We analyze P-wave codas from 12 deep Izu-Bonin earthquakes recorded by hundreds of Western United States seismograph stations for evidence of small-amplitude phases caused by near-source mantle discontinuities. For nearly every event, the dominant phase in the coda is the result of an S-to-P conversion from a nearly horizontal discontinuity ranging in depth from 650 to 690 km. This is interpreted as a thermally depressed spinel to perovskite and magnesiow¿stite phase transition. If the 660-km seismic discontinuity is also associated with a change in chemistry, it would be dynamically depressed by a subducting slab. We consistently see that there is no nearly horizontal discontinuity between 700 and 1000 km with shear wave velocity contrast exceeding 1%; this observation places constraints on the properties of a postulated chemical discontinuity separating the upper and lower mantles.¿ 1997 American Geophysical Union