碳酸盐岩Mg同位素分析方法及研究进展

Methods and research progress of Mg isotope analysis in carbonate rocks

Mg是中等挥发和流体活动性元素,也是生物所必需的元素,几乎参与了地球上所有的地球化学过程,包括物理、化学和生物循环。近20年来,随着多接收电感耦合等离子体质谱仪(MC-ICP-MS)高精度测定Mg同位素技术的快速发展,Mg同位素在碳酸盐岩研究中显示出非常广阔的应用前景,其中低Mg碳酸盐岩的研究对了解大陆地壳的化学演化具有非常重要的作用。然而,由于现有高Ca低Mg碳酸盐岩的分析方法并不能很好地将Ca完全去除,因此利用低Mg碳酸盐岩的Mg同位素特征进行地质研究仍然存在很大的局限性。通过对已有碳酸盐岩的Mg同位素分析方法进行归纳总结,认为可采取下列措施提高低Mg碳酸盐岩样品的Mg同位素分析精度,即上柱前完全溶解样品;化学分离过程中确保Mg与其他基质元素完全分离;淋洗基质过程中采用HF与HNO 3混合酸进行淋洗,同时提高Mg的进样量;采用新鲜的测试样品和标准溶液进行测试等。上述措施可为该分析方法的改进及其在碳酸盐岩类Mg同位素研究中的应用提供参考。

Mg is a biologically essential element that has a medium volatility and fluid activity.Almost all geochemical processes on the earth,including the physical,chemical,and biological cycles,depend on it.Mg isotope has demonstrated a very broad application promise in carbonate rocks over the past two decades due to the quick development of high precision Mg isotope determination by multi-reception inductively coupled plasma mass spectrometer(MC-ICP-MS).Among these,research on low-Mg carbonate rocks is crucial for comprehending the chemistry of continental crust and tracing its evolution.The application of Mg isotope features of low-Mg carbonate rocks in geological research is still relatively limited since the current analysis methods of high-Ca and low-Mg carbonate rocks cannot entirely eliminate Ca.The following measures can be taken to improve the accuracy of Mg isotope analysis of low-Mg carbonate rock samples.First,it is suggested to completely dissolve the samples before loading the column.Second,Mg should be completely separated from other matrix elements in the chemical separation process.Third,the mixed acid of HF and HNO 3 could be used to wash the matrix,and the loaded volume of Mg be increased.Fourth,fresh test samples and standard solutions are better for testing.The above measures may provide reference for the improvement of analysis methods and the application of Mg isotope in carbonate rocks in the future.