1. Where is subducted oceanic crust? more here

One hypothesis for the cause of LLSVPs is that they are thermochemical piles caused by accumulation of subducted oceanic crust at the CMB. However, although subducted oceanic crust is denser than its surroundings, it was unclear whether thin oceanic crust could provide enough negative buoyancy to overcome viscous stresses that act to stir the crust into the mantle. Our results find that viscous forces caused by mantle plume regions are stronger than the negative buoyancy of subducted oceanic crust, so the crust is easily stirred into the background mantle. A small amount of crustal material may collect at the base of plumes, but it is sufficiently entrained away into the plume and does not accumulate into larger-scale thermochemical structures. Therefore, it is difficult for a thin subducted oceanic crust (~6 km) to accumulate into large piles at the CMB with the same size as LLSVPs.

(a) Snapshot (at 1.0 Gyr) of the nondimensional temperature superimposed with oceanic crust superimposed (shown in green). (b) Logarithm of nondimensional viscosity at 1.0 Gyr. The black lines are contours of viscosity with an interval of 0.5. (c) Nondimensional temperature and oceanic crust at 2.0 Gyr. (d) Nondimensional temperature and oceanic crust at 2.8 Gyr

The results of our numerical melding shows that, under present-day Earth-like conditions, it is difficult for the subducted oceanic crust to accumulate into large thermochemical piles at the core-mantle boundary. The three panels are temperature, viscosity and composition respectively. We see that the major parts of the subducted oceanic crust are carried up by upwelling plumes and mixed with ambient mantle, instead of settle at the core-mantle boundary.