Experimental Study of Magma Degassing Processes

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(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