride)is carried out using 206 nm UV fsLA-MC-ICP-MS under the wet plasma cindition The results show that the ?65Cu of the copper sulfides(chalcopyrite CPY-1,CPY-SG;digenite DIG;covellite COV;tetrahedrite TET-G;bornite BOR-Q)are consistent with their solution results within the uncertainty(2SD)when using pure copper NWU-Cu-B as non-matrix-matched bracketing standard and calibrated using SSB combining Ga as doping element,the deviation are ranged from 0.03‰ to 0.13‰.The deviations of in situ ?65Cu value of carbonates(azurite AZU;malachite MAL-1,MAL-2)between bulk and in situ analyses are 0.06-0.12‰.The deviation of ?65Cu value of copper oxide(cuprite CUP-G)between bulk and in situ analyses is less than 0.10‰,and the deviation of ?65Cu of atacamite ATA-SG between bulk and in situ analyses is 0.25‰.According to the results,except the atacamite(ATA-SG)and the azurite(AZU)which have slightly larger deviation due to heterogeneity,the laser ablation analyzed?65Cu values of most samples are consistent with the solution values within the uncertainty after adding water vapor and N2 into plasma.In addition,the ?65Cu values of copper-bearing minerals using TC1725 as bracketing standard are also significantly optimized under the wet plasma condition.When TC1725 are used as bracketing standard to calibrate the ?65Cu value of carbonate minerals,the standard deviation of ?65Cu between in situ and bulk analyses are-0.17‰for the azurite AZU,-0.12‰ for the malachite MAL-1 and MAL-2 under wet plasma condition.The mean ?65Cu value of native copper(NC)calibrated by chalcopyrite TC1725 is also consistent with the solution value and the deviation is-0.05‰.These results indicate that the matrix effect of non-matrix-matched standardization has been significantly reduced under the wet plasma condition,whether using chalcopyrite or pure copper as bracketing standards.