Experimental study on H2 generation during serpentinization of komatiite
Abstract
Komatiite-hosted hydrothermal vent system has been considered the environment for the origin and early evolution of life on Earth because serpentinization of komatiite is predicted to generate H2-rich fluid that is essential for the earliest metabolisms such as methanogenesis (Takai et al., 2006; Martin, et al., 2008; Russell et al., 2010). However, the H2 productivity of komatiite is still uncertain because there is no natural example of komatiite-hosted hydrothermal system in the modern ocean. Furthermore, it is known that Al-depleted and Al-undepleted komatiites are dominant at 3.5 Ga and 2.7 Ga, respectively, which may be due to higher mantle temperature at the earlier Earth. Therefore, we experimentally evaluated the dependence property of Al content in komatiite with H2 generation during the serpentinization of komatiite. In the experiment, Al-undepleted (Al2O3 = 10 wt%) and Al-depleted (Al2O3 = 5 wt%) komatiites were dehydrated, decarbonated and remelted using an electric furnace under QFM condition at elevated temperatures up to 1600 degrees C, and then quenched and powdered, respectively. The powders of unaltered komatiites were reacted with NaCl-fluid in the gold-titanium reaction cell at 300 degrees C and 500 bars for 2000-3000 hours. The water/rock ratio was adjusted to four at the beginning of the experiment. The fluid in the reaction cell was sampled directly into an Ar-purged vial through a gold-lined tube with a titanium valve, by which the compositional evolution of fluid with time can be followed during the experiment. In addition, an experiment using San Calros olivine as an Al-free starting material was also conducted for comparison. These three experiments revealed that the final, steady-state H2 concentrations of fluids were approximately 2-3 mmol/kg (Al2O3 = 10 wt%), 20 mmol/kg (Al2O3 = 5 wt%) and 70 mmol/kg (olivine), indicating that H2 concentration increases with decreasing Al content in the starting material. It is therefore suggested that extremely H2-rich condition could be sustained by the serpentinization of Al-depleted komatiite, which would have the sufficient potential to sustain the H2-based early ecosystem. Considering the secular variation of komatiite composition, the emergence of life and its early evolution might be more likely to occur in the komatiite-hosted hydrothermal system in earlier ocean.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.B43G0504S
- Keywords:
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- 5215 PLANETARY SCIENCES: ASTROBIOLOGY / Origin of life;
- 5225 PLANETARY SCIENCES: ASTROBIOLOGY / Early environment of Earth