Abstract Mantle plumes are thought to generate Large Igneous Provinces (LIPs), the largest outpourings of lava in the geologic record, which are associated with long‐lasting climatic warming events and mass extinctions. However, the controls on mantle degassing by plumes remain poorly constrained. Here we use thermochemical plume models to compute the melt production and carbon release from compositionally heterogeneous plumes that sample peridotite and eclogite. We show that plume behavior is governed by the balance between thermal and compositional buoyancy. While excess temperature primarily controls melt production, carbon release is dominated by eclogite melting at depth and is maximized at intermediate eclogite fractions. Despite similar melt production, purely thermal plumes release about half as much CO2 ${mathrm{C}mathrm{O} }_{2}$ as eclogite‐bearing plumes. Our results highlight the significance of thermal and compositional buoyancy and the melting effects of eclogite in connecting thermochemical plumes to LIPs and their climate impacts.