High-precision Mg isotope analyses of low-Mg rocks by MC-ICP-MS

Abstract We present a method for precise measurement of Mg isotope ratios for low-Mg rock samples (where MgO 3  + HF step to remove undesired matrix elements (such as Ti, Al, Fe, and K) in low-Mg samples. We also establish that increasing the amount of Mg loaded to the chromatographic column minimized blank effects of organics leached from cation resin. All parameters that could affect the accuracy and precision of Mg isotope analyses were rigorously examined by two independent laboratories in Beijing and Hefei. The δ 26 Mg of mono-elemental Mg standard CAM-1 measured in the two laboratories were − 2.597 ± 0.042‰ (2σ, n = 49) and − 2.598 ± 0.039‰ (2σ, n = 79), respectively; in house standard IGGMg1 were − 1.742 ± 0.041‰ (2σ, n = 53) and − 1.749 ± 0.049‰ (2σ, n = 72), respectively. The average δ 26 Mg over ten months of two synthetic standards, made by doping IGGMg1 and IGGMg2 with matrix elements, agrees well with their recommended values, within error. The robustness of our method was further assessed by replicated analyses of sixteen rock standards with MgO contents from 0.28 wt.% to 49.4 wt.%. The δ 26 Mg of USGS rhyolite standards RGM-1 and RGM-2 are − 0.188 ± 0.031‰ (2σ, n = 35) and − 0.182 ± 0.041‰ (2σ, n = 72), respectively; granite standard GA is − 0.165 ± 0.038‰ (2σ, n = 57), G-2 is − 0.129 ± 0.045‰ (2σ, n = 34), GS-N is − 0.204 ± 0.059‰ (2σ, n = 33), GSP-2 is 0.042 ± 0.020‰ (2σ, n = 15), and GSR-1 is − 0.234 ± 0.016‰ (2σ, n = 17). Based on repeated analyses of standards, the long-term external precision of our method is better than ± 0.05‰ for δ 26 Mg. This precision allows us to distinguish the fractionation of Mg isotopes in low-Mg granites and rhyolites as well as that between mantle minerals.