Why is the geochemistry of hotspot basalts so complex? more here
Below we show that as the oceanic crust is subducted to the lowermost mantle, it takes multiple pathways. Some oceanic crust is directly entrained into mantle plumes, but a significant fraction enters the primordial piles. As a result, plumes forming on top of piles entrain a variable combination of relatively young oceanic crust directly from the subducting slab, older oceanic crust that has been stirred with ancient more primitive material and background, depleted mantle. Cycling of oceanic crust through mantle reservoirs can therefore reconcile observations of different recycled oceanic crustal ages and explain the chemical complexity of hotspot lavas.
(a)oceanic crust is directly entrain into mantle plumes
(b)some oceanic crust accumulates on top of the primordial piles
(c)the accumulation of oceanic crust sinks into the piles. At the same time, plumes entrain variables components.
(d-e)the oceanic crust within the piles is stirred with mantle flow
This video shows a numerical simulation of Earth's deep mantle. The top panel is temperature and the bottom panel is composition which includes three components: the more-primitive reservoir at the lowermost mantle (cyan), the subducted oceanic crust (yellow) and the depleted background mantle (black).