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

We present a method for precise measurement of Mg isotope ratios for low-Mg rock samples (where MgO < 1 wt.%) by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The efficiency of Mg purification is significantly improved by using a newly calibrated HNO3 + 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 δ26Mg 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 δ26Mg 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 δ26Mg 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 δ26Mg. This precision allows us to distinguish the fractionation of Mg isotopes in low-Mg granites and rhyolites as well as that between mantle minerals.