[SCG66-17] 数値シミュレーションによる中部沖縄トラフ伊平屋北海丘熱水域での鉱物沈殿プロセスのモデリング
キーワード：伊平屋北海丘、数値シミュレーション、TOUGH2、海底熱水系、海底熱水鉱床
Seafloor hydrothermal deposits have attracted attention as new metal resources. To explore seafloor hydrothermal deposits, an understanding of the genesis of deposits is required. However, the origin study of seafloor hydrothermal deposits is based on knowledge of previously formed deposits, and the detailed generation mechanism has not yet been clarified. In this study, we adopted hydrothermal flow simulation using TOUGH2 to clarify the mineral precipitation mechanism of the seafloor hydrothermal deposits and identify generation conditions by selecting the Iheya North Knoll as a case study field.
A three-dimensional numerical model was produced based on field observations such as drilling data, seismic survey, and heat flux measurement. In this study, we constructed a simple model composed of only four geological elements (Fig. 1): (1) conduit (high permeability), (2) caprock (low permeability), (3) sediment (low permeability), and (4) volcanic basement (medium permeability). The hydrothermal fluid of 350 °C was injected from the conduit bottom, and the hydrothermal fluid was set to discharge from the conduit surface (Fig. 1). The physical properties of rocks were set based on the values obtained by the seafloor drillings, and the permeability was set based on the values used in other seafloor hydrothermal systems.
As a result of the simulation, the hydrothermal fluid rising along the conduit was trapped by the caprock near the surface and flowed laterally in addition to the outflow from the seafloor. The calculated heat fluxes and temperatures generally corresponded with the measured values, even around the discharge zone in which the flow patterns tend to be complicated. Boiling occurred between the surface of the discharge area and 150 mbsf, which is consistent with the observation.
An electrical resistivity tomography survey at the Iheya North Knoll revealed that massive sulfide minerals were distributed in two layers above and below the sea floor (Ishizu et al., 2019). In this study, the origin of this two-layer structure was discussed as follows. Gas-rich hydrothermal fluids generated by boiling rises due to the density difference and discharges from the hydrothermal vents. In fact, all the hydrothermal vents found in the Iheya North Knoll are clear smokers that do not have a composition for precipitation of sulfide minerals. Accordingly, massive sulfide minerals on the seafloor were probably formed in the past. Meanwhile, the liquid phase-rich hydrothermal fluid containing a large amount of metal components is considered to flow laterally under the caprock due to the density difference. This lateral flow of the hydrothermal fluid was verified by a sensitivity analysis. A model without the conduit and a model without the caprock, which did not generate lateral flow of hydrothermal fluid, could not reproduce the trends of measured heat flux and temperature. Therefore, the lateral flow of hydrothermal fluid is considered to occur certainly under the seafloor. Additionally, boiling causes a decrease in the solubility of the metals in hydrothermal fluid, resulting in mineral precipitation. Therefore, the boiling of the laterally flowing hydrothermal fluid has probably formed massive sulfide minerals below the seafloor.
References
Ishizu, K., T. Goto, Y. Ohta, T. Kasaya, H. Iwamoto, C. Vachiratienchai, W. Siripunvaraporn, T. Tsuji, H. Kumagai, and K. Koike (2019), Internal structure of a seafloor massive sulfide deposit by electrical resistivity tomography, Okinawa Trough, Geophys. Res. Lett., doi: 10.1029/2019GL083749.
A three-dimensional numerical model was produced based on field observations such as drilling data, seismic survey, and heat flux measurement. In this study, we constructed a simple model composed of only four geological elements (Fig. 1): (1) conduit (high permeability), (2) caprock (low permeability), (3) sediment (low permeability), and (4) volcanic basement (medium permeability). The hydrothermal fluid of 350 °C was injected from the conduit bottom, and the hydrothermal fluid was set to discharge from the conduit surface (Fig. 1). The physical properties of rocks were set based on the values obtained by the seafloor drillings, and the permeability was set based on the values used in other seafloor hydrothermal systems.
As a result of the simulation, the hydrothermal fluid rising along the conduit was trapped by the caprock near the surface and flowed laterally in addition to the outflow from the seafloor. The calculated heat fluxes and temperatures generally corresponded with the measured values, even around the discharge zone in which the flow patterns tend to be complicated. Boiling occurred between the surface of the discharge area and 150 mbsf, which is consistent with the observation.
An electrical resistivity tomography survey at the Iheya North Knoll revealed that massive sulfide minerals were distributed in two layers above and below the sea floor (Ishizu et al., 2019). In this study, the origin of this two-layer structure was discussed as follows. Gas-rich hydrothermal fluids generated by boiling rises due to the density difference and discharges from the hydrothermal vents. In fact, all the hydrothermal vents found in the Iheya North Knoll are clear smokers that do not have a composition for precipitation of sulfide minerals. Accordingly, massive sulfide minerals on the seafloor were probably formed in the past. Meanwhile, the liquid phase-rich hydrothermal fluid containing a large amount of metal components is considered to flow laterally under the caprock due to the density difference. This lateral flow of the hydrothermal fluid was verified by a sensitivity analysis. A model without the conduit and a model without the caprock, which did not generate lateral flow of hydrothermal fluid, could not reproduce the trends of measured heat flux and temperature. Therefore, the lateral flow of hydrothermal fluid is considered to occur certainly under the seafloor. Additionally, boiling causes a decrease in the solubility of the metals in hydrothermal fluid, resulting in mineral precipitation. Therefore, the boiling of the laterally flowing hydrothermal fluid has probably formed massive sulfide minerals below the seafloor.
References
Ishizu, K., T. Goto, Y. Ohta, T. Kasaya, H. Iwamoto, C. Vachiratienchai, W. Siripunvaraporn, T. Tsuji, H. Kumagai, and K. Koike (2019), Internal structure of a seafloor massive sulfide deposit by electrical resistivity tomography, Okinawa Trough, Geophys. Res. Lett., doi: 10.1029/2019GL083749.