Fractures in geological formations may enable migration of environmentally relevant fluids, as in leakage of CO2 through caprocks in geologic carbon sequestration. We investigated geochemically induced alterations of fracture geometry in Indiana Limestone specimens. Experiments were the first of their kind, with periodic high-resolution imaging using X-ray computed tomography (xCT) scanning while maintaining high pore pressure (100 bar). We studied two CO2-acidified brines having the same pH (3.3) and comparable thermodynamic disequilibrium but different equilibrated pressures of CO2 (PCO2 values of 12 and 77 bar). High-PCO2 brine has a faster calcite dissolution kinetic rate because of the accelerating effect of carbonic acid. Contrary to expectations, dissolution extents were comparable in the two experiments. However, progressive xCT images revealed extensive channelization for high PCO2, explained by strong positive feedback between ongoing flow and reaction. The pronounced channel increasingly directed flow to a small region of the fracture, which explains why the overall dissolution was lower than expected. Despite this, flow simulations revealed large increases in permeability in the high-PCO2 experiment. This study shows that the permeability evolution of dissolving fractures will be larger for faster-reacting fluids. The overall mechanism is not because more rock dissolves, as would be commonly assumed, but because of accelerated fracture channelization. Fractured cores of Indiana Limestone were scanned with x-ray computed tomography (xCT) and the fracture geometry was analyzed as described in Deng et al. (2016). These fractured cores were used in experiments with CO2-acidified fluids and the evolution of fracture geometry and permeability were measured, as described in Deng et al. (2015). The 3D reconstructed images were used to generate aperature maps and statistical representations, which were used as the initial conditions for reactive transport models in Deng and Peters (WRR, submitted Dec 2017). The reactive transport simulations studied the effects of mineral heterogeneity using calcite mineral maps presented in Ellis and Peters (2016). ... Download the README.txt file for additional details about the dataset.