Nice find Don...I should've guessed you'd pull something up quickly.

This is a nice effort by Green II & co. to attempt to resolve this issue.

Anyway, here's a comment on this paragraph:

"When the temperature is high enough and the stress is very important, the olivine transforms quickly into spinel by nucleation and growth mechanism making it very weak and therefore subject to breaking under high stress by the olivine that has already been transformed into spinel and therefore denser and heavier. But this very important stress required is not known to exist, even in the mantle. Therefore, another factor must also be involved."

==> Note: The sentence:

"But this very important stress required is not known to exist, even in the mantle. Therefore, another factor must also be involved."

Some might argue that this study merely transformed the context of this paradox...from the plate tectonics context to the olivine transformation realm (if I understand what is being said).

Another thought is that if you have a blob of spinel enclosed by "lava" fluids then wouldn't the fracture halt at the fluid boundaries? Also, why would the fracture(s) create a net force vector versus just forming (smaller) cancelling force vectors (due to random directional fractures in the spinel)?

While the paper appears to be a nice attempt there still appears to be further conditions which must be satisfied....

I'm just flushing out all the (standard?) explanations and playing devil's advocate...my remarks are open to anyone....