Chemical Nature of Hydrothermal Fluids Generated by Serpentinization and Carbonation of Komatiite: Implications for H₂‐Rich Hydrothermal System and Ocean Chemistry in the Early Earth

Abstract

Abstract H 2 ‐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 CO 2 ‐rich conditions to further understand the chemical nature of hydrothermal fluids in the komatiite‐hosted seafloor hydrothermal systems in the early Earth. The H 2 concentration in fluid at 300°C is one order of magnitude lower than that under CO 2 ‐free conditions, which revealed that the carbonation of komatiites suppressed H 2 generation under CO 2 ‐rich conditions. The steady‐state H 2 concentrations 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 H 2 generation 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, H 2 ‐poor and acidic fluids were likely generated at low temperatures. In contrast, the H 2 ‐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.

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