Accurate Determination of Sr Isotopic Compositions in Clinopyroxene and Silicate Glasses by LA‐MC‐ICP‐MS

Abstract

The low‐Sr content (generally < 100 μg g−1) in clinopyroxene from peridotite makes accurate Sr isotopic determination by LA‐MC‐ICP‐MS a challenge. The effects of adding N2 to the sample gas and using a guard electrode (GE) on instrumental sensitivity for Sr isotopic determination by LA‐MC‐ICP‐MS were investigated. Results revealed no significant sensitivity enhancement of Sr by adding N2 to the ICP. Although using a GE led to a two‐fold sensitivity enhancement, it significantly increased the yield of polyatomic ion interferences of Ca‐related ions and TiAr+ on Sr isotopes. Applying the method established in this work, 87Sr/86Sr ratios (Rb/Sr < 0.14) of natural clinopyroxene from mantle and silicate glasses were accurately measured with similar measurement repeatability (0.0009–0.00006, 2SE) to previous studies but using a smaller spot size of 120 μm and low‐to‐moderate Sr content (30–518 μg g−1). The measurement reproducibility was 0.0004 (2s, n = 33) for a sample with 100 μg g−1 Sr. Destruction of the crystal structure by sample fusion showed no effect on Sr isotopic determination. Synthesised glasses with major element compositions similar to natural clinopyroxene have the potential to be adopted as reference materials for Sr isotopic determination by LA‐MC‐ICP‐MS.

Calibration and correction of LA-ICP-MS and LA-MC-ICP-MS analyses for element contents and isotopic ratios

“…For example, the application of shield torch can reduce the electric potential difference between the plasma and the tip of the sampling cone, thus the doubly charged ion yield can be reduced Gray, 1986). While the shield torch can significantly increase the polyatomic ion yield, for example, the interferences of Ca-related and TiAr þ ions on Sr isotopes can be elevated (Tong et al, 2015). The doubly charged ion yield and polyatomic ion yield can be optimized by adjusting instrument parameters such as flow rate of nebulizer gas, sampling depth and RF power .…”

“…The data reduction of Sr isotopic ratios includes corrections for background, isobaric interference and instrumental mass fractionation (Ramos et al, 2004;Tong et al, 2015;Vroon et al, 2008;Yang et al, 2009). The gas blank measurement was made to eliminate the effect of the small, persistent yet stable Rb and Sr backgrounds from the torch and cones, as well as Kr from the argon and helium supplies.…”

“…Besides trace elements of the silicate mineral , the carbonate , metal oxide and sulfide minerals and single melt/fluid inclusion (Zhang et al, 2011) can also be calculated; The time-dependent sensitivity drift in the analyses can be corrected by the measurements of quality control reference materials, which can be easily chosen by the user; The isotopic ratios of Li , Sr (Tong et al, 2015), Nd , Hf Liu et al, 2010a) and Os (Zhu et al, 2016) measured by LA-MC-ICP-MS can be calibrated and corrected; Data reduction of zircon UePb isotopic dating and trace element analyses can be quickly done at one time (Liu et al, 2010b); The used external standards can be automatically identified and easily changed (replace, add and delete) by the user; The unfinished data reduction can be processed continually as long as the relevant "data summery file" was saved; Data reduction for solution-ICP-MS analysis can also be done by ICPMSDataCal.…”

“…For example, the interferences of 168 Er 2þ on 84 Sr þ , 170 Er 2þ and 170 Yb 2þ on 85 Rb þ , 172 Yb 2þ on 86 Sr þ , and 174 Yb 2þ on 87 Sr þ can be corrected based on the measured signal intensity of the interference-free 167 Er 2þ , 173 Yb 2þ and the natural isotopic ratio of Er and Yb when Sr isotopic ratios of silicate glasses and minerals are analyzed by LA-MC-ICP-MS (Tong et al, 2015).…”

“…Isobaric interference of 87 Rb on 87 Sr can be corrected by two methods depending on Rb/Sr ratio of the samples (Jackson and Hart, 2006;Jochum et al, 2009;Tong et al, 2015). For Sr-rich samples with low Rb/ Sr ( 87 Rb/ 86 Sr < 0.005) (e.g., plagioclase and apatite), isobaric interference of 87 Rb on 87 Sr is negligible, and the natural ratio of 85 Rb/ 87 Rb can be used to correct the isobaric interference of 87 Rb on 87 Sr by the exponential law assuming Rb b ¼ Sr b: For samples with high Rb/Sr, however, interference of 87 Rb on 87 Sr should be corrected by one of two methods and its associated uncertainties propagated onto the quoted final ratio, (1) Rb b sam of each sample was calculated using a specified 85 Rb/ 87 Rb ratio back-calculated according to a reference material with known 87 Sr/ 86 Sr ratio (Jackson and Hart, 2006;Jochum et al, 2009;Tong et al, 2015), or (2) Rb b std calculated according to a reference material with known 87 Sr/ 86 Sr ratio and assuming a natural 87 Rb/ 85 Rb ¼ 0.38571 was applied to all samples, both of which incorporates the difference of mass fractionation between Rb and Sr. Following the interference corrections, mass fractionation of Sr isotopes was corrected by assuming 88 Sr/ 86 Sr ¼ 8.375209 (Jochum et al, 2009) and applying the exponential law.…”