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[SCG54-15] Lithium isotope ratios reveal differences in slab dehydration system between northeastern Japan and southwestern Japan
As the depth of the subducting oceanic plate increases, it dehydrates due to high temperature and pressure, and the water released is discharged into the mantle wedge. The Pacific Plate, which is subducting beneath northeastern Japan, was produced at a ridge about 130 million years ago, whereas the Philippine Sea Plate, which is subducting beneath southwestern Japan, is a young, hot plate that was produced about 20 million years ago. Based on this difference in thermal structure, it has been hypothesized that slab dehydration is more advanced in the forearc region of southwestern Japan than in northeastern Japan (Katayama, 20101). Nishio (2020)2 suggests that the large number of hot springs in the forearc region of southwestern Japan, excluding Kyushu, supports this hypothesis.. However, there was no geochemical evidence to support the idea that slab-derived fluids were rising on a large scale in the forearc region of southwestern Japan.
Lithium (Li) is the lightest of the alkali metals and is one of the elements that is easily distributed in the fluid phase. In a system where sediment and fluid coexist, the lithium concentration in the fluid phase increases rapidly as the temperature rises, but once lithium has been distributed into the fluid phase after experiencing high temperatures, it does not immediately return to the solid phase even when the temperature drops, and it remains in the fluid phase (You et al., 19963). As a result, fluids that have experienced high temperatures, such as deep-derived fluids, have a much higher lithium concentration than surface water that has not experienced high temperatures, so they are less susceptible to the influence of surface water contamination than other elements. (Nishio, 20202). In addition, the 7Li/6Li ratio makes an excellent geochemical thermometer. However, since multi-collector ICP mass spectrometers that can measure the 7Li/6Li ratio relatively easily and quickly became widely available on the market after 2000, there have been limited reports of data from hot spring water, etc.
In this study, groundwater containing hot spring water was collected and analyzed in the forearc regions of the Tohoku region, centered on Aomori and Iwate prefectures, and Hokkaido. The data was then compared with data from the forearc regions of southwestern Japan, such as Mie and Okayama prefectures, which had already been reported.
As a result, it was observed that the Cl/Li ratio and 7Li/6Li ratio of groundwater in northeastern Japan are higher than those in southwest Japan. The results of this study support the hypothesis that the amount of slab-derived fluids rising in the forearc region of northeastern Japan is less than that in southwestern Japan.
1) Katayama, I., Hirauchi, K., Nakajima, J., Variability of Subduction Processes beneath Japan, Journal of Geography 119 (2010) 205-223.
2) Nishio, Y., Unconventional Geothermal Fluid Science, In: 4D Integrated Kuroshio Basin Resource Science, Eds.: Sano, Y. and Tokuyama, H., Nakajima Publishing Co. (2022) pp40-51.
3) You, C.-F., Castillo, P.R., Gieskes, J.M., Chan, L.H., Spivack, A.J., Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones. Earth Planet. Sci. Lett. 140 (1996) 41-52.
Lithium (Li) is the lightest of the alkali metals and is one of the elements that is easily distributed in the fluid phase. In a system where sediment and fluid coexist, the lithium concentration in the fluid phase increases rapidly as the temperature rises, but once lithium has been distributed into the fluid phase after experiencing high temperatures, it does not immediately return to the solid phase even when the temperature drops, and it remains in the fluid phase (You et al., 19963). As a result, fluids that have experienced high temperatures, such as deep-derived fluids, have a much higher lithium concentration than surface water that has not experienced high temperatures, so they are less susceptible to the influence of surface water contamination than other elements. (Nishio, 20202). In addition, the 7Li/6Li ratio makes an excellent geochemical thermometer. However, since multi-collector ICP mass spectrometers that can measure the 7Li/6Li ratio relatively easily and quickly became widely available on the market after 2000, there have been limited reports of data from hot spring water, etc.
In this study, groundwater containing hot spring water was collected and analyzed in the forearc regions of the Tohoku region, centered on Aomori and Iwate prefectures, and Hokkaido. The data was then compared with data from the forearc regions of southwestern Japan, such as Mie and Okayama prefectures, which had already been reported.
As a result, it was observed that the Cl/Li ratio and 7Li/6Li ratio of groundwater in northeastern Japan are higher than those in southwest Japan. The results of this study support the hypothesis that the amount of slab-derived fluids rising in the forearc region of northeastern Japan is less than that in southwestern Japan.
1) Katayama, I., Hirauchi, K., Nakajima, J., Variability of Subduction Processes beneath Japan, Journal of Geography 119 (2010) 205-223.
2) Nishio, Y., Unconventional Geothermal Fluid Science, In: 4D Integrated Kuroshio Basin Resource Science, Eds.: Sano, Y. and Tokuyama, H., Nakajima Publishing Co. (2022) pp40-51.
3) You, C.-F., Castillo, P.R., Gieskes, J.M., Chan, L.H., Spivack, A.J., Trace element behavior in hydrothermal experiments: Implications for fluid processes at shallow depths in subduction zones. Earth Planet. Sci. Lett. 140 (1996) 41-52.