Dachine diamonds have C and N isotopic compositions indicating a sedimentary source. Disaggregated nitrogen defects severely limit diamond residence time in the mantle. Garnet and clinopyroxene inclusions are eclogitic and indicate a metal-rich source. Inclusion and xenocryst thermobarometry indicates formation at ~ 200–250 km depth. We suggest a model for diamond growth at the slab–mantle interface in a subduction zone.