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Earthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measuring temperature during frictional instabilities inhibit the investigation and interpretation of shear stress evolution. Here we conduct high velocity friction experiments on calcite‐bearing simulated faults, both on bare‐rock and on gouge samples, at 20–30 MPa normal stress, 1–6 m/s slip rate and 1–20 m total slip. Seismic slip pulses are reproduced by imposing boxcar and regularized Yoffe slip rate functions. We measured, together with shear stress, slip, and slip rate, the temperature evolution on the fault by employing an innovative two‐color fiber optic pyrometer. The comparison between modeled and measured temperature reveals that for calcite‐bearing faults the heat sink caused by decarbonation reaction controls the temperature evolution. In bare‐rocks, energy is dissipated as frictional heat, and temperature increase is buffered by the heat sink of the calcite decarbonation reaction. In gouges, energy is dissipated as frictional heat and for plastic deformation processes, balanced by the heat sink caused by the decarbonation reaction enhanced by the mechanochemical effect. Our results suggest that in calcite‐bearing rocks, a common fault zone material for earthquake sources in the continental crust at shallow depth, the type of fault materials (bare‐rocks vs. gouges) controls the energy dissipation during seismic slip. Plain Language Summary: During earthquakes, faults rocks lose strength, and therefore the ability to sustain shear stress as a consequence of slip, slip rate, and temperature resulting in dynamic weakening. The mathematical relationships between the decreasing strength and slip, slip rate, and temperature and the energy balance describing the partition of energy are of fundamental importance to model the propagation of an earthquake rupture. These relationships can be defined thanks to laboratory experiments that simulate seismic slip. Here, we tested calcite‐bearing fault rocks simulated as bare‐rock and gouges. During the experiments, temperature was monitored thanks to an innovative measuring system. Numerical models were done assuming all mechanical energy was converted into heat. By comparing all results above, we discovered that the mechanical energy in both bare‐rocks and gouges is converted to heat but limited by the occurrence of endothermic decarbonation reaction. In gouges, also an energy contribution for plastic deformation processes is required. Our work implies significant changes in the commonly accepted energy budget for earthquake propagation and show the importance of slip rate and temperature in driving together the dynamic weakening during seismic slip. Key Points: We measure temperature, shear stress, and slip rate to assess energy dissipation during seismic slip in calcite‐bearing fault materialsCalcite decarbonation compensates frictional heat production in all tested fault materialsMechanical energy is dissipated for frictional heat in bare‐rocks and for frictional heat and plastic deformation processes in gouges [ABSTRACT FROM AUTHOR]