The heat flux from an axial magma chamber into the convecting hydrothermal system at mid-ocean ridges is governed by the conductive boundary layer separating them. The nature of this critical interface has been modeled and seismically imaged, but its characteristics have hitherto rarely been documented in ocean crust. Here, hornfelsic rocks from the sheeted dike–gabbro transitions at two tectonic exposures of fast-spreading East Pacific Rise crust at the Pito and Hess Deeps and in the Oman and Troodos ophiolites are described. These rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700–1000 °C. These temperatures, in conjunction with their geological relationships, imply that the hornfels are preserved fragments of conductive boundary layers. Their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200–400 m. The heat flux across the hornfelsic aureoles (11–44 MW/km) is comparable to hydrothermal fluxes released along fast-spreading ridges, confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers.