Method For Magnesium Isotope Measurements And Its Application In The Study Of Bayan Obo Deposit

The thesis contains two aspects: （1） Method development of Mg isotope analysis for special samples such as Bayan Obo giant deposit ; （2） Application of the method in constraining of the origin of Bayan Obo deposit.Since the beginning of this century, Mg isotope system has been rapidly developed in medicine, biology, earth science and other fields benefiting from the development of MC-ICPMS mass spectrometry technology and the improvement of Mg isotope test accuracy. As the previous study of Mg isotopic analysis of different geological samples continue to be accumulated and summarized, Mg isotope could be used as an indicator in some geological events.Since the discovery of Bayan Obo Fe-Nb-REE deposit, the discussion and controversy of its genesis has never stopped. Given the Mg isotopic composition in igneous rocks and sedimentary carbonate rocks exist significant differences, the Mg isotope analysis of Bayan Obo sample may give new evidence of its genesis. But there is no Mg separation and purification method for the Bayan Obo samples yet, so the first task of this study is to improve the Mg isotope analysis method for the Bayan Obo sample.Fe, Mn, REE and Nb in the samples may affect the accuracy of Mg isotopic analysis. In this paper, Fe and Mn exsis as complex anions in in 10 M HCl +0.001‰H₂O₂, thus they can be separated from Mg by using AGMP-1 anion exchange resin; REE and Nb can also be separated from Mg by AG50W-12 cation resin. The study also found that the amount of Mg from the samples will also be a factor which can affect Mg isotope mesurement. When the amount of Mg was less than 50μg, Mg isotope values deviate from the true value, which may be due to solvent matrix casued by the existence of impurities from the resin or acid. Before the treatment of the samples, we must ensure that the Mg content of the sample is large enough （> 50μg）. Comparing the Mg isotopic compositions in dolomite from Bayan Obo samples and Proterozoic strata sample（Pingquan,Hebei）, we found that: 1) Mg isotopecomposition of REE-rich carbonatite veins （represented by Wu dyke） shows igneous rock characteristic; 2) Bayan Obo group dolomite and Pingquan dolomite samples have similar isotopic composition (δ²⁶Mg =- 1.52 1.66); 3) Mg isotope composition of micrite mounds dolomite samples in Sailinhudong is:δ²⁶Mg =- 1.92 1.98; 4) theδ²⁶Mg of main and east Mine dolomite samples plot between the ignous carbonatite veins and sediment dolomites, ranging from -0.4 to -1.27. Mg isotope data show that the dolomite in Bayan Obo ore is not normally deposited carbonate, nor is purely magmatic carbonate. Mictie mound in Salinhudong Group has no contact with Bayan Obe deposit in genesis either. Bayan Obo Mg isotope data tend to support the pattern of submarine volcanic eruption sedimentary.