ヘリウム同位体比とテクトニクス (1987年度日本地球化学会奨励賞受賞記念)
1988 年 22 巻 1 号 p. 1-11
詳細
1988 年 22 巻 1 号 p. 1-11
Recent achievements on helium isotope study are reviewed in the paper. Helium-3 is a unique nuclide which indicates a primordial signature, while most of helium-4 is produced by the radioactive decay of U and Th in crustal rocks. The 3He/4He ratios provide useful information on the origin of terrestrial gas and rock samples. It is confirmed that there is a close correlation between the 3He/4He ratio of gas and rock samples and the geotectonic structure of the sampling sites. The 3He/4He ratio in mid-ocean ridge basalts (MORB) as well as in thermal fluids emitting from a ridge crest is very uniform with the value of 1.2×10-5 for samples collected at global spreading centers. The MORB type helium is considered to be representative of the Earth's upper mantle. Volcanic gases and rocks from hot spot regions such as Iceland and Hawaii indicate higher 3He/4He ratios than that of MORB, suggesting their deeper origin. Geographical distributions of helium isotope ratios in Japan and New Zealand show a quite similar trend: low 3He/4He ratio in the frontal arc region and high in the volcanic arc. The mantle helium in the volcanic arc region may be associated with the diapiric uprise of a magma. The low 3He/4He ratio in the frontal arc may be attributed to the accumulation of radiogenic helium in crustal rocks.
