Chemical Nature of Hydrothermal Fluids Generated by Serpentinization and Carbonation of Komatiite: Implications for H <sub>2</sub> ‐Rich Hydrothermal System and Ocean Chemistry in the Early Earth
Chemical Nature of Hydrothermal Fluids Generated by Serpentinization and Carbonation of Komatiite: Implications for H <sub>2</sub> ‐Rich Hydrothermal System and Ocean Chemistry in the Early Earth
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科马提岩蛇纹石化和碳化产生的热液的化学性质：对早期地球富 H <sub>2</sub> 热液系统和海洋化学的影响
DOI：
10.1029/2021gc009827
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Takai Ken
中科院分区：
文献类型：
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作者：
Ueda Hisahiro;Shibuya Takazo;Sawaki Yusuke;Shozugawa Katsumi;Makabe Akiko;Takai Ken
富氢超镁铁质热液系统被认为是生命起源和早期演化的重要场所。在这项研究中，我们进行了两次热液蛇纹石化/碳化实验，在富含CO2的条件下，利用合成的科马提石，以进一步了解地球早期赋存于科马提石的海底热液系统中热液的化学性质。300℃时流体中的H_2浓度比无CO_2条件下低一个数量级，这表明在富CO_2条件下，科马提石的碳化作用抑制了H_2的生成。流体中的稳态H_2浓度随着温度的升高而增加，而运行产物中碳酸盐矿物中的铁含量随着温度的降低而增加。这种相关性表明，在不同温度下，碳酸盐矿物中Fe的掺入将限制科马提石蛇纹岩过程中H_2的生成。此外，液体的pH值也依赖于温度。高温(>；300°C)流体变为碱性，而低温流体变为酸性。热力学计算表明，水热反应过程中的高水岩比是导致酸性流体的主要原因。因此，很可能在低温下产生贫氢和酸性流体。相反，富氢和碱性热液形成于300℃以上，这表明在热液流体与周围海水之间的能量和电化学势的形成方面，高温海底热液系统可能比较低温度系统更有利于Hadean海洋生命的出现和早期演化。
H2‐rich ultramafic‐hosted hydrothermal systems are considered to be important places for the origin and early evolution of life. In this study, we conducted two hydrothermal serpentinization/carbonation experiments involving synthetic komatiite under CO2‐rich conditions to further understand the chemical nature of hydrothermal fluids in the komatiite‐hosted seafloor hydrothermal systems in the early Earth. The H2concentration in fluid at 300°C is one order of magnitude lower than that under CO2‐free conditions, which revealed that the carbonation of komatiites suppressed H2generation under CO2‐rich conditions. The steady‐state H2concentrations in the fluid increased with increasing temperature, while the Fe content of carbonate minerals in the run products increased with decreasing temperature. This correlation suggested that Fe incorporation into the carbonate minerals would limit the H2generation during the serpentinization of komatiites at each temperature. In addition, the pH of the fluid also depended on temperature. High‐temperature (>300°C) fluids became alkaline, whereas low‐temperature fluids became acidic. Thermodynamic calculations show that the acidic fluids were attributed to high water/rock ratio during the hydrothermal reactions. Consequently, H2‐poor and acidic fluids were likely generated at low temperatures. In contrast, the H2‐rich and alkaline hydrothermal fluids were generated at temperatures above 300°C, which suggests that high‐temperature seafloor hydrothermal systems may be more favorable than lower‐temperature systems for the emergence and early evolution of life in the Hadean ocean in terms of the formation of energy and electrochemical potentials between hydrothermal fluids and ambient seawater.