The Sub-Continental Mantle and Crust — Evidence from Kimberlite Xenoliths
The examination of kimberlite xenolith populations can provide considerable insight into the nature of the mantle and lower crust beneath the cratons where kimberlites exist; the models proposed below cannot be extrapolated to continental margins or oceanic areas. The upper mantle is here regarded as the zone of the earth between (a) the seismically-determined Mohorovicic discontinuity at approximately 35–40 km beneath the cratons, and (b) the seismically-determined beginning of the transition zone at 300–400 km depth. Petrologically these two boundaries are tentatively interpreted as (a) the break between crustal basic granulites and upper-mantle harzburgite (reviewed by Dawson 1977), and (b) the onset of the inversion of upper-mantle orthorhombic olivine to its denser, cubic polymorphringwoodite. The fact that ringwoodite has not yet been identified in the kimberlites gives a maximum value of 300 km to the depth sampled by kimberlite.