3:45 PM - 4:00 PM
[MGI28-02] Fluid migration in the Kumano forearc basin, Nankai Trough, estimated from fluid chemistry
Keywords:forearc basin, Nankai Trough, lithium, methane
During the International Ocean Discovery Program (IODP) Expedition 358 “Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)” in 2019, a drilling was conducted at the northwestern margin of the Kumano forearc Basin (Site C0025: 33°24′05″N, 136°20′09″E; water depth: 2040 m by using D/V Chikyu), and a sediment core sample composed of forearc basin sediments were collected from 420.2 to 574.3 meters below seafloor (mbsf). Site C0025 is located on the anticlinal dome of accretionary prism. Faults developed in the anticlinal dome could be a pathway for deep sourced fluid from the accretionary prism to the basin sediment. In order to clarify the fluid migration, we analyzed the stable hydrogen and oxygen isotopic composition of pore water (δD and δ18O), and the stable isotopic composition of dissolved Lithium (δ7Li), and compared those isotopic compositions to the chemical compositions of pore water analyzed on-board. Furthermore, we analyzed stable the carbon and hydrogen isotopic composition of methane (δ13C-CH4; δD-CH4) as well as the abundance of 13CH3D, a multiply substituted “clumped” isotopologue (Δ13CH3D), which is a geothermometer to constrain gas formation temperature.
At C0025, the Cl- concentration in the pore water (390-461 mM) was lower than that of seawater (ca. 560 mM). The low Cl– concentration suggests pore water freshening. The decrease of δD from -4.4‰ to -8.4‰ and increase of δ18O from -1.0 to -0.5‰ with the depletion of Cl– suggest the addition of freshwater derived from clay mineral dehydration. The Li+ concentration in the pore water (106-165 μM) were 4–6 times higher than in seawater, suggesting that Li+ was released at high temperatures (>50ºC). The temperature estimated by Na+/Li+ geothermometer showed 101-130℃. The Li+ concentration and δ7Li (+25.0- +19.3‰) suggest the release of Li from sediments at temperatures >60℃ and incorporation into authigenic smectite–illite. The smectite-to-illite transformation is a typical clay dehydration process, and occurs at 60-160℃. Since the in-situ temperature at the depth of drilling is below 25℃, the fluids experienced the clay mineral dehydration should migrate from deeper depths where the temperature is higher than the depth of drilling. Methane/ethane concentration ratio (C1/[C2+C3]: 365-83294) and δ13C-CH4 (-59‰; -64‰) indicate that the most of the methane is biogenic methane with a small amount of thermogenic methane. The δ13C-CH4 and δD-CH4 (-179‰; -183‰) indicate that the methane was formed by a hydrogenotrophic methanogenesis. The apparent formation temperature (93℃) estimated from Δ13CH3D (+3.97‰) suggests the mixing of biogenic and thermogenic methane. Assuming two end member mixing of biogenic and thermogenic methane, we estimated that as much as 70% of methane was microbially produced at ca. 60℃. These results suggest that experienced temperature for fluid is higher than 60℃.
Based on the thermal gradient in this region, the depth at which the temperature is >60℃ was estimated to be >2,000 mbsf. This depth corresponds to the accretionary prism located under the forearc basin sediments, indicating the migration of the fluid experienced >60℃ from the accretionary prism to the basin sediment. A strong Bottom Simulating Reflector (BSR) has been observed at about 400 mbsf in the forearc basin around Site C0025. The BSR indicates the presence of methane hydrate and free gas zone. Therefore, the fluids migrated from the accretionary prism may contribute the formation of methane hydrate and free gas zone in the forearc sediment.
At C0025, the Cl- concentration in the pore water (390-461 mM) was lower than that of seawater (ca. 560 mM). The low Cl– concentration suggests pore water freshening. The decrease of δD from -4.4‰ to -8.4‰ and increase of δ18O from -1.0 to -0.5‰ with the depletion of Cl– suggest the addition of freshwater derived from clay mineral dehydration. The Li+ concentration in the pore water (106-165 μM) were 4–6 times higher than in seawater, suggesting that Li+ was released at high temperatures (>50ºC). The temperature estimated by Na+/Li+ geothermometer showed 101-130℃. The Li+ concentration and δ7Li (+25.0- +19.3‰) suggest the release of Li from sediments at temperatures >60℃ and incorporation into authigenic smectite–illite. The smectite-to-illite transformation is a typical clay dehydration process, and occurs at 60-160℃. Since the in-situ temperature at the depth of drilling is below 25℃, the fluids experienced the clay mineral dehydration should migrate from deeper depths where the temperature is higher than the depth of drilling. Methane/ethane concentration ratio (C1/[C2+C3]: 365-83294) and δ13C-CH4 (-59‰; -64‰) indicate that the most of the methane is biogenic methane with a small amount of thermogenic methane. The δ13C-CH4 and δD-CH4 (-179‰; -183‰) indicate that the methane was formed by a hydrogenotrophic methanogenesis. The apparent formation temperature (93℃) estimated from Δ13CH3D (+3.97‰) suggests the mixing of biogenic and thermogenic methane. Assuming two end member mixing of biogenic and thermogenic methane, we estimated that as much as 70% of methane was microbially produced at ca. 60℃. These results suggest that experienced temperature for fluid is higher than 60℃.
Based on the thermal gradient in this region, the depth at which the temperature is >60℃ was estimated to be >2,000 mbsf. This depth corresponds to the accretionary prism located under the forearc basin sediments, indicating the migration of the fluid experienced >60℃ from the accretionary prism to the basin sediment. A strong Bottom Simulating Reflector (BSR) has been observed at about 400 mbsf in the forearc basin around Site C0025. The BSR indicates the presence of methane hydrate and free gas zone. Therefore, the fluids migrated from the accretionary prism may contribute the formation of methane hydrate and free gas zone in the forearc sediment.