17:15 〜 18:30
[SCG44-P03] 低温蛇紋岩化反応による水素生成における包有物の影響
キーワード：低温蛇紋岩化反応、水素、包有物
Low temperature serpentinization, a hydration reaction of ultramafic rocks occurring at < 100℃, may generate hydrogen near Earth’s surface environment. Fluids containing abundant hydrogen and methane formed by low temperature serpentinization (e.g., Lost City at the Mid-Atlantic Ridge) may have played key roles in abiotic synthesis of organic materials and therefore the origin of life in the Earth. However, the detail mechanism of the hydrogen generation during the reaction is poorly understood.
Previous experimental studies have demonstrated that fresh peridotites produced more hydrogen than serpentinites suggesting that dissolution of olivine and pyroxene generates hydrogen. On the other hand, Klein et al. (2019) suggests that olivine-hosted fluid inclusions in which serpentinization progresses during cooling from magmatic temperatures are the important reservoir of hydrogen and abiotic methane released during low temperature serpentinization in the Earth’s surface environments.
The objective of this study is to evaluate olivine-hosted inclusions as a potential source for hydrogen released during low temperature serpentinization experiments using different ultramafic rocks from the previous studies.
Harzburgites from Konde Hill (Soroako), Sulawesi, Indonesia and Horoman, Hokkaido were used in this study since they are the two samples that generated the greatest amount of hydrogen by low temperature serpentinization experiments. Although, both samples are relatively fresh harzburgites (< 0.7 wt% of water content), the generated amount of hydrogen was higher in Konde Hill (322 µmol/kg) than Horoman (146 µmol/kg). We observed the occurrence of olivine-hosted inclusions in the thin section samples using petrographic microscope under transmitted light and analyzed their mineralogical and gaseous compositions by confocal Raman spectroscopy.
Raman spectroscopy indicated that olivine-hosted inclusions are predominantly composed of some secondary minerals (i.e., serpentine, brucite) as well as methane in both samples. None of the samples show inclusions containing water molecules. These results suggest that serpentinization had occurred in the fluid inclusions with exhausting water. Whereas 33% of the gas inclusions contain hydrogen in the Konde Hill sample, none of the gas inclusions contain hydrogen in the Horoman sample. Based on the Raman spectroscopic analysis, we estimated the possible contribution of hydrogen in the gas inclusions to hydrogen generated during low temperature serpentinization experiments, which account for 230 µmol/kg. This amount of hydrogen may actually explain the difference between the two samples in the generated amounts of hydrogen during the experiments which can cover all of the differences measured in the previous study.
Therefore, this study suggests that olivine-hosted inclusions are an important source for the hydrogen generated during low temperature serpentinization. However, since the Horoman sample does not have any inclusion containing hydrogen, a significant amount of hydrogen was also likely produced by dissolution of olivine at the low temperature.
Reference
Klein, F., Grozeva, N. G., & Seewald, J. S. (2019). Proc. Natl. Acad, Sci. U.S.A., 116, 17666–17672.
Previous experimental studies have demonstrated that fresh peridotites produced more hydrogen than serpentinites suggesting that dissolution of olivine and pyroxene generates hydrogen. On the other hand, Klein et al. (2019) suggests that olivine-hosted fluid inclusions in which serpentinization progresses during cooling from magmatic temperatures are the important reservoir of hydrogen and abiotic methane released during low temperature serpentinization in the Earth’s surface environments.
The objective of this study is to evaluate olivine-hosted inclusions as a potential source for hydrogen released during low temperature serpentinization experiments using different ultramafic rocks from the previous studies.
Harzburgites from Konde Hill (Soroako), Sulawesi, Indonesia and Horoman, Hokkaido were used in this study since they are the two samples that generated the greatest amount of hydrogen by low temperature serpentinization experiments. Although, both samples are relatively fresh harzburgites (< 0.7 wt% of water content), the generated amount of hydrogen was higher in Konde Hill (322 µmol/kg) than Horoman (146 µmol/kg). We observed the occurrence of olivine-hosted inclusions in the thin section samples using petrographic microscope under transmitted light and analyzed their mineralogical and gaseous compositions by confocal Raman spectroscopy.
Raman spectroscopy indicated that olivine-hosted inclusions are predominantly composed of some secondary minerals (i.e., serpentine, brucite) as well as methane in both samples. None of the samples show inclusions containing water molecules. These results suggest that serpentinization had occurred in the fluid inclusions with exhausting water. Whereas 33% of the gas inclusions contain hydrogen in the Konde Hill sample, none of the gas inclusions contain hydrogen in the Horoman sample. Based on the Raman spectroscopic analysis, we estimated the possible contribution of hydrogen in the gas inclusions to hydrogen generated during low temperature serpentinization experiments, which account for 230 µmol/kg. This amount of hydrogen may actually explain the difference between the two samples in the generated amounts of hydrogen during the experiments which can cover all of the differences measured in the previous study.
Therefore, this study suggests that olivine-hosted inclusions are an important source for the hydrogen generated during low temperature serpentinization. However, since the Horoman sample does not have any inclusion containing hydrogen, a significant amount of hydrogen was also likely produced by dissolution of olivine at the low temperature.
Reference
Klein, F., Grozeva, N. G., & Seewald, J. S. (2019). Proc. Natl. Acad, Sci. U.S.A., 116, 17666–17672.