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

通过对斑晶中的玻璃包裹体进行微气体分析，可以得到maama中的挥发性成分，斑晶可以看作是“淬火岩浆”的一部分。建立了激光加热抽气-质谱联用系统，在100 μ m范围内测定玻璃中H_2O和CO_2的含量。本文采用了一束直径为10 μ m，能量为7W的ND-YAG激光在真空中熔融玻璃。释放出来的气体被收集在一个液态空气冷阱中。用捕集器把CO_2从H_2O中分离出来。为了避免H_2O吸附到萃取线壁上，将H_2O与加热的金属铀反应还原为H_2。所得的H_2和CO_2由静态运行的QMS测定。H_2O在0.04 ~ 99 ng范围内，CO_2在0.01 ~ 18 ng范围内均可获得线性校准曲线。根据坑的直径和深度估算了坑释放H_2O和CO_2的体积。熔融玻璃的质量由熔坑体积和玻璃密度计算得到。气体浓度由溶化玻璃的质量所决定的H_2和CO_2的量除以得到。用马里亚纳海槽玄武岩的玻璃状边缘（H_2O=1.37 wt%, CO_2=326 ppm）来测试该技术。在直径90 ~ 300 μ m、深度15 ~ 110 μ m的各坑中，H_2O的析出气体量在30 ~ 120 ng之间，CO_2的析出气体量在0.08 ~ 1.2 ng之间。6个独立分析的微量分析结果显示，H_2O <正负>0.2 (S.E.) wt%, CO_2 <正负>40 (S.E.) ppm。在假定包裹体具有上述气体成分的情况下，用目前的方法可以分析直径约为150微米的熔体包裹体。

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.