Yasuo Matsunaga, Tokyo Institute of Technology, School of Science, Tokyo, Japan, Wataru Kanda, Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan, Takao Koyama, Univ Tokyo, Tokyo, Japan, Shinichi Takakura, Geological Survey of Japan, AIST, Tsukuba Ibaraki, Japan and Tatsuji Nishizawa, Tokyo Institute of technology, Tokyo, Japan