硅铍石的包裹体及其形成条件研究

摘要

对硅铍石样品进行放大观察,可见大量杂乱分布的黄色透明针状固态包裹体、少量红色不透明粒状固态包裹体,气液包裹体和愈合裂隙普遍存在.通过激光拉曼光谱分析,测得硅铍石样品中气液包裹体的液态部分为含有CO2等气体的NaCl盐水溶液,气态部分的化学成分为CO2、水蒸气等.对硅铍石样品中气液包裹体的气体比和充填度进行估测,利用冷热台测出其均一温度和冰点分别为378.9～ 425.5℃和-12.1～-6.3℃.经过计算,得出硅铍石的近似形成温度为378.9～ 425.5℃,近似形成压力为234.44～428.56 MPa.%The formation conditions of phenakite were studied through investigating its gas-liquid inclusions by means of Laser Raman spectrum analysis and microscopic heating/cooling stage. The observation of phenakite samples revealed numerous randomly-distributed yellow acicular transparent solid inclusions, a few red granular opaque solid inclusions, lots of gas-liquid inclusions and healed cracks. The gas-liquid inclusions in phenakite were divided into two types according to their forms and substances. Type I included rich liquid two-phase H2O-NaCl inclusions (Ⅰa) and pure liquid H2O-NaCl inclusions ( Ⅰ b), whereas Type Ⅱ could be further divided into three-phase H2O-NaCl-CO2) inclusions containing liquid CO2( Ⅱ a) and two-phase H2O-NaCl-CO2 inclusions without liquid CO2 (Ⅱ b). Then some gas-liquid inclusions of different types with clear demarcated gas part and liquid part were chosen to do further test. Using Laser Raman spectrum analysis, the authors affirmed that phenakite is a kind of silicate which contains Be as the matrix for inclusions, and that the gas part of the gas-liquid inclusions contain such components as CO2 and water vapor, whereas the liquid part is NaCl aqueous solution with the dissolution of a little CO2. Two different gas-liquid inclusions were chosen for microscopic heating/ cooling stage test, and the results show that the homogenization temperatures of the inclusions are from 378. 9℃ to 425.5℃ by homogenization method, and their freezing points are about - 12.1 ～-6.3℃ by freezing method. These data were used to figure out the salinities of these inclusions, which are 9.60% ～16.14%. Calculations show that the homogenization pressures range from 234.44 MPa to 428.56 MPa. The formation temperature and the formation pressure of phenakite estimated from homogenization temperature and homogenization pressure are about 378.9～425.5℃ and 234.44～428.56 MPa respectively.