Carbon is commonly locked in the crust in two carbonate minerals: 1) calcite; and 2) dolomite. Pure, dry calcite is thermally stable to high temperatures, but can be assimilated by melts ascending from the mantle to the surface. Dolomite can decarbonate at high temperatures in addition to being consumed by subarc magmas. In this study, experiments containing carbonate with compositions between dolomite and calcite (with minor iron) give evidence for decarbonation at temperatures as low as 800 °C at 0.5 GPa, at nominally dry conditions, with increasing carbon dioxide release corresponding to increasing Mg/Ca ratios. Allowing these carbonates to interact with typical arc dacite and basaltic magmas at ~15 km depth and temperatures of 1000 and 1150 °C, respectively, depresses the liquidi, produces periclase and olivine with Mg-rich carbonate, expands the stability field of clinopyroxene, and releases CO2. Calculations indicate assimilation- and thermal breakdown-induced release of CO2 both increase with increasing Mg/Ca ratio of carbonate sediments. Extrapolating to conditions of natural systems with magmatic recharge suggests assimilation produces ≤1010–1012 g/y CO2, expelling as much as ~105 g CO2/m3 of carbonate, similar to that which can occur by thermal breakdown of carbonate at 600–800 °C, or potentially less depending on the heat, size and timescale of the aureole formation. Though more dolomitic systems assimilate more and thus release more crustal carbon to the atmosphere than more limestone-rich carbonate, our results indicate both assimilation and thermal breakdown processes can each contribute a significant and important flux of greenhouse gas to the atmosphere. Likely happening concurrently, these extra sources from the crustal carbon reservoir could affect climate, which may be particularly relevant during Earth's Eocene-Cretaceous warm period.