Abstract Mantle-derived peridotite xenoliths contain abundant olivine with rounded spinel and orthopyroxene inclusions, and orthopyroxene with rounded olivine and spinel inclusions. The shape-change of spinel, olivine and orthopyroxene inclusions from the primarily polyhedral outline to the spherical outline is governed by interfacial diffusion of oxygen in spinel (the most sluggish atom) due to reduction of total interfacial energy of the host-inclusion system. The critical radius of maximum rounded inclusions of spinel in olivine is a function of temperature and annealing time. Assuming that the activation energy for the interfacial diffusion is 40–70 kcal mol −1 and that the annealing time for the spinel lherzolite from Salt Lake Crater in Hawaii is 100 Ma, the annealing times for perioditites under the island arcs of Japan are estimated to be 1–10 Ma.