In Geochemistry Geophysics & Geosystems 8, Q03017 (2007) Australian National University researchers Jinshui Huang and Geoffrey Davies describe three-dimensional modeling of melting and mixing in the Earth's deep mantle . Their results jibe with the surviving ground truth visible at the Earth's surface -- only about 3% of the mantle has been spared melting at some point in the past. But that does not mean the whole ball of wax has been homogenized.
Upwelling solid mantle melts at mid-ocean ridges to form new crust, which eventually sinks back into the mantle at deep ocean trenches . But the brew is not well stirred-despite convective mixing
Their computer model shows how chemical patchiness in the mantle survives for about 2 billion years in spite of the surface turnover