A natural calcite reference material for microbeam Sr isotope analysis

Abstract

High-quality Sr isotope data are critically dependent on the quality of reference materials that are used for instrumental calibration and data monitoring during laser ablation (LA) multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) analysis. We have prepared a new natural calcite reference material MNP for in situ Sr isotope measurement by LA-MC-ICP-MS. Detailed textural examination and major element determination suggest that the MNP calcite reference material has no growth rim or zoning. Trace element analyses by LA-ICP-MS showed that the MNP calcite reference material has low Rb/Sr ratios, low amounts of heavy rare earth elements (HREEs) and high Sr contents. The MNP calcite reference material is homogeneous with respect to Sr isotopes based on a large number of measurements performed by single-grain solution nebulizer (SN)-MC-ICP-MS and LA-MC-ICP-MS. The traditional bulk isotope analyses by TIMS and SN-MC-ICP-MS yielded a mean ⁸⁷Sr/⁸⁶Sr ratio of 0.70617 ± 0.00005(2SD, n = 18). A total of 585 LA-MC-ICP-MS Sr isotope analyses on random calcite crystals show homogeneous Sr isotopic compositions with an average ⁸⁷Sr/⁸⁶Sr ratio of 0.70617 ± 0.00005 (2SD), identical with the solution-based data. These suggest that MNP calcite can be used as a potential reference material for microbeam Sr isotopic analysis.