To understand the influence of high CO2 contents in a hydrothermal fluid on ultramafic rock-hosted seafloor hydrothermal systems on the early Earth, the reaction between tremolite-bearing serpentinite exposed in the Hakuba-Happo area and a CO2-rich NaCl fluid at 200°C and 100 bars was monitored. The H2 concentration in the fluid reached a constant value of approximately 0.4 mmol/kg within 6312 hours. This concentration is lower than the H2 concentration in a natural hot spring in the Hakuba-Happo area. During the experiments, the total carbonic acid concentration (ΣCO2) in the fluid decreased, and magnesite was formed through the water–rock interaction. This is the first experimental report of magnesite formed from Ca-bearing ultramafic rocks, which is possibly attributed to the lower temperature condition compared to previous studies. Incorporation of ferrous iron into the magnesite probably suppressed iron oxidation in fluids and resultant H2 generation during serpentinization and carbonation at 200°C. Compilation of existing experimental data indicates that precipitated carbonate species depend on reaction temperature, initial fluid composition, and composition of reacted rock. The compilation further suggests that the amount of precipitated carbonate decreases and iron-poor calcite becomes predominant at higher temperatures. Therefore, the influence of the fluid CO2 on H2 generation at high temperatures becomes small even on the early Earth. The description of carbonate species formed in komatiites of Archean greenstone belts tells us that H2 generation in low-temperature hydrothermal fluids circulating peridotitic komatiites was possibly limited by the precipitation of Mg-rich carbonate and iron incorporated therein. On the other hand, experimental hydrothermal fluids co-existing with Mg-bearing carbonate during serpentinization of ultramafic rocks clearly have high Mg contents or become more magnesian, compared to modern basalt-hosted hydrothermal fluids. This implies that carbonation of ultramafic rocks would be one of the main sources of oceanic Mg on the early Earth.