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Ultramafic rocks are increasingly recognized as promising reservoirs for long-term carbon fixation through mineral carbonation. However, carbonation reactions are inherently self-limiting, as they involve solid volume increases of up to ~68%, which clog pore spaces, drastically reduce permeability, and inhibit further fluid infiltration. Geological processes capable of sustaining permeability during carbonation have therefore been invoked, including (1) continuous tectonic deformation that generates microfractures (Menzel et al. 2022, Nat. Commun.), and (2) metasomatic mass transfer from mantle rocks to the crust that reduces net solid volume (Okamoto et al. 2021, Commun. Earth Environ.). Despite their importance for both natural and engineered carbon storage, the dynamic coupling between metasomatic reactions, volumetric changes, and deformation remains poorly constrained by experiments.Here we investigate reaction–deformation coupling at the slab–mantle interface using a series of hydrostatic and shear deformation experiments conducted at 500 °C and 1.0 GPa in a Griggs-type piston-cylinder apparatus. Experimental assemblies consisted of a three-layer configuration in which a crustal lithology (pelitic schist from the Sanbagawa belt or quartzite) was sandwiched between harzburgite (Horoman peridotite) and serpentinite (Mikabu belt). Hydrostatic experiments were performed with pure H₂O, whereas shear experiments employed H₂O–CO₂ fluids (XCO₂ = 0.2) generated in situ by thermal decomposition of oxalic acid dihydrate.Hydrostatic experiments reveal that metasomatic reaction pathways and resulting textures are strongly controlled by the chemical composition of the adjacent crustal rock. In experiments involving pelitic schist, albite phenocrysts are preferentially replaced by Mg-rich saponite, while talc precipitates within dendritic fracture networks in the serpentinite. Mass balance calculations indicate that Mg absorption by Al-bearing minerals in the sedimentary rocks promotes progressive Mg extraction from mantle lithologies. Importantly, textural contrasts between lithologies indicate opposite volumetric responses: reaction-induced fracturing in serpentinite is associated with net solid volume reduction, whereas reactions in harzburgite proceed with solid volume expansion.Shear deformation experiments conducted along quartzite–serpentinite interfaces exhibit a pronounced reaction-duration dependence on mechanical behavior. Short time reaction (6 h), friction coefficients are relatively high. In contrast, long reaction duration (68 h) results in stable sliding with exceptionally low friction coefficients. Microstructural observations show the development of a reaction zone dominated by extensive carbonation (listvenite formation: quartz + magnesite) localized at the lithological interface. Deformation is strongly localized within the carbonation products, which display laminar fabrics and magnesite-filled fractures containing nanoscale porosity.Integrating hydrostatic and shear experiments, we suggest that metasomatic mass transfer is essential for sustaining carbonation reactions. Furthermore, the pronounced mechanical weakening observed in shear experiments may not be solely attributable to talc precipitation, but possibly also to the dehydration accompanying carbonation. Instead, the dynamic coupling between chemical reactions, solid volume changes, and deformation promotes fracture formation, permeability maintenance, and extreme rheological weakening. These processes provide a viable mechanism for overcoming reaction-induced pore clogging during long-term carbonation and have profound implications for carbon transport, storage efficiency, and the mechanical behavior of the slab–mantle interface.