Abstract Carbon sequestration in mafic and ultramafic rocks offers a pathway to mitigate CO2 ${text{CO} }{2}$ emissions, while mineralization could precondition rocks for critical mineral extraction. However, the reaction‐driven fracturing processes that increase permeability and weaken rock remain underexplored. Here, we expose serpentinized dunite samples to CO2 ${text{CO} }{2}$‐rich Mg(OH)2‐brine at 90°C and 3.5 MPa for up to 32 days, using X‐ray computed tomography and digital volume correlation to quantify morphological evolution and 3D solid displacements. Results reveal tensile microfractures nucleating within 11 days, progressing to core disaggregation, with localized damage concentrated along pre‐existing veins and flaw tips. Calculated crystallization pressures exceeding 100 MPa, together with fracture mechanics modeling, support magnesite formation within brucite‐filled veins as the primary driver for reaction‐driven fracturing. Raman spectroscopy and scanning electron microscopy help identify mineral changes. These insights into spatiotemporal dynamics of reaction‐driven fracturing can inform strategies for CO2 ${text{CO} }_{2}$ injection and mineralization in ultramafic formations.