Abstract We injected a CO2-rich hydrous fluid of seawater chlorinity into an ongoing, mildly reducing (H2(aq)≈3 mmol/kg) serpentinization experiment at 230 °C and 35 MPa to examine the changes in fluid chemistry and mineralogy during mineral carbonation. The chemistry of 11 fluid samples was measured, speciated, and compared with MgO–SiO2–H2O–CO2 (MSHC) phase equilibria to approximate the reaction pathway from serpentinization to carbonation. Although the overall system was in apparent disequilibrium, the speciated activities of dissolved silica (aSiO2(aq)) and carbon dioxide (aCO2(aq)) evolved roughly along MSHC equilibrium phase boundaries, indicative of 4 distinct mineral assemblages over time: (1) serpentine–brucite (± magnesite) before the injection, to (2) serpentine–talc–magnesite 2 h after the injection, to (3) quartz–magnesite (48 h after injection), and (4) metastable olivine–magnesite (623 h after injection) until the experiment was terminated. Inspection of the solid reaction products revealed the presence of serpentine, magnesite, minor talc, and magnetite, in addition to relict olivine. Although quartz was saturated over a short segment of the experiment, it was not found in the solid reaction products. A marked and rapid change in fluid chemistry suggests that serpentinization ceased and precipitation of magnesite initiated immediately after the injection. A sharp decrease in pH after the injection promoted the dissolution of brucite and olivine, which liberated SiO2(aq) and dissolved Mg. Dissolved Mg was efficiently removed from the solution via magnesite precipitation, whereas the formation of talc was relatively sluggish. This process accounts for an increase in aSiO2(aq) to quartz saturation shortly after the injection of the CO2-rich fluid. Molecular dihydrogen (H2(aq)) was generated during serpentinization of olivine by oxidation of ferrous iron before the injection; however, no additional H2(aq) was generated after the injection. Speciation calculations suggest a strong affinity for the formation of methane (CH4(aq)) at the expense of CO2(aq) and H2(aq) after the injection, but increased CH4(aq) formation was not observed. These findings suggest that kinetically fast mineral carbonation dominates over sluggish CH4(aq) formation in mildly reducing serpentinization systems affected by injection of CO2-rich fluids.