| dc.description.abstract | Transition zone discontinuities at roughly 410 and 660 km depth result from phase transitions in mantle minerals. Depending on local temperature and composition, these seismic discontinuities are either depressed or elevated relative to a global average. According to mineral physics, hot olivine-dominated regions should have a thinner transition zone. However, in hot garnet-dominated regions both the 410-
and 660-discontinuities may be depressed, resulting in no net change in transition zone thickness. Nevertheless, transition zone topography can provide essential clues for hotspot origin depth, which puts constraints on mantle convection. Hotspots fed by mantle plumes from the lower mantle is an argument in favour of whole-mantle convection. We have studied the transition zone in the mid-Pacific Ocean covering three known hotspots: Marqueses, Tahiti and Pitcairn. The transition zone is imaged with precursors of the SS-wave that reflect off the discontinuities. Imaging through slowness-time stacking is complicated by the large X-shaped Fresnel zone of underside reflections and presumed narrowness of mantle plumes. Therefore reverse time migration using the scalar wave equation and imaging techniques common in exploration seismics are applied to the same data set. Stacking results show that the transition zone is slightly thinner than average near the three hotspots, which might indicate a mantle plume origin with olivine-dominated discontinuities. Migrated images are blurred due to limited aperture, among other factors, but may show topography of the 410-discontinuity. | |
