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Reactions Between Komatiite and CO2-Rich Seawater at 250 and 350 °C

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Reactions Between Komatiite and CO2-Rich Seawater at 250 and 350 °C Ueda et al. Progress in Earth and Planetary Science (2016) 3:35 Progress in Earth and DOI 10.1186/s40645-016-0111-8 Planetary Science RESEARCHARTICLE Open Access Reactions between komatiite and CO2-rich seawater at 250 and 350 °C, 500 bars: implications for hydrogen generation in the Hadean seafloor hydrothermal system Hisahiro Ueda1,2*, Takazo Shibuya2,3,4, Yusuke Sawaki1, Masafumi Saitoh2,3,4, Ken Takai2,3,4,5 and Shigenori Maruyama6 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 aCO2-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 H2 generation in the fluids. The steady-state H2 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 H2 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 H2 concentration of the fluid at 350 °C corresponds to that of modern H2-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 H2-rich hydrothermal environments that were necessary for the emergence and early evolution of life in the Hadean ocean. In contrast, H2-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-H2O-CO2 system. Keywords: Komatiite, CO2-rich condition, Early Earth, Hydrothermal alteration, Serpentinization Introduction Russell et al. 2014; Nakamura and Takai 2014; Shibuya et Deep-sea hydrothermal environments have been consid- al. 2016). Molecular hydrogen (H2)generationduring ered as a favorable environment for the emergence and serpentinization is caused by the reduction of water in early evolution of life on Earth (e.g., Yanagawa and Kojima conjunction with the oxidation of ferrous iron in silicates. 1985; Russell and Hall 1997). In particular, H2-rich hydro- This process is written as the following simplified reac- thermal fluids generated through the serpentinization of tion (e.g., Allen and Seyfried 2003; Seyfried et al. 2007; ultramafic rocks would have driven prebiotic chemical McCollom and Bach 2009): evolution and the development of biotic energy metabo- 2ðÞ FeO þ H O→ðÞFe O þ H ; lisms (Takai et al. 2006; Amend and McCollom 2009; rock 2 2 3 rock 2 where the ferric iron-bearing solid phase generally pre- * Correspondence: [email protected] 1Department of Earth and Planetary Sciences, Tokyo Institute of Technology, cipitates as magnetite (Fe3O4). Therefore, H2 generation 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan is strongly related to magnetite formation during 2 Laboratory of Ocean-Earth Life Evolution Research (OELE), Japan Agency for serpentinization (McCollom and Bach 2009). Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan Among the various prospects for the seafloor hydrother- Full list of author information is available at the end of the article mal systems in the early Earth (e.g., Kump and Seyfried © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Ueda et al. Progress in Earth and Planetary Science (2016) 3:35 Page 2 of 14 2005; Shibuya et al. 2010), two types of serpentinization 2009). The experiments revealed the chemical compos- are hypothesized to be dominant in the Hadean ocean: ition of high-temperature hydrothermal fluids and CO2 a komatiite-hosted high-temperature type at oceanic absorption ability of komatiite through serpentinization islands/plateaus (Takai et al. 2006; Yoshizaki et al. 2009; under CO2-rich conditions. The results demonstrate the Shibuya et al. 2015) and a low-temperature alkaline significance of komatiite-hosted hydrothermal systems type hosted by komatiites or peridotites at off-ridge for seawater chemistry and provide insight into the H2- ocean floors (e.g., Russell et al. 2010, 2014; Shibuya et rich hydrothermal environments in the CO2-rich Hadean al. 2016). Based on the geological records, Archean ocean. oceanic crust was likely much thicker than modern equivalents owing to the higher potential mantle Methods/Experimental temperature at that time (Ohta et al. 1996; Komiya et Synthesis of komatiite al. 2002; Komiya 2004; Moores 2002; Shibuya et al. The komatiite used in the experiments was synthesized 2012). This thick lid of oceanic crust probably limited from a mixture of 12 reagents (SiO2,TiO2,Al2O3,Fe2O3, the exposure of mantle peridotites on the seafloor, thus MnO, MgO, CaCO3,Na2CO3,K2CO3,P2O5,NiO,and suggesting that komatiite volcanism would have been Cr2O3). The chemical composition of the mixed reagents much more abundant than the exposed mantle peridotites was adjusted to Al-depleted (Barberton-type) komatiite that are frequently observed near modern slow-spreading (Smith et al. 1980; Wei et al. 1990; Yoshizaki et al. 2009) ridges without a sufficient magmatic supply (Takai et al. because the volcanism of Al-depleted komatiite was likely 2006). Therefore, it has been pointed out that H2-rich more predominant than Al-undepleted komatiite in the seafloor hydrothermal environments would have been Hadean (Shibuya et al. 2015). The mixed powder was predominantly driven by komatiite volcanism in the placed in a Pt-Rh crucible at 1000 °C for 1 h in an elec- Hadean ocean (Takai et al. 2006). On the other hand, tronic furnace to decarbonate the reagents. The mixture the low-temperature alkaline type could provide distinct- was melted at 1600 °C for 0.5 h at the oxygen fugacity of ive chemical environments such as large pH gradients in quartz-fayalite-magnetite (QFM) buffer regulated by a the seawater-hydrothermal fluid mixing zones, which may H2-CO2 gas mixture (Canil 1997). To create a spinifex also have been advantageous for the possible development texture of olivine, the temperature was lowered to 1450 °C of proton-motive energy metabolisms (Russell et al. 2014; over 1.5 h. Next, the crucible was quenched to room Shibuya et al. 2016). temperature, thus yielding a fresh spinifex-textured Previously, hydrothermal alteration experiments have komatiite (Fig. 1). The composition and texture of the been conducted to understand the potential of H2 gener- synthetic komatiite and its mineral/glass phases were ation during the serpentinization of komatiites under analyzed using X-ray diffraction (XRD), X-ray fluorescence CO2-free conditions (Yoshizaki et al. 2009; Shibuya et al. (XRF), and an electron probe microanalyzer (EPMA) 2015; Suzuki et al. 2015). However, as suggested by the- (Table 1). In the interstitial glass, dendritic olivine crystals oretical calculations (Walker 1985; Kasting 1993; Sleep configurate a spinifex texture. Broadly depending on size, and Zahnle 2001) and geological records (Lowe and Tice these crystals have Mg# values ranging from 89 to 94. 2004; Ohmoto et al. 2004; Shibuya et al. 2007, 2012, Small amounts of chromian spinels are disseminated in 2013a), the atmospheric CO2 levels in the early Earth the vicinity of the olivine crystals. Owing to such a mineral were likely much higher than the modern level. Further- assemblage, almost all Ca in the komatiite was partitioned more, it has been experimentally demonstrated that car- into the interstitial glass (Fig. 1 and Table 1). The synthe- bonate formation during the serpentinization of olivine sized komatiite was crushed in a tungsten mortar and under CO2-rich conditions suppresses H2 generation in sieved to obtain a <100 μm powder. To remove any con- fluids (Jones et al. 2010; Klein and McCollom 2013; tamination of organic materials during sample prepar- Neubeck et al. 2014). Recently, some experiments have ation, the powdered komatiite was washed with acetone been conducted in komatiite-CO2-H2O systems (Lazar and distilled water several times and dried for 12 h in an et al. 2012; Hao and Li 2015), but H2 generation in oven at 50 °C. fluids was not the objective in these studies. There- fore, the potential for hydrogen generation through Experimental system the serpentinization of komatiites in the Hadean has The Inconel alloy autoclave used for the hydrothermal not yet been experimentally evaluated under CO2-rich alteration experiment is based on the study by Seyfried conditions. et al. (1979). It resists corrosion and withstands high- In this study, we conducted two experiments that temperature and high-pressure conditions (up to 600 °C simulate the reactions between komatiite and CO2-rich and 600 bars; Fig. 2). The reaction cell is made of a gold seawater at 250 and 350 °C, 500 bars, using
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