High-precision magnesium isotope analysis of geological and environmental reference materials by multiple-collector inductively coupled plasma mass spectrometry

Rationale: High-precision magnesium (Mg) isotopic analysis for geological and environmental reference materials is a prerequisite to ensure data quality before using Mg isotopes to trace geochemical and environmental processes. However, the Mg-isotopic ratios of many commonly used reference materials, especially sediments, have rarely been reported. Furthermore, published values for some commonly used reference materials exhibit a significant inconsistency across laboratories and thus need more data comparison. Methods: We developed different Mg purification schemes for silicate rocks, high-Ca carbonates and carbonatites, and high-Mn samples because of their significantly different matrices. We then used synthetic solutions to evaluate potential effects on measurement using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The accuracy and precision of our procedures were assessed by measurement on both synthetic solutions and well-studied geostandards. Results: The three different schemes for routine, high-Ca, and high-Mn samples can remove matrices efficiently with nearly 100% Mg yield. However, the presence of acid molarity and concentration mismatch, matrix elements, and fluctuations in room temperature can significantly affect the precision and accuracy of Mg isotope analysis, and must be avoided. The Mg isotopic ratios of reference materials obtained in this study are identical to the previously published values within +/- 0.06 parts per thousand, verifying that our procedures are robust. Conclusions: This study presented a thorough set of tests for high precision and accuracy of Mg isotope measurements using MC-ICP-MS, which demonstrate reproducibility and accuracy better than 0.05 parts per thousand for delta(25) Mg values and 0.06 parts per thousand for delta Mg-26 values. We reported high-quality Mg isotopic data for 16 geological and environmental reference materials to aid the inter-laboratory calibration of Mg isotope measurements in the future.