10:15 〜 10:30
[SGC37-06] Recycling mercury in a convergent margin of subducting Philippine Sea Plate at Southwest Japan
キーワード：中央構造線、水銀安定同位体比
Mercury actively behaves as aqueous and gaseous species in the geosphere because of high volatility. Mercury concentration and stable isotope ratios of soils was analyzed to detect the deep mercury emission in relation to subduction of Philippine Sea Plate beneath Southwest Japan. The Soils were sampled at 840 locations, from middle Kinki and Shikoku, Beppu (volcanogenic hot spring area) and Arima (deep fluid-derived hot spring area). Mercury stable isotopes were analyzed for selected 50 soils among those.
Bulk concentration of mercury ranged from 4 to 8900 ppb, with median of 166 ppb. The mercury concentrations generally are low in the Ryoke granitic rock areas (Median: 80 ppb), and high in the outer zone in the south of Median Tectonic Line (Median: 210 ppb). The concentrations are occasionally higher in the soils from hot spring area (>500 ppm); especially high concentrations were determined for the soils from Beppu (>8800 ppb) and Arima (>5000 ppb). The mercury concentrations occasionally became higher in the soils taken from/near active faults than the surrounding soils. The occurrences of anomalously high concentration among the surrounding background level suggested that the mercury would be upwelling with hydrothermal solution and/or through the active faults.
The soil mercury had the d202Hg within the narrow range between 0 and -2.2 (+-0.17) permil with average of -1.0 permil. The values are almost in the similar ranges of the cinnabars of the three mercury mines in the studied area (-0.7 to +0.2 permil, Takeuchi and Minami, 2011), the Nankai Trough sediments (range: -0.8 to -0.2 permil, average: -0.6 permil, Masuda et al., 2018) and shallow groundwaters in Osaka Plain (range: -0.8 to -0.4 permil, average: -0.5‰, unpublished), suggesting the same source of those mercury. While considerable numbers of the soils gave the enrichment of light isotopes than those previously reported geological materials. Mass Dependent Fractionation (MDF) can occur via volatilization of mercury of hydrothermal systems, in which light stable isotopes are lost from the fluid. Thus, certain amounts of volatilized mercury from hydrothermal fluids would be trapped by the soils at/near hot spring area and active faults.
MIF (Mass Independent Fractionation) changes the ratios of odd isotopes (199Hg and 201Hg) to even isotopes due to magnetic isotope effect or nuclear volume effect, which occurs via photochemical or biochemical reaction. D199Hg and D201Hg of most soil samples are close to zero (0+-0.1), indicating not considerably affected by MIF, although positive and negative shifts along 1:1 line were observed, indicating photochemical reaction at certain stage(s) of geologic mercury cycle. The positive shifts are observed for the soils near the high temperature hot springs (Arima, Beppu and Dogo), and the largest shift was observed for that from Arima (0.3+-0.1 for D199Hg and 0.2+-0.1 for D201Hg). It implies that a certain amount of mercury was derived from the marine deposits, which contained photochemically reacted mercury in the atmosphere and ocean. The negative shift was observed for the soils taken from the Ryoke granitic areas. This negative shift cannot be explained at present, while may be caused by the contamination of terrestrial components, which contains atmospheric mercury.
To documenting the whole process of mercury cycle at the study area, we need more research work. However, our results suggest that one of the possible origins of the soil mercury in the middle to southern Kinki and Shikoku regions is subducting ocean sediments.
Bulk concentration of mercury ranged from 4 to 8900 ppb, with median of 166 ppb. The mercury concentrations generally are low in the Ryoke granitic rock areas (Median: 80 ppb), and high in the outer zone in the south of Median Tectonic Line (Median: 210 ppb). The concentrations are occasionally higher in the soils from hot spring area (>500 ppm); especially high concentrations were determined for the soils from Beppu (>8800 ppb) and Arima (>5000 ppb). The mercury concentrations occasionally became higher in the soils taken from/near active faults than the surrounding soils. The occurrences of anomalously high concentration among the surrounding background level suggested that the mercury would be upwelling with hydrothermal solution and/or through the active faults.
The soil mercury had the d202Hg within the narrow range between 0 and -2.2 (+-0.17) permil with average of -1.0 permil. The values are almost in the similar ranges of the cinnabars of the three mercury mines in the studied area (-0.7 to +0.2 permil, Takeuchi and Minami, 2011), the Nankai Trough sediments (range: -0.8 to -0.2 permil, average: -0.6 permil, Masuda et al., 2018) and shallow groundwaters in Osaka Plain (range: -0.8 to -0.4 permil, average: -0.5‰, unpublished), suggesting the same source of those mercury. While considerable numbers of the soils gave the enrichment of light isotopes than those previously reported geological materials. Mass Dependent Fractionation (MDF) can occur via volatilization of mercury of hydrothermal systems, in which light stable isotopes are lost from the fluid. Thus, certain amounts of volatilized mercury from hydrothermal fluids would be trapped by the soils at/near hot spring area and active faults.
MIF (Mass Independent Fractionation) changes the ratios of odd isotopes (199Hg and 201Hg) to even isotopes due to magnetic isotope effect or nuclear volume effect, which occurs via photochemical or biochemical reaction. D199Hg and D201Hg of most soil samples are close to zero (0+-0.1), indicating not considerably affected by MIF, although positive and negative shifts along 1:1 line were observed, indicating photochemical reaction at certain stage(s) of geologic mercury cycle. The positive shifts are observed for the soils near the high temperature hot springs (Arima, Beppu and Dogo), and the largest shift was observed for that from Arima (0.3+-0.1 for D199Hg and 0.2+-0.1 for D201Hg). It implies that a certain amount of mercury was derived from the marine deposits, which contained photochemically reacted mercury in the atmosphere and ocean. The negative shift was observed for the soils taken from the Ryoke granitic areas. This negative shift cannot be explained at present, while may be caused by the contamination of terrestrial components, which contains atmospheric mercury.
To documenting the whole process of mercury cycle at the study area, we need more research work. However, our results suggest that one of the possible origins of the soil mercury in the middle to southern Kinki and Shikoku regions is subducting ocean sediments.