[SCG59-03] Thermal characteristics of slab fluid in Hongu area, southwest Japan: Insights form thermochronology and fluid inclusion analyses
Keywords:thermochronology, fluid inclusion, slab-derived fluid, Hongu area
Non-volcanic thermal fluid activities have been reported in subduction zones, which are derived from dehydration of the subducting slab (e.g., Ito et al., 1983, GCA; Jarrard, 2003, GGG; Hacker, 2008, GGG). In the Japanese Islands, slab-derived fluids are identified such as in Iwaki region (Togo et al., 2014, EPS) of NE Japan Arc, Kii peninsula (Morikawa et al., 2016, GCA) and Arima region (Nishimura et al., 2006, J. Hot Spring Sci.; Kusuda et al., 2014, EPS) in SW Japan Arc. Understanding of fundamental features of these slab-derived fluids, e.g., the thermal and chemical characteristics, are important because slab-derived fluids can be related to seismicity (Zhao et al., 1996, Science; Obara, 2002, Science), formation of ore deposits (Cox, 2005), development of geothermal resources (Adachi et al., 2014, J. Hot Spring Sci.), and stability of underground facilities (AIST, 2016).
This study attempted estimating of thermal characteristics of slab-derived fluids in Hongu region, southwest Japan, based on thermochronology and fluid inclusion analyses. We measured homogenization temperatures of fluid inclusions of quartz veins to estimate temperature of the thermal fluids. Thermochronometries, e.g., fission-track (FT) analyses on apatite and FT, (U-Th)/He and U-Pb analyses on zircon, were also applied to the host rocks to constrain the timing, duration, and spatial range of the thermal fluid activities.
Homogenization temperatures of the primary inclusions were estimated to be 140-210 deg. C. Apatite FT ages of 12-9 Ma, zircon (U-Th)/He ages of 24-9 Ma, zircon FT ages of 30-18 Ma, and zircon U-Pb ages of 77-68 Ma were also obtained. These cooling ages produced no spatial variation according to the distance from the quartz veins. These dating results can be explained by the three scenarios: 1) apatite FT ages of the all samples were totally reset at ~10 Ma by thermal fluid activities, 2) the thermal fluid activities occurred prior to ~10 Ma and the apatite FT ages reflect the regional exhumation at ~10 Ma, and 3) the thermal fluid activities occurred after ~10 Ma but had no effect on these apatite FT ages due to the short duration and/or low temperature. The scenario 1) is less possible because such a huge thermal event could have affected on ages of the higher-temperature thermochronometers. However, it is difficult to distinguish the scenarios 2) and 3) using our data set.
Thermal characteristics of the thermal fluid activities in the scenario 2) and 3) are summarized as below. In the scenario 2), the fluid activities occurred at the depth of 2-3 km or deeper, assuming a surface temperature of 10-20 deg. C and geothermal gradient of ~30 deg. C. Rise in temperature associated with the event is calculated at 20-130 deg. C from the ambient temperature and homogenization temperatures. The timing, duration, spatial range of the thermal fluid activities are not constrained. In the scenario 3), on the other hand, the upper limit of the duration of fluid activities can be constrained although the timing and spatial range are not. Based on annealing function of Ketcham et al. (1999, Amer. Min.), the upper limits are a few 10 years for 150 deg. C and one month for 200 deg. C. For further understanding of thermal characteristics of slab-derived fluids, further studies adopting the same approaches are desired in more slowly exhumated region, such as in Rokko area (Sueoka et al., 2010, J. Geogr.), where thermal anomalies related to recent events are easier to detect due to the older cooling ages of the host rocks.
Acknowledgements: This study was carried out under a contract with METI (Ministry of Economy, Trade and Industry) as part of its R&D supporting program for developing geological disposal technology.
This study attempted estimating of thermal characteristics of slab-derived fluids in Hongu region, southwest Japan, based on thermochronology and fluid inclusion analyses. We measured homogenization temperatures of fluid inclusions of quartz veins to estimate temperature of the thermal fluids. Thermochronometries, e.g., fission-track (FT) analyses on apatite and FT, (U-Th)/He and U-Pb analyses on zircon, were also applied to the host rocks to constrain the timing, duration, and spatial range of the thermal fluid activities.
Homogenization temperatures of the primary inclusions were estimated to be 140-210 deg. C. Apatite FT ages of 12-9 Ma, zircon (U-Th)/He ages of 24-9 Ma, zircon FT ages of 30-18 Ma, and zircon U-Pb ages of 77-68 Ma were also obtained. These cooling ages produced no spatial variation according to the distance from the quartz veins. These dating results can be explained by the three scenarios: 1) apatite FT ages of the all samples were totally reset at ~10 Ma by thermal fluid activities, 2) the thermal fluid activities occurred prior to ~10 Ma and the apatite FT ages reflect the regional exhumation at ~10 Ma, and 3) the thermal fluid activities occurred after ~10 Ma but had no effect on these apatite FT ages due to the short duration and/or low temperature. The scenario 1) is less possible because such a huge thermal event could have affected on ages of the higher-temperature thermochronometers. However, it is difficult to distinguish the scenarios 2) and 3) using our data set.
Thermal characteristics of the thermal fluid activities in the scenario 2) and 3) are summarized as below. In the scenario 2), the fluid activities occurred at the depth of 2-3 km or deeper, assuming a surface temperature of 10-20 deg. C and geothermal gradient of ~30 deg. C. Rise in temperature associated with the event is calculated at 20-130 deg. C from the ambient temperature and homogenization temperatures. The timing, duration, spatial range of the thermal fluid activities are not constrained. In the scenario 3), on the other hand, the upper limit of the duration of fluid activities can be constrained although the timing and spatial range are not. Based on annealing function of Ketcham et al. (1999, Amer. Min.), the upper limits are a few 10 years for 150 deg. C and one month for 200 deg. C. For further understanding of thermal characteristics of slab-derived fluids, further studies adopting the same approaches are desired in more slowly exhumated region, such as in Rokko area (Sueoka et al., 2010, J. Geogr.), where thermal anomalies related to recent events are easier to detect due to the older cooling ages of the host rocks.
Acknowledgements: This study was carried out under a contract with METI (Ministry of Economy, Trade and Industry) as part of its R&D supporting program for developing geological disposal technology.