Jap team maps 'semi-infinite' rare earth reserves, page-16

Not sure your main cause for opposition holds water. The ionic clays that make China so rich in REEs have grades from 0.05% to 0.3%, but nonetheless they are economically exploitable because they are cheap to process. Certainly doesn't hurt that they have low radioactivity.

I think they take the top ten meters of the mud.
Also not sure sea-bottom muds and ionic clays are comparable from the standpoint of processing simplicity; tempting to assume, but I suspect not.
I'll have to do more dredging.