[SCG75-P05] Helium measurements by passive diffusion samplers hanged in a borehole in Beppu, Japan
Keywords:helium, isotope ratio, passive diffusion sampler, borehole in Beppu
Beppu is located 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 situated 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). Therefore, it is worthwhile to attempt to find depth profile of helium signal in this area.
The sampling devices were installed every 50 m from near bottom of the well to the surface in the periods of July 13th-15th, August 21st-24th, 2015 and May 31st-June 3rd, 2016. 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) are gradually lower, as setting depth becomes shallow. The 3He/4He ratios ranges from 1.0-2.2 Ra under water surface to 6.3-7.1 Ra (Ra=1.4×10-6) at the bottom of a borehole. The high 3He/4He ratios are within range reported for mantle-derived magma at subduction zones (e.g., Hilton et al., 2002). The MORB-type helium could enter the borehole with hot spring water around the bottom. The observed variation in the 3He/4He ratios are the result of binary mixing of magma and air components.
Allis R.G. and Yusa Y. (1989) Fluid flow processes in the Beppu geothermal system, Japan. Geothermics, 18, 743-759.
Dame et al. (2005) Developing a new, passive diffusion sampler suite to detect helium anomalies associated with volcanic unrest. Bull. Volcanol., 77:23, doi10.1007/s00445-015-0912-4.
Hilton et al. (2002) Noble gases and volatile recycling at subduction zones, in Noble Gases in Geochemistry and Cosmochemistry, D. Porcelli, C.J. Ballentine, R. Wieler (Eds.), Rev. Mineral. Geochem., 47, 319-370.
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.