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The boundary between Earth’s rigid lithosphere and the underlying, ductile asthenosphere is marked by a distinct seismic discontinuity. A decrease in seismic-wave velocity and increase in attenuation at this boundary is thought to be caused by partial melt. The density and viscosity of basaltic magma, linked to the atomic structure^,, control the process of melt separation from the surrounding mantle rocks^,,,,. Here we use high-pressure and high-temperature experiments and in situ X-ray analysis to assess the properties of basaltic magmas under pressures of up to 5.5 GPa. We find that the magmas rapidly become denser with increasing pressure and show a viscosity minimum near 4 GPa. Magma mobility—the ratio of the melt–solid density contrast to the magma viscosity—exhibits a peak at pressures corresponding to depths of 120–150 km, within the asthenosphere, up to an order of magnitude greater than …