5:15 PM - 6:30 PM
[SCG55-P03] Hydrothermal experiments in olivine-orthopyroxene-H2O system at 200-400C: implications for hydrogen generation at high temperatures
Keywords:serpentinization, hydrogen
Hydrogen and hydrocarbon production during serpentinization provide significant impacts on microbial activities within the oceanic lithosphere. At the slow- to ultraslow spreading ridges, the alkaline hydrothermal vents with a special ecosystem has been discovered such as Lost City in Mid Atlantic Ridge. These hydrothermal vents are relatively low-temperatures, compared to the those in the fast-spreading ridges. Recently, it is suggested that serpentinization proceeds by seawater penetrating to the deep high-temperature fracture zones, and the bending faults, which potentially higher extent of hydration and hydrogen production. However, there has been few studies on serpentinization and hydrogen generation at temperature exceeding 300C.
In this study, we conducted hydrothermal experiments on serpentinization, focusing on (1) temperature, (2) silica (orthopyroxene) and (3) mass transport (batch vs. flow-through). The experiments were conducted in Ol-H2O system and Ol-Opx-H2O system. Batch type experiments were conducted for 576 hours at 200, 250, 280, 300, 350C at the vapor-saturated pressure and 400℃ at 30 MPa. Flow-through type experiments were conducted at 35 MPa and flow rate of 0.1 ml/min at 300C and 350C for the Ol-H2O system and 300C and 380C for the Ol-Opx-H2O system for 207-284 hours. After each run, the water content and magnetic susceptibility were measured, and microstructures were observed by using SEM.
In the Ol-H2O system, the extent of the serpentinization is highest at 300, and serpentinization did not proceed at 400C. In contrast, in the Ol-Opx-H2O system, the rate of the serpentinization was 1.5 times larger at 300C and >10 times larger at 400C than in the Ol-H2O system. In the flow-through type experiments, the reaction progress was 10 times larger than in the batch type experiments for both systems. These results indicates that the serpentinization proceeded after orthopyroxene faster in particular at high temperatures, and leaching of SiO2 by flow-through systems. The EDS analyses suggest that serpentinine minerals do not contain Fe3+ in any conditions. In contrast, magnetite linearly increased with the progress of serpentinization in the Ol-H2O system, whereas it does not increase in the Ol-Opx-H2O system. This can be understood in the activity diagram of silica activity and oxygen fugacity that iron is included in serpentine minerals in high silica activity. Our results suggest that serpentinization proceeds in harzburgite even at high temperature of >350, but the iron contents in orthopyroxene does not contribute to the hydrogen production. The production of hydrogen energy occurs at olivine-rich rocks at relatively low temperature.
In this study, we conducted hydrothermal experiments on serpentinization, focusing on (1) temperature, (2) silica (orthopyroxene) and (3) mass transport (batch vs. flow-through). The experiments were conducted in Ol-H2O system and Ol-Opx-H2O system. Batch type experiments were conducted for 576 hours at 200, 250, 280, 300, 350C at the vapor-saturated pressure and 400℃ at 30 MPa. Flow-through type experiments were conducted at 35 MPa and flow rate of 0.1 ml/min at 300C and 350C for the Ol-H2O system and 300C and 380C for the Ol-Opx-H2O system for 207-284 hours. After each run, the water content and magnetic susceptibility were measured, and microstructures were observed by using SEM.
In the Ol-H2O system, the extent of the serpentinization is highest at 300, and serpentinization did not proceed at 400C. In contrast, in the Ol-Opx-H2O system, the rate of the serpentinization was 1.5 times larger at 300C and >10 times larger at 400C than in the Ol-H2O system. In the flow-through type experiments, the reaction progress was 10 times larger than in the batch type experiments for both systems. These results indicates that the serpentinization proceeded after orthopyroxene faster in particular at high temperatures, and leaching of SiO2 by flow-through systems. The EDS analyses suggest that serpentinine minerals do not contain Fe3+ in any conditions. In contrast, magnetite linearly increased with the progress of serpentinization in the Ol-H2O system, whereas it does not increase in the Ol-Opx-H2O system. This can be understood in the activity diagram of silica activity and oxygen fugacity that iron is included in serpentine minerals in high silica activity. Our results suggest that serpentinization proceeds in harzburgite even at high temperature of >350, but the iron contents in orthopyroxene does not contribute to the hydrogen production. The production of hydrogen energy occurs at olivine-rich rocks at relatively low temperature.