Most of the 52 water samples from Lake Mashu, Hokkaido, Japan show large anomalies in their 3He 4He ratios (up to 4.6 times the atmospheric ratio) and 4He 20Ne ratios, indicating continuous injection of mantle He due to hot spring activity at the lake bottom. The 3He 4He ratio of the injected mantle He is estimated at 3He 4He = 9.43 ± 0.17 × 10-6, which is close to the highest value reported for volcanic gases from the Japanese Islands. Accumulation of mantle He in the near-bottom water can be reasonably explained by a diffusion process with a constant He flux from the lake bottom of f4He = 9.2 × 107 atoms/cm2/s and f3He = 8.7 × 102 atoms/cm2/s. The heat/3He ratio (1.7 × 10-7 J/atom) falls in the range of values reported for submarine hydrothermal systems (0.4-26 × 10-7 J/atom), whereas the C 3He ratio (1.8 × 1011) is about two orders of magnitude higher than the MORB average (2 × 109). The mantle He injected from the lake bottom has a fairly short mean residence time in the lake water, and is estimated at about 2-4 years. Overturn of the lake water, which occurs twice a year, probably plays an important role in the rapid transportation of the mantle He from the bottom to the surface.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science