Upward migration of subducted oceanic crust from deep in the upper mantle is discussed based on high-pressure experimental data. Numerical calculation reveals that a subducted oceanic crust, which is negatively buoyant in the present upper mantle, can ascend from the transition zone and be observed as a discrete magma source at the surface only when it has been broken into fragments comparable in dimension with its thickness and involved in a solid diapir of heated peridotite. For example, a solid diapir of heated peridotite with a temperature excess of 200 ¿C can contain up to 15~20 vol % of fragments of oceanic crust when it passes through the upper mantle, and it may cause extensive basaltic volcanism at the surface. ¿ 1998 American Geophysical Union