From CO2 to Mg Carbonates in Ultramafic Rocks: Isotopic and Kinetic Constraints from Fluid-Limited Serpentinization

Ophicarbonates provide an important natural record of mineral carbonation during serpentinization of ultramafic rocks and therefore offer insight into the mechanisms and limits of CO2 fixation in low-temperature geological environments. This paper presents a synthesis and process-oriented reinterpretation of stable-isotope published and previously unpublished data, petrographic, and mineralogical evidence for carbonate formation under fluid-limited serpentinization conditions. Using mineralogical constraints together with a compiled δ13C–δ18O dataset that includes legacy measurements from the 1980s–1990s, we evaluate how multi-stage carbonate precipitation reflects evolving water–rock ratio, redox state, transport limitation, and deformation-controlled permeability. Particular attention is given to systematic differences between vein-hosted carbonates and dispersed intergranular or scattered-grain ophicarbonates, as these textural–isotopic relationships help identify fluid flux, carbon source, and reaction progress in ultramafic systems. The analysis shows that carbonation does not proceed uniformly but is restricted to overlapping reactive windows controlled by fluid availability, nucleation kinetics, and permeability evolution. These constraints help explain why carbonation may either intensify or stall during progressive serpentinization. The Author further discuss why kinetic barriers and Mg–Ca partitioning may redirect carbonate mineralogy toward calcite or metastable Mg-rich phases even where dolomite or magnesite may be thermodynamically favored. The results highlight the importance of coupling isotopic signatures with petrographic context in reconstructing carbonation pathways and provide a broader framework for understanding natural mineral sequestration of carbon in heterogeneous serpentinite systems.