Garnet-controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

Deep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower-upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660km depth. We use subsequent forward modelling of mineral assemblages, seismic velocities, and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature-dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite-rich assemblages at undisturbed thermal conditions. Neglecting this ultralow-velocity zone at upwelling sites can cause a miscalculation of the lower-upper mantle boundary on the order of 20km.