We know that subduction is the key way to get carbon into the diamond-forming regions of the deep Earth. But which rocks in the subducting slab take carbon down?
Margo Regier studied diamonds from West Africa and showed that carbonated igneous oceanic crust (not sediment) is the primary carbon-bearing reservoir --- and diamond source --- in slabs that are subducted to deep lithospheric and transition zone depths (less than 660 km).
In contrast, diamonds that form below 660 km (in the lower mantle) crystallised from convecting-mantle carbon --- after the dehydrating slab triggered carbon mobilisation from its metallic hosts.
These results are consistent with experimental evidence that demonstrates major obstacles to transporting carbonated oceanic crust to lower-mantle depths. Read more here
Ferropericlase inclusions in a diamond from Kankan in West Africa. Image by Anetta Banas / University of Alberta.