Determination of volatiles in magma by micro-gas-analysis of melt inclutions in phenocrysts

斑晶中的玻璃包裹体可被视为“淬火岩浆”的一部分，通过微量气体分析可获得玛玛的挥发分。组装了激光加热气体提取和QMS分析系统，用于在100 μ m尺度上测定玻璃中的H_2O和CO_2含量。用光束直径为10 μ m、能量为7 W的ND-YAG激光器在真空中熔化玻璃。放出的气体收集在液体空气冷阱中。用捕集器分离CO_2和H_2O。为了避免H_2O吸附在引出管壁上，H_2O与加热的铀金属反应被还原成H_2。H_2和CO_2用静态操作的QMS测定。H_2O和CO_2的线性范围分别为0.04-99 ng和0.01-18 ng。根据坑径和坑深估算了释放H_2O和CO_2的坑体积。熔融的玻璃的质量由凹坑体积和玻璃的密度计算。利用马里亚纳海槽玄武岩的玻璃质边缘（H_2O= 1.37wt%，CO_2=326 ppm）进行了实验研究。直径为90 ~ 300 μ m、深度为15 ~ 110 μ m的蚀坑中，H_2O的释放量为30-120 ng，CO_2的释放量为0.08-1.2 ng。微量元素分析结果为1.4 <plus-minus>± 0.2（S.E.）wt%的H_2O和390 <plus-minus>40（S.E.）ppm的CO_2浓度。采用本技术，可以获得直径约为100 mm的熔融包裹体。假设夹杂物具有上述气体组成，则可以分析150 μ m的气体。

The volatile compositions in maama can be obtained by micro-gas-analysis of glass inclusions in phenocrysts which can be regarded as a part of a "quenched magma". The system for gas extraction by laser heating followed by QMS analysis was assembled to determine H_2O and CO_2 contents in glasses on a 100 mum scale. A ND-YAG laser with a beam diameter of 10 mum And the energy of 7W was used to melt glasses in vacuum. The evolved gases were collected in a liquid air cold trap. CO_2 was separated from H_2O by trap work. In order to avoid the absorption of H_2O onto walls of the extraction line, H_2O was reduced to H_2 by reaction with a heated uranium metal. The resulting H_2 and CO_2 were determined by a statically operated QMS. The linear calibration curves were obtained in the range between 0.04-99 ng for H_2O and 0.01-18 ng for CO_2. The pit volume which released H_2O and CO_2 was estimated from the diameter and depth of the pit. The mass of the glass melted was calculated from the pit volume and the density of the glass. The gas concentrations were obtained by dividing the amount of H_2 and CO_2 determined by the mass of the glass melted.Glassy margin of the Mariana Trough basalt (H_2O=1.37 wt% and CO_2=326 ppm) was used to test this technique. The amount of the evolved gases from each pit with a size ranging from 90 to 300 mum in diameter and from 15 to 110 mum in depth was between 30-120 ng for H_2O and between 0.08-1.2 ng for CO_2. The microanalysis showed 1.4<plus-minus>0.2 (S.E.) wt% for H_2O and 390<plus-minus>40 (S.E.) ppm for CO_2 based on 6 independent analyses. With the present technique, a melt inclusion with a diameter of ca. 150 mum can be analyzed, assuming the inclusion has the above gas composition.