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[SCG54-01] Geothermal resources and geochronology: what Quaternary granite indicates
Keywords:geothermal resources, Quaternary granite, U-Pb dating
Japan is regarded as a nation rich in geothermal resources, however, its annual geothermal output is only 610 MWe as of March 2021, which is merely a half of the output generated in the Geysers geothermal area, California. Here, I evaluate as a standpoint of geochronology the possibility of large-scale geothermal power generation at three areas in Japan (Kakkonda, Ogachi and Kurobe) that I assume as high potential areas in Japan.
The Kakkonda geothermal field, which involves a Quaternary granite underneath, seems to be a good candidate for a large-scale geothermal exploitation. The 0.1 Ma Kakkonda Granite with its temperature of >500 ℃ at depths of >3700 m is regarded as a heat source of this area (Ito et al., 2013a). The Hachimantai geothermal area (also called as the Sengan geothermal area) contains granitic rocks of <1 Ma (Ito et al., 2013a; Arai et al., 2019) besides the 0.1 Ma Kakkonda Granite, and is also assumed to have wide high-temperature zones (>300 ℃ at a depth of 2 km) (Tamanyu, 2000; Akatsuka et al., 2022). Considering the age and the size of Quaternary plutons underneath, the Hachimantai geothermal area seems to have a greater geothermal potential than the Geysers (Ito, 2016).
The Ogachi geothermal field, where Central Research Institute of Electric Power Industry conducted a Hot Dry Rock geothermal exploitation experiment in the 1990s (Ito, 2003), is situated in the Sanzugawa caldera rich in geothermal manifestations. Although zircon fission-track ages of lapilli tuffs from the caldera-forming Torageyama Formation showed ~1 Ma (Ito, 2000), it has been widely accepted that the caldera was formed at 6–3 Ma based on whole-rock K-Ar ages (Takeno, 1988). Recently, the author reevaluated the age of the Sanzugawa caldera by applying zircon U-Pb dating, confirming that the western part of the caldera was formed at ~1.5 Ma (Ito, 2023). Therefore, the Sanzugawa caldera is a young Quaternary caldera and magmatic resurgence since the caldera formation has been still active, causing uplift in the central part of the caldera (Ito, 2023). It seems that the Sanzugawa caldera contains a young pluton underneath as Kakkonda and is also a good candidate for a large-scale geothermal exploitation.
Northern Japan Alps, famous for its large hydropower generation, is the site where the world’s youngest exposed granite (Kurobegawa granite) is situated (Ito et al., 2013b). Zircon U-Pb ages are ~1.2 Ma in the southern part of the granite and as young as ~0.8 Ma in the western side of the central to northern part (Ito et al., 2021). Cold water is stored in fracture zones in the southern part of the granite whereas geothermal manifestations exist where the ~0.8 Ma pluton is exposed. This indicates that the 1.8–1.1 Ma pluton underneath the Geysers geothermal power plants (Schmitt et al., 2003) cannot act as a heat source, and moreover that a pluton hot and young (younger than 0.8 Ma) may exist just beneath the areas where the ~0.8 Ma Kurobegawa granite is exposed. This (the place where the ~0.8 Ma Kurobegawa granite is exposed) seems to be the right place where a large-scale geothermal exploitation is possible as Kakkonda and Ogachi (Ito, 2013).
References:
Akatsuka et al., 2022. Geothermics, 105, 102485.
Arai et al, 2019. Proceedings of the 2019 Annual Meeting of the Geothermal Research Society of Japan, A20.
Ito, 2000. Journal of the Geothermal Research Society of Japan, 22, 9–21
Ito, 2003. Journal of Geophysical Research, 108(B9), 2426, doi:10.1029/2001JB001671.
Ito, 2013. Proceedings of the 2013 Annual Meeting of the Geothermal Research Society of Japan, A05.
Ito, 2016. Journal of the Geothermal Research Society of Japan, 38, 53–60.
Ito, 2023. Frontiers in Earth Science, 10:964773.
Ito et al, 2013a. Journal of Volcanology and Geothermal Research, 265, 1–8.
Ito et al., 2013b. Scientific Reports, 3, 1306.
Ito et al., 2021. Scientific Reports, 11, 22059.
Schmitt et al., 2003. Geochimica et Cosmochimica Acta, 67, 3443–3458.
Takeno,1988. Report of Geological Survey of Japan, 268, 191–210.
Tamanyu, 2000. Proceedings of the World Geothermal Congress 2000, 1823–1828.
The Kakkonda geothermal field, which involves a Quaternary granite underneath, seems to be a good candidate for a large-scale geothermal exploitation. The 0.1 Ma Kakkonda Granite with its temperature of >500 ℃ at depths of >3700 m is regarded as a heat source of this area (Ito et al., 2013a). The Hachimantai geothermal area (also called as the Sengan geothermal area) contains granitic rocks of <1 Ma (Ito et al., 2013a; Arai et al., 2019) besides the 0.1 Ma Kakkonda Granite, and is also assumed to have wide high-temperature zones (>300 ℃ at a depth of 2 km) (Tamanyu, 2000; Akatsuka et al., 2022). Considering the age and the size of Quaternary plutons underneath, the Hachimantai geothermal area seems to have a greater geothermal potential than the Geysers (Ito, 2016).
The Ogachi geothermal field, where Central Research Institute of Electric Power Industry conducted a Hot Dry Rock geothermal exploitation experiment in the 1990s (Ito, 2003), is situated in the Sanzugawa caldera rich in geothermal manifestations. Although zircon fission-track ages of lapilli tuffs from the caldera-forming Torageyama Formation showed ~1 Ma (Ito, 2000), it has been widely accepted that the caldera was formed at 6–3 Ma based on whole-rock K-Ar ages (Takeno, 1988). Recently, the author reevaluated the age of the Sanzugawa caldera by applying zircon U-Pb dating, confirming that the western part of the caldera was formed at ~1.5 Ma (Ito, 2023). Therefore, the Sanzugawa caldera is a young Quaternary caldera and magmatic resurgence since the caldera formation has been still active, causing uplift in the central part of the caldera (Ito, 2023). It seems that the Sanzugawa caldera contains a young pluton underneath as Kakkonda and is also a good candidate for a large-scale geothermal exploitation.
Northern Japan Alps, famous for its large hydropower generation, is the site where the world’s youngest exposed granite (Kurobegawa granite) is situated (Ito et al., 2013b). Zircon U-Pb ages are ~1.2 Ma in the southern part of the granite and as young as ~0.8 Ma in the western side of the central to northern part (Ito et al., 2021). Cold water is stored in fracture zones in the southern part of the granite whereas geothermal manifestations exist where the ~0.8 Ma pluton is exposed. This indicates that the 1.8–1.1 Ma pluton underneath the Geysers geothermal power plants (Schmitt et al., 2003) cannot act as a heat source, and moreover that a pluton hot and young (younger than 0.8 Ma) may exist just beneath the areas where the ~0.8 Ma Kurobegawa granite is exposed. This (the place where the ~0.8 Ma Kurobegawa granite is exposed) seems to be the right place where a large-scale geothermal exploitation is possible as Kakkonda and Ogachi (Ito, 2013).
References:
Akatsuka et al., 2022. Geothermics, 105, 102485.
Arai et al, 2019. Proceedings of the 2019 Annual Meeting of the Geothermal Research Society of Japan, A20.
Ito, 2000. Journal of the Geothermal Research Society of Japan, 22, 9–21
Ito, 2003. Journal of Geophysical Research, 108(B9), 2426, doi:10.1029/2001JB001671.
Ito, 2013. Proceedings of the 2013 Annual Meeting of the Geothermal Research Society of Japan, A05.
Ito, 2016. Journal of the Geothermal Research Society of Japan, 38, 53–60.
Ito, 2023. Frontiers in Earth Science, 10:964773.
Ito et al, 2013a. Journal of Volcanology and Geothermal Research, 265, 1–8.
Ito et al., 2013b. Scientific Reports, 3, 1306.
Ito et al., 2021. Scientific Reports, 11, 22059.
Schmitt et al., 2003. Geochimica et Cosmochimica Acta, 67, 3443–3458.
Takeno,1988. Report of Geological Survey of Japan, 268, 191–210.
Tamanyu, 2000. Proceedings of the World Geothermal Congress 2000, 1823–1828.