Precise magnesium isotope analyses of high-K and low-Mg rocks by MC-ICP-MS

Abstract

This study presents a chemical protocol for the separation of Mg that is particularly adapted for diverse igneous rock samples, especially for high-K and low-Mg rocks. This protocol is based on a combination of three procedures: the removal of K from the samples using the precipitation procedure, followed by the separation of Fe and Ca using 2 mL of AG50W-X12 cation exchange resin, and finally the removal of Al, Ti, Na, and Fe elements using 0.5 mL of AG50W-X12 cation exchange resin. The effect of acidity and concentration mismatch as well as the matrix effect were rigorously evaluated using Nu Plasma II MC-ICP-MS with a wet plasma mode. The δ²⁶Mg mean values of the GSB and Alfa Mg standard solutions are −2.05 ± 0.05‰ (2 standard deviation (2s), n = 106) and −3.91 ± 0.04‰ (2s, n = 51), respectively, which serve as in-house Mg standards in the State Key Laboratory of Continental Dynamics (SKLCD). The average δ²⁶Mg value of one multi-elemental Mg standard GSB-1, made by adopting GSB with matrix elements, agrees well with the recommended value after precipitation and chromatographic separation. The long-term reproducibility, assessed by repeated measurements of Mg standard solutions and reference materials (RMs), is ±0.06‰. The robustness of our method is further assessed by the replicated analyses of fifteen rock standards and olivine grains with large variations in MgO and K₂O content.