We study the effects of a subducting lithospheric slab on the 410 and 660 km mantle seismic discontinuities in the Izu-Bonin region using data collected from networks of vertical component seismometers in the United Kingdom and northwestern United States. The consequences of a phase change origin for these discontinuities is tested by examining the effect of laterally changing temperature in and near the subducting slab. We analyse the data collected from 21 earthquakes with depths between 40 and 548 km. Based on arrival time and slowness we identify reflections and mode conversions in slant stacks with the aim of mapping the 410 and 660 km discontinuities. The combination of UK and US array data permits interrogation of the discontinuity properties on both sides of and also inside the slab. Results indicate a 410 minimum depth of 350 km in the interior of the slab and a 660 maximum depth of 700 km. A phase change origin for the discontinuities is consistent with the effects of lateral temperature changes on discontinuities with Clapeyron slopes of opposite signs. Here we see more topography on the 410 than the 660 consistent with thermodynamic Clapeyron slope estimates. 410 elevation suggests temperatures in the slab of 600¿75 ¿C at a depth of 350 km. We see weak evidence for a broader 410 near the slab interior, consistent with expected thermochemical effects.¿ 1997 American Geophysical Union