Mid-ocean ridges are places where interactions between seawater and oceanic crust take place. Hydrothermal interactions govern the chemistry of the oceans while hydrothermal vent fields host unique and diverse biological communities even in barren ocean floor settings, and are candidates for the birthplace of the earliest life forms. This paper presents the important roles of the hydrothermal carbonatization of Archean oceanic crust that is one of the characteristic seafloor alterations in the early Earth. Based on the mineralogical, geochemical, and geological features of calcite in the Archean greenstones, the CO₂ flux from ocean to oceanic crust was estimated to be two orders of magnitude larger than the modern value, which points to the significance of seafloor hydrothermal carbonatization in the Archean carbon cycle. Furthermore, thermodynamic calculations of phase equilibria in the high-temperature alteration zone indicate that the hydrothermal fluid was alkaline due to the presence of calcite in the alteration minerals under a high-CO₂ condition, and predict a generation of SiO₂-rich and Fe-poor hydrothermal fluids in the subseafloor hydrothermal system. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life.