16:00 〜 16:15
[SCG59-09] Hydrothermal experiment simulating petit-spot hydrothermal systems
キーワード：熱水活動、プチスポット火山、アウターライズ、太平洋
Petit-spot volcanoes are small volcanoes that form in association with plate flexure1. Since petit-spot volcanoes are the last volcanism before oceanic plate subduction, the interaction between petit-spot magma and the oceanic plate is important for understanding the physical properties of the subducting plate. In addition, since petit-spot volcanoes are the only volcanoes in the region surrounding outer-rise, hydrothermal activity at petit-spot volcanoes may be a biogeochemically significant factor. However, although the evidence of hydrothermal activity at petit-spot volcanoes has been reported2, no active hydrothermal sites have been observed. Therefore, in this study, we conducted hydrothermal experiments simulating petit-spot hydrothermal systems to estimate the geochemical characteristics of hydrothermal activity at petit-spot volcanoes.
The initial solid phase for the hydrothermal experiments was petit-spot lava and pelagic sediment collected from a petit-spot volcano where traces of hydrothermal activity had been found. The petit-spot lava is an almost aphyric basanite, and the total organic carbon content of the sediment is 0.6%. The initial liquid phase was a 3.5% NaCl aqueous solution in an amount approximately five times that of the initial solid phase. Because the CO2 content of petit-spot magma before degassing has been estimated to be 10–100 times higher than that of mid-ocean ridge magma, NaHCO3 and HCl were added to the initial material to adjust the CO2 content in the initial liquid phase to 0.5 mol/kg, which is 10–100 times higher than that in hydrothermal fluids at mid-ocean ridges. The inside of the reaction cell inside was gold-lined, so the reaction materials only came into contact with gold. The initial solid phase was either the petit-spot lava or a mixture of the petit-spot lava and the sediment, and the experiments were carried out at 250°C or 350°C for approximately 20 days at 500 bar.
As a result of hydrothermal reactions, olivine contained in the petit-spot lava and phillipsite contained in the sediment were decomposed, and calcite, analcime, and smectite-group clay minerals were crystallized. Clinopyroxene and phlogopite in the petit-spot lava, and quartz and plagioclase in the sediment remained. The chemical composition of the liquid phase was generally within the range observed in natural high-temperature hydrothermal fluids (>300°C)3. The liquid phases of the runs in which the sediment was included as an initial material exhibited lower pH and higher methane content than those in the liquid phases of the runs without the sediment, indicating the generation of organic acids and methane from organic matter in the sediment. Due to the decrease in pH, dissolved metal contents, such as Fe, Mn, Ca, and Ba, increased in the liquid phases of the runs with the sediment. In contrast, the Mo content in the liquid phase decreased in these runs compared to those without sediment. This may indicate that the formation of highly soluble molybdate was suppressed by the reducing conditions maintained by organic matter in the sediment. Therefore, the low Mo/Mn ratio of hydrothermal Fe–Mn oxides collected from a petit-spot volcano2 may reflect the involvement of sediments in water–rock interactions. The Mn/Fe ratio of the liquid phase was lower in the run conducted at 250°C than in the run conducted at 350°C, which is consistent with the fact that the Mn/Fe ratio of hydrothermal Fe–Mn oxides decreases in the later stages of hydrothermal activity at a petit-spot volcano2. These results consistent with the characteristics of petit-spot hydrothermal activity suggested by the natural Fe–Mn oxides indicate that the hydrothermal experiments were able to reproduce petit-spot hydrothermal system.
1. Hirano & Machida. Communications Earth & Environment 3, 110 (2022).
2. Azami et al. Communications Earth & Environment 4, 191 (2023).
3. Diehl & Bach. Geochemistry, Geophysics, Geosystems 21, e2020GC009385 (2020).
The initial solid phase for the hydrothermal experiments was petit-spot lava and pelagic sediment collected from a petit-spot volcano where traces of hydrothermal activity had been found. The petit-spot lava is an almost aphyric basanite, and the total organic carbon content of the sediment is 0.6%. The initial liquid phase was a 3.5% NaCl aqueous solution in an amount approximately five times that of the initial solid phase. Because the CO2 content of petit-spot magma before degassing has been estimated to be 10–100 times higher than that of mid-ocean ridge magma, NaHCO3 and HCl were added to the initial material to adjust the CO2 content in the initial liquid phase to 0.5 mol/kg, which is 10–100 times higher than that in hydrothermal fluids at mid-ocean ridges. The inside of the reaction cell inside was gold-lined, so the reaction materials only came into contact with gold. The initial solid phase was either the petit-spot lava or a mixture of the petit-spot lava and the sediment, and the experiments were carried out at 250°C or 350°C for approximately 20 days at 500 bar.
As a result of hydrothermal reactions, olivine contained in the petit-spot lava and phillipsite contained in the sediment were decomposed, and calcite, analcime, and smectite-group clay minerals were crystallized. Clinopyroxene and phlogopite in the petit-spot lava, and quartz and plagioclase in the sediment remained. The chemical composition of the liquid phase was generally within the range observed in natural high-temperature hydrothermal fluids (>300°C)3. The liquid phases of the runs in which the sediment was included as an initial material exhibited lower pH and higher methane content than those in the liquid phases of the runs without the sediment, indicating the generation of organic acids and methane from organic matter in the sediment. Due to the decrease in pH, dissolved metal contents, such as Fe, Mn, Ca, and Ba, increased in the liquid phases of the runs with the sediment. In contrast, the Mo content in the liquid phase decreased in these runs compared to those without sediment. This may indicate that the formation of highly soluble molybdate was suppressed by the reducing conditions maintained by organic matter in the sediment. Therefore, the low Mo/Mn ratio of hydrothermal Fe–Mn oxides collected from a petit-spot volcano2 may reflect the involvement of sediments in water–rock interactions. The Mn/Fe ratio of the liquid phase was lower in the run conducted at 250°C than in the run conducted at 350°C, which is consistent with the fact that the Mn/Fe ratio of hydrothermal Fe–Mn oxides decreases in the later stages of hydrothermal activity at a petit-spot volcano2. These results consistent with the characteristics of petit-spot hydrothermal activity suggested by the natural Fe–Mn oxides indicate that the hydrothermal experiments were able to reproduce petit-spot hydrothermal system.
1. Hirano & Machida. Communications Earth & Environment 3, 110 (2022).
2. Azami et al. Communications Earth & Environment 4, 191 (2023).
3. Diehl & Bach. Geochemistry, Geophysics, Geosystems 21, e2020GC009385 (2020).