Previous modeling of carbonate subduction by high‐pressure experimentation has allowed to propose scenarios for bulk carbon return to the mantle, but the detailed transfer mechanisms have seldom been studied. We monitored carbonate – silicate reactions by combining high‐pressure experiments and synchrotron‐based x‐ray diffraction. Carbonates break down at moderate pressure and high temperature and CO2 is trapped at grain boundaries. Further isothermal compression yields melting, which may control continuous carbon introduction, first in the mantle wedge, and next, away from the wedge. Carbon presence has been discussed in a variety of magmatic contexts, under the oceanic lithosphere (hotspots, petit spots, fossil ridges). We suggest the presence of a global carbon‐rich layer under the oceanic lithosphere that is steadily fed by subduction processes. This layer can be the source of mechanical weakening of the lithosphere‐asthenosphere boundary under the oceans. Therefore, carbon‐induced compression melting may be a key mechanism of modern‐style plate tectonics.