Experimental Study of Magma Degassing Processes

(1)弧熔岩成分含水玻璃的红外吸收光谱研究采用高压仪器合成了弧熔岩成分（拉斑玄武岩、高铝玄武岩和英安岩）含水玻璃。用氢测压法精确分析玻璃的总含水量，然后用FTIR分析得到2.8mum （H_2O分子和OH的拉伸）、2.2mum （OH的拉伸和弯曲）和1.9mum （H_2O分子的拉伸+弯曲）吸收带的摩尔吸收系数。(2)低压下玄武岩熔体溶解度的实验测定硅酸盐熔体溶解度是了解岩浆上升和泡化的最基本信息。采用气-介质内压装置，在1200℃、10-80 MPa条件下制备了含拉斑岩成分的水饱和玄武岩熔体。对淬火后的玻璃进行含水量分析，以确定其水溶性。将所得数据与较高压力下较多的自然值相结合，得到了在较大压力范围（10-1000 MPa）下玄武岩熔体水溶性的经验方程。(3)岩浆脱气和氢同位素变化根据1991 ~ 1992年云仙火山和1707年富士火山玻璃质岩石的含水量和氢同位素比值，考虑了火山的岩浆脱气过程。云仙岩浆在穹丘形成之前已经失去了大部分水分。穹丘熔岩和下伏岩浆的脱气主要受扩散控制，导致D/H比随着含水量的降低而增加。1707年富士火山喷发的岩屑样品显示出D/H比值随着Fe^<2+>/Fe^<3+>比值的减小而增大的相关性。这种相关性被解释为表明，在喷发时，水与岩浆中的Fe^<2+>反应导致了氢的大量损失。结果表明，水含量与氢同位素变化的相关性可用于评价岩浆脱气过程。少

(1)Infrared absorption spectroscopy of hydrous glasses of arc lava compositionsHydrous glasses of arc lava compositions (tholeiitic basalt, high-alumina basalt and dacite) were synthesized using a high pressure apparatus. The glasses were precisely analyzed for total water contents with hydrogen manometry, followed by FTIR analysis to obtain molar absorption coefficients of absorption bands at 2.8mum (molecular H_2O and OH stretching), 2.2mum (OH stretching and bending), and 1.9mum (stretching + bending of molecular H_2O).(2)Experimental determination of water solubility in basaltic melts at low pressureWater solubility in silicate melts is the most basic information in understanding the magma ascent and vesiculation. Using a gas-medium internal pressure apparatus, water-saturated basaltic melts of tholeiitic composition were prepared under 1200ﾟC and 10-80 MPa. The quenched glasses were analyzed for water contents to determine the water solubility. The data were combined with the lite … More rature values for higher pressures to obtain an empirical equation describing satisfactorily the water solubility in basaltic melts under a wide range of pressure, i.e., 10-1000 MPa.(3)Magma degassing and hydrogen isotopic variationsMagma degassing processes for the 1991-1992 Mt.Unzen volcano and 1707 Mt.Fuji volcano were considered on the basis of water contents and hydrogen isotopic rations of the glassy rocks. The Unzen magma had lost most of its water before dome formation. Degassing of water from the dome lava and underlying magma was controlled mainly by diffusion, resulting in increase in the D/H ratio as water content decreases. The scoria samples from the 1707 Fuji eruption showed the correlation that the D/H ratio increases as the Fe^<2+>/Fe^<3+> ratio decreases. This correlation was interpreted to indicate that significant loss of hydrogen took place at the time of eruption by reaction of water with Fe^<2+> in the magma. It was shown that the correlation between water contents and hydrogen isotopic variations can be used to evaluate the magma degassing processes. Less