Carbonated melts constitute a key medium in the global deep carbon cycle: their impact on the geochemical signature of deep rocks is well studied because of their role as metasomatic agents in the deep mantle. However, their physical properties and in particular their electrical conductivity at high temperature and high pressure remain poorly constrained. In this study, we investigated the effect of chemical composition on the electrical conductivity of carbonated melts. We characterized this effect for various temperatures (1000–1700 °C) and pressures (1 to 4 GPa). Measurements show a very high electrical conductivity (> 100 S·m− 1) with weak temperature, pressure and chemical composition dependence. Carbonated melts are five orders of magnitude more conductive than mantle olivine, and up to two orders of magnitude more conductive than basalts at similar T and P. The electrical conductivity of molten carbonates follows an Arrhenius law and the different parameters were determined. A common activation volume was defined with ΔV = 0.275 J/bar. As a result, we are able to calculate the electrical conductivity for larger temperature and pressure ranges for the melt compositions considered here. By combining the Nernst–Einstein and Eyring equations, a remarkably simple correlation was established between electrical conductivity and viscosity. The viscosity of carbonated melts, which is a key parameter defining the rate of metasomatic fluids flowing in the earth's mantle, can therefore be calculated as a function of pressure and temperature. We used these new data to interpret the high electrical conductivity recently observed in the mantle under the Brazilian craton. The anomalously elevated conductivity most likely images the process of lithospheric rejuvenation involving 0.03 to 0.2% of carbonated melt.