FIGURE 5. Schematic diagram of the upwelling asthenospheric mantle beneath mid-ocean ridges.
From the following article:
Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean
Chuan-Zhou Liu, Jonathan E. Snow, Eric Hellebrand, Gerhard Brügmann, Anette von der Handt, Anette Büchl & Albrecht W. Hofmann
Nature 452, 311-316(20 March 2008)
doi:10.1038/nature06688
Both enriched and refractory mantle are distributed among the 'matrix' of normally depleted mantle. The enriched mantle and the normally depleted mantle can be sampled by E-MORB and N-MORB, respectively. However, it is hard to sample the refractory mantle with inherited ancient depletion signals because it contributes little to the genesis of MORB. Therefore, they are not a part of the 'MORB source' sensu stricto, although the latter has been generally taken to be synonymous with asthenospheric mantle.
