In-situ Measurement Of Osmium Isotope Ratio In Sulfides And Its Geological Applicationof Mantle Research

Due to the special geochemical behavior of Re and Os during mantle melting,ReOs isotopic system has been broadly applied on research of mantle evolution.It is commonly inferred that Re is a moderately incompatible element which will enrich in melt,while Os is considered to be a compatible element which will maintain in residue and record original information of mantle.Because most of Os in mantle was controlled by sulfide,in-situ measurement of osmium isotope ratio in them can provide direct evidence about mantle evolution.Therefore,this thesis focus on how to in-situ determinate the Os isotope ratio in sulfide and how to apply this measurement in geological research,including preparation of reference material,improvement of data reduction,optimization of cups configuration and geological application.1.Preparation of reference materialIn general,there is not a commercialized sulfide-matrix reference material for insitu measurement of Os isotope ratio.More and more research demonstrate that pressed tablet from ultra-fine powder is suitable for prepare matrix-matched reference material which also conserve most component of powder.Therefore,we choose the domestic sulfide reference material which spiked Re or Os standard solution as original powder.Then,these powders were further pulverized by a planetary ball mill adopting ethyl alcohol as grinding aids.By this way,the particle size of most powders can be reduced to nanoscale.Then,these ultra-fine powders were pressed under high pressure to tablets or fused under high temperature and pressure to pellets.In general,the pressed tablets CR-1P and CO-1P were respectively used for isobaric interference and time-drift calibration.The pellets COR-1F,POR-1F and pressed tablet Mix-1P were used for verification experiment.The results of 187Re/185 Re for CR-1P and 187Os/188 Os for CO-1P agree with the reference value greatly.Both intensity of signal and ratio of different isotope signal were quite stable,which demonstrated the homogeneity of pressed tablets from ultra-fine powder meeting requirements of reference material.2.Improvement of data reductionFor in-situ measurement of Os isotope ratio using LA-MC-ICP-MS,the interference and mass fractionation are vital for accurate results.Due to Re only occur two isotpe which means the mass fractionation factor of Re cannot obtain from other interferencefree isotope pairs for interference calibration.In previous research,some people have tried to adopt desolation nebulizer system to add Ir aerosol as pseudo-internal standard to fix out this problem.Generally,this method not only ignore the mass fractionation during laser ablation,but also complicate analysis procedures.In order to overcome these disadvantages,different analysis routine and data reduction strategy were adopted in our research.Specifically,the Re mass fractionation factors were measured by analyzing pure Re sulfide pressed tablet CR-1P and then applied to samples via linear interpolation.And the calibration of Os mass fractionation is simpler which can be fixed out by adopting 189Os/188 Os as the internal standard.Both the results of solution and laser ablation analysis demonstrate that our method can effectively correct the isobaric interference and mass fractionation and then provide accurate 187Os/188 Os results.According statistics of solution and laser ablation analysis results,an equation was yielded to evaluate the relationship between degree of interference correction and accuracy of final result which is beneficial for assessing uncertainty of results.3.Optimization of analytical methodDuring determination of Os isotope ratio in some low-Os sulfide,we have to encounter that signal intensity is not enough.If we adopt Faraday cups as collectors for low signal intensity,the uncertainty of results will climb rapidly.Therefore,we need to change the cup configuration to ensure accuracy and precision of results by adopting ion counter.However,the employment of ion counter will bring some new problem including: calibration of effective yield,dead time and high voltage plateau.Before every LA-MCICP-MS analysis session,it is best to calibrate effective yield and dead time by solution experiment result of Au and Dy standard solution.Considering the yield of different ion counters will deteriorate over the time of analysis,both interference and mass fractionation calibration are adopting method of standard sample bracket(SSB).And the results of the tablet CO-1P and the pellet COR-1F demonstrate that cup configuration of all ion counters can receive 50 times less signal intensity than before.4.Geological applicationThere are huge amounts of mantle xenoliths occurring in North China Craton(NCC),and which has been a most important area to research destruction of craton.These mantle xenoliths directly record many geological information of melt/fluid.In this research,the chose samples was the mantle xenoliths from Damaping area which located at the northern part of NCC.In order to identify melt/fluid record clearly,this sample was performed details petrological observations and in-situ major and trace-element,in-situ Sr and Os isotope composition analysis.The sample DMP-331 is an lherzolite containing a lot of large clinopyroxene(>0.5mm)which indicates it may process mantle metasomatism.Based on the petrological features,the sulfides can be classified into two types: intragranular and intergranular sulfide.There is an intragranular was surrounded by an clinopyroxene.Both Mg#(88.4-88.9)and(La/Yb)N-Ti/Eu in the clinopyroxene demonstrate that it was the product of silicic melt.According to the rare earth elements(REE)of clinopyroxene,we can calculate the REE of melt which equilibrate with this clinopyroxene.However,the REE distribution of melt is not agree with that of host basalt which means this clinopyroxene is from earlier siliceous melt.Both in-situ Sr isotopic composition(0.70389-0.70419)in clinopyroxene and in-situ Os isotopic composition(0.1677)in intragranular sulfide are higher than that of the whole peridotites in this area,which indicate the earlier siliceous melt contain some recycling crust material.