Bent-shaped plumes and horizontal channel flow beneath the 660km discontinuity

Recent high-resolution seismic imaging of the transition zone topography beneath the Hawaiian archipelago shows strong evidence for a 1000 to 2000km wide hot thermal anomaly ponding beneath the 660km boundary west of Hawaii islands [Q. Cao et al. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii. Science (2011), 332, 1068-1071]. This scenario suggests that Hawaiian volcanism may not be caused by a stationary narrow plume rising from the core-mantle boundary but by hot plume material first held back beneath the 660km discontinuity and then entrained under the transition zone before coming up to the surface. Using a cylindrical model of iso-chemical mantle convection with multiple phase transitions, we investigate the dynamical conditions for obtaining this peculiar plume morphology. Focusing on the role exerted by pressure-dependent thermodynamic and transport parameters, we show that a strong reduction of the coefficient of thermal expansion in the lower mantle and a viscosity hill at a depth of around 1800km allow plumes to have enough focused buoyancy to reach and pass the 660km depth interface. The lateral spreading of plumes near the top of the lower mantle manifests itself as a channel flow whose length is controlled by the viscosity contrast due to temperature variations {increment}η_T. For small values of {increment}η_T, broad and highly viscous plumes are generated that tend to pass through the transition zone relatively unperturbed. For higher values (10²≤{increment}η_T≤10³), we obtain horizontal channel flows beneath the 660km boundary as long as 1500km within a timescale that resembles that of Hawaiian hotspot activity. This finding could help to explain the origin of the broad hot anomaly observed west of Hawaii. For a normal thermal anomaly of 450K associated with a lower mantle plume, we obtain activation energies of about 400kJ/mol and 670kJ/mol for {increment}_ηT=10² and 10³, respectively, in good agreement with values based on lower mantle mineral physics. If an increase of the thermal conductivity with depth is also included, our model can predict both long channel flows beneath the 660km discontinuity and also values of the radial velocity of local blobs sinking in the lower mantle of around 1cm/yr, in good agreement with those inferred for the sinking rate of cold remnants of the subducted lithosphere.