Mantle helium flux from the bottom of Lake Mashu, Japan

Abstract

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 f 4He = 9.2 × 10 7 atoms/cm 2/s and f 3He = 8.7 × 10 2 atoms/cm 2/s. The heat/ 3He ratio ( 1.7 × 10 −7J/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 × 10 11 ) is about two orders of magnitude higher than the MORB average ( 2 × 10 9 ). 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.