A low viscosity layer in the upper mantle, the Asthenosphere, is a requirement for plate tectonics 1 . The seismic low velocities and the high electrical conductivities of the Asthenosphere are attributed either to sub-solidus water-related defects in olivine minerals 2- 4 or to a few volume percents of partial melt 5- 8 but these two interpretations have shortcomings: (1) The amount of H 2O stored in olivine is not expected to be higher than 50 ppm due to partitioning with other mantle phases 9 , including pargasite amphibole at moderate temperatures 10 , and partial melting at high temperatures 9 ; (2) elevated melt volume fractions are impeded by the too cold temperatures prevailing in the Asthenosphere and by the high melt mobility that can lead to gravitational segregation 11, 12 . Here we determined the electrical conductivity of CO 2-H 2O-rich melts, typically produced at the onset of mantle melting. Electrical conductivity modestly increases with moderate amounts of H 2O and CO 2 but it dramatically increases as CO 2 content exceeds 6 wt% in the melt. Incipient melts, long-expected to prevail in the asthenosphere 10, 13- 15 , can therefore trigger its high electrical conductivities. Considering depleted and enriched mantle abundances in H 2O and CO 2 and their effect on the petrology of incipient melting, we calculated conductivity profiles across the Asthenosphere for various plate ages. Several electrical discontinuities are predicted and match geophysical observations in a consistent petrological and geochemical framework. In moderately aged plates (>5Ma), incipient melts most likely trigger both the seismic low velocities and the high electrical conductivities in the upper part of the asthenosphere, whereas for young plates 4 , where seamount volcanism occurs 6 , higher degree of melting is expected.