What is geochemical evidence for mantle convection (or subduction)?

Answer 1

Basically the evidence is in magmas from OIBs (ocean island basalts) and MORBs (mid ocean ridge basalts) OIBs have excess water and other trace and isotopic evidence that proves the material interacted with surface elements recently. MORBs are very uniform and have little waters and volitles. The uniform nature of MORBs show the well-mixed nature of the mantle.

Answer 2

Evidence: Volcanoes. When there's subduction, magma rises to the surface to form vocanoes.