5:15 PM - 6:30 PM
[SCG60-P05] Depth profile of helium concentration in a hot-spring well in Beppu, Japan
Keywords:helium, isotope ratios, hot spring, depth profile of concentration
Beppu is situated on east end of subsidence of the Beppu-Shimabara Graben in Kyushu Island, southwest Japan (Matsumoto, 1979), and is a famous area as a geothermal system. The geothermal system is located on the eastern flanks of the Tsurumi-Garandake volcanic center and spread until the coastline to the east. The geothermal activity is mostly concentrated in two areas, on the northern and southern sides of the fan deposit. These two areas are known as the Kamegawa and Beppu thermal zones, which are along with two faults, the Kamegawa and Asamigawa faults, respectively (Allis & Yusa, 1989).
The sampling devices were installed every 50 m from near bottom of the well to the surface in the periods of July 13-15 and August 21-24, 2015. The collected gases were measured by a noble gas mass spectrometer (Helix SFT; GV Instrument) installed at Atmosphere and Ocean Research Institute, University of Tokyo.
Helium concentrations and isotope ratios (3He/4He) is gradually lower, as setting depth becomes shallow. The highest in the isotope ratio shows 6.79 and 7.08 Ra (Ra=1.4E-6) around the bottom, and its high ratio can be of mantle origin. The screen of the borehole ranges 278-300 m, mantle helium could enter the well with hot spring water through the screen, and could go to the surface.
Allis R.G. and Yusa Y. (1989) Fluid flow processes in the Beppu geothermal system, Japan. Geothermics, 18, 743-759.
Matsumoto Y. (1979) Some problems on volcanic activities and depression structure in Kyushu, Japan. The MEMOIRS of the Geological Society of Japan, 16, 127-139.