The in-situ Sr isotope determination of solid samples has the advantages of high spatial resolution and convenient and rapid sample preparation.However,due to the lack of column chemistry separation of matrix elements from Sr,the interference correction(especially the isobaric interference from 87Rb on 87Sr)becomes a big issue for high precise in-situ Sr isotope analytical results.In this study,we evaluated the influence of isobaric interference systematically and adopted an improved strategy for isobaric interference correction of Rb.Specifically,as an external standard for data calibration,an in-house silicate standard(Rb-std)was fused from pure oxide powders.The new standard(Rb-std)shows very low contents of Sr(<1 ppm),Yb(<0.09 ppm)and Er(<0.05 ppm),which directly avoid the trouble of interference of doubly charged ions and isobar,when we determinate the mass fractionation factors of Rb(βRb).Our improved method can provide more accurate data than previous researches,in which the mass fractionation were calculated from an external reference material StHs6/80-G with high Rb(30.07 ppm),Sr(482 ppm),Yb(1.13 ppm)and Er(1.18 ppm).Then,these βRb from Rb-std were applied by linear interpolation to the unknown samples for correction of 87Rb on 87Sr.The analytical results of serial international reference materials(BIR-1G,BCR-2G,BHVO-2G,T1-G and ATHO-G)demonstrate that our improved method can raise the upper limit of Rb/Sr for in-situ Sr isotope determination and reduce the relative standard deviation of results.This improvement of the upper limit of Rb/Sr(<0.33)will expand not only range of microanalysis for silicate minerals,but also bulk Sr isotope determination of volcanic rock combining with rapid fusion technique.
Isotope plays an important role in both tracing and dating in earth science, especially 87Rb-86 Sr system. With the development of earth science, whole-rock analysis can＇t sufficiently meet the requirements for scientific research and the micro-analysis becomes more and more significant. Laser ablation multi-collector inductively-coupled plasma mass-spectrometry（LA-MC-ICP-MS） has been extensively applied in micro-zone analysis due to its low sample-consumption, high accuracy, in situ and low requirements on matrix, but it is still difficult to accurately measure Sr isotope compositions especially for the samples with high Rb/Sr ratios and low Sr contents as it is restricted by severe quality discrimination and various types of mass spectrum interferences. Consequently, thermal ionization mass-spectrometry（TIMS）, as the most accurate and precise method to analyze isotopic ratios, is still the most popular method of analyzing Sr ratios, especially for the samples with low Sr contents. This paper makes a systematic review on the high-precision Sr isotope analyses of low-Sr geological samples, including the micro-sampling technique, ultra-low procedural blank chemical method and TIMS measurement technique. The combination of ultra-low procedural blank and TIMS can be used to perform high-precision micro-analysis of the samples with ng magnitude, which will be undoubtedly an important direction for Rb-Sr geochronology, geochemistry and environmental studies.
Jie LinYongsheng LiuHaihong ChenLian ZhouZhaochu HuShan Gao
Laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS),which is a rapidly developing analytical technique for the analyses of trace elements and isotopes,plays an important role in advancing the study of Earth science,especially with respect to micro-geochemistry.This article reviews the application of LA-ICP-MS in the elemental analysis of solid geological samples.Although LA-ICP-MS has been widely used in the spatial resolution analysis of element compositions and rapid bulk analysis of whole-rock and soil samples,the analysis accuracy is restricted by numerous factors,including the instrumental conditions,the elemental fractionation and matrix effects,the lack of sufficient matrix-matched reference materials,and the strategies for quantitative calibration and sensitivity drift correction.According to the type of samples and the analyte elements,the analysis accuracy can be improved through the optimization of instrument conditions and the adoption of suitable correction strategies and reference materials.
