New Potential Apatite Reference Materials for Direct Sr Isotopic Analysis by LA-MC-ICP-MS

High-quality Sr isotopic data are critically dependent on the quality of reference materials that are used for instrumental calibration and data monitoring during LA-MC-ICP-MS analysis. Two natural apatite samples (SL-7 and SM139-1) were investigated here to test their suitability as in-situ Sr isotopic analysis reference materials. Major and trace element analyses by EPMA and LA-ICP-MS show that apatites are all characterized by high Cl (1.80 similar to 2.46 wt%), high Sr contents (> 3752 mu g g(-1)) and extremely low Rb/Sr ratios. A total of 120 and 126 LA-MC-ICP-MS analyses for SL-7 and SM139-1 apatite show consistent and homogeneous Sr isotopic compositions with average Sr-87/Sr-86 ratio of 0.70521 +/- 0.00016 (2SD, n = 120) and 0.70509 +/- 0.00015 (2SD, n = 126), respectively, which are in good agreement with solution-based data determined by TIMS or SN-MC-ICP-MS (0.705219 +/- 0.000019 (2SD, n = 13) and 0.705114 +/- 0.000041 (2SD, n = 13), respectively), suggesting that SL-7 and SM139-1 apatites can be used as potential reference materials for microbeam Sr isotopic analysis.