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The plume composition at Enceladus contains clues about conditions and processes in the interior. We present new geochemical interpretations of Cassini mass spectrometry data from the plume gas and salt-rich ice grains. It is found that self-consistency between the data sets can be achieved with a derived range of 10(-4.6) to 10(-3.2) for the activity of CO2 in Enceladus' ocean. This range is compatible with long-term buffering by reduced or oxidized seafloor rocks containing quartz, talc, and carbonate minerals in the MgO-FeO-SiO2-CO2-H2O system. Reaction path modeling shows that these types of rocks can be produced from accreted CO2-rich fluids reacting with hydrous chondritic rocks over an intermediate regime of carbonation. These results, together with previous findings of silica and H-2 at Enceladus, support the hypothesis of a heterogeneous structure for the rocky core (carbonated upper layer, serpentinized interior), which provides a geochemical gradient for habitability.