Gabbroic sills of the widespread, ca. 720 Ma Franklin large igneous event intruded sedimentary strata of the Neoproterozoic Shaler Supergroup exposed in the Minto Inlier on Victoria Island in the western Arctic. The mafic magmatism occurred during breakup of the supercontinent Rodinia and preceded Sturtian glaciation. Calc-silicate metamorphic reactions produced CO2 from decameter-scale contact aureoles in carbonate rocks containing variable amounts of silicates. Numerical modeling of the reactions and fluid flow was done for host-rock permeabilities (k) ranging from 10–18 to 10–14 m2. Metamorphic assemblages and aureoles widths are best reproduced with k between 10–18 and 10–17 m2; however, the lower k produces much broader fracture zones next to the sills than those observed in the field. When k ≥ 10–16 m2, the contribution of advective heat transport produces aureoles that are too thick. With k = 10–17 m2, the contribution of CO2 to the atmosphere from the aureole of one 50-m-thick sill with an area of 50,000 km2 would have been only 8 ppm (by weight) during the 600 yr metamorphic episode. The sedimentary basin would have to have been more permeable for a higher flux of metamorphic CO2 to the atmosphere. The results demonstrate that host-rock permeability must be considered before changes in the budgets of atmospheric carbonic gases can be attributed to metamorphic degassing from sedimentary basins.