火山气体的成分和总量研究

ON COMPOSITIONS OF VOLCANIC GASES ERUPTED AND TOTAL VOLATILE LOSS

近年来 ,随着全球变化研究的不断深入 ,火山气体研究作为全球气候变化的一个分支迅速发展。文章介绍了目前火山气体的测试方法 :(1)尽管目前利用“差异法”获得火山喷出的H2 O含量的研究还较普遍 ,但是 ,利用离子探针技术直接测定玻璃基质和岩浆包裹体中水含量代替“差异法”已成为必然。 (2 )目前主要采用电子探针对玻璃基质和岩浆包裹体中Cl、S和F百分含量进行定量测试 ,通常选定工作电压为 15～ 2 0kV ,电流 6～ 15nA ,束斑直径分别为 1～ 10 μm(包裹体 )和 15～ 2 0 μm(基质玻璃 )。 (3)岩浆包裹体和玻璃基质中CO2 和CH4 的含量主要利用红外光谱和激光拉曼探针测定 ,另外基质玻璃中CO2 和CH4 的含量可通过XRF方法分析。文中系统分析了估算火山气体成分和总量过程中选择待测斑晶、岩浆包裹体和基质玻璃的标准 ,阐述了利用“岩石学方法”估算火山喷出气体总量的质量守恒原理 ,指出应用这一方法的前提条件是 :(1)火山气体主要来源于上升岩浆的减压出溶 ;(2 )岩浆上升过程中没有经过大规模的同化混染作用。因此 ,利用“岩石学方法”估算的火山气体总量是实际喷发量的最小值。

The great progresses in the study of volcanic gases have been achieved worldwide along with the research of global change. This paper introduces the methods of volatile measurement, and systematically analyzes the criteria of selections of phenocryst minerals, melt inclusions and matrix glasses to be determined in order to precisely estimate volcanic gas compositions and volatile emission. The rationale and premise of the 'petrologic method' which is used to calculate the total weight of volatiles are discussed. Finally, some suggestions are made for the research on the volatiles in future. The concentrations of chlorine, fluorine and sulfur in glass inclusions and matrix glasses are analyzed using the wavelength dispersive electron microprobe. Normally, the analytical conditions are set at accelerating voltage 15～20kV, beam current 6～15nA for glass inclusions and matrix glasses and 10～15nA for mineral crystals. Electron beams of 1～10 and 15～20μm\|diameter\|wide are used for inclusions and matrix glass analyses, respectively. Elements F, Cl and S are determined for 20s, 30s and 250s, respectively. The elements sodium and fluorine are determined first because of their strong volatile behavior during analyses. Relative analytical precision is 10%～12% for S, 10%～15% for F and 7%～10% for Cl based on the repeated analyses of the reference glasses. The detection limits are 220×10 -6 for S, 350×10 -6 for Cl and 1000×10 -6 for F on the basis of the peak\|to\|background ratio. Water contents are determined by 'difference method' . The relative proportion of S 6+ /S total is determined on the basis of the determinations of the shift of the S kα wavelength peak position.