Upwelling process of mantle helium in northeast Japan

Abstract:

Northeast Japan has an island arc structure where the Pacific plate subducts from the east. The helium isotope ratio is a good indicator to discriminate the origin of fluid carring helium, because the ratios in the mantle and crust are clearly different from each other. We performed a detailed study of helium isotope ratios in northeast Japan, and clarified the boundaries of the higher and lower helium isotope ratio distributions. The crustal components are dominant in the fore-arc region, whereas the mantle components dominate along the volcanic front and in the back-arc region. It is suggested that the clear contrast of the helium isotope ratio between the fore-arc and back-arc regions is due to the difference in whether helium isotope ratio of the wedge mantle or radiogenic helium-4 production rate in the crust. Specifically, we found the following characteristics; 1) comparison of the distribution between helium isotope ratios and faults revealed that the area along faults shows higher helium isotope ratios, 2) distribution of the helium isotope ratios is correlated with that of Li/Cl ratios which is the indicator of slab-derived water or magmatic water, suggesting that the mantle helium is transported by the slab-derived aqueous fluid. Important constraints on mantle helium upwelling are concluded as follows; a) the slab-derived water as the carrier of mantle helium, b) the faults, tectonic lines and volcanoes as the flow paths of slab-derived water, and c) crustal helium-4 contamination during upwelling process.

*This research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (Secretariat of NRA), Japan.