Purification of Mg from extremely low-Mg felsic rocks for isotopic ratio determination by MC-ICP-MS

Abstract

Magnesium isotopes are powerful tracers in various geological processes. The high precision determination of Mg isotopes has however been hampered by difficulties in purifying Mg from extremely low-Mg rocks due to their high matrix/Mg ratios. Here, we present an improved and easily operated column procedure to separate Mg from extremely low-Mg (MgO down to 0.05 wt%) felsic rocks. The purification was carried out in three-step chromatographic procedure using a single column filled with AG50W-X8 resin. Matrix elements with high equilibrium distribution coefficient, such as Fe, Al, and Ca, were eluted with 1.5 M HNO₃ quantitatively after step I. Then, K, Na, Ti, Al, Ca and Mn were efficiently removed in 1 M HNO₃ by duplicate column procedures (step II and step III). Given that the high cation-resin ratio q for extremely low-Mg samples can drift the Mg peak to an earlier elution, a wide Mg cut is set for step II to avoid Mg loss. The total yield of Mg through the three-step procedure is >99.5% for the extremely low-Mg samples. Magnesium isotopic compositions of synthetic standards and well-studied geological reference materials (GSP-2, GSR-1, RGM-2, GBW07113) were measured using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The long-term reproducibility is ±0.06‰, and the δ²⁶Mg values are consistent with theoretical and previously reported values within errors. Notably, we observed negative Mg isotopic deviation when the amount of Mg purified was ≤5 μg, thus ≥10 μg Mg mass is recommended for loading on the columns. The δ²⁶Mg values of the extremely low-Mg rock standards JR-1 (−0.22 ± 0.03‰, 2SD, N = 9) and JR-3 (−0.08 ± 0.05‰, 2SD, N = 10) are reported for the first time, which can be used to calibrate the inter-laboratory bias in future studies on extremely low-Mg igneous rocks.