Improved in-situ Determination of Sr Isotope Ratio in Silicate Samples Using LA-MC-ICP-MS and Its Wider Application for Fused Rock Powder

Abstract

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.