There has been much debate on how the convection cells in the mantle are arranged: do they extend all the way to the outer core or are the different layers of convection cells? This seems to suggest that the convection goes very deep.

Also a topic of debate is how far down oceanic crust can get sucked into the mantle. We know that there are many deviations from the classic subduction zone model of a descending slab of mafic oceanic crust. Sometimes the slabs get detached and overridden, or the subducting slab may shallow and underplate the continental crust (such as the western US). Getting this oceanic material into the deep mantle may help explain hot-spot volcanism like Hawaii and Yellowstone. Does the sunken crust change the nature of the convection cells? Can it insulate the mantle above from the heat of the core? Can this play a role in the formation of kimberlite formation (diamond bearing volcanism that rises quickly from the mantle and into the crust)? Does it help explain carbonitites, which are carbonate rich magmas (think limestone volcanos) that are an important source for Rare Earth Elements. They are fairly rare and typically occurred only during specific time periods in earth’s history.

Remember that the current oceanic crust is fairly new, there is no oceanic crust older than the Jurassic. That means there are billions of years of oceanic crust that has been recycled into the mantle. That’s a lot of material!

As for the water, that is quite interesting. Lubrication, in my opinion, is not the implication here. Water has a way of making magma melt more easily. In the classic subduction model, as the the slab descends, it dewaters. That water gets added to the material at the lower crust, which helps it melt, which generates magma, which rises to create volcanos at the surface, such as the Cascade Range. (basically it changes the conditions for phase change.) If a similar process still works at the depths of the core mantle boundary, this too could help explain hot spots, kimberlites, and carbonites.

https://phys.org/news/2018-03-diamond-inclusions-free-boundary-upper.html

Article mentions implications for plate tectonics. Lubrication?

This solid science is refreshing compared to the never ending, flip-flopping and conjecture of Cosmology.