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

摘要

The low-Sr content (generally<100gg(-1)) in clinopyroxene from peridotite makes accurate Sr isotopic determination by LA-MC-ICP-MS a challenge. The effects of adding N-2 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 N-2 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, Sr-87/Sr-86 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 120m and low-to-moderate Sr content (30-518gg(-1)). The measurement reproducibility was 0.0004 (2s, n=33) for a sample with 100gg(-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.