Hot spot volcanics seem to tap a deeper and more primitive source. Volatiles may play an important role in crust - mantle interaction. Near surface metamorphism in rift zones with local high geothermal gradients, while cool subducting eclogites lower the geothermal gradient. The depleted peridotite may make up the mesosphere the lower, denser mantle, while lighter fertile peridotite makes up the asthenosphere - the upper 650 km. The implication is that the mantle 6 crust differentiation is largely a one way process.
Further discussion requires thermodynamics.