Reactions between komatiite and CO2-rich seawater at 250 and 350 °C, 500 bars: implications for hydrogen generation in the Hadean seafloor hydrothermal system

Abstract

To understand the chemical nature of hydrothermal fluids in the komatiite-hosted seafloor hydrothermal system in the Hadean, we conducted two hydrothermal serpentinization experiments involving synthetic komatiite and a CO 2 -rich acidic NaCl fluid at 250 and 350 °C, 500 bars. During the experiments, the komatiites were strongly carbonated to yield iron-rich dolomite (3-9 wt.% FeO) at 250 °C and calcite (<0.8 wt.% FeO) at 350 °C, respectively. The carbonation of komatiites suppressed H 2 generation in the fluids. The steady-state H 2 concentrations in the fluid were approximately 0.024 and 2.9 mmol/kg at 250 and 350 °C, respectively. This correlation between the Fe content in carbonate mineral and the H 2 concentration in the fluid suggests that the incorporation of ferrous iron into the carbonate mineral probably limited magnetite formation and consequent generation of hydrogen during the serpentinization of komatiites. The H 2 concentration of the fluid at 350 °C corresponds to that of modern H 2 -rich seafloor hydrothermal systems, such as the Kairei hydrothermal field, where hydrogenotrophic methanogens dominate in the prosperous microbial ecosystem. Accordingly, the high-temperature serpentinization of komatiite would provide the H 2 -rich hydrothermal environments that were necessary for the emergence and early evolution of life in the Hadean ocean. In contrast, H 2 -rich fluids may not have been generated by serpentinization at temperatures below 250 °C because carbonate minerals become more stable with decreasing temperature in the komatiite-H 2 O-CO 2 system.

Weak hydrothermal carbonation of the Ongeluk volcanics: evidence for low CO2 concentrations in seawater and atmosphere during the Paleoproterozoic global glaciation

“…Such CO 2 -rich seawater provided seafloor hydrothermal vent environments that would sustain the emergence and early evolution of life (Takai et al 2006;Russell et al 2010Russell et al , 2014Shibuya et al 2015Shibuya et al , 2016. Furthermore, chemical exchange reactions would have had particular characteristics as the then CO 2 -rich seawater interfaced oceanic lithosphere (e.g., intense CO 2 fixation into the crust) (Nakamura and Kato 2004;Rouchon and Orberger 2008) resulting in the generation of novel high-temperature hydrothermal fluids (e.g.., alkaline, silica-rich, metal-poor fluid, and Mg-rich fluid) (Shibuya et al 2010;Ueda et al 2016). These processes would have played a significant role in controlling the chemistry of early oceans.…”

Experimental and Simulation Efforts in the Astrobiological Exploration of Exooceans

“…Experimental studies have revealed that high concentrations of CO 2 and thus lower pH of early Earth's oceans (Halevy and Bachan 2017) had the potential to elevate in situ pH and reduce metal concentrations in hydrothermal fluids even at high temperatures, in contrast to their modern equivalents (Shibuya et al 2013). Additionally, the experimental studies show that H 2 concentrations of hydrothermal fluids in komatiite-hosted systems would be comparable to those in modern ultramafic-hosted hydrothermal systems Ueda et al 2016). These experimental studies suggest the existence of H 2 -rich alkaline hydrothermal vents on early Earth (Macleod et al 1994).…”

“…The vents would add to the general habitability of the oceans by providing nutrients and an energy source. In addition to in situ studies of terrestrial hydrothermal vent fluids and ocean floor drilling (Beaulieu et al 2015;Tivey 2011;Alt 2013), laboratory based experiments using large-scale reactors are conducted to reproduce closed-system water-rock interactions (Yoshizaki et al 2009;Shibuya et al 2013;Ueda et al 2016), which are essential for our understanding of the ocean floor on these icy moons. Efforts are also being taken to develop microfluidics devices for a more controlled setting.…”

Serpentinite-hosted Hydrothermal System on the Early Earth

“…Solid curves indicate solutions with 1 mol/L of Ca⊖Mg chloride. Ranges of hydrothermal fluids in this study and previous studies are plotted as individual symbols (Jones et al, 2010;Neubeck et al, 2014;McCollom et al, 2016;Ueda et al, 2016Ueda et al, , 2017 Neubeck et al, 2014;McCollom et al, 2016;Ueda et al, 2016Ueda et al, , 2017…”

“…。カンラン石を用いた高 二酸化炭素濃度下の熱水実験は 30 ～ 320°C の幅 広い温度範囲で行われている (Jones et al, 2010;Neubeck et al, 2014;McCollom et al, 2016;Ueda et al, 2017 (Lazar et al, 2012;Hao and Li, 2015;Ueda et al, 2016 (Alt., 1995;Edmond et al, 1979;Elderfield and Schultz, 1996;Kelley et al, 2001 (Rividi et al, 2010;Rintamäki, A., 2016) and those formed in hydrothermal experiments Ueda et al, 2016Ueda et al, , 2017 (Jones et al, 2010;Neubeck et al, 2014;McCollom et al, 2016;Ueda et al, 2016Ueda et al, , 2017) ． Fig. 7 Calcite⊖dolomite⊖magnesite stability fields as functions of temperature and mMg/ (mMg + mCa) value estimated from experiments at 275⊖420°C and their extrapolation to 0°C (modified after Tribble et al, 1995) .…”